Assessment of disinfection of water containing high concentration of ammonia by chlorination

摘要

Surface waters in Bangladesh and many developing countries are characterized by high concentrations of organic matter and ammonia, particularly during the dry season. In the presence of high concentration of ammonia, break point chlorination often becomes impractical at treatment plant because of very high chlorine dose requirements; therefore, "chloramination" (i.e., chlorination at chlorine to ammonia ratio below "break point") is most often carried out. It has been found that "chloramination" at low chlorine to ammonia molar ratios (e.g., less than 0.6:1.0) could be effective for disinfection for water containing lower level of fecal contamination, even when initial ammonia concentration is relatively low. However, "chloramination" may not be effective for water with higher level of fecal contamination. Even before breakpoint is reached, a noticeable amount of free chlorine has been found to exist in water; and both mono-chloramine and this free chlorine act as disinfectant. For a known initial ammonia concentration, the optimum disinfection should be achieved at chlorine to ammonia weight ratio of around 5.0:1.0 to 6.25:1.0. But if ammonia concentration in water is high, chloramination at such chlorine to ammonia ratio would produce very high concentrations of chloramines, exceeding the WHO/USEPA guideline values. For water with such high initial ammonia concentration, a lower "chlorine to ammonia" ratio should be considered for chloramination.