ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT
VOLUME 2
MAIN WASTEWATER TREATMENTPROCESSES 2.6
NIREAS VOLUME 2 [2.6] 1 2.6 DISINFECTION
2.6.1 INTRODUCTION 2.6.2 DISINFECTION WITH CHLORINE 2.6.2.1Introduction 2.6.2.2 Health effects of chlorine 2.6.2.3 Common commercial products 2.6.2.4 Mechanism of disinfectant action 2.6.2.5 Parameters affecting disinfectant action 2.6.2.6 Kinetic disinfection by chlorination 2.6.2.7Selection of chlorination method & equipment 2.6.2.8 Comparison of different chlorination methods 2.6.2.9 Advantages & disadvantages of disinfection by chlorination 2.6.2.10Designing a chlorination system 2.6.2.11 Troubleshooting guide – chlorination 2.6.3 DISINFECTION BY ULTRA VIOLET RADIATION 2.6.3.1 Introduction 2.6.3.2 Health effects of UV radiation 2.6.3.3 Mechanism of disinfectant action 2.6.3.4 Parameters affecting disinfectant action 2.6.3.5 Calculations of disinfection by UV radiation 2.6.3.6 Selection of UV disinfection method & equipment 2.6.3.7 UV disinfection system installation and installation requirements 2.6.3.8 UV disinfection system operation and maintenance 2.6.3.9 Advantages & disadvantages of disinfection by UV radiation 2.6.3.10
Designing a UV disinfection system
2.6.4OZONATION
NIREAS VOLUME 2 [2.6] 2 2.6.4.1Introduction
2.6.4.2 Mechanism of disinfectant action 80 2.6.4.3 Parameters affecting disinfectant action 2.6.4.4 Calculations of disinfection by ozonation 2.6.4.5 Selection of ozonation method & equipment 2.6.4.6 Operation and maintenance 2.6.4.7 & disadvantages of disinfection by ozonation 2.6.5 DISINFECTION WITH CHLORINE DIOXIDE 2.6.5.1 Introduction 2.6.5.2 Mechanism of disinfectant action 2.6.5.3 Disinfection method & equipment 2.6.5.4 Advantages & disadvantages of disinfection by ClO2 2.6.6 COMPARISON BETWEEN DISINFECTION METHODS
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2.6
DISINFECTION
2.6.1 INTRODUCTION Disinfection is considered to be the primary mechanism for the inactivation/destruction of pathogenic organisms to prevent the spread of waterborne diseases to downstream users and the environment. It is important that wastewater be adequately treated prior to disinfection in order for any disinfectant to be effective. Table bellow lists some common microorganisms found in domestic wastewater and the diseases associated with them.
1-1 Infectious agents potentially present in untreated domestic wastewater
(Tchobanoglous G., 1998)
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Preliminary and primary treatment processes used in wastewater treatment (e.g., coarse and fine screens, grit chambers, and primary clarifiers) are capable of removing or destroying a large number of bacteria. Typical removal efficiencies for various treatment operations in a wastewater treatment plant are reported in the following table.
1-2 Removal or destruction of bacteria by different treatment processes Process
Percent Removal
Coarse screens
0-5
Fine screens
10-20
Grit chambers
10-25
Plain sedimentation
25-75
Chemical precipitation
40-80
Trickling filters
90-95
Activated sludge
90-98
Chlorination
98-99.999
(Eddy, 1999)
Disinfection is most commonly accomplished by the use of 1) Chemical agents 2) Physical agents 3) Mechanical means 4) Radiation
The most common chemical agents used in disinfection are chlorine and its compounds. Ozone is highly effective but it does not leave residual. Acids and alkalies are sometimes used since pH .11 or pH ,3 are toxic to most bacteria. Bromine, iodine, phenols, alcohols, and hydrogen peroxide are other common chemical disinfection agents. Heat and light (especially UV light) are effective physical disinfection agents. However, using heat and UV light to disinfect large quantities of effluents is cost prohibitive. The presence of suspended matter in effluents can also reduce the efficacy of UV radiation.
A survey conducted in 1998 reveals the following distribution of disinfection technology use in the United States:
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1-3 Percentage of water treatment systems using different disinfection techniques for U.S. municipal services (1998)
(Felipe Solsona, Juan Pablo Méndez, 2003)
i. DISINFECTION WITH CHLORINE
2.6.1.1 Introduction Water disinfection by chlorination, massively introduced worldwide in the early twentieth century, set off a technological revolution in wastewater treatment, complementing the known and used process of filtration. The keys to its success are its easy accessibility in almost all of the world’s countries, reasonable cost, capacity for oxidation –the mechanism for destroying organic matter- , and residual effect.
Although chlorine and chlorine-related substances are not perfect disinfectants, they have a number of characteristics that make them highly valuable:
• They have broad-spectrum germicidal potency. • They show a good degree of persistence in networks, when reuse of treated wastewater takes place.Their easily measurable residual properties can be monitored in networks after treatment and/or delivery to site of final disposal. • The feeding equipment is simple, reliable and inexpensive. At the small community level, there are also a number of “appropriate technology” devices that local operators are able to handle easily. • Chlorine and chlorine-based compounds are easily found, even in remote areas of developing countries. • This method is economic and cost-effective.
The following chlorine-related compounds for water disinfection can be found in the market:
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• gaseous chlorine • chlorinated lime • sodium hypochlorite • calcium hypochlorite
The amount of disinfectant that will be needed will depend upon the water flow to be treated, the required dosage according to the water quality and the country’s treated wastewater standards.
2.6.1.2
More 2[2.5.1] Health effects of chlorine
The reaction of the human body to chlorine depends on the concentration of chlorine present in air, and on the duration and frequency of exposure. Effects also depend on the health of an individual and the environmental conditions during exposure. When small amounts of chlorine are breathed in during short time periods, this can affect the respirational system. Effects vary from coughing and chest pains, to fluid accumulation in the lungs. Chlorine can also cause skin and eye irritations. These effects do not take place under natural conditions. When chlorine enters the body it is not very persistent, because of its reactivity. Pure chlorine is very toxic, even small amounts can be deadly. During World War I chlorine gas was used on a large scale to hurt or kill enemy soldiers. The Germans were the first to use chlorine gas against their enemies. Chlorine is much denser than air, causing it to form a toxic fume above the soil. Chlorine gas affects the mucous membrane (nose, throat, eyes). Chlorine is toxic to mucous membranes because it dissolves them, causing the chlorine gas to end up in the blood vessels. When chlorine gas is breathed in the lungs fill up with fluid, causing a person to sort of drown. Exposure for 30 to 60 min in atmospheric air, containing from 40 to 60 ppm chlorine is dangerous and in higher concentrations (approximately 1000 ppm) is immediately lethal.
2.6.1.3 Common commercial products Commercial chlorine products are obtained by different methods, which determine their concentration of active chlorine, presentation and stability. “Active chlorine” is the percentage by weight of molecular chlorine rendered by a molecule of the compound. If, for example, a certain solution contains 10% active chlorine, this is equivalent to 10 g of chlorine gas being bubbled (and totally absorbed) in 100 ml (100 g) of water without any loss, hence the “10%.” The word “active”
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means that this chlorine is ready to enter into action; it is prepared and “waiting” to attack the organic matter or any other substance that it is capable of oxidizing.
The comparative table below lists the major properties of each commercial chlorine product:
1-4 Major properties of most common commercial chlorine products
(Felipe Solsona, Juan Pablo Méndez, 2003)
2.6.1.4
Mechanism of disinfectant action
The addition of chlorine in water, causes the formation of hypochlorous acid (HOCL), which is a direct indicator of disinfectant power. MORE{2.5.2] Main reactions take place when adding chlorine in water are given bellow.
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a.First stage
The reaction in the case of gaseous chlorine is as follows: CL2 + H2O HCL + HOCL For sodium hypochlorite (NaOCl), the reaction that takes place is: NaOCl + H2O Na+ + OH- + HOCl With calcium hypochlorite and the active portion of chlorinated lime, the reaction is as follows: Ca(OCl)2 + 2H2O Ca++ + 2OH- + 2HOCl When ammonia is present in the water, chemical disinfection produces compounds such as chloramines, dichloramines and trichloramines. The chloramines serve as disinfectants also, but they react very slowly.
b.Second stage The disinfecting agent is hypochlorous acid (HOCl), which splits into hydrogenous ions (H+) and hypochlorite (OCl-) and takes on its oxidizing properties: HOCl H+ + OClHypochlorous acid (HOCl) and hypochlorite (OCl-) as strong oxidizing agents, cause destruction of microorganisms (acts by oxidation of sulfhydryl groups).
2.6.1.5
Parameters affecting disinfectant action
Initial mixing The importance of initial mixing on the disinfection process cannot be overstressed. It has been shown that the application of chlorine in a highly turbulent regime (Rn≥104)1 will result in kills two orders of magnitude greater than when chlorine is added separately to a conventional rapid-mix reactor under similar conditions.
Contact time
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Depending on the characteristics and the species of microorganisms to be eliminated, different contact times are required.
1-5 Disinfection time for several different types of pathogenic microorganisms with chlorinated water, containing a chlorine concentration of 1 mg/L (1 ppm) when pH = 7,5 and T = 25 °C Fecal pollutant
Required contact time
E. coli 0157 H7 bacterium
< 1 minute
Hepatitis A virus
about 16 minutes
Giardia parasite
about 45 minutes
Cryptosporidium
about 7 days
The size of the chlorine dose will be obtained by studying the chlorine demand and the expected concentration of residual chlorine, as usually defined by each countryâ&#x20AC;&#x2122;s treated wastewater effluent quality standards. In this connection and as a reference figure, it is considered
that a
concentration of 2 mg/l of free residual chlorine in the treated wastewater effluent after a 60 minute contact period is a guarantee of satisfactory disinfection.
pH level Hypochlorous acid (HOCl) and hypochlorite ions (OCl-) are both present to some degree when the pH of the wastewater is between 6 and 9. When the pH value of the chlorinated water is 7.5, 50% of the chlorine concentration present will consist of undissolved hypochlorous acid and the other 50% will be hypochlorite ions. The different concentrations of the two species make a considerable difference in the bactericidal property of the chlorine, inasmuch as these two compounds have different germicidal properties. As a matter of fact, HOCl efficiency is at least 80 times greater than that of OCl-.
That is the reason why, when monitoring chlorine, it is advisable to monitor the pH level as well, for this will give an idea of the real bactericidal potential of the disinfectants that are present. It is important to mention that the WHO recommends a pH < 8 for appropriate disinfection with chlorine.
Turbidity Turbidity is another significant element in disinfection. Excessive turbidity will reduce the effectiveness of chlorine absorption and at the same time will protect bacteria and viruses from its
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oxidizing effects. For that reason, the WHO recommends a turbidity of less than 5 NTU, with under 1 NTU as the ideal.
Temperature The temperature of the water to be disinfected can have a significant effect on chlorine efficiency. The time needed for disinfection becomes longer as the temperature of the water gets lower. There is a noticeable difference in the killing rate of bacteria between 2 and 20°C.
Organic matter Certain types of organic compounds, when found in water, exert a high chlorine demand , affecting negatively chlorination process. Also, when the water to be disinfected contains organic materials known as “precursors,” (organic matter, humic acids, etc.) disinfection by-products (DBPs) may be produced. The most characteristic constituents of chlorination DBPs are the trihalomethanes (THM) and haloacetic acids (HAAs).
Formation of DBPs is of great concern because of the
potential impact of these compounds on public health and the environment. THM derivatives of methane are considered as carcinogenic according to E.E. directive (98/83).
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MORE 1-6 Representative disinfection byproducts resulting from the chlorination of wastewater containing organic and selected inorganic constituents
(Eddy, 1999)
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2.6.1.6
Kinetic disinfection by chlorination
Considering that all of the previous parameters affecting disinfection are stable, disinfectant action of chlorine depents only by contact time and residual chlorine concentration in the outlet of chlorination tank.
