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Chloramine
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| Figure 1: Typical chloramines formation in a conventional treatment process. |
Chloramines are a family of oxidants formed by the reaction of chlorine and ammonia. In water treatment, chloramines are primarily used as a secondary disinfectant to provide a residual in the distribution system; however, chloramines are occasionally used as a primary disinfectant.
At full-scale water treatment applications, ammonia is most frequently added to the water after chlorine (after achieving primary disinfection) and prior entering the distribution system. Where alternative primary disinfectants, such as ozone or chlorine dioxidež or in some groundwater applications where primary disinfection is not required, ammonia may be added before chlorine or chlorine and ammonia may be added simultaneously. Simultaneous application is sometimes referred to as preformed chloramination. In some applications, natural ammonia is present in the source water.
Downstream residual chloramine concentrations make chloramination concurrent with other treatment processes. Chloramine residuals are common during filtration to inhibit microbial (biofilm) growth on filter media that could increase filter head loss (pressure) build up. Figure 1 shows typical chlorine and ammonia application points for chloramine formation in a conventional treatment process.
The ratios at which chlorine and ammonia are fed control the species of chloramines present. At chlorine-to-ammonia-nitrogen (Cl2:NH3-N) mass ratios less than 5:1, monochloramine (NH2Cl) is the predominant species. Monochloramine is also the preferred species, as it is a more powerful oxidant and is less likely to cause taste and odor problems in the distribution system than the other species. At ratios between 5:1 and 7.6:1, dichloramine (NHCl2) and trichloramine (NHCl3) are the dominant species. At mass ratios greater than 7.6:1, the breakpoint reaction occurs in which all of the available ammonia is oxidized to nitrogen gas and free chlorine is the dominant chlorine species present. The pH of chloraminated water is oftentimes at 8 and above as monochloramine stability is enhanced in that range.
Although weaker than chlorine and chlorine dioxide, monochloramine oxidizes precursors of disinfection byproducts (DBPs), inactivates microorganisms, and controls biofilm. Chloramination is often an attractive alternative to chlorine for secondary disinfection because it minimizes the formation of DBPs. The effectiveness of chloramination is dependent on chloramine concentration (C), contact time (T), pH, and temperature. The product of concentration and time (CT) is the most important operational parameter in disinfection and inactivation. Total organic carbon (TOC) and ultraviolet absorbance (UV) are two measures of DBP-reactive materials and of chloramine demand.
Free ammonia can be present in chloraminated distribution systems as a result of 1) poor control of the Cl2:NH3-N ratio at the treatment plant and 2) chloramine degradation in the distribution system. Nitrification occurs when this free ammonia is oxidized by ammonia oxidizing bacteria to nitrite (partial nitrification), and nitrite is subsequently oxidized by nitrite-oxidizing bacteria to nitrate (complete nitrification).
Chloramination has been demonstrated to form currently unregulated DBPs like cyanogen chloride and N-nitrosodimethylamine, and impact the stability of existing pipe scale in previously chlorinated distribution systems.