Impact of Giardia inactivation requirements on ClO2 by-products

Chlorite, chlorate, and trihalomethane (THM) formation was measured followi ng the disinfection of drinking water using chlorine dioxide either alone, or followed by chlorine or monochloramine. Chlorine dioxide doses were sele cted to provide a range of Giardia inactivation levels as calculated using CT values (disinfectant concentration x contact time) presented in the US S urface Water Treatment Rule. Experiments were conducted at bench-scale in b atch reactors, using water obtained from two river sources. Dose requiremen ts did not vary greatly when disinfecting at 1 degrees C versus 25 degrees C, since the higher CT requirements in the cold (due to slower disinfection kinetics) were offset by greater chlorine dioxide stability at low tempera ture. Higher doses were required at pH 9 compared to pH 6, due to chlorine dioxide disproportionation at the higher pH. Greater formation of chlorite and chlorate was observed at the higher pH. Temperature exerted a relativel y small effect on chlorite and chlorate formation, following the trend obse rved for required dose. When free chlorine was applied as a secondary disin fectant following primary disinfection with chlorine dioxide, subsequent ch lorite concentrations decreased at pH 6, but not at pH 9, suggesting that h ypochlorous add, and not hypochlorite, was responsible for the oxidation of chlorite. THM formation at pH 9 arising from secondary chlorination was re duced by up to 30 mu g l(-1) due to prior application of chlorine dioxide a t concentrations corresponding to as low as 0.5 log Giardia inactivation.