Chloramines and Monochloramines

Chloramines and Monochloramines

Chloramines are very common drinking water contaminants, and most people are familiar with the foul taste & odor they create—at least from swimming pools, if not from drinking water. This Bulletin is to explain what chloramines are, where they come from, and what to do about them.

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: all proteins, fragments of proteins, and amino acids; all manner of DNA and RNA and their fragments; many metabolic waste products like urea and uric acid, and others.

Monochloramine is the simplest and most common member of the group, often produced intentionally from the reaction of pure chlorine and pure ammonia:

HOCl + NH3 ⇒ NH2Cl + H2O
aqueous ammonia mono- water chlorine chloramine

If the chlorine level is maintained, the other two hydrogen atoms (H) will also be replaced by
chlorine, in steps:
HOCl + NH3 ⇒ H2O + NH2Cl (monochloramine)
+ HOCl ⇒ H2O + NHCl2 (dichloramine—very smelly)
+ HOCl ⇒ H2O + NCl3 (trichloramine—only at low pH)
+ HOCl ⇒ H+
+ Cl¯ + N2 (nitrogen gas, which bubbles away)

Any extra hydrogen atoms in organic amines (like proteins and amino acids) are replaced by chlorine in the same way. The last reaction in the sequence is very important and is called the “Breakpoint Reaction” because it marks the point at which the last remaining oxidizable contaminants in the water are destroyed or “broken,” thus permitting disinfection to begin.

Reactions with simple chemicals are much faster than reactions with micro-organisms, and disinfection cannot begin until all of the “side-reactions” are finished and a “residual” of “free available chlorine” (FAC) remains. If the free chlorine level is not maintained or increased, as in many swimming pools where the chlorinator does not keep pace with the influx of ammonia and urea from swimmers, disinfection will suffer and the dark, heavy, acrid odor of chloramines will build up.

Chloramines in general are undesirable in drinking water because they smell and taste bad. Monochloramine in particular is tolerated because it is useful as a secondary disinfectant, and it is the least smelly of the group. 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 regrowth 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). After more than 20 years of research into minimizing the production of these DBPs, water-works operators now usually use free chlorine (or chlorine dioxide or ozone) only in the early steps of the treatment. Then, at the end, just as the finished, purified water is about to leave the plant and go out into the mains, pure ammonia may be added to convert the free chlorine residual into monochloramine. 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 to 1 ppm of free chlorine or 1 to 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 and 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 and beverage service.

Unfortunately, monochloramine 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 any filters that are just barely adequate at removing ordinary free chlorine will not be effective at removing monochloramine, and only products with significant capacity or kinetics in reserve will give satisfactory performance. Everpure filters, such as the CB20-312, CB20-302, C124R and C224R have enhanced chemical kinetics so they can reduce monochloramine adequately.

Combination filters or systems containing granular activated carbon in addition to precoat filtration reduce monochloramine even better. However, present demands of busy food service outlets can require a steady flow for several minutes at a time, and use is growing. Achieving better than 80% reduction of chloramines during continuous flow 100% of the time requires much larger filters and use of special grades of carbon, such as are found in our large carbon blocks and radial flow cartridges.

Both ammonia and monochloramine can be toxic to aquatic life such as fish and shellfish so we do not recommend reliance on activated carbon filters for treatment of water that is to be filling aquariums. Even chemicals like sodium thiosulfate, that destroy chlorine, which are sold by pet and aquarium stores, are not adequate when 100% removal of chloramines is a life-or-death matter. That is because chloramines have two reactive atoms that must be dealt with: the chlorine atom(s) and the nitrogen atom. The chlorine must be chemically “reduced,” but the nitrogen must be “oxidized,” and it is not possible to do both at once.

The preferred treatment is to add more chlorine (bleach) to oxidize the nitrogen part to N2 (the Breakpoint Reaction shown earlier), and only then to remove/destroy the free available chlorine (HOCl) that remains using activated carbon filters or chemicals like sodium thiosulfate. The Breakpoint Reaction takes a little time, so it is important to wait for five minutes after adding the extra chlorine bleach and then test for free chlorine (not “total chlorine”). If the FAC concentration is not greater than a trace level—less than 0.5 ppm—add more bleach, wait another five minutes, and test again.

How much chlorine bleach should it take, altogether? About 2 drops per gallon (or 4 liters) for each ppm (same as mg/L) of monochloramine in the water. If the water has 2.0 ppm of monochloramine, it would take 3 or 4 drops per gallon (4 L), depending on the freshness of the bleach and the size of the drops. Therefore, use no more than 4 drops per gallon (4 L) of tap water at first, and only another drop or two if a test for FAC indicates more is needed. Then you can destroy the residual free chlorine with confidence.

 

(This Technical Bulletin is courtesy of Pentair Everpure, Inc.)