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2.7.1 Biosolids Production 2.7.1 Preatreatment Grinding Screening Degritting Blending Storage 2.7.2 Thickening Flotation thickening Gravity thickening Centrifugal thickening Gravity – belt thickening Rotary-Drum Thickening 2.7.3 Stabilization Chemical stabilization Stabilization via anaerobic digestion Stabilization via aerobic digestion Autothermal thermophilic aerobic digestion (ATAD) Composting Comparison of stabilization methods 2.7.4 Conditioning Chemical conditioning Thermal conditioning 2.7.5 Mixing 2.7.6 Sludge dewatering

NIRAS VOLUME 2 [2.7] 2 Vacuum filtration Pressure filtration Centrifuge dewatering Hydroclones Drying beds Reed beds Sludge lagoons 2.7.7 Dry sludge volume reduction Heat drying Incineration 2.7.8 Sludge handling Transfer of sludge Disposal in water Disposal in land Application as soil conditioner 2.7.9 Calculations 2.7.10 General flow diagramm 2.7.11 Glossary

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2.7 BIOSOLIDS HANDLING The wastewater treatment unit processes described to this point remove solids and BOD from the waste stream before the liquid effluent is discharged to its receiving waters. What remains to be disposed of is a mixture of solids and wastes, called process residuals, more commonly referred to as sludge or biosolids. Disposal of biosolids is becoming an environmental concern. New treatments, disinfection processes, and disposal methods are available to help systems comply with increased regulations. The most costly and complex aspect of wastewater treatment can be the collection, processing, and disposal of sludge, because the quantity of sludge produced may be as high as 2% of the original volume of wastewater, depending somewhat on the treatment process being used. Because sludge can be as much as 97% water content and because the cost of disposal will be related to the volume of sludge being processed, one of the primary purposes or goals of sludge treatment (along with stabilizing it so it is no longer objectionable or environmentally damaging) is to separate as much of the water from the solids as possible. Sludge treatment methods may be designed to accomplish both of these purposes.

Note: Sludge treatment methods are generally divided into three major categories: thickening, stabilization, and dewatering.

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2.7.1 Biosolids Production Sludge forms initially as a 3 to 7% suspension of solids; with each person typically generating about 10-15 L of sludge per week, the total quantity generated each day, week, month, and year is significant. Because of the volume and nature of the material, sludge management is a major factor in the design and operation of all water pollution control plants.

Note: Wastewater solids account for more than half of the total costs in a typical secondary treatment plant.

Wastewater sludge is generated in primary, secondary, and chemical treatment processes.

In primary treatment, the solids that float or settle are removed. The floatable material makes up a portion of the solid waste known as scum. Scum is not normally considered sludge; however, it should be disposed of in an environmentally sound way. The settleable material that collects on the bottom of the clarifler is known as primary sludge. Primary sludge can also be referred to as ran' sludge because it has not undergone decomposition. Raw primary sludge from a typical domestic facility is quite objectionable and has a high percentage of water, two characteristics that make handling difficult.

Solids not removed in the primary clarifier are carried out of the primary unit. These solids are known as colloidal suspended solids. The secondary treatment system (e.g., trickling filter, activated sludge) is designed to change those colloidal solids into settleable solids that can be removed. Once in the settleable form, these solids are removed in the secondary clarifier. The sludge at the bottom of the secondary clarifier is called secondary sludge. Secondary sludges are

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light and fluffy and more difficult to process than primary sludges—in short, secondary sludges do not dewater well.

The addition of chemicals and various organic and inorganic substances prior to sedimentation and clarification may increase the solids capture and reduce the amount of solids lost in the effluent. This chemical addition results in the formation of heavier solids, which trap the colloidal solids or convert dissolved solids to settleable solids. The resultant solids are known as chemical sludges.

Sources of solids from conventional wastewater-treatment plants Unit operation or process

Types of solids


Coarse solids

Grit removal

Grit and scum


Grit and scum

Primary sedimentation

Primary solids and scum

Biological treatment

Suspended solids

Secondary sedimentation

Secondary biosolids and scum

Solids processing facilities

Solids, compost and ashes

(Eddy, 1999)

Characteristics of solids and sludge produced during wastewater treatment Solids or sludge Description Screenings Grit


Primary sludge

Sludge from chemical precipitation

Screenings include all types of organic and inorganic materials large enough to be removed on bar racks. The organic content varies, depending on the nature of the system and the season of the year Grit is usually made up of the heavier inorganic solids that settle with relatively high velocities. Depending on the operating conditions, grit may also contain significant amounts of organic matter, especially fats and grease Scum consists of the floatable materials skimmed from the surface of primary and secondary settling tanks and from grit chambers and chlorine contact tanks, if so equipped. Scum may contain grease, vegetable and mineral oils, animal fats, waxes, soaps, food wastes, vegetable and fruit skins, hair, paper and cotton, cigarette tips, plastic materials, condoms, grit particles, and similar materials. The specific gravity of scum is less than 1.0 and usually around 0.95 Sludge from primary settling tanks is usually gray and slimy and, in most cases, has an extremely offensive odor. Primary sludge can be readily digested under suitable conditions of operation Sludge from chemical precipitation with metal salts is usually dark in color, though its surface precipitation may be red if it contains much iron. Lime sludge is grayish-brown. The odor of chemical sludge may be objectionable, but is not as objectionable as the odor of primary sludge. While chemical sludge is somewhat slimy, the hydrate of iron or aluminum in it makes it gelatinous. If the sludge is left in the tank, it undergoes

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Activated sludge

Trickling-filter sludge

Aerobically digested biosolids

Anaerobically digested biosolids


decomposition similar to primary sludge, but at a slower rate. Substantial quantities of gas may be given off and the sludge density increased by long residence times in storage Activated sludge generally has a brown flocculent appearance. If the color is dark, the sludge may be approaching a septic condition. If the color is lighter than usual, there may have been underaeration with a tendency for the solids to settle slowly. Sludge in good condition has an inoffensive “earthy� odor. The sludge tends to become septic rapidly and then has a disagreeable odor of putrefaction. Activated sludge will digest readily alone or when mixed with primary sludge Humus sludge from trickling filters is brownish, flocculent, and relatively inoffensive when fresh. It generally undergoes decomposition more slowly than other undigested sludges. When trickling-filter sludge contains many worms, it may become inoffensive quickly. Trickling-filter sludge digests readily Aerobically digested biosolids are brown to dark brown and have a flocculent appearance. biosolids The odor of aerobically digested sludge is not offensive; it is often characterized as musty. Well-digested aerobic sludge dewaters easily on drying beds Anaerobically digested biosolids are dark brown to black and contain an exceptionally large digested biosolids quantity of gas. When thoroughly digested, they are not offensive, the odor being relatively faint and like that of hot tar, burnt rubber, or sealing wax. Primary sludge, when anaerobically digested, produces about twice as much methane gas as does waste activated sludge. When drawn off onto porous beds in thin layers, the solids first are carried to the surface by the entrained gases, leaving a sheet of comparatively clear water. The water drains off rapidly and allows the solids to sink down slowly onto the bed. As the solids dry, the gases escape, leaving a well-cracked surface with an odor resembling that of garden loam Composted solids are usually dark brown to black, but the color may vary if bulking agents such as recycled compost or wood chips have been used in the composting process. The odor of well-composted solids is inoffensive and resembles that of commercial garden-type soil conditioners

(Eddy, 1999) Typical Water Content of Sludges Percent Moisture Water Treatment Process kg Water/kg Sludge Solids of Sludge, % Generated Primary sedimentation 95 19 Trickling filter Humus, low rate



Humus, high rate





Activated sludge

(Frank R. Spellman, 2009)

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2.7.2 Preatreatment The pretreatment of sludge is often necessary before dewatering or thickening can take place. It includes degritting, grinding, screening, blending and storage prior to further treatment.


Sludge grinding involves shearing of large sludge solids into smaller particles. This method is used to prevent problems with operation of downstream processes. Inline grinders reduce cleaning and maintenance down time of equipment. The grinders can shear sludge solids to 6 to 13 mm, depending on design requirements.

Inline grinder (upper left); incisors of a grinder (lower left); typical grinder installation (right)

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Sludge screening is an alternative to grinding and takes place in order to remove fibrous materials from sludge. Screen openings normally range from 3 to 6 mm although openings up to 10 mm can be used.

Sludge screenings press: (a) schematic and (b) view of a large installation.

(Eddy, 1999)

Sludge screenings press

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In some plants where separate grit-removal facilities are not used ahead of the primary sedimentation tanks, or where the grit-removal facilities are not adequate to handle peak flows and peak grit loads, it may be necessary to remove the grit before further processing of the sludge. As a result, there is reduced wear on influent pumping systems and primary sludge pumping, piping and thickening systems Where further thickening of the primary sludge is desired, a practical consideration is sludge degritting. The most effective method of degritting sludge is through the application of centrifugal forces in a flowing system to achieve separation of the grit particles from the organic sludge. Such separation is achieved through the use of cyclone degritters, which have no moving parts. The sludge is applied tangential to a cylindrical feed section, thus imparting a centrifugal force. The heavier grit particles move to the outside of the cylinder section and are discharged through a conical feed section. The organic sludge is discharged through a separate outlet.


Sludge blending involves homogenization of all sludge streams . Sludge is generated in primary, secondary, and advanced wastewater-treatment processes. Primary sludge consists of settleable solids carried in the raw wastewater. Secondary sludge consists of biological solids as well as additional settleable solids. Sludge produced in the advanced wastewater may consist of biological and chemical solids. Sludge is blended to produce a uniform mixture to downstream operations and processes. Uniform mixtures are most important in short-detention-time systems, such as sludge dewatering, heat treatment, and incineration.


Before sludge undergoes treatment such as dewatering or thickening, it must be stored and pretreated. Sludge storage is an important, integral part of every wastewater sludge treatment and disposal system. Sludge storage provides many benefits including equalization of sludge flow to downstream processes, allowing sludge accumulation during times of non-operation of sludgeprocessing facilities, and allowing a uniform feed rate that enhances thickening, conditioning, and dewatering operations.

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2.7.3 Thickening Thickening is practiced in order to remove as much water as possible before final dewatering of the sludge. It is usually accomplished by floating the solids to the top of the liquid (floatation) or by allowing the solids to settle to the bottom (gravity thickening). Other method of thickening are by centrifuge, pressure filtration, or vacuum filtration. These processes offer a low-cost means of reducing the volumetric loading of sludge to subsequent steps.

Flotation thickening

Flotation thickening is used most efficiently for waste sludges from suspended-growth biological treatment process, such as the activated sludge process. Recycled water from the flotation thickener is aerated under pressure. During this time, the water absorbs more air than it would

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under normal pressure. The recycled flow together with chemical additives (if used) are mixed with the flow. When the mixture enters the flotation thickener, the excess air is released in the form of fine bubbles. These bubbles become attached to the solids and lift them toward the surface. The accumulation of solids on the surface is called the float cake. As more solids are added to the bottom of the float cake, it becomes thicker and water drains from the upper levels of the cake. The solids are then moved up an inclined plane by a scraper and discharged. The supernatant leaves the tank below the surface of the float solids and is recycled or returned to the wastestream for treatment. Typically, flotation thickener performance is 3 to 5% solids for waste activated sludge with polymer addition and 2 to 4% solids without polymer addition.

