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    Lesson 7:

    Disinfection

    Objective

    In this lesson we will answer the following questions:

    What disinfection requirements must be met in treating drinking water?

    How does chlorination fit into the water treatment process?

    How does chlorination work chemically?

    What factors influence the efficiency of chlorination?

    What equipment is used for chlorination?

    What other methods can be used to disinfect water?

    Reading Assignment

    Along with the online lesson, read Chapter 7: Disinfection, in your textbook Operation of Water

    Treatment Plants Volume I .

    Lecture

    Introduction

    What is Disinfection?

    Before water treatment became common, waterborne diseases could spread quickly through a

    population, killing or harming hundreds of people. The table below shows some common, water-

    transmitted diseases as well as the organisms (pathogens) which cause each disease. More

    information on water-borne pathogens can be found in ENV 108.

    Pathogen Disease Caused

    Bacteria:

    Anthrax anthrax

    Escherichia coli E. coliinfection

    Myobacterium

    tuberculosis

    tuberculosis

    Salmonella salmonellosis,

    paratyphoid

    http://water.me.vccs.edu/courses/ENV108/Env108.html
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    Vibrio cholerae cholera

    Viruses:

    Hepatitis Virus Hepatitis A

    Polio Virus polio

    Parasites:

    Cryptosporidium cryptosporidiosisGiardia lamblia giardiasis

    The primary goal of water treatment is to ensure that the water is safe to drink and does not contain

    any disease-causing microorganisms. The best way to ensure pathogen-free drinking water is to

    make sure that the pathogens never enter the water in the first place. However, this may be a

    difficult matter in a surface water supply which is fed by a large watershed. Most treatments plants

    choose to remove or kill pathogens in water rather than to ensure that the entire watershed is free of

    pathogens.

    Pathogens can be removed from water through physical or chemical processes. You may

    remember that some previously discussed treatment processes, notably sedimentation and filtration,

    can remove a large percentage of bacteria and other microorganisms from the water by physical

    means. Storage can also kill a portion of the disease-causing bacteria in water.

    This lesson will be concerned with disinfection, which is the process of selectively destroying or

    inactivating pathogenic organisms in water, usually by chemical means. Disinfection is different from

    sterilization, which is the complete destruction of all organisms found in water and which is usually

    expensive and unnecessary. Disinfection is a required part of the water treatment process whilesterilization is not.

    Testing and Requirements

    The goal of disinfection is to remove or inactivate all disease-causing organisms in water. However,

    testing for each type of pathogen individually would be costly and inefficient. Instead, operatorsfocus on three indicators of pathogen removal efficiency. The first two have been discussed in

    previous lessons - Giardia and viruses. The third test, total coliform, is the most frequently used

    indicator of disinfection efficiency.

    Coliform bacteriaare often found in the guts of warm-blooded animals such as humans, but can

    also be found in plants, soil, water, or air. It is relatively simple to test for the number of coliform

    bacteria found in water, and their presence indicates that other pathogenic bacteria are also likely to

    be present. If disinfection removes all of the coliforms from the water, then the operator can safely

    assume that the other disease-causing microorganisms have also been removed.

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    You will remember that the standards for the removal of Giardiaand viruses are 99.9% and

    99.99%, respectively. After disinfection, standards for total coliform require that water should have

    0 coliforms per hundred millimeters of water sampled. If less than 40 samples of water are tested

    per month, then no more than one sample can test positive for coliform bacteria. If forty or more

    samples are taken more month, then no more than 5% of the samples can be positive.

    Chlorination

    Purpose

    Chlorinationis the application of chlorine to water to accomplish some definite purpose. In this

    lesson, we will be concerned with the application of chlorine for the purpose of disinfection, but you

    should be aware that chlorination can also be used for taste and odor control, iron and manganese

    removal, and to remove some gases such as ammonia and hydrogen sulfide.

    Chlorination is currently the most frequently used form of disinfection in the water treatment field.

    However, other disinfection processes have been developed. These alternatives will be discussed

    at the end of this lesson.

