ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT
VOLUME 2
MAIN WASTEWATER TREATMENTPROCESSES 2.4
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2.4 SECONDARY TREATMENT 2.4.1 AEROBIC SUSPENDED GROWTH BIOLOGICAL TREATMENT PROCESS 2.4.1.1 Equipment 2.4.1.2 Conventional activated-sludge process 2.4.1.3 Step aeration process 2.4.1.4 Complete-Mix Activated-Sludge Process 2.4.1.5 Tapered aeration process 2.4.1.6 Extended aeration process 2.4.1.7 Contact stabilization process 2.4.1.8 Oxygen activated sludge Process 2.4.1.9 Sequencing Batch Reactor Process (SBR) 2.4.1.10 Processes for biological nitrogen removal 2.4.1.11 Processes for biological phosphorus removal 2.4.1.12 Suspended growth aerated lagoons and ponds 2.4.1.13 Oxidation ditch 2.4.1.14 Membrane biological reactors (MBR) 2.4.1.15 Design parameters 2.4.1.16 Aerobic suspended growth biological treatment processes sunopsis 2.4.1.17 General Operation and maintenance 2.4.2 AEROBIC ATTACHED GROWTH BIOLOGICAL TREATMENT PROCESSES 2.4.2.1 Trickling filters (TF) 2.4.2.2 Packed-bed filters - Intermittent Sand filters (ISFs) 2.4.2.3 Packed-bed filters - Recirculating Sand filters (RSFs) 2.4.2.4 Packed-bed filters - Textile filters 2.4.2.5 Rotating Biological Contactors (RBC’s) 2.4.3 COMBINED AEROBIC TREATMENT PROCESSES
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2.4.3.1 Fixed film activated sludge 2.4.3.2 Fluidized Bed BioReactor Process (FBBR) 2.4.4 ANAEROBIC BIOLOGICAL TREATMENT PROCESSES 2.4.4.1 Anaerobic contact process (Suspended growth) 2.4.4.2 Anaerobic upflow sludge blanket processes (UASB) 2.4.4.3 Anaerobic filter processes (Attached growth) 2.4.4.4 Anaerobic fluidized-bed reactor (Attached & Suspended growth-AFBR) 2.4.4.5 Design Parameters 2.4.5 NATURAL WASTEWATER TREATMENT PROCESSES 2.4.5.1 Constructed Wetlands - Free Water Surface (FWS) 2.4.5.2 Constructed Wetlands - Subsurface Systems Horizontal Flow (HF) 2.4.5.3 Constructed Wetlands - Subsurface Systems Vertical Flow (VF) 2.4.5.4 Floating Aquatic Plant Systems 2.4.5.5 Stabilisation Ponds (WSPs) 2.4.6 CLARIFICATION PROCESSES 2.4.6.1 General 2.4.6.2 Secondary clarification 2.4.7 GLOSSARY
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2.4 SECONDARY TREATMENT Two basic processes, fixed-bed (attached growth) and fluid-bed (suspended growth), are used in conventional secondary biological treatment. The trickling filter has fixed-bed of stone, or plastic packing material that provides a growth surface for zoogleal bacteria and other organisms. The intermittent or recirculating sand filter and the rotating biological contactor system are other examples of the fixed-bed technique. The activated-sludge processes and sewage lagoons are fluid-bed systems. The activatedsludge process uses mechanical aeration and returns a percentage of the active sludge to the process influent. Lagoons or stabilization ponds and oxidation ditches do not routinely waste sludge, but multipond systems can have recirculation.
When designing secondary treatment processes, environmental engineers must consider the organic loading, microorganism concentration, contactor retention time, artificial aeration, liquids– solids separation, effluent quality, and process costs.
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2.4.1 AEROBIC SUSPENDED GROWTH BIOLOGICAL TREATMENT PROCESS Suspended growth biological treatment processes use continuous agitation and artificially supplied aeration of settled sewage together with recirculation of a portion of the active sludge that settles in a separate clarifier back to the aeration tanks. These processes vary in detention time, the method of mixing and aeration, and the technique of introducing the waste and recirculated sludge into the aeration tank. Clarified wastewater discharged from the primary clarifier is delivered into the aeration basin where it is mixed with an active mass of microorganisms (referred to as activated sludge) capable of aerobically degrading organic matter into carbon dioxide, water, new cells, and other end products. Aerobic biological oxidation of organic wastes
(David H.F. Liu, Bela G. Liptak, 1999)
After a specific treatment time, the mixed liquor passes into the secondary clarifier, where the sludge settles under quiescent conditions and a clarified effluent is produced for discharge. The process recycles a portion of settled sludge back to the aeration basin to maintain the required activated-sludge concentration (expressed in terms of mixed-liquor, volatile SS [MLVSS] concentration). The process also intentionally wastes a portion of the settled sludge to maintain the required SRT for effective organic (BOD) removal.
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2.4.1.1 Equipment The equipment requirements for suspended growth biological treatment processes includes an aeration tank, aeration, system settling tank, return sludge, and waste sludge. These are discussed in the following.
Aeration Tank The aeration tank is designed to provide the required detention time (depends on the specific modification) and ensure that the activated sludge and the influent wastewater are thoroughly mixed. Tank design normally attempts to ensure that no dead spots are created.
Aeration Aeration can be mechanical or diffused. Mechanical aeration systems use agitators or mixers to mix air and mixed liquor. Some systems use a sparge ring to release air directly into the mixer. Diffused aeration systems use pressurized air released through diffusers near the bottom of the tank. Efficiency is directly related to the size of the air bubbles produced. Fine bubble systems have a higher efficiency. The diffused air system has a blower to produce large volumes of lowpressure (5 to 10 psi) air, air lines to carry the air to the aeration tank, and headers to distribute the air to the diffusers, which release the air into the wastewater.
Settling Tank Activated sludge systems are equipped with plain settling tanks designed to provide 2 to 4 hours of hydraulic detention time.
Return Sludge The return sludge system includes pumps, a timer or variable speed drive to regulate pump delivery, and a flow measurement device to determine actual flow rates.
Waste Activated Sludge In some cases, the waste activated sludge withdrawal is accomplished by adjusting valves on the return system. When a separate system is used, it includes pumps, a timer or variable-speed drive, and a flow measurement device.
2.4.1.2 Conventional activated-sludge process
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Process The activated-sludge process is an aerobic, continuousflow, secondary treatment system that uses sludge-containing, active, complex populations of aerobic microorganisms to break down organic matter in wastewater. Activated sludge is a flocculated mass of microbes comprised mainly of bacteria and protozoa. In the activated-sludge process, bacteria are the most important microorganisms in decomposing the organic material in the influent. During treatment, aerobic and facultative bacteria use a portion of the organic matter to obtain energy to synthesize the remaining organic material into new cells. Only a portion of the original waste is actually oxidized to low-energy compounds such as nitrate, sulfate, and carbon dioxide; the remainder of the waste is synthesized into cellular material. In addition, many intermediate products are formed before the end products.
A conventional activated sludge process includes the following: •
Aeration tank. Aerobic oxidation of organic matter is carried out in this tank. Primary effluent is introduced and mixed with return activated sludge (RAS) to form the mixed liquor, which contains 1500–2500 mg/L of suspended solids. Aeration is provided by mechanical means. An important characteristic of the activated sludge process is the recycling of a large portion of the biomass. This makes the mean cell residence time (i.e., sludge age) much greater than the hydraulic retention time. This practice helps maintain a large number of microorganisms that effectively oxidize organic compounds in a relatively short time. The detention time in the aeration basin varies between 4 and 8 hours.
•
Sedimentation tank. This tank is used for the sedimentation of microbial flocs (sludge) produced during the oxidation phase in the aeration tank. A portion of the sludge in the clarifier is recycled back to the aeration basin and the remainder is wasted to maintain a proper F/M (food to microorganisms ratio).
Following figure is a conventional activated-sludge plant flow diagram. A return of activated sludge at a rate equal to about 25% of the incoming wastewater flow is normal; however, plants operate with recirculation rates from 15 to 100%. The mixture of primary clarifier overflow and
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activated sludge is called mixed liquor. The detention time is normally 6 to 8 hr in the aeration tank. Conventional activated-sludge plant flow sheet
(David H.F. Liu, Bela G. Liptak, 1999)
In a conventional plant, the oxygen demand is greatest near the influent end of the tank and decreases along the flow path. Plants built before the process was well understood provided uniform aeration throughout the tank. A conventional plant cannot accommodate variations in hydraulic and organic loadings effectively, and the final clarifier must be sized to handle a heavy solids load. Usually aeration units are in parallel so that a shutdown of one unit does not totally disrupt plant operation.
The conventional activated-sludge process is susceptible to shock and toxic loading conditions since longitudinal mixing is absent in aeration tanks. Summary method • Employing the conventional activated sludge modification requires primary treatment.
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• Conventional activated sludge provides excellent treatment; however, a large aeration tank capacity is required, and construction costs are high. In operation, initial oxygen demand is high. • The process is also very sensitive to operational problems (e.g., bulking)
WWTP in Psyttalia, Athens, Greece (5.600.000 PE- 730.000 m3/day)
2.4.1.3 Step aeration process Process Modifications have evolved as the activated-sludge plant has become more widely used and are described in the following paragraphs. One technique that furnishes more uniform oxygen demand throughout the aeration tank is introducing the primary settled waste at several points in the aeration tank instead of at a single point as in the conventional process. This modification is step aeration.The percentage of settled, activated sludge returned to the aeration tank is usually greater than in the conventional process (about 50% typically), and the detention time is reduced
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to 3 or 4 hr since the loading is more evenly distributed in the tank. Additional piping and pumps are required to distribute the waste to several locations; however, the improved performance is considered to be worth the expense. This operation mode increases the flexibility of the process to handle shock and toxic loading conditions.
Step-aeration type activated-sludge plant flow sheet
(David H.F. Liu, Bela G. Liptak, 1999)
Advantages •
Flexibility in operation
•
Appropriate for many types of municipal wastewater
•
Tolerance in toxic/huge loads
•
Compatible with all aeration types
•
Ability for nitrification/denitrification
•
Easy to upgrade/ convert to another activated sludge type process
Disadvantages
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•
Higher constructing cost comparing to conventional activated sludge process
•
Complex operation
•
Problems with equalization of flows
Summary method •
Step aeration requires primary treatment.
•
It provides excellent treatment.
•
Operation characteristics are similar to conventional.
•
It distributes organic loading by splitting influent flow.
•
It reduces oxygen demand at the head of the system.
•
It reduces solids loading on the settling tank
Operation Step-feed aeration actually is a step-feed process based on conventional activated sludge loading guidelines. The difference between step-feed and conventional operation is that in conventional activated sludge, the primary effluent and return sludge are introduced at one point only, the entrance to the aeration tanks. In step-feed aeration, the return sludge is introduced separately and, in many cases, allowed a short reaeration period by itself at the entrance to the tank. The primary effluent is admitted to the aeration tanks at several different locations. These locations distribute the waste load over the aeration tank and reduce oxygen sags in an aerator. If you introduce the influent near the outlet end of the aeration tank, the process will become similar to contact stabilization. Step-feed aeration distributes the oxygen demand from the wastewater over the entire aerator instead of concentrating it at the inlet end. Some of its advantages over conventional operation include less aeration volume to treat the same volumes of wastewater, better control in handling shock loads, and the potential for lower applied solids to the secondary clarifiers. When a conventional plant is operating above design loads or the secondary clarifiers cannot handle the solids load, switching to step-feed aeration or contact stabilization allows the operator to maintain more solids under aeration with a lower applied solids concentration to the secondary clarifiers. Successful operation requires transfer of wastes into the activated sludge solids in the short time interval before the waste reaches the effluent end of the aeration tank. Step-feed aeration can be operated on a variable basis by using combinations of different modes . Selection of the proper mode depends on influent characteristics (flow rate and waste strength) and the capabilities of the plant to handle these characteristics.
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. When dealing with a bulking sludge, the problem is usually caused by inadequate dissolved oxygen levels, which are conducive to the growth of filamentous organisms. Rising sludge usually results from high dissolved oxygen levels in the aerator and prolonged secondary clarifier detention times, which will cause denitrification. When attempting to cure a bulking or rising sludge problem, an increase in return sludge flows may be helpful to reduce solids on a short-term basis. This procedure should not be relied on to solve the problem. Increased return sludge flows will cause the sludge blanket in the secondary clarifier to drop initially, yet the blanket could rise as the result of excessive clarifier underflew rates. Try to find the return rate that produces the lowest sludge blanket and seek to correct the bulking or rising problem by modifying the aeration system operational mode. Your job as an operator is to select the best mode that will do the best job for the normal wastewater characteristics and meet effluent requirements. The step-feed mode of operation is controlled by many of the same procedures used for the conventional process. An exception is that the mixed liquor suspended solids determinations must be made at each point of wastewater addition. The purpose of this is to measure the waste content and dilution factor provided by the primary effluent in order to determine the total pounds of solids in the aeration tank.
2.4.1.4 Complete-Mix Activated-Sludge Process Process Completely mixed, activated-sludge system is an extension of step aeration and provides a uniform oxygen demand throughout the aeration tank. Mechanical aerators also provide mixing for this unit. The SS concentration in the mixed liquor is two to three times the concentration in most conventional plants. Aeration detention times are reduced to 2 to 4 hr. The sludge recycling ratio is generally high because the greater flow improves mixing.
NIREAS VOLUME 2 [2.4] 12 Completely mixed, activated-sludge treatment flow sheet
(David H.F. Liu, Bela G. Liptak, 1999)
Mixing intensity in the aeration tank of the completely-mixed, activated-sludge process
is
sufficiently high to yield a uniform mixed liquor that can smooth out and dilute load variations. As a result, the completely-mixed, activated-sludge process is resistant to shock and toxic loadings and is used widely for treating industrial wastewater. The aeration equipment is equally spaced for good mixing. Advantages/disadvantages
Advantages •
Great experience in construction and operation worldwide
•
Appropriate for many types of municipal wastewater
•
Tolerance in toxic/huge loads
•
Compatible with all aeration types
Disadvantages
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•
Susceptible to sludge bulking
Summary method •
The process may or may not include primary treatment.
•
It distributes waste, return, and oxygen evenly throughout the tank.
•
Aeration may be more efficient.
•
It maximizes tank use.
•
It permits a higher organic loading.
Operation The complete mix mode of operation is a design modification of tank mixing techniques that is made to ensure equal distribution of applied waste load, dissolved oxygen, and return sludge throughout the aeration tank. The theory of this modification is that all parts of the aeration tank should be similar in terms of amounts of food, organisms, and air. This is accomplished by providing diffuser location and application points of influent and return sludge to the aerator at several locations. Providing a similar condition throughout the entire aeration tank allows a food/organism ratio of 1/1 and still produces effluent qualities comparable to conventional operation. Generally, smaller aeration tanks are more completely mixed than larger ones. Usually aeration is more efficient in a complete mix process.
2.4.1.5 Tapered aeration process Process A less popular alternative to distributing the load to the aeration tank is to provide different quantities of oxygen along the tank length, related to the oxygen demand that gradually decreases along the tank length. This modification is tapered aeration. The flow sheet for tapered aeration is the same as that in for conventional activated-sludge plant. The disadvantage of tapered aeration is that although it is more economical due to reduced air quantities, it can only be designed for one loading. Tapered aeration plant flow sheet
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(David H.F. Liu, Bela G. Liptak, 1999)
Advantages •
Great experience in construction and operation worldwide
•
High removal of ammonia
•
Easy to convert to another activated sludge process
Disadvantages •
Design and implementation of different oxygen doses along the tank length may present some difficulties
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2.4.1.6 Extended aeration process Process The most popular activated sludge treatment process, is extended aeration treatment process. The extended-aeration process is similar to the conventional activated-sludge process except it operates in the endogenous respiration phase to reduce excess process sludge. As a result, the aeration basin is generally much larger. In extended aeration, as the name implies, the activatedsludge detention time is increased by a factor of 4 or 5 compared to conventional activated sludge, thus a total detention time of 24 to 40 hr is implemented. The primary clarifier in this process is not necessary
and only a preliminary wastewater treatment unit
is needed to remove coarse
materials and to protect treatment equipment. After extended-aeration process a secondary final settling clarifier is used, with a typical surface settling rate of 14 to 28 m3 per m2, for 4 hr detention time. The extended-aeration period reduces or eliminates the requirement for disposing excess sludge and is therefore a popular system for small plants. Wastewater treatment facilities minimize this amount by operating the process in the endogenous respiration phase with the SRT maintained in the range of 20–40 days. As a result, the cost incurred with sludge disposal is reduced.
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This system requires less aeration than conventional treatment and is mainly suitable for small communities that use package treatment.
Extended-aeration plant flow sheet
NIREAS VOLUME 2 [2.4] (David H.F. Liu, Bela G. Liptak, 1999)
17
The effluent produced is generally low in BOD and well nitrified.
Advantages/Disadvantages
Advantages •
Tolerance in toxic/huge loads
•
Compatible with all aeration types
•
Ability for nitrification/denitrification
•
High quality effluent
•
No primary sedimentation is necessary
•
Production of Stabilized sludge
Disadvantages •
Higher constructing cost comparing to conventional activated sludge process, due to larger aeration tanks
•
Increased power consumption due to intensive aeration
•
Mainly applicable for medium to small facilities
Operation & Maintenance An operational problem related to nitrification is a drop in pH which treatment facilities can correct by adding lime slurry to the aeration basin.
Summary method •
The process does not require primary treatment.
•
It is frequently used for small flows such as schools and housing subdivisions.
•
It uses 24-hour aeration.
•
It produces the least amount of waste activated sludge.
•
The effluent is low in BOD (the process is capable of achieving 95% or greater removal of BOD).
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•
The effluent is low in organic and ammonia nitrogen
2.4.1.7 Contact stabilization process
Process The contact-stabilization, activated-sludge process uses two separate tanks or compartments (contact and reaeration) to treat wastewater. This process first delivers the wastewater (usually without primary settling) into the aerated contact tank where it mixes with the stabilized sludge that rapidly removes suspended, colloidal, and a portion of the dissolved BOD (entrapment of suspended BOD in sludge flocs and adsorption of colloidal and dissolved BOD by sludge flocs). These reactions yield approximately 90% removal of BOD within 15 min of contact time . The mixed liquor then passes into the secondary clarifier where sludge is separated from clarified effluent. The settled sludge is recycled back to the reaeration tank where organic matter stabilization occurs. The resulting total aeration basin volume is typically 50% less than that of the conventional activated-sludge process.
In a typical contact stabilization process, the small contact tank, with detention times of about ½ hr binds the insoluble organic matter in the activated sludge. Clarification separates the contactsettled sludge from the supernatant. The smaller sludge volume is then aerated for an additional 3 or 4 hr. Since the total sewage flow is aerated for a shorter period (with only the returned activated sludge being aerated for longer periods), the aeration tank capacity is smaller than in conventional plants. With the activated sludge in two tanks, the plant is not out of operation when the contact tank is disabled. Sludge recycling percentages of 30 to 60% are normal with contact stabilization.
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(David H.F. Liu, Bela G. Liptak, 1999) Advantages/disadvantages Advantages •
Lower oxygen doses are needed
•
No MLSS loss when hydraulic overloads occur
Disadvantages •
Reduced ability for nitrification
•
Complex operation
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Contact stabilization does not require primary treatment.
•
During operation, the organisms collect organic matter (during contact).
•
Solids and activated sludge are separated from flow via settling.
•
Activated sludge and solids are aerated for 3 to 6 hr (stabilization).
•
Return sludge is aerated before it is mixed with influent flow.
•
The activated sludge oxidizes available organic matter.
•
Although the process is complicated to control, it requires less tank volume than other modifications and can be prefabricated as a package unit for small flows
•
A disadvantage is that common process control calculations do not provide usable information.
Operation Operation of an activated sludge plant on the basis of contact stabilization requires two aeration tanks. One tank is for separate reaeration of the return sludge for at least four hours before it is permitted to flow into the other aeration tank to be mixed with the primary effluent requiring treatment. Overall loading factors are the same as for conventional activated sludge. If the solids content in aeration tank "A" (mixed liquor aerator) and aeration tank "B" (return sludge reaeration only) are combined, the loading ratio of food/microorganisms is the same as conventional operation. However, if you only look at aeration tank Ά" where the load is applied, the food/microorganism ratio is nearly double the usual load ratio for conventional activated sludge. Contact stabilization attempts to have organisms assimilate (take in) and store large portions of the influent waste load in a short time (30 to 90 minutes). The activated sludge is separated from the treated wastewater in the secondary clarifier and returned to the reaeration tank "B." No new food is added to the reaeration tank and the organisms must use the waste material they collected and stored in the first aeration tank. When the stored food is used up, the organisms begin searching for more food and are ready to be returned to tank "A." Process controls for a contact stabilization plant are the same as those described for a conventional plant in this chapter. The contact stabilization system with its off-stream reservoir of organisms in aeration tank "B" avoids a complete solids wash-out when high flows occur or a kill of microorganisms when toxic wastes reach the plant. When calculating loading guidelines, the solids in the reaeration tank may be ignored. You must realize that the effluent quality will not be the same as for loadings on the conventional process, but the results from the calculations can be used to operate your plant and to reveal any trends.
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2.4.1.8 Oxygen activated sludge Process Process The oxygen-activated-sludge process uses high-purity oxygen instead of air. The aeration tanks are usually covered, and the oxygen is recirculated, reducing the oxygenation requirements. This process must vent a portion of the gas accumulated inside the aeration tank to remove carbon dioxide. However, adjusting the pH of the mixed liquor may still be needed. Since the amount of oxygen added in the oxygen-activated-sludge process is approximately four times greater than that available in the conventional activated-sludge process, the BOD loading applied is higher, yielding a small aeration basin volume. Experimental evidence also indicates that oxygen-activated sludge settles better than airactivated sludge. A facility for generating and supplying high-purity oxygen is needed at a treatment site.
Activated-sludge contactor using pure oxygen.
(David H.F. Liu, Bela G. Liptak, 1999)
Advantages/disadvantages
Advantages •
Tolerance in toxic/huge loads
•
Applicable to many types of municipal wastewater
•
Easy control and regulation of DO
•
Production of Stabilized sludge
Disadvantages •
Low ability for nitrification
•
Increased power consumption due to oxygen production
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•
Complex operation
•
Nocardia foaming formation
Summary method •
The process requires primary treatment.
•
It permits higher organic loading.
•
Higher solids levels are required.
•
It operates at higher F/M ratios.
•
It uses covered tanks.
•
The use of pure oxygen poses potential safety hazards.
•
Oxygen production is expensive.
2.4.1.9 Sequencing Batch Reactor Process (SBR) Equipment Preliminary Treatment Preliminary treatment requirements depend upon the makeup of the wastewater, the collection system type and length, and any other factor that may affect or occasionally degrade the quality of the plant's effluent. In most instances preliminary treatment at an SBR facility includes screening and removal of grit and grease.
Screens Screening devices are frequently installed ahead of SBRs to remove debris from the wastewater stream. SBRs are vulnerable to sticks, rags, and plastic material that can jam pumps or block automatic valves from opening or closing. If problem-causing debris is not found in the wastestream, then screens would not be necessary. Where screening devices are required, automatic fine screens or hand-raked bar racks may be installed. The type of screening device needed depends on the type and quantity of debris to be removed from the wastewater.
Grit Removal If grit is present in the wastewater, it is going to end up in the SBR's tanks. Grit removal is usually essential for waste-streams from combined sewers.
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Grease Removal Soaps, oils, and grease, if not removed prior to the SBR tanks, will accumulate into large floating balls and scum on the surface of the SBRs. This problem produces odors, hinders decant operations, degrades the effluent quality, and promotes insect breeding. Hand skimming of the reactors and chlorine contact chamber is a labor-intensive (and therefore expensive) process. The unsightly appearance and foul odors from floating grease and scum give visitors and regulators a bad impression of plant operation. Regardless of the type of collection system serving the SBR plant some form of grease removal prior to the SBR tanks should be provided if grease is present in the wastestream.
Primary Treatment Primary treatment is not provided in most SBR processes, but may be installed or kept in retrofitted activated sludge plants, or where excessive heavy solids loading is anticipated. Primary treatment would consist of primary sedimentation tanks or clarifiers.
Reactors 1. Number of Reactors The SBR activated sludge process requires at least two reactors. One reactor is filling and reacting while the other is settling and decanting. Some operators believe that there should be at least three reactors so that two will always be available for operation even if one tank is out of service for extended maintenance periods. 2. Tank Configuration Reactors have been constructed in various forms and shapes, including rectangular, circular, and oval. They may be equipped with straight or sloped side walls. 3. Reactor Depths The wastewater level in an SBR reactor reaches its maximum height during the fill stage, and it drops to its minimum level after the decant (drawdown) stage. After drawdown, a large portion of the tank's contents still remain in the reactor. For this reason SBR reactors are quite deep. When the reactor has been filled to its highest working water level, this is called the TOP WATER LEVEL (TWL). At the end of the decant cycle, after effluent has been drawn off to a prescribed depth, this depth is called the BOTTOM WATER LEVEL (BWL). The water depth of most SBR
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reactors ranges from 3.3 to 6.1 m for TWL, and 2.1 to 4.2 m for BWL. The operating range (from BWL to TWL) of the reactors is from 0.9 to 1.8 m of water depth. 4. Decanters Decanters are installed in each reactor to remove the mixed liquor following treatment. Several types of decanting equipment are produced by several different manufacturers. Some decanters use fixed-elevation submersible pumps while others use a siphon. Other decanters may be elevated above the reactor's water surface in a so-called park mode. Decanters of the launder type convey the treated effluent from near the surface of the reactor after the settling period. Some decanters are designed to move downward with the fall of the water level in the reactor. Decanter working levels are from the TWL down to the prescribed set point of the tank water elevation for the BWL. 5. Aeration/Mixing Mixing or aeration of the reactor may be accomplished by mechanical mixers or by blowers supplying air to either jet or fine/coarse bubble diffusers. In some instances, a combination of mixing devices is used in the reactors to conserve energy. One disadvantage of using air diffusers in SBRs is the possibility of overaeration. At reactor BWLs, the lower head over air diffusers can greatly increase the air flow rates. 6. Wasting 7. The settled activated sludge blanket in the reactor ranges from 1.5 to 3.6 m in depth, depending on the depth of the reactor. Usually the sludge blanket measures 40 to 50 % of the TWL. Wasting of excess activated sludge from each reactor may be accomplished in either of two ways: (1) gravity drawoff lines can be used to remove mixed liquor either during the aeration or settling periods, or (2) a submersible pump can be used. In some designs, the pump can be raised or lowered to various levels in the reactor. The waste activated sludge (WAS) is processed through other sludge handling facilities such as aerobic digesters or sludge drying beds. Sequencing Control SBRs are controlled by a microprocessor controller called a programmable logic controller (PLC). The PLC controls the sequencing of reactor cycles for each stage of treatment by operating pneumatic, solenoid, or motorized valves. The PLC regulates aeration or mixing devices, and it controls the decanting equipment through level sensors, automatic timers, and flowmeters. The PLC may also be programmed to control other plant equipment such as screens, influent pumps,
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and chlorinators. These components of automated equipment and the computer SOFTWARE 30 are supplied by the manufacturers of the SBRs. The PLC hardware is built in modules and replacement of a faulty module is not difficult. An internal battery powers the PLC in case of a power failure. If both the battery and the main power supply fail, the software is backed up by a memory chip and can easily be reloaded.
Sludge Wasting Sludge wasting is normally required to control reactor mixed liquor solids. This, in turn, controls the F/M (food to microorganism ratio) or solids retention time (SRT) sludge age for the activated sludge process. An exception may be found in a few very small, low-flow plants with long sludge ages; these plants may function for long periods (several months) without the need of wasting solids. SBR plants with sludge ages of 30 to 45 days produce a fairly stable (oxidized) sludge that may not require additional oxidation. These plants generally waste directly to drying beds or compost systems for disposal of the activated sludge solids. Plants operating with lower sludge ages require additional oxidation of the wasted solids in separate facilities. The additional treatment may consist of aerobic or anaerobic digestion or wet combustion. Additional treatment is needed because waste sludges with lower sludge ages or solids retention times (SRT) contain a larger amount of residual volatile organic material, which could cause odors and other nuisances. How the SBR plant is operated depends on the number of reactors that are available, the configuration or design of the reactors and, as previously mentioned, the quality of the effluent required to be produced.
Post Treatment Usually the only post treatment of the SBR plant's effluent is disinfection before discharge. Chlorine or chlorine compounds are the most widely used disinfecting agent .A significant factor affecting SBR effluent disinfection is the intermittent release of treated effluent from the reactors. After the settling period in the reactor, the decant cycle takes place. Depending on the decant equipment and the operation program in the PLC, a very rapid withdrawal of effluent from the reactor may occur. The effluent flow rate may be as high as four times normal plant inflow. Therefore, disinfection facilities must be sized to accommodate the high flow releases. In particular, tankage must be available to provide adequate contact time with the disinfection agent, and
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disinfection chemical addition equipment must be sized to provide the correct dosage rates to achieve adequate disinfection. Process The sequencing batch reactor (SBR) is a single, fill-anddraw, completely-mixed reactor that operates under batch conditions. Recently, SBRs have emerged as an innovative wastewater treatment technology. SBRs can accomplish the tasks of primary clarification, biooxidation, and secondary clarification within the confines of a single reactor. A typical treatment cycle consists of the following five steps: fill, react, settle, draw, and idle. Depending on the mode of operation, SBRs can achieve good BOD and nitrogen removal.
1. Wastewater
fills the tank,
mixing
with
biomass
settles
during the
that
previous
cycle. 2. Air is added to the tank to aid biological growth and felicitate waste reduction. 3. Mixing
and
Aeration
stop
during this stage to allow solids to settle. 4. Clarified effluent is discharged. 5. Sludge can be removed during this stage.
NIREAS VOLUME 2 [2.4] 27 1. Fill process
2. React process
NIREAS VOLUME 2 [2.4] 28 3. Settling process
4. Draw/decant process
NIREAS VOLUME 2 [2.4] 5. Idle/sludge waste process
29
Design capacity SBRs are uniquely suited for wastewater treatment applications characterized by low or intermittent flow conditions.
Sequencing batch reactor systems are particularly practical for smaller municipal flows for three reasons: 1. The SBR plant's process controls are implemented by a PROGRAMMABLE LOGIC CONTROLLER (PLC).37 Once the SBR system is put online and the start-up bugs have been worked out (normally performed by the contractor and equipment supplier), the PLC automatically controls routine process sequencing. 2. The SBR process is very stable due to the high sludge age associated with long sludge retention times (SRT). Since the treatment processes all take place in a single tank, high storm flows are less likely to cause sludge washouts. 3. Constructing an SBR plant usually costs less than a conventional activated sludge plant due to the elimination of secondary clarifiers and, in most instances, return sludge facilities. In some plants separate sludge digestion facilities are not needed due to the light organic
NIREAS VOLUME 2 [2.4] 30
loading and the high sludge age maintained in the SBR process. Sludge is wasted directly from the reactors to drying beds or composting processes
Sequencing batch reactor (SBR) activated-sludge process schematic diagram
(Eddy, 1999)
NIREAS VOLUME 2 [2.4] 31 View of a typical SBR reactor
(Eddy, 1999)
NIREAS VOLUME 2 [2.4] 32 View of movable weir used to decant contents of SBR reactor
(Eddy, 1999) Advantages/disadvantages
Advantages •
Simple process
•
Flexibility in operation
•
Appropriate for many types of wastewater
•
Equalization, primary clarification (in most cases), biological treatment, and secondary clarification can be achieved in a single reactor vessel.
•
Minimal footprint
•
Potential capital cost savings by eliminating clarifiers and other equipment
Disadvantages •
Complex operation
•
Increased hydraulic loadings may disturb the process
•
A higher level of sophistication of timing units and controls is required (compared to conventional systems), especially for larger systems
NIREAS VOLUME 2 [2.4] 33
•
Higher level of maintenance (compared to conventional systems) associated with more sophisticated controls, automated switches, and automated valves
•
Potential of discharging floating or settled sludge during the draw or decant phase with some SBR configurations
•
Potential plugging of aeration devices during selected operating cycles, depending on the aeration system used by the manufacturer
•
Potential requirement for equalization after the SBR, depending on the downstream processes
Operation
1. STAGE 1—FILL OR FEED The fill or feed stage admits raw wastewater or primary effluent. The fill process allows the reactor water level to rise from the BWL, which is the level of the reactor after the decant or idle cycle. The reactor is filled to or nearly to capacity, which is the TWL. The maximum level during fill ranges from 60 to 100 percent of the reactor volume. The same range applies for the draw (decant) stage. The BWL during the idle period ranges from 60 to 80 percent of the tank volume. The time (length) of the cycle is controlled by the PLC responding to a timer or water level sensors in the reactor. Fill cycles vary considerably due to the process control mode and flow rates at that particular time. For example, PLCs may be programmed to accelerate sequencing process cycles during wet weather periods. During the fill cycle, some degree of treatment is taking place within the reactor. The activated sludge microorganisms that remained in the reactor after the last draw cycle continue to break down and METABOLIZE wastes.
2. STAGE 2—REACT OR AERATION The start of the react or aeration cycle may depend on the type of aeration devices that are used to provide dissolved oxygen to the mixed liquor. It is desirable to immediately start mixing the reactor to resuspend the settled activated sludge
and mix the influent with the mixed liquor. Mechanical aerators may start as soon as the fill cycle begins and continue operating until the end of the react cycle. Diffused air systems may be programmed not to start aeration until the reactor is almost to the TWL. This prevents overaeration
NIREAS VOLUME 2 [2.4] 34
and inefficient blower operation due to the low head over the diffusers. Some SBRs with diffused air aeration are designed to operate at reduced air flow rates during the fill cycle to facilitate mixing. Then, when the reactor reaches a predetermined water level, the air flow rates are increased to maintain the desired dissolved oxygen levels and mixing in the reactor. The treatment that occurs during the fill and react cycles is what would occur in a standard aeration tank where microorganisms convert the nutrients (organic material) to cell growth and provide some oxidation. The timing of the cycle, which is programmed into the PLC, is designed to achieve the particular plant's effluent quality requirements.
3. STAGE 3—SETTLE All mixing or agitation of the reactor is stopped. This permits the activated sludge floe to form and settle to the bottom of the reactor, creating a solids blanket and leaving a clear supernatant above the settled solids. The rate of solids separation is determined by the condition of the activated sludge within the reactor. This may be observed by obtaining a sample of mixed liquor from the reactor near the end of the react cycle. The sample may be analyzed in the laboratory to determine the SLUDGE VOLUME INDEX (SVI), or a simple JAR TEST may be used to measure sludge settling. The SBR frequently produces a better effluent more efficiently than a conventional system because in the settle cycle of the reactor the wastewater is completely quiescent (still). The length of the settle cycle is based on the settling rates (SVI) usually found in the reactor. Cycle length may be controlled by elapsed time or by sludge blanket sensors. Sludge blanket sensors can be set to signal the PLC when the bottom of the supernatant layer or top of the sludge blanket reaches a predetermined level. SVIs may differ from reactor to reactor in the same wastewater treatment plant and from day to day in the same reactor. SVIs will be affected by such factors as influent wastewater strength, presence of filamentous bacteria, and presence of compounds that are toxic to the activated sludge microorganisms. 4. STAGE 4—DECANT (Withdrawal of Effluent) The decant cycle provides the time to remove the upper layer of clarified wastewater from the reactor; this liquid is the plant effluent. The decant equipment is automatically actuated and starts the removal of effluent. Surface material (scum or floating debris) is prevented from leaving the reactor with the effluent by means of weirs or baffles. This prevents degrading the effluent due to an increase in suspended solids. The decant cycle lowers the reactor's wastewater liquid level
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from TWL down to the predetermined BWL. When the BWL is reached, the decant equipment is automatically taken out of service. Depending on the decant equipment, decant cycles may be short or long, but this cycle typically requires 45 minutes. Shorter or longer cycles may be programmed. 5. STAGE 5—IDLE An idle cycle is used in SBR plants that have several reactors in service. This cycle lets one reactor completely fill during its fill cycle before switching to another reactor. Since idle is not a necessary stage, it is sometimes omitted from the process program. During the idle cycle or at the end of the decant cycle, waste sludge may be removed from the reactor. 2.4.1.10
Processes for biological nitrogen removal
Nitrogen can be removed from incoming wastewater by chemical–physical means (e.g., breakpoint chlorination or air stripping to remove ammonia) or by biological means, which consist of nitrification followed by denitrification. Indicative concentrations of nitrogen compounds in wastewater are given in the next table :
Compound
Concentration (mg/L)
TN
20-85*
TCN
8-35
Ammonia
12-50
NO2- , NO3-
-
*In many parts of the world with limited water supplies, total nitrogen concentrations in excess of 200 mg/L as N have been measured in domestic wastewater.
Nitrification Nitrification is the term used to describe the two-step biological process in which ammonia (NH4-N) is oxidized to nitrite (NO2-N) and nitrite is oxidized to nitrate (NO3-N). The need for nitrification in wastewater treatment arises from water quality concerns over (1) the effect of ammonia on receiving water with respect to DO concentrations and fish toxicity, (2) the need to provide nitrogen removal to control eutrophication, and (3) the need to provide nitrogen control for water-reuse applications including groundwater recharge.
NIREAS VOLUME 2 [2.4] 36
Aerobic autotrophic bacteria are responsible for nitrification in activated sludge and biofilm processes. Nitrification, as noted above, is a two-step process involving two groups of bacteria. In the first stage, ammonia is oxidized to nitrite by one group of autotrophic bacteria. In the second stage, nitrite is oxidized to nitrate by another group of autotrophic bacteria. It should be noted that the two groups of autotrophic bacteria are distinctly different.