Dissolved Air Flotation Thickener

(John M. Stubbart, 2006 )

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Dissolved Air Flotation System, schematic drawing

Dissolved Air Flotation System operation Dissolved Air Flotation System installation

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Gravity thickening

Gravity thickening has been widely used on primary sludge for many years because of its simplicity and inexpensiveness. In gravity thickening, sludge is concentrated by the gravityinduced settling and compaction of sludge solids. It is essentially a sedimentation process. Sludge flows into a tank that is similar to the circular clarifiers used in primary and secondary sedimentation.

The solids in the sludge settle to the bottom where a scraping mechanism removes them to a hopper. The type of sludge being thickened has a major effect on performance. The best results can be achieved with primary sludge. Purely primary sludge can be thickened from 1-3% to 10% solids, 2 to 4% solids from waste activated sludge, 7 to 9% solids from trickling filter residuals, and 4 to 9% from combined primary and secondary residuals As the proportion of activated (secondary) sludge increases, the thickness of settled solids decreases. There are various designs for sludge thickeners. Scraping mechanism in a gravity thickener

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Schematic diagram of a gravity thickener: (a) plan and (b) section

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(Eddy, 1999)

Performance of Conventional Gravity Thickening

Type of Solids

Primary (PRI)

Feed, % total solids

Thickened Solids, % total solids





Rotating biological contactor (RBC)



Waste-activated solids (WAS)












PRI + lime





Trickling filter (TF)

PRI + (WAS + iron)

NIRAS VOLUME 2 [2.7] PRI + (WAS + aluminum salts)


Anaerobically digested PRI + WAS





(John M. Stubbart, 2006 )

Some advantages and disadvantages of gravity thickening are listed bellow :

Advantages •

Gravity thickening equipment is simple to operate and maintain

Gravity thickening has lower operating costs than other thickening methods such as dissolved air flotation (DAF) gravity belt, or centrifuge thickening. For example, an efficient gravity thickening operation will save costs incurred in downstream solids handling steps.

In addition, facilities that land-apply liquid biosolids can benefit from thickening in several ways, as follows:

Truck traffic at the plant and the farm site can be reduced.

Trucking costs can be reduced.

Existing storage facilities can hold more days of biosolids production.

Smaller storage facilities can be used.

Less time will be required to transfer solids to the applicator vehicle and to incorporate or surface-apply the thickened solids.

Crop nitrogen demand can be met with fewer passes of the applicator vehicle, reducing soil compaction.

Disadvantages •

Scum buildup can cause odors.

Grease may build up in the lines and cause a blockage. This can be prevented by quick disposal or a backflush.

Septic conditions will generate sulfur-based odors. This can be mitigated by minimizing detention times in the collection system and at the plant, or by using oxidizing agents.

Supernatant does not have solids concentrations as low as those produced by a DAF or centrifuge thickener. Belt thickeners may produce supernatant with lower solids concentrations depending on the equipment and solids characteristics.

More land area is needed for gravity thickener equipment than for a DAF gravity belt or centrifuge thickener.

Solids concentrations in the thickened solids are usually lower than for a DAF gravity belt or centrifuge thickener.

NIRAS VOLUME 2 [2.7] Photograph of a basin thickener installation


(E. S. Tarleton, R. J. Wakeman, 2007)

Factors Affecting Gravity Thickening Performance Factor Nature of the solids feed


Affects the thickening process because some solids thicken more easily than others. Freshness of feed solids High solids age can result in septic conditions High volatile solids Hamper gravity settling due to reduced particle specific gravity concentrations High hydraulic loading rates Increase velocity and cause turbulence that will disrupt settling and carry the lighter solids past the weirs. Solids loading rate If rates are high, there will be insufficient detention time for settling. If rates are too low, septic conditions may arise. Temperature and variation • High temperatures will result in septic conditions. in temperature of thickener Extremely low temperatures will result in lower settling contents velocities. If temperature varies, settling decreases due to stratification • As the temperature of the sludge increases, the rate of biological activity increases and the sludge tends to gasify and rise at a higher rate. As a result, during summertime operation settled sludge has to be removed at a faster rate from the thickener. High solids blanket depth Increases the performance of the settling by causing compaction of the lower layers, but it may result in solids being carried over the weir Solids residence time An increase may result in septic conditions. A decrease may result in only partial settling Mechanism and rate of Must be maintained to produce a smooth and continuous flow.

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solids withdrawal Chemical treatment Presence of bacteriostatic agents or oxidizing agents Cationic polymer addition Use of metal salt coagulants

Otherwise, turbulence, septic conditions, altered settling, and other anomalies may occur. Chemicals-such as potassium permanganate, polymers, or ferric chloride-may improve settling and/or supernatant quality Allows for longer detention times before anaerobic conditions create gas bubbles and floating solids. Helps thicken waste-activated solids and clarify the supernatant Improves overflow clarity but may have little impact on underflow concentration (John M. Stubbart, 2006 )

Gravity Thickening Troubleshooting Guide Indicators

Septic odor, rising solids

Thickened solids not thick enough

Torque overload of solids collecting mechanism

Surging flow Excessive biological growths on surfaces and weirs (slimes, etc.) Oil leak

Probable Cause

Check or Monitor

Thickened solids pumping rate is too slow; thickener overflow rate is too low

Check thickened solids pumping system for proper operation; check thickener collection mechanism for proper operation

Oil seal failure

Oil seal


Increase pumping rate of thickened solids; increase influent flow to thickener – a portion of the secondary effluent may be pumped to thickener to bring overflow rate to 16-24 m3/m2d; chlorinate influent solids. Overflow rate is too high; Check overflow rate; use Decrease influent solids thickened solids dye or other tracer to flow rate; decrease pumping rate is too high; check for circulation. pumping rate of short-circuiting of flow thickened solids; check. through tank effluent weirs and repair or re-level; check influent baffles and repair or relocate. Heavy accumulation of Probe along front of Agitate solids blanket in solids; foreign object collector arms front of collector arms jammed in mechanism: with water jets; increase improper alignment of solids removal rate; mechanism attempt to remove foreign object with grappling device; if problem persists, drain thickener and check mechanism for free operation Poor influent pump Pump cycling Modify pump cycling; programming reduce flow and increase time. Inadequate cleaning Frequent and thorough program cleaning of surfaces; apply chlorination. Replace seal

NIRAS VOLUME 2 [2.7] 19 Noisy or hot bearing or universal joint Pump overload

Excessive wear: improper alignment; lack of lubrication Improper adjustment of packing; clogged pump Waste-activated solids

Fine solids particles in effluent

Alignment; lubrication

Replace, lubricate, or align joint or bearing as required Check packing; check for Adjust packing; clean trash in pump. pump Portion of waste- Better conditioning of the activated solids (WAS) in WAS portion of the thickener effluent solids; thicken WAS in a flotation thickener

(John M. Stubbart, 2006 )

Centrifugal thickening

Centrifuges are used both to thicken and to dewater sludges. Their application in thickening is limited normally to waste-activated sludge, because centrifuges have inlet assemblies that clog easily. Thickening by centrifugation involves the settling of sludge particles under the influence of centrifugal forces. The basic type of centrifuge used for sludge thickening is the solid-bowl centrifuge. Schematic diagram of a centrifuge used for sludge thickening

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(Eddy, 1999)

Typical Centrifuge system

Gravity – belt thickening

The development of gravity-belt thickeners stemmed from the application of belt presses for sludge dewatering. In belt-press dewatering, particularly for sludges having solids concentrations less than 2%, effective thickening occurred in the gravity drainage section of the press. Systems are often designed for a maximum of 5 to 7 % thickened solids. The equipment developed for thickening consists of a gravity belt that moves over rollers driven by a variable-speed drive unit. The sludge is conditioned with polymer and fed into a feed/distribution box at one end, where the sludge is distributed evenly across the width of the moving belt. The water drains through the belt

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as the concentrating sludge is carried toward the discharge end of the thickener. The sludge is ridged and furrowed by a series of plow blades placed along the travel of the belt, allowing the water released from the sludge to pass through the belt. After the thickened sludge is removed, the belt travels through a wash cycle. The gravity-belt thickener has been used for thickening wasteactivated sludge, anaerobically and aerobically digested sludge, and some industrial sludges. Polymer addition is required. Testing is recommended to verify that the solids can be thickened at typical polymer dosages.

Gravity-belt thickener - schematic diagram

(Eddy, 1999)

Polymer dosages for thickening waste-activated sludge range from 3 to 7 kg of dry polymer per tn of dry solids.

Main factors that affect gravity belt thickener performance include belt type, chemical conditioning, belt speed, and hydraulic and solid loadings.

Gravity-belt thickener during operation

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Rotary-Drum Thickening

Rotary media-covered drums are also used to thicken sludges. A rotary-drum thickening system consists of a conditioning system (including a polymer feed system) and rotating cylindrical screens . Polymer is mixed with dilute sludge in the mixing and conditioning drum. The conditioned sludge is then passed to rotating-screen drums, which separate the flocculated solids from the water. Thickened sludge rolls out the end of the drums, while separated water decants through the screens. Some designs also allow coupling of the rotary-drum unit to a beltfilter press for combination thickening and dewatering. Rotary-drum thickeners can be used as a prethickening step before belt-press dewatering.

NIRAS VOLUME 2 [2.7] Rotary-drum thickener


(Eddy, 1999)

2.7.4 Stabilization The purpose of sludge stabilization is to :

• • • • • •

reduce, or eliminate the potential for putrefaction ; stabilize the organic matter eliminate the offensive odours eliminate pathogenic organisms to permit reuse or disposal volume reduction production of usable gas (methane) to improve the dewaterability of sludge

The means to eliminate these nuisance conditions is mainly related to the biological reduction of the volatile content and the addition of chemicals to the solids or biosolids to render them unsuitable for the survival of microorganisms.

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The equipment required for stabilization depends on the specific process used. Sludge stabilization processes include:

Chemical stabilization (chlorine or lime oxidation)

Anaerobic digestion

Aerobic digestion

Autothermal thermophilic aerobic digestion (ATAD)


When designing a stabilization process, it is important to consider the sludge quantity to be treated, the integration of the stabilization process with the other treatment units, and the objectives of the stabilization process. The objectives of the stabilization process are often affected by existing or pending regulations. If sludge is to be applied on land, pathogen reduction has to be considered.

Chemical stabilization

Chemical stabilization is a process whereby the sludge matrix is treated with chemicals in different ways to stabilize the sludge solids. Two common methods employed are lime stabilization, and the use of chlorine.