    Prechlorination and Postchlorination

    Like several other water treatment processes, chlorination can be used as a pretreatment process

    (prechlorination) or as part of the primary treatment of water (postchlorination). Treatment

    usually involves either postchlorination only or a combination of prechlorination and

    postchlorination.

    Prechlorination is the act of adding chlorine to the raw water. The residual chlorine is useful in

    several stages of the treatment process - aiding in coagulation, controlling algae problems in basins,

    reducing odor problems, and controlling mudball formation. In addition, the chlorine has a much

    longer contact time when added at the beginning of the treatment process, so prechlorination

    increases safety in disinfecting heavily contaminated water.

    Postchlorination is the application of chlorine after water has been treated but before the water

    reaches the distribution system. At this stage, chlorination is meant to kill pathogens and to provide

    a chlorine residual in the distribution system. Postchlorination is nearly always part of the treatment

    process, either used in combination with prechlorination or used as the sole disinfection process.

    Until the middle of the 1970s, water treatment plants typically used both prechlorination and

    postchlorination. However, the longer contact time provided by prechlorination allows the chlorine

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    to react with the organics in the water and produce carcinogenic substances known as

    trihalomethanes. As a result of concerns over trihalomethanes, prechlorination has become much

    less common in the United States. Currently, prechlorination is only used in plants where

    trihalomethane formation is not a problem.

    Location in the Treatment Process

    During prechlorination, chlorine is usually added to raw water after screening and before flash

    mixing. Postchlorination, in contrast, is often the last stage in the treatment process. After flowing

    through the filter, water is chlorinated and then pumped to the clearwell to allow a sufficient contact

    time for the chlorine to act. From the clearwell, the water may be pumped into a large, outdoor

    storage tank such as the one shown below. Finally, the water is released to the customer.

    Photo Credit: Virginia Department of Health

    Chlorination Chemistry

    Introduction

    When chlorine is added to water, a variety of chemical processes take place. The chlorine reacts

    with compounds in the water and with the water itself. Some of the results of these reactions

    (known as the chlorine residual) are able to kill microorganisms in the water. In the following

    sections, we will show the chemical reactions which occur when chlorine is added to water.

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    Chlorine Demand

    When chlorine enters water, it immediately begins to react with compounds found in the water. The

    chlorine will react with organic compounds and form trihalomethanes. It will also react with

    reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions.

    Let's consider one example, in which chlorine reacts with hydrogen sulfide in water. Two different

    reactions can occur:

    Hydrogen Sulfide + Chlorine + Oxygen Ion Elemental Sulfur + Water +

    Chloride Ions

    H2S + Cl2+ O2- S + H2O + 2Cl

    -

    Hydrogen Sulfide + Chlorine + Water Sulfuric Acid + Hydrochloric Acid

    H2S + 4Cl2+ 4 H2O H2SO4+ 8 HCl

    I have written each reaction using both the chemical formula and the English name of each

    compound. In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create

    elemental sulfur, water, and chloride ions. The elemental sulfur precipitates out of the water and

    can cause odor problems. In the second reaction, hydrogen sulfide reactions with chlorine and

    water to create sulfuric acid and hydrochloric acid.

    Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric

    acid which have no disinfecting properties. The total amount of chlorine which is used up in

    reactions with compounds in the water is known as the chlorine demand. A sufficient quantity of

    chlorine must be added to the water so that, after the chlorine demand is met, there is still some

    chlorine left to kill microorganisms in the water.

    Reactions of Chlorine Gas With Water

    At the same time that chlorine is being used up by compounds in the water, some of the chlorinereacts with the water itself. The reaction depends on what type of chlorine is added to the water as

    well as on the the pH of the water itself.

    Chlorine may be added as to water in the form of chlorine gas, hypochlorite, or chlorine dioxide.

    All types of chlorine will kill bacteria and some viruses, but only chlorine dioxide will effectively kill

    Cryptosporidium, Giardia, protozoans, and some viruses. We will first consider chlorine gas,

    which is the most pure form of chlorine, consisting of two chlorine atoms bound together.

    Chlorine gas is compressed into a liquid and stored in metal cylinders. The gas is difficult to handle

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    since it is toxic, heavy, corrosive, and an irritant. At high concentrations, chlorine gas can even be

    fatal.