Main bacteria commonly noted for nitrification in wastewater treatment are the autotrophic bacteria Nitrosomonas and Nitrobacter, which oxidize ammonia to nitrite (NO2-N) and then to nitrate (NO3-N), respectively. There are two nitrification systems in suspended growth reactors:
1. Combined carbon oxidation-nitrification (single-stage nitrification system). This process is characterized by a high BOD5/TKN ratio and has a low population of nitrifiers. Most of the oxygen requirement is exerted by heterotrophs . 2. Two-stage nitrification. Nitrification proceeds well in two-stage activated sludge systems. In the first stage, BOD5 is removed, while nitrifiers are active in the second stage. Nitrification is pH-sensitive and rates decline significantly at pH values below 6.8. At pH values near 5.8 to 6.0, the rates may be 10 to 20 % of the rate at pH 7.0 . Optimal nitrification rates occur at pH values in the 7.5 to 8.0 range. A pH of 7.0 to 7.2 is normally used to maintain reasonable nitrification rates, and for locations with low-alkalinity waters, alkalinity is added at the wastewatertreatment plant to maintain acceptable pH values. The amount of alkalinity added depends on the initial alkalinity concentration and amount of NH4-N to be oxidized. Alkalinity may be added in the form of lime, soda ash, sodium bicarbonate, or magnesium hydroxide depending on costs and chemical handling issues.
Nitrifying organisms are sensitive to a wide range of organic and inorganic compounds and at concentrations well below those concentrations that would affect aerobic heterotrophic organisms. In many cases, nitrification rates are inhibited even though bacteria continue to grow and oxidize ammonia and nitrite, but at significantly reduced rates. In some cases, toxicity may be sufficient to kill the nitrifying bacteria. Nitrifiers have been shown to be good indicators of the presence of organic toxic compounds at low concentrations.
NIREAS VOLUME 2 [2.4] 37
Denitrification Nitrification must be followed by denitrification to remove nitrogen from wastewater. The biological reduction of nitrate to nitric oxide, nitrous oxide, and nitrogen gas is termed denitrification.
The most common process used for biological nitrogen removal in municipal wastewater treatment is called Modified Ludzak-Ettinger (MLE) process. The process consists of an anoxic tank followed by the aeration tank where nitrification occurs. Nitrate produced in the aeration tank is recycled back to the anoxic tank. Because the organic substrate in the influent wastewater provides the electron donor for oxidation reduction reactions using nitrate, the process is termed substrate denitrification. Further, because the anoxic process precedes the aeration tank, the process is known as a preanoxic denitrification.
Substrate driven (preanoxic denitrification) process
(Eddy, 1999)
NIREAS VOLUME 2 [2.4] 38
In the second process shown on next Figure, denitrification occurs after nitrification and the electron donor source is from endogenous decay. The process illustrated is generally termed a postanoxic denitrification as BOD removal has occurred first and is not available to drive the nitrate reduction reaction. When a postanoxic denitrification process depends solely on endogenous respiration for energy, it has a much slower rate of reaction than for the preanoxic processes using wastewater BOD.
Endogenous driven (postanoxic denitrification)
(Eddy, 1999)
The denitrification preanoxic and postanoxic processes described employ heterotrophic bacteria for nitrate reduction, but other pathways for biological nitrogen Removal exist. Ammonia can be converted to nitrogen gas by novel autotrophic bacteria under anaerobic conditions and by heterotrophic-nitrifying bacteria under aerobic conditions. A wide range of bacteria has been shown capable of denitrification, but similar microbial capability has not been found in algae or fungi. Bacteria capable of denitrification are both heterotrophic and autotrophic. Pseudomonas species are the most common and widely distributed of all the denitrifiers and have been shown to use a wide array of organic compounds including hydrogen, methanol, carbohydrates, organic acids, alcohols, benzoates, and other aromatic compounds.
NIREAS VOLUME 2 [2.4] 39 2.4.1.11
Processes for biological phosphorus removal
Process In wastewater treatment plants, phosphorus is removed by chemical means (e.g., P precipitation using iron or aluminum) and by microbiological means. The principal advantages of biological phosphorus removal are reduced chemical costs and less sludge production as compared to chemical precipitation. All the processes incorporate aerobic and anaerobic stages and are based on phosphorus uptake during the aerobic stage and its subsequent release during the anaerobic stage .
The commercial processes can be divided into mainstream and sidestream processes.
The most popular processes are described below.
Mainstream Processes 1. (Anaerobic/Oxic) Process The A/O process consists of a modified activated sludge system, which includes an anaerobic zone (detention time =0.5–1 h) upstream of the conventional aeration tank (detention time =1–3 h). During the anaerobic phase, inorganic phosphorus is released from the cells as a result of polyphosphate hydrolysis. The energy liberated is used for the uptake of BOD from wastewater. Removal efficiency is high when the BOD/phosphorus ratio exceeds 10. During the aerobic phase, soluble phosphorus is taken up by bacteria that synthesize polyphosphates, using the energy released from BOD oxidation. The A/O process results in phosphorus and BOD removal from effluents and produces a phosphorus-rich sludge. The key features of this process are the relatively low SRT (solid retention time) and high organic loading rates
A/O process
(Eddy, 1999)
NIREAS VOLUME 2 [2.4] 40
2. Bardenpho Process This system, removes nitrogen by nitrification窶電enitrification as well as phosphorus. The process consists of two aerobic and two anoxic tanks followed by a sludge settling tank. Tank 1 is anoxic and is used for denitrification, using wastewater as a carbon source. Tank 2 is an aerobic tank utilized for both carbonaceous oxidation and nitrification. The mixed liquor from this tank, which contains nitrate, is returned to tank 1. The anoxic tank 3 removes by denitrification the nitrate remaining in the effluent. Finally, tank 4 is an aerobic tank used to strip the nitrogen gas that results from denitrification, thus improving mixed liquor settling.
Bardenpho Process
(Eddy, 1999)
Operation The Bardenpho process is used to remove between 90 and 95 % of all the nitrogen present in the raw wastewater by recycling nitrate-rich mixed liquor from the aeration basin to an anoxic zone located ahead of the aeration basin. Denitrification takes place in the anoxic zone in the absence of dissolved oxygen. Further denitrification may be obtained by adding a second anoxic basin for the removal of nitrate remaining after recycling. The degree of nitrate removal depends on the rate of recycling the mixed liquor from the aeration basin. Some plants have three recycle pumps that allow pumping of two, four, or six times the average dry weather flow back to the anoxic zone. Usually pumping four times the average dry weather flow is sufficient to achieve satisfactory nitrate removal. The correct recycle flow can be determined by monitoring the nitrate level in the effluent of the first anoxic basin. If the nitrate concentration in the effluent rises above about 1 mg/L, the recycle rate is too high because not enough detention time is provided in the anoxic zone for denitrification to occur.
NIREAS VOLUME 2 [2.4] 41
If phosphorus removal is desired, a fermentation stage (tank) is added before the first anoxic zone. The return activated sludge is mixed with the influent to produce an organism stress condition in the absence of dissolved oxygen and nitrate. This stress condition allows phosphorus to be removed biologically in subsequent aeration basins
Sidestream Processes PhoStrip is a sidestream process that is designed for phosphorus removal by biological as well by chemical means.Sidestream return activated sludge is diverted to an anaerobic tank called an anaerobic phosphorus stripper, where phosphorus is released from the sludge. The phosphorusrich supernatant is treated with lime to remove phosphorus by chemical precipitation. The solid retention time in the anaerobic tank is 5–20 h. Phosphorus uptake in the aeration basin is ensured when the DO level is .2 mg/L. The PhoStrip process can help achieve an effluent total P concentration of ,1 mg/L if the soluble BOD5 / soluble P is low (12–15). PhoStrip Process
(Gabriel Bitton, 2005) 2.4.1.12 Equipment
Suspended growth aerated lagoons and ponds
NIREAS VOLUME 2 [2.4] 42
Process Aerated lagoons or ponds are similar to facultative ponds in waste stabilization pond systems, with the difference that natural oxygenation is enhanced by mechanical air injection to achieve high rates of organic degradation and nutrient removal. As the oxygenation does not depend on algae activity and photosynthesis, ponds can be deeper (thus smaller in surface) and are suited for colder climates. There are two types of aerated ponds: common aerated lagoons are enhanced facultative ponds, while completely mixed aerated ponds are in essence activated sludge systems without sludge. The effluent of aerated ponds may be reused or used for recharge, but settled sludge requires a further treatment or correct disposal.
The smaller area requirement means that it is appropriate for both rural, and peri-urban environments. However, the use of aerators also increases the complexity of the systems and the involved the need for technical material and energy.
Schematic view of an artificially aerated facultative lagoon (partially mixed)
NIREAS VOLUME 2 [2.4] 43
Design Parameters Aerated facultative ponds/lagoons : The design of aerated facultative pond is very similar to that of facultative ponds, with an aerobic zone close to the surface and a deeper, anaerobic zone. But there are no requirements in term of surface area as the process is independent of photosynthesis. The two main design criteria are HRT and depth. The HRT should be adopted in order to allow a satisfactory removal of BOD and is usually 4 to 10 days for organic loads of 20 to 30 g BOD/m3day The depth of the pond should be planned keeping in mind the compatibility with the aeration system and the need of an aerobic layer of approximately 2 meters to oxidise the gases from the anaerobic decomposition of the bottom sludge. Usually, the depth varies from 2,5 to 4 m. The amount of oxygen to be supplied by the aerators for the aerobic degradation/stabilisation of the organic matter should normally be equal to the total ultimate influent BOD . However, such lagoons are generally designed using empirical methods: a HRT of 4 to 5 days results in 70 to 90% BOD5 removal in a partially mixed aerated lagoon by power requirements of approximately 4 W/m3.
Completely mixed aerated ponds/lagoons : Completely mixed aerated lagoons are essentially aerobic. The aerators serve not only to guarantee the oxygenation of the medium, but also to maintain the suspended solids (biomass) dispersed in the liquid medium. These systems are also called flow-through lagoons or CSTR (completely-stirred tank reactor) lagoons. Aerated ponds act similarly to aeration tanks in activated sludge processes. The main difference is that solids are not recirculated. Biomass and solids from the raw sewage are maintained together in suspension.
NIREAS VOLUME 2 [2.4] 44
This enhances the contact between bacteria contained in the biomass (responsible for the degradation) and the raw sludge to be degraded. Hence, the efficiency of completely aerobic ponds increases in comparison to partially mixed ponds and allows a reduction in volume. Land requirements for this system are the smallest within ponds systems. The typical HRT of a completely mixed aerated lagoon is in the order of 2 to 4 days. This time is enough for an efficient removal of the suspended solids . A HRT of 4 days, resulting in 70 to 90% BOD5 removal, requires about 20 W/m3 of energy. Aerated ponds have removal capabilities similar to facultative lagoons, except that nitrification of ammonia-nitrogen can be nearly completed in warm seasons, while cold weather will halt that process. Some minimal phosphorus and nitrogen removal (10 to 20 %) can be anticipated. Faecal coliform removal of 1 to 2 logs/100 ml is likely. Despite the good efficiency of aerated lagoons in removing organic matter from the wastewater, the quality of their effluent is not satisfactory for direct discharge into the environment. Completely mixed aerated lagoons should be followed by settling ponds, which may be either several short HRT ponds (i.e. 2 day), requiring frequent de-sludging ; or a single 10-day facultative pond with sufficient depth to allow long-term sludge storage. Aerators should be positioned carefully to avoid dead areas where solids are able to settle out. Small aerators rather than fewer large ones provide more evenly spread mixing, and rounded pond corners also help in avoiding dead areas. A clay, asphalt, compacted earth, or another impervious material should be used for construction to prevent leaching and infiltration into the groundwater. A protective berm or fence should also be built to protect the lagoon from runoff and erosion.
Design capacity Adapted for almost all wastewater (also industrial) in rural or urban areas. However, electricity supply needs to be continuous and a foul odour might be a problem in urban areas.
Effluent quality •
Aerated facultative ponds: 70 to 90 % BOD; HRT: 4 to 10 day
•
Completely mixed aerated ponds: 70 to 90 % BOD; HRT: 2 to 4 day; high phosphorus, nitrogen and ammonia removal.
Cost Investment costs are moderate to high, but expert design is required. Due to the large energy consumption for mixing and aeration, operation and maintenance is expensive. The aeration
NIREAS VOLUME 2 [2.4] 45
devices also increase the complexity of the unit and thus the vulnerability for technical failure (due to lack of replacement/spare parts or engineering skills). In consequence, the costs are generally higher than for WSP systems, depending on the local context and the availability of electricity. In some cases, solar-power driven aeration systems may be worth considering.
Advantages/disadvantages
Advantages •
Good resistance against shock loading
•
Can treat high loads
•
High reduction in BOD and pathogens
•
No real problems with insects or odours if designed correctly
•
Less land required than for simple pond systems (e.g. WSP)
•
The treated water can be reused or discharged if a secondary maturation/settling pond follows the aerated lagoon/completely mixed aerated pond
Disadvantages •
Sludge requires secondary treatment and/or appropriate discharge
•
Requires expert design and construction supervision
•
Requires a constant energy/electricity source for continuous aeration; the technique does not work in cases of power failure
Operation & Maintenance The influents to aerated ponds need to be screened or settled as any large object could damage the aeration system. The pond itself must also be fenced off so no coarse objects (e.g. garbage) can be thrown in. In the case of aerated facultative lagoons, sludge needs to be dug out every 2 to 5 years and either post-treated (e.g. anaerobic digestion, composting) or correctly disposed. The same applies for completely mixed systems: The sludge of the sedimentation ponds (retaining the solids contained in the effluent of the ponds) also needs to be desludged and post-treated. Skilled staff is required permanently to repair and maintain aeration machinery .
NIREAS VOLUME 2 [2.4] 46
Response to Abnormal Conditions Ponds-Troubleshooting guide
NIREAS VOLUME 2 [2.4] 47
NIREAS VOLUME 2 [2.4] 48
2.4.1.13
Oxidation ditch
Equipment
Process The oxidation ditch is a variation of the aerated raw sewage lagoon in that the process combines settling and aerobic biological oxidation in a single unit. Oxidation ditches are effective in treating the waste of small communities. Similar to lagoons, construction and operating costs are low and they can be constructed rapidly. The energy requirement for treatment is small, and operator attention is minimal. Oxidation ditches operate on higher loadings than aerated ponds. A circulation rate of about 0,3 m/sec maintains the solids in suspension. Oxygenation is supplied by an aeration rotor system, which is a power unit of either angle-iron or cage design.
Oxidation ditch flow sheets. A, Single ditch unit, B, Multiple ditch unit
NIREAS VOLUME 2 [2.4] 49
(David H.F. Liu, Bela G. Liptak, 1999)
The single-ditch unit in Part A in next Figure operates in the following sequence: First, the aeration rotor is turned off when the overflow level of the ditch is reached. After sludge settling occurs in the ditch, additional raw waste is pumped in displacing a like volume of supernatant, and this cycle repeats. When the detention time of raw waste sewage is at least 24 hr and sufficient oxygen is present, the quantity of excess sludge is small. An alternating anoxic and oxic environment is established in the channel depending on the distance from the aeration device. Consequently, the oxidation ditch can achieve good nitrogen removal via nitrification and denitrification. Some oxidation ditches use intrachannel clarifiers to separate the sludge from the mixed liquor.
Oxidation ditch activated sludge system
NIREAS VOLUME 2 [2.4] 50
MORE Part B shows the multiple-ditch configuration. Ditches B and C are alternately used for settling while ditch A operates continuously. When ditch B is used for settling, the gates connecting pond A with B are closed, and the aeration rotor in ditch B is shut off. When the ditch is not used for settling, the aeration rotor is turned on, and the ditch functions in an auxiliary treatment capacity. After settling occurs in either ditch B or C, the gates to the ditch are opened, and the supernatant is discharged as in the single-ditch unit. After the supernatant is discharged, the settled sludge in the ditch is resuspended and distributed by the aeration rotor in that ditch.
Advantages/disadvantages
Advantages •
Tolerance in toxic/huge loads
•
High quality effluent
•
No primary sedimentation is necessary
•
Production of Stabilized sludge
•
Lower power consumptions comparing to extended aeration
Disadvantages •
Higher constructing cost comparing to conventional activated sludge process, due to larger aeration tanks
NIREAS VOLUME 2 [2.4] 51
•
Not easy to upgrade/expand an existing facility
Summary method •
The process does not require primary treatment.
•
The oxidation ditch process is similar to the extended aeration process.
NIREAS VOLUME 2 [2.4] 52 Response to Abnormal Conditions
Oxidation ditch bruss rotor – Troubleshooting guide
NIREAS VOLUME 2 [2.4] 53 2.4.1.14
Membrane biological reactors (MBR)
Process The membrane bioreactor (MBR) process is a technology that consists of a suspended growth biological reactor integrated with an ultrafiltration membrane system, using the hollow fiber membrane. Essentially, the ultrafiltration system replaces the solids separation function of secondary clarifiers and sand filters in a conventional activated sludge system. Ultrafiltration membranes can also be immersed in an aeration tank, in direct contact with mixed liquor. Due to it being a very technical solution; it needs expert design and skilled workers. Furthermore it is a costly but efficient treatment possibility. With the MBR technology, it is possible to upgrade old wastewater plants.
Design Parameters When designed accordingly, these systems can also provide an advanced level of nutrient removal. In an MBR system, the membranes are submerged in an aerated biological reactor. The membranes have porosities ranging from 0.035 Îźm to 0.4 Îźm (depending on the manufacturer), which is considered between micro and ultrafiltration.This level of filtration allows for high quality effluent to be drawn through the membranes and eliminates the sedimentation and filtration processes typically used for wastewater treatment. Because the need for sedimentation is eliminated, the biological process can operate at a much higher mixed liquor concentration. This dramatically reduces the process tankage required and allows many existing plants to be upgraded without adding new tanks.
Pre-treatment To avoid unwanted solids in the waste stream, which enters the membrane tank, fine screening is an essential pre-treatment step. This minimises an accumulation of solids and protects the membrane from damaging debris and particles, extends the membrane life, reduces operating costs and guarantees a higher sludge quality as well as a trouble free operation.
Membrane During MBR wastewater treatment, solid–liquid separation is achieved by Microfiltration (MF) or Ultrafiltration (UF) membranes. A membrane is simply a two-dimensional material used to separate components of fluids usually on the basis of their relative size or electrical charge. The capability of a membrane to allow transport of only specific compounds is called semi-permeability (sometimes also permselective). This is a physical process, where separated components remain
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chemically unchanged. Components that pass through membrane pores are called permeate, while rejected ones form concentrate or retentate.There are five types of membrane configuration which are currently in operation: •
Hollow fibre (HF)
•
Spiral-wound
•
Plate-and-frame (i.e. flat sheet (FS))
•
Pleated filter cartridge
•
Tubular
Membrane bioreactor systems have two basic configurations: (1) the integrated bioreactor that uses membranes immersed in the bioreactor and (2) the recirculated MBR in which the mixed liquor circulates through a membrane module situated outside the bioreactor.
Schematic diagram of membrane bioreactors: (a) integrated MBR with an immersed membrane module, and (b) bioreactor with an external membrane separation unit
(Eddy, 1999)
Membranes immersed in the bioreactor are mounted in modules (sometimes called cassettes) that can be lowered into the bioreactor. The modules are comprised of the membranes, support structure for the membranes, feed inlet and outlet connections, and an overall support structure.
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The membranes are subjected to a vacuum (less than 50 kPa) that draws water (permeate) through the membrane while retaining solids in the reactor. To maintain TSS within the bioreactor and to clean the exterior of the membranes, compressed air is introduced through a distribution manifold at the base of the membrane module. As the air bubbles rise to the surface, scouring of the membrane surface occurs; the air also provides oxygen to maintain aerobic conditions.
Typical membrane membrane bundle in position to be placed in a membrane bioreactor
(Eddy, 1999)
Besides submerged membranes, in in-line, recirculated MBR design activated sludge from the bioreactor is pumped to a pressure-driven tubular membrane where solids are retained inside the membrane and water passes through to the outside. The driving force is the pressure created by high cross velocity through the membrane. The solids are recycled to the activated-sludge basin. The membranes are backwashed systematically to remove solids and cleaned chemically to control pressure buildup.
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By replacing solids separation by gravity settling in secondary clarifiers, membranes avoid issues of filamentous sludge bulking and other floc settling and clarification problems, and the aeration tank MLSS concentration is no longer controlled by secondary clarifier solids loading limitations. The MBR systems can operate at much higher MLSS concentrations (15,000 to 25,000 mg/L) than conventional activatedsludge processes. Although high concentrations of MLSS as noted above have been reported, MLSS concentrations in the range of 8000 to 10,000 mg/L appear to be most cost-effective when all factors are considered.
Design capacity Applicable in conventional wastewater plants.
Effluent quality •
BOD mg/L <5
•
COD mg/L <30
•
NH3 mg/L <1
•
TN mg/L <10
•
Turbidity NTU <1
Cost Although MBR capital and operational costs (membranes, oxygen utilisation, expert design, etc.) exceed the costs of conventional process, it seems that the upgrade of conventional process occurs even in cases when conventional treatment works well. This can be related to increase of water prices and the need for water reuse as well as with more stringent regulations on the effluent quality. Integrated MBR system immersed in the bioreactor
Advantages/disadvantages
NIREAS VOLUME 2 [2.4] 57 Advantages •
Secondary clarifiers and tertiary filtration processes are eliminated, thereby reducing plant footprint. In certain instances, footprint can be further reduced because other process units such as digesters or UV disinfection can also be eliminated/ minimised (dependent upon governing regulations).
•
Can be designed to prolong sludge age, hence lower sludge production
•
The Highest effluent quality from all other secondary treatment processes
•
High loading rate capability
Disadvantages •
Very High operation, maintenance and capital costs (membranes)
•
Membrane complexity and fouling
•
Energy costs
•
Need for expertise personnel
Operation & Maintenance Most MBRs employ chemical maintenance cleaning on a weekly basis, which lasts 30–60 min, and recovery cleaning when filtration is no longer durable, which occurs once or twice a year. A deposit that cannot be removed by available methods of cleaning is called “irrecoverable fouling”. This fouling builds up over the years of operation and eventually determines the membrane life-time. All O&M tasks have to be done by skilled workers.
Fouling Modern systems (e.g. KUBOTA systems) are maintained with chemicals, i.e. it is not necessary to remove the membranes from the membrane tank. Organic fouling can be cleaned with as sodium hypochlorite and inorganic fouling with oxalic acid . Fouling occurs as a consequence of interactions between the membrane and the mixed liquor, and is one of the principal limitations of the MBR process. Fouling of membranes in MBRs is a very complex phenomenon with diverse interlinkages among its causes, and it is very difficult to localise and define membrane fouling clearly. The main causes of membrane fouling are:
•
Adsorption of macromolecular
•
Growth of biofilms on the membrane surface
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â&#x20AC;˘
Precipitation of inorganic matter
â&#x20AC;˘
Aging of the membrane Fouling mechanisms
Integrated MBR system with an immersed membrane module
2.4.1.15
Design parameters
Organic loading (F/M)
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The basic criterion when designing suspended growth biological treatment processes is the organic loading. The organic loading or food to microorganism (F/M) ratio is the amount of biodegradable organic material available to an amount of microorganisms per unit of time. This ratio can be expressed more concisely as follows:
or
where:
F/M = Organic loading, kg BOD5 per kg mixed-liquor SS (MLSS) day BOD5 = Biological oxygen demand, mg/l MLSS = Mixed-liquor SS, mg/l V = Contactor volume, m3 Q = Wastewater flow, m3/d
The concentration of biodegradable organic material is expressed as BOD5. For municipal wastewater, the BOD5 ranges from 200 to 400 mg/l. The volume of wastewater to be treated is based on historical flow measurements plus an estimation of any increase or decrease anticipated during the life of the treatment plant.
MLSS The viable microorganisms in the activated-sludge process are expressed in terms of MLSS. MLSS is not the concentration of viable microorganisms but an indication of the microorganisms present in the system. Environmental engineers use the MLSS concentration because measuring the actual number of viable organisms in the system is difficult. The organic loading equation represents the ratio of the weight of organic material fed to the total weight of microorganisms available for oxidation.
Environmental engineers choose the organic loading on the basis of the desired effluent quality. If the organic loading is maintained at a high level, the effluent quality is poor, and solids (excess microorganisms) production is high. As the organic loading is reduced, however, the quality of the effluent improves, and the sludge production decreases.
NIREAS VOLUME 2 [2.4] 60 Effect of organic loading on organics removal efficiency and excess sludge production Organic Loading
Design parameter
BOD5 removal efficiency Excess sludge produced (kg/kg BOD5 removed)
(kg BOD5/kg MLSS-day) 0,1(a)
0,3(b)
0,5(b)
1,0(c)
1,5(c)
95%
90%
90%
75%
70%
0,2
0,4
0,5
0,6
0,7
MLVSS The organic portion of MLSS is represented by MLVSS, which comprises nonmicrobial organic matter as well as dead and live microorganisms and cellular debris. The MLVSS is determined after heating of dried filtered samples at 600â&#x20AC;&#x201C;650 oC, and represents approximately 65â&#x20AC;&#x201C;75 % of MLSS.
Hydraulic (Liquid) Retention Time (HRT or LRT) Hydraulic retention time is the average time spent by the influent liquid in the aeration tank of the activated sludge process;
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V = aeration tank volume, L SSe = suspended solids in wastewater effluent, mg/L Qe = Wastewater effluent, L/d SSw = suspended solids in wasted sludge, mg/L Qw = Quantity of wasted sludge, L/d
Sludge age may vary from 5 to 40 days in activated sludge processes. It varies with the season of the year and is higher in the winter than in the summer season.
SVI The SVI value indicates the ability of microorganisms separate from the wastewater after contact. The SVI is defined analytically as the volume in milliliters occupied by 1 g of MLSS after a 1 L sample has settled in a graduated cylinder for 30 min. The SVI value for an activated-sludge system varies with the concentration of microorganisms maintained in the contactor.
NIREAS VOLUME 2 [2.4] 62 Field test for determining sludge volume index (SVI)
NIREAS VOLUME 2 [2.4] 63 Effect on MLSS concentration on settled volume for a constant SVI value
(David H.F. Liu, Bela G. Liptak, 1999)
The above table shows that the same SVI value of 100 can be observed for MLSS concentration from 500 to 8000 mg/l, yet the volume occupied by the MLSS after 30 min of settling is in the same proportion as the MLSS concentration. Therefore, the SVI value is meaningful only in indicating separation characteristics of solids at a particular concentration. If the same 30-min MLSS volume were required for a concentration of 8000 mg/l compared to 500 mg/l, the SVI value would have to be 6 compared to 100 at 500 mg/l MLSS. The SVI value is of operational importance since it reflects changes in the treatment system. Any increase of SVI with no increase of MLSS concentration indicates that the solids settling characteristics are changing and a plant upset can occur.
Next figure shows the relationship between the MLSS concentration, SVI, and the recycling ratio (R/Q). The amount of recycled flow depends largely on the settling characteristics of the MLSS. For example, if the SVI value is 400 and the required MLSS concentration is 2000 mg/l, a recycling ratio of about 3.5 is required. On the other hand, if the SVI is 50, the recycling ratio required is about 0.2. This relationship demonstrates that the settling characteristics of the formed biological solids are important to the successful operation of the activated-sludge process. For municipal wastewater, environmental engineers use an SVI value of approximately 150 and a MLSS concentration of 2000 mg/l for design. To achieve the required MLSS concentration in the contactor they use a recycling ratio of about 0,5.
NIREAS VOLUME 2 [2.4] Relationship between SVI, recycling ratio, and MLSS concentration
64
(David H.F. Liu, Bela G. Liptak, 1999) Artificial aeration The activated-sludge process design must provide oxygenation and mixing to achieve efficient results. Current methods of accomplishing both oxygenation and mixing include 1) compressed-air diffusion, 2) sparge-turbine aeration, 3) low-speed surface aerators, and 4) motor-speed surface aerators.
NIREAS VOLUME 2 [2.4] 65 Artificial oxygenation and mixing devices
(David H.F. Liu, Bela G. Liptak, 1999)
Air diffusers were the earliest aeration devices used. These devices compress air to the hydrostatic pressure on the diffuser and release it as small air bubbles. The larger the number and the smaller the size of the air bubbles produced, the better the oxygen transfer. Releasing air bubbles beneath the surface also results in airlift mixing of the contactor contents.
Typical porous air diffusers: (a) aluminum oxide disk, (b) ceramic dome
(Eddy, 1999)
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Some advantages and disadvantages of various fine pore diffusers are listed bellow :
Advantages •
Exhibit high oxygen-transfer efficiencies
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Exhibit high aeration efficiencies (mass oxygen transferred per unit power per unit time)
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Can satisfy high oxygen demands
•
Are easily adaptable to existing basins for plant upgrades
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Result in lower volatile organic compound emissions than
nonporous diffusers or
mechanical aeration devices
Disadvantages •
Fine pore diffusers are susceptible to chemical or biological fouling, which may impair transfer efficiency and generate high head loss. As a result, they require routine cleaning. (Although not totally without cost, cleaning does not need to be expensive or troublesome.)
•
Fine pore diffusers may be susceptible to chemical attack (especially perforated membranes). Therefore, care must be exercised in the proper selection of materials for a given wastewater.
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Because of the high efficiencies of fine pore diffusers at low airflow rates, airflow distribution is critical to their performance, and selection of proper airflow control orifices is important.
•
Because of the high efficiencies of fine pore diffusers required airflow in an aeration basin (normally at the effluent end) may be dictated by mixing, not oxygen transfer.
•
Aeration basin design must incorporate a means to easily dewater the tank for cleaning. In small systems where no redundancy of aeration tanks exists, an in situ, non-process interruptive method of cleaning must be considered.
Combining compressed-air and turbine mixing eliminates the problems of clogging experienced with diffusers and adds versatility to the mixing and oxygen transfer. With the sparge-turbine aerator, the mixing and oxygenation can be varied independently within an operating range. The additional development of aeration devices resulted in the elimination of compressors. The low-speed surface aerator uses atmospheric oxygen by causing extreme liquid turbulence at the surface. It is nearly twice as efficient in oxygen transfer as diffusers or sparge turbines.
NIREAS VOLUME 2 [2.4] 67 Surface Mechanical Aerator with Vertical Axis
The motor-speed surface aerator is the latest aeration device. This device operates at the liquid surface but does not have a gear reducer between the motor and impeller. Because no gear reducer is used, the cost is significantly less than the low-speed surface aerator. Unfortunately, the oxygen transfer efficiency and liquid pumpage rate are also significantly reduced. The device has been used extensively to supplement oxygen requirements for oxidation ponds.
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68 Design criteria of Activated sludge processes Suspended Growth Process -Activated Sludge Type Conventional Complete mix aeration Step feed aeration Contact stabilization High-purity oxygen aeration Oxidation ditch Sequencing batch reactor Extended aeration MBR
Liquid Retention Time- LRT (hr) 4-8
Solids Retention Time- SRT (days) 5-15
F/M (kg BOD5/kgMLVSS-day)
Volumetric Loading (kg BOD/m3-day)
0,2-0,4
0,3-0,65
0,2-0,6 0,2-0,4
0,8-2 0,60-1
3-5 3-5 0,5-1 (contact tank) 3-6 (stabilization tank)
5-15 5-15
5-15
0,2-0,6
0,95-1,2
1-3 15-36
1-4 10-30
0,25-1 0,05-0,1
1,5-3,2 0,1-0,3
12-50 18-36
10-30 20-40
4-6
5-20
0,05-0,1 0,05-0,1 (kg COD/ kgMLVSS-day) 0,1-0,4
0,1-0,3 0,1-0,3 (kg BOD/m3-day) 1,2-3,2
BOD5 REMOVAL (%)
MLSS (mg/L)
Recycling Ratio
1500-3000
0,25-0,75
2500-4000 2000-3500
85-95
1000-3000 (contact tank) 4000-10000 (stabilization tank) 2000-8000 3000-6000 1500-5000 3000-6000
99%
4000-16000
0,25-1 0,25-0,75
0,5-1,5 0,25-0,5 0,75-1,5 n.a. 0,5-1,5 n.a.
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Effluent quality Activated-sludge process design is based on the desired effluent quality. Successful operation of the activated-sludge process with an F/M ratio of 0.35 kg BOD5 per kg MLSS per day and efficient liquid–solids separation should yield effluent containing an average of 20 mg/l of SS and 20 mg/l of BOD5. For municipal wastewater, activated-sludge treatment removes the following major pollutants in the percentages listed:
90+% BOD5 (biological oxygen demand) 70+% COD (chemical oxygen demand) 90+% SS (suspended solids) 30+% P (phosphorus) 35+% N (nitrogen)
If a more efficient liquid–solids separation device, such as a granular-media filter, removes the remaining effluent solids (≈20 mg/l), an effluent quality of 10 mg/l or less of BOD5 and SS can be achieved.
NIREAS VOLUME 2 [2.4] 70 2.4.1.16
Aerobic suspended growth biological treatment processes sunopsis
Design capacity High-tech centralized system, not adapted for small communities. Almost every wastewater can be treated as long as it is biodegradable. Usually applied in densely populated areas for treatment of domestic wastewater.
Effluent quality Removal 80 to almost 100% BOD and TSS. High nitrogen removal. P accumulated in biomass and sludge. Low pathogen removal. HRT of some hours up to several days.
Costs Construction and maintenance costs are very high as activated sludge treatment units are highly mechanised. Also operation is expensive due to the requirement of permanent professional operation, high electricity consumption (pumping and aeration) and costly mechanical parts
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Overall Advantages/disadvantages
Advantages •
Little land required
•
High effluent quality
•
Efficient centralized systems
Disadvantages •
Requires large amount and continuous supply of energy
•
Technical complexity
•
Not all parts and materials may be available locally
•
Not suitable for application on community level
•
Very high construction, operation & maintenance costs
•
Mixing of industrial effluent with domestic wastewater can lead to toxicity and major malfunctioning and make the recycling of nutrients almost impossible
2.4.1.17
General Operation and maintenance Operation
PROCESS START-UP PROCEDURES General Procedures for starting the activated sludge process are outlined in this section. Start-up help should be available from the design engineer, vendors, nearby operators, or other specialists. The equipment manufacturers or contractor should be under contract for start-up instruction and assistance. During start-up, they should be present to be sure that any equipment breakdowns are not caused by improper start-up procedures. The operator may have several options in the choice of start-up procedures with regard to number of tanks used and procedures to establish a suitable working culture in the aeration tanks. The method described in this section is recommended because it provides the longest possible aeration time, reduces chances of solids washout, and provides the opportunity to use most of the equipment for a good test of its acceptability and workability before the end of the warranty. First Day
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First, start the air blowers and have air passing through the diffusers before primary effluent is admitted to the aeration tanks. This prevents diffuser clogging from material in the primary effluent and is particularly important if fine bubble diffusers are used. Fill aeration tanks to the normal operating water depth, thus allowing the aeration equipment to operate at maximum efficiency. Using all of the aeration tanks will provide the longest possible aeration time. You are trying to build up a microorganism population with a minimum amount of seed organisms, and you will need all the aeration capacity available to give the organisms a chance to reach the settling stage. When aeration tanks have been filled, begin filling the secondary clarifiers. Use of all the secondary clarifiers will provide the longest possible detention time to reduce washout of light solids containing rapidly growing organisms and will encourage solids buildup. When the secondary clarifiers are approximately three-fourths full, start the clarifier collector mechanism and return sludge pumps. During start-up, return sludge pumping rates must be adjusted to rapidly return the solids (organisms) to the aeration tanks and to keep the sludge blanket in the secondary clarifier as low as possible. The solids should never remain in the secondary clarifiers longer than 1.5 hours. Trouble also may develop if the return sludge pumping rate is too high (greater than 50 percent of the raw wastewater flow), because the high flows through the clarifier may not allow sufficient time for solids to settle to the bottom of the clarifier. A conventional activated sludge plant usually operates satisfactorily at return sludge rates of 20 to 30 percent of raw wastewater flow; however, current designs often provide for return capacities of 50 to 100 percent of the wastewater flow at larger plants. The return rate selected should be based upon returning organisms to the aerator where they can treat the incoming wastes. A thin sludgewill require a higher return percentage than a thick one. However, increasing the return percentage with a thin sludge can produce a thinner sludge. Addition of a coagulant or flocculant aid at the end of the aeration tank will hasten solids buildup and improve effluent during start-up. When the secondary clarifiers are full and begin to overflow, start effluent chlorination to disinfect the plant effluent and to protect the health of the receiving water users. Filling the aeration tanks and aerating the wastewater starts the activated sludge process. The AEROBES" in the aeration tank have food and are now being supplied with oxygen; consequently this worker population will begin to increase. After two or three hours of aeration, you should check the dissolved oxygen (DO) of the aeration tanks to see if enough air is being supplied.
