Formation of chlorination by-products in drinking water treatment plants using breakpoint chlorination

Abstract

In drinking water treatment plants generally chlorination is applied for disinfection using Cl2, ClO2, or NaOCl reagents. In Hungary due to the relatively high ammonium ion concentration of source water originating from deep aquifiers, not only the disinfection but simultaneously the ammonium ion removal is also a crucial task to prevent the nitrite formation. For this purposes the breakpoint chlorination is used which needs approximately 10 times more chlorine than the disinfection resulting in enhanced formation of organic and inorganic by-products. Chlorination by-product formation was investigated in twelve drinking water treatment plants applying breakpoint chlorination. Trihalomethanes were detected in the highest concentration (14.7 μg/L to 143 μg/L), followed by haloacetic acids and haloacetonitriles. In 50% of the investigated water treatment plants, the concentration of trihalomethanes in finished water exceeded the Hungarian parametric value, but it was the same range as in water supplies using chlorine for disinfection. The concentration of trihalomethanes, haloacetic acids, haloacetonitriles and adsorbable organic halides in finished water was found to correlate with residual free chlorine (r > 0.5) and raw water temperature (r > 0.48). At high bromide concentration (cBr- > 0.2 mg/L) of the raw water the proportion of brominated by-products (trihalomethanes and haloacetic acids) increased up to 50%. Chlorate concentration was particularly high at drinking water treatment plants using hypochlorite (0.66–2.0 mg/L).By-products are mostly generated at breakpoint chlorination, but their additional formation within the water distribution system is also significant, especially where the concentration of free chlorine is high and the residence time is long. Elimination (potentially biodegradation) of haloacetic acids and haloacetonitriles was observed at several sampling sites. The efficiency of the granulated activated carbon filters in removing adsorbable organic halides, trihalomethanes and haloacetic acids amounted generally under 25%.