For the calculation of required chlorine dosage, the following equation of Collins-Selleck (White, 1978) is used : N/No=(1+0,23 Ct)-3
Where,
N=Fecal coliforms in the inlet of chlorination tank No=Fecal coliforms in the outlet of chlorination tank C=concentration of residual chlorine in the outlet of chlorination tank (mg/L) t=contact time of chlorine in chlorination tank (min)
1-7 Typical chlorine dosages, based on combined chlorine unless otherwise indicated, required to achieve different effluent total coliform disinfection standards for various wastewaters based on a 30-min contact time
(Eddy, 1999)
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2.6.1.7
Selection of chlorination method & equipment
The choice of the chlorine doser or feeder depends on three elements:
• The characteristics of the chlorine product to be used. • The chlorine dose to be added • The treated wastewater flow to be disinfected.
1-8 Most widely used equipment for chlorination
(Felipe Solsona, Juan Pablo Méndez, 2003).
Chlorine gas
Disinfection by gaseous chlorine is inexpensive and the most widely used technology in the world.
Chlorine gas feeders work under two principles: by vacuum through pipe injection and under pressure by means of diffusion in open channels or pipes. The most commonly used is the vacuum system.
Vacuum gas chlorinators
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This system consists of a gas cylinder, a regulator with a rotameter (feed rate indicator) and an injector. It operates through the vacuum produced by the water flow-activated venturi injector that ejects a mixture of water and gas at the application point, where the gas diffuses and dissolves. The system should be equipped with anti-return valves to keep water from entering the chlorine pipes and corroding the equipment if the operation is interrupted for any reason.
1-1 Chlorine gas vacuum equipment
(Felipe Solsona, Juan Pablo MĂŠndez, 2003). 1-2 Gas chlorinator
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(John M. Stubbart, 2006 ) 1-3 Vacuum-solution feed chlorination
(Office of Water Programms, College of Engineering and Computer Science, California State University, 2008)
Pressurized gas chlorinators Use of this type of chlorinator is usually recommended when there is no possibility of employing a pressure differential or when there is no electric power to operate a booster pump that would produce the necessary pressure differential for the operation of vacuum chlorinators. The system consists of a diaphragm activated by a pressurized regulator while a rotameter indicates the chlorine feed rate. A regulator controls the progression of the chlorine gas toward the diffuser.
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1-4 Pressurized chlorine gas feeding equipment
(Felipe Solsona, Juan Pablo MĂŠndez, 2003). ď&#x201A;§
Gas chlorinator installation and installation requirements
To install a gas chlorination system, it is first necessary to determine the most suitable type of chlorinator. The factors that determine the gas chlorinator to be installed are the capacity to supply the necessary amount of chlorine per unit of time (kg/h) and the operational flexibility.
MORE The typical feeding rates for the smallest vacuum chlorinators range from approximately 10 to 100 g/h. The most common devices have maximum operating capacities of 2 kg/h, 5 kg/h and 10 kg/h, making it possible to serve medium-sized to large cities. The smallest pressurized chlorinators have a capacity of between 10 to 150 g/h.
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The maximum continuous feeding rate must be calculated according to the lowest environmental temperature forecast because the pressure of the chlorine gas in the cylinder varies according to that temperature. The environmental temperature must be above â&#x20AC;&#x201C;5 °C for a continuous chlorine gas feeding rate of 120 g/h. As for the installation requirements and precautions, since the most precise way to determine the effective chlorine gas feeding rate being dosed is by measuring the weight of the chlorine consumed, appropriate scales must be used. Correct weighing will make it possible to calculate the exact amount of chlorine being dosed over a given period of time and also when and how soon the cylinders should be replaced.
All chlorine gas installations must be equipped with chains or other anchoring devices well attached to a wall to keep the chlorine cylinders from being accidentally tipped over. Since chlorine is a dangerous gas, it must be handled carefully. For most safety and economy, gas chlorination systems must be designed and installed by experienced personnel and located far away from laboratories, storage areas, offices, operating areas, etc., to avoid contamination from possible leakage.
The figure below shows a typical floor plan for a small gas chlorination facility.
1-5Typical floor plan for a small gas chlorination facility
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(Felipe Solsona, Juan Pablo MĂŠndez, 2003)
The chlorine cylinders must be stored in a separate room designed specifically for that purpose and kept away from direct sunlight to avoid their heating. Installations must be properly ventilated, always at the floor level because chlorine is heavier than air. Since one-ton cylinders are placed in a horizontal position, cranes must be available to replace them and an anchoring system to keep them from rolling.
MORE In the case of pressurized chlorination systems, it is important for the contact chamber, whether channel or tank, to be designed to carry a minimum water head of 0.5 meters over the diffuser to ensure that all of the chlorine gas is dissolved and avoid its loss in the air. Since the pressure of the chlorine gas in the cylinder itself activates this type of chlorinator, there is no need for external electric power. This is an advantage when there is no source of hydraulic or electric power to produce the pressure differential required by a vacuum chlorinator. Relatively little electric power is needed to operate vacuum chlorinators, only enough to introduce the water flow through the ejector (venturi). The needed water flow and differential pressure can be produced by electric or hydraulic means with the aid of a small 1 to 1.5 HP auxiliary (booster) pump. In choosing electrically-operated equipment, the reliability and stability of the power source is an important consideration. In both systems, as a safety measure, a manual pressure relief valve is inserted between the chlorinator and the diffuser to discharge (outside the building) any remaining chlorine gas when cylinders are replaced. In this connection, all large treatment plants must always have a leak detection system and a stock of chlorine neutralizing products on hand. Care must be taken with the materials used in chlorination equipment because they react differently to oxidation. The following table shows the resistance of some of the most common materials.
1-9Resistance of some materials to different forms of chlorine
(Felipe Solsona, Juan Pablo MĂŠndez, 2003)
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ď&#x201A;§
Operation and maintenance of Gas chlorinators
Vacuum chlorinators need to be regularly inspected and maintained by trained operators. The manufacturerâ&#x20AC;&#x2122;s instructions must be followed to ensure that they operate properly and to avoid costly repairs and accidents. This type of system is generally long-lasting and relatively free from problems. Extreme care must be taken to keep moisture out of the gaseous chlorine in the feeding system, for moist chlorine gas will rapidly corrode or destroy the equipment: the plastic parts, metal fittings, valves, flexible connections, etc. The materials used in the chlorination system, including spare parts and accessories, must be appropriate for the handling of moist and dry gaseous chlorine.
MORE Ferric chloride scaling on the pipes, generally due to impurities in the chlorine, must be removed regularly. An appropriate quantity of spare parts must be available at all times. Flexible connections must be replaced as recommended by the manufacturer. Lead gaskets between the cylinder and the chlorinator should be used only once. When the joints between cylinder and chlorinator must be opened to replace cylinders, or for any other reason, the gaskets must be replaced by new ones recommended by the manufacturer. The reuse of used gaskets is probably the most common cause of chlorine gas leakage.
The same care must be taken with pressurized chlorination equipment. It is also necessary to keep in mind that a counter pressure of more than 10 m of water column will cause problems in the diffusion of the chlorine in the pipes; in that case, vacuum-type chlorinators should be chosen. It is common practice for an operator to check and, if necessary, adjust the chlorine gas dose three or four times during an eight-hour shift. Care should be taken not to extract more than 18 kg of
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chlorine gas a day from a single cylinder; more will result in the freezing of the cylinder due to a rapid fall in pressure, known as the “Joule- Thompson effect.”
An experienced operator should take less than 15 minutes to routinely replace an empty cylinder with a full one. For safety reasons, at least two operators should be present for this operation.
Because of its extreme toxicity and corrosiveness, strict safety regulations govern the use of gaseous chlorine. In the case of fire, the tanks or cylinders should be removed first because their fire resistance is guaranteed only up to 88 °C (with a 30-bar internal pressure). Because steel will burn in the presence of chlorine, care must be taken not to crack the containers (by not using a hammer to unblock or unfreeze valves). Moist chlorine is highly corrosive: a chlorine leak will cause external corrosion and the entry of water into pipes carrying chlorine will cause them to corrode inside. Gas masks must be used when handling the containers in any of the areas where chlorine is stored and it should be recalled that masks with carbon filters have a limited service life.
Chlorine Solutions
All chorine-based products, except for chlorine gas, are liquid or, if solid, can be dissolved and used as a solution. Hypochlorite disinfection is the most popular method used in rural areas. It is simple, easy and inexpensive and there are many available devices using the appropriate technology.
There are several ways to feed a solution and dosers can be classified according to their driving force, which can be of two kinds: atmospheric pressure and positive or negative pressure.
Atmospheric pressure feeders Some of the devices that work under atmospheric pressure have been designed with a varying head (paddle wheel feeder or the Archimedes wheel for example).
The most popular devices, however, are those that operate under the “constant head” principle, which are more precise and reliable. A constant head system is composed of two elements: a constant head tank of a stock solution to be fed and a regulating mechanism. Three of the most recommended systems which can be built from materials that are easily obtained locally, are Float valve in a box system, Floating tube with hole system and Bottle/glass system.
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Float valve in a box system
The heart of this system is a float valve similar to those used in toilet cisterns. One or two tanks hold the stock solution to be fed and the float valve is placed in a small box. The system, although quite simple and cheap, is fairly accurate
1-6Float valve in a box system
(Felipe Solsona, Juan Pablo Méndez, 2003).
Floating tube with hole system
The basic element is a PVC tube with one or more holes. The tube is attached to any kind of floating device and the hole should be situated some centimeters below the solution level so that the solution enters the delivery tube and flows at the desired feeding rate toward the application point
1-7 Floating tube with a hole feeder
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(Felipe Solsona, Juan Pablo Méndez, 2003).
Bottle/glass system
It consists of a tank containing the stock solution, a dosing element, connections and a regulating valve. The system is precise, inexpensive and easy to build and operate. The dosing range is from 2 to 10 l/h, making it applicable to small communities of up to 20,000 inhabitants.
1-8 Bottle/glass dosing device
(Felipe Solsona, Juan Pablo Méndez, 2003).
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1-9Typical mixers for the addition of chlorine: (a) in-line turbine mixer and (b) injector pump type
(Eddy, 1999)
1-10 Typical diffusers used to inject chlorine solution: (a) single injector for small pipe, (b)dual injector for small pipe, (c) across the pipe diffuser for pipes larger than 0.9 m (3 ft) in diameter, (d) diffuser system for large conduits, (e) single across-the-channel diffuser, and (f ) typical hanging nozzle type chlorine diffuser for open channels.
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(Eddy, 1999) ď&#x201A;§
Chlorine solution feeders installation and installation requirements
These systems should be built of materials that are resistant to the corrosion caused by a strong hypochlorite solution. The solution tank can be made of high-density polyethylene (PEHD), fiberglass or asbestos-cement. The floater can be PVC or wood. No aluminum, steel, copper or
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stainless steel should be used because they are rapidly destroyed. This device, like all constant head systems, is easy to install. Its application is limited to cases where the hypochlorite solution can flow by gravity toward the mixing site, whether channel or chlorine contact chamber, or directly toward a storage tank. The installation should include an airspace in the discharge pipe to avoid possible siphoning. The system should also be designed in such a way that there is no chance of having the contents of the solution tank discharge all at once accidentally into the mixture channel or the contact chamber if an accessory or pipe is broken or any other type of spill occurs. The installation design should facilitate handling of the chlorine compounds and solution mixtures and adjustment of the dosing. A water faucet should be located conveniently for use in preparing stock solutions and for general hygiene. ď&#x201A;§
Chlorine solution feeders operation and maintenance
These devices are easy to operate, maintain and repair and do not require the care of specialized operators. The latter can be easily trained over a short period of time. Continuous oversight is needed, however, to make sure that the equipment, particularly the submerged hole, is kept clean, that the size of the dose is appropriate, that the tank solution has not run out or its concentration been weakened, that there is no change in the water flow, etc. For that reason, it must be cleaned periodically and a filter must be used to trap all particulate material.
Great care must be taken when preparing the hypochlorite solution by hand, as explained earlier. When using calcium hypochlorite, the concentration of the solution must be between 1% and 3% available chlorine to impede the excessive formation of calcium scaling and sediments. Sodium hypochlorite solutions can have a 10% concentration. Higher concentrations are not advisable because they lose their strength rapidly and if too high they can crystallize.