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Check the DO throughout the aerator. If possible, use a DO probe and determine the DO along both sides of the aerator at both the water surface and the bottom of the tank. Oxygen must be available for the aerobes throughout the tank. If the DO is less than 1.0 mg/L, increase the air supply. If the DO is greater than 3.0 mg/L the air supply may be decreased, but not to the point where the tank would stop mixing. There will probably be an excess amount of DO at first due to the limited number of organisms initially present to use it. After a biological culture of aerobes is established in the aeration tanks, sufficient oxygen must be supplied to the aeration tank to overcome the following demands: 1. DO usually is low in both influent wastewater and return sludge to the aerator. 2. Influent wastewater may be septic, thus creating an immediate oxygen demand. 3. Organisms in the presence of sufficient food create a high demand for oxygen. The effluent end of the aerator should have a dissolved oxygen level of at least 1.0 mg/L. DO in the aerator should be checked every two hours until a pattern is established. Thereafter, DO should be checked as frequently as needed to maintain the desired DO level and to maintain aerobic conditions in the aerator. Daily flow variations will create different oxygen demands. Until these patterns are established, you will not know whether just enough or too much air is being delivered to the aeration tanks. Frequently excess air is provided during early mornings when the inflow waste load is low. Air supply may be too low during the afternoon and evening hours because the waste load tends to increase during the day. Turn on the water spray system as soon as possible because foaming will be severe until there is a sufficient buildup of activated sludge solids. If spray water is not available at start-up or the water spray pump is not working, you may need to apply commercial defoamers to the surface of the aerator to control foam. Second Day Collect a sample from the aeration tank and run a 30-minute settleability test using a 1,000-mL graduated cylinder. If possible, use a 2,000-mL cylinder with a five-inch (125-mm) diameter to obtain better results. Results of the 30-minute settleability test indicate the flocculating, settling, and compacting characteristics of the sludge. Observe the sludge settling in the sample for approximately one hour. It will probably have the same color as the primary effluent during the first few days. After a few minutes in the cylinder, very fine particles will start forming with a light buff color. The particles remain suspended, but settling, similar to fine particles of dust in a light beam. After one hour, a small amount of these particles may have settled to the bottom of the cylinder to
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a depth of 10 or 20 mL, but most are still in suspension. This indicates that you are making a start toward establishing a good condition in the aeration tank, but many more particles are needed for effective wastewater treatment. Third Through Fifth Days During this period of operation the only controls applied to the system usually consist of maintaining DO concentrations in the system and maintaining proper sludge return rates. A sampling program should be started in accordance with Section 11.58, "Recordkeeping," to develop and record the necessary data required for future plant control. Aeration of wastewater to maintain DO will require some time before settling will produce a clear liquid over the settled solids. Time is required for organisms to grow to the point where there are sufficient numbers to perform the work neededâ&#x20AC;&#x201D;to produce an activated sludge organism culture. Usually within 24 to 72 hours of aeration you will note that the settleable solids do not fall through the liquid quite so rapidly, but the liquid remaining above the solids is clearer. The active solids (organisms) are light and may wash out of the clarifier to some extent. Try to retain most of them because a rapid solids buildup will not occur unless they are retained. A good garden soil will add organisms and solids particles for start-up. Mix the soil with water and hose in the lighter slurry, but try to avoid a lot of grit. A truckload of activated sludge from a neighboring treatment plant also will help to start the process. Hopefully, you will not have to treat design flows during plant start-up. More time is needed both for aeration and clarification until you have collected enough organisms in your return sludge to enable you to produce a clear effluent after a short period of mixing with the influent followed by settling. Sixth Day A reasonably clear effluent should be produced by the sixth day. Solids buildup in the aeration tank should be closely checked using the 30-minute settleable solids test during the first week. Results of this test indicate the flocculating, settling, and compacting characteristics of the sludge. Suspended solids buildup is very slow at first but increases as the waste removal efficiency improves. This buildup should be carefully measured and evaluated each day. Microorganisms in the system are so varied and small that it is impossible to count them. To obtain an indication of the size of the organism population in the aeration tank, the solids are measured either in mg/L or in pounds of dry solids. Suspended solids determinations for aerator mixed liquor will give the desired information in mg/L, and the total pounds of solids may be calculated on the basis of the size of the aerator. The suspended solids test conducted on activated sludge plant mixed liquor normally requires a grab sample obtained at the effluent end of the aerator. The sample should be
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collected at the same time every day, preferably during peak flows, in order to make day-to-day comparisons of the results. Collect the mixed liquor sample approximately 1.5 m from the effluent end of the aeration tank and 0.4 to 0.6 m below the water surface to ensure a good sample. A return sludge sample also should be collected at this time every day to determine its concentration. Close observation of the suspended solids buildup and results from the 30-minute settleability test will indicate the solids growth rate, condition of solids in the aerator, and how much sludge should be returned to ensure proper return of the organisms to the aerator. It will be necessary to return all of the sludge for 10 to 15 days or longer if the wastewater is weak. Results from the 30-minute settleability test can be used to calculate the return sludge rate. If the volume of settled sludge in the cylinder is indicative of the amount of sludge settling in the secondary clarifier, the volume of return sludge should be approximately equal to the ratio of the volume occupied (in milliliters) by the settleable solids from the aeration tank effluent to the volume of the clarified liquid (in milliliters) after settling for 30 minutes in a 1,000-milliliter graduated cylinder.
ROUTINE OPERATIONAL CONTROL Operational Strategy An excellent effluent can be produced by the activated sludge process if the operator has a plan of operation or operational strategy and understands how the microorganisms remove wastes from the water being treated. This lesson provides a brief plan for operating an activated sludge plant on a day-to-day basis. There are three areas of major concern for the operator of an activated sludge plant: 1. What enters the plant 2. The environment for treating the wastes 3. What leaves the plant All three of these areas are closely related or are influenced by the other two areas. Influent Characteristics As the influent flows and waste concentrations change, the environment changes in the aeration tank and secondary clarifiers where the wastes are treated. If the activated sludge process is in balance (a good secondary effluent with BOD and suspended solids levels less than 20 or 25 mg/L) for routinely experienced high flows, then the plant should perform as expected. If flows increase significantly, a switch to step-feed aeration may be necessary to avoid loss of the
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activated sludge bacteria. This solution will be effective if it causes a reduction in the mixed liquor suspended solids fed to the secondary clarifiers. If the influent solids loading increases, the mixed liquor suspended solids may have to be increased to treat these wastes by reducing the sludge wasting rate.
Aeration Tank Environment If a good effluent is being produced, maintain a DO of 2 to 4 mg/L and thorough mixing throughout the entire aeration tank. If significant changes occur in the influent solids, adjust the mixed liquor suspended solids by regulating the waste sludge rate. If there is a white foam on the aeration tank surface, reduce sludge wasting rates. If there is a thick, dark foam on the tank surface, increase the sludge wasting rates. The higher the F/M, the more food and therefore a higher DO is needed.
Secondary Clarifier The settleability test is a good indication of how solids will settle in a clarifier. Adjust the return sludge rate so the sludge blanket will stay as low as possible in the clarifier. Remember that the sludge blanket can increase or rise with either an increase or a decrease in the return sludge rate as a result of either an increase or decrease in detention time. This increase or decrease in detention time also can be caused by changes in the influent flow. For these reasons you must keep good records and experiment to find the best return rate for the conditions in your plant.
Plant Effluent The turbidity test using a TURBIDITY METER is a quick way to determine the quality of your plant effluent. When your plant is operating properly, try to determine why. Plot trend charts describing influent characteristics, aeration tank and secondary clarifier conditions, and effluent characteristics. When problems start to develop, try to determine why and correct the situation. Remember that influent and process environmental conditions (such as temperature) are continuously changing and you must adjust for these changes.
How to Control the Process The effectiveness of the activated sludge treatment process in reducing the waste load depends on the amount of activated sludge solids in the system and the health of the organisms that are a
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part of the solids. Successfully maintaining control of the solids and health of the organisms requires continuous (seven days a week) observation and checking by the plant operators. SLUDGE AGE is one of the methods used by operators to determine and maintain the desired amount of activated sludge solids in the aeration tank. Sludge age is recommended for operational control because suspended solids are relatively easy to measure. In addition, sludge age considers two factors vital to successful operation: (1) solids (food) entering the treatment process, and (2) solids (organisms) available to treat the incoming waste (food). A critical point to recognize is that the solids test is capable of indicating both the amount of food carried by the inflow to the process and the number of organisms available to treat the waste. NOTE: The activated sludge process we are describing in this example plant is controlled on the basis of sludge age, but other process control options may be used.
Our example conventional activated sludge plant has an allowable effluent BOD of 20 mg/L. A sludge age of five days will serve as a satisfactory loading target during start-up for this plant. After the plant is in operation, various sludge ages may be tried in an effort to improve the quality of the plant effluent. How activated sludge is wasted can have an important impact on the best sludge age for your plant. If activated sludge is wasted to the primary clarifiers, the wasted organisms could be mistaken for food entering the treatment process. If activated sludge is wasted to a gravity, belt, or flotation thickener, not as many organisms would be in the effluent from the primary clarifier. Therefore, plants wasting to the primary clarifier may have a lower sludge age than plants wasting to a gravity, belt, or flotation thickener. Temperature also influences sludge age. The warmer the weather the more active the organisms, so the sludge age can be lowered by reducing the MLSS (mixed liquor suspended solids).
Always remember that you must maintain the DO in the aerator and more air will be required when aeration tank solids increase in concentration and activity.
Wasting Activated Sludge The amount of activated sludge wasted may vary from 1 to 20 % of the total incoming flow. Normally, waste activated sludge is expressed in kg per day or kg of solids removed from the aeration system. Continuous wasting is preferred. Try not to change your sludge wasting rate by
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more than 10 or 15 % from one day to the next. The main purpose is to maintain a sludge age that produces the best effluent. Wasting is normally accomplished by diverting a portion of the return sludge to a primary clarifier, gravity thickener, gravity belt thickener, dissolved air flotation thickener (DAFT), aerobic digester, or anaerobic digester. Normal operations in a conventional activated sludge plant will concentrate return sludge and waste sludge solids three to four times as much as the solids concentration of the mixed liquor. This may provide return sludge with a concentration of 2,000 to 10,000 mg/L, or 0.2 to 1.0 percent in terms of total solids. If the waste sludge line discharges directly to the anaerobic digestion system, it would contain 10 to 20 times as much water as should be entering the anaerobic system with that amount of solids. Operating an anaerobic digester would be difficult under this condition. It would be wiser to waste to the primary clarifiers where combining with primary sludge minimizes the addition of excess water to the digester. Wasting activated sludge will occur in the effluent whether or not it is controlled. In all activated sludge plants, wasting must be controlled by the operator. Mixed liquor suspended solids that need to be wasted accumulate from two sources. The first is the suspended solids in the plant flow from the primary clarifiers or raw wastewater. The second and main source is the new cell production by the microorganisms. For every kg of BOD or solids removed by treatment in the activated sludge system, a part of that kg will remain in the system as microorganisms. The rate of production of excess sludge will depend on the type of process being operated and the nature of the waste load. The high-rate activated sludge plant is capable of producing 0.75 kg of sludge volatile matter for every kg of BOD removed. The conventional plant runs around 0.55 kg of sludge volatile matter per kg of BOD removed in the activated sludge system. The extended aeration plant drops down to about 0.15 kg of sludge volatile matter per kg of BOD removed. Excessive silt or inert material may increase sludge production beyond that indicated by the BOD test.
Initial Inspection At the start of every shift, the shift operator goes through the following steps. 1. Review the log book. a. Has anything unusual happened? If so, were any corrections required and has the problem been corrected? b. Check the status of the return sludge pumps and waste sludge pumps.
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c. What are the levels of DO in the aerators? Maintain a DO of 1.0 to 3.0 mg/L in all aerators. NOTE: Higher DO levels may be required to maintain DO in the secondary clarifiers. d. If chlorinating the return sludge flow for control of filamentous growths, be sure to check the pounds of chlorine applied per day.
2. Visually inspect the activated sludge process. a. Is there any foam on the surface of the aerators? If so, is it white or a thick, dark foam? b. Do the secondary clarifiers appear normal? Are they turbid or are there any solids on the surfaces? Check the depth of the sludge blanket in each clarifier. If the sludge blanket is high, be sure the return sludge pumps are working properly. NOTE: In this plant the sludge blanket is kept as low as possible in order to maintain aerobic conditions in the secondary clarifiers. c. Inspect the effluent from the secondary clarifiers. The effluent should be clear and free of suspended solids and floatables. 3. Review the lab results.
a. SVI and microorganisms. If the SVI is high, something is wrong. Collect a sample of aerator mixed liquor and examine under a microscope for the typical numbers, types, and condition of the mixed liquor microorganisms. If there is an excess or lack of any type of microorganism, carefully inspect the laboratory data. Look for unusual values for: (1) pounds of volatile suspended solids under aeration, (2) sludge wasting rates, (3) air usage, and (4) dissolved oxygen levels in the reaerator and aerators. If the microscopic examination of the mixed liquor indicates an excess of normal filamentous microorganisms, then chlorination of the return activated sludge should be started. b. Kg of volatile matter under aeration. Check the suspended solids levels in mg/L and percent volatile matter in the aerators. c. Sludge wasting rate. Adjust the sludge wasting rate to maintain the desired pounds of volatile matter under aeration.
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d. Air usage. Examine air usage in cubic m3 per day needed to maintain desired DO levels in the aerators. Air required serves as an excellent indication of influent BOD.
Maintenance
A comprehensive preventive maintenance program is an essential part of plant operations. Proper maintenance will ensure longer and better equipment performance than equipment that is given little, if any, care. This section should be used as a guideline in performing the required maintenance on activated sludge process equipment.
Need for Understanding Equipment Always read and understand manufacturers' literature before starting, operating, maintaining, or shutting down equipment. You may be very capable of operating the activated sludge process, but if your equipment does not perform, life can be very difficult. Proper shutdown procedures may be overlooked in manufacturers' literature and Î&#x; & Î&#x153; manuals. If proper shutdown procedures are not followed, equipment can be damaged and not start again properly. This section will identify important steps to follow when shutting down aeration system equipment, when attempting to handle abnormal conditions or troubleshooting problems, and when maintaining equipment. Remember that these steps are general and you must prepare your own detailed list if one is not available. There are many equipment manufacturers in business today andthey are continually improving their products; therefore the lists in this section are presented to guide you in the preparation of your own lists for the equipment in your plant.
Mechanical equipments (pumps, aerates, mixers) require continuous maintenance and control, and supply of oxygen and sludge is essential. Control of concentrations of sludge and oxygen levels in the aeration tanks is required and technical appliances (e.g. pH-meter, temperature, DOmeter, inverters for gearing of blowers etc.) need to be maintained carefully. To make sure that optimal living conditions for the required bacteria are guaranteed and a satisfying effluent quality is met, the influent as well as the effluent should be supervised and controlled constantly (e.g. by a centralized computerized monitoring system).
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SHUTDOWN Surface Aerators Aerator shutdown is required when any maintenance service is performed to prevent injury to maintenance personnel and possible damage to equipment. 1. Turn the ON-OFF-AUTO switch to OFF. 2. Turn the main power breaker to OFF. 3. Lock out and tag the main power breaker in the OFF position. Maintenance service may be performed now. REMEMBER, wear an approved flotation device or fall arrest system, depending on the depth of water in the aeration basin.
Positive Displacement Blowers Blower shutdown is required when any maintenance service is performed in order to prevent injury to maintenance personnel and possible damage to equipment. 1. Turn the ON-OFF-AUTO or ON-OFF switch to OFF. 2. Turn the main power breaker to OFF. 3. Lock out and tag the main power breaker in the OFF position. 4. Close the discharge and suction valves. When the blower is shut down, ensure that the check valve closes and seats. Many blowers run backward for a moment or two after shutdown and this reverse operation creates a vacuum condition that will pull liquid up the headers and into the air distribution system. Maintenance service may be performed now. Centrifugal Blowers
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Blower shutdown is REQUIRED when any maintenance service is performed to prevent injury to maintenance person-. nel and possible damage to equipment. 1. Depress the stop button. 2. Open the bypass valve. 3. Closethedischargevalve. 4. Let the auxiliary oil pump run for a 10-minute, post-lubrication period. This allows the bearings and gears to cool gradually. 5. Turn the control panel power to OFF. 6. Turn the main power breakers to OFF. 7. Lock out and tag the main power breakers in the OFF position. Maintenance service may be performed now.
Air Distribution System Periodically, the air distribution system may need to be shut down for repairs, modifications, or cleaning. If your distribution system is composed of different pipes serving different plant processes, and if regulating/isolation valves have been installed, the section to be serviced may be shut down effectively by closing and tagging the regulating/isolation valve. Keep in mind that when closing down a section of the distribution system, an increase of air flow, head loss, and system pressure in the active sections of the distribution system may occur. Where only one or no regulating/isolation valve is installed or there is only one distribution line, the blower must be shut down before service on the distribution system begins. CAUTION: Before service begins on the distribution system, the regulating/isolation valve(s) or the blower must be locked out and tagged to prevent air from entering the system. Do not attempt to repair even the smallest leaks or discrepancies in the distribution system unless the system is shut down. Although 6.0 to 10.0 psi (0.4 to 0.7 kg/ cm2) system pressure may not seem like much, SERIOUS BODILY INJURY may result from escaping air or foreign material. Air Headers and Diffusers
NIREAS VOLUME 2 [2.4] 83
Before shutting down the header(s) or diffusers, mark the valve position of the butterfly valve or count the number of turns that a gate valve is open and record it. This will provide a ready reference for positioning the valve when the header(s) and diffusers are returned to service. Close the header(s) regulating/isolation valve.
Motors Motors should be greased after about 2,000 hours of operation. The motor must be stopped when greasing begins. Remove filler and drain plugs, free the drain hole of any hardened grease, add new grease through the filler hole until it starts to come out of the drain hole. Start the motor and let it run for about 15 minutes to expel any excess grease. Stop the motor and install the filler and drain plugs. After about five years of operation, the motor windings may tend to deteriorate due to moisture and heat. Have the motor inspected and removed from service for repair by an authorized motor repair facility.
GEAR REDUCER Generally all new oil-lubricated equipment has a break-in period of about 400 hours. After this time the oil should be drained from the gear reducer, the reservoir flushed, and new oil added. This procedure removes fine metal particles that have worn off of the internal components as a result of the initial close tolerances during the break-in period. If large quantities of fine metal particles are found after the break-in period, the manufacturer should be consulted. A high-quality, turbine-type oil is normally used in the gear reducer assembly. After an oil change has been completed and with the reducer assembly inspection plates removed, inspect the gears for proper operation and the oil for proper flow. Grease-lubricated bearings should be greased about every 500 hours of operation, depending on service conditions. NOTE: More damage is done to bearings by overgreasing than undergreasing. Oil for gears and bearings should be changed after about 1,400 hours of operation under normal service and more frequently when required. NOTE: The proper type of oil is a necessity. If it is too thin or too thick, it will impede proper functioning of bearings and gears.
COUPLING AND IMPELLER
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Every 6 to 12 months the aerator should be stopped and all bolts and nuts on the impeller and coupling should be retorqued according to the manufacturer's specifications. This is also a good time to inspect the metal surfaces for deterioration such as cracks or worn components. While the unit is off, check the impeller and shaft for proper alignment. After performing routine maintenance, the unit and surrounding area must be wiped or washed. Be sure to remove any spilled oil or grease. Dispose of oil- and grease-soiled rags in a covered container to avoid a fire hazard.
Air Filters When filter cleaning is scheduled, remove the filters from the filter chamber and check the inside of the chamber. Remove any sand, dirt, paper, water, or other debris. Removing air filters can produce substantial quantities of airborne dust. Wear protective clothing and eye and respiratory protection if excessive dust exposure may occur. Usually filters may be cleaned by using a relatively high-pressure stream of clean water or by steam cleaning. The filter manufacturer's equipment manual should be consulted for the recommended method of cleaning your particular type of filter. Allow the filters to dry, securely install the filters in the filter chamber, and ensure that no tools or other items are left in the filter chamber that could be drawn into the blowers.
Blowers Generally, all new oil-lubricated equipment has a break-in period of about 400 hours. After this time the oil should be drained from the blower, the reservoir and filter cleaned and flushed, and new oil added. This procedure removes the fine metal particles that have worn off of the internal components as a result of the initial close tolerances during the break-in period. If large quantities of fine metal particles are found after the break-in period, the equipment manufacturer should be consulted. Grease-lubricated bearings should be greased about every 500 hours of operation, depending on service conditions. NOTE: More damage is done to bearings by overgreasing than undergreasing Oil for gears and bearings should be changed and the filters cleaned about every 1,400 hours of operation under normal service, more frequently when required. NOTE: The proper grade of oil is a necessity. If it is too thin or thick it will impede proper functioning of bearings and gears. Maintenance on blower motors is similar to maintenance of surface aerator motors.
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After performing routine maintenance, the units and surrounding area must be wiped or washed, removing any spilled oil or grease. Dispose of oil-soiled rags in a covered container. A routine should be established whereby the blower, pressure relief valve, and blower suction and discharge valves are operated to prevent the equipment from SEIZING up and becoming inoperable. During this operation the equipment should be checked for proper alignment. Blowers that are not routinely in service should be operated at least six hours per week on a given day. Pressure relief valves should be checked at least once a month by manually lifting the valve off of the valve seat. Test the pressure relief valve for the correct setting by slowly closing the blower discharge valve slightly, with the blower operating, until the pressure relief valve opens. Observing the air system pressure gauge while noting when the relief valve opens will alert you to the pressure setting at that time. Instructions for making adjustments on the pressure relief valve and the proper setting for your blower installation should be contained in the manufacturer's manual. NOTE: When testing the pressure relief valve, do not close the blower discharge valve completely or serious damage to the blower may result. Use this test procedure only if the pressure relief valve is located between the blower and the discharge valve. Never use any oil or grease on the pressure relief valve. Due to the heat generated by compressing air, oil and grease lubricants will harden and may keep the pressure relief valve from opening. A metal seizing inhibitor may be applied and is generally available from most hardware stores. Blower suction and discharge valves should be operated once a month, with the blower off, by fully closing and opening the valves. A metal seizing inhibitor should also be applied to the gear assembly of each valve. The blower check valve should be checked once a month for free operation by opening and closing the valve while checking that the valve seats properly when it is in the closed position.
Air Distribution System Depending on the harshness of the environment surrounding the distribution piping, an inspection schedule should be established for the entire piping system from at least once monthly to once every six months. During this inspection, look for loose pipe support clamps or cracked pipe support welds, shifting of pipe out of original position due to structural settling, loose nuts and bolts on fittings, stuck or difficult to operate valves, and damage from corrosion.
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Exterior pipe, fittings, and valve surfaces should be painted with a primer coat and a finish coat. Painting should be done as frequently as needed to prevent and inhibit corrosion. Meters should be calibrated at least once a year to ensure meter accuracy. All valves in the distribution system, including meter valves, should be fully opened and fully closed once a month (with blowers off) to ensure free operation. For more detailed information on maintenance of your meters, fittings, valves, and pipes consult your manufacturer's manual, the plant operation and maintenance manual, and Chapter 15, "Maintenance," of this training manual.
Air Headers Due to the severe environment that the headers are exposed to, a maintenance program should be adopted so that header failure and resultant lengthy downtime can be minimized or avoided. The following activities should be scheduled on a monthly and yearly basis. 1. Monthly a. Fully close and open all regulating/isolation valves to ensure free operation on coarse bubble diffusers, but Î&#x203A;/OTon porous media diffusers. b. Apply 3 to 5 shots of grease to the upper pivot swing joint O-ring cavity (swing header). 2. Yearly a. Raise headers, clean and check for loose bolts, nuts, and fittings. Secure if loose. b. Apply 3 to 5 shots of grease to the pivot joint O-ring cavity (swing header). c. Check for corrosion. Properly prepare pipe and paint with epoxy coating where needed. Hydraulic fluid in the swing header hoist should be changed at least once yearly. A weatherproof tarpaulin should be provided to cover the hoist to protect it from the elements when it is not in use. Secure the tarpaulin to the hoist to prevent the wind from blowing it into the tanks.
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Diffusers Maintenance activities that should be scheduled on a monthly and yearly basis are listed in this section. 1. Monthly Increase the air flow to the diffusers 2 to 3 times normal for about 15 minutes to blow out the biological growths that have accumulated around the diffuser orifices. 2. Yearly, or as conditions warrant Raise the air header from the tank and clean diffusers, inspect for damage, and replace as needed. Diffusers that are allowed to remain in a defective condition reduce the diffusers' effectiveness and will result in inefficient wastewater treatment.
NIREAS VOLUME 2 [2.4] 88 Response to Abnormal Conditions Major operational problems of the activated-sludge process are caused by bulking sludge, rising sludge, and Nocardia foam.
Bulking sludge A bulking sludge has poor settleability and compactability and is usually caused by excessive growth of filamentous microorganisms. These organisms that grow in filamentous form instead of flocs will not settle, or will incorporate large volumes of water into their cell structure, making their density near that of water . Factors such as waste characteristics and composition, nutrient contents, pH, temperature, and oxygen availability can cause sludge bulking. The absence of certain components in the wastewater such as nitrogen, phosphorus, and trace elements can lead to the development of a bulking sludge. This absence is critical when industrial wastes are mixed with municipal wastewater for combined treatment. Wide fluctuations in pH and DO are also known to cause sludge bulking. At least 2 mg/l of DO should be maintained in the aeration basin under normal operating conditions. Wastewater treatment facilities should check the F/M ratio to insure that it is within the recommended range. They should also check the additional organic loads received from internal sources such as sludge digesters and sludge dewatering operations to avoid internal overloading conditions, especially under peak flow conditions. Chlorination of the return sludge effectively controls filamentous sludge bulking. Chlorine doses in the range of 2â&#x20AC;&#x201C;3 mg/l of Cl2 per 1000 mg/l of MLVSS are suggested. However, high doses can be necessary under severe conditions (8 to 10 mg/l of Cl2 per 1000 m/l of MLVSS). Also bulking sludge can be avoided by the alteration of the dissolved-oxygen concentration in the aeration tank and the addition of nutrients and growth factors to favour other microorganisms etc.
To prevent sludge bulking from occurring, the following items should be carefully controlled in an activated sludge plant: 1. SLUDGE AGE. Carefully review plant records and maintain a sludge age that produces the best quality effluent. Watch influent solids loadings, maintain desired level of solids in the aerator, and carefully regulate waste sludge rates. Generally, bulking may be cured by increasing the sludge age. 2. DO LEVEL. Prevent low levels of DO from developing. Mixed liquor DO tests are a quick and simple test, or a DO probe installed in the aeration tank wall will give you a continuous reading.
NIREAS VOLUME 2 [2.4] 89
There is no valid excuse for low DO concentrations during normal conditions if sufficient oxygenation capacity is available, unless a slug of waste with an excessive oxygen demand is received. 3. LENGTH OF AERATION PERIOD. Bulking caused by the aeration period being too short is usually the result of a design problem, unless the operator has formed the habit of returning too high a volume of return sludge. To correct this problem, reduce the return sludge rate and thicken the return sludge solids concentration by COAGULATION (if necessary). In this way, you still return the same number of organisms to meet the new food (waste) entering the aerator, but effectively reduce the total flow through the aerator and clarifier. 4. FILAMENTOUS GROWTH. The growth of filamentous organisms may be caused by incorrect sludge age or nutritional imbalances, such as a shortage or abundance of nitrogen, phosphorus, or carbon. If filamentous growths are allowed to become well established, they create a difficult problem to overcome. Control may be achieved by increasing MLSS (more microorganisms, which will increase sludge age), by maintaining higher DOs and, in special instances, supplementing a nutrient deficiency. Control by chlorination of return sludge, as discussed earlier, may also be used.
Rising sludge Rising sludge is usually caused by the release of gas bubbles entrapped within sludge flocs in the secondary clarifier. Nitrogen gas bubbles formed by denitrification of nitrite and nitrate under anoxic secondary clarifier conditions are known to cause sludge rising. Oversaturation of gases in the aeration tank can also cause sludge rising in the secondary clarifier, especially when aeration tank depth is significantly deeper than that of the secondary clarifier. Reducing the sludge retention time in the secondary clarifier is effective in controlling rising sludge. Close monitoring and control of aeration in the aeration tank can also reduce rising sludge in the secondary clarifier.
NIREAS VOLUME 2 [2.4] 90 Rising sludge in an aeration tank
MORE Rising sludge is not to be confused with bulking. The sludge settles and compacts satisfactorily on the bottom of the clarifier, but after settling it rises to the top of the secondary tank in patches or small particles the size of a pea. Rising sludge usually produces a fine scum" or froth (brown in color) on the surface of the aeration and secondary tanks. Rising sludge is caused by DENITRIFICATION or SEPTICITY and results from too long a detention time in the secondary clarifiers. The secondary clarifiers should be equipped with scum baffles and skimmers to prevent these solids from escaping in the plant effluent. Denitrification is most common when the sludge age is high (extended aeration). When this type of activated sludge flows from the aerator to the secondary clarifier or becomes short of oxygen, the organisms first use the available dissolved oxygen, and then the oxygen in the nitrate compounds resulting in the release of nitrogen gas. Denitrification is an indication of good treatment, providing the sludge in the settleability test stays on the bottom of the cylinder for at least one hour, but floats to the surface in two hours. If it floats up too early in the settleability test, the sludge age should be reduced or the food-tomicroorganism ratio should be increased. This solution will be successful if the nitrifying bacteria are washed out of the system. If the sludge stays down for an hour in the settleability test but
NIREAS VOLUME 2 [2.4] 91
problems are still present in the secondary clarifier, increase the return sludge rates to move the solids out of the clarifier at a faster rate. Under some circumstances this will not help and better results might be obtained by decreasing the return sludge rate, so be careful. Rising sludge also may be controlled by increasing the load to the aerator by removing a primary clarifier from service if more than one is being used. During low flow periods, raw wastewater may be discharged directly to the aerator. Another option is to check the possibility of installing aeration diffusers in a tapered pattern. The tapered pattern is used to meet higher initial oxygen demands.
Nocardia foam Nocardia foam
is associated with a slow-growing, filamentous microorganisms of the Nocardia
genus. Some factors causing Nocardia foaming problems are low F/M ratio, long SRT, and operating in the sludge reaeration mode. Reducing SRT is the most common means of controlling Nocardia foaming problems. Nocardia can also be controlled by avoiding the recycling of the skimmed foam or the addition of a chemical agent (e.g. polymers or chlorine) on the surface
MORE Aerator foaming or frothing has been a problem for some plants. There have been many theories presented on the cause, such as surfactants (detergents), polysaccharides, and overaeration. Whatever the cause, there is a definite relationship between froth buildup on the aerator and the amount of suspended solids in the mixed liquor and air supply to the aerator. Operators may have to control different types of foam. The foam may be unstable and easy to control or the foam could be persistent and difficult to control. Unstable foam may be caused by nutrient deficiencies or solids from dewatering processes (recycled solids). Polymer overdosing can be a cause of foam. Floating sludges and floating scum also are types of foams. These unstable foams are usually kept down using water sprays. Persistent foams are often called filamentous or Nocardia foam and are~difficult to control. These foams are brown, stable, viscous, and usually scum-like in appearance. These foams are often associated with high MCRT values. The higher the concentration of filaments, the greater the tendency for foaming. Also the higher the MLSS concentration, the more susceptible an aeration basin is to foaming. The aeration rate directly influences foaming and the height of foam. Filamentous growth rates tend to increase with temperature. Many plants have experienced foaming problems during seasonal temperature changes in the spring or fall. Apparently, the optimum pH for filamentous growth is around a pH of 6.5. For control of foaming:
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1. Maintain higher mixed liquor suspended solids concentrations. 2. Reduce air supply during periods of low flow while still maintaining DO. 3. Return supernatant to the aeration tank during low flows (be cautious in this methodâ&#x20AC;&#x201D; supernatant should be returned slowly and steadily because too much supernatant could cause an excess oxygen demand). These solutions apply only to detergent foam. In some extended aeration systems or nitrification systems, a froth builds up that can sometimes be controlled by higher sludge wasting rates (reduction of mixed liquor suspended solids, MLSS). Frothing from filamentous organisms is usually present in aeration basins. When the number of filaments becomes excessive, the organisms may form a thick, dark brown scum or froth on the surface of the aeration basin. Microscopic examination of the foamy scum can confirm the presence of filamentous organisms . Filamentous foam has been controlled by MCRT control, RAS/MLSS chlorination, direct foam chlorination, selective foam wasting, use of water sprays, and selector technology. An MCRT of less than six days has been effective. MCRT can be reduced by slowly increasing the wasting rate with care to remain in compliance. Chlorination of RAS/MLSS or return activated sludge or both has been effective in controlling filamentous foam. If a stable foam has already formed, direct foam chlorination is the most effective method of killing foam-forming microorganisms in the foam. A foam trap is installed in the mixed liquor effluent or aeration tank and a highly concentrated chlorine spray is applied directly to the foam-forming microorganisms. Periodic chlorination of return sludge or MLSS can be useful as a preventive measure to control the number of filaments below the foaming threshold.
NIREAS VOLUME 2 [2.4] 93 Nocardia foam
Typical non-Nocardia froth on activated-sludge aeration tank
(Eddy, 1999)
NIREAS VOLUME 2 [2.4] 94
Septic Sludge Septic sludge may be produced when any type of sludge remains too long in such places as hoppers and channels. It is likely to cause a'foul odor, rises slowly, and sometimes rises in clumps. Even small amounts can upset an aerator. Septic sludge may occur in poorly designed or constructed hoppers, wet wells, channels, or pipe systems. This occurs when activated sludge is allowed to be deposited and anaerobic decomposition starts. Septic sludge deposits also may develop on the floor of the aerator due to insufficient air rates that are not keeping the tank completely mixed. A high solids load also can cause septic problems. To effectively control septic sludge, aerators must be thoroughly mixed and sludge must be pumped frequently. In channels and pipelines, a velocity over 0.45 m/ sec will prevent the formation of sludge deposits that could become septic. Sludge going septic in the secondary clarifier may develop from four causes: 1. Return sludge rate too low, thus holding the solids in the final clarifier too long and allowing them to become septic 2. Clarifier collection mechanism turned off, thus the sludge is not being moved to the draw-off hopper 3. Sludge draw-off lines plugged, obstructed, or used infrequently 4: Return sludge pump off or a valve closed A good operator checks the system several times a day. In most new activated sludge plants the secondary clarifiers have air lift samplers or photocells to indicate sludge blanket level in the tank. Whenever the final clarifier sludge blanket level changes, an immediate investigation should be undertaken. In any of the cases above, the correction is quite obviousâ&#x20AC;&#x201D;restore suitable return sludge flow as soon as possible. To control biological growths and the production of odours in the settling tank, the accumulated solids must be flushed out periodically.
Aeration If aeration or movement of water in the reactors is not visible or have totally stopped, this mean air distribution is not good. Check the flow rate and backpressure of the blower. If capacity and pressure are correct, check the air header and distributors for leakages. Overloads
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Some wastewater treatment plants become seriously overloaded during certain times of the year, such as during the canning season. Under the overloaded conditions, the operator has to work very hard to meet the daily, sevenday average, and monthly NPDES permit requirements. Both the blowers and return sludge pumps will work at full capacity the entire month. Ferric chloride must be added in the primary clarifier to reduce the solids and BOD loadings on the activated sludge process. Toward the end of the month, a portion of the effluent had to be diverted to storage oxidation ponds in order for the plant to meet permit requirements. An activated sludge plant can accept quite a shock load now and then without adverse effects to the system, but it cannot survive a continuous series of shock loads. Many factors may change that the operator cannot anticipate or control but must compensate for by adjusting the operational controls. For example, a conventional activated sludge plant has operated satisfactorily for several weeks. The secondary clarifier had good clarity of 1.7 m with a Secchi disc, and the effluent BOD and suspended solids were running from 5 to 18 mg/L. The aeration tanks had been maintained at 6,000 kg of mixed liquor suspended solids with a volatile content of 78.5 %, and sludge age of five days. A minimum DO of 2.8 mg/L had been measured in the last two-thirds of the aerator. Sludge wasting had been at a rate of 900 kg/day from the system. This week the situation has changed; the clarity in the secondary tanks has dropped to 0.5 m. The suspended solids in the secondary clarifier effluent have remained about the same, but the BOD test started five days ago came out at 38 mg/L. If a COD test had been run at the time the BOD was started, an operational correction could have been made at that time. Overall, the plant effluent has definitely deteriorated from the previous week. Only you and your records can determine the cause and what corrective action should be taken. Has plant flow increased or decreased? Have air rates been maintained? Have you received some toxic or untreatable slug dose in the influent? Are your sludge return pump and lines clear? Has the BOD load to the aeration tank changed? Have mixed liquor solids been the same? These are just a few of the conditions that may change effluent quality. The difficult decision after determining the cause or probable cause isâ&#x20AC;&#x201D;should a change be made? This is where an operator's thorough knowledge of plant processes pays off. If you know the situation is unusual and will only last a couple of days, minor changes may quickly improve the effluent quality
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But if the condition occurred before and lasted several weeks according to past records, a process change may be necessary to compensate for it. This is where experience with your plant and records plays an important role in activated sludge operation. By keeping accurate records, you can find the desirable operating range in terms of efficiency of waste removal and cost of operation. Usually each plant will have some mixed liquor suspended solids concentration where the plant will function best. This concentration should produce a clear final effluent, with low suspended solids and BOD of 8 to 20 mg/L. However, depending on plant design, type of waste, and season of year, the best mixed liquor suspended solids concentration might be found to be anywhere from 1,000 to 4,000 mg/L. When a satisfactory mixed liquor suspended solids concentration is found for a specific plant under certain conditions, the operator should attempt to maintain this level until something changes. If the mixed liquor suspended solids are allowed to start building up, the final effluent will begin to deteriorate by becoming turbid. When the mixed liquor suspended solids are allowed to increase too high for the conventional activated sludge plant, other problems can develop. The previous return sludge rate for the plant flow would not be sufficient. Return rates may have to be increased considerably. If the return sludge rate was not increased, the activated sludge in the final clarifiers would build a higher blanket. The deep blanket in the final tank could cause solids to be swept over the weirs during peak flow. Another limiting factor is aeration equipment. The amount of oxygen supplied to the aerator also limits the microorganism mass that can be maintained in an aerobic state. A high oxygen demand in the aerator can be created by a high solids content in the plant influent. The other factor is the organisms themselves. If insufficient food is available, only a limited number of organisms will develop energy to multiply. This is where the struggle for survival begins. When food supply is low, the microorganisms begin to feed upon themselves (ENDOGENOUS RESPIRATION). This is the period of most complete oxidation, and new sludge production is at a minimum. Extended aeration plants are designed to operate under these conditions that tend to increase solids in the plant effluent
Always be alert for the possibility of toxic dumps, accidental spills (particularly the midnight variety), storms, or other up-sewer factors that may change the influent flow or waste characteristics. A frequent problem is the increased flows from storm infiltration or other sources. These flows may create shorter aeration times or loss of activated sludge solids from the final clarifiers due to a hydraulic overload. To compensate for this condition, regulate return and waste sludge rates to hold as much of the solids as possible in the aerator discharge rate rather than all
NIREAS VOLUME 2 [2.4] 97
at once. Certain industries such as canneries create seasonal problems, which the operator should prepare for in advance.
