By Rob Herman
Chloramine isn’t a naturally occurring constituent in drinking water. However, formed by the reaction between chlorine and ammonia, it’s used as a disinfectant in many municipal drinking water supplies. Some municipal supplies contain chloramine simply because they use chlorine to disinfect, and ammonia at low concentrations is naturally present in the source water. Many municipal water supplies use chloramine to limit formation of disinfection by-products (DBPs). This is true for both ground and surface water supplies. Municipal water supplies usually apply this disinfectant by adding the chlorine to a flowing stream first for an initial kill of any pathogens and then add ammonia. The ammonia stabilizes the chlorine and reduces formation of chlorine DBPs. The chloramine is then present in the distribution system and acts as a residual disinfectant.
More than one type
There are three types of chloramine that can be formed by the reaction of chlorine with ammonia: monochloramine, dichloramine and trichloramine. Dichloramine and trichloramine are less stable than monochloramine and are present only at low pH values with high chlorine-to-nitrogen ratios. At pH levels above 8, monochloramine is the only chloramine of any consequence. The U.S. Environmental Protection Agency has also limited its discussion of chloramines to monochloramine so the term chloramine herewith will refer to monochloramine only.
Since formation of chloramine is one source that limits formation of DBPs, its use has significantly increased in the last several years. Water supplies with high natural organic matter (NOM) have adopted use of chloramine in significant numbers.
Like chlorine, chloramine can be detected by some consumers as having a bitter off-taste. This has prompted the NSF Drinking Water Joint Committee to include chloramine in ANSI/NSF Standard 42—Aesthetic Effects, which required developing a new protocol for evaluation of its removal with point-of-use/point-of-entry (POU/POE) drinking water treatment units. There has been some confusion in the marketplace that a system with NSF certification under ANSI/NSF Standard 42 for chlorine reduction will also remove chloramine. This definitely isn’t the case since chlorine is much easier to reduce in drinking water than chloramine.
Basic removal mechanism
Chloramine may be removed by the action of activated carbon. The carbon causes the disassociation of chloramine. The primary mechanism of removal isn’t by the typical adsorption as seen with most organic contaminants, but with a “catalytic” conversion. The theoretical reaction is the following two-step reaction:
Step 1: NH2Cl + H2O + C* → NH3 + H+ + Cl- + CO*
Step 2: 2NH2Cl + CO* → N2 + 2H+ + 2Cl- + H2O + C*
C* and CO* represent surface carbons and surface carbon oxides, respectively, of the activated carbon media.
In actual use, steady-state reduction of chloramine doesn’t actually occur but the reduction efficiency decreases over time. This may be caused by incomplete conversion of the CO* back to C* surface carbons in the second reaction and because of competition by other reactions with the CO* and C* surface carbons. The catalytic reaction isn’t a rapid process and is highly dependent on the type of carbon used.
Factors affecting removal
Water characteristics that affect chloramine reduction include:
The stability of the chloramine affects the rate of the catalytic reaction. The higher the pH of the water, the more stable chloramine is—until a pH of 10 is reached. Most water supplies, however, don’t reach this high of a pH. Typical pH values for chloraminated water supplies are around 8 or 9.
The reaction is thermodynamic in nature and is affected by changes in temperature. Very low temperatures can reduce effectiveness of the system.
The flow rate through the treatment system determines contact time between the chloramine and carbon. Since the catalytic reaction isn’t rapid, any changes in flow rate would have a significant effect on the ability of a system to remove chloramine. The flow rate in use shouldn’t exceed the NSF certified rate.
It’s apparent through testing experience at NSF that the amount of time the system rests greatly affects removal of chloramine. This is because the carbon is able to complete some of the reactions during the rest period and recover. Actual use rates and patterns differ significantly depending on application of the systems. Residential (POU) systems would normally see long periods of rest, while commercial systems may have rest periods measured in minutes. The proposed ANSI/NSF Standard 42 requires all systems with greater than a 1 gallon per minute (gpm) flow rate to be tested with minimal rest periods. Systems with less than 1 gpm are allowed longer rest periods seen in POU applications.
Since the catalytic reaction is a relatively slow process, changes in influent concentration can greatly affect performance. Chloramine levels that greatly exceed 3 milligrams per liter (mg/L)—influent level in test protocol—are uncommon but will directly affect performance.
Hardness only has a significant effect if precipitation of carbonates occur. This precipitation blinds the active sites on the carbon and reduces the ability of the carbon to catalyze the chloramine.
Natural organic matter
Waters with high levels of NOM can cause blinding of active sites on the carbon and hinder performance. The test protocol requires greater than 1 mg/L of total organic carbon (TOC) to be present throughout the test.
Addition of chloramine
The protocol that’s being adopted into ANSI/NSF Standard 42 will evaluate POU and POE systems for chloramine reduction. The protocol is on track to become an official part of ANSI/NSF Standard 42 in November 2000 and certified products should be available a short time after that. An unusual part of the protocol is the ability for a manufacturer to test for chloramine reduction and then also claim chlorine reduction. This is possible because the mechanism for chlorine reduction is similar to the chloramine mechanism, although the reaction is much faster and fewer steps are involved. This fact was demonstrated in validation testing of carbon systems at NSF for both chlorine reduction and chloramine reduction.
Validation data in combination with understanding of the mechanism of chlorine and chloramine—monochloramine in this discussion—reduction allow the conclusion that if an activated carbon system reduces chloramine, the systems’ performance for chlorine will be greater. The new protocol in ANSI/NSF Standard 42 will allow a system with test data that meets requirements of the standard for chloramine to also claim chlorine reduction to the same capacity and reduction efficiency. However, the standard won’t allow a system to claim chloramine reduction based on chlorine reduction data.
Adoption of chloramine into ANSI/NSF Standard 42—Aesthetic Effects will allow manufacturers to validate and certify their claims for chloramine reduction providing assurance to consumers that the product will perform as claimed.
About the author
Rob Herman is technical manager of NSF International’s Drinking Water Treatment Unit Program and has been with NSF since 1985. He can be reached at (800) 673-7275, (734) 913-5787 (fax) or email: http://[email protected]