Pressure feeders ď&#x201A;§
Diaphragm pump feeding system
Positive pressure feeders work by raising the chlorine solution above atmospheric pressure and subsequently injecting it into a water pipe. The most important positive pressure system is the highly popular diaphragm feeding pump.
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Negative pressure feeding or suction pumps, for their part, operate on the principle that the chlorine solution is suctioned out by the vacuum created by a venturi or by connecting the feeder to an adduction pipe. The venturi is the most used negative pressure system. It is installed in the pressurized water supply pipe itself or in an alternate line, as can be seen later.
These pumps are equipped with a chamber housing two one-way valves, one at the point of entry and the other at the exit. The solution is added to the chamber through the intake valve as the diaphragm, powered by an electric motor expands, and is expelled outside the chamber by the exit valve as the diaphragm contracts. The flexible diaphragm is made of material resistant to the corrosive effects of hypochlorite solutions.
1-11 Dosing device with diaphragm pump in pipe under negative pressure (adduction pipe)
(Felipe Solsona, Juan Pablo MĂŠndez, 2003). ď&#x201A;§
Suction feeders (venturi - type)
The suction feeder in widest use employs a venturi device that makes it possible to feed chlorinated solutions through pressurized pipes. This type of chlorinator is based on the same principle as that of the ejector used in gas chlorinators. The vacuum created by the water flow through the venturi pipe suctions the hypochlorite solution and discharges it directly into the main water stream or a secondary one. The feed is regulated by adjusting a needle valve installed between the venturi device and the rotameter.
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1-12 Typical installation of a venturi – type suction feeding system
(Felipe Solsona, Juan Pablo Méndez, 2003).
1-13Typical Deep-Well Chlorination System
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(John M. Stubbart, 2006 )
1-14 Calcium hypochlorite chlorination system
(David H.F. Liu, Bela G. Liptak, 1999)
Diaphragm pump feeders installation and installation requirements
Diaphragm pumps are usually powered by electric motors; hydraulically-powered pumps are less common. The latter can be used when there is no reliable source of electric power. An advantage of this system is that the hypochlorite feeding speed can be calibrated to the water flow speed by using a special device. A disadvantage of using hydraulic power is its mechanical complexity, which often causes operating and maintenance problems. Relatively little power, generally from ¼ to ¾ HP, is needed to operate the hypochlorinator. In choosing this type of chlorinator, it is important to consider the reliability and quality of the power source to be used. A well-designed installation should protect the chemical products against the effects of sunlight and provide the necessary conditions for easy management and mixing of chemical solutions. It should also be well-ventilated and very high temperatures and moisture should be avoided. The installation
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should be designed to facilitate its operation and maintenance and reduce potential chlorine risks. A separate room is recommended for storing the hypochlorite because of its corrosive and reactive nature. The figure below shows the diagram of a typical calcium hypochlorite chlorination installation.
1-15 Typical calcium hypochlorite chlorination installation
(Felipe Solsona, Juan Pablo MĂŠndez, 2003)
ď&#x201A;§
Diaphragm pump feeders operation and maintenance
The capacity of diaphragm pumps can be regulated to adjust the hypochlorite solution feed by adjusting either the frequency or length of the pump stroke. Most hypochlorinators use variable speed motors to regulate the frequency or length of the pump stroke. Some employ mechanical means to adjust its length and a few make use of both methods. Control of the pump stroke frequency appears to be the method of choice of most small water supply systems because of its simplicity. Starting and stopping, as well as the feed rate, tend to be controlled manually, although starting and stopping can also be controlled automatically using a magnetic switch connected directly to the water pump regulator. Complicated control systems that adjust the feed rate automatically are not generally recommended for use by small communities. Chlorinators of this kind are simple to operate and maintain, but do require continuous and appropriate maintenance.
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The feed can be exact and uniform if the equipment valves are kept free from precipitates and scaling. A concentration of 1 to 3% is recommended for calcium hypochlorite solutions in order to reach an economic balance between the costs of pumping and of preventing calcium precipitation in the check valves and diaphragm chamber. Special care must be taken when the water is hard, containing high contents of dissolved solids, or when using dissolved chlorinated lime. The use of sodium hypochlorite solutions with a concentration of less than 10% is recommended in order to avoid precipitates and maintain the stability of the chlorine. Because the diaphragm pump is made up of metal pieces, these can corrode and reduce its service life. For that reason, the pump must be replaced periodically. The check valves are exposed to calcium scaling and so must be cleaned with an acid solution to avoid their deficient operation or having to replace them more frequently due to a loss of elasticity as a result of oxidation. Hypochlorite solutions must also be handled with care. Inasmuch as they are highly corrosive, the tools and containers used to prepare them must be made of plastic or ceramic or other corrosionresistant materials. Personnel must be trained to handle spills and in the correct equipment operation and maintenance procedures. ď&#x201A;§
Venturi type suction feeders installation and installation requirements
The venturi device functions efficiently within a relatively narrow range of operation. For that reason, care must be taken in its selection to ensure that the hydraulic requirements of the device match the characteristics of the water supply system (maximum and minimum flow). Venturi devices should not be used for wide fluctuations in flow and pressure outside their range of operation. They should also be resistant to strong hypochlorite solutions, whose oxidizing potential could attack and rapidly deteriorate the device. Venturi devices can be installed in the wall or directly on the pipes, depending on their design. Their installation is so simple that specialists are not needed. All of the flexible plastic pipes should be appropriately installed to facilitate the operation and maintenance of the devices. Beforehand, a filter should be installed in the device and it should be arranged in such a way that the venturi can be easily removed to clean any precipitates or scaling that could obstruct it. As in the case of all hypochlorinators, special precautions must be taken in designing the chlorination and storage installations because of the reactive nature of chlorine solutions. The venturi device does not require a great deal of water pressure to perform. In some cases, however, a reliable source of electric power will be needed to flush a small amount of water through the venturi to create the necessary vacuum. ď&#x201A;§
Venturi type suction feeders operation and maintenance
NIREAS VOLUME 2 [2.6] 31
Venturi hypochlorinators are not very precise, particularly when the flow varies widely, making it necessary to frequently adjust the feed. Acrylic venturi devices are better because they permit the operator to visually determine when they need cleaning and are also resistant to hypochlorite. All venturi devices are liable to calcium scaling due to the hypochlorite solution or the presence of hard water. They should be cleaned on a routine basis and, if necessary, use acid to remove the hardest scaling and other precipitates or sediments. Most joint gaskets, retaining valves springs and joints deteriorate over time because of being in contact with hypochlorite; they should, therefore, be replaced periodically. These spare parts should be made of proper materials and be on hand at all times.
Solid chlorine
Solid calcium hypochlorite feeders Calcium hypochlorite feeders are manufactured for large and small flows. The former are volumetric or gravimetric feeders that drop a measured amount (in volume or weight) into a small dissolution tank (always accompanied by mixing), where it dissolves and is later fed at the application point. The use of these devices is not popular, for when large flows are to be treated chorine gas is the choice. To disinfect small flows (typical in medium-sized and small communities), devices operating through tablet erosion or the direct feeding of solid calcium hypochlorite pills are preferred.
The concentration of active chlorine in these presentations is between 65 and 70%, unlike the 33% concentration of powdered calcium hypochlorite, marketed under different brands. ď&#x201A;§
Tablet and pill erosion feeder
Erosion feeders use high concentration calcium hypochlorite tablets (HTH) that can be obtained from distributors or prepared locally by mechanically compressing powdered calcium hypochlorite. This system has found an important niche in the disinfection of water supply systems for small communities and households. The devices are easy to handle and maintain and are cheap and durable, as well. The tablets are safer than the hypochlorite solutions and the gaseous chlorine and are easier to handle and store. Erosion feeders gradually dissolve hypochlorite tablets at a preset rate while a water current flows around them. This mechanism provides the necessary chlorine dose to disinfect the water. As the tablets dissolve, they are replaced by new ones that fall
NIREAS VOLUME 2 [2.6] 32
into the chamber by gravity. The concentrated chlorine solution feeds a tank, an open channel or a reservoir, as the case may be.
1-16 Calcium Hypochlorite tablets erosion feeders
(Felipe Solsona, Juan Pablo MĂŠndez, 2003).
1-17 Calcium Hypochlorite pill chlorine feeder
NIREAS VOLUME 2 [2.6] 33
(Felipe Solsona, Juan Pablo Méndez, 2003).
Many Pill or Erosion feeders can be found in the market.
1-1 Calcium Hypochlorite tablets erosion feeder – Commercial model
Solid chlorine feeders installation and installation requirements
NIREAS VOLUME 2 [2.6] 34
Only minimum specialized training is needed to install these feeders. In most cases, it is enough to train a operator in a basic knowledge of plumbing and piping. However, although the feeding devices are made of noncorrosive materials and have no movable parts, the manufacturerâ&#x20AC;&#x2122;s instructions must be followed to ensure their durability and adequate operation in accordance with the specifications. Attention must also be paid to the water temperature, on which the tabletsâ&#x20AC;&#x2122; solubility depends.
ď&#x201A;§
Solid chlorine feeders operation and maintenance
Tablet and pill erosion feeders are simple to operate. The equipment can be calibrated easily, but not very precisely by adjusting the immersion depth of the column of tablets or the speed or flow of the water that is flushed through the dissolution chamber. Once the feeder has been calibrated and if there are no major variations in the flow, it will normally require little attention, except to check to see that the container is filled with tablets to ensure continuous dosing. The tablet feeder mechanism should be inspected on a regular basis to check for obstructions. It must be well cleaned, returned to its proper position and then calibrated. Inspection and replenishment of tablets will depend on the specific installation, the chlorine feed and the volume of water treated.
Operators can be trained rapidly because the device is easy to operate. Safety wise, hypochlorite tablets are usually easier and safer to handle and store than other chlorine compounds; even so, it is necessary to take some minimum precautions.
NIREAS VOLUME 2 [2.6] 35
2.6.1.8
Comparison of different chlorination methods
1-10 Comparative table of advantages and disadvantages of different chlorination methods
(Felipe Solsona, Juan Pablo MĂŠndez, 2003).
NIREAS VOLUME 2 [2.6] 36
2.6.1.9
Advantages & disadvantages of disinfection by chlorination
Advantages 1. Chlorination is a well established technology 2. Presently chlorine is more cost effective than either UV or ozone disinfection (except when dechlorination is required and fire code requirements must be met) 3. The chlorine residual that remains in the wastewater effluent can prolong disinfection even after initial treatment and can be measured to evaluate the effectiveness 4. Reliable and effective method against a wide spectrum of pathogenic organisms 5. Chlorination has flexible dosing control 6. Chlorination can eliminate certain noxious and unpleasant odours from wastewater effluent
Disadvantages 1. Chlorine residual even at low concentrations is toxic to aquatc life and may require dechlorination 2. All forms of chlorine are highly corrosive and toxic. As a result, storage, shipping and handling require increased safety precautions. 3. Chlorine
produces
hazardous
by-products
(DBPâ&#x20AC;&#x2122;s),e.g.trihalomethanes. 4. Level of total dissolved solids is increased in the treated effluent 5. Chlorine isnâ&#x20AC;&#x2122;t effective against certain types of pathogenic organisms such as cryptosporidium parvum, cysts, Giarfia lamblia and eggs of parasitic worms.