Temperature Changes The activated sludge system is influenced by temperature changes similar to the response of trickling filters to temperature changes in spring and fall. During the summer, the activated sludge plant may operate satisfactorily in a certain loading range and air rates, but in winter the best loading ranges and air rates change and the plant requires less air and more solids under aeration. Usually a temperature change is not significant unless it raises or lowers the wastewater temperature more than 6째C. Temperature is an important factor in oxidation relative to sludge accumulation. A high temperature produces a rapid microorganism growth rate and more waste storage in the organism cell with less oxidation. Therefore, greater biological activity will result.in more overall sludge production but the sludge may be thinner than usual. During the colder winter months, operators increase the solids under aeration (MLSS) to provide more microorganisms to treat the biochemical oxygen demand. When the weather warms in the spring and summer, the microorganisms become more active. If there is poor settling of the activated sludge in the secondary clarifier, try increasing the wasting rate (by no more than ten percent per day) until you see an increase in settling and improved effluent quality
NIREAS VOLUME 2 [2.4] 98
Activated sludge processes â&#x20AC;&#x201C; performance evaluation and troubleshooting
(California State University, 2008)
NIREAS VOLUME 2 [2.4] 99
(California State University, 2008)
(California State University, 2008)
NIREAS VOLUME 2 [2.4] 100
Abnormal surface aerator operation Item
Abnormal Condition
Possible Cause Moisture Winding breakdown
Operator Response Have electrician check motor. Have motor rewound.
Degree of impeller submergence results in Motor
High or uneven amperage
amperage draw in excess of
Adjust aerator.
motor amperage design Inspect and Excessive motor bearing or
lubricate bearings
gear reducer friction
and gears. Overhaul if needed. Repair or replace
Gear Reducer
Lack of proper lubrication
oil pump. Change oil.
Bearing or gear noise
Remove obstruction in oil line. Cracked coupling
Shaft Coupling
Unusual noise and vibration
Loose coupling bolts/nuts as a result of vibration
Replace coupling. Align impeller shaft. Torque bolts. Use "locking" nuts. Align impeller shaft. Torque blade bolts.
Loose blades Impeller
Use lock-washers. Align impeller.
Unusual noise and vibration Cracked blades
(California State University, 2008)
Replace. Torque bolts. Align.
NIREAS VOLUME 2 [2.4] 101 Abnormal blower operation Item Unusual noise or vibration Air system pressure
Air flow
Abnormal Condition Coupling misaligned
Possible Cause Incorrect installation
Loose nuts, bolts, or screws Low pressure
Vibration Bypass valve open, leaks or breaks in distribution piping
High pressure -
Diffusers came off air header Blockage or partially closed valve in distribution piping Plugged diffusers
Low total flow
High ambient temperatures
System oil pressure
Blower air control malfunction
Low pressure
Oil level too low Oil filter dirty Check valve sticks open Incorrect oil type
High pressure
Incorrect oil type
Oil discharge pressure
Low pressure
Oil temperature
Low temperature High temperature
Suction lift too high Air or vapor in oil Coupling slipping on pump shaft Oil cooler water flow too high Oil cooler water flow too low Incorrect oil type Insufficient oil circulation
Bearings
Hot bearing(s)
Blower speed too high Defective bearing(s)
Motor
Will not start Noisy High temperature
Oil cooler water flow too low Overload relay tripped Noisy bearing Restricted ventilation Electrical
(California State University, 2008)
Operator Response Align coupling with blower at operating temperature according to manufacturer. Tighten. Close valve, repair leaks or breaks. Replace diffusers. Remove blockage or open valve. Blow out or remove and clean. Add more air if needed. Repair or replace control. Add oil. Replace. Replace valve. Drain and refill with proper oil type. Drain and refill with proper oil type. Reduce lift. Purge air at filter. Secure coupling. Throttle water flow. Increase water flow. Drain and refill with proper oil type. Replace oil filter, check oil lines for restrictions. Reduce speed to recommended RPM. Check bearing(s) for clearance, hot spots, cracks or other damage. Repair or replace. Increase water flow. Correct and reset. Check and lubricate. Check openings and duct work for obstructions. Check for grounded or shorted coils and unbalanced voltages between phases.
NIREAS VOLUME 2 [2.4] 102 Abnormal air distribution system operation Item Meter(s)
Seals, gaskets, and tlex connections Pipe
Valves
Abnormal Condition
Operator Response High, low, or no indication Loose movement Tighten or replace. Out of calibration Calibrate. Dirt in mechanism Clean. Pointer dragging on scale plate Adjust pointer. Bypass valve open or leaking Close or repair. Meter piping leaks Tighten or replace. Meter piping plugged Clean piping. Leaking Loose bolts or fittings Tighten. Blown out Replace. { Worn Corrosion
Possible Cause
Usual deterioration Condensate
Sludge inside pipe
Vacuum action by blower operating in reverse
Dirt
No or inefficient air filtration
Difficult to operate or frozen
Hardened grease
Corrosion
(California State University, 2008)
Replace. Drain traps daily, install additional traps, flush pipe, paint pipe, and remove standing water from around pipe. Flush pipe, install check valve on blower, or repair check valve. Install filters. Clean filters more frequently. Remove old grease 3and apply seizing inhibitor. Operate valves monthly. Drain condensate traps daily. Apply seizing inhibitor.
NIREAS VOLUME 2 [2.4] 103 Abnormal air header operation Item Valve
Abnormal Condition Valve leaks at stem
Possible Cause Loose stem packing nut Defective packing
Valve will not seat closed
Corrosion
Butterfly rubber seat defective
Butterfly or gate has come off valve system Swing header _ _ Air leaks from joint pivot joints
Defective O-ring
â&#x20AC;˘
Fixed header couplings or unions
Air leaks from couplings, unions, or end caps
Loose joint Insufficient grease in joint Cracked joint PVC has defective glue bond
Pipe has leak through thread Horizontal header Uneven water motion (roll) in tank
Header not perfectly level, thus allowing more air to one side
Header pipe
Interior corrosion
O-rings or gaskets defective or connections loose Moisture
Exterior corrosion
Moisture Electrolysis
Operator Response Tighten nut. Secure distribution system and replace packing. Secure distribution system and clean or replace valve. Secure distribution system and replace rubber seat. Secure distribution system and replace. Close header valve, pull header from tank with crane, and replace O-ring. Tighten. Apply 3 to 5 shots of grease. Replace. Remove, clean with PVC solvent, bond, and allow bond to cure. Remove, apply Teflon tape, tighten. Level header with surveyor's level (tank empty) or use Mason's level. Replace O-rings or gaskets. Tighten connections. Use PVC or galvanized pipe. Use PVC, galvanized pipe, or paint pipe with an epoxy coating. Use a sacrificial (magnesium) anode or coat surface.
(California State University, 2008) Abnormal air diffuser operation Item Fine-bubble diffuser
Coarse-bubble diffuser
Abnormal Condition Exterior clogged
Possible Cause Biological growth
Interior clogged
Dirt from distribution system
Exterior clogged
Biological growth
Cracked
Overtightened when installed, structural failure Inefficient pretreatment, normal conditions Clogged diffusers Inadequate diffuser arrangement Too few diffusers
Fine- and coarse- Accumulation of rags, hair, bubble diffusers string Insufficient diffusion pattern or oxygen transfer
Operator Response Raise air header, remove diffuser, scrub and wash diffuser. Raise air header, remove diffuser, scrub and wash diffuser. Install filters, clean filters more frequently. Raise air header, scrub and wash diffuser. Once a month increase air flow 2 to 3 times normal for 15 minutes to "blow out" diffuser orifices. Replace. Yearly, raise headers and clean diffusers. Clean the diffusers. Modify diffuser arrangement. Add diffusers or install a
NIREAS VOLUME 2 [2.4] different type.
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2.4.2 AEROBIC ATTACHED GROWTH BIOLOGICAL TREATMENT PROCESSES In an attached growth or fixed-film biological process, microorganisms are attached to a solid substratum where they reach relatively high concentrations. The support materials include gravels, stones, plastic, sand, or activated carbon particles. Two important factors that influence microbial growth on the support material are the flow rate of wastewater as well as the size and geometric configuration of particles.
Fixed-biofilm processes offer the following advantages :
1. They allow the development of microorganisms with relatively low specific growth rates (e.g., methanogens). 2. They are less subject to variable or intermittent loadings. 3. They are suitable for small reactor size. 4. For fixed-film processes such as trickling filters, the operational costs are lower than for activated sludge.
However, in industrial systems, fixed-biofilm processes lead to biofouling, the undesirable overgrowth of microorganisms on surfaces. Biofouling is controlled by using physical or chemical methods or a combination of both. Physical methods include physical removal of biofilms or application of low-intensity electric fields or ultrasound energy across the biofilm. Chemical control involves the use of oxidizing agents (e.g., peroxides, halogens, ozone), and nonoxidizing biocides (e.g., surface active agents, aldehyde-based chemicals, or phenol derivatives).
NIREAS VOLUME 2 [2.4] 105
2.4.2.1 Trickling filters (TF) Equipment
(California State University, 2008)
Process A trickling filter is an attached-growth, biological process that uses an inert medium to attract microorganisms, which form a film on the medium surface. A rotatory or stationary distribution mechanism distributes wastewater from the top of the filter percolating it through the interstices of the film-covered medium. As the wastewater moves through the filter, the organic matter is adsorbed onto the film and degraded by a mixed population of aerobic microorganisms. The oxygen required for organic degradation is supplied by air circulating through the filter induced by natural draft or ventilation. A light-weight, highly-permeable medium with a large specific surface area (e.g., plastic modules) is conducive to microorganism buildup and ensures unhindered movement of wastewater and air. A porous underdrain system at the bottom of the filter collects treated effluent
NIREAS VOLUME 2 [2.4] 106
and circulates air. The filter recirculates and mixes a portion of the effluent with the incoming wastewater to reduce its strength and provide uniform hydraulic loading .
Schematic cross-section of a trickling filter
3D Cross section of a stone media trickling filter
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 2 [2.4] 107 3D Cross section of a typical trickling filter ( California State University, 2008)
Typical underdrain system for tower filter
(Eddy, 1999)
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Single-stage and multistage filter arrangements are both used because many recirculation schemes and options of intermediate settling between multistage filters are available. The recirculation method is much less a factor in plant performance than the recirculation ratio.
Installation of synthetic media in trickling filter
(California State University, 2008)
NIREAS VOLUME 2 [2.4] 109
Typical distributor used to apply wastewater to trickling filter packing
(Eddy, 1999)
Typical single-stage, trickling filter, recirculation flow sheets.
NIREAS VOLUME 2 [2.4] 110
(David H.F. Liu, Bela G. Liptak, 1999)
Above figure shows the single-stage filter recirculation flow diagrams. Sludge from the final clarifier is usually recirculated to a point before the primary settling tank. The recirculation flow is also taken from in front of and behind the final clarifier to a point either before or after the primary settling. All units must be designed for total hydraulic flow and organic loading.
Next Figure shows several flow routings used for multistage filters. All sludge returned from the intermediate and final clarifiers that is not wasted (excess) is returned to a point before the primary settling tanks.
NIREAS VOLUME 2 [2.4] 111 Typical multistage, trickling-filter, recirculation flow sheets
(David H.F. Liu, Bela G. Liptak, 1999)
NIREAS VOLUME 2 [2.4] 112
Wastewater treatment facilities often recirculate the treated effluent from the clarifier to :
• Reduce the possibility of organic shock loadings by diluting the incoming wastewater • Maintain uniform hydraulic loadings especially under low and intermittent flow conditions • Achieve an extensive film coverage and a relatively uniform film thickness through the filter • Reduce the nuisances of odor and flies
Hydraulic & Organic loading rate Trickling filters are classified according to the hydraulic and organic loading applied. Filters are categorized as follows: the low rate is 1,5 to 4 m3 per m2 per day, the intermediate rate is 4 to 10 m3 per m2 per day, the high rate is 10 to 30 m3 per m2 per day, and super-rate units are greater than 30 m3 per m2 per day. The effluent from a low-rate trickling filter is usually low in BOD and well nitrified.
NIREAS VOLUME 2 [2.4] 113 Underdrains The underdrains used in trickling filters support the filter medium, collect the treated effluent and the sloughed biological solids, and circulate the air through the filter. Precast blocks of vitrified clay or fiberglass grating arranged on a reinforced concrete floor can be used as the underdrain system for a rock-media trickling filter. Precast concrete beams supported by columns or posts can be used as the underdrain and support system for a plastic-media trickling filter. The floor should be sloped towards either central or peripheral collection channels at a 1 to 5% grade for improved liquid flow.
Filter media The ideal medium used in a trickling filter should have the following properties: high specific surface area, high void space, light weight, biological inertness, chemical resistance, mechanical durability, and low cost. Plastic media are reported to be highly effective for BOD and SS removal over a range of loadings. Furthermore, lighter and taller filter structures can be constructed to house plastic media, reducing land requirements.
Clarifiers Clarifiers used in trickling filters remove large and heavily sloughed biological solids or humus without providing thickening functions. Therefore, the design of these clarifiers is similar to the design of primary settling tanks. The overflow rate is based on the influent flow plus the recirculation flow.
NIREAS VOLUME 2 [2.4] 114 Design Parameters
Trickling filters design parameters Attached Growth Process Trickling Filter type Low rate Intermediate rate High rate Super high rate Roughing Two stage
Filter Medium Rock & Slag Rock & Slag Rock Plastic Plastic & Redwood Rock & Plastic
Hydraulic Loading (m3/m2_hr)
BOD5 Loading (kg/m3_d)
BOD5 removal (%)
Recycling Ratio
0,05-0,15 0,15-0,40 0,35-1,55 0,45-3
0,08-0,22 0,20-0,5 0,45-2,4 0,60-3,2
80-90 50-70 65-85 65-80
0 0-1 1-2 1-2
2-8
1,6-8
0,35-1,55
1-2
40-65 85-95
1-4 0,5-2
Design capacity De-centralised to semi-centralised. The system is usually applied in communities for treatment of municipal-domestic wastewater. It can be applied for bigger and smaller communities.
Effluent quality •
Removal BOD: 65 to 90 %.
•
Low TSS removal.
•
Total Coliforms: 1 to 2 log units
•
N: 0 to 35%.
•
P: 10 to 15 %.
Costs Intermediate; investment costs depend on type of filter materials and feeder pumps used; moderate operational costs determined by electricity consumption of feeder pumps. However, skilled labour is required for construction and maintenance (e.g. prevent clogging, ensure adequate flushing, monitor hydraulic and organic loads, control filter flies, etc. To bring the water to the top of the filter and for the rotary sprinkler system, energy is required (even though the requirements are low compared to other aerated systems such as activated sludge).
NIREAS VOLUME 2 [2.4] 115
Advantages/disadvantages
Advantages Trickling filters are attractive to small communities because of:: •
easy operation,
•
low maintenance costs,
•
reliability.
•
They are able to withstand shock loads of toxic inputs.
•
Can be operated at a range of organic and hydraulic loading rates
•
The sloughed biofilms can also be easily removed by sedimentation.
•
High effluent quality in terms of BOD and suspended solids removal; in combination with a primary and tertiary treatment also in terms of pathogens
Disadvantages •
High organic loading may lead to filter clogging as a result of excessive growth of slime bacteria in biofilms. Excessive biofilm growth can also cause odor problems in trickling filters. Clogging restricts air circulation, resulting in low availability of oxygen to biofilm microorganisms
•
The systems does not work during power failures, unless feeding is carried by dosometric siphons
•
Pre-treatment and treatment of excess sludge required
•
Experts required for design, construction and maintenance
•
Not all parts and materials may be available locally
Operation & Maintenance To prevent clogging and excessive sloughing and removing the dead sludge, the bacterial film has to be flushed away once in five to seven years or more. This can be done using high hydraulic loading rates > 0.8 m3/m3hr and temporal collection of the effluent. The rotary distributor may also require regular cleaning or technical maintenance. Moisture control of the filter is very important on one hand to prevent odour (i.e., if too dry) and on the other hand to prevent nesting of flies and mosquitoes. Constant hydraulic loading can be maintained through suction level controlled pumps or dosing siphons. This may be problematic at night when the water flow is reduced or when there are power failures. Recirculation of effluent may also be required to avoid low flow conditions , but a too strong flow overload would flush out the microbes. Besides drying out, excessive odour can
NIREAS VOLUME 2 [2.4] 116
also arise when anaerobic conditions arise due to excessive organic loadings or insufficient aeration.
Modifications have helped improve the BOD removal of trickling filters. The following are some of these improvements: •
Alternating double filtration (ADF), which consists of alternating two filters for receiving the waste
•
Slowing down wastewater distribution
•
Use of plastic materials in the filter for increased surface area and improvement of air circulation
•
Management of odors problems by increasing air flow by means of forced ventilation.
Pre-Start •
A trickling filter should be checked carefully before a new one is started or an existing one is placed in service again to be certain all the essential parts will work properly when wastewater is applied. A new plant is seldom started up without some unexpected, frustrating problems. Some careful inspecting ahead of time can prevent many of these situations. Filter bearings often come packed in a special grease to prevent damage during transportation. This packing grease must be removed and replaced with the proper grease before start-up. If at all possible, you should arrange to be present when your new equipment is serviced. You should see that the correct oil and amount of oil are used in all oil reservoirs. Many contractors will put motor oil in everything and consider it serviced. For future reference, record the amount and type of oil each reservoir holds.
•
After the oil has been installed in a distributor, check the arms for even adjustment and level. Rotate the unit by hand and observe for smooth turning. Any vibration or roughness should be corrected before putting the unit in service. If the distributor has adjustable orifices, get the design specifications and a rule and check out the orifice settings. File the specification sheet for future reference. In a trickling filter plant with fixed-spray nozzles, each nozzle should be checked to ensure that it is free of foreign objects.
•
In order to prevent damage to pumps, crawl into the under-drain system of the filter and remove any debris (rocks, pieces of wood, and other debris). Check painted surfaces for damaged areas. Touch these up before they get wet to prevent corrosion and further damage. A few nicks and scratches in a distributor arm can seriously affect the life of the original protective coatings.
NIREAS VOLUME 2 [2.4] 117
•
Check all valves in the system for smooth operation. On sliding-gate valves, see that the gates seat properly. There are adjustable wedges and stops on this type of valve. With the valve adjusted, set the lock nut on the stem to prevent jamming or closing the gate too tightly. These small precautions will yield years of trouble-free valve operation.
•
In addition to the general items covered in this section, you should be certain that the correct manufacturer's manual has been furnished for each piece of equipment. Read each manual carefully and follow the given recommendations. Obtain the oils and greases recommended; or, if you buy from one oil company, have their representative furnish you a written list of the company's products that are equivalent to those recommended by the equipment manufacturer. Finally, remember to remove any trash on or in the media.
Placing a Filter in Service •
Try to schedule the starting of trickling filters during late April through early June (depends on local conditions). This procedure will produce the most slime growth during the shortest period of time. Problems avoided will include wet weather flows in the spring, odors in the summer, and dormant bacteria in the winter.
•
When you have checked out all equipment mechanically, starting up the trickling filter portion of the plant is very simple. Start the wastewater flow to the filters, observing the rotating arms carefully for smooth operation, speed of rotation, and even distribution of the wastewater over the media. Time the speed of rotation, record the flow rate, and log them for future reference.
•
NOTE: Starting up recirculation may be tricky in some plants. The pump may run out of water before the return from the filter has begun. You may have to block the channels (launders) in the clarifier and build up extra water before starting the pump. Conversely, shutting off recirculation will result in a surge of water because the pump is no longer removing water, but water is still returning from the filter. The solution of a recirculating ball valve can also be chosen.
•
Recirculating Ball Valves are constructed of PVC and rubber components for corrosion resistance. The valve redirects 100% of the incoming flow to the recirculation tank during periods in which the ball is not seated, and 0% when the ball is seated.
NIREAS VOLUME 2 [2.4] 118 Recirculation ball valve (Orenco systems Inc.)
•
For fixed nozzles, observe the spray pattern. Usually, some debris will show up to plug some of the nozzles, the amount depending on how thoroughly the plant was checked out prior to start-up. Be sure to keep the nozzles clear so that the wastewater is distributed over all of the filter media. Regular care is required to keep fixed nozzles working properly.
•
Several days will pass before any slime growth starts to develop on the filter media, and up to several weeks will pass before full development occurs. During this period, lower efficiencies of waste removal may be expected. Time of year, weather conditions, and strength of the wastewater are all factors that will affect the time needed for slime growth development.
•
Growth may be accelerated by recirculating wastewater through the trickling filter prior to treating the main wastewater flow stream. Waste activated sludge may also be added to the recirculated flow to encourage slime growth development.
•
During this period of slime growth development, an unstable effluent will be produced. This effluent will exert a pollutional load on the receiving waters. Heavy chlorination is usually used during this time to reduce the pollutional load and the health hazard to some extent.
NIREAS VOLUME 2 [2.4] 119
•
In some locations, such as where fish are threatened, the use of chlorine in this manner may be restricted. If an older plant is being phased out, it may be possible to load the new facilities lightly or intermittently until a full slime growth is established.
Daily Operation •
Once growth on the media has been established and the plant is in normal operation, very little routine operational control is required. Careful daily observation is important. Items to be checked daily are: Any indication of ponding Filter flies Odors Plugged orifices Roughness or vibration of the distributor arms Leakage past the distributor turntable seal Splash beyond the filter media Cleanup of slimes not on media
•
Operation of clarifiers is interconnected with trickling filter operation. If the recirculation pattern permits, it is a good idea to return filter effluent to the primary clarifier. This is a very effective odor-control measure because it adds oxygen to incoming wastewater that is often septic. In some plants, increasing the recirculation rate will increase the hydraulic loading on the clarifier. Be sure the hydraulic loading remains within the engineering design limits. If the hydraulic loading is too low, septic conditions may develop in the clarifier. Excessively high loadings may wash solids out of the clarifier.
•
Recirculation during low-inflow periods of the day and night may help to keep the slime growths wet, minimize fly development, and wash off excessive slime growths. Reduce or stop recirculation during high-flow periods, if necessary, to avoid clarifier problems from hydraulic overloading. Recirculation of final clarifier effluent dilutes influent wastewater and recirculation improves slime development on the media. Proper recirculation rates help to control snail populations on the media.
•
You should, by evaluating your own operating records, adjust the process to obtain the best possible results for the least cost. Power costs are a large item in a plant budget. In order to conserve energy, use the lowest recirculation rates that will yield good results. Be careful not to cause ponding, reduced BOD removal efficiency, or other problems that result from recirculation rates that are too low. Also, reduced hydraulic loadings mean better settling in the clarifiers. This results in less chlorine usage in plants that disinfect the final effluent,
NIREAS VOLUME 2 [2.4] since organic matter exerts a high chlorine demand. If filter effluent, rather than secondary
120
clarifier effluent, is recirculated, the hydraulic loading on the secondary clarifier is not affected.
Shutdown of a Filter •
Always take a few minutes to plan what you are going to do before shutting down a major plant process or piece of equipment, such as a trickling filter, regardless of the seriousness of the problem or the need for immediate action. Items that must be considered are listed below.
•
What is the incoming flow? Could a shutdown be scheduled at a better time such as during lower flows or when more operators are available to perform the work?
•
How will a shutdown affect the rest of the plant? When the process or equipment is placed back on line after a shutdown, will it cause development of a hydraulic surge which will overload other processes (clarifiers) or equipment (such as chlorinators)?
•
If the filter is to be shut down for maintenance, are the necessary tools and other items (such as funnels, buckets, and lubricants) available?
•
Are there any other tasks that should be performed while the unit is off the line? For example, does one of the recirculation pumps need repacking?
•
To shut down a trickling filter, consider the following step-by-step procedures if they apply to your treatment plant: Inspect your plant to be sure there are no abnormal conditions hindering the effectiveness of other operating areas and process units. If the filter to be taken out of service has filter influent and recirculation pumps that supply only the filter being shut down, reduce the pump speed to the minimum range. Reducing the speed of a pump will tend to relieve a part of the surge created to the remaining process units when the filter is shut down. Also, due to the reduced load when the pump is started again, the life of variable-speed pumps using belt drives will be extended. Stop the influent flow (feed) and recirculation pumps for the filter. Allow the distributor arms to stop moving. Secure the distributor arms. Open the end gates. Restart the pump in order to flush the arms for a few minutes. Do not try to open the gates or stop the arms when the arms are still moving. Stop the influent flow (feed) and recirculation pumps for the filter and close the pump discharge valves. Tag and lock out the pump motor starters. The filter distributor will stop rotating soon because no water is flowing out the outlet orifices.
NIREAS VOLUME 2 [2.4] WARNING. Never attempt to stop a rotating distributor by standing in front of it or
121
grabbing it with your hands. Check the remaining plant parts for proper operation, particularly wet wells and distribution or diversion structures between the other filters and clarifiers for normal water levels and position of flow control valves. Once the distributor arm has stopped rotating, remove debris and rags from the distributor arm orifice plates. Also, remove from the top of the media any debris and rags that could have been dumped during flushing of the distributor arms. •
If the filter is to be left out of service for several days or longer, the following steps should be taken. Close the filter underdrain outlet gates to prevent flow from other units from entering the underdrain channel. Drain or pump down the underdrain channel to prevent odors and insects from developing in the captured (stagnant) wastewater. Hose down the distributor arms, side walls, vent ducts, and underdrain channels. Remove any grit or debris from the main underdrain collection channel. Inspect the underdrains and remove any debris in order to prevent stoppages. Check the oil level in the distributor turntable for proper level and the possible presence of water. Inspect the turntable seal. Consider removal of material (biomass) from media if growths are very heavy. If not removed, excessive growths may cause ponding when the filters are restarted. After drying, the material can be removed by the use of a leaf rake. Most of the remaining material will be flushed out when the unit is put back in service.
•
These steps take a small amount of extra time, but they can prevent unnecessary mistakes or prevent your plant effluent from violating discharge requirements.
Get familiar with your plant
Study those areas influencing how the plant will be operated and maintained. These areas should include: 1. Site a. Access to the filter. Consider roads for maintenance equipment and walkways for personnel.
NIREAS VOLUME 2 [2.4] 122
b. Overhead clearance. Determine locations and distances to electrical power and telephone lines. Be sure they will not interfere with the boom of a crane lifting a turntable. c. Trees and shrubs near filters. Trees should not be planted or allowed to remain close to open filters because leaves will plug the voids in the media, cause ponding, and also prevent ventilation. Evergreen trees are recommended over deciduous trees (trees that lose their leaves). Be sure that trees are planted where their roots cannot get into plant piping. d. Location of hose bibs (high-pressure water faucets). Place hose bibs at convenient locations for washing down the filter and other maintenance jobs. 2. Trickling filter structure a. Access (walking surface) to turntable seals and also oil drain, fill, and level plugs. Be sure sufficient space is provided for necessary maintenance work. b. Layout of underdrain grills, channels, and channel slopes. Access and space must be provided for flushing out solids, carrying solids away, and also proper ventilation. c. Location of valves and gates. Provisions must be made to allow flooding of the filter media and also dewatering of effluent control boxes and underdrain collector channels. Be sure valves are located between observation and sampling manholes and the filter in order to allow and observe flooding of the filter. d. Access to effluent boxes. Access and space must be available for removal of effluent box covers or grates and also maintenance of slide gates. e. Center column support. Support should be wide enough for timbers and jacks to be used to raise the distributor from the turntable for race maintenance. f. Covered trickling filters. (1)The operator cannot easily see if the distributor arm is moving under the cover. Some type of device that causes a light to flash when the arm passes a certain point should be placed on the end of one of the arms. Then the operator can determine the speed of rotation of the distributor arm by watching the flashing light. (2)If the filter is completely covered, a forced-air ventilation system is needed. If odors cause complaints from neighbors, an odor-scrubbing device will be needed also. The odor scrubbing fans should be installed so the forced-air ventilation will be in the same direction as natural air currents. These ventilation fans must be equipped with airtight seals to the drive motors to avoid corrosion problems. (3)Proper materials must be used to avoid corrosion of the roof structure.
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g. Walls should extend a minimum of 1,2 to 1,5 feet above the surface of the trickling filter media to prevent staining/ spraying of the outside of the filter tower. h. Vents should be designed to prevent staining/spraying of the outside of the filter towers. They should also be designed to close if forced ventilation is used. 3. Equipment a. Distributors (1)Adjustable orifice plates should be installed on both the leading and trailing edges of the distributor arms. (2)Safety stops should be installed to prevent the end-gate handle from catching in the media during flushing of the distributor arm. (3)Turnbuckles on guy rods must have sufficient thread length to make necessary adjustments. b. Valves (1)Valves must seat properly against design heads to prevent leakage back into the channel during de-watering operations. (2)A protective coating must be applied to all gates and frames. (3)Stop nuts must be installed on all valve stems.
Response to Abnormal Conditions Ponding Ponding results from a loss of open area in the filter. If the voids are filled, flow tends to collect on the surface in ponds. Ponding can be caused by excessive organic loading without a corresponding high recirculation rate. Perhaps the most common source of ponding is from the lack of good primary clarification prior to the filter. Another cause of ponding can be the use of media that are too small or not sufficiently uniform in size. In nonuniform media, the smaller pieces fit between the larger ones and thus make it easier for the slimes to plug the filter. If this condition exists, replacement of the media is the most satisfactory solution. Other causes of ponding include a poor or improper media permitting cementing or breakup, accumulation of fibers or trash in the filter voids (spaces between media), a high organic growth rate followed by a shock load and rapid, uncontrolled sloughing, or an excessive growth of insect larvae or snails, which may accumulate in
NIREAS VOLUME 2 [2.4] 124
the voids. The cause of ponding must be identified since the problem may increase rapidly and take over large areas of the filter. Increasing the hydraulic loading is likely to flush off some of the heavier portions of the biological film and may slowly cure this condition. This can be achieved by increasing the recirculation rate or adjusting the orifices on the distributor assembly so that it distributes flow more evenly. Minor ponding, which may occur from time to time, can be eliminated by any of several methods, including the following: Spray the filter surface with a high-pressure water stream. Sometimes stopping a rotary distributor over the ponded area will flush the growth from the voids. One way to do this is to shut off the flow momentarily, wait for the distributor to stop, move the distributor to the problem area, and then restart the flow while keeping the distributor over the ponded area. Hand turn or stir the filter surface with a rake, fork, or bar. Remove any accumulation of leaves or other debris. Dose the filter with chlorine at about 5 mg/L for several hours. If done during a period of low flow, the amount of chlorine used is held to a minimum. If it is possible to flood the filter, keeping the media submerged for 24 hours will cause the growth to slough somewhat. Keep the surface of the media covered, but do not let the water rise high enough to get into the distributor bearings. Under these conditions, the growths tend to become anaerobic and loosen or liquify. After the holding period, carefully release the wastewater in order to avoid violating NPDES effluent discharge requirements. Shut off the flow to the filter for several hours. The growth will dry and can be removed by the use of a leaf rake. Most of the remaining material will be flushed out when the unit is put back in service. Be sure to keep in mind that your primary purpose is to turn out an effluent of consistently good quality. With this in mind, the above corrective actions are listed in order, starting with procedures that will least affect the effluent. If at all possible, ponding should be corrected before it becomes serious. Items 4 and 5 are drastic measures. However, the job must be done so that full efficiency of the filter is restored. In some cases more chlorine will be needed for effluent disinfection. Where dechlorination is required to protect fish in the receiving stream, more chemicals also will be needed for this purpose until the filters are again operating normally.
Odors Since operation of trickling filters is an aerobic process, no serious odors should exist unless odorproducing compounds are present in the wastewater in high concentrations. The presence of foul odors indicates that anaerobic conditions are predominant. Anaerobic conditions are usually
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present under that portion of slime growth that is next to the media surface. As long as the surface of the slime growth (zoogleal film) is aerobic, odors should be minor. Corrective measures should be taken immediately if foul odors develop. The following are guidelines for maintaining trickling filters to prevent odor problems. Do everything possible (such as prechlorination or pre-aeration) to maintain aerobic conditions in the sewer collection system and in the primary treatment units. Check the ventilation in the filter. Heavy biological growths or obstructions in the underdrain system will interfere with proper ventilation. Examine the ventilation facilities such as the draft tube or other inlets for stoppages. If necessary, force air into underdrains using mechanical equipment such as fans or compressors. Natural ventilation through a filter will occur if the vents are open and the difference between air temperature and filter temperature is greater than (2°C). Increase the recirculation rate to provide more oxygen to the filter bed and increase sloughing. Keep the wastewater splash from the distributor away from exposed structures, grass, and other surfaces. If slime growths appear on sidewalks, inside walls of the filter or distributor splash plates, remove them immediately. In some cases during hot weather, odors will be noticeable from filters in good condition. If these odors are a serious problem (close neighbors), the situation can sometimes be resolved with one of the commercially available masking agents For covered filters, a forced-air ventilation system and odor control of the exhaust air stream is usually provided. Refer to the plant Ο & Μ manual or the manufacturer's literature for proper operation of this equipment. A covered filter and odor control system do not substitute for good operation and housekeeping procedures. The other points covered in this section will still apply. However, where uncovered filters have become a problem (such as in a nearby housing development), the addition of a cover and an odor control system could solve the problem. Before a filter is covered, an investigation of potential corrosion control and odor scrubbing controls must be undertaken to prevent further problems.
Filter Flies The tiny, gnat-size filter fly (psychoda) is the primary nuisance insect connected with trickling filter operations. They are occasionally found in great numbers and can be an extremely difficult problem to plant operating personnel as well as nearby neighbors. Preferring an alternately wet and dry environment for development, the flies are found most frequently in low-rate filters and are
NIREAS VOLUME 2 [2.4] 126
usually not much of a problem in high-rate filters. Control usually can be accomplished by the use of one or more of the following methods. Increase the recirculation rate. A continuous hydraulic loading of 8 m3/ m2 _ day) or more will keep filter fly larvae washed out of rock filter media. Synthetic media will require higher hydraulic loadings or the use of weekly flushings by turning on all the filter pumps. Keep orifice openings clear, including end gates of the distributor arms. The gates can be opened slightly to obtain a flushing action on the walls. Apply approved insecticides with caution to filter walls and to other plant structures. Flood the filter for 24 hours at intervals frequent enough to prevent completion of the life cycle. This cycle is as short as seven days in hot weather. A poor effluent will result from this practice so it should be carefully monitored. Dose with about 1 mg/L chlorine for a few hours each week. The chlorine will cause some of the slime layer to slough off. Too much chlorine will remove too much of the slime layer, reducing BOD removal and lowering the effluent quality of the plant. Shrubbery, weeds, and tall grass provide a natural sanctuary for filter flies. Good grounds maintenance and cleanup practices will help to minimize fly problems.
Sloughing One of the most common problems with trickling filter operation is the periodic uncontrolled sloughing of biological slime growths from the filter media. Increasing the recirculation pumping rate to the filter on a weekly basis may help to induce controlled sloughing rather than to allow the slime growths to build up. During this flushing process, slow down the rotation of the distributor arms by adding more speed-retarder orifices to produce a slower rotation and thus cause a longer flush that penetrates deeper into the filter media. This will help prevent the buildup of the slime biomass.
Poor Effluent Quality Check the organic load on the filter when the treated effluent quality is poor. Measure both the soluble and total BOD in the final effluent. The results will indicate if the poor effluent is caused by BOD associated with escaping solids (high total BOD) or whether the poor effluent results from the trickling filter BOD removal capacity being exceeded (high soluble BOD).
Cold Weather Problems Cold weather usually does not offer much of a problem to wastewater flowing in a pipe or through a clarifier. Occasionally, however, wastewater sprayed from distributor nozzles or exposed in thin
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layers on the media may reach the freezing point and cause a buildup of ice on the filter. Several measures can be taken to reduce ice problems on the filter. Decrease the amount of recirculation (because influent is usually warmer than recycled flows), provided sufficient flow will remain to keep the filter working properly. Operate TWO-STAGE FILTERS" in parallel rather than in series. Adjust or remove orifices and splash plates to reduce the spray effect. Construct wind screens, covers, or canopies to reduce heat losses. Break up and remove the larger areas of ice buildup. Partially open the end gates to provide a stream rather than a spray along the retaining wall. Add hot water or steam to the filter influent if necessary.
Although the efficiency of the filter unit is reduced during periods of icing, it is important to keep this unit running. Taking the unit out of service will not only reduce the quality of the effluent but may lead to additional maintenance problems, such as ice forming, with the possibility of structural damage. Also, moisture may condense in the oil and damage the bearings.
NIREAS VOLUME 2 [2.4] 128 Trickling Filters Troubleshooting Guide (California State University, 2008)
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2.4.2.2 Packed-bed filters - Intermittent Sand filters (ISFs)
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Packed bed filters (PBFs) incorporating naturally occurring treatment media such as sand and gravel have been used successfully for treating small to medium volume wastewater flows for decades. These filters produce high quality effluent that is superior to that discharged by the majority of most municipal treatment facilities with conventional activated sludge processes. Over the past three decades, two types of packed bed sand filters have been most commonly usedâ&#x20AC;&#x201D;the single-pass filter (ISF) and the recirculating filter (RSF).
