Chloramines are increasingly recognised as a viable alternative to traditional chlorine compounds to provide a sterilising effect in potable water. However, the inadvertent formation of chloramines can cause significant concern when appropriate measures are not in place to manage their deleterious effects.
In order to reduce the formation of disinfection by-products (DBP) from the reaction of chlorine with residual organics in potable water, the application of alternative disinfectants has become increasingly widespread. Although approximately 200 times less effective than chlorine as a sterilant, monochloramine has emerged as one of the leading alternative disinfectants for municipal water supplies. Monochloramine offers two advantages. Firstly, it is less reactive avoiding the creation of DBP’s. Secondly, it is a more persistent disinfectant remaining in the public water supply throughout the distribution system up to the faucet.
However, its persistence in the supply and its tendency to form breakdown products below a pH of 7.5 causes taste and odour issues for consumers.
FORMATION OF CHLORAMINES
The use of traditional sterlising agents, particularly sodium hypochlorite can cause inorganic chloramines to form in water, where ammonia is present. This reaction is represented by the following equation (Morris, 1967):
NH3 + HOCl = NH2Cl + H2O
As ammonia has more than one hydrogen that can be replaced by a chlorine atom, it reacts with an excess of hypochlorous acid to form dichloramine (Morris, 1967; Gray et al., 1979):
NH2Cl + HOCl = NHCl2 + H2O
Dichloramine is relatively instable, and frequently will decompose to monocloramine, or additional chlorine added to the stream will force the creation of trichloramine, a more stable form of chloramine. This reaction proceeds as follows (Morris and Isaac, 1983):
NHCl2 + HOCl = NCl3 + H2O
The presence of the various chloramine species is driven by the pH of the water; at low pH levels (pH<4) trichloramine is the predominant species. For a less acidic water (pH 4-7), dichloramine will be present. At a neutral or higher pH (>7), monochloramine will serve as the dominant species.
Chloramines are of concern to a variety of water users. Where water is used for the production of beverages, or medical use, such as dialysis, the presence of chloramines is considered unpleasant to taste and harmful in respective cases. In drinking water applications, residual chloramines post their own byproduct formation concerns (e.g. nitrosoamines). Thus, removal of the residual chloramine is often desired. With the majority of applications being at neutral pH, most applications desire to optimize the removal of monochloramine.
As opposed to traditional organic removal on activated carbon via physical adsorption, the mechanism for removal of chloramine is catalytic reaction with the carbon surface. The stoichiometry of the dechloramination reactions on carbon are well known and are shown in the formulae below.
C* + NH2Cl + H20 → NH3 + H+ + Cl- + C*O
C*O + 2NH2Cl → C* + 2H+ + 2Cl- + H20 + N2
While traditional activated carbons can facilitate these reactions, over the past 20 years the development of activated carbons with enhanced catalytic activity helped to drive the use of activated carbon for chloramine reduction. Furthermore, catalytically enhanced carbons based on coconut shell, such as Jacobi’s Aquasorb CX-MCA, have proven to show superior performance over all carbon types, including catalytically enhanced coal based carbons, for this application. One study, utilizing a 25cm³ bed of GAC in a 2.5-cm diameter column operating at a flow rate of 3.3 bed volumes per minute and with a challenge water prepared with reference to ANSI-NSF 42-2002 chloramine testing protocol at pH 9, and inlet concentration of ~3mg/L chloramine, demonstrated a 65% improvement in chloramine reduction after 10,000 bed volumes treated as compared to a standard coal-based carbon, and a 24% improvement over a catalytic coal-based carbon after the same quantity of water treated.
One of the additional benefits of a catalytic coconut based activated carbon is its stability to chemicals. When subject to sustained chemical attack by aqueous phase oxidizing chemical, such as chloramines, the surface of activated carbon degrades releasing fines into treated water. Other studies have demonstrated that AquaSorb® CX-MCA produces significantly less backwash fines than the coal based carbon as a direct result of mechanical stability of a coconut shell.
To see the details of these studies and to learn more about chloramine reduction please utilize our contact us form to request additional information.
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