Chapter 3: Chloramination

Chapter 2 covered disinfection byproducts. Many water agencies have switched from the direct use of chlorine to chloramines to better manage the formation of DBP’s while also maintaining adequate disinfection residuals against harmful microbes and ensuring compliance with Stage I and Stage II Disinfection Byproduct Rules. This chapter will describe the chloramination process in more detail.  The reader is encouraged to review the section on Chlorine and Chlorination breakpoint zones in Chapter 2 before you start.

Chloramination is the process of adding chloramine to drinking water to disinfect it and kill germs. Chloramine has been used by water systems for almost 90 years, and its use is closely regulated. Chloramines are a group of chemical compounds that contain chlorine and nitrogen. They are formed when chlorine and ammonia are added to water. Chloramines are also known as secondary disinfection.

Chloramines are a weaker disinfectant than chlorine, but they are more stable and provide longer-lasting disinfection. As we noted in Chapter 3, many utilities have adopted chloramination to avoid the formation of trihalomethanes. Chloramines are made up of three chemical species:

• Monochloramine (NH2Cl).
• Dichloramine (NHCl2).
• Trichloramine (NCl3).

Chloraminated water that meets the EPA standard is safe for drinking and other general household activities such as bathing, cooking, laundry, and cleaning. The water can also be used for gardening (the water is safe for plants) and for watering lawns with no adverse effects. As with chlorine, chloramine should be removed from the water used in kidney dialysis machines. However, chloraminated water that meets the EPA standard is safe for kidney dialysis patients to drink, since the digestive process neutralizes chloramine. Chloramine (and chlorine) is toxic to fish and amphibians at levels used for drinking water. Unlike chlorine, chloramine does not rapidly dissipate on standing or by boiling. Therefore, fish owners must neutralize or remove chloramine from water used in aquariums or ponds. Treatment products are readily available at aquarium supply stores.

Advantages of using chloramine

There are a number of operational and compliance benefits to using chloramine. Chloramine can provide the following benefits:

• Since chloramine is not as reactive as chlorine, it forms fewer disinfection byproducts. Some disinfection byproducts, such as the trihalomethanes (THMs) and haloacetic acids (HAAs), may have adverse health effects and are closely regulated.
• Because a chloramine residual is more stable and longer lasting than free chlorine, it provides better protection against bacterial regrowth in systems with large storage tanks and dead-end water mains.
• Chloramine, like chlorine, is effective in controlling biofilm, which is a coating in the pipe caused by bacteria. Controlling biofilm also tends to reduce coliform bacteria concentrations and biofilm induced corrosion of pipes.
• Because chloramine does not tend to react with organic compounds, many systems will experience fewer taste and odor complaints when using chloramine.
• Chloramine technology is relatively easy to install and operate. It is also among the less expensive disinfectant alternatives to chlorine.

Disadvantages to using chloramine

Drawbacks to the use of chloramine can include potential water quality problems (e.g., nitrification and corrosion) if the treatment process is not carefully controlled and the system’s operational practices are not appropriately adjusted for the new disinfectant. Chloramine can change the chemical properties of the water, which can impact corrosion of lead and copper. Nitrification in the distribution system can also occur when using chloramine. Nitrification can have a detrimental effect on water quality (such as loss of disinfectant residual). Nitrification results from the bacterial oxidation of ammonia (conversion of ammonia into nitrite and then nitrate) but can be controlled by optimizing the chloramination process or by applying occasional free chlorination practices. Each system considering a switch to chloramine should determine whether or not it has the resources to properly maintain and monitor its water quality so that these issues do not become problems.

Chloramination Breakpoint Curve

We will start our discussion considering the simultaneous addition of ammonia and free chlorine to treatment plant influent in order to minimize THM formation. Assume chlorine demand has already been met. Refer to Zone I in Figure 4.1.  In this zone, it is primarily monochloramine  that is forming according to the reaction.[latex]〖NH〗_3+HOCl→〖NH〗_2 Cl+H_2 O[/latex]

Monochloramine is the preferred chloramine for treatment because it is stable and presents the least taste and odor problems.  The unit weight of Chlorine is 70 and the unit weight of Nitrogen (in the ammonia) is 14. When chloramine treatment is employed, the ratio of chlorine to nitrogen in the free ammonia (NH3) will always be 5:1 because 5 mg/L of chlorine will always combine with 1.0 mg/L of ammonia (70 ÷ 14 = 5). The maximum total chlorine residual occurs at this ratio.  Note there is no free ammonia left (you still have ammonium ion however). You will always target a 5:1 ratio to avoid excess free chlorine. Treatment operators typically work in the range of 3:1 to 5:1.

In Zone I we are creating primarily monochloramine residual. At the chlorine to nitrogen ratio of 5:1, there is no free chlorine residual. It is all combined chlorine residual, primarily monochloramine. In addition, there is no free ammonia left at this ratio. We are maximizing chloramine residual at this point.

Figure 3.1 Chloramination Breakpoint Curve.