Process Intermittent Sand Filters (ISFs), are a subcategory of trickling filters, but their main difference is that they operate at much lower hydraulic and organic loads. ISFs have 0,6 m deep filter beds of carefully graded media. Sand is a commonly used medium, but anthracite, mineral tailings, bottom ash, etc., have also been used. The surface of the bed is intermittently dosed with effluent that percolates in a single pass through the sand to the bottom of the filter. After being collected in the underdrain, the treated effluent is transported to a line for further treatment or disposal. The two basic components of an ISF system are a primary treatment unit(s) (a septic tank or other sedimentation system) and a sand filter. Next Figure shows a schematic of a typical ISF process:
Typical ISF wastewater treatment process
ISFs remove contaminants in wastewater through physical, chemical, and biological treatment processes. Although the physical and chemical processes play an important role in the removal of many particles, the biological processes play the most important role in sand filters. ISFs are
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typically built below grade in excavations 1 to 1,3 m deep and lined with an impermeable membrane where required. The underdrain is surrounded by a layer of graded gravel and crushed rock with the upstream end brought to the surface and vented. Pea gravel is placed on top of the graded gravel, and sand is laid on top of the pea gravel. Another layer of graded gravel is laid down, with the distribution pipes running through it. A flushing valve is located at the end of each distribution lateral. Lightweight filter fabric is placed over the final course of rock to keep silt from moving into the sand while allowing air and water to pass through. The top of the filter is then backfilled with loamy sand that may be planted with grass. Buried ISFs can also be designed for single homes. Typical cross section of an ISF
NIREAS VOLUME 2 [2.4] 133 Distribution pipes in a Typical Sand Filter installation
The most important environmental factors that determine the effectiveness of treatment are media reaeration and temperature. Reaeration makes oxygen available for the aerobic decomposition of the wastewater. Temperature directly affects the rate of microbial growth, chemical reactions, and other factors that contribute to the stabilization of wastewater within the ISF. Filter performance is typically higher in areas where the climate is warmer compared to areas that have colder climates. Discussed below are several process design parameters that affect the operation and performance of ISFs.
The Degree of Pretreatment An adequately sized, structurally sound, watertight septic tank will ensure adequate pretreatment of typical domestic wastewater.
Media Size The effectiveness of the granular material as filter media is dependent on the size, uniformity, and composition of the grains. The size of the granular media correlates with the surface area available to support the microorganisms that treat the wastewater. This consequently affects the quality of the filtered effluent.
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Media Depth Adequate sand depth must be maintained in order for the zone of capillarity to not infringe on the upper zone required for treatment.
Hydraulic Loading Rate In general, the higher the hydraulic load, the lower the effluent quality for a given medium. High hydraulic loading rates are typically used for filters with a larger media size or systems that receive higher quality wastewater.
Organic Loading Rate The application of organic material in the filter bed is a factor that affects the performance of ISFs. Hydraulic loading rates should be set to accommodate the varying organic load that can be expected in the applied wastewater. As with hydraulic loading, an increase in the organic loading rate results in reduced effluent quality.
Dosing Techniques and Frequency It is essential that a dosing system provide uniform distribution (time and volume) of wastewater across the filter. The system must also allow sufficient time between doses for reaeration of the pore space. Reliable dosing is achieved by pressure-dosed manifold distribution systems.
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Design Parameters Design criteria for ISFs Item
Design criteria
Pretreatment Filter medium Material Effective size Uniformity coefficient Depth Underdrains Type Slope Size Hydraulic loading Organic loading Pressure distribution Pipe size Orifice size Head on orifice Lateral spacing Orifice spacing Dosing Frequency Volume/orifice Dosing tank volume
Minimum level: septic tank or equivalent Washed durable granular material 0,25-0,75 mm <4.0 0,45-0,9 m Slotted or perforated pipe 0%-0.1% 63-100 mm 40-60 L/m2_day 3-10 gr BOD5/ m2_day 20-50 mm 3-6 mm 1-2 m 0,3-1,2 m 0,3-1,2 m 12-48 times/day 0,5-1 L/orifice/dose 0.5-1.5 flow/day
(Tchobanoglous G., 1998) Design capacity Ideal for very small decentralized systems, up to 1000 PE.
Effluent quality •
BOD = 5 mg /L
•
TSS = 5 mg /L
Removal •
TN = 50 to 80 %;
•
TP = 5 to 20 %;
•
FC ≤ 2 to 3 log;
Cost The cost of an ISF system depends on the labor, materials, site, capacity of the system, and characteristics of the wastewater. The main factors that determine construction costs are land and
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media, which are very site-specific. The operation and maintenance costs for ISFs typically range between 2 to 4 euros per PE per year. Consequently, the energy costs of sand filters are lower than most decentralized wastewater treatment technologies, except for lagoons
Advantages/disadvantages
Advantages •
ISFs can produce a high-quality effluent (even in tertiary level) that can be used for drip irrigation or can be surface-discharged after disinfection
•
Drainfields can be small and shallow.
•
ISFs have low-energy requirements.
•
ISFs are easily accessible for monitoring and do not require skilled personnel to operate.
•
No chemicals are required.
•
If sand is not feasible, other suitable media can be substituted and may be found locally.
•
Construction costs for ISFs are moderately low, and the labor is mostly manual.
•
The treatment capacity can be expanded through modular design
•
ISFs can be installed to blend into the surrounding landscape.
Disadvantages •
The land area required may be a limiting factor (almost 3-4 m2/p.e.)
•
Regular (but minimal) maintenance is required.
•
Odor problems could result from open-filter configurations and may require buffer zones from inhabited areas.
•
If appropriate filter media are not available locally, costs could be higher
•
Clogging of the filter media is possible, if organic overloading occur
•
ISFs could be sensitive to extremely cold temperatures
•
Sand filters are typically built onsite with locally available materials, and the quality of installation is partially contingent on the consistency of these materials, and the knowledge and ability of the installing contractor.
Operation
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At high loading rates (>60 L/m2_day), the sand must be replaced every 2-5 years. At lower loading rates, the system will operate properly for a longer time. If higher loading rates are necessary, recirculating the waste is an attractive alternative to the single-pass design.
Maintenance Maintenance includes inspecting all components and cleaning and repairing when needed. The daily operation and maintenance (O&M) of large filter systems is generally minimal when the ISF is properly sized. Primary O&M tasks require minimal time and include monitoring the influent and effluent, inspecting the dosing equipment, maintaining the filter surface, checking the discharge head on the orifices, and flushing the distribution manifold annually. In addition, the pumps should be installed with quick disconnect couplings for easy removal. The septic tank should be checked for sludge and scum buildup and pumped as needed. In extremely cold temperatures, adequate precautions must be taken to prevent freezing of the filter system by using removable covers or a thin surface layer of rock material. Following Table lists the typical O&M tasks for ISFs: O&M tasks for ISFs (US EPA, 1999)
Response to Abnormal Conditions
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Ponding Ponding results from a loss of open area in the filter. If the voids are filled, flow tends to collect on the surface in ponds. Ponding can be caused by excessive organic loading without a corresponding high recirculation rate. Perhaps the most common source of ponding is from the lack of good preatreatment in septic tank prior to the filter . For this reason, a common practice is the use of effluent filters. A good effluent filter prevents large solids from leaving the tank, dramatically improving the quality of effluent and extending sandfilter life. Orenco’s Biotube® effluent filters can reduce TSS by about 70%.
Effluent filter in the septic tank Left- Installation in the septic tank Right- Effluent filter during maintenance labour out of septic’s tank manifold
Sloughing If organic overloads occur, one problem with ISFs operation is the uncontrolled sloughing of biological slime growths from the filter media. If this phenomenon takes place very often, there is a strong indication that an upgrade/extension of the treatment plant is necessary.
Poor Effluent Quality Check the organic load on the filter when the treated effluent quality is poor. Measure both the soluble and total BOD in the final effluent. The results will indicate if the poor effluent is caused by
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BOD associated with escaping solids (high total BOD) or whether the poor effluent results from the ISF BOD removal capacity being exceeded (high soluble BOD).
Cold Weather Problems Occasionally, and in very cold weather, wastewater sprayed from distributor nozzles or exposed in thin layers on the media may reach the freezing point and cause a buildup of ice on the filter. Several measures can be taken to reduce ice problems on the filter. Construct wind screens, covers, or canopies to reduce heat losses. Break up and remove the larger areas of ice buildup. Partially open the end gates to provide a stream rather than a spray along the retaining wall. Add hot water or steam to the filter influent if necessary. Adequate precautions must be taken to prevent freezing of the filter system by using removable covers or a thin surface layer of rock material above distribution system
Although the efficiency of the filter unit is reduced during periods of icing, it is important to keep this unit running. Taking the unit out of service will not only reduce the quality of the effluent but may lead to additional maintenance problems, such as ice forming, with the possibility of structural damage.
2.4.2.3 Packed-bed filters - Recirculating Sand filters (RSFs) Process Recirculating sand/gravel filters operating principal is the same as for ISFs, except the fact that effluent from the filter recirculates in a rate between the range of 2–10 times. Multiple-pass recirculating sand/gravel filters (RSFs or RGFs) have been most popular in applications with medium to large wastewater flows. They are ideal wastewater treatment systems for parks, restaurants, schools, office complexes, and large developments, and they are especially suited for small to medium size communities .
Recirculation means cycling wastewater through the filter a number of times, allowing for continued filtering and increased bacterial decomposition. To achieve acceptable treatment levels a minimum recirculation rate is 4 times. Wastewater moves from the sewer network into a septic tank where solids settle out and some organic matter is decomposed. Effluent moves, usually by gravity, to the recirculation tank. Here effluent that has been recirculated through the filter is mixed with septic tank effluent. Effluent is pumped repeatedly through a lined filter and then back (by
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gravity or pump) to the recirculation tank. In the filter, biological treatment occurs as the effluent passes the surfaces of the filter media. Treated effluent is collected at the bottom and returned to the recirculating tank where the cycle begins again. After the effluent has gone through the filter several times a controlling mechanism or recirculating ball valve, sends the effluent for final treatment. Typical RSF wastewater treatment process
Typical RSF wastewater treatment process (3D representation) (Orenco Systems Inc)
Design Parameters Typical multiple-pass recirculating sand/gravel filter design criteria are given bellow :
NIREAS VOLUME 2 [2.4] Design criteria for RSFs
141 Item
Design criteria
Pretreatment Filter medium Material Effective size Uniformity coefficient Depth Underdrains Type Slope Size Hydraulic loading Organic loading Pressure distribution Pipe size Orifice size Head on orifice Lateral spacing Orifice spacing Dosing Frequency Volume/orifice Dosing tank volume
Minimum level: septic tank or equivalent Washed durable granular material 1-5 mm <2,5 0,45-0,9 m Slotted or perforated pipe 0%-0.1% 63-100 mm 120-200 L/m2_day 10-40 gr BOD5/ m2_day 20-50 mm 3-6 mm 1-2 m 0,3-1,2 m 0,3-1,2 m 48-120 times/day 3-12 L/orifice/dose 0.5-1.5 flow/day
(Tchobanoglous G., 1998) Design capacity Ideal for small decentralized systems, up to 5000 PE.
Effluent quality •
BOD < 10 mg /L
•
TSS < 10 mg /L
Removal •
TN = 50 to 70 %;
•
TP = 5 to 20 %;
•
FC ≤ 2 to 3 log;
Cost The cost of an RSF system depends on the labor, materials, site, capacity of the system, and characteristics of the wastewater. The main factors that determine construction costs are land and media, which are very site-specific. The operation and maintenance costs for RSFs typically range between 4 to 6 euros per PE per year.
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Advantages/disadvantages
Advantages •
RSFs can produce a high-quality effluent (even in tertiary level) that can be used for drip irrigation or can be surface-discharged after disinfection
•
Drainfields can be small and shallow.
•
RSFs have low-energy requirements.
•
RSFs are easily accessible for monitoring and do not require skilled personnel to operate.
•
No chemicals are required.
•
If sand is not feasible, other suitable media can be substituted and may be found locally.
•
Construction costs for RSFs are moderately low, and the labor is mostly manual.
•
The treatment capacity can be expanded through modular design
•
RSFs can be installed to blend into the surrounding landscape.
Disadvantages •
The land area required may be a limiting factor (almost 1-2 m2/p.e.)
•
Regular (but minimal) maintenance is required.
•
Odor problems could result from open-filter configurations and may require buffer zones from inhabited areas.
•
If appropriate filter media are not available locally, costs could be higher
•
Clogging of the filter media is possible, if organic overloading occur
•
RSFs could be sensitive to extremely cold temperatures
•
Sand filters are typically built onsite with locally available materials, and the quality of installation is partially contingent on the consistency of these materials, and the knowledge and ability of the installing contractor.
Operation At high loading rates (>150 L/m2_day), the sand must be replaced every 2-5 years. At lower loading rates, the system will operate properly for a longer time. If higher loading rates are necessary, recirculating the waste is an attractive alternative to the single-pass design.
Maintenance
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143
Response to Abnormal Conditions - Same as ISFs -
2.4.2.4 Packed-bed filters - Textile filters Process The efforts to improve loading capacities of PBFs and serviceability have led to extensive research into a wide variety of media (e.g., foam, glass, styrene, plastic products, expanded clays, zeolite, limestone, furnace slag, peat, etc.). Over the past decade, this research has led to the development of an advanced technology for packed bed filters that uses an engineered textile medium assembled in a variety of configurations.
Wastewater treatment with textile filter process - Flow diagramm
The textile bed filter treats wastewater with the same processes as the recirculating sand filter.The two basic components of the textile bed filter are a process tank/recirculating tank and textile filter unit. The wastewater percolates both through and between the textile media. A visible biological film attaches onto the filter medium. Due to the high porosity within the filter aerobic conditions exist that are ideal for microbes that break down the organics in the liquid and convert ammonia to nitrate. Other conditions exist that result in further nitrogen reduction within the media. Eighty percent of the filtrate recirculates back to the high-carbon, low-oxygen (anoxic) environment of the
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process tank that is ideal for microbes that reduce nitrates to nitrogen gas (denitrification). Harmless nitrogen gas is then released freely back into the atmosphere. The textile filters usually come in prepackaged modules that can be placed on top of the process tank, or wherever is best suited. A textile bed filter prepackaged module (Orenco Systems Inc)
NIREAS VOLUME 2 [2.4] 145 Distribution system during operation in a textile filter (Orenco Systems Inc)
Design Parameters Textile provides all the benefits inherent in the packed bed filter design but overcomes the relevant limitations listed for ISFs & RSFs such as (1) land area, (2) media quality or accessibility, (3) installation quality and (4) serviceability. Land area â&#x20AC;&#x201D; The land area needed is significantly smaller than that for sand filters because loading rates are 5 to 20 times higher (typically, 0,6-1,2 m3/m2 . Thus, the footprint area for a textile filter serving a given PE is 5 to 20 times lower compared to RSFs. If the textile filter is positioned over the processing tank, virtually no additional area is required. Media quality and availability â&#x20AC;&#x201D; The manufactured textile medium ensures consistent quality and availability. Installation quality â&#x20AC;&#x201D; Lightweight textile medium (60 kg/m3) and small filter size make premanufactured treatment units practical, eliminating onsite construction and reducing installation time, labor, and construction errors. These characteristics make textile systems ideal for costsaving self-help programs and particularly suited for difficult-to-access and remotely located sites.
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Serviceability — Special configurations allow for ease of maintenance and cleaning without expensive or large excavation equipment, or the need for replacing the medium. A single-family residential filter can now be cleaned and serviced in as little as an hour.
Textile must be specifically engineered for treatment of wastewater, for high mechanical and physical properties, endurance and longevity. Porosity, attached growth surface area, and waterholding capacity contribute to the textile media’s treatment performance.
Porosity — The porosity of the textile media is several times greater than that of sand, gravel, and other particle-type mediums. The more porous the medium, the greater its hydraulic conductivity, the greater its air space (which enhances the capacity of passively ventilated systems and free air movement), and the greater its capacity for the accumulation of solids and biomass development. Surface area — Textile media can be blended with a variety of fibers to achieve relatively large total surface area per unit volume (m2/m3). In current media blends, the typical attached-growth surface area is 4-8 times greater than recirculating sand or gravel filter media. Expanding the biomass growth area provides a greater surface potential for air and effluent to interface and come in contact with the biomass. Water-holding capacity — The water-holding capacity of textile media also varies considerably depending on the media density, type of material, and blend of fibers. The water-holding capacity in textile media is also several times greater than expected in the sands and gravels used in filters.
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Water-holding capacity performs a key function in the treatment process. Together with the programmed dosing time and frequency, it governs the effluent retention time within the filter and ultimate effluent quality. In Next Figure , complex fiber structure and void space of textile fibers is compared to that of typical 0.30-mm and 1.5-mm sand particles. Textile fiber porous structure, relative to sand and gravel particles (Orenco Systems Inc)
Design Parameters for Textile Filters Process
SRT
F/M
Loading
Type
(days)
(kg BOD/kgMLVSS_d)
kg BOD/m2_d
PBF-Textile Filters
365+
0,005-0,01
0,2
Design capacity Textile filters’ effluent provide consistent, high quality wastewater treatment: better than 10/10 cBOD5/TSS mg/L. Consequently, they have proven to be an ideal solution in the following, diverse applications: •
New onsite and decentralized wastewater treatment systems for up to 5000 PE
•
Repairs and reclamation projects
•
Jurisdictions requiring nutrient reduction with proper configuration
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•
Seasonal or periodically used facilities
•
Facilities with extreme variations in daily flows
•
Wherever water reuse is essential
•
Failing conventional collection and treatment systems which can replace
Wastewater treatment facility with Textile filters in Greece, Heraklion, Crete (800 PE)
Effluent quality Next Figure illustrates the relative levels of effluent quality achieved by textile filter units (Orenco Systems Inc) throughout
evaluations conducted and reported on by facilities such as the
University of California, Davis Campus; NSF International; and NovaTec Consultants, Inc. of Vancouver, British Columbia in the US. The graph represents over 360 data days of composite sampling over a time span of more than two years.
NIREAS VOLUME 2 [2.4] 149 Effluent quality achieved relative to actual hydraulic loading rates
General effluent characteristics for medium loads •
BOD < 10 mg /L
•
TSS < 10 mg /L
Removal •
TN = 50 to 70 %;
•
TP = 5 to 20 %;
•
FC ≤ 2 to 4 log;
Cost The cost of an Textile bed filter system is general higher when compared to RSF or ISF, and it depends on the labor, materials, site, capacity of the system, and characteristics of the wastewater. The main factor that determine construction costs is media, which is very important.. The operation and maintenance costs for Textile bed filters typically range between 4 to 6 euros per PE per year.
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Advantages/disadvantages
Advantages •
Quick startup
•
Consistent and reliable treatment - easily serviced
•
Extremely high quality components - very long warrantees and lifetime (>20 years)
•
The treatment process is passive
•
High quality effluent ideal for many water-reuse applications
•
Minimum cost of operation - Low energy consumption
•
Adequate storage during power outages (6-12 hrs)
•
The technology is readily "scalable" - system can be implemented in "modules"
•
The system is “fail-safe”
•
Excess sludge production is negligible.
•
Efficient performance when occasional hydraulic and biologic overloads occur
•
Ability to treat septic tank effluent
•
Low visual impact - aesthetic installation
•
Low noise levels (>45 dB)
•
No offensive odours
•
Leach fields can be small and shallow.
•
Low energy requirements.
•
Need the smaller land area than any other decentralised system (up to 1000 PE)
•
Ease in construction
•
Can handle toxic loads when found in waste, but for a limited amount
Disadvantages •
Media may need replacing after several years of use (>10 years)
•
Higher area requirements comparing to suspended growth systems for medium to big plants
NIREAS VOLUME 2 [2.4] 151 Comparison of Textile Filter technology with suspended growth processes
A/A
Parameter
TEXTILE FILTERS
EXTENDED
SBR
AERATION Very good
Very good
90-95%
90-95%
Treatment efficiency
Very good 95-99%
2
Economy
Intermediate-High 5-10 lower energy consumption High energy energy than extended aeration consumption consumption
3
Simplicity Maintenance
1
4
Manholes, deodorizing
-
wells,
Electricity 5
utdowns
Requires maintenance checks
Very simple.
No usual checks, no wells opening. Deodorizing is Requires accomplished through compost opening. biofilter.
daily Requires & maintenance checks
often
inspection
and
daily &
wells
The system can with stand 6-12 The systems can with stand a maximum hours without electricity without of 3-6 hours. After 12 hours biomass is decomposed. After 24 hours severe odor consequences. problems during restart.
6
High loadings during peak and rain water injection (up to 5Qmax)
Loadings are equalized in the Loading are equalized as soon as there is equalizing dosing tank, without processing volume available in the any consequences. aeration tank.
7
Chemicals, oil products cleaning powders
No remarkable consequences
Processing can be damaged completely due to biomass destruction and final sedimentation failure.
8
Sludge management
No excess sludge production.
High amounts of sludge production.
Screening, fats-oils
No screening material and fats removals. The accumulate in the Usual screening materials removal. Fats septic tank and are removed with removals and often system cleaning sludge every 3-5 years.
Noise
Silent operation (can notified over 10 meters).
9
10
not
be Machinery noise. Four silent operations extra equipment is necessary with increased cost.
NIREAS VOLUME 2 [2.4] Operation & Maintenance
152
Inspect the septic tank for liquid depth, color of scum and effluent, and sludge and scum thickness.(After the first yearâ&#x20AC;&#x2122;s measurement of septic tank sludge and scum thickness, measurements only need to be taken about every three years.). Measurements of solids accumulation help to determine when the tank needs pumping. The pumping system should be inspected annually to ensure that all pumps are operating properly.The inspection should include: 1.
Verify that there are no obvious holes or leaks in the pump tank riser.
2.
Verify that the float cords are neatly wrapped inside the riser so they cannot interfere
with the operation of the floats. 3.
Verify the high water alarm works by lifting the top float up.
4.
Verify the programmable timer settings are correct.
5.
Read and record the elapsed timer meter and or cycle counter if the control panel is
equipped with these devices.
Response to Abnormal Conditions If a strong or offensive odor is emitted form the filter, measure the DO levels in the filtrate and recirculation chamber and adjust the recirculation time if necessary. The following textile filter items should be checked annually: 1.
Check the organic build-up on the media under the orifices.
2.
Clean and flush the manifold.
3.
Check residual pressure against start-up value.If the pressure is more than 20%
higher than the start-up value, perform additional cleaning and verify that all orifices are clear. 4.
Inspect for ponding. The filter should not be saturated. Effluent should move freely
through the media. If there is a build-up of oil and grease that is causing the ponding, scrape a sample from the biomat and have it analyzed by a lab.
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2.4.2.5 Rotating Biological Contactors (RBCâ&#x20AC;&#x2122;s) Equipment The equipment that makes up an RBC includes the rotating biological contactor (the media, either standard or high density), a center shaft, drive system, tank, baffles, housing or cover, and a settling tank. The RBC consists of circular sheets of synthetic material (usually plastic) mounted side by side on a shaft. These sheets offer large amounts of surface area for growth of the biomass.
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RBC system
The center shaft provides support for the disks of media; they must be strong enough to support the weight of the media and the biomass; experience has shown that a major problem is the collapse of the support shaft. The drive system provides the motive force to rotate the disks and shaft. The drive system may be mechanical or air driven, or a combination of each. When the drive system does not provide uniform movement of the RBC, major operational problems can arise. The tank holds the wastewater in which the RBC rotates. It should be large enough to allow variation of the liquid depth and detention time. Baffles are required for proper adjustment of the loading applied to each stage of the RBC process. Adjustment can be made to increase or decrease the submergence of the RBC. RBC stages are normally enclosed in some type of protective structure [cover] to prevent the loss of biomass due to severe weather changes (e.g., snow, rain, temperature, wind, sunlight). In many instances, this housing greatly restricts access to the RBC. The settling tank removes the sloughing material created by the biological activity and is
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similar in design to the primary settling tank. The settling tank provides 2- to 4-hour detention times to permit settling of lighter biological solids.
Process A rotating biological contactor (RBC) is an attachedgrowth, biological process that consists of a basin(s) in which large, closely spaced, circular disks mounted on horizontal shafts rotate slowly through wastewater. The disks are made of high-density polystyrene or PVC for durability and resistance. Corrugation patterns increase surface area and structural integrity. Bacterial growth on the surface of the disks leads to the formation of a biofilm layer that eventually covers the entire wetted surface of the disks. The rotating disks are partially submerged in the wastewater. In this way, the biofilm layer is alternatively exposed to the wastewater from which the organic matter is adsorbed and the air from which the oxygen is absorbed. A schematic of an RBC system
Both aerobic and anaerobic microorganisms can live in the biofilm and contribute to the removal of pollutant form the water.
NIREAS VOLUME 2 [2.4] 156 Biofilm growth in RBCâ&#x20AC;&#x2122;s
Rotation also provides a means for removing excess bacterial growth on the disksâ&#x20AC;&#x2122; surfaces and maintaining suspension of sloughed biological solids in wastewater. A final clarifier removes sloughed solids. Partially submerged RBCs are used for carbonaceous BOD removal, combined carbon oxidation and nitrification, and nitrification of secondary effluent. Completely submerged RBCs are used for denitrification.
Plastic disc media and biological contactor drum (California State University, 2008)
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Next figure shows typical arrangements of RBCs. In general, an RBC system is divided into a series of independent stages or compartments by baffles in a single basin or separate basins arranged in series.
Typical arrangements of RBCs. A, Compartmentalization in a single basin using baffles; B, Basins arranged in series
(David H.F. Liu, Bela G. Liptak, 1999)
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Compartmentalization creates a flow pattern with little longitudinal mixing in the flow direction (i.e., a plugflow pattern), increasing overall removal efficiency of an RBC . It can also promote separation of bacterial species at different stages, achieving optimal performance. For example, autotrophic bacteria responsible for nitrification can concentrate at later stages in an RBC system designed for combined carbon removal and nitrification where the mixed liquor BOD is low. Consequently, nitrification performance is more reliable and stable.
Example of an underground RBC for the decentralised treatment of domestic wastewater
Biological discs A biological disc unit consists of a series of closely spaced, large-diameter, expanded polystyrene discs mounted on a horizontal shaft. The discs are partially immersed in wastewater and rotated. From the microorganisms present in the wastewater, a biological growth develops on the surface of the discs. As the discs rotate, the bacteria alternately passes through the wastewater and the air. Operating in this manner, the discs provide support for microbial growth and alternately contact this growth with organic wastewater pollutants and air. The rotational speed, which controls the contact intensity between the biomass and the wastewater, and the rate
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of aeration can be adjusted according to the organic load in the wastewater. Biological disc units are available in sizes up to 4 m in diameter. The residence time of the wastewater in the disc sections and the rotational speed of the discs determine unit BOD removal efficiency. Installing a number of discs in a series of stages improves the residence time distribution and yields a greater BOD removal efficiency. Staged operation is advantageous when the wastewater contains several types of biodegradable materials because staging enhances the natural development of different biological cultures in each stage. For example, the discs in the later stages are dominated by nitrifying bacteria that oxidize ammonia after most of the carbonaceous BOD has been removed. Staged operation also permits the use of intermediate solids separation units at strategic points. The process is claimed to be stable under hydraulic surges and intermittent flows. Because of the high buoyancy of the disc materials and the low rotational speeds, the power consumption is low. For sewage flow capacities of 1,5 to 36 m3/d, commercial packaged unit is available. It includes a feed mechanism, a section of bio-disk surfaces, and an integral clarifier tank with sludge removal mechanism. Depending on the nature of the influent sewage and on the total flow capacity, the length of the package unit varies from 3 to 13 m.
A RBC system before installation
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Design Parameters RBC design is often based on empirical design curves supplied by RBC manufacturers. Once the environmental engineer estimates the surface loading L (L/m2-day) required to achieve a BOD removal efficiency, the required disk surface area A(m2) for a total flow Q(L/day) is calculated as follows: A = Q/L RBC’s Design Parameters RBC's - Treatment Level Secondary Combined carbon oxidation/nitrification Nitrification
Hydraulic Loading (L/m2_day)
BOD5 Loading (kg/m2_d)
BOD5 removal (%)
Liquid Retention Time (hr)
Effluent BOD (mg/L)
80-160
3,5-10
85-95
0,7-1,5
15-20
30-80
2,5-7,5
40-100
0,5-1,5
85-95 85-95
1,5-4 1,2-3
7-15 7-15
Effluent NH3 (mg/L)
<2 1-2
Design capacity Optimum use for small to intemediate flows, in small communities.
Cost Observed costs for RBCs are highly variable depending on climate and location. Generally, RBCs involve medium to high capital costs as not all materials may be locally available and motor and special material for rotation is required. Another cost factor may be manufacture and implementation, which requires skilled experts . Operation and maintenance costs are relatively medium, because operation requires a continuous electricity only for the rotation of disk’s low energy shaft. Also supervision requires semi-skilled labour and professional operator.
Advantages/disadvantages
Advantages •
High contact time and high effluent quality (both BOD and nutrients)
•
High process stability, resistant to shock hydraulic or organic loading
•
Short contact periods are required because of the large active surface
•
Low space requirement
•
Well drainable excess sludge collected in clarifier
•
Process is silent compared to dosing pumps for aeration
•
No risk of channelling
•
Low sludge production
NIREAS VOLUME 2 [2.4] 161 Disadvantages •
Continuous electricity supply required (but uses less energy than activated sludge processes for comparable degradation rates)
•
Contact media not available at local market
•
High investment and medium to high operation and maintenance costs
•
Must be protected against sunlight, wind and rain (especially against freezing in cold climates)
•
Odour problems may occur
•
Requires permanent skilled technical labour for operation and maintenance
Operation Start-Up Prior to plant start-up, become familiar with and understand the contents of the plant Ο & Μ manual. If you have any questions, ask the design engineer or the manufacturer's representative. These persons should instruct the operator on the proper operation of the plant and the maintenance of the equipment.
Pre-Start Checks for New Equipment Before starting any equipment or allowing any. wastewater to enter the rotating biological contactor treatment process, check the following items: 1. TIGHTNESS OF BOLTS AND PARTS Inspect the following for tightness in accordance with manufacturer's recommendations. a. Anchor bolts b. Mounting studs c. Bearing caps Check any torque limitations. d. Locking collars e. Jacking screws f. Roller chain Be sure chain is properly aligned. g. Media
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Unbalanced media may cause slippage. h. Belts Use matched sets on multiple-belt drives. 2. LUBRICATION OF EQUIPMENT Be sure the following parts have been correctly lubricated with proper lubricants in accordance with manufacturer's recommendations. a. Mainshaft bearings b. Roller chain c. Speed reducer 3. CLEARANCES FOR MOVING PARTS a. Between media and tank wall b. Between media and baffles or cover support beams c. Between chain casing and media d. Between roller chain, sprockets, and chain casing 4. PROPER INSTALLATION OF SAFETY GUARDS Be sure safety guards are properly installed over chains and other moving parts. Procedure for Starting Unit Actual start-up procedures for a new unit should be in your plant Ο & Μ manual and provided by the manufacturer. A typical starting procedure is outlined below.
1. Switch on power, allow shaft to rotate one turn, turn off the power, and lock out and tag the main breaker. Inspect and correct if necessary during this revolution: a. Movement of chain casing. b. Unusual noises. c. Direction of media rotation. Where wastewater flow is parallel to the rotating media shaft, the direction of rotation is not critical. If the wastewater flow is perpendicular to the rotating media shaft, the media should be moving through the wastewater against the direction of flow 2. Switch on power and allow shaft to rotate for 15 minutes. Check the following: a. Chain drive sprocket alignment. b. Noises in bearings, chain drives, and drive package. c. Motor amperage. Compare with NAMEPLATE6 value. d. Temperature of mainshaft bearing (by hand) and drive package pillow block. If too hot for the hand, use a PYROMETER or thermometer. Temperature should not exceed 93°C.
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e. Tightness of shaft bearing cap bolts. Tighten to manufacturer's recommended torque. f. Determine number of revolutions per minute for drum and record for future reference. 3. Open inlet valve and allow wastewater to fill the tank (all four stages if in one tank). Open the outlet valve to allow water to flow through the tank. Make inspections listed in Steps 1 and 2 again while drum is rotating. Shut off power and lock out and tag the main breaker to make any corrections. 4. Check the relationship between the clarifier inlet and the rotating biological contactor outlet for hydraulic balance. This means that you want to be sure the tank containing the biological contactor will not overflow and cause stripping of the biomass. Development of biological slimes can be encouraged by regulating the flow rate and strength of the wastewater applied to nearly constant levels by the use of recirculation, if available. Maintaining building temperatures at 18째C or higher will help. The best rotating speed is one that will shear off growth at a rate that will provide a constant hungry and reproductive film of microorganisms exposed to the wastewater being treated. Allow one to two weeks for an even growth of biological slimes (biomass) to develop on the surface of the media with normal strength wastewater. After start-up, a slimy growth (biomass) will appear. During the first week, excessive sloughing will occur naturally. This sloughing is normal and the sloughed material is soon replaced with a fairly uniform, shaggy, brown-to-gray appearing biomass with very few or no bare spots. Follow the same start-up procedures whether a plant is starting at less than design flow or at full design flow. Start-up during cold weather takes longer because the organisms in the slime growth (biomass) are not as active and require more time to grow and reproduce.
Operation Rotating biological contactor treatment plants are not difficult to operate and produce a good effluent provided the operator properly and regularly performs the duties of inspecting the equipment, testing the influent and effluent, observing the media, maintaining the equipment, and taking corrective action when necessary.
7.120 Inspecting Equipment
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This treatment process has relatively few moving parts. There is a drive train to rotate the shaft and there are bearings upon which the shaft rotates. Rotating biological contactor equipment should be inspected on a regular basis. Include the following tasks when inspecting equipment.
1. Feel the outer housing of the shaft bearing to see if it is running hot. Use a pyrometer or thermometer if the temperature is too hot for your hand. If temperature exceeds 200째F (93째C), the bearings may need to be replaced. Also, check for proper lubrication and be sure the shaft is properly aligned. The longer the shaft, the more critical the alignment. 2. Listen for unusual noises in the motor bearings. Locate the cause of any unusual noises and correct. 3. Feel the motors to determine if they are running hot. If hot, determine the cause and correct. 4. Look around the drive train and shaft bearing for oil spills. If oil is visible, check oil levels in the speed reducers and chain drive system. Also, look for damaged or worn out gaskets or seals. 5. Inspect the chain drive for alignment and tightness. 6. Inspect the belts for proper tension. 7. Be sure all guards over moving parts and equipment are in place and properly installed. 8. Clean up any spills, messes, or debris.
Testing Influent and Effluent Wastewater analysis is required to monitor overall RBC plant and process performance. Because there are few process control functions to be performed, only a minimal analysis is required to monitor and report daily performance. To determine if the rotating biological contactors are operating properly, you should measure the following influent and effluent water quality indicators: (1) BOD, (2) suspended solids, (3) pH, and (4) dissolved oxygen (DO). Performance is best monitored by analysis of a 24-hour COMPOSITE SAMPLE3 for BOD and suspended solids on a daily basis. DO and pH should be measured using GRAB SAMPLES9 at specific times. Actual frequency of tests may depend on how often you need the results for plant
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control and also how often your licence permit requires you to sample and analyze the plant effluent.
DISSOLVED OXYGEN The DO in the wastewater being treated beneath the rotating media will vary from stage to stage. A plant designed to treat primary effluent for BOD and suspended solids removal will usually have 0.5 to 1.0 mg/L DO in the first stage. The DO level should increase to 1 to 3 mg/L at the end of the first stage. A plant designed for nitrification to convert ammonia and organic nitrogen compounds to nitrate usually will have four stages and DO levels of 4 to 8 mg/L. The difference between an RBC unit designed for BOD removal and one designed for nitrification is the design flow applied per square meter of media surface area. DO in the first stage of a nitrification unit will be more than 1 mg/L DO and often as high as 2 to 3 mg/L.
EFFLUENT VALUES Typical BOD, suspended solids, and ammonia and nitrate effluent values for rotating biological contactors depend on NPDES permit requirements and design effluent values. As flows increase, effluent values increase because a greater flow is applied to each square foot of media while the time the wastewater is in contact with the slime growths is reduced. Also, the greater the levels of BOD, suspended solids, and nitrogen in the influent, the greater the levels in the plant effluent. If an analysis of effluent sample results reveals a decrease in process efficiency, look for three possible causes: 1. Reduced wastewater temperatures 2. Unusual variations in flow or organic loadings 3. High or low pH values (less than 6.5 or greater than 8.5) Once the cause of the problem has been identified, possible solutions can be considered and the problem corrected.
TEMPERATURE Wastewater temperatures below 13째C will result in a reduction of biological activity and in a decrease in BOD or organic material removal. Not much can be done by the operator except to
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wait for the temperatures to increase again. Under severe conditions, provisions can be made to heat the building, the air inside the RBC unit cover, or the RBC unit influent. Solar heat can be used effectively to maintain temperature in buildings and enclosures without drying out the biological slime growths. Ceilings should be kept low to effectively use available heat. If existing buildings have high ceilings, large vaned fans can be mounted on the ceilings to direct heat downward.