    When chlorine gas enters the water, the following reaction occurs:

    Chlorine + Water Hypochlorous Acid + Hydrochloric Acid

    Cl2+ H2O HOCl + HCl

    The chlorine reacts with water and breaks down into hypochlorous acidand hydrochloric acid.

    Hypochlorous acid may further break down, depending on pH:

    Hypochlorous Acid Hydrogen Ion + Hypochlorite Ion

    HOCl H++ OCl-

    Note the double-sided arrows which mean that the reaction is reversible. Hypochlorous acid maybreak down into a hydrogen ion and a hypochlorite ion, or a hydrogen ion and a hypochlorite ion

    may join together to form hypochlorous acid.

    The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will depend on

    the water's pH. A higher pH facilitates the formation of more hypochlorite ions and results in less

    hypochlorous acid in the water. This is an important reaction to understand because hypochlorous

    acid is the most effective form of free chlorine residual, meaning that it is chlorine available to kill

    microorganisms in the water. Hypochlorite ions are much less efficient disinfectants. So

    disinfection is more efficient at a low pH (with large quantities of hypochlorous acid in the water)

    than at a high pH (with large quantities of hypochlorite ions in the water.)

    Hypochlorites

    Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known

    as a bleach. Hypochlorites are less pure than chlorine gas, which means that they are also less

    dangerous. However, they have the major disadvantage that they decompose in strength over timewhile in storage. Temperature, light, and physical energy can all break down hypochlorites before

    they are able to react with pathogens in water.

    There are three types of hypochlorites - sodium hypochlorite, calcium hypochlorite, and commercial

    bleach:

    Sodium hypochlorite(NaOCl) comes in a liquid form which contains up to 12% chlorine.

    Calcium hypochlorite(Ca(OCl)2), also known as HTH, is a solid which is mixed with

    water to form a hypochlorite solution. Calcium hypochlorite is 65-70% concentrated.

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    Commercial bleach is the bleach which you buy in a grocery store. The concentration of

    commercial bleach varies depending on the brand - Chlorox bleach is 5% chlorine while

    some other brands are 3.5% concentrated.

    Hypochlorites and bleaches work in the same general manner as chlorine gas. They react with

    water and form the disinfectant hypochlorous acid. The reactions of sodium hypochlorite and

    calcium hypochlorite with water are shown below:

    Calcium hypochlorite + Water Hypochlorous Acid + Calcium Hydroxide

    Ca(OCl)2+ 2 H2O 2 HOCl + Ca(OH)2

    Sodium hypochlorite + Water Hypochlorous Acid + Sodium Hydroxide

    NaOCl + H2O HOCl + NaOH

    In general, disinfection using chlorine gas and hypochlorites occurs in the same manner. The

    differences lie in how the chlorine is fed into the water and on handling and storage of the chlorine

    compounds. In addition, the amount of each type of chlorine added to water will vary since each

    compound has a different concentration of chlorine.

    Chloramines

    Some plants use chloramines rather than hypochlorous acid to disinfect the water. To produce

    chloramines, first chlorine gas or hypochlorite is added to the water to produce hypochlorous acid.

    Then ammonia is added to the water to react with the hypochlorous acid and produce a

    chloramine.

    Three types of chloramines can be formed in water - monochloramine, dichloramine, and

    trichloramine. Monochloramine is formed from the reaction of hypochlorous acid with ammonia:

    Ammonia + Hypochlorous Acid Monochloramine + Water

    NH3+ HOCl NH2Cl + H2O

    Monochloramine may then react with more hypochlorous acid to form a dichloramine:

    Monochloramine + Hypochlorous Acid Dichloramine + Water

    NH2Cl + HOCl NHCl2+ H2O

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    Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:

    Dichloramine + Hypochlorous Acid Trichloramine + Water

    NHCl2+ HOCl NCl3+ H2O

    The number of these reactions which will take place in any given situation depends on the pH of the

    water. In most cases, both monochloramines and dichloramines are formed. Monochloramines

    and dichloramines can both be used as a disinfecting agent, called a combined chlorine residual

    because the chlorine is combined with nitrogen. This is in contrast to the free chlorine residual of

    hypochlorous acid which is used in other types of chlorination.

