Chlorination of water
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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.
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, manganese 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 has 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 chlorine reacts with the water itself. The reaction depends on what type of chlorine is added to the water as well as on 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 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 may break 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 time while 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.
· 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.
The pH of water has a definite effect on the efficiency of chlorine as well as on the corrosive properties of water (covered later in this chapter.) For now, we will consider only the effect of pH on sanitation.
It can be seen in Table 2.a that free chlorine is most efficient in pH ranges below the ideal range of 7.2-7.6. Some pool operators do, however, maintain pH levels higher than the ideal range. They should also maintain appropriately higher FAC levels to provide the same concentration of the active HOCL form.
For example, at a pH of 8.0, 21% (about 1/5 of the FAC is in the active form. At that pH level, it would take 2.5 ppm of FAC to provide about 0.5 ppm of HOCI. At a pH of 7.5, about 1/2 (50%) of the FAC is in the active HOCI form. At that pH level, it would take only 1.0 ppm of FAC to provide the same 0.5 ppm of HOCI. For this reason, many authorities recommend that the pH of pools be maintained in the range between 7.2 and 7.6 and as close to 7.5 as practical. These conditions are also considered to be most comfortable for the swimmers' eyes and skin.
HOCI
|
H*
|
OCI-
|
Hypochlorous Acid
Killing Agent
Active, but unstable form
|
Hydrogen Ion
|
Hypochlorite Ion
Inactive, but stable form
|
% Chlorine as HOCI
|
pH
|
% Chlorine as OCI-
|
90
|
6.5
|
10
|
73
|
7.0
|
27
|
66
|
7.2
|
34
|
45
|
7.6
|
55
|
21
|
8.0
|
79
|
10
|
8.5
|
90
|
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:
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
Monochloramine + Water
NH3 + HOCl NH2Cl + H2O
NH2Cl + H2O
Monochloramine may then react with more hypochlorous acid to form a dichloramine:
Monochloramine + Hypochlorous Acid Dichloramine + Water
Dichloramine + Water
NH2Cl + HOCl NHCl2 + H2O
NHCl2 + H2O
Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:
Dichloramine + Hypochlorous Acid Trichloramine + Water
Trichloramine + Water
NHCl2 + HOCl NCl3 + H2O
NCl3 + H2OThe 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 chlorine residual.
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 will be 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 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 of chlorination.
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
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 produces 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 of chlorine 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 dose 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 system.
It is also important to understand the breakpoint curve when changing chlorine dosages. 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.
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 time, the initial mixing of chlorine into the water, and the location of chlorination within the treatment process. The most efficient process will have a high chlorine residual, a long contact time, 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 since 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 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.
Finally, and most intuitively, the number and type of microorganisms in the water will influence 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.
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