INFLUENT VARIATIONS When large daily influent flow or organic (BOD) variations occur, a reduction in process efficiency is likely to result. Before corrective steps are taken, the exact extent of the problem and resulting change in process efficiency must be determined. In most cases, when the influent flow or organic peak loads are less than three times the daily average values during a 24-hour period, little decrease in process efficiency will result. In treatment plants where the influent flow or organic loads exceed design values for a sustained period, the effluent BOD and suspended solids must be measured regularly during this period to determine if corrective action is required. During periods of severe organic overload, the bulkhead or baffle between stages one and two may be removed. This procedure provides a larger amount of media surface area for the first stage of treatment. If the plant is continuously overloaded and the effluent violates the NPDES permit requirements, additional treatment units should be installed. A possible short-term solution to an organic overload problem might be the installation of facilities to recycle effluent; however, this would cause a greater increase of any hydraulic overload.
pH Every wastewater has an optimum pH level for best treatability. Domestic wastewater pH varies between 6.5 and 8.5 and will have little effect on organic removal efficiency. However, if this range is exceeded at any time (due to industrial waste discharges, for example), a decrease in efficiency is likely. To adjust the pH toward 7.0, either pre-aerate the influent or add chemicals. If the pH is too low, add sodium bicarbonate or lime. If the pH is too high, add acetic acid. The amount of chemical to be added depends on the characteristics of the water and can best be determined by adding chemicals to samples in the lab and measuring the change in pH. Always wear appropriate safety gear (goggles or face shield, impervious gloves, protective clothing) when handling chemicals.
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When dealing with nitrification, pH and alkalinity are very critical. The pH should be kept as close as possible to a value of 8.4 when nitrifying. The alkalinity level in the raw wastewater should be maintained at a level at least 7.1 times the influent ammonia concentration to allow the reaction to go to completion without adversely affecting the microorganisms. Sodium bicarbonate can be used to increase both the alkalinity and pH. Another cause of pH variations could be the addition of SUPERNATANT from a digester. The supernatant should be tested for pH and suspended solids. Without testing the supernatant, you will not know what kind of load you are placing on the rest of the plant. Sometimes it is best to drain supernatant at low flows to the plant. Caution should be taken to avoid overloading the process. If the supernatant pH is too low, supernatant could be drawn off during high flows when these flows can be used for dilution and NEUTRALIZATION. Observing the Media Rotating biological contactors use bacteria and other living organisms growing on the media to treat wastes. Because of this, you can use your senses of sight and smell to identify problems. The slime growth or biomass should have a brown-to-gray color, no algae present, a shaggy appearance with a fairly uniform coverage, and very few or no bare spots. The odor should not be offensive, and certainly there should be no sulfide (rotten egg) smells.
BLACK APPEARANCE If the appearance becomes black and odors that are not normal do occur, then this could be an indication of solids or BOD overloading. These conditions would probably be accompanied by low DO in the plant effluent. Compare previous influent suspended solids and BOD values with current test results to determine if there is an increase. To solve this problem, place another rotating biological contactor unit in service, if possible, or try to pre-aerate the influent to the RBC unit. Also, review the operation of the primary clarifiers and sludge digesters to be sure they are not the source of the overload.
WHITE APPEARANCE A white appearance on the disc surface also might be present during high loading conditions. This might be due to a type of bacteria that feeds on sulfur compounds. The overloading could result from industrial discharges containing sulfur compounds upon which certain sulfur-loving bacteria thrive and produce a white slime biomass. Corrective action consists of placing another
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RBC unit in service or trying to pre-aerate the influent to the unit. During periods of severe organic or sulfur overloading, remove the bulkhead or baffle between stages one and two. Prechlorination of plant influent also will control sulfur-loving bacteria. Another cause of overloading may be sludge deposits that have been allowed to accumulate in the bottom of the bays. To remove these deposits, drain the bays, wash the sludge deposits out, and return the unit to service. Be sure the orifices in the baffles between the bays are clear.
SLOUGHING If severe sloughing or loss of biomass occurs after the startup period and process difficulty arises, the causes may be due to the influent wastewater containing toxic or INHIBITORY SUBSTANCES that kill the organisms in the biomass or restrict their ability to treat wastes. To solve this problem, steps must be taken to eliminate the toxic substance even though this may be very difficult and costly. Biological processes will never operate properly as long as they attempt to treat toxic wastes. Until the toxic substance can be located and eliminated, loading peaks should be dampened (reduced) and a diluted, uniform concentration of the toxic substance allowed to reach the media in order to minimize harm to the biological culture. While the corrections are made at the plant, dampening may be accomplished by regulating inflow to the plant. Be careful not to flood any homes or overflow any low manholes. Toxic wastes may be diluted using plant effluent (until the toxic material reaches the effluent) or any other source of water supply. Another problem that could cause loss of biomass is an unusual variation in flow or organic loading. In small communities, one cause may be high flow during the day and near zero flow at night. During the day, the biomass is receiving food and oxygen and starts growing; then the night flow drops to near zeroâ&#x20AC;&#x201D;available food is reduced and nearly stops. The biomass starts sloughing off again due to lack of food.
Possible solutions to sloughing of the biomass due to excessive variations in plant flow or organic loading include throttling peak conditions and recycling from the secondary clarifier or RBC effluent during low flows. Be very careful when throttling plant inflows that low-elevation homes are not flooded or that manholes do not overflow. Usually, RBC units do not have provisions for any recycling from the secondary clarifier. If low flows at night are creating operation problems due to lack of organic matter, a possible solution is the installation of a pump to recirculate water from the secondary clarifier. If recirculation is provided, try to maintain a hydraulic loading rate of greater than 40 to 60 L/m2. A flow equalization tank can be used to provide fairly continuous or even flows.
NIREAS VOLUME 2 [2.4] 169 Control of Snails Snails are not a problem in RBCs used to remove carbonaceous biochemical oxygen demand (CBOD) because the growth of "bugs" (microbes) removing CBOD is high and the microbial slime (treating the wastewater) consumed by snails is quickly replaced by new growth. Snails are a problem when the RBC is expected to remove nitrogenous biochemical oxygen demand (NBOD) because snails remove slow-growing nitrifying bacteria and interfere with nitrification. Snail shells are a problem when they clog pipes and pumps. Chlorination is commonly used to control snails on RBCs. One approach is to take an RBC train (the RBCs treating a particular slug or flow of wastewater) off line. Add chlorine to the water to a concentration of 60 to 70 mg/L, rotate the RBC in the superchlorinated solution for two to three days, and then return the RBC to service. Chlorination also can be used to control filamentous microorganisms. The superchlorinated water can be dechlorinated before it is discharged from the facility by applying sulfur dioxide. When the RBC is returned to service, it will take a period of time for the biomass to recover and reach full treatment capacity. Another approach to control snails is to increase the pH to 10. A pH of 10 will kill snails without harming the microbial growth on the RBCs. The pH can be increased by adding caustic soda, sodium hydroxide, or lime and maintaining the RBC exposure for eight hours. Operators may have to increase the pH to 10 every one to two months to control the snails. Adjustment of the wastewater application rate at upstream trickling filters where snails lay their eggs will wash away a large proportion of the eggs and thereby help to reduce the population of snails in RBCs. Also, if secondary solids are being recycled to the head of the plant, snail eggs and snails may be recycled along with the solids. Some operators reduce this transfer of snails by sending the solids from the secondary clarifier directly to anaerobic digesters.
Maintenance Many RBC operating problems are caused by shaft failures, disk breakage, bearing failures, and organic overloadings. By adopting proper design, operation, and maintenance practices, wastewater treatment facilities can mitigate many of these problems. For example, many RBC systems are enclosed to eliminate disk exposure to UV light, reduce temperature effects, and protect the equipment. These facilities can control odor problems by reducing organic loading or increasing the oxygen supply using supplemental air diffusers in the basin. During operation, the
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system must be supervised by professional operators . Although RBC units are operation- and maintenance-intensive, they do not require seeding with bacterial cultures (as do anaerobic processes such as anaerobic baffled reactors, septic tanks, upflow anaerobic sludge blanket reactors or anaerobic digesters) and the start-up phase is therefore considerably shorter. However, it takes 6 to 12 weeks for the biofilm to establish for a good treatment performance .
Rotating biological contactors are usually preceded by preliminary treatment processes consisting of screening and grit removal and by primary settling. Grit and large organic matter, if not removed, can settle beneath the drums and form sludge deposits. These deposits can reduce the effective tank volume, produce septic conditions, scrape the slimes from the media, and possibly stall the unit. Some rotating biological contactor plants have aerated flow equalization tanks between the primary clarifiers and the rotating biological contactors. Flow equalization tanks may be installed to equalize or balance highly fluctuating flows and to allow for the dilution of strong wastes and neutralization of highly acidic or alkaline wastes. These equalization tanks are capable of reducing or eliminating shock loads and providing pre-aeration to the RBC.
Break-In Maintenance AFTER 8 HOURS OF OPERATION 1. Recheck tightening torque of capscrews in all split-tapered bushings in the drive package. 2. Visually inspect hubs and capscrews for general condition and possibility of rubbing against an obstruction. 3. Inspect
belt
drive
(drive
package)
and
tighten
as
needed.
AFTER 24 HOURS OF OPERATION 1. Inspect all chain drives. AFTER 40 HOURS OF OPERATION 1. Inspect all belt drives in drive packages. AFTER 100 HOURS OF OPERATION 1. Change oil in speed reducer. Use manufacturer's recommended lubricants. 2. Clean magnetic drain plug in speed reducer. 3. Check all capscrews in split-tapered bushings and set-screws in drive package output sprocket and bearing for tightness. 4. Inspect all belt drives in drive packages. AFTER 3 WEEKS OF OPERATION
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1. Change oil in chain casing. Be sure oil level is at or above the mark on the dipstick. Use manufacturer's recommended lubricants.
Preventive Maintenance Program Interval Procedure Daily
1. Check for hot shaft and bearings. Replace bearings if temperature exceeds 93째 and
there are problems. Daily
2. Listen for unusual noises in shaft and bearings.
Identify cause of noise and correct if necessary. Weekly 3. Grease the mainshaft bearings and drive bearings. Use manufacturer's recommended lubricants. Add grease slowly while shaft rotates. When grease begins to ooze from the housing, the bearings contain the correct amount of grease. Add six full strokes where bearings cannot be seen. 4 wk.
4. Inspect all chain drives.
4 wk.
5. Inspect mainshaft bearings and drive bearings.
4wk.
6. Apply a generous coating of general purpose grease to mainshaft stub ends, mainshaft bearings, and end collars.
3 mo.
7. Change oil in chain casing. Use manufacturer's recommended lubricants. Be sure oil level is at or above the mark on the dipstick.
3 mo.
8. Inspect belt drive.
6 mo.
9. Change oil in speed reducer. Use manufacturer's recommended lubricants.
6 mo.
10. Clean magnetic drain plug in speed reducer.
6 mo. 11. Purge the grease in the double-sealed shaft seals of the speed reducer by removing the plug located 180 degrees from the grease fitting on both the input and output seal cages. Pump grease into the seal cages and then replace the plug. Use manufacturer's recommended grease. 12 mo. 12. Grease motor bearings. Use manufacturer's recommended grease. To grease motor bearings, stop motor and remove drain plugs. Inject new grease with pressure gun until all old grease has been forced out of the bearing through the grease drain. Run motor until all excess grease has been expelled. This may require up to several hours running time for some motors. Replace drain plugs.
NIREAS VOLUME 2 [2.4] 172 Housekeeping Properly designed systems have sufficient turbulence so solids or sloughed slime growths should not settle out on the bottom of the bays. If grease balls appear on the water surface in the bays, they should be removed with a dip net or screen device. If media comes apart, squeeze the two unbonded sections together with a pair of pliers. Take another pair of pliers and force a heated nail through the media. The heat from the nail will melt the plastic and make a plastic weld between the two sections of media.
Response to Abnormal Conditions Abnormal operating conditions may develop under the following circumstances: 1. High or low flows 2. High or low solids loading 3. Power outages
When your plant must treat high or low flows or solids (organic) loads," abnormal conditions develop as the treatment efficiency drops. For solutions to these problems, refer to Section 7.12, "Operation," and Table 7.2. One advantage of RBC units is the fact that high flows usually do not wash the slime growths off the media; consequently, the organisms are present and treating the wastewater during and after the high flows. A power outage requires the operator to take certain precautions to protect the equipment and the slime growths while no power is available. If the power is off for less than four hours, nothing needs to be done. If the power outage lasts longer than four hours, the RBC shaft needs to be turned about one-quarter of a turn every four hours. Turning prevents all the slime growth from accumulating on the bottom portion of the plastic disc media. Before attempting to turn the shaft, lock out and tag the power in case the outage ends abruptly. To turn the shaft, remove the belt guard using extreme care. Turn the shaft by using the spokes on the pulley or a strap wrench on the shaft. Do not expose your fingers or hands to the pinch point between the belt(s) and the pulley(s). Place a wedge-shaped block between the belts and belt pulley or sheave to hold the shaft and media in the desired location. Actually, the shaft is very delicately balanced and easy to
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rotate. Do not try to weld handles or brackets to the shaft to facilitate turning because this will throw the shaft off balance. WARNING. If the shaft starts to roll back to its original position before you get the block properly inserted, do not try to stop the shaft. Let it roll back and stop. If you try to stop the shaft from rolling back, you could injure yourself and also damage the belts and the belt's pulley or sheave. Gently spray water on the slime growth that is not submerged frequently enough to keep the biomass moist whenever the drum is not rotating. If the power outage lasts longer than 12 hours, more than normal sloughing will occur from the media when the unit is placed back in service. When the sloughing becomes excessive, increase the sludge pumping rate from the secondary clarifier.
Shutdown and Restart The rotating biological contactor may be stopped by turning off the power to the drive package.. Do not allow one portion of the media to be submerged in the wastewater being treated for more than four hours. Occasionally, spray the media not submerged to prevent the slime growth from drying out whenever the drum is not rotating. If the tank holding the wastewater being treated must be drained, a portable sump pump may be used. A sump is usually located at the end of the unit by the motor. Pump the water either to the primary clarifier or to the inlet end of an RBC unit in operation. A trough running the full length of the tank allows the solids to be pumped out. While the tank is empty, inspect for cracks and any other damage and make necessary repairs. Try to keep the slime growths moist to minimize sloughing and a reduction in organism activity when the process starts again. A loss in process efficiency can result if the slimes are washed off the media. Do not wash the slime growth off the media because you will be washing away the organisms that treat the wastewater. If the unit is to be out of service for longer than one day, the slimes may be washed off the media to prevent the development of odor problems
NIREAS VOLUME 2 [2.4] 174 RBCs – Troubleshooting guide
NIREAS VOLUME 2 [2.4] 175 Roller chain drive â&#x20AC;&#x201C; Troubleshooting guide
NIREAS VOLUME 2 [2.4] 176 Bearings and motors – Troubleshooting guide
2.4.3 COMBINED AEROBIC TREATMENT PROCESSES Several treatment process combinations have been developed that couple fixed-bed (attached growth) and fluid-bed (suspended growth), processes. Combined processes have resulted as part of plant upgrading; they have also been incorporated into new treatment plant designs (MBBR systems, etc). Combined processes have the advantages of the two individual processes, which can include (1) the stability and resistance to shock loads of the attached growth process, (2) the volumetric efficiency and low energy requirement of attached growth process for partial BOD removal, (3) the role of attached growth pretreatment as a biological selector to improve activatedsludge settling characteristics, and (4) the high-quality effluent possible with activated-sludge treatment.
2.4.3.1 Fixed film activated sludge It is a similar process to activated sludge, but the fixed biomass combines aerobic, anaerobic and anoxic zones and increases the Sludge Retention Time, promoting better nitrification compared to simple suspended growth systems.
Process Integrated fixed film activated sludge (IFAS) is a relatively new technology that describes any suspended growth system that incorporates an attached growth media within the suspended growth reactor. Biofilm carriers are generally divided in ‘dispersed media’ or ‘fixed media’. This technique, used in highly developed wastewater treatment plants, can be used as an
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upgrade for existing facilities or can be constructed newly. The design needs expert knowledge and the system must be operated by skilled labourers. Integrated Fixed-film Activated Sludge (IFAS) Technology provides for additional biomass within a wastewater treatment facility in order to meet more stringent effluent parameters or increased loadings without the direct need for additional tankage. Industry practice for upgrading wastewater treatment plants usually focuses on increasing the bioreactor volume to provide the additional bacterial population required to meet the system’s kinetic needs. However, designers often encounter clarifier solids loading limitations that put an upper limit on the amount of biomass that can be carried in the suspended growth system. The advantage of biofilm processes compared to activated sludge processes is that the anaerobic, anoxic and aerobic zones can be combined in a single stage (see also rotating biological contactor). IFAS systems allow for the additional bacterial population to exist on a fixed surface, thereby eliminating the need to increase the suspended growth population. The Integrated Fixedfilm Activated Sludge (IFAS) process combines the advantages of conventional activated sludge with those of biofilm systems by combining the two technologies in a single reactor (see also rotating biological contactor, anammox or trickling filters). Typically, an IFAS configuration will be similar to an activated sludge plant, with biomass carriers (described above) introduced into carefully selected zones within the activated sludge process. This allows two distinct biological populations to act synergistically, with the Mixed Liquor Suspended Solids (MLSS) degrading most of the organic load (BOD), and the biofilm creating a strongly nitrifying population for oxidation of the nitrogenous load (NH4+). There are a number of different approaches to IFAS implementation but the various configurations fall into one of two basic types:
1. “dispersed media” (e.g. MBBR – Moving Bed Biofilm Reactor) entrapped in the aeration basin, and 2. “fixed media”, such as structured sheet media or knitted fabric media, fixed-in-place in the aeration basin
Schematic design of an integrated fixed-film/activated sludge system. In this illustration, polypropylene finned cylinders are in use as a biofilm dispersed media
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Design Parameters
Different types of IFAS media
Design capacity These high-tech systems are mostly used for upgrading existing treatment plants to enable extensive nitrogen removal.
Cost
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IFAS vs. Conventional Activated Sludge •
For new installations, IFAS systems will generally require less volume and therefore have less capital cost than a conventional AS system.
•
For retrofits of existing activated sludge (AS) systems to address increased capacity or improved biological nutrient removal (BNR), IFAS systems represent a cost-avoidance associated with the additional volume of that would otherwise be required for additional AS capacity.
•
IFAS systems require little or no additional operational costs or operating staff over conventional AS. However, the need for oxygen supply remains.
Cost Comparison of Various IFAS Systems •
Dispersed systems require expenditures for additional components, such as mediaretaining sieves, air knives, and/or pumps for sponge regeneration.
•
Rather than solely using the media-specific surface areas a means of comparing various IFAS approaches, a true capital cost comparison of different IFAS media systems should look at the cost of removing a given NH3-N load.
Advantages/disadvantages
Advantages •
The fixed biomass combines aerobic, anaerobic and anoxic zones and increases the Sludge Retention Time, promoting better nitrification compared to simple suspended growth systems
•
System nitrification is also restored faster since a large mass of nitrifiers is retained on the fixed-film
•
Improved process stability
•
In various studies and discussions with practitioners, it has been noted that the Sludge Volume Index (SVI) improves and has less variation when IFAS upgrades are implemented
•
Reduced Sludge Production
Disadvantages •
Large energy requirements (e.g. for aeration). Even still the same for conventional activated sludge processes.
•
Mechanical spare parts are not locally available
•
High construction and operation costs
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•
Requires expert knowledge
All IFAS systems, whether based on dispersed or fixed media, require adequate preliminary treatment design and operation. Primary clarification or fine screening will prevent ragging and material build-up on the media in the aeration basin and clogging of the dispersed media and retaining screens . All operational activities must be carried out by skilled labourers.
IFAS-‘Dispersed Media’ - Moving Bed Biofilm Reactor Process (MBBR) Process In the MBBR bio film technology the bio film grows protected within engineered plastic carriers, which are carefully designed with high internal surface area. These bio film carriers are suspended and thoroughly mixed throughout the water phase. With this technology it is possible to handle extremely high loading conditions without any problems of clogging, and treat industrial and municipal wastewater on a relatively small footprint. The process consists of adding bio film carriers (small cylindrical shaped polyethylene carrier elements - specific density of 0.96 g/cm3) in aerated or nonaerated basins to support biofilm growth. The small cylinders are about 10 mm in diameter and 7 mm in height with a cross inside the cylinder and longitudinal fins on the outside. The biofilm carriers are maintained in the reactor by the use of a perforated plate at the tank outlet. Air agitation or mixers are applied in a manner to continuously circulate the packing. The packing may fill 25 to 50 % of the tank volume. The specific surface area of the packing is about 500 -700 m2/m3 of bulk packing volume. The MBBR does not require any return activated sludge flow or backwashing. A final clarifier is used to settle sloughed solids. The MBBR process provides an advantage for plant upgrading by reducing the solids loading on existing clarifiers. The presence of packing material discourages the use of more efficient fine bubble aeration equipment, which would require periodic drainage of the aeration and removal of the packing for diffuser cleaning; on the contrary, medium bubble aeration diffusers are prefered.
NIREAS VOLUME 2 [2.4] 181 Typical flow diagram for MBBR process
(Eddy, 1999) Bio film carriers
Design Parameters
Typical process design parameters for a moving-bed biofilm reactor
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(Eddy, 1999)
Operation & Maintenance
Biocarriers filling instructions when upgrading an existing treatment plant CHECK THAT THE AIR HOLES IN THE SPARGER PIPES ARE FACING DOWNWARDS. CHECK THAT THERE ARE NO OPENINGS OUTSIDE THE STRAINERS THAT THE BIOMEDIA CAN PASS THROUGH. FILL THE BIO REACTORS WITH WATER WHILE RUNNING THE AIR BLOWER WITH FULLY OPEN DIFFUSER VALVES AND CHECK THAT THE AIR DIFFUSERS ARE DELIVERING EQUAL AND UNIFORMLY DISTRIBUTED AMOUNT OF AIR FROM EACH DIFFUSER. START AND RUN THE SLUDGE PUMP IN RECIRCULATION MODE FROM ALL DRAIN VALVES. FILL REQUIRED BIOMEDIA AS PER THE QUANTITIES SPECIFIED IN TECHNICAL STUDIES CONTINUE AERATION UNTIL GOOD MIXING IS OBTAINED, MEANING VISIBLE MOVEMENT OF BIOMEDIA ON THE SURFACE GOOD MIXING IS NORMALLY OBTAINED AFTER 2 TO 3 DAYS.
Biofilm generation
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TIME FOR GENERATION OF BIO FILM ON CARRIER ELEMENTS WILL VARY DEPENDING UPON TYPE OF WASTEWATER AND TEMPERATURE. MUNICIPAL WASTEWATER
3 – 4 WEEKS
FOOD INDUSTRY
1 – 2 WEEKS
PAPER / PULP INDUSTRY
3 – 4 DAYS
AS TIME GOES BY, THE BIO FILM WILL DEVELOP AND BECOME MORE EFFICIENT AND ROBUST. IT WILL REACH ITS MAXIMUM CAPACITY AFTER ONE YEAR OF NORMAL OPERATION.
Foam control SEVERE FOAMING MAY BE ENCOUNTERED IN THE INITIAL PHASES OF TREATMENT DUE TO UNDER LOAD CONDITIONS BUT USUALLY THIS DISAPPEARS AFTER SOME WEEKS OF OPERATION. FOAM DOES NOT HINDER TREATMENT, IT IS A NUISANCE WHICH CAN BE CORRECTED THROUGH USE OF ANTI-FOAM CHEMICALS. OR BY SPRAYING FRESH WATER. FOAMING IS CAUSED BY SURFACE TENSIONS IN THE WASTEWATER. FOAMING DURING STARTUP IS REDUCED BY REDUCING AERATION.
Shut down and restart IF IT BECOMES NECESSARY TO STOP THE LOAD ON THE BIOREACTORS FOR A SHORTER PERIOD, FOR INSTANCE ONE WEEK, ALL REACTORS SHOULD BE MODERATELY AERATED TO KEEP AEROBIC BIO MEDIA CONDITION. IF IT BECOMES NECESSARY TO STOP THE LOAD ON THE BIOREACTORS FOR A LONGER PERIOD, IT IS RECOMMENDED TO AERATE THE REACTORS THE FIRST WEEK AFTER THE STOP, AND TO RESTART AERATION 2 DAYS BEFORE THE REACTORS ARE RE-LOADED WITH ORGANIC MATTER. BEFORE WASTEWATER AGAIN CAN BE PUMPED TO THE BIOLOGICAL STAGE, IT IS IMPORTANT THAT ALL THE BIO MEDIA IS IN GOOD SUSPENSION IN ALL BIOREACTORS FIRST.
NIREAS VOLUME 2 [2.4] 184 Response to Abnormal Conditions •
IF AERATION OR MOVEMENT OF WATER IN THE REACTORS IS NOT VISIBLE OR HAVE TOTALLY STOPPED, THIS MEAN AIR DISTRIBUTION IS NOT GOOD. CHECK THE FLOW RATE AND BACKPRESSURE OF THE BLOWER. IF CAPACITY AND PRESSURE ARE CORRECT, CHECK THE AIR HEADER AND DISTRIBUTORS FOR LEAKAGES.
•
TO CONTROL BIOLOGICAL GROWTHS AND THE PRODUCTION OF ODOURS IN THE SETTLING
TANK,
PERIODICALLY.
THE
ACCUMULATED
SOLIDS
MUST
BE
FLUSHED
OUT
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There are now more than half a dozen, and counting, different variations of processes in which a fixed packing material is placed in the aeration tank of the activated-sludge process. Three typical examples of fixed packing processes include the Ringlace® and BioMatrix® processes, Bio-2Sludge® process, and submerged RBCs.
Placement of Ringlace® packing in an activated sludge reactor: (a) schematic of placement of packing in activated-sludge reactor and (b) isometric view of packing placed in activatedsludge reactor.
(Eddy, 1999)
Rotating biological contactor units have been installed in activated sludge. The submerged rotating biological contactor (SRBC) is operated at approximately 85 % submergence. The SRBC units can be as large as 5.5 m diameter with a surface area of 28,800 m2.. The rotation is driven by aeration and may be mechanically assisted. The submerged operation reduces the load on the packing shaft.
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2.4.3.2 Fluidized Bed BioReactor Process (FBBR) Process In fluidized-bed bioreactors the wastewater is fed upward to a bed of 0.4 to 0.5 mm sand or activated carbon. Bed depths are in the range of 3 to 4 m. The specific surface area is about 1000 m2/m3 of reactor volume, which is greater than any of the other fixedfilm packing. Upflow velocities are 30 to 36 m/h. Effluent recirculation is necessary to provide the fluid velocity within the necessary treatment detention times. Hydraulic retention times in FBBRs range from 5 to 20 min. As the biofilm increases in size, the packing becomes lighter and accumulates at the top of the bed where it can be removed and agitated periodically to remove excess solids. A schematic of an FBBR system is shown on next figure.
Schematic of fluidized bed biological reactor (FBBR)
(Eddy, 1999)
For aerobic applications recirculated effluent is passed through an oxygenation tank to predissolve oxygen. Adding air to the fluidized-bed reactor would discharge packing to the effluent. For municipal wastewater treatment, FBBRs have been used mainly for post denitrification. Aerobic FBBRs are frequently used to treat groundwater contaminated with hazardous substances. In these applications activated carbon is used for the packing to provide both carbon adsorption and biological degradation.
The main advantages for the FBBR technology in this application are :
NIREAS VOLUME 2 [2.4] 187 1. it provides an extraordinarily long SRT for microorganisms necessary to degrade the xenobiotic and toxic compounds; 2. shock loads or nonbiodegradable toxic compounds can be absorbed onto the activated carbon; 3. high-quality effluent is produced low in TSS and COD concentration; 4. the oxygenation method prevents stripping and emission of toxic organic compounds to the atmosphere; and 5. the system operation is simple and reliable
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2.4.4 ANAEROBIC BIOLOGICAL TREATMENT PROCESSES Process Anaerobic treatment applies to both wastewater treatment and sludge digestion. This section discusses only anaerobic wastewater treatment. Anaerobic wastewater treatment is an effective biological method for treating many organic wastes. The microbiology involved in the process includes facultative and anaerobic microorganisms, which, in the absence of oxygen, convert organic materials into gaseous end products such as carbon dioxide and methane.
The end products of anaerobic degradation are gases, mostly methane (CH4), carbon dioxide (CO2), and small quantities of hydrogen sulfide (H2S) and hydrogen (H2). The process involves two distinct stages: acid fermentation and methane fermentation.
MORE In acid fermentation, the extracellular enzymes of a group of heterogenous and anaerobic bacteria hydrolyze complex organic waste components (proteins, lipids, and carbohydrates) to yield small soluble products. These simple, soluble compounds (e.g., triglycerides, fatty acids, amino acids, and sugars) are further subjected, by the bacteria, to fermentation, b-oxidations, and other metabolic processes that lead to the formation of simple organic compounds, mainly short-chain (volatile) acids (e.g., acetic [CH3COOH], propionic [CH3CH2COOH], butyric [CH3- CH2-CH2COOH]) and alcohols. In the acid fermentation stage, no COD or BOD reduction is realized since this stage merely converts complex organic molecules to shortchain fatty acids, alcohols, and new bacterial cells, which exert an oxygen demand. In the second stage, short-chain fatty acids (other than acetate) are converted to acetate, hydrogen gas, and carbon dioxideâ&#x20AC;&#x201D;a process referred to as acetogenesis. Subsequently, several species of strictly anaerobic bacteria bring about methanogenesisâ&#x20AC;&#x201D;a process in which hydrogen produces methane from acetate and carbon dioxide reduction. In this stage, the stabilization of the organic material truly occurs. Two stages of anaerobic treatment occur as sequential processes; however, both stages occur simultaneously and synchronously in an active, well-buffered system. The main concern of a wastewater treatment facility in operating an anaerobic system is that the various bacterial species function in a balanced and sequential way. Hence, although other types of microorganisms may be present in the reactors, attention is focused mostly on the bacteria.
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The anaerobic wastewater treatment processes discussed in this section include the anaerobic contact process, the USB reactor, the anaerobic filter, and the AFBR.
Advantages/disadvantages
The major advantages of anaerobic treatment over aerobic treatment are as follows: •
The biomass yield for anaerobic processes is much lower than that for aerobic systems; thus, less biomass is produced per unit of organic material used. This reduced biomass means savings in excess sludge handling and disposal and lower nitrogen and phosphorus requirements.
•
Since aeration is not required, capital costs and power consumption are lower.
•
Methane gas produced in anaerobic processes provides an economically valuable end product.
•
The reduction of sludge and aeration energy consumption each result in savings that are greater than the cost of the energy required by the anaerobic process. In addition, a substantial part of the energy requirements for anaerobic processes can be obtained from exhaust gas.
•
Higher influent organic loading is possible for anaerobic systems than for aerobic systems because the anaerobic process is not limited by the oxygen transfer capability at highoxygen utilization rates in aerobic processes.
However, the disadvantages associated with the anaerobic process are as follows: •
Energy is required by elevated reactor temperatures to maintain microbial activity at a practical rate. (Generally, the optimum temperature for anaerobic processes is 35°C.) This disadvantage is not serious if the methane gas produced by the process can supply the heat energy.
•
Higher detention times are required for anaerobic processes than aerobic treatment. Thus, an economical treatment time can result in incomplete organic stabilization.
•
Undesirable odors are produced in anaerobic processes due to the production of H2S gas and mercaptans. This limitation can be a problem in urban areas.
•
Anaerobic biomass settling in the secondary clarifier is more difficult to treat than biomass sedimentation in the activated-sludge process. Therefore, the capital costs associated with clarification are higher.
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â&#x20AC;˘
Operating anaerobic reactors is not as easy as aerobic units. Moreover, the anaerobic process is more sensitive to shock loads.
NIREAS VOLUME 2 [2.4] 191 2.4.4.1 Anaerobic contact process (Suspended growth) Process The anaerobic contact process is a suspended-growth process, similar in design to the activatedsludge process except that anaerobic conditions prevail in the former process. Next figure shows the process schematic :
Anaerobic contact process
(David H.F. Liu, Bela G. Liptak, 1999)
The anaerobic contact process is comprised of two parts. The contact part involves thorough mixing of the wastewater influent with a well-developed anaerobic sludge culture. The separation part involves the settling out of anaerobic sludge from the treated wastewater and recycling back to the contact reactor. The process usually has a vacuum degasifier placed following the aerobic reactor to eliminate gas bubbles that cause SS in the clarifier to float.
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2.4.4.2 Anaerobic upflow sludge blanket processes (UASB) Process UASB reactor is essentially a suspended-growth reactor, but it is also a fixed-biomass process. Next figure shows the process schematic :
Upflow sludge blanket reactor
(David H.F. Liu, Bela G. Liptak, 1999)
This USB system is based on the development of a sludge blanket. In this sludge blanket, the component particles are aggregated to withstand the hydraulic shear of the upwardly flowing wastewater without being carried upwards and out of the reactor. The sludge flocs must be structurally stable so that hydraulic shear forces do not break them into smaller portions that can be washed out, and they should also have good settlement properties The wastewater is fed at the bottom of the reactor, and active anaerobic sludge solids convert the organics into methane and carbon dioxide. The anaerobic biomass is distributed over the sludge blanket and a granular sludge bed. The sludge solids concentration in the sludge bed is high—100.000 mg/l SS—and does not vary over a range of process conditions. The sludge solids concentration in the sludge blanket is lower and depends on process conditions. The reactor can include an internal baffle system, usually referred to as a gas–liquid separator, above the sludge blanket to separate the biogas, sludge, and liquid.
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UASB reactors are separated in three phases: granules, liquid and gas (left). They can be constructed circular or rectangular (right)
Design capacity
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Centralised or decentralised at community level, for industrial wastewater or blackwater. The system requires a continuous and stable water flow and energy.
Effluent quality •
Removal of 60 to 90 % BOD; 60 to 80 % COD and 60 to 85 % TSS
•
Low pathogen reduction minimal removal of nutrient (N and P)
Cost The significantly lower level of technology required by the UASB process in comparison with conventional advanced aerobic processes means that they are also cheaper in construction and maintenance. Capital costs for construction can be estimated as low to medium . Operation costs are low, as usually no costs arise other than desludging costs and the operation of feeding pump
Advantages/disadvantages
Advantages •
High treatment efficiency for high-strength wastewater
•
Biogas can be used for energy (but usually requires scrubbing first)
•
No aeration system required (thus little energy consumption)
•
Low sludge production, treated sludge is stabilised (can be used for soil fertilisation)
•
Effluent is rich in nutrients and can be used for agricultural irrigation
•
Low land demand, can be constructed underground and with locally available material
•
Reduction of CH4 and CO2 emissions
•
Low odour emissions in case of optimum operation
Disadvantages •
Not resistant to shock loading and sensitive to organic load fluctuations.
•
Requires skilled staff for construction, operation and maintenance (control of feeding pump and influent organic load)
•
Insufficient pathogen removal without appropriate post-treatment
•
Long start-up phase
•
Constant source of electricity and water flow is required
•
Not adapted for cold regions
Operation & Maintenance
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The construction, the start-up phase as well as the maintenance of UASB requires skilled staff. UASB reactors require several months to start up. Granular sludge forms when bacteria aggregate, form chains and coagulate into flocs or granules. The sludge not only needs to form but also needs to adapt to the characteristics of the specific wastewater. As domestic or municipal wastewater already contains the composition of nutrients and micronutrients required for bacterial activity and growth, they are generally less problematic than industrial wastewaters. High organic loading in connection with lower hydraulic loading rates quicken the granulation process in the starting phase. To keep the blanket in proper position, the hydraulic load must correspond to the upstream velocity and must correspond to the organic load. The latter is responsible for development of new sludge . This means that the flow rate must be controlled and properly geared in accordance with fluctuation of the organic load. A permanent operator is also required to control, monitor and repair the reactor and the dosing pump. Desludging is infrequent and excess sludge needs to be removed only every few years (2 to 3 years).
NIREAS VOLUME 2 [2.4] 196 2.4.4.3 Anaerobic filter processes (Attached growth) Process In an anaerobic filter reactor, the growth-supporting media is submerged in the wastewater. Anaerobic microorganisms grow on the media surface as well as inside the void spaces among the media particles. The media entraps the SS present in the influent wastewater that can be fed into the reactor from the bottom (upflow filter) or the top (downflow filter) as shown in the process schematics in next figure. Anaerobic filter reactor
(David H.F. Liu, Bela G. Liptak, 1999)
Thus, the flow patterns in the filter can be either PF or completely mixed depending on recirculation magnitude. Periodically backwashing the filter solves bed-clogging and high-headloss problems caused by the accumulation of biological and inert solids An advantage of using the filter process for industrial wastewater treatment is that the filter reactor can retain the active biomass within the system for an extended time period. The long sludge-retention time maintained by the reactor allows ample time for aerobic microorganisms to remove organics in the wastewater, and there is no appreciable loss of the active biomass from the system until the filter is saturated.
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In addition, the anaerobic filter minimizes operational concerns of sludge wasting and disposal because the synthesis rate of excess biomass under anaerobic conditions is low. Because it can retain a high concentration of active biomass within the system for an extended time period, the anaerobic filter can easily adapt to varied operating conditions (e.g., without significant changes in effluent quality and gas production due to fluctuations in parameters such as pH, temperature, loading rate, and influent composition). Also, intermittent shutdowns and complications in industrial treatment will not damage the filter since it can be fully recovered when it is restarted at a full load.
A problem associated with the filterâ&#x20AC;&#x2122;s ability to retain the biomass for a long time period is the close control of biomass holdup. Although periodic backwashing of the filter is a feasible method for maintaining the biomass holdup nat the required level, more efficient techniques are needed.
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2.4.4.4 Anaerobic fluidized-bed reactor (Attached & Suspended growth-AFBR) Process The AFBR is an expanded-bed reactor that retains media in suspension from drag forces exerted by upflowing wastewater. Fluidization of the media particles provides a large surface area where biofilm formation and growth can occur.