    Chloramines are weaker than chlorine, but are more stable, so they are often used as the

    disinfectant in the distribution lines of water treatment systems. Despite their stability, chloramines

    can be broken down by bacteria, heat, and light. Chloramines are effective at killing bacteria and

    will also kill some protozoans, but they are very ineffective at killing viruses.

    Breakpoint Chlorination

    The graph below shows what happens when chlorine (either chlorine gas or a hypochlorite) is

    added to water. First (between points 1 and 2), the water reacts with reducing compounds in the

    water, such as hydrogen sulfide. These compounds use up the chlorine, producing no chlorineresidual.

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    Next, between points 2 and 3, the chlorine reacts with organics and ammonia naturally found in the

    water. Some combined chlorine residual is formed - chloramines. Note that if chloramines were to

    be used as the disinfecting agent, more ammonia would be added to the water to react with the

    chlorine. The process would be stopped at point 3. Using chloramine as the disinfecting agent

    results in little trihalomethane production but causes taste and odor problems since chloramines

    typically give a "swimming pool" odor to water.

    In contrast, if hypochlorous acid is to be used as the chlorine residual, then chlorine willbe added

    past point 3. Between points 3 and 4, the chlorine will break down most of the chloramines in the

    water, actually lowering the chlorine residual.

    Finally, the water reaches the breakpoint, shown at point 4. The breakpointis the point at which

    the chlorine demand has been totally satisfied - the chlorine has reacted with all reducing agents,

    organics, and ammonia in the water. When more chlorine is added past the breakpoint, the

    chlorine reacts with water and forms hypochlorous acid in direct proportion to the amount of

    chlorine added. This process, known as breakpoint chlorination, is the most common form of

    chlorination, in which enough chlorine is added to the water to bring it past the breakpoint and to

    create some free chlorine residual.

    Chlorine Dioxide

    There is one other form of chlorine which can be used for disinfection - chlorine dioxide. We have

    not discussed chlorine dioxide previously because it disinfects using neither hypochlorous acid nor

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    chloramines and is not part of the breakpoint chlorination process.

    Chlorine dioxide, ClO2, is a very effective form of chlorination since it will kill protozoans,

    Cryptosporidium, Giardia, and viruses that other systems may not kill. In addition, chlorine

    dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and

    water. Chlorine dioxide can be used to remove sulfide compounds and phenolic tastes and odors.

    When chlorine dioxide is used, trihalomethanes are not formed and the chlorination process is

    unaffected by ammonia. Finally, chlorine dioxide is effective at a higher pH than other forms ofchlorination.

    So why isn't chlorine dioxide used in all systems? Chlorine dioxide must be generated on site,

    which is a very costly process requiring a great deal of technical expertise. Unlike chlorine gas,

    chlorine dioxide is highly combustible and care must be taken when handling the chlorine dioxide.

    Efficiency

    Residual and Dosage

    A variety of factors can influence disinfection efficiency when using breakpoint chlorination or chloramines.

    One of the most important of these is the concentration of chlorine residual in the water.

    The chlorine residual in the clearwell should be at least 0.5 mg/L. This residual, consisting of

    hypochlorous acid and/or chloramines, must kill microorganisms already present in the water and

    must also kill any pathogens which may enter the distribution system through cross-connections or

    leakage. In order to ensure that the water is free of microorganisms when it reaches the customer,

    the chlorine residual should be about 0.2 mg/L at the extreme ends of the distribution system. This

    residual in the distribution system will also act to control microorganisms in the distribution system

    which produce slimes, tastes, or odors.

    Determining the correct dosage of chlorine to add to water will depend on the quantity and type of

    substances in the water creating a chlorine demand. The chlorine dose is calculated as follows:

    Chlorine Dose = Chlorine Demand + Chlorine Residual

    So, if the required chlorine residual is 0.5 mg/L and the chlorine demand is known to be 2 mg/L, then 2.5 mg/L ofchlorine will have to be added to treat the water.

