Effectiveness of chlorination of potable water in the presence of ammonia

Abstract

All surface water treatment plants in Bangladesh use chlorine for disinfection. In the presence of high concentration of ammonia, “break point chlorination” becomes impractical because of very high chlorine dose requirements; therefore, “chloramination” is most often carried out. In the presence of high and variable concentration of ammonia, it becomes difficult to adjust chlorine dose to achieve desired disinfection at treatment plants. The primary objective of this research was to assess the effectiveness of chlorination of potable water in the presence of ammonia. In order to understand the disinfection capability of chlorine in the presence of ammonia, batch disinfection experiments were carried out at different chlorine to ammonia ratios (varying from 0.60:1 to 1.4:1.0) using raw water containing different levels of fecal coliform and ammonia concentrations. It has been found that “chloramination” at low chlorine to ammonia molar ratios (less than 0.6:1.0) may be effective (i.e., zero residual FC) for disinfection for water containing lower level of contamination (FC up to 3,000 cfu/100 mL), even when initial ammonia concentration is relatively low (0.5 mg/L). However, “chloramination” may not be effective for water with higher level of contamination. During “chloramination”, mono-chloramine (NH2Cl) is the most dominant species followed by di-chloramine (NHCl2). Mono-chloramine concentration reaches its peak at chlorine to ammonia molar ratio of 1.2:1 to 1.5. Before breakpoint is reached, a noticeable amount of free chlorine exists in water, and its concentration increases with increasing chlorine dose. Presence of free chlorine along with chloramines in water samples from Saidabad Water Treatment Plant (SWTP) confirms simultaneous presence of free chlorine and chloramines in water during “chloramination”. Mono-chloramine appears to be an effective disinfectant. But during “chloramination”, some free chlorine also exists in water, and both mono-chloramine and this free chlorine act as disinfectant.

At relatively lower ammonia concentration (below 0.5 mg/L), it is better to go for “break point” chlorination, rather than chloramination. For a fixed initial ammonia concentration, peak mono-chloramine concentration is reached at chlorine to ammonia ratio of about 1.20:1.0 to 1.50:1, which should be the optimum chlorine to ammonia ration for effective disinfection. But for higher initial ammonia concentration (≥ 5 mg/L), chloramination at such chlorine to ammonia ratio would produce very high concentrations of chloramines, exceeding the WHO/USEPA guideline values (3.0 mg/L for mono-chloramines, and 4.0 mg/L for total chloramines). For such high initial ammonia concentration, a lower “chlorine to ammonia” ratio should be considered for chloramination. For fixing chlorine dose for disinfection of water containing high concentration of ammonia, the maximum chlorine to ammonia ratio that could be applied without exceeding WHO/USEPA guideline values needs to be determined.