Now refer to Zone II. We see that for ratios between 5:1 and 7:1 we start losing chlorine or monochloramine residual. This is what is happening. As the chlorine to nitrogen ratio increases, the pH starts to decrease. The monochloramine starts to react with free chlorine to form dichloramine (NHCl2), which in turn reacts with free chlorine to form trichloramine (NCl3). Both of these forms of chloramines are unstable disinfectants and cause taste and odor problems. Trichloramine is a much weaker disinfectant.[latex]NH_2 Cl+HOCL →NH〖Cl〗_2+H_2 0[/latex][latex]NH〖Cl〗_2+HOCL→N〖Cl〗_3+H_2 O[/latex]And there’s more. The chloramines end up being converted to nitrogen (N₂) and nitrate ([latex]NO_3^-[/latex]).[latex]3NH〖Cl〗_2→N〖Cl〗_3+N_2+3HCl[/latex][latex]NH〖Cl〗_2+2HOCl+H_2 O→NO_3^-+H^++4HCl[/latex][latex]2NH_2 Cl+HOCL→N_2+3HCl+H_2 O[/latex]

Treatment operators providing chloramine residual, want to stay out of this zone.

At the beginning of Zone III, the total chlorine residual is at a minimum at a ratio of 7.6:1. This is referred to as the “breakpoint”. The combined chlorine residual at the breakpoint is zero. 1 mg/l of additional free chlorine added will become 1 mg/l of free chlorine residual Operating at ratios greater than the breakpoint is called “breakpoint chlorination”. Total ammonia in this zone is zero.

Chloramination operations: For chloramine disinfection:

• The ideal operating point is at the (5:1 Ratio of Chlorine to Ammonia-N).
• At lower ratios, unreacted ammonia exists, and will give rise to bacterial growth in the distribution system called “Nitrification.”
• At higher ratios, more dichloramine and trichloramine exist: more odors and even less disinfection.

Other Approaches to Produce Chloramines

The previous example assumed we wanted to minimize THM formation. Simultaneous (concurrent) addition of free ammonia and chlorine is the preferred approach for this. But it’s not the only approach. A treatment plant may employ enhanced coagulation to remove Disinfection Byproduct precursors, allowing plant operators to perhaps apply ammonia and chlorine separately, depending on factors such as taste and odor control. Sometimes it may be necessary to apply free chlorination at the beginning of the treatment process and ammonia at the end of the process to meet CT requirements as shown in Figure 4.2.

Figure 3.2: Chloramination Disinfection Example

Nitrification

Nitrification in a water distribution system is the transformation of free ammonia into nitrate. It is basically the nitrogen cycle which occurs in nature where nitrifying bacteria (Nitrosomonas) use the ammonia as a food source to grow and create nitrites ([latex]NO_2^-[/latex]). Other nitrifying bacteria (Nitrobacters) consume the nitrites and produce Nitrates ([latex]NO_3^-[/latex]) in the process.

During chloramination operations, if the chlorine to nitrogen ratio falls below 5:1, unreacted free ammonia will be available as a bacterial food source. The production of nitrite will cause a loss of disinfectant residual by nitrite dechlorination, and further compound bacterial growth which could lead to regulatory violations.

Nitrification Reactions:

[latex]NH_3+O_2→NO_2^-+3H^++2e^-[/latex]

[latex]NO_2^-+H_2 O→NO_3^-+2H^++2e^-[/latex]

Dechlorination Reaction with Nitrite: [latex]NO_2^-+HOCl→NO_3^-+HCl[/latex]

Conditions in a distribution system which are vulnerable to nitrification would include the presence of free ammonia in a dark environment with long detention times, warmer temperatures on the order to 25^o C to 30^o  C and a pH between 7.5 and 8.5.

Review Questions

1. Describe chloramination.

2. Outline the uses of chloramination.

3. Explain the formation of chloramines.

Test Questions

1. What process requires chloramines be removed from water?
2. boilers.
3. metal plating operations.
4. dialysis.
5. oil refining.

2. What process is affected by chloramine’s ability to react with and change some of the natural plant flavors?
3. dialysis.
4. brewing beer.
5. paper manufacturing.
6. commercial baking.

3. Which disinfection by-product has been discussed in reference to chloramines?
4. chlorite.
5. THM.
6. bromate.
7. NDMA.

4. Which has a more effective disinfecting power?
5. monochloramine.
6. trichloramine.
7. dichloramine.
8. N-nitroso dimethylamine.

5. Which is ammonia-oxidizing bacteria?
6. Nitrobacter.
7. E. coli.
8. Cryptosporidium.
9. Nitrosoma.

6. Which is nitrite-oxidizing bacteria?
7. Nitrobacter.
8. E. coli.
9. Cryptosporidium.
10. Nitrosoma.

7. What is caused by incomplete or partial nitrification?
8. nitrate.
9. nitrite.
10. ammonia.
11. NDMA.
12. Which health problem is caused by very high levels of NDMA?
13. malformation of the brain.
14. gastrointestinal disruptions.
15. liver disease.
16. kidney disorders.

9. Which describes the total concentration of chlorine in water, including the combined chlorine and the free available chlorine?
10. monochloramine.
11. chlorine residual.
12. breakpoint chlorination.
13. total chlorine.

References

• Centers for Disease Control and Prevention (CDC): Disinfection with Chlorine and Chloramine
• United States Environmental Protection Agency (EPA): Information About Chloramine in Drinking Water
• Image by Wikimedia is licensed under CC BY-SA 3.0 
• Image by the EPA is in the public domain