2.6.1.10 Designing a chlorination system Data Maximum inflow :
0,1 m3/sec
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Effluent to be disinfected :
secondary treatment effluent
Fecal coliforms in tank inlet :
106 /100 mL
Required Fecal coliforms in tank outlet :
102 /100 mL
Contact time in chlorination tank :
30 min
Disinfectant to be used :
sodium hypochlorite (NaCLO),
chlorine concentration 15% w.w., specific weight 1,20 kg/L (Office of Water Program Operations, US EPA) Answer
Net chlorination tank volume : 0,1 m3/sec x 30 min (60 sec/min) = 180 m3
Chlorination tank design :
Parameters of design Minimum required tank’s length to width ratio : > 40/1 Acceptable tank’s depth : 0,8 – 2 m Meandering path of effluent in the tank
Considering •
tank’s depth equal to 1,5 m
•
8 channels, 1,5 meters wide each
Minimum channel’s length •
180 m3 / (1,5 m x 1,5 m x 8 channels) =10 m
•
Choosen length of each channel
: 10 m
Tank’s length to width ratio •
8 channels x 10 m / 1,5 m = 53,33/1 > 40/1
Chlorine dose :
NIREAS VOLUME 2 [2.6] 38
N/No=(1+0,23 Ct)-3
Solving to Residual Chlorine concentration C, C = 2,98 mg/L
Solving to Initial Chlorine concentration Co, according to following equation, C = 0,7 Co e-0,003 t Co = 4,65 mg/L
Solving to sodium hypochlorite (NaCLO) maximum demand per hour Q NaCLO (kg/hr) = 0,1 m3/sec x 4,65 mg/L / 15% w.w. = 11,16 kg/hr or Q NaCLO (L/hr) = 11,16 kg/hr / 1,20 kg/L= 9,3 L/hr 2.6.1.11 Troubleshooting guide – chlorination 1-11 Troubleshooting guide – chlorination INDICATOR/OBSERVATI
PROBABLE
ON
CAUSE
1.Low chlorine gas pressure at chlorinator.
CHECK OR
SOLUTION
MONITOR
1a.Insufficient number of cylinders connected to system. Supply valve closed or partly closed.
1a. Reduce feed rate and note if pressure rises appreciably after short period of time. If so, 1a. is the cause.
1b. Stoppage or flow restriction
1b. Reduce feed rate
1a. Connect enough cylinders to the system so that chlorine feed rate does not exceed the withdrawal rate from the cylinders. Icing or very cold conditions can be noted at the cylinder/ container valve if inadequate supply is the problem. 1b. Valve out, evacuate line
NIREAS VOLUME 2 [2.6] 39
2.No chlorine gas pressure at chlorinator.
3.Chlorinator will not feed chlorine.
between cylinders and chlorina-tors. Gas pressurereducing valve closed/ malfunctioning.
and note if icing and cooling effect on supply lines continues.
then disassemble chlorine header system to point where cooling begins, locate stoppage and clean with solvent.
2a. Chlorine cylinders empty, not connected to system, or supply valve closed.
2a. Visual inspection of system gauges.
2a. 1. Replace empty cylinders. 2. Connect cylinders. 3. Open supply valve.
2b. Plugged or damaged pressurereducing valve.
2b. Inspect valve. High chlorine pressure upstream of valve, low pressure downstrea m. 3a. Check chlorine supply and pressure gauges.
2b. Repair the reducing valve after shutting off supply valves, evacuating gas in the header system.
3a. No chlorine supply.
3b. 1. 3b. 1. Inadequate Check injector vacuum. injector supply pump for proper output. 2. Injector diaphragm ruptured.
2. Injector diaphragm.
3a. 1. Restore chlorine supply to chlorinator. 2. Check chlorine pressurereducing valve (CPRV) on evaporator on chlorine supply header. 3b. 1. Start injector supply pump, obtain proper output in flow and psi.
2. Replace injector diaphragm, adjust injector to
NIREAS VOLUME 2 [2.6] obtain proper vacuum for operating chlorina-tion system.
40
3c. Air leak in chlorinator.
3d. Plugged diffuser.
4. Chlorine gas escaping from chlorine pressurereducing valve (CPRV).
5. Inability to maintain chlorine feed rate without icing of chlorine system.
3e. V-notch orifice of chlorinator (chlorine) out of adjustment/ disengaged. 4a. Main diaphragm of CPRV ruptured due to improper assembly or fatigue.
5a. Insufficient supply.
5b. Insufficient evaporator capacity.
3c. Check chlorinator component s for secureness and proper connection s. 3d. Check back pressure on chlorine water supply to contact basin.
3c. Retighten connections, replace faulty diaphragms, or ruptured tubing, defective seals, or O-rings.
3e. Travel of V-notch orifice.
3e. Adjust or reconnect orifice, lubricate stem.
4a. Place ammonia bottle near termination of CPRV vent line to confirm leak. 5a. Check chlorine supply header pressure gauge. 5b. Check evaporator temperatur e and in/out pressure.
4a. Disassemble valve and diaphragm; reassemble correctly.
3d. Clean diffuser.
5a. Add more cylinders to meet chlorine feed demand.
5b. 1. Place another evaporator in service. 2. If operating a chlorina-tion
NIREAS VOLUME 2 [2.6] 41
5c. 1. CPRV dirty (supply manifold). 2. Restriction in line. 3. Withdrawal rate too high.
6. Chlorine evaporator system unable to maintain water bath temperature sufficient to keep external CPRV open.
6a. Heating element malfunction.
6b. Solenoid valve mal function
5c. 1. Check chlorine pressure downstream of CPRV. 2. Chlorine system supply line pressures. 3. Feed rate. 6a. Evaporator water bath temperatur e. 6b. Solenoid valve.
system with separate prechlorination and postchlorina-tion unitsâ&#x20AC;&#x201D;place pre-system on gas from cylinder supply, leave postsystem on evaporators. 3. Clean water bath on evaporator, ensure evaporator power is ON, ensure heater element is functioning properly, ensure solenoid is not malfunctioning and allowing water to circulate through evaporator. 5c. 1. Clean CPRV. 2. Locate and remove restriction. 3. Lower withdrawal rate or place more chlorine containers on line.
6a. Remove and replace heating element.
6b. Repair/replace defective
NIREAS VOLUME 2 [2.6] 42
7. Wide variation in chlorine residual produced in effluent.
7a. Variation in chlorine demand.
solenoid. 7a. Analyze 7a. Program chlorine postchlorina-tion demand of feed rates plant during day to effluent to meet chlorine determine demand and demand supply desired during chlorine residual various to meet flow disinfection periods. requirements.
7b. Chlorine contact basin.
7b. 1. Determine detention time for various portions of day. 2. Dye test basin at peak flows. 3. Solids deposit in basin. 4. Sample location
7b. 1. Maintain minimum of thirty minutes of chlorine contact time with effluent in basin. 2. Baffle basin or mix to prevent shortcircuiting. 3. Clean contact basin to avoid solids resuspending during peak flows and increasing chlorine demand. 4. Sample other locations for best application point.
7c. Chlorine diffuser.
7c. Chlorine diffuser for blockage, damage, and proper location for even chlorine dispersion.
7c. 1. Clean diffuser orifices. 2. Replace broken or damaged parts on diffuser. 3. Change location or style of diffuser for better mixing of
NIREAS VOLUME 2 [2.6] chlorine with effluent.
43
8.Chlorine residual analyzer recorder controller does not control chlorine residual properly.
7d. Inadequate feed rate adjustment of postchlorinators.
7d. Monitor effluent for chlorine residual.
7d. Reprogram chlorine feed rates to meet demand conditions.
7e. Flowproportioning chlorine control devices not working properly.
7e. 1. Check flowmeter output. 2. Check flowmeter proportioni ng output device. 3. Check chlorinator controller for in/out response.
7e. 1. Recalibrate flowmeter to correctly measure plant flow. 2. Maintain equipment so that flowmeter reading is correctly transmitting to chlorinator controller equivalent readings by mechanical cam, air, or electronic signals. 3. Adjust chlorinator controller so that chlorinator follows plant flow signal.
7f. Malfunction of auto control.
7f. Verify that chlorinator feed rate is what is required at given flow.
7f. Repair control system. May require manufacturer's field service personnel to perform repairs.
8a. Electrodes fouled.
8a. Visual inspection.
8a. Clean electrodes.
8b. Insufficient potassium
8b. Potassium
8b. Adjust potassium
NIREAS VOLUME 2 [2.6] 44
iodide being added for amount of residual being measured. 8c. Buffer additive system malfunctioning.
8d. Malfunctioning of analyzer cell.
8e. Poor mixing of chlorine at point of application.
8f. Chlorinator rotameter tube range is improper size.
8g. Loop time too long.
iodide dosage.
iodide feed to correspond with residual being measured.
8c. See if pH of sample going through cell is maintained. 8d. Disconnect analyzer cell and apply a simulated signal to recorder mechanism . 8e. Set chlorine feed rate at constant dosage and analyze a series of grab samples for consistency . 8f. Check tube range to see if it gives too small or too large an incremental change in feed rate.
8c. Repair buffer additive system.
8g. Check loop time.
8g. Reduce loop time by doing the following: 1. Move injector closer to point of
8d. Call authorized service personnel to repair electrical components.
8e. Install mixing device to cause turbulence at point of application.
8f. Replace rotameter tube with a proper range of feed rates.
NIREAS VOLUME 2 [2.6] 45
9. Coliform count fails to meet required standards for disinfection
9a. Chlorine residual too low..
9a. Chlorine residual.
9b. Inadequate 9b. chlorine residual Continuous control. ly record residual in effluent.
9c. Inadequate chlorination equipment capacity.
9c. Check capacity of equipment.
9d. Solids buildup in contact chamber. 9e. Shortcircuiting in contact chamber.
9d. Visual inspection
9f. Coliform regrowth in piping/sample station.
9f. Effluent coliform sampling station.
9e. Contact time.
application. 2. Increase velocity in sample line to analyzer cell. 3. Move cell closer to sample point. 4. Move sample point closer to point of application. 9a. 1. Increase chlorine feed rate. 2. Increase chlorine contact time. 9b. Use chlorine residual analyzer to monitor and control the chlorine dosage automatically. 9c. Replace equipment as necessary to provide treatment based on maximum flow through plant 9d. Clean contact chamber to reduce solids buildup. 9e. 1. Install baffling in contact chamber. 2. Install mixing device in contact chamber. 9f. Modify system as needed to provide adequate
NIREAS VOLUME 2 [2.6] 46
10. Chlorine residual too high in plant effluent to meet requirements.
11. BREAKOUT (breakaway) OF CHLORINE
9g. High chlorine demand, effluent appears low in solids.
9g. Nitrite lock. or 9g. Breakpoint chlorination .
10a. Chlorine feed rate too high.
10a. Chlorine residual.
10b. Malfunctioning chlorine residual control.
10b. Operation of residual control system/ analyzer. 11a. Chlorine feed rate.
11a. Overfeeding chlorine.
11b. Insufficient injector water flow.
11b. Injector flow or supply/ pump system.
11c. Excess mixing.
11c. Chlorine mixer speed.
chlorine residual to prevent regrowth. 9g. 1. Reduce level of nitrification. 2. Add supplemental alkalinity. 9g. 1. Increase chlorine until breakpoint is reached. 2. Decrease chlorine until chlorine:ammoni a-N ratioisbelow5:1. 10a. Reduce chlorine feed rate. 10b. Repair/calibrate chlorine feed/residual control loops. 11a. Decrease chlorine feed rate to minimize breakout and maximize application efficiency. 11b. Adjust injector to maximum water flow. Place additional pumps on line to provide maximum capacity for dissolving chlorine 11c. Decrease mixer speed if variable speed, if not, turn unit
NIREAS VOLUME 2 [2.6] 47
11d. Inadequate 11 d. mixing. Chlorine mixer speed.
11e. Inadequate 11e. Depth diffuser of diffuser submergence. at application point.
11f. Inadequate diffuser size
11f. Diffuser size/design ,
off. Excess mixing may cause release of chlorine under breakout conditions. 11 d. Increase mixer speed to encourage dispersion of chlorine solution and increase dissolution capacity of flow stream. 11e. Lower diffuser to increase flow level over diffuser discharge point and provide additional contact between flow and chlorine solution. 11f. Install a larger diffuser to disperse chlorine solution over a larger flow area.
CHLORINE SUPPLY SYSTEM (must be performed by trained and qualified operators) 12. Discoloration at joint, 12. Chlorine 12. Check 12. Disassemble cadmium plating gone. leak. suspect and Green or reddish colored area with repair/replace deposits. fumes of defective joint ammonia as quickly as solution. possible. 13. Small drop of liquid on joint.