Lime stabilization The lime stabilization process can be used to treat raw primary, waste activated, septage and anaerobically digested sludge. The process involves mixing a large enough quantity of lime with the sludge to increase the pH of the mixture to 12 or more. This pH is maintained for at least 2 hr. This normally reduces bacterial hazards and odor to a negligible value, improves dewatering performance and provides satisfactory means of stabilizing the sludge prior to ultimate disposal. If quicklime, CaO (or any compound high in quicklime), is added to sludge, it initially reacts with water to form hydrated lime. This reaction is exothermic and can result in substantial temperature rise.

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When sludge pretreatment with lime is used prior to dewatering, dewatering has been accomplished by a pressure-type filter press. Lime pretreatment is seldom used with centrifuges or belt-filter presses because of abrasive wear and scaling problems.

Advantages •

A rich soil-like product results with substantially reduced pathogens.

Disadvantages •

The product mass is increased by the addition of the alkaline material.

Also Many odorous, volatile off-gases are produced from lime stabilization method, especially ammonia, which require collection and treatment in odor-control systems such as chemical scrubbers or biofilters.

Typical lime dosages for pretreatment sludge stabilization Lime dosage* Type of sludge

Primary Waste activated Anaerobically digested mixed Septage


g Ca(OH)2/ kg

concentration, %

dry solids
















*Amount of Ca(OH)2 required to maintain a pH of 12 for 30 min

(Eddy, 1999)

In lime posttreatment hydrated lime or quicklime is mixed with dewatered sludge in a pugmill, paddle mixer, or screw conveyor to raise the pH of the mixture. Quicklime is preferred because the exothermic reaction of quicklime and water can raise the temperature of the mixture above 50°C, sufficient to inactivate worm eggs.

Advantages •

dry lime can be used; therefore, no additional water is added to the dewatered sludge, and there are no special requirements for dewatering

NIRAS VOLUME 2 [2.7] 26


scaling problems and associated maintenance problems of lime-sludge dewatering equipment are eliminated

Disadvantages •

lime posttreatment of anaerobically digested sludge may release odorous gases, such as trimethyl amine

A lime posttreatment stabilization system consists typically of a dry lime feed system, dewatered sludge cake conveyor, and a lime-sludge mixer. Good mixing is especially important to ensure contact between lime and small particles of sludge.

Typical lime post treatment system

(Eddy, 1999)

Stabilization by chlorine Stabilization by chlorine addition has been developed and is marketed under the registered trade name "Purifax". The chemical conditioning of sludge with chlorine varies greatly from the more traditional methods of biological digestion or heat conditioning. First, the reaction is almost instantaneous. Second, there is very little volatile solids reduction in the sludge. Chlorine oxidation also occurs in a closed vessel. In this process chlorine (100 to 1000 mg/L) is mixed with a recycled solids flow. The recycled flow and process residual flow are mixed in the

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reactor. The solids and water are separated after leaving the reactor vessel. The water is returned to the wastewater treatment system, and the treated solids are dewatered for disposal.

Advantages •

The process can be operated intermittently.

Disadvantages •

Production of extremely low pH and high chlorine content in the supernatant.

Stabilization via anaerobic digestion

The purpose of digestion is to attain both of the objectives of sludge treatment – a reduction in volume and the decomposition of highly putrescible organic matter to relatively stable or inert organic and inorganic compounds. Additionally, anaerobic sludge digestion produces a valuable by-product in the form of methane gas (the primary constituent of natural gas, which we can bum for heat or convert to electricity). Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. It is, therefore, anaerobic decomposition. The solid matter in raw sludge is about 70% organic and 30% inorganic or mineral. Much of the water in wastewater sludge is "bound" water which will not separate from the sludge solids. The facultative and anaerobic organisms break down the complex molecular structure of these solids setting free the "bound" water and obtaining oxygen and food for their growth.

MORE The anaerobic degradation of domestic sludge occurs in two steps. In the first step, acid forming bacteria attack the soluble or dissolved solids, such as the sugars. From these reactions organic acids, at times up to several thousand ppm, and gases, such as carbon dioxide and hydrogen sulfide are formed. This is known as the stage of acid fermentation (acidogenesis) and proceeds rapidly. It is followed by a period of acid digestion in which the organic acids and nitrogenous compounds are attacked and liquefied at a much slower rate. In the second stage of digestion, known as the period of intensive digestion (methanogenesis), stabilization and gasification, the more resistant nitrogenous materials, such as the proteins, amino-acids and others, are attacked. The pH value must be maintained from 6.8 to 7.4. Large volumes of gases with a 65 % or higher percentage of methane are produced. The organisms which convert organic acids to methane and carbon dioxide gases are called methane

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formers. The solids remaining are relatively stable or only slowly putrescible, can be disposed of without creating objectionable conditions and have value in agriculture.

The reduction of organic matter as measured by the volatile solids indicates the completeness of digestion. Raw sludge usually contains from 60% to 70% volatile solids while a well digested sludge may have as little as 50%. This would represent a volatile solids reduction of about 50%.

Most anaerobic digestion systems are designed to operate in the mesophilic temperature range, between 30 and 38°C . Other systems are designed for operation in the thermophilic temperature range of 50 to 57°C. It has been found that sludge digestion proceeds in almost any range of temperature likely to be encountered, but the time taken to complete digestion varies greatly with the temperature. Also rapid changes in temperature are detrimental. Digester temperature should not vary more than ± 1 °C per day. Pumping excessive quantities of thin sludge can cause significant decreases in digester temperature .

Typical anaerobic digester, schematic view

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The methane forming organisms are extremely sensitive to changes in temperature. At a temperature of 12 °C about 90% of the desired digestion is completed in about 55 days. As the temperature increases, the time decreases, so that at 24 °C the time is cut to 35 days, at 30 °C to 26 days, and at 35 °C to 24 days. The theoretical time for sludge digestion at 35 °C is one half that at 15 °C. Of course, above values are not exactly the same, for all types of sludge and are depending on the sludge composition and origin.

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Anaerobic digester sizing is based on providing sufficient residence time in well-mixed reactors to allow significant destruction of volatile suspended solids (VSS) to occur. Sizing criteria that have been used are (1) solids retention time SRT, the average time the solids are held in the digestion process, and (2) the hydraulic retention time t, the average time the liquid is held in the digestion process. For soluble substrates, the SRT can be determined by dividing the mass of solids in the reactor (M) by the mass of solids removed daily (M/d). The hydraulic retention time t is equal to the volume of liquid in the reactor (L3) divided by the quantity of biosolids removed (L3/d). For digestion systems without recycle, SRT = t. Biogas process

A well digested sludge should be black in color, have a not unpleasant tarry odor and, when collected in a glass cylinder, should appear granular in structure and show definite channels caused by water rising to the top as the solids settle to the bottom.

The quantity of gases produced should be relatively constant if the feed rate is constant . Sharp decreases in total gas production may indicate toxicity in the digester. The gas is usually about 65% methane, about 35% carbon dioxide and inert gases such as nitrogen. An increasing percentage of carbon dioxide may be an indication that the digestion process is not proceeding properly.

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Total gas production is usually estimated from the percentage of volatile solids reduction. Typical values vary from 0,75 to 1,12 m3/kg of volatile solids destroyed. Gas production can also be estimated crudely on a per capita basis. The normal yield is 15 to 22 L/person*d in primary plants treating normal domestic wastewater. In secondary treatment plants, the gas production is increased to about 28 L/person*d.

Low pressure Biogas storage tank

Methane gas at standard temperature and pressure (20째C and 1 atm) has a lower heating value of 36.000 kJ/m3 or 10 kWh/m3 . Because digester gas is only 65% methane, the lower heating value of digester gas is approximately 24.000 kJ/m3 or 6,7 kWh/m3. By comparison, natural gas, which is a mixture of methane, propane, and butane, has a heating value of 37,300 kJ/m3 or 10,4 kWh/m3. Anaerobic Digesters in a facility

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Large plants, digester gas may be used as fuel for boiler and internal-combustion engines which are, in turn, used for pumping wastewater, operating blowers, and generating electricity. Hot water from heating boilers or from engine jackets and exhaustheat boilers may be used for sludge heating and for building heating, or gas-fired sludge-heating boilers may be used. Energy recovery is more efficient if prime movers are designed to run hot because heat rejected at high temperatures can be put to a greater variety of uses than heat rejected at low temperatures .

Digester gas can be used in cogeneration. Cogeneration is generally defined as a system for generating electricity and producing another form of energy (usually steam or hot water). Digester gas can be used to power an engine-generator to generate electricity, and the jacket water from the internal-combustion engine can then be used for digester or building heating.

Electricity engine-generator from biogas (diesel cycle)

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Because digester gas contains hydrogen sulfide, nitrogen, particulates, and water vapor, the gas frequently has to be cleaned in dry or wet scrubbers before it is used in internal-combustion engines. Hydrogen sulfide concentrations in excess of approximately 100 ppm by volume may require the installation of hydrogen sulfide removal equipment.

Stabilization via aerobic digestion

Equipment used for aerobic digestion includes an aeration tank (digester), which is similar in design to the aeration tank used for the activated sludge process. Either diffused or mechanical aeration equipment is necessary to maintain the aerobic conditions in the tank. Solids and supernatant removal equipment is also required. In operation, process residuals (sludge) are added to the digester and aerated to maintain a dissolved oxygen (DO) concentration of 1 mg/L. Aeration also ensures that the tank contents are well mixed. Generally, aeration continues for minimum 20 days of retention time. Periodically, aeration is stopped and the solids are allowed to settle. Sludge and the clear liquid supernatant are withdrawn as needed to provide more room in the digester. When no additional volume is available, mixing is stopped for 12 to 24 hours before solids are withdrawn for disposal. Process control testing should include alkalinity, pH, percent solids, percent volatile solids for influent sludge, supernatant, digested sludge, and digester contents.

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A typical operational problem associated with an aerobic digester is pH control. When pH drops, for example, this may indicate abnormal biological activity or low influent alkalinity. This problem is corrected by adding alkalinity (e.g., lime, bicarbonate).

MORE Aerobic digestion is an extension of the activated sludge aeration process whereby waste primary and secondary sludge are continually aerated for long periods of time. In aerobic digestion the microorganisms extend into the endogenous respiration phase. This is a phase where materials previously stored by the cell are oxidized, with a reduction in the biologically degradable organic matter. This organic matter, from the sludge cells is oxidized to carbon dioxide, water and ammonia. The ammonia is further converted to nitrates as the digestion process proceeds. Eventually, the oxygen uptake rate levels off and the sludge matter is reduced to inorganic matter and relatively stable volatiles. The primary advantage of aerobic digestion is that it produces a biologically stable end product suitable for subsequent treatment in a variety of processes. Volatile solids reductions similar to anaerobic digestion are possible.

The advantages most often claimed for aerobic digestion are: •

A humus-like, biologically stable end product is produced.

The stable end product has no odors, therefore, simple land disposal, such as lagoons, is feasible.

Capital costs for an aerobic system are low, when compared with anaerobic digestion and other schemes.

Aerobically digested sludge usually has good dewatering characteristics. When applied to sand drying beds, it drains well and redries quickly if rained upon.

The volatile solids reduction can be equal to those achieved by anaerobic digestion.

Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion. Most tests indicated that BOD would be less than 100 ppm. This advantage is important because the efficiency of many treatment plants is reduced as a result of recycling high BOD supernatant liquors.

There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system is more stable. As a result, less skillful and costly labor can be used to operate the facility. In comparison with anaerobic digestion, more of the sludge basic fertilizer values are recovered.

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The major disadvantage associated with aerobic digestion is high power costs. This factor is responsible for the high operating costs in comparison with anaerobic digestion. Two other minor disadvantages are the lack of methane gas production and the variable solids reduction efficiency with varying temperature change.

Design criteria for aerobic digesters Parameter



at 20 oC



at 15 oC



Volatile solids loading


1,6 – 4,8

Oxygen requirements

kg O2/kg VSS


Mechanical aerators

kW/103 m3


Diffused air mixing

m3/m3 * hr

1,2 – 2,4






Energy requirements for mixing

Dissolved oxygen residual in liquid Reduction of volatile suspended solids (Eddy, 1999)

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Autothermal thermophilic aerobic digestion (ATAD)

(ATAD) represents a variation of both conventional and high-purity oxygen aerobic digestion. In the ATAD process, the feed sludge is generally prethickened and the reactors are insulated to conserve the heat produced from the oxidation of volatile solids during the digestion process. Thermophilic operating temperatures (generally in the range of 55 to 70째C) can be achieved without external heat input by using the heat released by the exothermic microbial oxidation process. Approximately 5-6 kWh of heat is produced per kg of volatile solids destroyed. Because supplemental heat is not provided (other than the heat introduced by aeration and mixing), the process is termed autothermal.

Autothermal thermophilic aerobic digester (ATAD) system schematic

(Eddy, 1999)

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Advantages •

retention times required to achieve a given suspended solids reduction are decreased significantly (to about 5 to 6 d) to achieve volatile solids reductions of 30 to 50 percent, similar to conventional aerobic digestion

simplicity of operation

greater reduction of bacteria and viruses are achieved as compared to mesophilic anaerobic digestion

when the reactor is well mixed and maintained at 55°C and above, pathogenic viruses, bacteria viable helminth ova, and other parasites can be reduced to below detectable levels

Disadvantages •

objectionable odors are formed

poor dewatering characteristics of ATAD biosolids

lack of nitrification.

Because the ATAD system is capable of producing the best stabilized sludge, it is growing in popularity.


The purpose of composting sludge is to stabilize the organic matter, reduce volume, and eliminate pathogenic organisms. In a composting operation, dewatered solids are usually mixed with a bulking agent (e.g., hardwood chips) and stored until biological stabilization occurs. The composting mixture is ventilated during storage to provide sufficient oxygen for oxidation and to prevent odors. After the solids are stabilized, they are separated from the bulking agent. The composted solids are then stored for curing and applied to farmlands or other beneficial uses. Expected performance of the composting operation for percent volatile matter reduction and percent moisture reduction ranges from 20 to 30% and 40 to 60% respectively.

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Futhermore it should be noted that sludges that have been subjected to Chemical stabilization, should not be composted, because chemical stabilization produces environments that are unsuitable for microorganisms survival and will not support life of composting vacteria unless the sludges are neutralized and favorable conditions exist.


Although composting may be accomplished under anaerobic or aerobic conditions, essentially all municipal wastewater biosolids composting applications are under mostly aerobic conditions (composting is never completely aerobic). Aerobic composting accelerates material decomposition and results in the higher rise in temperature necessary for pathogen destruction. Aerobic composting also minimizes the potential for nuisance odors.

During composting, microorganisms break down organic matter in wastewater solids into carbon dioxide, water, heat, and compost. As the organic material in the sludge decomposes, the compost heats to temperatures in the pasteurization range of 50 to 70째C , and enteric pathogenic organisms are destroyed. To ensure optimal conditions for microbial growth, carbon and nitrogen must be present in the proper balance in the mixture being composted. The ideal carbon-to-nitrogen ratio ranges from 25

NIRAS VOLUME 2 [2.7] 39

to 35 parts carbon for each 1 part of nitrogen by weight. A lower ratio can result in ammonia odors. A higher ratio will not create optimal conditions for microbial growth causing degradation to occur at a slower rate and temperatures to remain below levels required for pathogen destruction. Wastewater solids are primarily a source of nitrogen and must be mixed with a higher carboncontaining material such as wood chips, sawdust, newspaper, or hulls. In addition to supplying carbon to the composting process, the bulking agent serves to increase the porosity of the mixture. Porosity is important to ensure that adequate oxygen reaches the composting mass. Oxygen can be supplied to the composting mass through active means such as blowers and piping or through passive means such as turning to allow more air into the mass. The proper amount of air along with biosolids and bulking agent is important. The anticipated daily production of biosolids from a wastewater-treatment facility will have a pronounced effect on the alternate composting systems available for use. Biosolids that are stabilized by aerobic or anaerobic digestion prior to composting may result in reducing the size of the composting facilities by up to 40%.

During the composting process, three separate stages of activity and associated temperatures are observed: mesophilic, thermophilic, and cooling .In the initial mesophilic stage, the temperature in the compost pile increases from ambient to approximately 40째C with the appearance of fungi and acid-producing bacteria. As the temperature in the composting mass increases to the thermophilic range of 40 to 70째C, these microorganisms are replaced by thermophilic bacteria, actinomycetes, and thermophilic fungi. It is in the thermophilic temperature range that the maximum degradation and stabilization of organic material occur. The cooling stage is characterized by a reduction in microbial activity, and replacement of the thermophilic organisms with mesophilic bacteria and fungi. During the cooling period, further evaporative release of water from the composted material will occur, as well as stabilization of pH and completion of humic acid formation.

Phases during composting as related to carbon dioxide respiration and temperature

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(Eddy, 1999)

Microbial succession during composting

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(Gabriel Bitton, 2005)

The two principal methods of composting worldwide may be classified as agitated or static. In the agitated method the material to be composted is agitated periodically to introduce oxygen, to control the temperature, and to mix the material to obtain a uniform product. In the static method, the material to be composted remains static and air is blown through the composting material. The most common agitated and static methods of composting are known as the windrow and static pile methods, respectively.

Aerated Static Pile

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The aerated static pile system consists of a grid of aeration or exhaust piping over which a mixture of dewatered sludge and bulking agent is placed. In a typical static pile system, the bulking agent consists of wood chips, which are mixed with the dewatered sludge by a pug-mill type or rotatingdrum mixer or by movable equipment such as a front-end loader. Material is composted for 21 to 28 days and is typically followed by a curing period of 30 days or longer. Typical pile heights are generally about 2 to 2,5 m . A layer of screened compost is often placed on top of the pile for insulation. Disposable corrugated plastic drainage pipe is commonly used for air supply and each individual pile is recommended to have an individual blower for more effective aeration control. Screening of the cured compost usually is done to reduce the quantity of the end product requiring ultimate disposal and to recover the bulking agent. For improved process and odor control, many facilities cover or enclose all or significant portions of the system.

Composting systems : aerated static pile

(Eddy, 1999)

Windrow In a windrow system, the mixing and screening operations are similar to those for the aerated static pile operation. Windrows are constructed from 1 to 2 m high and 2 to 4,5 m at the base .The rows are turned and mixed periodically during the composting period. Supplemental mechanical aeration is used in some applications. The composting period is about 21 to 28 d. Under typical operating conditions, the windrows are turned a minimum of five times while the temperature is maintained at or above 55째C.

NIRAS VOLUME 2 [2.7] 43

In windrow composting, aerobic conditions are difficult to maintain throughout the cross-sectional area of the windrow. Thus, the microbial activity within the pile may be aerobic, facultative, anaerobic, or various combinations thereof, depending on when and how often the pile is turned. Turning of the windrows is often accompanied by the release of offensive odors. The release of odors occurs typically when anaerobic conditions develop within the windrow. Specialized equipment is available to mix the sludge and bulking agent and to turn the composting windrows. Some windrow operations are covered or enclosed, similar to aerated static piles.

Composting systems : compost windrows (left) ; equipment for turning and reforming compost windrows (right)

(Eddy, 1999)

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Design considerations for aerobic sludge composting processes •

The compost mix should be about 40% dry solids to ensure ade quate composting in windrow and static-pile composting.

Preparation includes conveying the finished compost from the active composting area to the curing, screening, and preparation areas. Trommel screens and belt shredders are used frequently; shredding can precede or follow curing. In some cases, double screening is preferable, especially for the horticultural market to meet product quality requirements. Particle size of the finished product for general use ranges typically from 6 to 25 mm

The initial C/N ratio should be in the range of 25:1 to 35:1 by weight. At lower ratios, ratio ammonia is given off. Carbon should be checked to ensure it is readily biodegradable

The volatile solids of the composting mix should be greater than 30% of the total solids content.

Moisture content of the composting mixture should be not greater than 60% for static pile and windrow composting.

pH of the composting mixture should generally be in the range of 6 to 9. To achieve optimum aerobic decomposition, pH should remain in the 7 to 7,5 range

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For best results, temperature should be maintained between 50 and 55°C for the first few days and between 55 and 60°C for the remainder of the active composting period. If the temperature is allowed to increase beyond 65°C for a significant period of time, biological activity will be reduced.

If properly conducted, it is possible to kill all pathogens, weeds, and seeds during the pathogens composting process. To achieve this level of control, the temperature must be maintained between 60 and 70°C for 24 h (thermophilic composting).

Comparison of stabilization methods

Relative degree of attenuation achieved with various sludge stabilization processes Degree of attenuation Process Pathogens Putrefaction Odor potential Alkaline stabilization Good Fair Fair Anaerobic digestion Fair Good Good Aerobic digestion Fair Good Good Autothermal thermophilic aerobic digestion Excellent Good Good (ATAD) Composting Fair Good Poor to fair Composting (thermophilic) Excellent Good Poor to fair (Eddy, 1999)

2.7.5 Conditioning Before any of the sludge can proceed to dewatering processes, it must be conditioned. Sludge conditioning involves chemical or thermal conditioning to improve the efficiency of the downstream processes.

In other words, conditioning improves dewaterability.

Chemical conditioning

Chemical conditioning involves use of inorganic chemicals or organic polyelectrolytes, or both. The most commonly used inorganic chemicals are ferric chloride and lime.

Many chemicals have been used such as sulfuric acid, alum, chlorinated copper, ferrous sulfate, and ferric chloride with or without lime, and others.

NIRAS VOLUME 2 [2.7] 46

Organic polymers, introduced during the 1960's, are used for both sludge-thickening and dewatering processes. Their advantage over inorganics is that polymers don't greatly increase the amount of sludge production: 1 kg of inorganic chemicals added will produce 1 kg of extra sludge. The disadvantage of polymers is their relatively high cost.

These polymers are of three basic types: •

Anionic (negative charge) -- serve as coagulants aids to inorganic Aluminum and Iron coagulants by increasing the rate of flocculation, size, and toughness of particles.

Cationic (positive charge) -- serve as primary coagulants alone or in combination with inorganic coagulants such as aluminum sulfate.

