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7/28/2019 Lesson 7_ Disinfection (2)
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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. coli infection
Myobacterium
tuberculosis
tuberculosis
Salmonella salmonellosis,
paratyphoid
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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 bacteria are 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 ofGiardia and 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
Chlorination is 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 acid and 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 offree 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 breakpoint is 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 time is 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 value is 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.
Chlorinators are 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 feeder since 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 controller which 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 chlorinators are 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, orUV, 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 forGiardia,
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/7/28/2019 Lesson 7_ Disinfection (2)
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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