13. Chlorine leak/condensation.
13. Check suspect area with fumes of ammonia solution.
13. If chlorine leak confirmed, repair/replace defective joint as quickly as possible.
NIREAS VOLUME 2 [2.6] 48
14. Corrosion at gas gauges or pressure switches
14. Chlorine leak/condensati on
14. Check suspect area with fumes of ammonia solution.
14. If chlorine leak confirmed, repair/replace defective gauges/ pressure switches as quickly as possible.
(Office of Water Program Operations, US EPA)
ii. DISINFECTION BY ULTRA VIOLET RADIATION
2.6.1.12 Introduction The practical applications of ultraviolet radiation started in 1901, when this light was first produced artificially. The technique was considered for use in drinking water disinfection when quartz was found to be one of the few materials that are almost totally transparent to ultraviolet radiation, allowing it to be used as protective casing for the lamps.
The popularity of chlorine and chlorine compounds, together with their low cost, slowed the production of UV equipment until the 1950s and, in fact, even until 1970, when the lamps became reliable and long-lasting. The concern aroused by the identification of disinfection by-products (DBP), particularly those associated with chlorine disinfection, led many water systems to shift to the use of UV.
The glaring disadvantage of UV radiation cancels out its major advantage with regard to DBPs: it does not leave treated water with any disinfectant residual
NIREAS VOLUME 2 [2.6] 49
to cope with future contamination of the distribution or household water systems.
2.6.1.13 More
Health effects of UV radiation
Small amounts of UV are beneficial for people and essential in the production of vitamin D. UV radiation is also used to treat several diseases, including rickets, psoriasis, eczema and jaundice. On the contrary, prolonged human exposure to UV radiation may result in acute and chronic health effects on the skin, eye and immune system. Over the longer term, UV radiation induces degenerative changes in cells of the skin, fibrous tissue and blood vessels leading to premature skin aging, photodermatoses and actinic keratoses. Another long-term effect is an inflammatory reaction of the eye. In the most serious cases, skin cancer and cataracts can occur.
2.6.1.14 Mechanism of disinfectant action The disinfection mechanism s based on a physical phenomenon whereby short wave UV radiation acts on the genetic material (ADN) of the microorganisms and the viruses, destroying them rapidly without producing any major physical or chemical changes in the treated water. UV radiation energy waves are the range of electromagnetic waves 100 to 400 nm long (between the X-ray and visible light spectrums). The division of UV radiation may be classified as Vacuum UV (100-200 nm), UV-C (200-280 nm), UV-B (280-315 nm) and UV-A (315-400 nm). In terms of germicidal effects, the optimum UV range is between 245 and 285 nm. UV disinfection utilizes either: low-pressure lamps that emit maximum energy output at a wavelength of 253.7 nm; medium pressure lamps that emit energy at
NIREAS VOLUME 2 [2.6] 50
wavelengths from 180 to 1370 nm; or lamps that emit at other wavelengths in a high intensity “pulsed” manner. UV inactivation is thought to occur as a result of the direct absorption by the microorganism
of
the
UV
radiation,
bringing
about
an
intracellular
photochemical reaction that changes the biochemical structure of the molecules (probably of the nucleic acids) that are essential to the microorganism’s survival. It has been shown that irrespective of the duration and intensity of the dosage, the expending of the same total energy will result in the same degree of disinfection.
Most UV disinfection equipment uses a minimum exposure (in the wastewater) of 60 mWs/cm2. This is enough to inactivate most of the pathogenic bacteria and viruses, but perhaps not enough for certain pathogenic protozoa, protozoan cysts and nematode eggs that can require up to 200 mWs/cm2 for total inactivation. MORE… The following tables contain a summary of published data of UV radiation dosages for the elimination of some pathogens, indicators, or other microorganisms that give an idea of the range and order of the exposure magnitude.
1-12 UV doses for multiple Log reductions for various spores
(Harold Wright, Gail Sakamoto, / Revised and expanded by Gabriel Chevrefils, Eric Caron, 2006)
1-13 UV doses for multiple Log reductions for various bacteria
NIREAS VOLUME 2 [2.6] 51
(Harold Wright, Gail Sakamoto, / Revised and expanded by Gabriel Chevrefils, Eric Caron, 2006)
Table 1-13 continued
NIREAS VOLUME 2 [2.6] 52
(Harold Wright, Gail Sakamoto, / Revised and expanded by Gabriel Chevrefils, Eric Caron, 2006)
As already stated, UV light has the capacity to treat water without producing major physical or chemical changes in the treated water. This disinfection process does not add any new substances to the water, therefore eliminating the risk of the formation of DBPs. The dosage and frequency used for the
NIREAS VOLUME 2 [2.6] 53
disinfection are not known to produce any related substances. Nor does overdosing UV light produce any harmful effect. Even so, the operator of UV disinfection equipment must use protective goggles and clothing to avoid exposure to the high power radiation characteristic of ultraviolet light.
2.6.1.15 Parameters affecting disinfectant action UV wavelengths are very similar to those of sunlight. Since UV radiation is energy in the form of electromagnetic waves, its effectiveness is not limited by chemical water quality parameters. For instance, it appears that pH, temperature, alkalinity, and total inorganic carbon do not impact the overall effectiveness of UV disinfection. The most important parameters affecting water disinfection by UV radiation are:
Wavelength The germicidal portion is between 240 and 280 nm (nanometers) with maximum disinfecting efficiency existing at close to 260 nm. These limits fall within what is known as the ultraviolet - C range (100-280 nm), which is different from the ultraviolet - A (315-400 nm) and ultraviolet - B (280-315 nm) ranges.
Intensity of radiation The closer the emission point is to the water, the more intense the radiation will be and, accordingly, the more efficient the disinfection. A rule of thumb states that the water depth should be no more than 75 mm to ensure that the UV rays reach every part of it properly.
Temperature While water temperature has little or no influence over the effectiveness of ultraviolet disinfection, it does affect the operative yield of an ultraviolet lamp when submerged in the water.
Maximum estimated radiated energy is
achieved when UV lampâ&#x20AC;&#x2122;s temperature takes value around 41oC.
Suspended Solids, Turbidity, Chemical compounds
NIREAS VOLUME 2 [2.6] 54
The water itself absorbs ultraviolet energy, suspended or dissolved solids, turbidity and color absorb even more. The concentration of solids in suspension should not be more than 10 ppm, the level at which the water starts to experience problems of ultraviolet light absorption. Water turbidity should be as low as possible and turbidities of over 5 NTU should be avoided at all costs. Also many types of chemical compounds (proteins, phenols, etc), absorb amounts of ultraviolet energy, reducing available radiation for the destruction of microorganisms and viruses.
Dissolved Solids, pH Whereas total dissolved solids and pH doesnâ&#x20AC;&#x2122;t seem to have a direct impact on UV disinfection process, in the long term they show an effect in the creation of deposits on UV lamps, reducing UV radiation intensity and therefore the disinfection efficiency. As a result, a more often cleaning of UV lamps is necessary.
Particle size For similar number of suspended particles, the greater the average particle size, the lower the efficiency of the disinfection by ultraviolet radiation. Relative studies (Ho, 1981), showed that disinfection rates were higher for primary and secondary effluents rather than secondary effluents by activated sludge method. Higher disinfection rates for primary and secondary effluents is explained by the fact that activated sludge effluent contains particles with larger diameter .
Ferric compounds Iron has proven to absorb significant amounts of UV radiation, and contribute in the creation of deposits on UV lamps, reducing UV radiation intensity and therefore the disinfection efficiency.It is strongly recommended that concentration of ferrous iron in water to be disinfected, should not be higher than 0,5 ppm.
Type of microorganisms
NIREAS VOLUME 2 [2.6] 55
Ultraviolet radiation is measured in microwatts per square centimeter (μW/cm2) and the dose in microwatt seconds per square centimeter (μWs/cm2) (radiation x time). Resistance to the effects of radiation will depend upon the type of microorganism involved. Nevertheless, the dosage of UV light required to destroy the most common microorganisms (coliform, pseudomona, etc.) is between 6,000 and 10,000 μWs/cm2. Standards for UV dosage in different countries range between 10,000 and 70,000 μWs/cm2. Exposure time As in the case of any other disinfectant, the exposure time is vital to ensure a good performance. It is not easy to determine the exact contact time (as this depends on the type of flow and the characteristics of the equipment used), but the period of time should be related to the needed dosage (remember the concept of C x T and the explanation given). In any case, the normal exposure time is from 10 to 20 seconds.
Exposure in sunlight A large number of microorganisms, have the ability to self repair the harmful effects caused by UV radiation, when exposed to natural sunlight. This phenomenon is known as ‘photorepair process’ and it is proved that harmed microorganisms have the ablity to increase their population by up to 1 scale of magnitude, when process of photorepair takes place (Harris, 1987).
Disinfection by products by UV radiation Continuous wave UV radiation at doses and wavelengths typically employed in drinking water applications, does not significantly change the chemistry of water nor does it significantly interact with any of the chemicals within the water
(USEPA, 1996).Therefore, no
natural physiochemical features of the water are changed and no chemical agents are introduced into the water. In addition, UV radiation does not produce a residual. As a result, formation of THM or other DBPs with UV disinfection is minimal .
NIREAS VOLUME 2 [2.6] 56
2.6.1.16 Calculations of disinfection by UV radiation The degree to which the destruction or inactivation of microorganisms occurs by UV radiation is directly related to the UV dose. The UV dosage is calculated as:
D=Ixt Where: D = UV Dose, mWĂ&#x2014;s/cm2 I = Intensity, mW/cm2 t = Exposure time, s
Research indicates that when microorganisms are exposed to UV radiation, a constant fraction of the living population is inactivated during each progressive increment in time. This dose-response relationship for germicidal effect indicates that high intensity UV energy over a short period of time would provide the same kill as a lower intensity UV energy at a proportionally longer period of time. The UV dose required for effective inactivation is determined by site-specific data relating to the water quality and log removal required. MORE Based on first order kinetics, the survival of microorganisms can be calculated as a function of dose and contact time. For high removals, the remaining concentration of organisms appears to be solely related to the dose and water quality,
and
not
dependent
on
the
initial
microorganism
density.
Tchobanoglous (Tchobanoglous, 1997) suggested the following relationship between coliform survival and UV dose: N = f x Dn Where: N = Effluent coliform density, /100mL
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D = UV dose, mW×s/cm2 n = Empirical coefficient related to dose f = Empirical water quality factor
The empirical water quality factor reflects the presence of particles, color, etc. in the water. For water treatment, the water quality factor is expected to be a function of turbidity and transmittance (or absorbance). UV demand of water is measured by a spectrophotometer set at a wavelength of 254 nm using a 1 cm thick layer of water. The resulting measurement represents the absorption of energy per unit depth, or absorbance. Percent transmittance is a parameter commonly used to determine the suitability of UV radiation for disinfection. The percent transmittance is determined from the absorbance (A) by the equation: Percent Transmittance = 100 x 10-A
Typical absorbance values for various wastewaters at 254 nm are given bellow.
1-14 Typical absorbance values for various wastewaters at 254 nm
Primary effluent
0,5 – 0,8/ cm
Percent Transmittance 15-30 %
Secondary effluent
0,3 – 0,5/ cm
30-50 %
Nitrified Secondary effluent
0,25 – 0,45/ cm
35-55 %
Filtered Secondary effluent
0,2 – 0,4/ cm
40-65 %
Microfiltered Secondary effluent
0,15 – 0,3/ cm
50-70 %
Reverse Osmosis effluent
0,05 – 0,2/ cm
65-90 %
Effluent type
Absorbance
(Eddy, 1999)
The National Science Foundation’s (NSF) Standard 55 for ultraviolet water treatment systems recommends that UV disinfection systems should not be used if the UV transmittance is less than 75 percent (NSF, 1991).
