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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 x 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 1deg.C versus 25 deg.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 mu g l-1 due to prior application of chlorine dioxide at concentrationscorresponding to as low as 0.5 log Giardia inactivation. [ABSTRACT FROM AUTHOR]

This objective of this study was to explore the practicality of monitoring naturally occurring organisms to predict drinking water treatment plant performance, in this case for the reduction of Cryptosporidium. Surface and ground water from seven drinking water treatment plants across North America that use chlorine dioxide were surveyed for aerobic and anaerobic bacterial spore concentrations. The concentrations of total spores were usually high enough in both raw and treated water to allow 4- to 5-log reductions to be observed across the treatment train by filtering up to 2 l of sample. These results suggested that naturally occurring treatment-resistant spores could be candidates as indicators of treatment performance. However, to be useful as indicators for Cryptosporidium reduction, the organisms would have to exhibit similar resistances to disinfection (chlorine dioxide in this case) in order to be useful. The inactivation kinetics of seven of the most common species were determined, and all were observed to be considerably more susceptible to chlorine dioxide inactivation than Cryptosporidium as reported in the literature. This study therefore did not identify an appropriate ambient microbial indicator for Cryptosporidium control.