Anaerobic fluidized-bed reactor (AFBR)
(David H.F. Liu, Bela G. Liptak, 1999)
The media particles have a high density resulting in a settling velocity that is high enough so that high-liquid-velocity conditions can be maintained in the reactor. However, the media particlesâ&#x20AC;&#x2122; overall density decreases as biomass growth accumulates on the surface area. The decrease in density can cause the bioparticles to rise and be washed out of the reactor. To prevent this situation, the reactor controls fluidized-bed height at a required level by wasting a corresponding amount of overgrown bioparticles. The wasted bioparticles can then be received by a mechanical device that separates the biomass from the wasted media particles. The cleaned particles can then be returned to the reactor, while the separated biomass is wasted as sludge. The AFBR combines a suspended-growth system and an attached-growth system since biomass growth attaches to the media particles which are suspended in the wastewater. The reactor
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recycles a portion of the effluent flow ensuring uniform bed fluidization and sufficient substrate loading. An advantage of the AFBR is that it employs small fluidized media that provide a high biomass holdup in the reactor, reducing hydraulic retention time. The AFBR also prevents bed-clogging and high-pressure drops—complications associated with anaerobic filters. Due to the flexibility provided by bed-height control in an AFBR, a constant biomass concentration can be maintained in the reactor independent of substrate loadings. Another advantage of the AFBR is that it is insensitive to variations in influent pH, temperature, and waste loading because it maintains a high biomass holdup and completely mixed conditions inside the reactor. The AFBR has been applied to a variety of industrial treatment processes with substrates such as molasses, synthetic sucrose, sweet whey, whey permeate, glucose, and acid whey.
The choice for media types should be based on the following media characteristics: •
A large surface area for microbial growth
•
A large void space to accommodate the accumulation of biological and inert solids and minimize short-circuiting
•
Inertness to biological and chemical reactions
•
Resistance to abrasion and erosion
•
A light weight
Small media should be used since they provide large surface-to-volume ratios, and thus, a greater surface area for biofilm growth without increasing reactor volume. Small media are also easier to fluidize, reducing the circulation requirements which decreases the shearing effects and allows a more quiescent environment for optimal biofilm growth. Silica sand, anthracite coal, activated carbon, stainless-steel wire spheres, and reticulated polyester foams are some of the media that can be considered for AFBR applications.
NIREAS VOLUME 2 [2.4] 200 2.4.4.5 Design Parameters Design parameters of Anaerobic treatment processes Anaerobic treatment process type
Liquid Retention Time- LRT (hr)
Volumetric Loading (kg COD/m3-day)
COD REMOVAL (%)
Anaerobic contact process Anaerobic upflow sludge blanket reactor (UASB) Anaerobic filter
2-10
0,45-2,5
75-90
4-12 24-48
4-12 1-5
75-85 75-85
Anaerobic fluidized-bed reactor (AFBR)
5-10
5-10
80-85
Optimal Temperature (째C)
Optimal pH
30-35 (mesophilic) 49-57 (thermophilic)
6,8-7,5
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2.4.5 NATURAL WASTEWATER TREATMENT PROCESSES 2.4.5.1 Constructed Wetlands - Free Water Surface (FWS) Constructed wetlands are a treatment step of Decentralised wastewater treatment systems and they can even be used as a tertiary treatment system for polishing after activated sludge or trickling filter plants. Basically, there are three different types of constructed wetlands (CWs). They are classified according to the water flow regime as: •
Free-surface constructed wetlands (FWS)
•
Horizontal flow constructed wetlands (HF)
•
Vertical flow constructed wetlands (VF)
A free-surface constructed wetland (also called free water surface flow or FWS) is a series of flooded planted channels or a basin that aims to replicate the naturally occurring processes of a natural wetland, marsh or swamp. As water slowly flows through the wetland, particles settle, pathogens are destroyed, and organisms and plants utilise the nutrients. It is especially appropriate for pre-treated and settled wastewater, or as treatment stage in hybrid constructed wetlands. Pre-treating of wastewater in e.g. a septic tank or biogas settler is necessary to avoid excess accumulation of solids and garbage. Because of the open water surface, there is a risk of mosquito breeding if not properly designed. Plants grown on the wetland may be used for composting or energy production and the effluent can be used for aquaculture and irrigation. This system is appropriate for small sections of urban areas (e.g. decentralised treatment for a community or several housings or small industries) or even more appropriate for peri-urban and rural communities because of the land surface required.
NIREAS VOLUME 2 [2.4] 202 Functional schematic of a free-surface wetland
Process In a free-surface constructed wetland (also known as surface flow CW or free water surface CW), water flows above ground and plants are rooted in the sediment layer at the base of the basin or floating in the water. Typically, there is a basin or channels lined with an impermeable layer (clay or geotextile). The substrate consists of rocks, gravel and soil. The basin is planted advantageously with native plants. Compared to subsurface wetlands (horizontal flow or vertical flow), free-surface CWâ&#x20AC;&#x2122;s can be vegetated with emergent, submerged and floating plants .
Plants for free-surface flow constructed wetlands
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Design Parameters The wetland is flooded with wastewater to a depth of 10 to 45 cm above ground level. As the water slowly flows through the wetland, simultaneous physical, chemical and biological processes filter solids, degrade organics and remove nutrients from the wastewater. To avoid clogging and the excess accumulation of solids and garbage, pre-treatment is necessary. Pre-treatment of wastewater separates solid materials (e.g. faeces or kitchen slop) as well as grease or oil from the liquid. Depending on the situation, there are several possibilities such as grease trap, septic tank, biogas settler, anaerobic baffled reactor, imhoff tank, or UASB reactor. Pre-treated wastewater enters the basin via a weir or a distribution pipe. It is important for the treatment effect that it is distributed over the whole width. Once in the pond, the wastewater flows slowly through the basin and the heavier sediment particles settle, also removing nutrients that are attached to particles. Plants, and the communities of microorganisms that they support (on the stems and roots), take up nutrients like nitrogen and phosphorus. Chemical reactions may cause other elements to precipitate out of the wastewater. Pathogens are removed from the water by natural decay, predation from higher organisms, sedimentation and UV irradiation. Although the soil layer below the water is anaerobic, the plant roots release oxygen into the area immediately surrounding the root hairs, thus creating an environment for complex biological and chemical activity .
Free-surface CWâ&#x20AC;&#x2122;s normally require more surface than a subsurface system (e.g. a horizontal flow or vertical flow wetland). This is because the porous subsurface filter medium in subsurface systems provides a greater contact area for treatment activities. Consequently, compared to a subsurface filter, free-surface wetlands are designed bigger for the same volume of wastewater.
Design capacity Depending on the volume of water, and therefore the size, wetlands can be appropriate for small sections of urban areas or more appropriate for peri-urban and rural communities phosphorus .
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Cost The capital costs of constructed wetlands are highly dependent on the costs of sand since the bed has to be filled with sand and on the cost of land. Financial decisions on treatment processes should not primarily be made on capital costs, but on net present value or whole-of-life costs, which includes the annual costs for operation and maintenance . Compared to other intensive (high-rate) aerobic treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work extensively. That means treatment may require more land and time, but you can safe costs because of lower operation, which requires no or only little electrical energy and operators can be trained people from the community (low-skilled people). It also means that there is no need for sophisticated equipment, expensive spare parts or chemicals. Constructed wetlands are usually cheaper to build than high-rate aerobic plants but for larger plants, they are usually more expensive in terms of capital costs. For large-scale treatment plants of more than 10 000 PE in areas where land is available cheaply, surface-flow and waste stabilisation ponds have lower capital costs than subsurface-flow constructed wetlands (horizontal and vertical). Surface-flow constructed wetlands have also often lower maintenance and repair costs than in subsurface systems. On the other hand, if land area is not available or ground prices high, the large surface can be a big disadvantage.
Advantages/disadvantages
Advantages •
One of the main advantages of CWs are, that they are natural systems and thus not require chemicals, energy or high-tech infrastructure. Moreover, thy are suited to be combined with aquaculture or sustainable agriculture (irrigation).
•
Simple construction
•
Low operation and maintenance costs (1 check per 20 days)
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High reliability
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The lowest energy consumption among all wastewater treatment processes. In some cases no energy is needed at all
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They are able to withstand shock loads of toxic inputs.
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Growth of flora and fauna in local area, Aesthetic view
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No excess sludge is produced
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Can be built and repaired with locally available materials
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Construction can provide short-term employment to local labourers
Disadvantages
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•
Large & flat areas required for the construction
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Need for disposal or old reeds (every 5 to 10 years)
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May facilitate mosquito breeding
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Long start up time to work at full capacity
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Requires expert design and supervision
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Moderate capital cost depending on land, liner, etc.
•
Not very tolerant to cold climates
Operation & Maintenance In general the O&M requirements for constructed wetlands are relatively simple (no high-tech appliances or chemical additives), allowing community organisations or a private, small-scale entrepreneur to manage the system after adequate capacity building and with technical support. Regular maintenance should ensure that water is not short-circuiting, or backing up because of fallen branches and leaves or garbage. Vegetation may have to be cut back or thinned out periodically. In general free-surface CW’s are easier to regulate than subsurface systems. On the other hand they have not a great cold temperature tolerance and odour and mosquito problems can occur if operated and maintained incorrectly. A number of systems have had problems with clogging and unintended surface flows.. An important part of O&M is to empty the sludge of the pre-treatment facilities (e.g. septic tank). This should be done in a proper and safe way (see human-powered emptying and transport and motorised emptying and transport). The filter bed of the constructed wetland may also be changed sometimes. The old material, full of earth and organic matter may be directly used as soil amendment or composted first, similar to a planted drying bed.
NIREAS VOLUME 2 [2.4] 206 2.4.5.2 Constructed Wetlands - Subsurface Systems Horizontal Flow (HF) A horizontal flow constructed wetland (horizontal flow CW) is a planted filter bed for secondary or tertiary treatment of wastewater (e.g. greywater or blackwater). After primary treatment for solids removal in e.g. in a septic tank or imhoff tank, the wastewater is fed at the inlet zone and flows horizontally through the porous filter medium (sand or gravel) until it reaches the outlet zone. The water is treated by a combination of biological and physical processes. The effluent of a wellfunctioning constructed wetland can be used for irrigation and aquaculture or safely been discharged to receiving water bodies. Design and implementation of requires expert knowledge. Horizontal flow CW are relatively inexpensive to build where land is affordable and can be maintained by the local community as no high-tech spare parts, electrical energy or chemicals are required.
Process A horizontal subsurface flow constructed wetland is a large gravel and sand-filled channel that is planted with aquatic vegetation. As wastewater flows horizontally through the channel, the filter material filters out particles and microorganisms degrade organics. The water level in a Horizontal Subsurface Flow Constructed Wetland is maintained at 5 to 15 cm below the surface to ensure subsurface flow. To avoid clogging of the wetland, pre-treatment is necessary. This separates solid materials (e.g. faeces or kitchen slop) as well as grease or oil from the liquid. Depending on the situation, there are several possibilities such as grease trap, septic tanks, anaerobic baffled reactors, imhoff tanks, biogas settlers, or UASB reactors.
NIREAS VOLUME 2 [2.4] Horizontal flow constructed wetland
207
Design Parameters Pre-treated wastewater flows slowly through the porous medium under the surface of the bed in a horizontal path until it reaches the outlet zone. At the outlet, the water level is controlled with an adjustable standpipe.A common design suggests a water level of about 60 cm which is maintained at ca. 5 to 15 cm below the surface of the CW to avoid anaerobic conditions in the bed. An important role of treatment efficiency is the oxygen supply. Horizontal filter beds have a very small external oxygen transfer and a smaller inlet compared to a vertical flow constructed wetland. Therefore they require a larger area. If topography allows for gravity flow, horizontal flow filters are not dependent on energy and can be operated by gravity.
The treatment process of constructed wetlands is based on a number of biological and physical processes (adsorption, precipitation, filtration, nitrification, predation, decomposition, etc.).
For designing the filter bed, expert knowledge is required. Parameters such as hydraulic load or organic loading (e.g. BOD) and experience of the designer specify the size of the filter as well as the filter substrate.
Plants
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Plants in constructed wetlands are an important part of the design. Plants are aesthetically pleasant and serve as a habitat for wildlife. Dead plant material is a natural insulation layer and protects the filter bed during winter in cold climates. Furthermore, the vegetation transfers oxygen to the filter zone and plants and its roots provide an appropriate habitat for microbiological growth in the root zone. But the most essential function of the vegetation, i.e. the roots system is to maintain the permeability in the filter. A very common plant for horizontal constructed wetlands is Phragmites australis, a type of reed. The plant should build a deep root zone to transfer at least a minimum of oxygen into the filter body. Bamboo or papyrus should also be possible, but that has not been investigated yet.
Substrate The filter bed should be wide and shallow with a slope of ~1%. Small, round, evenly sized gravel (3â&#x2C6;&#x2019;32 mm in diameter) should be used to fill the bed to a depth of 0.5â&#x2C6;&#x2019;1 m. Sand is also acceptable but more prone to clogging . The inlet and the outlet zone should be constructed with coarse gravel.
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An almost completed horizontal flow CW. Coarse gravel avoids a clogging of the inlet and outlet pipes. The inspection chamber allows a periodically check of the treated water quality and water level in the filter body
Design capacity It can be applied for single households or small communities as a secondary or tertiary treatment facility of grey- or blackwater. Effluent can be reused for irrigation or is discharged into surface water.
Effluent quality Removal •
BOD = 80 to 90 %;
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TSS = 80 to 95 %;
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TN = 15 to 40 %;
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TP = 30 to 45 %;
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FC ≤ 2 to 3 log;
NIREAS VOLUME 2 [2.4] 210 Cost The capital costs of constructed wetlands are highly dependent on the costs of sand (since the bed has to be filled with sand), and on the cost of land. Financial decisions on treatment processes should not primarily be made on capital costs, but on net present value or whole-of-life costs, which includes the annual costs for operation and maintenance. Compared to other intensive (high-rate) aerobic treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work extensively. That means treatment may require more land and time, but you can safe costs because of lower operation, which requires no or only little electrical energy and operators can be trained people from the community (low-skilled people). Furthermore, there is no need for sophisticated equipment, expensive spare parts or chemicals. Constructed wetlands are usually cheaper to build than high-rate aerobic plants but for larger plants, they are usually more expensive in terms of capital costs. For large-scale treatment plants of more than 10 000 PE in areas where land is available cheaply, free-surface-flow constructed wetlands and waste stabilisation ponds have lower capital costs than subsurface-flow constructed wetlands (horizontal and vertical) due to the high amounts of sand and gravel fill required for the bed of the sub-surface flow constructed wetland. Plants and liners may substantially add to the costs if they are unavailable locally. Moreover, design and construction of subsurface-flow constructed wetland requires skilled technical staff. However, the cost may be reduced if the material is acquired locally.
Advantages/disadvantages
Advantages Same as FWS plus â&#x20AC;˘
Requires less space than a FWS
â&#x20AC;˘
Does not have the mosquito problems compared to the FWS
DIsadvantages Same as FWS
Operation & Maintenance In general the O&M requirements for constructed wetlands are relatively simple (no high-tech appliances or chemical additives), which may allow a community organisation or a private, small-
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scale entrepreneur to manage the system after adequate capacity building and with technical support . However, a CW will always require some maintenance for the duration of its life. This aspect is frequently overlooked in decision-making processes. With time, the gravel will become clogged with accumulated solids and bacterial film. The material may have to be replaced every 8 to 15 or more years. Maintenance activities should focus on ensuring that primary treatment effectively lowers organics and solids concentrations before entering the wetland. A very critical situation occurs, when the filter smells like â&#x20AC;&#x153;rotten eggsâ&#x20AC;?. This is an indicator for anaerobic conditions. In this case the filter should be rested and the loads must be readjusted. Pre-treatment facilities need to be checked regularly if they work properly and they have to be emptied frequently and sludge must be discharged correctly.
2.4.5.3 Constructed Wetlands - Subsurface Systems Vertical Flow (VF) A vertical flow constructed wetland (vertical flow CW) is a planted filter bed for secondary or tertiary treatment of wastewater (e.g. greywater or blackwater). Pre-treated wastewater (e.g. from a septic tank or an Imhoff tank) is distributed over the whole filter surface and flows vertically through the filter. The water is treated by a combination of biological and physical processes. On the bottom of the filter, there is a drainage system which collects the treated wastewater. A vertical flow constructed wetland needs a specific filter surface of 1 to 4 m2 per population equivalent, depending on the climate. Normally, sand and gravel is used to construct the filter body. The filtered water of a well functioning constructed wetland can be used for irrigation, aquaculture, groundwater recharge or is discharged in surface water. To design a vertical flow constructed wetland, expert knowledge is recommended. They are relatively inexpensive to build where land is affordable and can be maintained by the local community.
Process In vertical filter beds wastewater is intermittently applied (either by pump or self-acting syphon device) onto the surface and then drains vertically down through the filter layers towards a drainage system at the bottom. In some cases, the distribution pipes are covered with gravel to avoid open water puddles. The treatment process is characterised by intermittent short-term loading intervals (4 to 12 doses per day) and long resting periods during which the wastewater percolates through the unsaturated substrate, and the surface dries out. The intermittent batch loading enhances the oxygen transfer and leads to high aerobic degradation activities. Therefore, vertical filters always need pumps or at least siphon pulse loading, whereas horizontal flow constructed wetlands can be operated without pumps (if topography allows). The treatment
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process of constructed wetlands is based on a number of biological and physical processes (adsorption, precipitation, filtration, nitrification, predation, decomposition, etc.) .
To avoid clogging, pre-treatment is necessary. This separates solid materials (e.g. faeces or kitchen slop) as well as grease or oil from the liquid. Depending on the situation, there are several possibilities such as grease trap, septic tank, anaerobic baffled reactor, imhoff tank, biogas settler, or UASB reactor.
Vertical flow constructed wetland
Vertical flow constructed process flow
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Design Parameters Due to the high oxygen supply into the filter, the rates of nitrification are higher than in a horizontal flow filter.
Plants Most common plants for vertical constructed wetlands are Phragmites australis, Typha cattalis and Echinochloa Pyramidalis . Bamboo or papyrus should also be possible, but have not been investigated yet .
Substrate The provision of a suitably permeable substrate in relation to the hydraulic and organic loading is the most critical design parameter of subsurface flow constructed wetlands. Most treatment problem occur when the permeability is not adequately chosen for the applied load.The drainage pipes at the base are covered with gravel. On top of this gravel layer, there is a sand layer (40-80 cm thick) which contains the actual filter bed of the subsurface flow CW. On top of the sand layer there is another gravel layer (about 10 cm), in order to avoid water accumulating on the surface. The top gravel layer does not contribute to the filtering process.
Design recommendations regarding the substrate to be used in subsurface flow filters are: •
The sand should have a hydraulic conductivity of about 10-4 to 10-3 m/s.
•
The filtration sand layer needs to have a thickness of 40 to 80 cm.
•
The substrate should not contain loam, silt or other fine material, nor should it consist of material with sharp edges
What filter material should be used depends on the local conditions and the experiences of the design engineer. Sand is monstly recommended as a substrate, because it is the most suitable substrate for the application of subsurface flow CWs for wastewater or greywater treatment especially in developing countries. In cold climates (annual average < 10°C), an area of 4 m2/p.e. is necessary. In warmer climates (annual average > 20°C), 1,2 m2/p.e. is enough, if the filter is designed correctly .
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Design capacity It can be applied for single households or small communities as a secondary or tertiary treatment facility of grey- or blackwater. Effluent can be reused for irrigation or is discharged into surface water .
Effluent quality Removal •
BOD = 75 to 90%;
•
TSS = 65 to 85%;
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TN < 60%;
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TP < 35%;
•
FC ≤ 2 to 3 log;
Cost For large-scale treatment plants of more than 10 000 PE in areas where land is available cheaply, free-surface-flow constructed wetlands and waste stabilisation ponds have lower capital costs than subsurface-flow constructed wetlands (horizontal and vertical) due to the high amounts of sand and gravel fill required for the bed of the sub-surface flow constructed wetland. Plants and liners may substantially add to the costs if they are unavailable locally (EAWAG/SANDEC 2008). Moreover, design and construction of subsurface-flow constructed wetland requires skilled technical staff. However, the cost may be reduced if the material is acquired locally.
Advantages/disadvantages
Advantages Same as HF plus •
Less clogging than in HF
•
Less area needed than in HF or FWS for the same hydraulic and organic loadings
DIsadvantages
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Same as HF plus â&#x20AC;˘
Dosing system requires more complex engineering
Operation & Maintenance -Same as HF -
2.4.5.4 Floating Aquatic Plant Systems Process Wastewater treatment systems have been designed that are based on the use of floating aquatic plants. The floating aquatic plant systems include water hyacinth systems and duckweed systems, as described below. Water Hyacinth Systems Water hyacinth systems are similar in concept to free water surface wetlands, as described previously, but are based on using water hyacinths rather than emergent wetlands vegetation. The water hyacinth (Eichhornia crassipes) is a perennial freshwater macrophyte that is native to the Amazon region of South America. As much as 50 % of the plantâ&#x20AC;&#x2122;s biomass is in the form of roots, which can extend to a depth of up to 0,6 m below the surface of the water. Water hyacinth wastewater treatment systems consist of wastewater ponds containing floating water hyacinth plants, as shown in next Figure . Water Hyacinth System
A minimum of primary treatment is required prior to water hyacinth treatment. The water hyacinth plants transmit oxygen to the water through the roots. Bacterial growth attached to the plant roots accomplishes much of the treatment, in addition to physical and chemical processes. The water
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hyacinth multiplies rapidly in proper conditions, and the plant matter must be regularly harvested and disposed. Water hyacinth systems require mosquito control measures to prevent nuisance conditions from developing. Water hyacinth systems usually produce secondary effluent. If they are aerated, they can achieve ammonia reduction by nitrification .There are relatively few water hyacinth systems still in operation.
Duckweed Systems Duckweed (Lemna spp.) is another floating aquatic plant that has been used in wastewater treatment systems. Duckweed plants are considerably smaller than water hyacinth, and they do not transmit oxygen to the water through their roots. Duckweed wastewater treatment systems consist of ponds covered with the plants, similar to previous Figure The duckweed plants are susceptible to movement by breezes; therefore, floating baffles are used to keep the plants in place. The duckweed largely functions as a pond covering, providing shade to prevent algae growth. Influent to the duckweed system must have received at least primary treatment; however, duckweed systems are typically used as effluent polishing systems to remove algal suspended solids downstream of algal pond treatment systems. No nitrogen removal is expected from duckweed systems.
2.4.5.5 Stabilisation Ponds (WSPs) Process Wastewater Stabilisation Ponds or non aerated Lagoons (WSPs) are artificial man-made lagoons in which blackwater, greywater or faecal sludge are treated by natural occurring processes and the influence of solar light, wind, microorganisms and algae. The ponds can be used individually or in series of an anaerobic, facultative and aerobic (maturation) pond. WSPs are low-cost for O & M and BOD and pathogen removal is high. However, large surface areas and expert design are required. The effluent still contains nutrients (e.g. N and P) and is therefore appropriate for the reuse in agriculture (irrigation) or aquaculture (e.g. fish- or macrophyte ponds) but not for direct recharge in surface waters.
In a first pond (anaerobic pond), solids and settleable organics settles to the bottom forming a sludge, which is, digested anaerobic by microorganism. In a second pond (facultative pond), algae growing on the surface provide the water with oxygen leading to both anaerobic digestion and
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aerobic oxidation of the organic pollutants. Due to the algal activity, pH rises leading to inactivation of some pathogens and volatilisation of ammonia. The last ponds serves for the retention of stabilised solids and the inactivation of pathogenic microorganisms via heating rise of pH and solar disinfection.
The treatment, in opposition to conventional treatment processes such as activated sludge system takes days to week, but WSPs provide a good option for a (semi-) centralised treatment in developing countries because of the low capital and particularly low O & M (Operation and Maintenance) costs. In addition, it is one of the few low-cost natural processes which provides good treatment of pathogens. Experience from around the world has shown that WSPs are very often the most cost-effective wastewater treatment method, but their major disadvantage is that availability of large areas of land far away from homes and public spaces is required.
Design Parameters Anaerobic ponds require approximately 4 m2/m3 daily flow and facultative aerobic ponds require 25 m2/m3 daily flow. WSPs make use of the sun, wind, gravity, and biological activity to achieve treatment. The principles behind WSP operation are simple and they place no strain on technical resources or labour. However, both the process design and the physical design of WSPs have to be carried out very carefully by competent design engineers since WSPs are more than just holes in the ground.
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Typical scheme of a waste stabilisation system: An anaerobic, facultative and maturation pond in series
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The different types of WSP can be used individually, but the most efficient and common system generally consists of three ponds in series: first an anaerobic; then a facultative pond and finally an aerobic or maturation pond. Only slightly polluted wastewater may be discharged directly into primary facultative ponds. This can be done also with more heavily polluted wastewater in situations when anaerobic ponds are unacceptable because of odour nuisance. In essence, anaerobic and facultative ponds are designed for BOD (Biological Oxygen Demand) removal and maturation ponds for pathogen removal, although some BOD removal occurs in maturation ponds and some pathogen removal in anaerobic and facultative ponds. Depending on the requirement for the final effluent, only anaerobic and facultative ponds are necessary in some instances.
MORE Anaerobic Treatment Ponds (APs) are deep ponds (2 to 5 m) devoid of dissolved oxygen, where sludge is deposited on the bottom and anaerobic bacteria break down the organic matter by anaerobic digestion, releasing methane and carbon dioxide. Viruses, bacteria, helminth, Ascaris eggs and other pathogens can also be inactivated by sedimentation when associated with solids. N, P and K can also be reduced by sludge formation and the release of ammonia into the air. However, the main function of anaerobic ponds is BOD removal, which can be reduced 40 to 85%.
As a complete process, the anaerobic pond serves to: •
Settle undigested material and non-degradable solids as bottom sludge
•
Dissolve organic material
•
Break down biodegradable organic material
APs can receive organic loads usually in the range of 100 to 350 g BOD/m3/day . They should not be operated below 10°C, and the load, which can be treated increases linearly with temperature rise (e.g. 100 g/m3/day at 10°C and 300g/m3/day at 20°C). The design temperature should be the mean of the coldest month of the year. A HRT of one day should be sufficient for a BOD5 lower than 300 mg/m3/day at 20°C, but the recommended HRT range varies from 2 to 5 . For highstrength industrial wastes, up to three anaerobic ponds in series might be necessary. The optimum pH for digestion lies at 6 to 8 and acidic wastewaters thus require neutralising prior to treatment. Due to its toxicity to anaerobic bacteria, ammonia concentrations should not exceed >80 mg NH3-/L.
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Facultative Treatment Ponds (FPs) are the simplest of all WSPs and consist of large shallow ponds (depth of 1 to 2m) with an aerobic zone close to the surface and a deeper, anaerobic zone. There are two types of facultative ponds: primary facultative ponds that receive raw wastewater (after grit removal), and secondary facultative ponds receiving settled wastewater usually from the anaerobic pond. In primary facultative ponds, the functions of anaerobic and secondary facultative ponds are combined. This type of pond is designed generally for the treatment of only slightly polluted wastewater and in sensitive locations where anaerobic pondsâ&#x20AC;&#x2122; odour would be unacceptable. FPs are designed for BOD removal on the basis of low surface loading (unlike anaerobic ponds which are designed according to their volumetric load) and can treat water in the BOD range of 100 to 400 kg/ha/day corresponding to 10 to 40 g/m2/day at temperatures above 20°C . The facultative ponds are covered by algae. The algae grow using the sunlight and they produce oxygen in excess to their own requirements, which they transfer to the water. It is this excess of oxygen that is used by bacteria to further break down the organic matter via aerobic digestion (oxidation) transforming the organic pollutants into CO2. Additionally to aerobic and anaerobic digestion of BOD, in the facultative ponds "sewage BOD" is converted into "algal BOD". The algal production of oxygen occurs near the surface of aerobic ponds to the depth to which light can penetrate (i.e. typically up to 500 mm). Additional oxygen can be introduced by wind due to vertical mixing of the water. Oxygen is unable to be maintained at the lower layers if the pond is too deep, and the colour too dark to allow light to penetrate fully or if the BOD and COD in the lower layer is higher than the supply. As a result of the photosynthetic activities of the pond algae, there is a diurnal variation in the concentration of dissolved oxygen. At peak sun radiation, the pond will be mostly aerobic due to algal activity, while at sunrise the pond will be predominantly anaerobic. Peak algae activity also results in a pH rise to above 9 since carbonate and bicarbonate ions react to provide more carbon dioxide for the algae, leaving an excess of hydroxyl ions. A pH above 9 for 24 hours can provide a 100% kill of E. coli and thus, most pathogenic bacteria. At high pH, ammoniac, coming from the hydrolysis of organic nitrogen is transformed to ammonia, which is volatilised to the air. There is little evidence for nitrification and denitrification. But ammonia, as well as phosphorus is also incorporated into new algal biomass and part of this is settled to the ground in non-biodegradable death algae material. Phosphorus can also be removed by precipitation as inorganic P, but it can also return through mineralization and resolubilisation into the water column.
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As a complete process, the facultative pond serves to: •
Further treat wastewater through sedimentation and aerobic oxidation of organic material
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Reduce odour
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Reduce some disease-causing microorganisms if pH raises
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Store residues as bottom sludge
FPs lose ammonia into the air at high pH; and settle some nitrogen and phosphorus in the sludge. FPs can result in the removal of 80 to 95% of the BOD5 (WSP 2007), which means an overall removal in the order of 95% over the two ponds (AP and FP). Total nitrogen removal in WSP systems can reach 80% or more, and ammonia removal can be as high as 95%. The HRT for a facultative pond lies between 5 to 30 days. Sometimes two or more consecutively smaller facultative ponds are constructed instead of a very large one, because it is more practical for desludging. To remove the algae from aerobic pond, effluents’ rock filtration, grass plots, floating macrophytes and herbivorous fish can be used, but most commonly, the effluent flows directly in a final maturation pond.
Whereas anaerobic and facultative ponds are designed for BOD removal, maturation or polishing ponds (MPs) are essentially designed for pathogen removal and retaining suspended stabilised solids. The size and number of maturation ponds depends on the required bacteriological quality of the final effluent. The principal mechanisms for faecal bacterial removal in facultative and maturation ponds are HRT, temperature, high pH (> 9), and high light intensity. Faecal bacteria and other pathogens die off due to the high temperature, high pH or radiation of the sun leading to solar disinfection. Regarding virus removal, little is definitely known but it is generally recognised that it occurs by adsorption on to settable solids (including the pond algae) and consequent sedimentation in the anaerobic and facultative pond. Some macroorganisms such as protozoan cysts and helminth eggs are also removed by sedimentation. Maturation ponds are shallower (1 to 1.5 m), with 1 m being optimal. The recommended hydraulic retention time is 15 to 20 days . If used in combination with algae and/or fish harvesting, this type of pond is also effective at removing the majority of nitrogen and phosphorus from the effluent.
NIREAS VOLUME 2 [2.4] 222 Design Parameters of Wastewater Stabilisation Ponds Pond
BOD Removal Pathogen Removal
HRT
Anaerobic Pond
50 to 85%
1 to 5 days
Facultative Pond
80 to 95%
5 to 30 days
Maturation Pond
60 to 80%
90%
15 to 20 days
Design capacity Almost all wastewaters (including heavily loaded industrial wastewater) can be treated, but the higher the organic load, the higher the required surface. In the case of high salt content, the use of the water for irrigation is not recommended.
Effluent quality â&#x20AC;˘
Removal 90% BOD and TSS;
â&#x20AC;˘
high pathogen reduction and relatively high removal of ammonia and phosphorus;
Cost According to the International Water and Sanitation Centre (IRC), stabilisation ponds are the most cost-effective (semi-)centralised wastewater treatment technology for the removal of pathogenic microorganisms. However, this depends on the availability of land and its price. Stabilisation ponds also have the advantage of very low operating costs since they use no energy compared to other wastewater treatment technologies and only low-tech infrastructure. This makes them particularly suitable for developing countries where many conventional wastewater treatment plants have failed because water and sewer utilities did not generate sufficient revenue to pay the electricity bill for the plant. However, expert design is still required. Further, the ponds can be combined with aquaculture to locally produce animal feed (e.g. duckweed) or fish (e.g. fishponds). Biogas (methane and carbon dioxide) may also be recovered for use when anaerobic ponds are covered with a floating plastic membrane .
NIREAS VOLUME 2 [2.4] 223 Advantages/disadvantages
Advantages •
Can be built and repaired with locally available materials
•
No external energy required for operation
•
Low in construction and very low operating costs
•
High reduction in pathogens
•
Can treat high-strength wastewater to high quality effluent
•
Generally reliable and well-functioning
•
Effluent can be reused in aquaculture or for irrigation in agriculture
Disadvantages •
Requires large open land surfaces far away from homes and public spaces
•
Requires expert design and supervision
•
May promote breeding of insects in the pond (e.g. flies, mosquitoes)
•
De-sludging (normally every few years) and correct disposal of the sludge needs to be guaranteed
•
If the effluent is reused, salinity needs to be monitored
•
If the nutrients in the effluent can not be reused (e.g. in agriculture), discharge can cause eutrophication
•
Anaerobic ponds can cause bad odours if poorly designed
•
Not always appropriate for colder climates
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Operation & Maintenance Solids in the raw wastewater, as well as biomass produced, will settle out in first-stage anaerobic ponds and it is common to remove sludge when it has reached half depth in the pond. This usually occurs after 1 up to 10 or 20 years of operation. In certain instances, anaerobic ponds become covered with a thick scum layer, which is thought to be beneficial but not essential, and may give rise to increased fly breeding. To prevent scum formation, excess solids and garbage need to be removed before the wastewater enters the ponds; and pre-treatment (with grease traps) is essential to maintain the ponds. Care should be taken to ensure that plant material does not fall into the ponds as this increases the BOD content of the water. Unless it is the purpose of the pond, vegetation or macrophytes should be removed as it may provide a breeding habitat for mosquitoes and prevent light from penetrating the water column. The WHO does not promote pond systems if appropriate mosquito control measures are not guaranteed. If the water is reused for irrigation, the salinity of the effluent should be controlled regularly in order to prevent negative impact on the soil structure.
Area requirements for different types of natural wastewater treatment processes Technology Free Water Surface Wetlands Subsurface Flow Wetlands Vertical Flow Wetlands Stabilization Ponds Water Hyacinth System Duckweed System
2.4.6 CLARIFICATION PROCESSES
Unit Area Requirement m2/1000 PE 2000-3000 2000-3000 670-1000 800-1200 1000 3500
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2.4.6.1 General Settling processes remove settleable solids by gravity settling either prior to or after biological or chemical treatment and between multiple-stage biological or chemical treatment steps. In larger tanks, mechanical scrapers accumulate the solids at an underflow withdrawal point, whereas in smaller and some older systems, a hopper bottom is used for solids collection. Solids move down the sloped tank bottom by gravity in hopper-bottom tanks. Both circular and rectangular tank shapes are used. A rectangular or square tank uses the land area more efficiently and environmental engineers can save construction costs by nesting units and using common walls. With circular tanks, this cannot be done. Settling tanks are commonly designed based on the overflow rate, the unit volume of flow per unit of time divided by the unit of tank area (m3 per day per m2).
Typical overflow rates are : •
25 m3 per m2 for primary settling
•
40 m3 per m2 for intermediate settling
•
30 to 40 m3 per m2 for final clarifiers after activatedsludge units
•
28 to 40 m3 per m2 for final clarifiers after trickling filters
The detention times for settling range from 1 to 2.5 hr for average flows depending on the processes before or after the settling step.
Traditional chemical precipitation uses either iron or aluminum salts to form a floc, which is then settled. Lime also clarifies. This process step can reduce the SS up to 85%. The accumulated chemical sludge is removed by gravity flow or pumping to conditioning or disposal or both. The chemicals and sewage are flash-mixed in a mixing tank that has only a few minutes detention time followed by 30 to 90 min detention in a flocculation tank that is slowly agitated to aid floc growth. Settling usually follows the flocculation tank.
2.4.6.2 Secondary clarification Process A secondary clarifier must have an adequate clarification capacity to insure that SS discharge requirements are met.
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Also since maintaining proper SRTs is important in the operation of activated-sludge processes, secondary clarifiers in the activated-sludge process must have an adequate thickening capacity to produce the required underflow density for sludge recirculation
Biological solids removal is essentially accomplished by gravity settling. However, biological solids can settle differently depending on their origins and characteristics. The sloughed solids produced from trickling filters and RBCs are generally large and heavy. Therefore, their settling motion is discrete and can be described by Stokesâ&#x20AC;&#x2122; Law. On the other hand, biological flocs produced in activated-sludge processes undergo some flocculation with neighboring particles during the settling process. As flocculation occurs, the mass of particles increases and settles faster. As a result, the settling process is classified as flocculant settling.
The design of most clarifiers falls into one of the following categories: horizontal flow, solids contact, or inclined surface.
Wastewater treatment facilities can increase existing clarifier capacity by installing inclined tubes or parallel plates.
Horizontal flow clarifiers In horizontal-flow clarifiers, sedimentation occurs in specially designed basins. These basins are known as settling tanks, settling basins, sedimentation tanks, sedimentation basins, or clarifiers. They can be rectangular, square, or circular. The most common basins are rectangular tanks and circular basins with a center feed.
Flow patterns in sedimentation basins
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(David H.F. Liu, Bela G. Liptak, 1999)
Basins are usually made of reinforced concrete. The bottom slopes slightly to make sludge removal easier. In rectangular tanks, the bottom slopes toward the inlet end, whereas in circular or square tanks, the bottoms are conical and slope toward the center of the basin.
The selection of any shape depends on the following factors:
• Size of installation • Regulation preference of regulatory authorities • Local site conditions • Preference, experience, and engineering judgement of the designer and plant personnel
A typical circular clarifier
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NIREAS VOLUME 2 [2.4] Overflow in a circular clarifier
229
The advantages and disadvantages of rectangular clarifiers over circular clarifiers follow.
