Recently, a small public water system in Florida wanted to begin using chloramine disinfection for its distribution system. The system had experienced a violation of the EPA disinfection byproducts rule: It had recorded a trihalomethane (THM) level of 197 ppb.

System leaders asked for information about chloramination so that they could change to chloramine secondary disinfection as seamlessly as possible. We forwarded the information and explained to the operators that chloramines were an excellent way to provide a disinfecting residual in distribution systems where disinfection byproduct (DBP) formation is a concern, or where it is difficult to maintain a free chlorine residual.

Matter of substitution

Chloramines are derivatives of ammonia created through the substitution of one, two or three chlorine atoms for hydrogen atoms. The desired disinfection agent in this process is monochloramine, an inorganic compound with the formula NH2Cl.

The term chloramine refers to a family of organic compounds with the formulas RNCl2 and R2NCl where R is an organic group. Other chloramines are dichloramine (NHCl2) and nitrogen trichloride (NCl3). Monochloramine is prepared through a chemical reaction between ammonia and hypochlorous acid under mildly alkaline conditions:

NH3 + HOCl → NH2Cl + H2O

In the reaction, hypochlorous acid (HOCl) undergoes attack by the nucleophile ammonia (NH3). At lower pHs, further chlorination of the ammonia molecule occurs, creating species other than monochloramine.

A growing solution

Water systems are replacing chlorine with chloramine as an agent for disinfection residual in distribution systems because it is much more stable and does not dissipate from the treated water before it reaches consumers.

Monochloramine also has a much lower tendency than free chlorine to convert organic materials into chlorocarbons, such as chloroform and carbon tetrachloride, which are trihalomethane compounds (THMs) identified as carcinogens. In 1979, the U.S. EPA began regulating THM levels in U.S. drinking water.

Research studies have shown that chloramination significantly reduces THM formation in drinking water supplies. Chloramination can reduce THM concentrations in finished water by 10 to 95 percent, although 40 to 80 percent is most common.

Besides increased residual activity in the distribution system and reduction of THMs and other byproducts associated with chlorine, monochloramine as a secondary disinfectant in some cases can control bacterial biofilm regrowth in distribution lines. In some instances it can also reduce taste and odor problems associated with chlorine.

The right recipe

The information we forwarded outlined a recommended maximum acceptable concentration of 3.0 mg/L for chloramines in drinking water. The maximum contaminant level (MCL) is 4.0 mg/L, based on a risk evaluation for monochloramine.

Production of monochloramine, dichloramine (NHCl2), and trichloramine (NCl3) depends on the pH, the ratio of chlorine to ammonia-nitrogen, and to a lesser extent on temperature and contact time. The ideal pH for the formation of monochloramine is 8.3. A lower pH favors formation of dichloramine (pH 4-6) and trichloramine (pH <4.4). A chlorine to ammonia-nitrogen ratio of 3:1 to 5:1 is optimum for monochloramine formation.

Chloramine is considered to have moderate biocidal activity against bacteria and low biocidal activity against viruses and protozoan cysts. In a model distribution system study, results suggested that biofilms can be controlled using monochloramine levels from 2.0 to 4.0 mg/L. The American Water Works Association recommends a goal of 2.0 mg/L combined chlorine residual for water leaving the treatment plant and a level of 1.0 mg/L combined chlorine throughout the distribution system.

Better taste

Chlorine residual compounds can also be responsible for taste and odor problems in drinking water. The taste and odor of monochloramine are less objectionable than those of hypochlorous acid, hypochlorite ion, dichloramine and trichloramine.

While monochloramine is unlikely to result in complaints about taste and odor at concentrations of 3.0 mg/L in drinking water, dichloramines may cause complaints at concentrations as low as 0.5 mg/L.

The fact is that odor is more closely related to the ratio of dichloramine to monochloramine than to absolute concentrations. Problems with taste and odor may result when the concentration of dichloramine exceeds 20 percent of the monochloramine concentration.

Issues with nitrification

One issue with using chloramine secondary disinfection is nitrification in the distribution system. Nitrification is a microbiological process in which ammonia is oxidized sequentially to nitrite and nitrate. The addition of ammonia in the production of chloramine provides the source of nitrogen, which under certain conditions can be used by bacteria to produce nitrites and nitrates.

Two groups of chemolithotrophic bacteria commonly found in water oxidize ammonia into nitrite and nitrate. When incomplete nitrification occurs, an accumulation of nitrite can result. Nitrite in a water supply is undesirable because of health concerns, such as methaemoglobinaemia, and because nitrite accelerates the decomposition of monochloramine and interferes with chlorine residual measurements.

A total chlorine residual of about 2.0 mg/L helps limit nitrification; in some instances a periodic burnout of the distribution system with free chlorine appears to be needed to kill off nitrifying populations.

Successful application

Experience demonstrates that chloramination significantly reduces THM formation in drinking water supplies. In fact, after implementing chloramination, the small water system in this example recorded THM levels of 10 ppb.

The use of monochloramine as a secondary disinfectant in drinking water also can yield advantages such as increased residual activity in the distribution system, control of bacterial biofilm regrowth, and reduction of taste and odor problems.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.