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(1-e-kIt) Where: Nt = total number of surviving disperse coliform bacteria at time t No = total number of disperse coliform bacteria prior to UV light application (at time 0) Npo = total number of disperse coliform bacteria prior to UV light application containing at least one coliform bacterium (at time 0) k
= inactivation rate coefficient, cm2/mJ
I
= average intensity of UV light in bulk solution, mW/cm2
t
= exposure time, sec
58
For UV doses greater than 10 mJ/cm2, (i.e. as is typically applied for wastewater disinfection) the following equation can be used for modeling the log-linear inactivation of disperce coliform bacteria in a batch system. (Qualls B.G., 1985) : Nt=No x e-kIt Where: Nt = total number of surviving disperse coliform bacteria at time t No = total number of disperse coliform bacteria prior to UV light application (at time 0) k
= inactivation rate coefficient, cm2/mJ
I
= average intensity of UV light in bulk solution, mW/cm2
t
= exposure time, sec
However, previus equation is applicable only to disperse organisms, assuming perfectly mixed conditions, and therefore the fact that all organisms receive the same intensity of UV light. Emerick et al. (Emerick, 2000) demonstrated the applicability of the following modeling equation for describing the inactivation of both disperce and particle associated coliform bacteria when knowledge of the applied intensity to the bulk liquid medium is known:
Nt=No x e-kIt+
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2.6.1.17 Selection of UV disinfection method & equipment UV lamps Producing UV radiation requires electricity to power UV lamps. The lamps typically used in UV disinfection consist of a quartz tube filled with an inert gas, such as argon, and small quantities of mercury. Ballasts control the power to the UV lamps. UV lamps operate in much the same way as fluorescent lamps. UV radiation is emitted from electron flow through ionized mercury vapor to produce UV energy in most units. The difference between the two lamps is that the fluorescent lamp bulb is coated with phosphorous, which converts the UV radiation to visible light. The UV lamp is not coated, so it transmits the UV radiation generated by the arc.
Low-pressure and Medium-pressure UV lamps Both low-pressure and medium-pressure lamps are available for disinfection applications. Lowpressure lamps emit their maximum energy output at a wavelength of 253.7 nm, while medium pressure lamps emit energy with wavelengths ranging from 180 to 1370 nm. The intensity of medium-pressure lamps is much greater than low-pressure lamps. Thus, fewer medium pressure lamps are required for an equivalent dosage. For small systems, the medium pressure system may consist of a single lamp. Although both types of lamps work equally well for inactivation of organisms, low-pressure UV lamps are recommended for small systems because of the reliability associated with multiple low-pressure lamps
Quartz and Teflon sleeve UV lamps Typically, low-pressure lamps are enclosed in a quartz sleeve to separate the water from the lamp surface. This arrangement is required to maintain the lamp surface operating temperature near its
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optimum of 40oC. Although Teflon sleeves are an alternative to quartz sleeves, quartz sleeves absorb only 5 percent of the UV radiation, while Teflon sleeves absorb 35 percent. Therefore, Teflon sleeves are not recommended.
Service Life Although the lamps rarely burn out, they are generally replaced when they have lost from 25% to 30% of the UV light they produced when new. Low-pressure lamps have a duration of 10,000 hours while medium-pressure lamps have a duration of no more than 5,000 hours, which, in practical terms and considering the need for their replacement when they have reached 70-75 % of their normal intensity, means that their service life is from nine months to one year and from four to six months of uninterrupted operation respectively. The useful life of the quartz sleeve is about 4 to 8 years.
UV reactor design Most conventional UV reactors are available in two types; closed vessel (pressure system) and open channel (gravitational system).
Open channel
UV reactors are also distinguished in two basic types of
chambers for exposing the water to UV radiation: those in which the lamps are submerged in the water and those that remain outside the water. Lamps in closed vessel UV reactors are always submerged in the water.
The submerged lamp UV disinfection equipment can have two basic water flow configurations: either parallel or perpendicular to the length of the lamps.
1-18 UV reactor, type of closed vessel â&#x20AC;&#x201C; flow parallel to the length of the lamps
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(Felipe Solsona, Juan Pablo Méndez, 2003)
1-19 UV reactor, type of open channel – (a) flow parallel to the length of the lamps- (b) flow vertical to the length of the lamps
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(Tchobanoglous G., 1998)
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1-2 UV reactor, type of open channel â&#x20AC;&#x201C; flow parallel to the length of the lamps-commercial model
1-3 UV reactor, type of closed vessel â&#x20AC;&#x201C; flow parallel to the length of the lamps-commercial model
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1-4 Modules arrangement in bank
A modern UV disinfection system may include the following elements: •
A non-corrosive chamber that hosts the system
•
UV lamps
•
Mechanical wipers, ultrasonic cleaners or any other self-cleaning system
•
Sensors connected to alarm systems for monitoring UV light intensity
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•
Safety shut-off in case of high or low flow rates, high or low lamp intensity or elevated system component temperatures
•
Lamp-out monitors
•
Electronic ballasts
•
Telemetry systems for remote installations
A modern UV disinfection system is demonstrated in the video bellow:
http://www.youtube.com/watch?v=o_g1led_2Vw
2.6.1.18 UV disinfection requirements
system
installation
and
installation
The installation of a typical UV disinfection system is shown in the figures above. The lamp is cased in a protective quartz sleeve. With the old systems, it was difficult to keep the lamp or the sleeves clean because of calcium carbonate scaling, sediments, organic materials or iron that limited its penetration and germicidal power. Today, almost all of the systems have sleeve wipers to reduce that problem.
Electric power is an essential requirement. Consumption varies according to the condition of the water to be treated; 22 watts/hour per cubic meter of water treated is considered optimum.
Inasmuch as UV light does not leave any residual, the electric power source must be extremely reliable for the entire time it takes for the water to flow through the disinfection unit. Communities
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that have no reliable electric power source should install an independent emergency power source to ensure that disinfection is not interrupted at any time. The system may be installed either in or outside a shelter that protects it from adverse climatic conditions and vandalism. In the former case, a shelter helps to protect the equipment from temperature extremes or other conditions that could damage it or affect its operation.
UV disinfection equipment does not need much space because the necessary contact/exposure time is very short. Although it is one of the disinfection systems that occupies the least space, sufficient space should be left to replace the lamps and to store enough lamps to cover two years of operation.
2.6.1.19 UV disinfection system operation and maintenance The operational and maintenance requirements of UV disinfection systems are minimum, but crucial to an adequate yield. The quartz sleeves or Teflon pipes must be kept free from sediments or other deposits that would reduce the radiation, thus contributing to scaling. In small systems that are generally cleaned by hand, the quartz sleeve should be wiped at least once a month and, in special circumstances, two or three times a week. The lamps should be replaced at regular intervals to guarantee at least 30,000 microwattssecond/cm2 in the area of exposure at all times. These will vary from lamp to lamp, but generally are programmed for the average interval when their intensity drops to less than 70% of its nominal potential. Lamps may need to be replaced more frequently in cold water.
Inasmuch as UV light leaves no residual disinfectant whatsoever, the entire system must be thoroughly disinfected with an appropriate chemical disinfectant before activating the unit for the first time. Any external contamination of the distribution system due to return siphoning or a crossed connection must also be remedied and the system disinfected before starting it up again.
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2.6.1.20 Advantages & disadvantages of disinfection by UV radiation Advantages 1. UV disinfection is effective at inactivating most viruses, spores, and cysts. 2. UV disinfection is a physical process rather than a chemical disinfectant, which eliminates the need to generate, handle, transport, or store toxic/hazardous or corrosive chemicals. 3. There is no residual effect that can be harmful to humans or aquatic life. 4. UV disinfection is user-friendly for operators. 5. UV disinfection equipment requires less space than other methods.
Disadvantages 1. Organisms can sometimes repair and reverse the destructive effects of UV through a "repair mechanism," known as photo reactivation, or in the absence of light known as "dark repair." 2. A preventive maintenance program is necessary to control fouling of tubes. 3. Turbidity and total suspended solids (TSS) in the wastewater can render UV disinfection ineffective. 4. UV disinfection with low-pressure lamps is not as effective for secondary effluent with TSS levels above 30 mg/L. 5. UV disinfection is not as cost-effective as chlorination, but costs are competitive when chlorination dechlorination is used and fire codes are met.
2.6.1.21 Designing a UV disinfection system Data
Minimum inflow :
0,07 m3/sec
Maximum inflow :
0,245 m3/sec
Effluent to be disinfected :
Filtered Secondary effluent
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Minimum transmittance :
55%
Minimum design dose (in the end of UV lamps service life) :
60 mJ/cm2
Answer
1. Selection of system characteristics •
Horizontal lamp configuration
•
System headloss coefficient per bank= 1,8 (from manufacturer)
•
Lamp/sleeve diameter = 20 mm
•
Lamp/sleeve length = 1500 mm
•
UV lamps maximum axial distance = 75 mm
•
UV Lamp inpout power = 85 W
•
UV Lamp outpout power at 254 nm = 30 W
•
UV Lamp flow range = 0,3 to 1,4 L/sec (from manufacturer)
•
Construction of a 2 channel system is recommended, with 2 banks in each channel
•
The system should be capable of applying minimum dose of 60 mJ/cm2 in the end of UV lamps service life,within the range of 0,07 to 0,245 m3/sec
2. Calculation of number of UV lamps required -
For Minimum transmittance equal to 55% and the geometry of UV lamps mentioned above, initial intensity of UV light is given bellow : Ii=17,52 mW/cm2
-
Considering a 20% loss of power in the end of UV lamp service life, final intensity of UV light is given bellow :
NIREAS VOLUME 2 [2.6] If=14,01 mW/cm2
69
-
For Minimum design dose of 60 mJ/cm2, minimum exposure time t, is given bellow:
t=4,28 sec
-
For Minimum exposure time t of 4,28 sec, minimum net volume of the 2 channel system Vn, is given bellow: Vn=1,05 m3
-
Equivalent Volume of water per UV lamp: Vd=7,5 cm x 7,5 cm x 150 cm – π x 12 x 150 = 7966 cm3 or 7,966 L
-
Minimum number of UV lamps required:
UV lamps=Vn/Vd = 132 UV lamps
-
Minimum number of UV lamps per channel required:
66 UV lamps
-
Check that the design falls within the manufacturer recommended range:
Minimum inflow = 70 L/sec /132 UV lamps = 0,53 L/sec/ UV lamp
Maximum inflow = 245 L/sec /132 UV lamps = 1,85 L/sec/ UV lamp
At maximum inflow, hydraulic loading rate falls outside the manufacturer’s
NIREAS VOLUME 2 [2.6] recommended range, so a greater number of UV lamps is
70
necessary to be installed.
-
Check that the design falls within the manufacturer recommended range for 192 UV lamps installation :
Minimum inflow = 70 L/sec /192 UV lamps = 0,36 L/sec/ UV lamp
Maximum inflow = 245 L/sec /192 UV lamps = 1,27 L/sec/ UV lamp
Both of the above hydraulic loading rates falls within the manufacturerâ&#x20AC;&#x2122;s recommended range for the UV disinfection system.
-
Configure the UV disinfection system :
Typically 4, 8 or 16 UV lamps per module are available. For an 8 lamp module, 6 modules are required per bank, for a total of 48 lamps per bank or 96 lamps per channel or 192 lamps in total.
-
Estimation of hydraulic losses through the UV system :
Each Channels net width : 6 x 7,5 cm = 45 cm Each Channels net depth : 8 x 7,5 cm = 60 cm Each Channels cross-sectional area : 0,45 m x 0,60 m = 0,27 m2
Each Channels net cross-sectional area substracting crosssectional area of quartz sleeves =0,27 m2 - Ď&#x20AC; x (0,01 m)2 x 48 = 0,255 m2
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Maximum velocity in each channel : uc= 0,245 m3/sec / 2 / 0,255 m2 = 0,48 m/sec Headloss per UV channel :
hc= 1,8
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water would give it an unpleasant taste; at the same time, it is also a disadvantage because, as has already been stated, a disinfectant residual is needed to ensure water quality until it reaches the consumer.
Despite its excellent properties, its use is restricted to large cities with highly contaminated water sources; small and medium-sized communities use it very little. Ozonationâ&#x20AC;&#x2122;s main drawbacks for them are its high initial and operating costs, as well as problems in its operation and maintenance. Even so, when the most accessible water sources are highly contaminated (biologically and chemically), ozonation may be the most recommendable method for oxidizing the organic substances and primary disinfection, provided that a secondary chlorination is always added to maintain the residual effect during water distribution.