Nonionic (equal amounts of positively and negatively charged groups in monomers) -- serve as coagulant aids in a manner similar to that of both anionic and cationic polymers.

Typical levels of polymer addition for belt-filter press and solid-bowl centrifuge sludge dewatering

Type of sludge

kg/tn dry solids Belt-filter Solid-bowl press centrifuge 1-4 1-2,5 2-8 2-5 2-8 4-10 5-8 2-5 3-5

Primary Primary & Waste activated Primary and trickling filter Waste activated Anaerobically digested primary Anaerobically digested primary and air 1,5-8,5 waste activated Aerobically digested primary and air 2-8 waste activated (Eddy, 1999)

2-5 -

Thermal conditioning

Heat treatment is a process that has been used for the conditioning and stabilization of sludge, but it is seldom used in new installations. There are two basic processes for thermal treatment of sludge.

One, wet air oxidation, is the flameless oxidation of sludge at temperatures of 230 °C to 300 °C and pressures of about 80 atm. The other type, heat treatment, is similar but carried out at temperatures of 230 °C to 300 °C and pressures of 10 to 20 atm.

NIRAS VOLUME 2 [2.7] 47

Wet air oxidation (WAO) reduces the sludge to an ash and heat treatment improves the dewaterability of the sludge. The lower temperature and pressure heat treatment is more widely used than the wet air oxidation process.

Gases released from the thermal process are passed through a catalytic after-burner at 340 °C to 380 °C or deodorized by other means. In some cases these gases have been returned through the diffused air system in the aeration basins for deodorization.

Advantages •

thermal treatment is a more readily dewaterable sludge than this produced with chemical conditioning. Dewatered sludge solids of 30 to 40 % (as opposed to 15 to 20 % with chemical conditioning) have been achieved with heat treated sludge at relatively high loading rates on the dewatering equipment (2 to 3 times the rates with chemical conditioning).

the process also provides effective disinfection of the sludge.

Disadvantages •

Unfortunately, the heat treatment process ruptures the cell walls of biological organisms, releasing not only the water but some bound organic material. This returns to solution some organic material previously converted to particulate form and creates other fine particulate matter. The breakdown of the biological cells as a result of heat treatment converts these previously particulate cells back to water and fine solids. This aids the dewatering process, but creates a separate problem of treating this highly polluted liquid from

the cells.

Treatment of this water or liquor requires careful consideration in design of the plant because the organic content of the liquor can be extremely high.

2.7.6 Mixing Mixing plays an important role in most of the stabilization and/or conditioning processes. Without well-mixed systems, the processes cannot acceptable levels of efficiency. Mixing may be either intermittent or continuous, but however effected it provides all working organisms their proper food requirements and helps maintain uniform temperature. Intermittent mixing allows separation and removal of supernatant from a single stage digester. With continuous mixing the digestion proceeds at a higher rate throughout the entire tank, thus reducing the tank capacity needed. Such continuous mixing requires a second digester or storage tank into which digesting sludge may be

NIRAS VOLUME 2 [2.7] 48

moved to make room for fresh sludge in the first digester and to make possible separation and removal of supernatant in the secondary digester.

2.7.7 Sludge dewatering Digested sludge removed from the digester is still mostly liquid. Sludge dewatering is used to reduce volume by removing the water to permit easy handling and economical reuse or disposal. There are both mechanical and thermal techniques for achieving this. Dewatering processes include : •

vacuum filters

filter presses



drying beds

reed beds

sludge lagoons

Vacuum filtration

The vacuum filter for dewatering sludge is a drum over which is laid the filtering medium consisting of a cloth of cotton, wool, nylon, dynel, fiber glass or plastic, or a stainless steel mesh, or a double layer of stainless steel coil springs. The drum with horizontal axis is set in a tank with about one quarter of the drum submerged in conditioned sludge. Valves and piping are so arranged that, as a portion of the drum rotates slowly in the sludge, a vacuum is applied on the inner side of the filter medium, drawing out water from the sludge and holding the sludge against it. The application of the vacuum is continued as the drum rotates out of the sludge and into the atmosphere. This pulls water away from the sludge, leaving a moist mat or cake on the outer surface. This mat is scraped, blown or lifted away from the drum just before it enters the sludge tank again.

There are three principal types of rotary vacuum filters : rotary drum, coil, and belt.

Drum and belt vacuum filters use natural or synthetic fiber materials. On the drum filter, the cloth is stretched and secured to the surface of the drum. In the belt filter, the cloth is stretched over the

NIRAS VOLUME 2 [2.7] 49

drum and through the pulley system. The installation of a blanket requires several days. The cloth (with proper care) will last several hundred to several thousand hours. The life of the blanket depends on the cloth selected, the conditioning chemical, backwash frequency, and cleaning (e.g., acid bath) frequency. Filter media must always passes through a washing zone, in order to remove fine particles from the media and to reduce the possibility of media blinding too early.

The filter drum is a maze of pipe work running from a metal screen and wooden skeleton and connecting to a rotating valve port at each end of the drum. The drum is equipped with a variable speed drive to turn the drum from 1/8 to 1 rpm. Normally, solids pickup is indirectly related to the drum speed. The drum is partially submerged in a vat containing the conditioned sludge. Submergence is usually limited to 1/5 or less of filter surface at a time.

Vacuum filter operating schematic

(California State University, 2006)

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Rotary drum vacuum filter

(California State University, 2006) Representations of rotary vacuum drum filters. (a) bottom fed, knife/scraper discharge ; (b) bottom fed, roller discharge; (c) bottom fed, string discharge; (d) bottom fed, belt discharge ; (e) top fed (Filtration Services); (f) internal drum

NIRAS VOLUME 2 [2.7] 51

(E. S. Tarleton, R. J. Wakeman, 2007)

NIRAS VOLUME 2 [2.7] 52

Rotary drum vacuum filter in operation

Operational considerations •

Conditioned sludge should be filtered as quickly as possible after the addifion of the chemicals and adequate mixing.

Continuous feeding is preferable to batch conditioning. In raw sludge filtration, fresh sewage solids and sludge filter more readily than stale or septic sludge.

Completely digested sludge usually filters more readily than partially digested sludge.

The concentration of sludge to be filtered is critical as sludge with the higher solid content usually filters more readily than that with a lower solid content.

The presence of mineral oils and wastes from dry cleaning establishments makes sludge filtration difficult. Such wastes should, therefore, be kept out of the sewer system and disposed of separately.

After every use the vacuum filter should be cleaned, and all sludge drained from the unit. This sludge and wash water should not be returned to the sludge storage tank but to the raw sewage channel or to a digester.

Sludge dewatered using vacuum filtration is normally chemically conditioned just prior to filtration. Sludge conditioning increases the percentage of solids captured by the filter and

NIRAS VOLUME 2 [2.7] 53

improves the dewatering characteristics of the sludge; however, conditional sludge must be filtered as quickly as possible after chemical addition to obtain these desirable results.

Pressure filtration

Pressure filtration differs from vacuum filtration in that the liquid is forced through the filter media by a positive pressure instead of a vacuum. Like vacuum filtration, a porous media is used in leaf filters to separate solids from the liquid. The solids are captured in the media pores; they build up on the media surface; and they reinforce the media in its solid-liquid separation action. Sludge pumps provide the energy to force the water through the media. Lime, aluminum chloride, aluminum chlorohydrate, and ferric salts have been commonly used to condition sludge prior to pressing . Leaf filters represent an attempt to dewater sludge in a small space quickly. But, when compared to other dewatering methods, they have major disadvantages, including: (1) batch operation, and (2) high operation and maintenance costs. Some other types of pressure filters include hydraulic and screw presses, which while effective in dewatering sludges, have a major disadvantage of usually requiring a thickened sludge feed. Sludge cakes as high as 75% solids using pressure filtration have been reported. It should be noted though, that secondary sludges do not dewater as readily as primary sludges because secondary sludges contain fine, low-density solids that have large surface areas and relatively large quantities of water associated with them.

Plate and Frame Filter Press Dewatering

Sludge can be dewatered using a plate and frame filter press. It works by pressing water out of sludge through the use of plates. Sludge flows in the spaces between the plates and water is pressed out (Pressure 14 to 17 atm). The plates are then separated and the cake falls out into a hopper or onto a conveyor belt.

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Belt Filter Press Dewatering

Sludge can be dewatered using a belt filter press. The sludge is pressed between belts into a cake. The cake is fed into a hopper or onto a conveyor belt

Belt filter press operational diagramm

(John M. Stubbart, 2006 )

Schematic of the belt arrangement on a belt press filter

NIRAS VOLUME 2 [2.7] 55

(E. S. Tarleton, R. J. Wakeman, 2007) A belt filter press and the produced dewatered sludge cake

Schematic diagram of a belt-press dewatering system

NIRAS VOLUME 2 [2.7] 56

(Eddy, 1999)

Photograph of a belt filter press

NIRAS VOLUME 2 [2.7] 57

Typical Data for Various Types of Sludges Dewatered on Belt Filter Presses Type of Wastewater Sludge Raw primary Raw waste-activated solids (WAS) Raw primary +WAS Anaerobically digested primary Anaerobically digested WAS Anaerobically digested primary + WAS Aerobically digested primary + WAS Oxygen-activated WAS Thermally conditioned primary +WAS

Total Feed Solids, %

Polymer, g/kg

Total Cake Solids, %




























(John M. Stubbart, 2006 )

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Centrifuge dewatering

Centrifuges are machines that separate solids from the liquid through sedimentation and centrifugal force. In a typical unit sludge is fed through a stationary feed tube along the centerline of the bowl through a hub of the screw conveyor. The screw conveyor is mounted inside the rotating conical bowl. It rotates at a slightly lower speed than the bowl. Sludge leaves the end of the feed tube, is accelerated, passes through the ports in the conveyor shaft, and is distributed to the periphery of the bowl. Solids settle through the liquid pool, are compacted by centrifugal force against the walls of the bowl, and are conveyed by the screw conveyor to the drying or beach area of the bowl. The beach area is an inclined section of the bowl where further dewatering occurs before the solids are discharged. Separated liquid is discharged continuously over adjustable weirs at the opposite end of the bowl.

Solid Bowl Scroll Centrifuge

(John M. Stubbart, 2006 )

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Scroll centrifuge

(California State University, 2006)

In centrifuge dewatering centrifugal force is used to accelerate the separation of solid and liquid phases of the liquid sludge stream. The process involves clarification of the sludge and its compaction. Centrifuges separate the sludge into dewatered sludge cakes and clarified liquid, which is called centrate.

Two factors usually determine the success of failure of centrifugation -- cake dryness and solids recovery. The effect of the various parameters on these two factors are listed below

To Increase Cake Dryness

To Increase Solids Recovery

1. Increase bowl speed

1. Increase bowl speed

2. Decrease pool volume

2. Increase pool volume

3. Decrease conveyor speed

3. Decrease conveyor speed

4. Increase feed rate

4. Decrease feed rate

5. Decrease feed consistency

5. Increase temperature

6. Increase temperature

6. Use flocculents

7. Do no use flocculents

7. Increase feed consistency

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Centrifugation has some inherent advantages over vacuum filtration and other processes used to dewater sludge.