    The chlorine demand will typically vary over time as the characteristics of the water change. By testing the

    chlorine residual, the operator can determine whether a sufficient dos e of chlorine is being added to treat the

    water. In a large system, chlorine must be sampled every two hours at the plant and at various points in the

    distribution sys tem.

    It is also important to understand the breakpoint curve when changing chlorine dos ages . If the water smells

    strongly of chlorine, it may not mean that too much chlorine is being added. More likely, chloramines are being

    produced, and more chlorine needs to be added to pass the breakpoint .

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    Contact Time

    Contact time is just as important as the chlorine residual in determining the efficiency of

    chlorination. Contact timeis the amount of time which the chlorine has to react with the

    microorganisms in the water, which will equal the time between the moment when chlorine is added

    to the water and the moment when that water is used by the customer. The longer the contact time,

    the more efficient the disinfection process is. When using chlorine for disinfection a minimum

    contact time of 30 minutes is required for adequate disinfection.

    The CT valueis used as a measurement of the degree of pathogen inactivation due to chlorination.

    The CT value is calculated as follows:

    CT = (Chlorine residual, mg/L) (Contact time, minutes)

    The CT is the Concentration multiplied by the Time. As the formula suggests, a reduced chlorine

    residual can still provide adequate kill of microorganisms if a long contact time is provided.

    Conversely, a smaller chlorine residual can be used as long as the chlorine has a longer contact time

    to kill the pathogens.

    Other Influencing Factors

    Within the disinfection process , efficiency is influenced by the chlorine residual, the type of chemical used for

    chlorination, the contact t ime, the initial mixing of chlorine into the water, and the location of chlorination withinthe treatment process. The most efficient process will have a high chlorine residual, a long contact t ime, and

    thorough mixing.

    Characteristics of the water will also affect efficiency of chlorination. As you will recall, at a high pH, the

    hypochlorous acid becomes dissociated into the ineffective hypochlorite ion. So lower pH values result in more

    efficient disinfection.

    Temperature influences chlorination just as it does any other chemical reaction. Warmer water can be treated

    more efficiently s ince the reactions occur more quickly. At a lower water temperature, longer contact times or

    higher concentrations of chemicals must be used to ensure adequate disinfection.

    Turbidity of the water influences disinfection primarily through influencing the chlorine demand.Turbid water tends to contain particles which react with chlorine, reducing the concentration of

    chlorine residual which is formed. Since the turbidity of the water depends to a large extent on

    upstream processes (coagulation, flocculation, sedimentation, and filtration), changes in these

    upstream processes will influence the efficiency of chlorination. Turbidity is also influenced by the

    source water - groundwater turbidity tends to change slowly or not at all while the chlorine demand

    of surface water can change continuously, especially during storms and the snow melt season.

    Finally, and most intuitively, the number and type of microorganisms in the water will influence

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    chlorination efficiency. Since cyst-forming microorganisms and viruses are very difficult to kill using

    chlorination, the disinfection process will be less efficient if these pathogens are found in the water.

    Chlorination Equipment

    Hypochlorinators

    The simplest method of continuous chlorination of systems less than 75 gpm is by the use of a

    hypochlorinator. Hypochlorinators are motor driven pumps which are used to added

    hypochlorite solutions to water. The pump pulls the hypochlorite solution out of a holding chamber

    and pumps it into the water to be treated. Where the pipe from the pump joins the pipe carrying

    the raw water, the Venturi effect creates a small vacuum and pulls the chlorine solution into the

    water.

    It is often necessary to increase or decrease the amount of chlorine added to the water as

    conditions change. Hypochlorinators allow you to adjust the amount of chlorine fed into the water in

    three ways. You can adjust the stroke length or machine speed by varying the pulley size. Both of

    these adjustments change the hypochlorinator feed rate - the speed at which the machine puts

    chlorine into the water. You can also adjust the amount of chlorine added by changing the strength

    of the hypochlorite solution.

    Chlorinators and Cylinders

    While hypochlorinators are usually used to perform continuous chlorination in smaller systems,

    chlorinators are more economical when the supply source is greater than 75 gpm and may

    sometimes be used in smaller systems as well. Anticipated pumping periods and chlorine demand

    (based on the chlorine residual test) determine whether a hypochlorinator or chlorinator should be

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    used in each situation.

