Impact of Giardia Inactivation Requirements on ClO2 By-Products

Chlorite, chlorate, and trihalomethane (THM) formation was measured following the disinfection of drinking water using chlorine dioxide either alone, or followed by chlorine or monochloramine. Chlorine dioxide doses were selected to provide a range of Giardia inactivation levels as calculated using CT values (disinfectant concentration × contact time) presented in the US Surface Water Treatment Rule. Experiments were conducted at bench-scale in batch reactors, using water obtained from two river sources. Dose requirements did not vary greatly when disinfecting at 1°C versus 25°C, since the higher CT requirements in the cold (due to slower disinfection kinetics) were offset by greater chlorine dioxide stability at low temperature. 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 relatively small effect on chlorite and chlorate formation, following the trend observed for required dose. When free chlorine was applied as a secondary disinfectant following primary disinfection with chlorine dioxide, subsequent chlorite concentrations decreased at pH 6, but not at pH 9, suggesting that hypochlorous acid, and not hypochlorite, was responsible for the oxidation of chlorite. THM formation at pH 9 arising from secondary chlorination was reduced by up to 30 µg l−1 due to prior application of chlorine dioxide at concentrations corresponding to as low as 0.5 log Giardia inactivation.