(1) It is simple, compact, totally enclosed, flexible, (2) It can be used without chemical aids, and the costs are moderate. (3) Industry particularly has accepted centrifuges in part due to their low capital cost, simplicity of operation, and effectiveness with difficult-to-dewater sludges.

The disadvantages associated with centrifugation are: (1) without the use of chemicals the solids capture is often very poor, and chemical costs canbe substantial; (2) trash must often be removed from the centrifuge feed by screening; (3) cake solids are often lower than those resulting from vacuum filtration; and (4) maintenance costs are high.

The poor quality of the centrate is a major problem with centrifuges. The fine solids in centrate recycled to the head of the treatment plant sometimes resist settling and as a result, their concentrations in the treatment system gradually build up. The centrate from raw sludge dewatering can also cause odor problems when recycled. Flocculents can be used to increase solids captures, often to any degree desired, as well as to materially increase the capacity (solids loading) of the centrifuges. However, the use of chemicals nullifies the major advantage claimed for centrifuges. The most effective centrifuges to dewater waste sludges are horizontal, cylindrical -- conical, solid bowl machines

Range of Expected Centrifuge Performance Feed, Type of Wastewater Solids

% total solids

Primary undigested Waste-activated solids (WAS) undigested Primary + WAS undigested Primary + WAS aerobic digested Primary + WAS anaerobic digested

Cake, % total solids











NIRAS VOLUME 2 [2.7] 61

Primary anaerobic digested



WAS aerobic digested



High-temperature aerobic



High-temperature anaerobic



Lime stabilized



(John M. Stubbart, 2006 ) Typical solid-bowl centrifuge dewatering installation

(Eddy, 1999) Hydroclones We can think of these machines as low-energy centrifuges. Hydroclones are employed for the separation of solid particles from mediurn- to low-viscosity liquids. Like their cyclone counterparts used in gas cleaning applications, hydroclones are simple in design, and the degree of separation can be altered by either varying loading conditions or changing geometric proportions. Unlike other types of solid-liquid separating equipment, they are better suited for classifying than for clarifying because high shearing stresses in a hydroclone promote the suspension of particles which oppose flocculation. However, by properly specifying dimensions and operating conditions, they can be used as thickeners in such a manner that the underflow contains mostly solid particles, while the clear overflow constitutes the largest portion of the liquid.

NIRAS VOLUME 2 [2.7] Features of a hydroclone


(Nickolas P. Cheremisinoff, 2002)

A hydroclone consists of an upper short cylindrical section (1) and an elongated conical bottom (2). The suspension is introduced into the cylindrical section (1) through the nozzle (3) tangentially, whence the fluid acquires an intensive rotary motion. The larger particles, under the action of centrifugal force, move toward the walls of the apparatus and concentrate on the outer layers of the rotating flow. Then they move spirally downward along the walls to the nozzle (4), through which the thickened slurry is evacuated. The largest portion of liquid containing small particles (clear liquid) moves in the internal spiral flow upward along the axis of the hydroclone. The cleared liquid is discharged through the nozzle (5) and fixed at the partition (6) and the nozzle (7). The actual flow pattern is more complicated than described because of radial and closed circulating flows.

Drying beds

This is one of two common methods of dewatering based upon thermal energy. Drying beds are generally used for dewatering of well digested sludges. Attempts to air dry raw sludge usually result in odor problems. Sludge drying beds consist of perforated or open joint drainage pipe laid within a gravel base. The gravel is covered with a layer of sand. Partitions around and between the drying beds are generally open to the weather but may be covered with ventilated green-house type enclosures where it is necessary to dewater sludge in wet climates. The drying of sludge on sand beds is accomplished by allowing water to drain from the sludge mass through the supporting sand to the drainage piping and natural evaporation to the air. As the sludge dries, cracks develop in the surface allowing evaporation to occur from the lower layers which accelerates the drying process.

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Typical Drying Bed

There are many design variations used for sludge drying beds, including the layout of the drainage piping, thickness and type of materials in the gravel and sand layers, and construction materials used for the partitions. The major variation is whether or not the beds are covered. Any covering structure must be well ventilated. In the past, some beds were constructed with flat concrete bottoms for drainage without pipes, but this construction has not been very satisfactory. Asphalt concrete (blacktop) has been used in some drying beds

The only sidestream is the drainage water. This water is normally returned to the raw sewage flow to the plant or to the plant headworks. The drainage water is not normally treated prior to return to the plant. Experience is the best guide in determining the 20 to 30 cm .

Sludge can be removed by shovel or forks at a moisture content of 60%, but if it is allowed to dry to 40% moisture, it will weigh only half as much and is still easy to handle. If the sludge gets too dry (10 to 20% moisture) it will be dusty and will be difficult to remove because it will crumble as it is removed. Many operators of smaller treatment plants use wheelbarrows to haul sludge from drying beds. Planks are often laid on the bed for a runway so that the wheelbarrow tire does not sink into the sand. Wheelbarrows can be kept close to the worker so that the shoveling distance is not great. Most plants use pick-up trucks or dump trucks to transport the sludge from the drying bed. Dump trucks have the advantage of quick unloading.

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Cross-Section of a Wedgewire Drying Bed

(John M. Stubbart, 2006 )

Advantages •

low cost

infrequent attention required

high-solids content in the dried product

Disadvantages •

large space required

effects of climatic changes on drying characteristics

labor-intensive sludge removal

drying beds attract insects, and emmit potential odors

Typical conventional sand drying bed: (a) plan and pictorial views and (b) cross sectional view. Insert—view of sludge drying beds with sludge in various stages of dryness

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(Eddy, 1999) Typical area requirements for open sludge drying beds for various types of biosolids* Type of biosolids


Sludge loading rate


kg dry solids/m2*yr



Primary and trickling-filter humus digested



Primary and waste-activated digested



Primary and chemically precipitated digested



Primary digested

*Corresponding area requirements for covered beds vary from about 70 to 75 percent of those for the open beds (Eddy, 1999)

Unplanted drying beds in Ghana (left) and rain protected unplanted drying bed at the Arcata wastewater treatment plant, USA (right).

Reed beds

Reed beds are similar in appearance to subsurface flow constructed wetlands, which consist of channels or trenches filled with sand or rock to support emergent vegetation. The difference between reed beds used for biosolids application and subsurface flow wetlands is that the liquid

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biosolids are applied to the surface of the beds (as compared to subsurface application) and the filtrate flows through the gravel to underdrains.

Typical Reed Bed

Unlike drying beds, reed beds do not need desludging before each new application as the root system of the plants maintains the permeability. The sludge is added intermittently once a week and only removed every 5 to 10 years.

Cross section of a reed bed for dewatering and storage of biosolids

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(Eddy, 1999)

Typically, reed beds are constructed of washed river-run gravel in the following layers (from bottom to surface) :

(1) a 250 mm deep drainage layer composed of 20 mm washed gravel (2) a 250 mm deep layer composed of 4 to 6 mm washed gravel, and (3) a 100 to 150 mm layer of sand (0.4 to 0.6 mm).

Sometimes an even coarser bottom layer is used. At least 1 m of freeboard above the sand layer is provided for a 10-year accumulation of sludge. “Phragmites australis� (reeds) are planted on 300 mm centers in the gravel layer just below the sand. Other wetland vegetation can be used, although reeds are the most popular. The first sludge application is made after the reeds are well established. Harvesting of the reeds is practiced typically in the winter by cutting the tops back to a level above the sludge blanket. Harvesting is necessary whenever the plant growth becomes too thick and restricts the even flow of sludge. The harvested material can be composted, burned, or landfilled. The design loading rates for reed beds range from 30 to 100 kg/m2*yr, depending on the nature of the sludge and climatic conditions. The liquid sludge is applied intermittently, as in sand drying beds. The typical sludge depth applied is 75 to 100 mm every week to 10 d.

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Planted sludge drying beds, also designated as reed beds or constructed wetlands, could minimise the need for frequent removal of dried sludge as they can be operated for several years before sludge removal becomes necessary

Sludge lagoons

Drying lagoons may be used as a substitute for drying beds for the dewatering of digested sludge. Lagoons are not suitable for dewatering untreated sludges, limed sludges, or sludges with a highstrength supernatant because of their odor and nuisance potential. The performance of lagoons, like that of drying beds, is affected by climate; precipitation and low temperatures inhibit dewatering. Lagoons are most applicable in areas with high evaporation rates. Dewatering by subsurface drainage and percolation is limited by increasingly stringent environmental and groundwater regulations. If a groundwater aquifer used for a potable water supply underlies the lagoon site, it may be necessary to line the lagoon or otherwise restrict significant percolation.

Biosolids are removed mechanically, usually at a solids content of 25 to 30 %. The cycle time for lagoons varies from several months to several years. Typically, biosolids are pumped to the lagoon for 18 months, and then the lagoon is rested for 6 months. Solids loading criteria range from 36 to 39 kg/m3*yr of lagoon capacity.


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Thls is a technique that relies both on the settling characteristics of sludge and solar evaporation. The considerable labor involved in sludge drying bed operation may be avoided by the use of sludge lagoons. These lagoons are nothing but excavated areas in which digested sludge is allowed to drain and dry over a period of months or even a year or more . They are usually dug out by bulldozers, or other dirt-moving equipment, with the excavated material used for building up the sides to confine the sludge. Depths may range from 0,5 to 2 m. Areas vary, and although drainage is desirable, it is not usually provided. Digested sludge is drawn as frequently as needed, with successive drawings on top of the previous ones until the lagoon is filled. A second lagoon may then be operated while the filled one is drying. After the sludge has dried enough to be moved, a bulldozer, or a tractor with an end-loader, may be used to scoop out the sludge Although lagoons are simple to construct and operate, there can be problems associated with sizing them. These problems largely arise from uncertainty in estimating the solar evaporative capacity. In semi-arid regions evaporation ponds are a conventional means of disposing of wastewater without contamination of ground or surface waters. Evaporation ponds as defined herein will refer to lined retention facilities. Successful use of evaporation for wastewater disposal requires that evaporation equal or exceed the total water input to the system, including precipitation. The net evaporation may be defined as the difference between the evaporation and precipitation during any time period. Evaporation rates are to a great extent dependent upon the characteristics of the water body. Evaporation from small shallow ponds is usually considered to be quite different than that of large lakes mainly due to differences in the rates of heating and cooling of the water bodies because of size and depth differences. Additionally, in semi-arid regions, hot dry air moving from a land surface over a water body will result in higher evaporation rates for smaller water bodies. The evaporation rate of a solution will decrease as the solids and chemical composition increase.

Sludge lagoons in summary can be characterized as anaerobic lagoons.

Some advantages and disadvantages of sludge lagoons are listed in the following sections:

Advantages •

More effective for rapid stabilization of strong organic wastes, making higher influent organic loading possible


Produce methane, which can be used to heat buildings, run engines, or generate electricity, but methane collection increases operational problems


Produce less biomass per unit of organic material processed. Less biomass produced equates to savings in sludge handling and disposal costs.