    Chlorinatorsare devices which introduce chlorine gas to water using liquid chlorine supplied in

    steel cylinders. The following sections will explain how the proper quantity of chlorine is delivered

    from the cylinder to the source water. But first we need to understand how the liquid chlorine is

    stored.

    Chlorine cylinders

    Liquid chlorine can be stored in 100 or 150 pound cylinders, ton containers, or 55 to 90 ton rail

    cars. In each case, the chlorine has been condensed into a liquid form, but expands back into a gas

    as it leaves the cylinder. Whenever a substance changes state from a liquid to a gaseous form, heat

    is required. The heat which is absorbed by the chlorine as it changes state in the cylinder comes

    from the surrounding air.

    If chlorine is drawn off from a cylinder too quickly, the temperature of the air surrounding the tank

    will drop and will cause frosting and lower gas flow. To prevent frosting, the draw off rate should

    be no greater than 350 pounds of gas/day for a 100-150 pound cylinder. If greater feed rate are

    required, several tanks can be connected using a manifold, which is a pipe joining the cylinders

    together so that chlorine gas is drawn from several cylinders at once.

    The only accurate way to determine the feed rate of chlorine from a cylinder is to weigh the cylinder

    over time. By subtracting the tare weight(the weight of an empty cylinder), the operator can

    determine how much chlorine gas remains in the cylinder so that empty cylinders can be replaced in

    a timely manner. If the cylinders are weighed over time, the feed rate of chlorine can be determined

    to ensure that the proper concentration of chlorine is being added to the water.

    Whenever dealing with gaseous chlorine, safety is an important issue. Ammonia should be kept

    handy for checking for leaks and storage buildings should be well ventilated. If the operator must

    walk through an area with chlorine in the air, he or she should use a breathing apparatus. If no

    breathing apparatus is available, the operator should keep his head high since chlorine is 2.5 times

    as heavy as air and will tend to sink to the ground.

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    Vacuum Chlorinators

    The most typical kind of chlorinator, a vacuum chlorinator, is shown below:

    In a vacuum chlorinator, chlorine gas is pulled from the cylinder into the source water by a

    vacuum. The vacuum is created by water flowing through the injector and creating a negative

    head. This negative head forces open the pressure regulating valve on the cylinder and allowschlorine gas to flow out of the cylinder and into the chlorinator.

    Once the gas has entered the chlorinator, the chlorine feed rate is measured using an indicator

    known as a rotameter. Just beyond the rotameter, the chlorine gas flows past a regulating device

    (a V-notch plug or a valve) which is used to adjust the chlorine feed rate.

    Then the chlorine gas is pulled into the injector, also known as an ejector. The injector consists of

    a pipe filled with flowing water. The flowing water pulls chlorine into the water, both chlorinating

    the source water and creating a vacuum in the chlorine line which pulls more chlorine gas out of the

    cylinder. This type of chlorinator is also known as a solution feedersince the chlorine gas isdissolved into a small amount of source water, which is then piped into the main line of water to be

    chlorinated.

    Chlorinators can be controlled manually (using the regulator) or with a controller. The most

    common type of controller is the flow proportional controllerwhich automatically feeds chlorine

    based on the flow rate of the water.

    Vacuum chlorinators are very safe since any break in the line with disrupt the vacuum and close the

    pressure regulating valve. As a result, chlorine leaks are very uncommon.

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    Direct Feed Chlorinators

    In a few cases, direct feed chlorinatorsare used instead of vacuum chlorinators. In a direct feed

    chlorinator, the chlorine gas is under pressure and is pumped directly into the main flow of water.

    There, the chlorine is evenly dispersed into the water using a diffuser, like the one shown below.

    Since the chlorine is under pressure, a pressurized water supply is not needed for use with a direct

    feed chlorinator. However, the pressurized chlorine is prone to leakage, so safety issues limit direct

    feed chlorinators to small installations or for use as emergency equipment.

    Other Disinfection Methods

    Types of Disinfection

    Up until this point, we have been concerned only withdisinfection using chlorine. However, a variety of othermethods can be used to disinfect water. The table below summarizes eight disinfection processes .

