Chloramines are very common drinking water contaminants, and everybody is familiar with the foul taste & odor they create—at least from swimming pools, if not from drinking water. Usually their presence is not intended, but about 25% of the largest public water supplies in the U.S. add the simplest one, mono-chloramine, to the water on purpose. They use chloramine instead of chlorine because it is more stable in water, allowing for disinfection over longer distances.
Chloramines are a family of disinfection by-products (DBPs), formed from the reaction of disinfectant chlorine with the nitrogen atom (N) in ammonia (NH3) or organic compounds containing a reactive nitrogen atom. There are many such biological chemicals in drinking water, mostly derived from the cellular debris from killed bacteria and algae. Mono-chloramine is the simplest and most common member of the group, often produced intentionally from the reaction of pure chlorine and pure ammonia.
Chloramines in general are undesirable in drinking water because they are toxic and because they smell and taste bad. However, mono-chloramine is tolerated because it is useful as a secondary disinfectant, and it is the least toxic and smelly of the group. Still, concentrations above 4.0 mg/L are prohibited. The usefulness of monochloramine comes from its comparative weakness as an oxidizing agent: it retains about 5% of free chlorine’s chemical power, which is not strong enough to use as a primary disinfectant, but it is still able to inhibit the re-growth of any survivors of disinfection. It is also too weak to corrode copper and brass plumbing materials, and therefore it lasts much longer in the mains—two or three days instead of just a few hours for free chlorine. Finally, monochloramine is chemically too weak to produce the other common disinfection byproducts—trihalomethanes (THMs), haloacetic acids (HAAs) and haloketones (HKs),, which may pose a health hazard.
Typically water utility companies use free chlorine (or chlorine dioxide or ozone) only in the early steps of water treatment. Then, at the end, just as the finished, treated water is about to leave the plant and go out into the water mains, pure ammonia is added to convert the free chlorine residual into chloramine. Without that final adjustment, the free chlorine would continue to produce unwanted THMs, etc. for several more hours and then be completely gone, leaving the system with no continuing protection.
Standard water treatment practice is to use ½ - 1 ppm of free chlorine or 1 – 2 ppm of monochloramine. Some systems attempt to counteract monochloramine’s weakness by using more of it, but all that does is increase the frequency of taste & odor complaints. There is not much difference between the smell of the two at low concentrations, but above 1 ppm the stink of monochloramine is very objectionable—much worse than free chlorine—and removing it is even more important than removing ordinary free chlorine, especially if the water is to be used for commercial food/beverage service.
Unfortunately, chloramine is more difficult to remove than plain free chlorine: it reacts only weakly and slowly with activated carbon, just like it does with everything else. That means that the water must stay in contact with the carbon much longer than if free chlorine alone was present. Many filters do not have enough carbon for the long contact time required to achieve removal. Only products with significant capacity will give satisfactory performance.