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Do not require additional energy, because they are not aerated, heated, or mixed

Less expensive to construct and operate

Ponds can be operated in series

Disadvantages •

They require a relatively large area of land

Odour emissions are possible to detect

2.7.8 Dry sludge volume reduction Of immediate concern is how we can reduce the volume of so-called “dry” sludge, at solids contents ranging anywhere from 30 to 60 %, even further.

Heat drying

Heat drying involves the application of heat to evaporate water and to reduce the moisture content of biosolids below that achievable by conventional dewatering methods. The advantages of heat drying include reduced product transportation costs, further pathogen reduction, improved storage capability, and marketability.

The classification of dryers is based on the predominant method of transferring heat to wet solids. These methods are convection, conduction, radiation, or a combination of both. Convection In convection (direct drying) systems the wet sludge directly contac the heat-transfer mechanism, usually hot gases. Direct (convection) dryers that have been used for drying municipal wastewater sludge are the flash dryer, rotary dryer, and fluidized-bed dryer.

Flash Dryer Flash drying involves pulverizing the sludge in a cage mill or by an atomized suspension technique in the presence of hot gases. The equipment is designed so that the particles remain in contact with the turbulent hot gases long enough to accomplish mass transfer of moisture from sludge to the gases.

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It is possible to achieve a moisture content of 8 to 10 % in this operation. The dried sludge may be used or sold as soil conditioner or it may be incinerated in a furnace in any proportion up to 100 percent of production. Rotary Dryer Rotary dryers have been used for the drying of raw primary sludge, waste-activated sludge, and digested primary biosolids. A rotary dryer consists of a cylindrical steel shell that is rotated on bearings and usually is mounted with its axis ata slight slope from the horizontal. The feed sludge is mixed with previously dried sludge cake in a blender located ahead of the dryer. It is possible to achieve a moisture content of 5 to 10 % in this operation.

Rotary sludge dryer

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(Eddy, 1999) Fluidized-Bed Dryer This new type of dryer has the capability of producing a pellet product, similar to that obtained from rotary drying systems. The dryer system has certain components that are similar to rotary dryers: product classification and cooling of the product before storage and loading. The heat required for evaporation is supplied by steam via an in-bed heat exchanger. A uniform temperature of 120째C is maintained in the bed through intimate contact between the sand granules and the fluidizing air.

NIRAS VOLUME 2 [2.7] View of a fluidized-bed reactor


(Eddy, 1999) Conduction In conduction (indirect) drying systems, a solid retaining wall separates the wet sludge from the heat-transfer medium, usually steam or another hot fluid. Indirect dryers are designed in either a horizontal or vertical configuration. Horizontal dryers employ paddles, hollow flights, or disks mounted on one or more rotating shafts to convey biosolids through the dryer. It is possible to achieve a moisture content of 5 to 10 % in this operation.

Radiation In radiation (infrared) drying systems, infrared lamps, electric resistance elements, or gas-fired incandescent refractories supply radiant energy that transfers to the wet sludge and evaporates moisture.

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Incineration of sludge involves the total conversion of organic solids to oxidized end products, primarily carbon dioxide, water, and ash. Incineration is used most commonly by medium- to large sized plants with limited disposal options.

In all types of incinerators, the gases from combustion must be brought to and kept at a temperature of 650 oC to 800 oC until they are completely burned. This is essential to prevent odor nuisance from stack discharge. It is also necessary to maintain effective removal of dust, fly ash and soot from the stack discharge. This may be done by a settling chamber, by a centrifugal separator, or by a Cottrell electrical precipitator. The selection depends on the degree of removal efficiency required for the plant location.

All types of sludge, primary, secondary, raw or digested sludge, may be dried and burned.

Raw primary sludge with about 70% volatile solids contains about 5 kwh per 1 kg of dry solids and when combustion is once started will bum without supplementary fuel , in fact an excess of heat is usually available. This process is called autogenous burn, and it takes place always when volatitle content of the sludge cake is high enough (>50-60%).

Digested sludge may or may not require supplementary fuel, depending on the moisture content of the cake and percent volatile solids or degree of digestion. Raw activated sludge generally requires supplementary fuel for drying and burning. In all cases, supplementary fuel is necessary to start operation and until combustion of the solids has been established.

Incineration of sludge has gained popularity throughout the world, especially at large plants. It has the advantages of economy, freedom of odor, independence of weather and the great reduction in the volume and weight of end product to be disposed of. There is a minimum size of sewage treatment plant below which incineration is not economical. There must be enough sludge to necessitate reasonable use of costly equipment. One of the difficulties in operating an incinerator is variations in tonnage and moisture of sludge handled.

The major advantages of incineration are •

maximum volume reduction thereby lessening disposal requirements

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destruction of pathogens and toxic compounds

energy recovery potential

Disadvantages include •

high capital and operating cost

highly skilled operating and maintenance staffs are required

the residuals produced (air emissions and ash) may have adverse environmental effects

disposal of residuals, which may be classified as hazardous wastes, if they exceed prescribed maximum pollutant concentrations.

There are two major incinerator technologies used in this process. They are (1) the multiple hearth incinerator, and (2) the fluidized bed incinerator. An incinerator is usually part of a sludge treatment system which includes sludge thickening, macerations, dewatering (such as vacuum filter, centrifuge, or filter press), an incinerator feed system, air pollution control devices, ash handling facilities and the related automatic controls. The operation of the incinerator cannot be isolated from these other system components. Of particular importance is the operation of the thickening and dewatering processes because the moisture content of the sludge is the primary variable affecting the incinerator fuel consumption.

Sewer sludge as sludge cake normally contains from 55 to 85% moisture. It cannot burn until the moisture content has been reduced to no more than 30%. The purpose of incineration is to reduce the sludge cake to its minimum volume, as sterile ash. There are three objectives incineration must accomplish:

1. dry the sludge cake 2.destroy the volatile content by burning 3.produce a sterile residue or ash

There are four basic types of incinerators used in wastewater treatment plants. They are the multiple hearth incinerator, the fluid bed incinerator , the electric furnace, and the cyclonic furnace

Multiple hearth incinerator The basic configuration and features of the multiple hearth incinerator are illustrated in next Figure. Sludge cake enters the furnace at the top. The interior of the furnace is composed of a

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series of circular refractory hearths, which are stacked one on top of the other. There are typically five to nine hearths in a furnace.

Typical multiple-hearth incinerator

(Eddy, 1999)

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Flames in the middle of an out hearth

(California State University, 2006)

Fluid bed incinerator The basic configuration and features of the fluid bed incinerator are illustrated in Next Figure. This technology has been around since the early 1960s. In this system, air is introduced at the fluidizing air inlet at pressures of 0,2 to 0,5 bar. The air passes through openings in the grid supporting the sand and creates fluidization of the sand bed. Sludge cake is introduced into the bed. The fluidizing air flow must be carefully controlled to prevent the sludge from floating on top of

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the bed. Fluidization provides maximum contact of air with sludge surface for optimum burning. The drying process is practically instantaneous. Moisture flashes into steam upon entering the hot bed. Some advantages of this system are that the sand bed acts as a heat sink so that after shutdown there is minimal heat loss. With this heat containment, the system will allow startup after a weekend shutdown with need for only one or two hours of heating. The sand bed should be at least 650 oC when operating. Typical fluidized-bed incinerator

(Eddy, 1999)

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The scheme of a fluidised-bed incinerator - Bubbling bed technology

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The scheme of a fluidised-bed incinerator - Circulating bed technology

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Electric furnace The basic features of the electric furnace are illustrated in next Figure. The electric furnace is basically a conveyor belt system passing through a long rectangular refractory lined chamber. Heat is provided by electric infrared heating elements within the furnace. Cooling air prevents local hot spots in the immediate vicinity of the heaters and is used as secondary combustion air within the furnace. The conveyer belt is made of continuous woven wire mesh chosen of steel alloy that will withstand the 700 oC to 850 oC. The sludge on the belt is immediately leveled to 2-3 cm. The belt speed is designed to provide burnout of the sludge without agitation.

Typical Electric furnace incinerator

(Nickolas P. Cheremisinoff, 2002)

Cyclone furnace The basic features of the cyclone furnace are illustrated in next Figure. The cyclonic furnace is a single hearth unit where the hearth moves and the rabble teeth are stationary. Sludge is moved

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towards the center of the hearth where it's discharged as ash. The furnace is a refractory lined cylindrical shell with a domed top. The air, heated with the immediate introduction of supplemental fuel creates a violent swirling pattern which provides good mixing of air and sludge feed. The air, which later turns into flue gas, swirls up vertically in cyclonic flow through the discharge flue in the center of the doomed roof. One advantage of these furnaces is that they are relatively small and can be placed in operation, at operating temperature within an hour.

Typical cyclone furnace incinerator

Synopsis of High temperature processes High-temperature processes should be considered where available land is scarce, stringent requirements for land disposal exist, destruction of toxic materials is required, or the potential exists for recovery of energy, either with wastewater solids alone or combined with municipal refuse.

High-temperature processes have potential advantages over other methods which include: •

Maximum volume reduction. Reduces volume and weight of wet sludge cake by approximately 95 %, thereby reducing disposal requirements.

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Detoxification. Destroys or reduces toxics that may otherwise create adverse environmental impacts

Energy recovery. Potentially recovers energy through the combustion of waste products, thereby reducing the overall expenditure of energy.

Disadvantages of high-temperature processes include: •

Cost. Both capital and operation and maintenance costs, including costs for fuel, are generally higher than for other disposal alternatives.

Operating problems. High-temperature operations create high maintenance requirements and can reduce equipment reliability.

Staffings. Highly skilled and experienced operators are required for high supplemental temperature processes. Municipal salaries and operator status may have to be raised in many locations to attract the proper personnel.

Environmental impacts. Discharges to atmosphere (particulates and other toxic or noxious emissions), surface waters (scrubbing water), and land (furnace residues) may require extensive treatment to assure protection of the environment.

2.7.9 Sludge handling At the end of the day what we are left with is ultimate sludge . There is no ultimate destruction of sludge, only ultimate sludge that we are left with. The final engineering solution we need to devise is how to ultimately handle this waste. It simply boils down to whether we select a so-called pollution prevention related technology or a final disposal option for the solid waste.

Like the liquid effluent from the treatment plant, there are two broad methods for the disposal of sludge : disposal in water, and disposal on land. But first of all, safe and correct transfer is the primary issue .

Transfer of sludge

Practices to Prevent Mud or Biosolids From Being Tracked Onto Public Roadways

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Vehicles transporting biosolids should be cleaned before they leave the wastewater treatment plant.

Concrete or asphalt off -loading pads at the storage facility will help keep equipment clean and make cleanup of drips or spills easier

The storage facility should have provisions to clean trucks and equipment when the need arises. Mud on tires or vehicles can be hand-scraped or removed with a high-pressure washer or with compressed air (as long as this does not exacerbate an existing dust problem).