    Disinfection

    Method

    Disinfection Process

    Advantages

    Disadvantages

    Uses

    Chlorine chemical reaction with pathogens

    a small dose kills bacteria rapidly; residual can be

    maintained

    in some cases, chlorination can cause the formation

    of trihalomethanes

    widespread use to disinfect water; also

    used in color, taste, and odor removal,

    improving coagulation, and killing algae.

    Iodine chemical reaction with pathogens

    good disinfectanthigh cost; harmful to pregnant

    emergency treatment of water supplies;

    disinfecting s mall, non-permanent water

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    women supplies

    Bromine chemical reaction with pathogens

    handling difficulties; residuals hard to obtain;

    supply is limited

    very limited use, primarily for treating

    swimming pool water

    Bases

    (sodium

    hydroxide

    and lime)

    chemical reaction with pathogens

    bitter taste in the water; handling difficulties

    sterilize water pipes

    Ozone chemical reaction with pathogens

    good disinfectant; better virucide than chlorine;

    oxidizes iron, manganese, sulfide, and organics;

    removes color, odor, and taste

    high cost; lack of residual; s torage difficulties;

    maintenance requirements ; s afety problems;

    unpredictable disinfection; no track record

    disinfection; treating iron and manganese,

    helping flocculation, removing algae,

    oxidizing organics, removing color,

    treating tas tes and odors

    Ultraviolet UV light causes biological changes which kill the

    pathogenslack of dangerous by-products lack of

    measurable residual; cost of operation; turbidity

    interferes with disinfection

    small or local systems and indus trial

    applications

    Ultrasonic sound waves destroy pathogens by vibration

    very expens ive

    Heat boiling water for about five minutes will des troy

    essentially all microorganismssimple, requires little

    equipment

    very energy intensive; expensive

    Individuals may boil their water for

    household quantities of water when

    quality of water is ques tionable

    In the past, water treatment plants have principally relied on the use of chlorine for disinfection. The

    prevalent use of chlorine has come about because chlorine is an excellent disinfecting chemical and,

    until recently, has been available at a reasonable cost.

    However, chlorine has several disadvantages. Chlorine is becoming more expensive and has been

    shown to be toxic to fish and other biota. In addition, chlorine can combine with organic

    substances in water to produce trihalomethanes, which are suspected of causing cancer.

    As a result, future water treatment may see an increased use of ozone or ultraviolet (UV) light.

    Both types of treatment are effective disinfecting agents and leave no toxic residual. We will

    consider ozone and UV disinfection briefly below.

    Ozone

    Oxygen in the air (O2) is composed of two oxygen atoms. Under certain conditions, three oxygen

    atoms can be bound together instead, forming ozone(O3).

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    Ozone has many advantages as a disinfectant. It kills all pathogenic organisms by a direct effect on

    their DNA. Disinfection with ozone occurs 30,000 times faster than with chlorine, so a prolonged

    contact time is unnecessary. And there is no harmful residual left in the system.

    The disadvantages of an ozone disinfection system include a corrosive nature, a high cost for the

    initial set-up, and a high electricity consumption.

    UV Light

    Ultraviolet, or UV, light is light outside the range usually detectable by the human eye. It can be

    used to deactivate protozoans so that they can't reproduce and to significantly reduce the

    concentration of bacteria in water.

    The picture below shows a UV disinfection setup:

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    The primary disadvantage of UV light is a high operating cost. In addition, anything which blocks

    UV light from reaching the water will result in a lack of treatment, so water must be free of turbidity

    before being treated with UV light.

    Choosing a Disinfection Method

    Of the many disinfection methods, five have been used extensively in water treatment. The table

    below lists some of the factors which may influence the choice of treatment method in a new plant.

