Chloramines FAQ/info

[BWI]

Chloramines


Chloramines are chemical complexes formed from the reaction between ammonia and chlorine being used to disinfect many municipal supplies. U like chlorine, chloramines do not combine with organics in the water to form potentially dangerous trihalomethanes (THMs). Chloramines can exist in three forms:

monochloramine (NH2Cl)
dichloramine (NHCl2)
nitrogen trichloride (NCl3)
The proportions of the chloramines depend on the physical and chemical properties of the water. Water containing chloramines may not be used for fish or for kidney dialysis applications.

A disagreeable taste/odour in drinking water is associated with the presence of chloramines, usually as monochloramine (NH2Cl), also called “combined chlorine.” A chloramines residual is most often the aftermath of the two-step ammonia plus chlorine disinfection method used at large, central municipal-type treatment plants. This particular method of disinfection produces much lower levels of trihalomethanes (THMs) as compared to chlorine alone. The ammonia/chlorine disinfection process proceeds at a slower chemical reaction rate versus free chlorine, and must be carried out at a relatively high pH (over 8.5 value) environment.

Chloramines (combined chlorine) are produced by the reaction of aqueous chlorine and ammonia. Order of oddition may be chlorine first, ammonia first or the two simultaneous. The tree species of chloramines can be produced. The percentage of each is dependent upon the ratio of chlorine to ammonia, chlorine dose, temperature, alkalinity and pH. Equations of 3.74 to 3.76 summarize the formation of these three compounds.

Under typical water treatment conditions, the application of up to 5:1 chlorine to ammonia-nitrogen (by weight) will result primarily in the formation of monochloramine at greater than 7. At lower pH values or higher chlorine-to-ammonia-nitrogen ratios, dichloramine may also form. Beyond the breakpoint, trichloramine may be formed. Dichloamine and especially trichloramine are odorous compounds, and therefore a chlorine-to-ammonia-nitrogen weight ratio of approximately 4:1 should be maintained to prevent the formation of dichloramine and trichloramine.

Chloramines have less disinfection power than ozone, hypochlorous acid, hypochlorite ion and chlorine dioxide for identical contact times, but they can be used when it is necessary to maintain a residual for long periods of time (e.g. distribution systems). Despite the fact that monochloramine has less disinfecting power than other disinfectants, it is more effective than chlorine for penetrating and inactivating biofilms growing on the surfaces of iron pipe.

Chloramines are used at some utilities to assist in the control of certain taste and odour problems caused by chlorination and to keep THM formation to a minimum. Because chloramines are slow-acting disinfectants and poor performance on viruses and protozoan cysts and oocysts, they are seldom used as primary disinfectants. In addition, some utilities using chloramines have experienced taste and odour problems and excessive algal growth in treatment basins.



Chloramine removal



Chloromines are pH-sensitive and can shift to dichloramines or trichloramines, depending upon the pH and the amount of ammonia in the water. Two very specialized applications, hemodialysis and aquariums, require essentially chloramines-free water. Chloramines have been shown to be extremely toxic to aquatic life. Concentrations of only 0.4 ppm (mg/L) can kill many fish. As little as 0.05 ppm (mg/L) is reported as lethal to trout. Chloramines in kidney dialysis water should be reduced to less than 0.1 ppm (mg/L) – the level recommended by the Association for Advancement of instrumentation and the Society for Artificial Internal Organs.

In large scale industrial treatment, eliminating chloramines is achieved by dropping the pH value below 6.0. Another method to remove chloramines is feeding a solution of sodium sulfide. At many beverage plants, superchlorination at a magnitude of 25 ppm (mg/L) is used to oxidize the ammonia. In ion exchange deionization, chloramines can be troublesome and tricky to deal with, causing low-quality ultrapure product water. The NH2Cl must be removed upstream ahead of the two-bed or mixed bed deionizer system.

Chloramine removal from domestic water supplies is limited to physical adsorption methods, since the chemical treatments mentioned above are not practical in the home or business setting. Regular granular activated carbon, with low ash content, operated at very low service flow rates, will remove some chloramines, but is quite limited in capacity. The use of preconditioned activated carbon material generally is the more successful treatment approach. There are special proprietary “impregnated carbon” products that have an enhanced capacity to pick up chloramines.

A second reported means to improve chloramines removal is to accentuate the carbon by regenerating (soaking) it with a strong chlorine solution (one pint of 5.25 per cent household bleach per cubic foot), followed by a long rinse down before placing the unit on stream. Another common practice is the “move-up” technique, where two cartridge (or one tank-type) units are installed in the series mode. As the first unit exhausts, the second one is moved up to the first position, and a new fresh carbon unit is placed in the second position. One other reported treatment scheme is the use of “partially used carbon” material that has already been subjected to dechlorination. This is reported to be more successful than the use of “virgin carbon.” All of these techniques suggest enhanced ability of carbon products to remove ammonia.



Info Provided by CWQA