:Advantages
• Less area occupied when multiple units are used • Economic use of common walls with multiple units • Easy covering of units for odor control • Less short circuiting • Lower inlet–outlet losses • Less power consumption for sludge collection and removal mechanisms
Disadvantages • Possible
dead spaces
• Sensitivity to flow surges • Collection equipment restricted in width • Multiple weirs required to maintain low-weir loading rates • High upkeep and maintenance costs of sprockets, chains, and fliers used for sludge removal
Square clarifiers combine the common-wall construction of rectangular basins with the simplicity of circular sludge collectors. These clarifiers have generally not been successful. Because effluent launderers are constructed along the perimeter of basins, the corners have more weir length per degree of radial arc. Thus, the flow is not distributed equally, resulting in large sludge depositions
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in basin corners. Corner sweeps added to circular sludge collection mechanisms to remove sludge settling in the corners have been a source of mechanical difficulty. Because of these problems, few square basins are constructed for water treatment. Circular settling tanks are often chosen because they use a trouble-free, circular sludge removal mechanism and, for small plants, can be constructed at a lower capital cost per unit surface area.
Solids-contact clarifiers In solids-contact clarifiers, incoming solids are brought in contact with a suspended sludge layer near the bottom. This layer acts as a blanket, and the incoming solids agglomerate and remain enmeshed within this blanket. The liquid rises upward while a distinct interface retains the solids below. These clarifiers have hydraulic performance and a reduced retention time for equivalent solids removal in horizontal flow clarifiers.
Inclined-surface clarifiers Inclined-surface basins, also known as a high-rate settler, use inclined trays to divide the depth into shallower sections. Thus, the depth of all particles (and therefore the settling time) is significantly reduced. Wastewater treatment plants frequently use this concept to upgrade the existing overloaded primary and secondary clarifiers. B. and C. clarifiers in next figure, show the operating principle of inclined surface clarifiers.
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Types of clarifiers. A. Circular-solids-contact clarifier. B. Parallel inclined plates in a circular clarifier. C. Tube settlers in a rectangular clarifier. D. Counter-current flow in tubes
(David H.F. Liu, Bela G. Liptak, 1999)
Inclined-surface clarifiers provide a large surface area, reducing clarifier size. No wind effect exists, and the flow is laminar. Many overloaded, horizontal-flow clarifiers are upgraded with this concept.
The major disadvantages of the inclined-surface clarifiers include:
• Long periods of sludge deposits on the inner walls can cause septic conditions. • The effluent quality can deteriorate when sludge deposits slough off. • Clogging of the inner tubes and channels can occur. • Serious short-circuiting can occur when the influent is warmer than the basin temperature.
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MORE Three design variations to the inclined-surface clarifiers are tube settlers, parallel-plate separators and lamella plates.
Tube settlers In these clarifiers, the inclined trays are constructed with thin-wall tubes. These tubes are circular, square, hexagonal, or any other geometric shape and are installed in an inclined position within the basin. The tubes are about 0,5 m long and are produced in modules of about 750 tubes. The incoming flow enters these tubes and flows upward. Solids settle on the inside of the tube and slide down into a hopper. The most popular commercially available tube settler is the steeply inclined tube settler. The angle of inclination is steep enough so that the sludge flows in a countercurrent direction from the suspension flow passing upward through the tube. Thus, solids drop to the bottom of the clarifier and are removed by conventional sludge removal mechanisms. Test results for alum-coagulated sludge indicate that solids remain deposited in the tubes until the angle of inclination increases to 60째 or more from the horizontal.
Parallel Plate Separators Parallel-plate separators have parallel trays covering the entire tank. The operational principles for these separators are the same as those for the tube settlers.
Lamella plates Lamella plates are installed parallel at a 45째 angle. In this design, water and sludge flow in the same direction. The clarified water is returned to the top of the unit by small tubes.
Design Parameters Design criteria for secondary clarifiers depending on the clarifier type
Type of clarifier
Weir Overflow Rate (m3/m_day)
Surface Overflow Rate (m3/m2_day)
Detention time (hr)
Standard basin
250
20-40
2-8
Upflow clarifier
175-350
55-100
1-2
Tube settler
Manufacturer (>> 100)
0,2
(David H.F. Liu, Bela G. Liptak, 1999) Design criteria for secondary clarifiers depending on the main biological treatment process
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Biological Treatment
Air-Activated Sludge
Oxygen-Activated Sludge
Extended Aeration
Trickling Filters
Overflow Rate
Solids Loading
(m3/m2_day)
(kg/m2_day)
18-36 (avg)
90-140 (avg)
45-54 (peak)
230 (peak)
18-36 (avg)
120-160 (avg)
45-54 (peak)
230 (peak)
9-18 (avg)
60-120 (avg)
27-36 (peak)
160 (peak)
18-27 (avg)
70-120 (avg)
45-54 (peak)
190 (peak)
18-36 (avg)
90-140 (avg)
45-54 (peak)
230 (peak)
18-27 (avg)
70-120 (avg)
36-45 (peak)
190 (peak)
Depth (m))
4-6,5
4-6,5
4-6,5
3-5
RBCs Secondary Effluent
Nitrified Effluent
(David H.F. Liu, Bela G. Liptak, 1999)
3-5
3-5
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GLOSSARY Activated carbon—Derived from vegetable or animal materials by roasting in a vacuum furnace. Its porous nature gives it a very high surface area per unit mass—as much as 1000 square meters per gram, which is 10 million times the surface area of 1 gram of water in an open container. Used in adsorption (see definition), activated carbon adsorbs substances that are not or are only slightly adsorbed by other methods. Activated sludge—The solids formed when microorganisms are used to treat wastewater using the activated sludge treatment process. It includes organisms, accumulated food materials, and waste products from the aerobic decomposition process. Adsorption—The adhesion of a substance to the surface of a solid or liquid. Adsorption is often used to extract pollutants by causing them to attach to such adsorbents as activated carbon or silica gel. Hydrophobic (waterrepulsing) adsorbents are used to extract oil from waterways in oil spills. Advanced wastewater treatment—Treatment technology to produce an extremely high-quality discharge. Aeration—The process of bubbling air through a solution, sometimes cleaning water of impurities by exposure to air. Aerobic—Conditions in which free, elemental oxygen is present. Also used to describe organisms, biological activity, or treatment processes that require free oxygen. Agglomeration—Floe particles colliding and gathering into a larger settleable mass. Air gap—The air space between the free-flowing discharge end of a supply pipe and an unpressurized receiving vessel. Algae bloom—A phenomenon whereby excessive nutrients within a river, stream, or lake causes an explosion of plant life that results in the depletion of the oxygen in the water needed by fish and other aquatic life. Algae bloom is usually the result of urban runoff (of lawn fertilizers, etc.). The potential tragedy is that of a "fish kill," where the stream life dies in one mass execution. Alum—Aluminum sulfate, a standard coagulant used in water treatment. Ambient—The expected natural conditions that occur in water unaffected or uninfluenced by human activities. Anaerobic—Conditions in which no oxygen (free or combined) is available. Also used to describe organisms, biological activity, or treatment processes that function in the absence of oxygen. Anoxic—Conditions in which no free, elemental oxygen is present. The only source of oxygen is combined oxygen, such as that found in nitrate compounds. Also used to describe biological activity of treatment processes that function only in the presence of combined oxygen. Aquifer—A water-bearing stratum of permeable rock, sand, or gravel. Aquifer system—A heterogeneous body of introduced permeable and less permeable material that acts as a wateryielding hydraulic unit of regional extent. Artesian water—A well tapping a confined or artesian aquifer in which the static water level stands above the top of the aquifer. The term is sometimes used to include all wells tapping confined water. Wells with water level above the water table are said to have positive artesian head (pressure), and those with water level below the water table negative artesian head. Average monthly discharge limitation—The highest allowable discharge over a calendar month. Average weekly discharge limitation—The highest allowable discharge over a calendar week. Baclcflow—Reversal of flow when pressure in a service connection exceeds the pressure in the distribution main. Backwash—Fluidizing filter media with water, air, or a combination of the two so individual grains can be cleaned of the material that has accumulated during the filter run. Bacteria—Any of a number of one-celled organisms, some of which cause disease. Bar screen—A series of bars formed into a grid used to screen out large debris from influent flow. Base—A substance that has a pH value between 7 and 14. Basin—A groundwater reservoir defined by the overlying land surface and underlying aquifers that contain water stored in the reservoir.
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Beneficial use of water—The use of water for any beneficial purpose. Such uses include domestic, irrigation, recreation, fish and wildlife, fire protection, navigation, power, and industrial uses, among others. The benefit varies from one location to another and by custom. What constitutes beneficial use is often defined by statute or court decisions. Biochemical oxygen demand (BOD5)—The oxygen used in meeting the metabolic needs of aerobic microorganisms in water rich in organic matter. Biosolids'—Solid organic matter recovered from a sewage treatment process and used especially as fertilizer or soil amendment; usually referred to in the plural [Merriam-Webster's Collegiate Dictionary, 10th ed., 1998). Biota—All the species of plants and animals indigenous to a certain area. Boiling point—The temperature at which a liquid boils. The temperature at which the vapor pressure of a liquid equals the pressure on its surface. If the pressure of the liquid varies, the actual boiling point varies. The boiling point of water is 212° Fahrenheit or 100° Celsius. Breakpoint—Point at which chlorine dosage satisfies chlorine demand. Breakthrough—In filtering, when unwanted materials start to pass through the filter. Buffer—A substance or solution that resists changes in pH. Calcium carbonate—Compound that is principally responsible for hardness. Calcium hardness—Portion of total hardness caused by calcium compounds. Carbonaceous biochemical oxygen demand (CBOD)—The amount of biochemical oxygen demand that can be attributed to carbonaceous material. Carbonate hardness—Caused primarily by compounds containing carbonate. Chemical oxygen demand (COD)—The amount of chemically oxidiz-able materials present in the wastewater. Chlorination—Disinfection of water using chlorine as the oxidizing agent. Clarifter—A device designed to permit solids to settle or rise and be separated from the flow. Also known as a settling tank or sedimentation basin. Coagulation—Neutralization of the charges of colloidal matter. Coliform—A type of bacteria used to indicate possible human or animal contamination of water. Combined sewer—A collection system that carries both wastewater and stormwater flows. Comminution—A process to shred solids into smaller, less harmful particles. Composite sample—A combination of individual samples taken in proportion to flow. Connate water—Pressurized water trapped in the pore spaces of sedimentary rock at the time it was deposited. It is usually highly mineralized. Consumptive use—(1) The quantity of water absorbed by crops and transpired or used directly in the building of plant tissue, together with the water evaporated from the cropped area. (2) The quantity of water transpired and evaporated from a cropped area or the normal loss of water from the soil by evaporation and plant transpiration. (3) The quantity of water discharged to the atmosphere or incorporated in the products of the process in connection with vegetative growth, food processing, or an industrial process. Contamination (water)—Damage to the quality of water sources by sewage, industrial waste, or other material. Cross-connection—A connection between a storm-drain system and a sanitary collection system, a connection between two sections of a collection system to handle anticipated overloads of one system, or a connection between drinking (potable) water and an unsafe water supply or sanitary collection system. Daily discharge—The discharge of a pollutant measured during a calendar day or any 24-hour period that reasonably represents a calendar day for the purposes of sampling. Limitations expressed as weight are total mass (weight) discharged over the day; limitations expressed in other units are average measurement of the day. Daily maximum discharge—The highest allowable values for a daily discharge.
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Darcy's law—An equation for the computation of the quantity of water flowing through porous media. Darcy's law assumes that the flow is laminar and that inertia can be neglected. The law states that the rate of viscous flow of homogeneous fluids through isotropic porous media is proportional to, and in the direction of, the hydraulic gradient. Detention time—The theoretical time water remains in a tank at a given flow rate. Dewatering—The removal or separation of a portion of water present in a sludge or slurry. Diffusion—The process by which both ionic and molecular species dissolved in water move from areas of higher concentration to areas of lower concentration. Discharge monitoring report (DMR)—The monthly report required by the treatment plant's National Pollutant Discharge Elimination System (NPDES) discharge permit. Disinfection—Water treatment process that kills pathogenic organisms. Disinfection byproducts (DBPs)—Chemical compounds formed by the reaction of disinfectants with organic compounds in water. Dissolved oxygen (DO)—The amount of oxygen dissolved in water or sewage. Concentrations of less than 5 parts per million (ppm) can limit aquatic life or cause offensive odors. Excessive organic matter present in water because of inadequate waste treatment and runoff from agricultural or urban land generally causes low DO. Dissolved solids—The total amount of dissolved inorganic material contained in water or wastes. Excessive dissolved solids make water unsuitable for drinking or industrial uses. Domestic consumption (use)—Water used for household purposes such as washing, food preparation, and showers. The quantity (or quantity per capita) of water consumed in a municipality or district for domestic uses or purposes during a given period, it sometimes encompasses all uses, including the quantity wasted, lost, or otherwise unaccounted for. Drawdown—Lowering the water level by pumping. It is measured in feet for a given quantity of water pumped during a specified period, or after the pumping level has become constant. Drinking water standards—Established by state agencies, the U.S. Public Health Service, and the Environmental Protection Agency (EPA) for drinking water in the United States". Effluent—Something that flows out, usually a polluting gas or liquid discharge. Effluent limitation—Any restriction imposed by the regulatory agency on quantities, discharge rates, or concentrations of pollutants discharged from point sources into state waters. Energy—In scientific terms, the ability or capacity of doing work. Various forms of energy include kinetic, potential, thermal, nuclear, rotational, and electromagnetic. One form of energy may be changed to another, as when coal is burned to produce steam to drive a turbine, which produces electric energy. Erosion—The wearing away of the land surface by wind, water, ice, or other geologic agents. Erosion occurs naturally from weather or runoff but is often intensified by human land-use practices. Eutrophication—The process of enrichment of water bodies by nutrients. Eutrophication of a lake normally contributes to its slow evolution into a bog or marsh and ultimately to dry land. Eutrophication may be accelerated by human activities, thereby speeding up the aging process. Evaporation—The process by which water becomes a vapor at a temperature below the boiling point. Facultative—Organisms that can survive and function in the presence or absence of free, elemental oxygen. Fecal coliform—The portion of the coliform bacteria group that is present in the intestinal tracts and feces of warmblooded animals. Field capacity—The capacity of soil to hold water. It is measured as the ratio of the weight of water retained by the soil to the weight of the dry soil. Filtration—The mechanical process that removes particulate matter by separating water from solid material, usually by passing it through sand. Floe—Solids that join to form larger particles that will settle better. Flocculation—Slow mixing process in which particles are brought into contact, with the intent of promoting their agglomeration. Flume—A flow rate measurement device. Fluoridation—Chemical addition to water to reduce incidence of dental caries in children.
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Food-to-microorganisms (F/M) ratio—An activated sludge process control calculation based on the amount of food (BOD5 or COD) available per pound of mixed liquor volatile suspended solids. Force main—A pipe that carries wastewater under pressure from the discharge side of a pump to a point of gravity flow downstream. Grab sample—An individual sample collected at a randomly selected time. Graywater—Water that has been used for showering, clothes washing, and faucet uses. Kitchen sink and toilet water is excluded. This water has excellent potential for reuse as irrigation for yards. Grit—Heavy inorganic solids, such as sand, gravel, eggshells, or metal filings. Groundwater—The supply of freshwater found beneath the Earth's surface (usually in aquifers) often used for supplying wells and springs. Because groundwater is a major source of drinking water, concern is growing over areas where leaching agricultural or industrial pollutants or substances from leaking underground storage tanks (USTs) are contaminating groundwater. Groundwater hydrology—The branch of hydrology that deals with groundwater: its occurrence and movements, its replenishment and depletion, the properties of rocks that control groundwater movement and storage, and the methods of investigation and use of groundwater. Groundwater recharge—The inflow to a groundwater reservoir. Groundwater runoff—A portion of runoff that has passed into the ground, has become groundwater, and has been discharged into a stream channel as spring or seepage water. Hardness—The concentration of calcium and magnesium salts in water. Head loss—Amount of energy used by water in moving from one point to another. Heavy metals—Metallic elements with high atomic weights, such as mercury, chromium, cadmium, arsenic, and lead. They can damage living things at low concentrations and tend to accumulate in the food chain. Holding pond—A small basin or pond designed to hold sediment-laden or contaminated water until it can be treated to meet water quality standards or used in some other way. Hydraulic cleaning—Cleaning pipe with water under enough pressure to produce high water velocities. Hydraulic gradient—A measure of the change in groundwater head over a given distance. Hydraulic head—The height above a specific datum (generally sea level) that water will rise in a well. Hydrologic cycle (water cycle)—The cycle of water movement from the atmosphere to the Earth and back to the atmosphere through various processes. These processes include precipitation, infiltration, percolation, storage, evaporation, transpiration, and condensation. Hydrology—The science dealing with the properties, distribution, and circulation of water. Impoundment—A body of water, such as a pond confined by a dam, dike, floodgate, or other barrier, that is used to collect and store water for future use. Industrial wastewater—Wastes associated with industrial manufacturing processes. Infiltration—The gradual downward flow of water from the surface into soil material. Infi.ltrationlinfl.ow—Extraneous flows in sewers; simply, inflow is water discharged into sewer pipes or service connections from such sources as foundation drains, roof leaders, cellar and yard area drains, cooling water from air conditioners, and other clean-water discharges from commercial and industrial establishments. Defined by Metcalf & Eddy (2003) as follows: Delayed inflow—Stormwater requiring several days or more to drain through the sewer system. This category can include the discharge of sump pumps from cellar drainage as well as the slowed entry of surface water through manholes in ponded areas. DirectJlow—Those types of inflow that have a direct stormwater runoff connection to the sanitary sewer and cause an almost immediate increase in wastewater flows. Possible sources are roof leaders, yard and areaway drains, manhole covers, cross-connections from storm drains and catch basins, and combined sewers. Infiltration—Water entering the collection system through cracks, joints, or breaks. Steady inflow—Water discharged from cellar and foundation drains, cooling water discharges, and drains from springs and swampy areas. This type of inflow is steady and is identified and measured along with infiltration.
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Total inflow—The sum of the direct inflow at any point in the system plus any flow discharged from the system upstream through overflows, pumping station bypasses, and the like. Influent—Wastewater entering a tank, channel, or treatment process. Inorganic chemical/compounds—Chemical substances of mineral origin, not of carbon structure. These include metals such as lead, iron (ferric chloride), and cadmium. Ion exchange process—Used to remove hardness from water. Jar test—Laboratory procedure used to estimate proper coagulant dosage. Langelier saturation index (LI)—A numerical index that indicates whether calcium carbonate will be deposited or dissolved in a distribution system. Leaching—The process by which soluble materials in the soil such as nutrients, pesticide chemicals, or contaminants are washed into a lower layer of soil or are dissolved and carried away by water. License—A certificate issued by the State Board of Waterworks/ Wastewater Works Operators authorizing the holder to perform the duties of a wastewater treatment plant operator. Lift station—A wastewater pumping station designed to "lift" the wastewater to a higher elevation. A lift station normally employs pumps or other mechanical devices to pump the wastewater and discharges into a pressure pipe called a force main. Maximum contaminant level (MCL)—An enforceable standard for protection of human health. Mean cell residence time (MCRT)—The average length of time a mixed liquor suspended solids particle remains in the activated sludge process. May also be known as sludge retention time. Mechanical cleaning—Clearing pipe by using equipment (bucket machines, power rodders, or hand rods) that scrapes, cuts, pulls, or pushes the material out of the pipe. Membrane process—A process that draws a measured volume of water through a filter membrane with small enough openings to take out contaminants. Metering pump—A chemical solution feed pump that adds a measured amount of solution with each stroke or rotation of the pump. Milligrams per liter (mg/L)—A measure of concentration equivalent to parts per million (ppm). Mixed liquor suspended solids—The suspended solids concentration of the mixed liquor. Mixed liquor volatile suspended solids (MLVSS)—The concentration of organic matter in the mixed liquor suspended solids. Nephelometric turbidity unit (NTU)—Indicates amount of turbidity in a water sample. Nitrogenous oxygen demand (NOD)—A measure of the amount of oxygen required to biologically oxidize nitrogen compounds under specified conditions of time and temperature. Nonpoint-source (NPS) pollution—Forms of pollution caused by sediment, nutrients, and organic and toxic substances originating from land-use activities that are carried to lakes and streams by surface runoff. Nonpoint-source pollution occurs when the rate of materials entering these water bodies exceeds natural levels. NPDES permit—National Pollutant Discharge Elimination System permit, which authorizes the discharge of treated wastes and specifies the conditions that must be met for discharge. Nutrients—Substances required to support living organisms. Usually refers to nitrogen, phosphorus, iron, and other trace metals. Organic chemicals/compounds—Animal- or plant-produced substances containing mainly carbon, hydrogen, and oxygen, such as benzene and toluene. Parts per million (ppm)—The number of parts by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations. Large concentrations are expressed in percentages. Pathogenic—Disease causing. A pathogenic organism is capable of causing illness. Percolation—The movement of water through the subsurface soil layers, usually continuing downward to the groundwater or water table reservoirs. 1 '•I
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pH—A way of expressing both acidity and alkalinity on a scale of 0 to 14, with 7 representing neutrality; numbers less than 7 indicate increasing acidity, and numbers greater than 7 indicate increasing alkalinity. Photosynthesis—A process in green plants in which water, carbon dioxide, and sunlight combine to form sugar. Piezometric surface—An imaginary surface that coincides with the hydrostatic pressure level of water in an aquifer. Point-source pollution—A type of water pollution resulting from discharges into receiving waters from easily identifiable points. Common point sources of pollution are discharges from factories and municipal sewage treatment plants. Pollution—The alteration of the physical, thermal, chemical, or biological quality of, or the contamination of, any water in the state that renders the water harmful, detrimental, or injurious to humans, animal life, vegetation, property or public health, safety, or welfare, or impairs the usefulness or the public enjoyment of the water for any lawful or reasonable purpose. Porosity—That part of a rock that contains pore spaces without regard to size, shape, interconnection, or arrangement of openings. It is expressed as percentage of total volume occupied by spaces. Potable water—Water satisfactorily safe for drinking purposes from the standpoint of its chemical, physical, and biological characteristics. Precipitate—A deposit on the Earth of hail, rain, mist, sleet, or snow; the common process by which atmospheric water becomes surface or subsurface water. The term precipitation is also commonly used to designate the quantity of water precipitated. Preventive maintenance (PM)—Regularly scheduled servicing of machinery or other equipment using appropriate tools, tests, and lubricants. This type of maintenance can prolong the useful life of equipment and machinery and increase its efficiency by detecting and correcting problems before they cause a breakdown of the equipment. Purveyor—An agency or person that supplies potable water. Radon—A radioactive, colorless, odorless gas that occurs naturally in the earth. When trapped in buildings, concentrations build up and can cause health hazards such as lung cancer. Recharge—The addition of water into a groundwater system. Reservoir—A pond, lake, tank, or basin (natural or human made) where water is collected and used for storage. Large bodies of groundwater are called groundwater reservoirs; water behind a dam is also called a reservoir of water. Reverse osmosis—Process in which almost pure water is passed through a semipermeable membrane. Return activated sludge solids (RASS)—The concentration of suspended solids in the sludge flow being returned from the settling tank to the head of the aeration tank. River basin—A term used to designate the area drained by a river and its tributaries. Sanitary wastewater—Wastes discharged from residences and from commercial, institutional, and similar facilities that include both sewage and industrial wastes. Schmutzdecke—Layer of solids and biological growth that forms on top of a slow sand filter, allowing the filter to remove turbidity effectively without chemical coagulation. Scum—The mixture of floatable solids and water removed from the surface of the settling tank. Sediment—Transported and deposited particles derived from rocks, soil, or biological material. Sedimentation—A process that reduces the velocity of water in basins so suspended material can settle out by gravity. Seepage—The appearance and disappearance of water at the ground surface. Seepage designates movement of water in saturated material. It differs from percolation, which is predominantly the movement of water in unsaturated material. Septic tanks—Used to hold domestic wastes when a sewer line is not available to carry them to a treatment plant. The wastes are piped to underground tanks directly from a home or homes. Bacteria in the wastes decompose some of the organic matter, the sludge settles on the bottom of the tank, and the effluent flows out of the tank into the ground through drains. Settleability—A process control test used to evaluate the settling characteristics of the activated sludge. Readings taken at 30 to 60 minutes are used to calculate the settled sludge volume (SSV) and the sludge volume index (SVI).
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Settled sludge volume (SSV)—The volume (in percent) occupied by an activated sludge sample after 30 to 60 minutes of settling; normally written as SSV with a subscript to indicate the time of the reading used for calculation (SSV60 or SSV30). Sludge—The mixture of settleable solids and water removed from the bottom of the settling tank. Sludge retention time (SRT)—See mean cell residence time. Sludge volume index (SVI)—A process control calculation used to evaluate the settling quality of the activated sludge; requires the SSV30and mixed liquor suspended solids test results to calculate. Soil moisture (soil water)—Water diffused in the soil. It is found in the upper part of the zone of aeration from which water is discharged by transpiration from plants or by soil evaporation. Specific heat—The heat capacity of a material per unit mass. The amount of heat (in calories) required to raise the temperature of 1 gram of a substance 1°C; the specific heat of water is 1 calorie. Storm sewer—A collection system designed to carry only stormwater runoff. Stormwater—Runoff resulting from rainfall and snowmelt. Stream—A general term for a body of flowing water. In hydrology, the term is generally applied to the water flowing in a natural channel as distinct from a canal. More generally, it is applied to the water flowing in any channel, natural or artificial. Some types of streams include: (1) ephemeral, a stream that flows only in direct response to precipitation and whose channel is at all times above the water table; (2) intermittent or seasonal, a stream that flows only at certain times of the year when it receives water from springs, rainfall, or surface sources such as melting snow; (3) perennial, a stream that flows continuously; (4) gaining, an effluent stream or reach of a stream that receives water from the zone of saturation; (5) insulated, a stream or reach of a stream that is separated from the zones of saturation by an impermeable bed so it neither contributes water to the zone of saturation nor receives water from it; (6) losing, an influent stream or reach of a stream that contributes water to the zone of saturation; and (7) perched, either a losing stream or an insulated stream that is separated from the underlying groundwater by a zone of aeration. Supernatant—The liquid standing above a sediment or precipitate. Surface tension—The free energy produced in a liquid surface by the unbalanced inward pull exerted by molecules underlying the layer of surface molecules. Surface water—Lakes, bays, ponds, impounding reservoirs, springs, rivers, streams, creeks, estuaries, wetlands, marshes, inlets, canals, gulfs inside the territorial limits of the state, and all other bodies of surface water, natural or artificial, inland or coastal, fresh or salt, navigable or nonnavigable, and including the beds and banks of all watercourses and bodies of surface water that are wholly or partially inside or bordering the state or subject to the jurisdiction of the state; except that waters in treatment systems which are authorized by state or federal law, regulation, or permit, and which are created for the purpose of water treatment, are not considered to be waters in the state. Thermal pollution—The degradation of water quality by the introduction of a heated effluent. Primarily the result of the discharge of cooling waters from industrial processes (particularly from electrical power generation); waste heat eventually results from virtually every energy conversion. Titrant—A solution of known strength of concentration; used in titration. Titration—A process whereby a solution of known strength (titrant) is added to a certain volume of treated sample containing an indicator. A color change shows when the reaction is complete. Titrator—An instrument, usually a calibrated cylinder (tube-form), used in titration to measure the amount of titrant being added to the sample. Total dissolved solids—The amount of material (inorganic salts and small amounts of organic material) dissolved in water and commonly expressed as a concentration in terms of milligrams per liter. Total suspended solids (TSS)—Total suspended solids in water, commonly expressed as a concentration in terms of milligrams per liter. Toxicity—The occurrence of lethal or sublethal adverse effects on representative sensitive organisms due to exposure to toxic materials. Adverse effects caused by conditions of temperature, dissolved oxygen, or nontoxic dissolved substances are excluded from the definition of toxicity. Transpiration—The process by which water vapor escapes from the living plant, principally the leaves, and enters the atmosphere.
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Vaporization—The change of a substance from a liquid or solid state to a gaseous state. Volatile organic compound (VOC)—Any organic compound that participates in atmospheric photochemical reactions except for those designated by the EPA Administrator as having negligible photochemical reactivity. Waste activated sludge solids (WASS)—The concentration of suspended solids in the sludge being removed from the activated sludge process. Wastewater—The water supply of a community after it has been soiled by use. Wastewater can also be defined as a community's spent water. Wastewater contains the impurities that were present when the water was obtained (water picks up impurities as it travels) and any impurities added through human uses. The term sewage is often used to refer to wastewater but is more properly applied to domestic or household wastewater. As mentioned, raw wastewater entering a treatment plant (or unit process) is referred to as influent. The treated water discharged from a wastewater treatment plant (or unit process) is known as effluent. Water cycle—The process by which water travels in a sequence from the air (condensation) to the Earth (precipitation) and returns to the atmosphere (evaporation). It is also referred to as the hydro-logic cycle. Water quality—A term used to describe the chemical, physical, and biological characteristics of water with respect to its suitability for a particular use. Water quality standard—Apian for water quality management containing four major elements: water use, criteria to protect users, implementation plans, and enforcement plans. An antidegradation statement is sometimes prepared to protect existing high-quality waters. Water supply—Any quantity of available water. Waterborne disease—A disease caused by a microorganism that is carried from one person or animal to another by water. Watershed—The area of land that contributes surface runoff to a given point in a drainage system. Weir—A device used to measure wastewater flow. Zone of aeration—A region in the Earth above the water table. Water in the zone of aeration is under atmospheric pressure and would not flow into a well. Zoogleal slime—The biological slime that forms on fixed-film treatment devices. It contains a wide variety of organisms essential to the treatment process.
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ASSIGNMENTS SECTION QUESTIONS
1. Which are the Two basic processes, used in conventional secondary biological treatment ?
2. Extended aeration is a Suspended growth biological treatment process. True or False?
3. Medium bubbles systems, have in general higher efficiency in oxygen transfer rather than fine bubbles systems. True or False?
4. In which way sludge age can be increased retention time in an activated sludge process ?
5. What F/M ratio means?
6. What is called mixed liquor ?
in order to be greater than the hydraulic
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7. Step aeration furnishes more uniform oxygen demand throughout the aeration tank. True or False?
8. Completely mixed activated-sludge system operates at a high SS concentration in the mixed liquor. True or False?\
9. In extended aeration treatment process aeration basin is generally much smaller comparing to conventional activated-sludge process. True or False?
10. Primary clarifier in extended aeration treatment process is not necessary. True or False?
11. Extended aeration treatment process requires less aeration, in terms of quantity, than conventional treatment process. True or False?
12. In which way Extended aeration treatment process reduces or eliminates the requirement for disposing excess sludge ?
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13. Operation of an activated sludge plant on the basis of contact stabilization requires two aeration tanks. True or False?
14. Oxygen-activated-sludge process permits higher organic loading than conventional treatment process. True or False?
15. How many reactors are required (at least) at the SBR activated sludge process and why?
16. Briefly refer to the stages of a typical treatment cycle in a sequencing batch reactor (SBR) process.
17. Oxidation ditch process does not require primary treatment. True or False?
18. Membrane bioreactor (MBR) process eliminates the need for sedimentation and filtration processes. True or False?
NIREAS VOLUME 2 [2.4] 245 19. Which are the two basic configurations for membrane bioreactor systems (MBR)?
20. Which secondary treatment process shows the highest effluent quality compared to others?
21. What is Hydraulic (Liquid) Retention Time (HRT or LRT) in an activated sludge process?
22. What is Solids Retention Time (SRT) in an activated sludge process?
23. Which activated sludge process shows the highest values in SRT?
24. What is the minimum threshold for DO concentration in effluent from an aeration tank?
25. High-rate activated sludge plant or extended aeration plant produces more sludge for every kg of BOD removed?
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26. Report at least 3 ways to prevent sludge bulking in activated sludge processes
27. Reducing the sludge retention time in the secondary clarifier is a correct measure in order to prevent sludge rising in activated sludge processes. True or False?
28. During the summer, the activated sludge plant may operate more satisfactorily in a certain loading range and air rates, than in winter. True or False?
29. For fixed-film processes such as trickling filters, the operational costs are lower than for activated sludge processes. True or False?
30. Trickling filter is an suspended-growth, biological process. True or False?
31. Trickling filter is an aerobic process. True or False?
NIREAS VOLUME 2 [2.4] 247 32. Trickling filters are in general able to withstand shock loads of toxic inputs. True or False?
33. Is pretreatment in trickling filters necessary?
34. Report at least 3 attached-growth processes
35. The efficiency of a trickling filter unit is reduced during periods of icing. True or False?
36. What is the main limiting factor in the construction of an ISF?
37. Is pretreatment in ISFs necessary?
38. What is the main difference between an ISF and a RSF?
NIREAS VOLUME 2 [2.4] 248 39. RSF can receive greater hydraulic loadings per unit of area, comparing to an ISF. True or False?
40. ISFs and RSFs can produce a high-quality effluent, even in tertiary level. True or False?
41. Refer to at least 3 limitations that a Textile filter overcomes comparing to ISFs & RSFs.
42. A Textile filter system is much simpler in operation and maintenance comparing to an activated sludge process. True or False?
43. A Textile filter system canâ&#x20AC;&#x2122;t produce a high-quality effluent, equivalent to tertiary level. True or False?
44. A Textile filter an ISF or an RSF system, when right designed, produce negligible or almost zero amounts of excess sludge. True or False?
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45. If the appearance of an Contactor is black, and odors that are not normal do occur, what would probably be the cause?
46. In an MBBR system, both attached and suspended growth processes do occur. True or False?
47. The fixed biomass increases the Sludge Retention Time, and reduces the sludge production compared to simple suspended growth systems. True or False?
48. MBBR process provides an advantage for plant upgrading by reducing the solids loading on existing clarifiers. True or False?
49. Where the biofilm formation takes place in a MBBR system?
50. Report at least 3 major advantages of anaerobic treatment over aerobic treatment
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51. Anaerobic excess sludge in the secondary clarifier is more difficult to treat than excess sludge in the activated-sludge process. True or False?
52. Report the three different types of constructed wetlands (CWs)
53. The capital costs of constructed wetlands are highly dependent on the cost of land. True or False?
54. Is pre-treatment in a constructed wetland necessary, and if it is, whats the purpose?
55. Free-surface CWâ&#x20AC;&#x2122;s normally require more surface than a subsurface constructed wetlands. True or False?
56. No excess sludge is produced from constructed wetlands (CWs). True or False?
57. Which type of constructed wetland is most susceptible to the mosquito problem?
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58. Which type of constructed wetland
requires the less area for the same hydraulic and
organic loadings?
59. Which is the most cost-effective (semi-)centralised wastewater treatment technology for the removal of pathogenic microorganisms?
60. How a Wastewater treatment facility can increase existing clarifier capacity?
61. Report at least 3 advantages of rectangular clarifiers over circular clarifiers
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SUGGESTED ANSWARS: 1. fixed-bed (attached growth) and fluid-bed (suspended growth) 2. True 3. False 4. Recycling of a large portion of the biomass, (activated sludge). This makes the mean cell residence time (i.e., sludge age) much greater than the hydraulic retention time. 5. F/M ratio means food to microorganisms ratio. It is the amount of biodegradable organic material available to an amount of microorganisms per unit of time 6. The mixture of primary clarifier overflow and activated sludge 7. True 8. True 9. False 10. True 11. True 12. Extended aeration treatment process reduces or eliminates the requirement for disposing excess sludge by operating in the endogenous respiration phase with the SRT maintained in the range of 20â&#x20AC;&#x201C;40 days. 13. True 14. True 15. SBR activated sludge process requires at least two reactors. One reactor is filling and reacting while the other is settling and decanting 16. Fill, react, settle, draw, and idle 17. True 18. True 19. (1) the integrated bioreactor that uses membranes immersed in the bioreactor and (2) the recirculated MBR in which the mixed liquor circulates through a membrane module situated outside the bioreactor 20. Membrane bioreactor process (MBR) 21. Hydraulic retention time is the average time spent by the influent liquid in the aeration tank of the activated sludge process 22. Solids Retention Time (SRT) or Sludge age is the mean residence time of microorganisms in the system 23. Extended aeration process 24. 1 mg/L 25. High-rate activated sludge plant
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26. - Increase the sludge age - Maintain DO levels at a minimum threshold of 2 mg /L - Reduce the return sludge rate - thicken the return sludge solids concentration by coagulation - Control by chlorination of return sludge 27. True 28. True 29. True 30. False 31. True 32. True 33. Yes 34. - Trickling filters - RBCs - PBFs- Sand/Gravel Filters - PBFs- Trickling Filters 35. True 36. The large land area required 37. Yes 38. Main difference between an ISF and a RSF is that the effluent from a RSF recirculates comparing to effluent from an ISF which is passes one time through media filter. 39. True 40. True 41. - land area - media quality - installation quality - serviceability 42. True 43. False 44. True 45. This could be an indication of solids or BOD overloading 46. True 47. True 48. True 49. In biofilm carriers, inside the aeration tank
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50. - capital costs and power consumption are lower - Methane gas produced in anaerobic processes provides an economically valuable end product - Higher influent organic loading is possible for anaerobic systems - Less excess sludge is produced in anaerobic systems 51. True 52. - Free-surface constructed wetlands (FWS) - Horizontal flow constructed wetlands (HF) - Vertical flow constructed wetlands (VF) 53. True 54. Yes. The purpose is to avoid clogging and the excess accumulation of solids and garbage. 55. True 56. True 57. FWS 58. VF 59. Stabilisation ponds (WSPs) 60. By installing inclined tubes or parallel plates. 61. • Less area occupied when multiple units are used • Economic use of common walls with multiple units • Easy covering of units for odor control • Less short circuiting • Lower inlet–outlet losses • Less power consumption for sludge collection and removal mechanisms
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