Use mud flaps on the back of dump trailers to preclude biosolids getting on tires or undercarriage during unloading operations

Install a temporary gravel access pad as necessary at the entrance/exit to avoid soil ruts and tracking of mud onto roads.

Public roadways accessing the site should be inspected each day during operational periods and cleaned promptly (shovel and sweep).

Disposal in water

Disposal in water is one option to consider. This is an economical but not common method because it is contingent on the availability of bodies of water adequate to permit it. At some seacoast cities, sludge either raw or digested is pumped to barges and carried to sea (the context of these discussions is strictly sewage) to be dumped in deep water far enough off shore to provide huge dilution factors and prevent any ill effects along shore.

Overall, this is an environmentally unfriendly option, and the bottom line is that it is no different that straight landfill, and in fact can be more environmentally damaging.

Disposal in land

Under land disposal the following methods may be included: •



Burial is used principally for raw sludge, where, unless covered by earth, serious odor nuisances are created. The sludge is run into trenches 0,5 to 1 m wide and about 0,5 m deep. The raw sludge in the trenches should be covered by at least 30 cm of earth. Where large areas of land are

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available, burial of raw sludge is probably the most economical method of sludge disposal as it eliminates the costs of all sludge treatment processes. It is, however, rarely used and even then as a temporary makeshift because of the land area required. The sludge in the trenches may remain moist and malodorous for years so that an area once used cannot be reused for the same purpose or for any other purpose for a long period of time.

The option of using sludge for Fill is confined almost entirely to digested sludge which can be exposed to the atmosphere without creating serious or widespread odor nuisances. The sludge should be well digested without any appreciable amount of raw or undigested mixed with it. Either wet or partially dewatered sludge, such as obtained from drying beds or vacuum filters can be used to fill low areas. Where wet sludge is used the area becomes a sludge lagoon. When used as a method of disposal, the lagoon area is used only until filled, and then abandoned. When used as a method of treatment, the sludge after some drying, is removed for final disposal and the lagoon reused. Lagoons used for disposal are usually fairly deep. Sludge is added in successive layers until the lagoon is completely filled. Final disposal of dgested sludge by lagoons is economical as it eliminates all dewatering treatments. It is applicable, however, only where low waste areas are available on the plant site or within reasonable piping distance .

It should be clear to you that the above options for sludge sill are temporary solutions, and they still have environmental trade-offs. In the end, they too represent environmentally unfriendly solutions and are end-of-pipe disposal technologies that add costs to treatment.

Application as soil conditioner

Soil conditioner

Raw primary sludge, unless composted, is unsatisfactory as a soil conditioner because of its effect on the soil and on growing plants, and because of the health hazards involved.

Raw activated sludge, after heat drying, is established as a superior sludge product. Such sludge retains most of its organic solids and it contains more nitrogen than other sludge.

Digested sludge from all sewage treatment processes are materials of moderate but definite value as a source of slowly available nitrogen and some phosphorous. They are comparable with farm yard manure except for a deficiency of potash. Their principal value is the humus content resulting

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in increased moisture-holding capacity of the soil and a change in soil structure which results in a greater friability.



The basic equation used to calculate percent solids is

Problem Determine the liquid volume before and after digestion and the percent reduction for 500 kg (dry basis) of primary sludge with the following characteristics: Primary


Solids, %



Volatile matter, %


60 (destroyed)

Specific gravity of fixed solids,Ss



Specific gravity of volatile solids,Sv



Solution Average specific gravity of all the solids in the primary sludge Sx= Volatile matter x Sv + Non Volatile matter x Ss Sx = 60% * 1 + 40% * 2,5 = 1,32 Specific gravity of the primary sludge S= Solids % * Sx + Water % * Sw S= 5% * 1,32 + 95% * 1 = 1,01 Volume of the primary sludge V=

= 9,9 m3

Average specific gravity of all the solids in the sludge after digestion Sx’= Volatile matter x Sv + Non Volatile matter x Ss Sx’ = 60%*40% * 1 + 40% * 2,5 = 1,24 Specific gravity of the sludge after digestion S’= Solids % * Sx + Water % * Sw S’= 10% * 1,24 + 90% * 1 = 1,024

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Volatile matter digested 500 kg *60% * 60% = 180 kg Volume of the sludge after digestion V=

= 3,125 m3

Percentage of sludge reduction after digestion 9,9 – 3,125 / 9,9 = 68,4 %

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General flow diagramm Generalized sludge-processing flow diagram

(Eddy, 1999)

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GLOSSARY Aerobic - Living or active in the presence of oxygen. Refers especially to microorganisms and/or decomposition of organic matter Anaerobic - Living or active in the absence of oxygen (e.g., anaerobic microorganisms). Biosolids - The organic solids product of municipal wastewater treatment that can be beneficially utilized. Wastewater treatment solids that have received processes to significantly reduce pathogens or processes to further reduce pathogens treatment, or their equivalents. The solids: liquid content of the product can vary: liquid biosolids, 1%-4% solids; thickened liquid biosolids, 4%- 12% solids; dewatered biosolids, 12%-45% solids; dried biosolids, >50% solids (advanced alkaline stabilized, compost, thermally dried). In general, liquid biosolids and thickened liquids can be handled with a pump. Dewatered/dried biosolids are handled with a loader. Cake - Dewatered biosolids with a solids concentration high enough (>12%) to permit handling as a solid material. (NOTE some dewatering agents might still cause slumping even with solids contents higher than 12%). Composting - The accelerated decomposition of organic matter by microorganisms, which is accompanied by temperature increases above arnbient; for biosolids, composting is typically a managed aerobic process. Dewatered biosolids - The solid residue (12% total solids by weight or greater) remaining after removal of water from a liquid biosolids by draining, pressing, filtering, or centrifuging. Dewatering is distinguished from thickening in that dewatered biosolids may be transported by solids handling procedures. Digestion - The decomposition of organic matter by microorganisms with consequent volume reduction. Anaerobic digestion produces carbon dioxide and methane, whereas aerobic digestion produces carbon dioxide and water. Lagoon - A reservoir or pond built to contain water, sediment, and/or manure usually containing 4% to 12% solids until they can be removed for application to land Land application - The spreading or spraying of biosolids onto the surface of land, the direct injection of biosolids below the soil surface, or the

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incorporation into the surface layer of soil. Also applies to manure and other organic residuals Stability - The characteristics of a material that contribute to its resistance to decomposition by microbes and to generation of odorous metabolites. The relevant characteristics include the degree of organic matter decomposition, nutrient, moisture, and salts content, pH, and temperature. For biosolids, compost, or animal manure, stability is a general term used to describe the quality of the material taking into account its origin, processing, and intended use.

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What is the primary function of sludge thickening ?


How does sludge temperature affect the efficiency of gravity thickeners and what measure should be taken during summertime operation to reduce gas production and rising sludge ?


Why are centrifuges not commonly used to thicken primary sludges?


List the factors affecting gravity belt thickening performance


What are the goals of stabilization ?

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List the factors affecting anaerobic digestion.


List the factors affecting aerobic digestion.


How does temperature affect aerobic digester permormance?


List two chemicals used to stabilize sludges

10. Why is continuous operation of a heat treatment unti desirable?

11. What is the primary objective of sludge dewatering?

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12. Typical permormance data indicate that secondary sludges do not dewater as readily as primary sludges. Why is this so?

13. Why does the filter media in vacuum filtration, pass through a washing zone?

14. Why are chemically stabilized sludges generally not composted?

15. What is an autogenous burn?

16. Thickening is practiced in order to increase weight of sludge as much as possible . True or false?

17. The best results can in gravity thickening can be achived with primary sludge. True or false?

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18. Gravity thickening has the lower operating costs than any other thickening method. True or false?

19. Purpose of sludge stabilization is to achieve low compressibility in the sludge. True or false?

20. When stabilization by lime takes place, the final product mass is increased. True or false?

21. Aerobic sludge digestion produces a valuable by-product in the form of methane gas. True or false?

22. Digester temperature can vary up to ¹ 5 °C per day in an anaerobic digester. True or false?

23. Aeration in aerobic digestion continues for minimum retention time. True or false?

5 days of

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24. Thermophilic operating temperatures cannot be achieved without external heat input


autothermal thermophilic aerobic digestion.

True or false?

25. The two principal methods of composting worldwide may be classified as agitated or static. True or false?

26. Autothermal






composting methods, can achieve the better degree of attenuation in pathogens removal. True or false?

27. In heat drying processes it is possible to achieve a moisture content of 5 to 10 % . True or false?

28. In incineration processes, total destruction of pathogens and toxic compounds cannot be achieved. True or false?

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29. Only High-temperature processes can achieve the maximum volume reduction of sludge. True or false?

30. 1 tn of primary sludge with 5% solids, means that total solids in dry basis in the sludge is 50 kg. True or false?


The primary function of sludge thickening is to reduce the sludge volume to be handled in subsequent processes.


As the temperature of the sludge increases, the rate of biological activity increases and the sludge tends to gasify and rise at a higher rate. As a result, during summertime operation settled sludge has to be removed at a faster rate from the thickener.


Centrifuges are not commonly used to thicken primary sludges, because they have inlet assemblies that clog easily

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Main factors that affect gravity belt thickener performance include belt type, chemical conditioning, belt speed, and hydraulic and solid loadings.


The purpose of sludge stabilization is to :

reduce, or eliminate the potential for putrefaction ; stabilize the organic matter

• • • • • 6.

eliminate the offensive odours eliminate pathogenic organisms to permit reuse or disposal volume reduction production of usable gas (methane) to improve the dewaterability of sludge

Main factors that affect anaerobic digestion include , sludge type, digestion time, digestion temperature, and mixing


Main factors that affect aerobic digestion include , sludge type, digestion time, digestion temperature, quantity of air supplied and volatile solids loading


Aerobic process is directly proportional to temperature. So, given the fact that desirable aerobic digestion temperatures are approximately 18 to 27 °C. as the temperature decreases, the rate of biological activity decreases as well.


Lime and chlorine

10. Continuous operation of a heat treatment unit is desirable because energy is not wasted in allowing the heat exchanger and reactor contents to cool down and be heated back to the desired temperature each day when operated as a batch process. 11. Sludge dewatering is used to reduce volume by removing the water to permit easy handling and economical reuse or disposal 12. Secondary sludges do not dewater as readily as primary sludges because secondary sludges contain fine, low-density solids that have

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large surface areas and relatively large quantities of water associated with them. 13. Filter media passes through a washing zone to remove fine particles from the media and to reduce the possibility of media blinding 14. Chemical stabilization produces environments that are unsuitable for microorganisms survival and will not support life of composting vacteria unless the sludges are neutralized and favorable conditions exist. 15. An autogenous burn occurs when the volatile content of the sludge cake is high enough that the cake will burn without the additional heat input from the burner 16. False 17. True 18. True 19. False 20. True 21. False 22. False 23. False 24. False 25. True 26. True 27. True 28. False 29. True 30. True

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Volume 2 part 5 final b