    Chlorine

    (Gas or

    Hypochlorite)

    Chlorine

    Dioxide

    Chloramine Ozone Ultraviolet

    Produces trihalomethanes? yes no yes sometimes no

    Produces other troublesome

    byproducts?

    yes yes yes yes sometimes

    Impacted by lime softening? yes no yes no yes

    Impacted by turbidity? somewhat somewhat somewhat somewhat yesMeets Giardia removal standards? no yes no yes no

    Meets Cryptosporidium removal

    standards?

    no no no yes no

    Meets virus removal s tandards? yes yes no yes yes

    Operator skill level low high low/medium high medium

    Applicable to large utilities ? yes yes yes yes no

    Applicable to small utilities ? yes yes yes yes yes

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    You may note that many of the disinfection methods do not meet standards for Giardia,

    Cryptosporidium, and virus removal. This does not mean that these disinfection methods cannot

    be used. When used in conjunction with filtration, all of the disinfection methods can be used to

    meet removal standards.

    Review

    Drinking water is disinfected to kill or inactivate waterborne pathogens. The most common form of

    disinfection is chlorination, although ozone and UV light are also used in some plants. Chlorine may

    be added to the water as chlorine gas or hypochlorite (both of which produce the disinfectant

    hypochlorous acid), as chlorine dioxide, or ammonia may be added with chlorine to form

    disinfectant chloramines.

    Chlorination may occur as a pretreatment process or as the final step in the treatment process. A

    sufficient quantity of chlorine must be used to both kill microorganisms already existing in the water

    and to maintain a chlorine residual throughout the distribution system. Chlorination efficiency

    depends on chlorine residual, contact time, type of chemical used, mixing effectiveness, location in

    the treatment process, and on characteristics of the water being treated.

    Breakpoint chlorination is a common form of disinfection in which chlorine is added to water until

    the chlorine demand has been satisfied and some free chlorine residual has been formed. The

    chlorine demand involves the reaction of chlorine with compounds in water, reducing the amount of

    chlorine available to kill microorganisms. Once all of these reactions have occurred, any additional

    chlorine added to the water will produce hypochlorous acid, a free chlorine residual.

    Disinfection equipment depends on the type of disinfectant used. Hypochlorite is added to water

    using a hypochlorinator. Gaseous chlorine is added to water using a chlorinator. Disinfection

    equipment used for chlorine dioxide, ozone, and UV light is more complex and requires a higher

    level of operator skill.

    New Formulas Used

    To calculate chlorine dose during breakpoint chlorination:

    Chlorine Dose = Chlorine Demand + Chlorine Residual

    To calculate CT value:

    CT = (Chlorine residual, mg/L) (Contact time, minutes)

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    References

    Alabama Department of Environmental Management. 1989. Water Works Operator Manual.

    Environmental Protection Agency. 1999. Alternative Disinfectants and Oxidants Guidance

    Manual.

    Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.

    Ragsdale and Associates. Version III. New Mexico Water Systems Operator Certification

    Study Guide. NMED Surface Water Quality Bureau: Santa Fe.

    Assignments

    Part 1 of your Assignment: Answer the following questions. Show all of your work and circle the

    answer for each math problem below. If there is insufficient information to find the answer, write

    "Insufficient information". When you are done, either email, mail or fax the assignment to your

    instructor. (Each question is worth 25 points)

    1. A chlorinator is set to feed 15 pounds of chlorine in 24 hours to a flow of 0.75 MGD. Find

    the chlorine dose in mg/L.

    2. Find the chlorine demand in mg/L for the water being treated in #1 with a chlorine dose of

    2.4 mg/L. The chlorine residual after 30 minutes of contact time is 0.8 mg/L.

    Part 2 of your Assignment: Work the following crossword puzzle that comes from definitions in

    your textbook. You may either print the puzzle out, complete it and mail or fax back to the

    instructor or you may send an email with the correct answers numbered accordingly. (Crossword

    worth 50 points)

    Quiz

    http://water.me.vccs.edu/courses/ENV110/crosswords/lesson7.pdfhttp://www.nmenv.state.nm.us/swqb/FOS/Training/WSOC_Study_Guide/Chapter_IV-Disinfection.pdfhttp://www.epa.gov/safewater/mdbp/pdf/alter/
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    Answer the questions in the Lesson 7 quiz . When you have gotten all the answers correct, print

    the page and either mail or fax it to the instructor. You may also take the quiz online and submit

    your grade directly into the database for grading purposes.

    http://water.me.vccs.edu/courses/ENV110/quiz7.htm