Ozone is also toxic to humans. The maximum allowable concentration in air for an 8-hr period is 0.1 ppm by volume.
2.6.1.23 Mechanism of disinfectant action Ozone disinfection consists of the addition to the water source of sufficient quantities of ozone as rapidly as possible, in order to satisfy the demand and maintain an ozone residual during a long enough period of time to ensure microorganism inactivation or destruction. Most water supply systems require a larger amount of ozone than of chlorine, because of its high oxidation potential. Ozone disinfection is generally aimed at maintaining a minimum residual of 0.4 to 0.5 ppm after 10 to 20 minutes of contact with the water.
Ozone disinfection is based on the high power of ozone as a general cell oxidizer, making it an efficient bacteria destroyer. Evidence suggests that it is just as effective against viruses, spores, resistant bacteria and mold cysts.
MORE The mechanisms of disinfection using ozone include:
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•
Direct oxidation/destruction of the cell wall with leakage of cellular constituents outside of the cell.
•
Reactions with radical by-products of ozone decomposition.
•
Damage to the constituents of the nucleic acids (purines and pyrimidines).
When ozone decomposes in water, the free radicals hydrogen peroxy (HO2) and hydroxyl (OH) that are formed have great oxidizing capacity and play an active role in the disinfection process. It is generally believed that the bacteria are destroyed because of protoplasmic oxidation resulting in cell wall disintegration (cell lysis).
2.6.1.24 Parameters affecting disinfectant action Ozon dosage The disinfecting capacity of ozone, unlike that of chlorine, does not depend so much on the length of time it is kept in the water (although this does have an effect), as on the dosage administered (in the C x T formula, the value of “C” predominates). The reason for this is that its high oxidizing potential makes ozone extremely unstable, even in distilled water; this means that only when the material with a high oxidizing capacity has been oxidized will some ozone remain and that for a short time only. Otherwise, it is quite possible that the demand for ozone may not have been completely satisfied.
Organic material When organic material is present, the chemistry is even more complex and ozone decomposition accelerates. With an oxidizing potential of 2.07 volts, ozone theoretically can oxidize most organic compounds and turn them into carbon dioxide and water. However, because it is selective as to the substances that it will rapidly oxidize, the kinetics of ozone’s reactions with many compounds will be too slow to convert them into carbon dioxide during the water treatment process. Inasmuch as the total demand for ozone is almost always larger than its supply, these reactions will cease long before all of the organic substances have been totally oxidized. In treating organic
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substances, ozone has been used mainly to rupture the multiple links as a preliminary treatment prior to filtration and as an aid to coagulation.
Turbidity, Suspended solids Another consideration to be kept in mind is that the effectiveness of ozone, like
that
of
other
disinfectants,
depends
on
its
contact
with
the
microorganisms; therefore, an effort should be made to keep them from clumping and protecting themselves (if the water is turbid). A system should also be used to enhance the contact with the ozone before the gas dissipates.
Disinfection by products by ozone disinfection The ozone concentration needed to disinfect drinking water is not known to have any adverse effect on health. However, like chlorine, ozone can produce by-products, such as bromates, bromoform, bromacetic acid, aldehydes, ketones and carboxylic acids. Of these, the aldeydes are probably the most troubling for human health.
MORE
1-15 Impact of wastewater constituents on the use of ozone for wastewater disinfection
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(Eddy, 1999)
2.6.1.25 Calculations of disinfection by ozonation Wastewater treatment facilities can introduce the ozone containing air or oxygen mixture produced by the ozone generator (1 or 2% ozone) into the water by injecting or diffusing it into a mixing chamber, spraying the water into an ozone-rich atmosphere, or discharging the ozone into a scrubber.
The ozone concentration needed for disinfection depends on the chemicals and contaminants in the water and the concentration of microorganisms.
For the disinfection of tertiary biological sewage treatment plant effluents, a dosage of 6 ppm is sufficient. For secondary effluent, 15 ppm is required. This dosage also reduces BOD and COD.
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Where: D= total required ozone dosage, mg/L U=transferred ozone dose, mg/L
76
The models that have been developed to describe the disinfection process
TE=ozone transfer efficiency (vary from 80 to 90%)
1-16 Typical ozone dosages required to achieve different effluent coliform disinfection standards for various wastewaters based on a 15min contact time
with chlorine, have also been adapted for ozone with some modifications (Eddy, 1999): N/No=[U/q]-n Where: N=Number of organisms remaining after disinfection No=Number of organisms remaining after disinfection U=transferred ozone dose, mg/L n=slope of dose response curve q=value of x intercept when N/No=1 or log N/No=0
MORE
The required ozone dosage must be increased to account for the transfer of the applied ozone to the liquid .The required dosage can be computed with the following equation :
D=U x
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(Eddy, 1999)
\ 2.6.1.26 Selection of ozonation method & equipment Ozonation systems have five basic components: the gas (air or pure oxygen) preparation unit; the ozone generator; the electric power source; the contactor and the surplus gas elimination unit. In most cases, a secondary disinfectant is added to the ozone to ensure the presence of a lasting disinfectant residual in the distribution system.
1-20 Ozonation process diagramm
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(Felipe Solsona, Juan Pablo Méndez, 2003)
The purpose of the gas preparation device is to dry and cool the gas containing the oxygen. Crown-discharge type generators use dry air or pure oxygen as the oxygen source for conversion into ozone.
MORE
Production of ozone by dry air When air is used, it is vital to dry it to a point of condensation of –65 °C to maximize the effect of the ozone and reduce to a minimum the formation of nitrogen oxides that accelerate electrode corrosion. The air should also be cooled because the ozone rapidly decomposes into oxygen at temperatures
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of over 30 °C. Chemical driers can also be used instead of refrigeration to dry the air. The cost is a little higher and varies considerably from place to place. But in the case of small systems, the simplicity of their operation and maintenance can offset that cost. Zeolite towers that act like a molecular screen have been used successfully to produce pure oxygen by eliminating the nitrogen in the air. Continuous improvements are being made to increase the ozone yield.
Production of ozone by pure oxygen This method of production is based on the fission of molecular oxygen in atomic oxygen, and
the formation of ozone according with the following
reaction. O2 O + O O2 + O O3 While the installation of the ozone system does not require much power, the air drying does. The combined power consumption is 25 and 30 kilowatts-hour of electricity per kilogram of ozone generated in the oxygen and air-fed systems, respectively.
Ozone generator The ozonation systems used for water treatment generate ozone at the application site and almost all of them do so by means of a crown discharge produced by the passage of oxygen or dry air between two dielectrics.
1-21 Dielectric ozone generator
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(Felipe Solsona, Juan Pablo MĂŠndez, 2003) 1-22 Basic ozonator configuration
The gas stream generated from air will contain about 0.5 to 3.0% ozone by weight, whereas pure oxygen will form approximately two to five times that concentration.
Electric power source
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The most commonly used electric power sources are low frequency (50 to 60 Hz) and high voltage (> 20.000 volts). Technological advances have produced devices that operate at high frequency (1,000 to 2,000 Hz) and 10,000 V, which are used for large water systems.
Contactors Ozonation systems use contactors to transfer the ozone that has been generated to the water for disinfection. The type of contactor chosen depends on the specific objective of the ozonation. These can be broken down into rapid
reaction
objectives:
such
as
microorganism
inactivation,
iron,
magnesium and sulfur oxidation, and flocculation improvement; and slow reaction objectives: the oxidizing of more difficult substances, such as pesticides, volatile organic substances and other complex organic substances that for kinetic reasons tend to require longer reaction times
Failures of ozone disinfection systems can generally be traced to injector failures and defects in the contactor design and construction. There are two basic contactor designs: one with bubble diffuser chambers and the other containing a turbine-agitated reactor. In the former, there can be a series of chambers separated by deflectors or baffles or the chambers can be arrayed in parallel, in which case the device is called a â&#x20AC;&#x153;multicolumnâ&#x20AC;? contactor. Studies have revealed that the multicolumn bubble diffuser produces the most efficient transfer.
Contact columns or chambers (usually filled with irregular pieces of plastic material to lengthen the exchange period and disperse the bubbles), static agitators and propeller or turbine diffusers can be used to accelerate the ozone gas solution and help to ensure mixing and contact. All type of contacts use the counterflow, in which the water flows downward and the air bubbles rise to maximize the contact time.
1-23 Separate chamber contactor with baffles and diffusers
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(Felipe Solsona, Juan Pablo MĂŠndez, 2003)
Destruction of the surplus ozone The dissolved ozone will reach a concentration directly proportional to the partial pressure exerted by the ozone on the water. As a result, even with a transfer efficiency of 90% (one of the highest attained), the escaping gas can contain from 500 to 1,000 ppm of ozone. The surplus ozone gas is frequently recirculated to a prior unit process to improve the oxidation or flocculation so that it can be used to the fullest. Despite the recirculation, (surplus) ozone is generally present in the escaping gas and should be destroyed or sufficiently diluted for safety reasons. In small treatment plants, ozone can be diluted with air, but large treatment plants use one of the three following methods to destroy surplus ozone:
1) thermal decomposition by raising the water temperature to over 300 °C 2) catalytic decomposition by making it flow through metal or metal oxides 3) absorption in moist granular activated carbon.
1-24Typical flow diagram for the application of ozone for disinfection.
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(Eddy, 1999)
2.6.1.27 Operation and maintenance
Ozonation equipment typically has low to medium maintenance requirements. The
airpreparation
system
requires
frequent
attention
for
air
filter
cleaning/changing and for assuring that the desiccant is drying the air properly. However, both are usually simple operations. Two factors which impact ozone generator operation and maintenance are the effectiveness of the air-preparation system and the amount of time that the generator is required to operate at maximum capacity. Maintenance of the ozone generators is commonly scheduled once a year. However, many plants perform this maintenance every six months. Typically, one man-week is necessary to service an individual ozone generation unit of the horizontaltube type. Dielectric replacement due to failure as well as breakage during maintenance may be as low as 1 percent to 2 percent. An average tube life of ten years can be expected if a feedgas dew point of - 60' is maintained and if the ozone generator is not required to operate for prolonged periods at its rated capacity. Plate-type ozone generators use window glass as dielectrics. However, the same attention to air preparation is taken as with the more expensive glass or ceramic tubes in order to avoid costly downtime.
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Operations and maintenance of the ozone contactor also requires attention. Turbines require electricity to power the drive motors, while porous diffusers require regular inspection and maintenance to insure a uniform distribution of ozone-rich gas in the contact chamber. It should be noted that serious safety problems exist with servicing some of these units. For example, even after purging the contact chambers with air, maintenance personnel entering the chambers should be equipped with a self-contained breathing apparatus, since the density of ozone is heavier than air and therefore is difficult to remove completely by air purging.
2.6.1.28 Advantages & disadvantages of disinfection by ozonation Advantages 1. From the viewpoint of biocide effectiveness, ozone is the strongest disinfectant used in water supply systems. 2. The contact times and concentrations for inactivating or killing waterborne pathogens are much lower than those of free chlorine or any other disinfectant 3. There are no harmful residuals that need to be removed after ozonation because ozone decomposes rapidly. 4. After ozonation, there is no regrowth of microorganisms, except for those protected by the particulates in the wastewater stream. 5. Ozone is generated onsite, and thus, there are fewer safety problems associated with shipping and handling. 6. Ozonation elevates the dissolved oxygen (DO) concentration of the effluent. The increase in DO can eliminate the need for reaeration and also raise the level of DO in the receiving stream.
Disadvantages 1. An important point of disinfection via oxidation is that a large part of the ozone will generally be consumed by other substances that are usually present in the water and that this demand must be satisfied before disinfection can be assured.
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2. Its main disadvantage is that ozone does not provide a stable residual. Therefore, it will be necessary to add a secondary disinfectant to provide the disinfectant residual . 3. For these reasons and because its cost is relatively high, ozone is rarely used for disinfection purposes alone; rather, it is used when other aspects of water treatment must be improved simultaneously with disinfection, through ozoneâ&#x20AC;&#x2122;s power of oxidation. 4. Ozonation is a more complex technology than is chlorine or UV disinfection, requiring complicated equipment and efficient contacting systems. 5. Ozone is very reactive and corrosive, thus requiring corrosion-resistant material such as stainless steel. 6. Ozone is extremely irritating and possibly toxic, so off-gases from the contactor must be destroyed to prevent worker exposure.
Summary Because of the above limitations, ozone tends to be combined with other disinfectants (secondary disinfectants) that have weaker but more lasting residuals, in order to impede the resurgence of microorganisms in the distribution system. Economically-speaking, ozone can be used most beneficially when it is employed for other water treatment purposes, at the same time as the disinfection, such as to decompose synthetic organic substances, eliminate phenols, avoid the formation of trihalomethanes, improve flocculation and for other similar purposes. As indicated earlier, ozone is such a strong oxidant that it is almost always used for multiple purposes in treating water supplies, instead of merely as a disinfectant.
iv. DISINFECTION WITH CHLORINE DIOXIDE
2.6.1.29 Introduction Chlorine dioxide (ClO2) is a disinfectant with a stronger biocidal capacity than that of chlorine and chlorine compounds. Its selective oxidating qualities make
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its application an alternative to be considered for cases where not only must the water be disinfected, but its organoleptic qualities must also be improved. It has a major effect on destroying organic substances that color the water or are trihalomethane (THM) precursors. Even so, its use as a disinfectant in wastewater treatment plants is limited by the complexity and sensitivity of its production and its relatively high cost.
2.6.1.30 Mechanism of disinfectant action Chlorine dioxide exists in the water as ClO2 (little or no dissociation) and, therefore, is able to permeate through bacterial cell membranes and destroy those cells. Its actions on viruses include the absorption and penetration of the protein coat of the viral capsid and reacting with the viral RNA, thus damaging the genetic capacity of the virus. Chlorine dioxide produces a smaller microbicidal effect than ozone, but it is a stronger disinfectant than chlorine.
By-products of disinfection with chlorine dioxide Where chlorine disinfectants react with different substances through oxidation and electrophilic substitution, chlorine dioxide reacts only via oxidation. That is why the use of chlorine dioxide can result in reduced THM formation in treated water. Production of high levels of THM in chlorine dioxide treated water can usually be attributed to poor chlorine dioxide generator performance, generally because of excess chlorine, a substance that participates significantly in THM formation.
Even so, the existence of DBP cannot be denied and the products formed by the reaction of chlorine dioxide with organic material in the water include chlorophenols and maleic, fumaric and oxalic acids. A study of the byproducts of chlorine dioxide in a pilot treatment project revealed the presence of more than 40 DBP, most of them of unknown toxicity. During the oxidation of organic material, the chlorine dioxide breaks down to a chlorite ion. Chlorite and the chlorates are precisely the most important DBPs produced by the use of this disinfectant.
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2.6.1.31 Disinfection method & equipment Chlorine dioxide is a yellowish-green gas that is stable and relatively soluble in water until it reaches concentrations of up to 2%.
Chlorine dioxide is not sold off the shelf, but must be generated on-site. Furthermore, it is used only as a primary disinfectant and its generating and management are complex and risky. For those reasons, its use is not recommended for small facilities.
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Two mechanisms are usually used to generate chlorine dioxide: by reacting sodium chlorite with chlorine gas (two chemical compounds system) or by reacting sodium chlorite with sodium hypochlorite and sulphuric acid (three chemical compounds system). 2NaClO2 + Cl2 2ClO2 + 2NaCl (two compounds) 2NaClO2 + NaOCl + H2SO4 2ClO2 + NaCl + Na2SO4 + H2O (three compounds)
There is no industrial standard for the performance of chlorine dioxide generators. Generator efficiency is defined not only in terms of the conversion of sodium chlorite into chlorine dioxide, but also of the generating of byproducts such as chlorate ion, free chlorine and surplus chlorite. If the generator fails to operate properly, it can produce these by-products in excessive amounts and reduce the expected results. Poor generator performance will also result in higher operating costs than desired.
For information about the installation, operation & maintenance of an Chlorine dioxide disinfection system, see the relative bibliography (California State University, 2008) (Felipe Solsona, Juan Pablo Méndez, 2003).
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2.6.1.32 Advantages & disadvantages of disinfection by ClO2
Advantages 1. Its bactericidal potential is relatively independent of the pH at between 4 and 10. 2. It works better than chlorine for treating spores. 3. It needs little contact time. 4. It is quite soluble. 5. No corrosion is produced at high concentrations, thus reducing maintenance costs. 6. It improves coagulation. 7. It is better than chlorine for removing iron and manganese. 8. Where chlorine disinfectants react with different substances through oxidation and electrophilic substitution, chlorine dioxide reacts only via oxidation. That is why the use of chlorine dioxide can result in reduced THM formation in treated water. 9. Apparently not affected by variations in pH
Disadvantages 1. Chlorine dioxide has limited residual properties; for that reason, chlorine is generally used as a secondary disinfectant to ensure additional protection of the water distribution system. 2. Complex method in the apply 3. Costs more than chlorine 4. Chlorite and chlorate by-products are formed 5. Must be generated on-site. 6. Trained workers are required for its operation and maintenance 7. Difficult to analyze in the laboratory.
NIREAS VOLUME 2 [2.6] v. COMPARISON BETWEEN DISINFECTION METHODS
89
1-17Mechanisms of disinfection using chlorine, UV and ozone
(Eddy, 1999)
All four alternative disinfection methods descripted previously, are compared in the following table.
1-18 Disinfection methods comparison Disinfection method Parameter Recommended wastewater quality SS (mg/L) BOD (mg/L) Turbidity (NTU) Effectiveness against Bacteria Viruses Parasites Residual effect
Chlorine
Ozone
UV
Chlorine Dioxide
<20 <20 <10
<15 <20 <5
<10 <20 <5
<20 <20 <10
High Moderate Low
High High High
High High NA
High High Moderate
High
None
None
Moderate
NIREAS VOLUME 2 [2.6] Practicality 90
Size of WWTP applicable Required area Process control Complexity Maintenance and cleaning Stability Temperature dependent
Low
Medium to Large High
Developing
Developing
Developing
Complex
Low to moderate
Moderate
Moderate to High
High
Moderate
Low
High
Moderate
Low
Low to moderate
Moderate
High
High
Moderate
High
Low to moderate
Moderate
Moderate
Moderate
Moderate
High
Low to moderate Moderate to High High
Moderate to High
None
Moderate
None
High
All plants High Well developed Low to moderate Low to moderate High(except gas clorine) Moderate to High
Medium to Large Moderate
All plants
Reliability Costs Operation Contruction (small to medium facility) Contruction (medium to large facility) High Energy consumption Adverse effects Dangerous emmisions Transportation risks On site risks Fish and macroinvertebrate toxicity Formation of toxic byproducts Disposal of cleaning products Likelihood of pathogens regrowth
Low to moderate None
High High
Moderate Moderate
None(except gas clorine) High Low to moderate
Potential leakage of O3 None Moderate
Low
High
High
None
None
High
Potential High
Unknown
Unknown
Potential High
None
None
Yes
None
Low to moderate
Moderate
High
Low to moderate
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ASSIGNMENTS SECTION ASSIGNMENTS SECTION 5 QUESTIONS
1. What is the purpose of disinfection? Why is this important?
2. Why is chlorine used for disinfection?
3. What happens when chlorine is added to waters containing ammonia and why is this significant?
4. How is the chlorine demand determined ?
5. How is the effectiveness of the chlorination process for a particular plant determined ?
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6. How chlorine gas feed can be controlled?
7. What are the hazards of chlorine gas?
8. What type of breathing apparatus is recommended when repairing a chlorine leak?
9. Why has chlorine dioxide not been widely used to treat wastewater?
10. Why should chlorinators be in a separate room?
11. Why is adequate ventilation important in a chlorinator room?
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12. Why should disinfection by chlorination be continuous?
13. What is the best piping material for conducting chlorine gas or liquid?
14. Why are the effluents from some treatment plants dechlorinated?
15. What happens when ultraviolet radiation is absorbed by the cells of microorganisms?
16. How is the number of UV banks per channel determined ?
17. What kinds of damage can the light from a UV lamp do to operators?
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18. The UV light intensity that reaches the pathogens in the wastewater is affected by which factors?
19. Why do operators need to periodically observe the UV wiping system process?
20. Why do UV systemas require extensive alarma systems?
21. The service life of UV lamps depends on which factors?
22. Why is ozone generated on site?
23. The effectiveness of ozone disinfection depends on which factors?
24. What are the key process control guidelines for ozone disinfection?
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SUGGESTED ANSWARS:
1. The purpose of disinfection is to destroy pathogenic organisms. This is important to prevent the spread of waterborne diseases
2. Chlorine is used for disinfection because it is relatively easy to obtain and cheap to manufacture . Even at low dosages, chlorine is extremely effective.
3. Since ammonia is present in all domestic wastewaters, the reaction of ammonia with chlorine is of great significance. When chlorine is added to waters containing ammonia, the ammonia reacts with hypochlorous acid (HOCL) to form chloramines : monochloramine, dichloramine, and trichloramine. The mono- and dichloramine forms
have definite
disinfection powers and are of interest in the measurement of chlorine residuals. Dichloroamine has a more effective disinfection power than monochloroamine.
4. Chlorine demand is equal to the chlorine dose minus the chlorine residual, or : Chlorine demand = Chlorine dose - Chlorine residual
5. The
ultimate
measure
of
chlorination
effectiveness
is
the
bacteriological result. The residual chlorine that yields satisfactory bacteriological results in a particular plant must be determined and used as a control in that plant.
6. Chlorine gas feed can be controlled by manual, start/stop, step-rate, timed-programa, flow-proportional, chlorine-residual, and compound â&#x20AC;&#x201C; loop controls.
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7. Chlorine gas is extremely toxic and corrosive in moist atmospheres.
8. When repairing a chlorine leak, self-contained air or supplied-air types of breathing apparatus are recommended. Self- contained air supply and demand â&#x20AC;&#x201C; breathing equipment must fit and be used properly. Pressure â&#x20AC;&#x201C; demand and redbreather kits may be safer. Pressuredemand units use more air from the air bottle, which reduces the time a person may work on a leak. There are certain physical constraints when using respiratory protection. Confirm requirements with your local safety regulatory agency.
9.
Due to the safety hazards of handling sodium chlorite, chlorite dioxide has not been widely used to treat wastewater.
10. Chlorinators should be in a separate room because chlorine gas leaks can damage equipment and are hazardous to personnel
11. Adequate ventilation is important in a chlorinator room to remove any leaking chlorine gas
12. Disinfection by chlorination must be continuous for the protection of downstream water users
13. The best piping material for conducting chlorine gas or liquid is seamless carbon steel.
14. The effluents from some treatment plants are dechlorinated to protect fish and other aquatic organisms from toxic chlorine residuals.
15. When UV radiation is absorbed by the cells of microorganisms, the genetic material is damaged in such a way that the organisms are no longer able to grow or reproduce
thus ultimately killing them.
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16. The number of UV banks per channel is determined by the required UV dosage to achieve the target effluent quality.
17. The light from a UV lamp can cause serious burns to the eyes and skin of operators.
18. UV light intensity that reaches the pathogens in the wastewater is affected by the condition of the UV
lamps and the quality of the
wastewater.
19. Operators need to periodically observe the UV wiping system process to ensure proper operation of the wiping action of a bank and the proper wiping cycle.
20. UV systems require extensive alarm systems to ensure continuous complete disinfection of the water being treated
21. The service life of UV lamps depends on : -
The level of suspended solids in the water to be disinfected and the fecal coliform level to be achieved
-
The frequency of ON/OFF cycles
-
The operating temperature of the lamp electrodes
22. Ozone is generated on site because it is unstable and decomposes to elemental oxygen in a short time after generation
23. The effectiveness of ozone disinfection depends on the susceptibility of the target organisms, the contact time, and the concentration of the ozone
24. The key process control quidelines for ozone disinfection are dose, mixing and contact time
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