Kinetics of tap water dechlorination and aquatic health impacts of selected dechlorination chemicals

The kinetics of seven dechlorination agents (DA) was studied to identify the efficacy of dechlorination under variable dosage, mixing (periodic/continuous), and pH conditions (6.5--8.6 and/or >9). Dechlorination reactions at 1x stoichiometric dosage (mg DA/mg Cl2) of the DAs are rapid in most cases, but not instantaneous and in some cases needed more than stoichiometric dosage to ensure a near zero residual level (0.02 mg/L total residual chlorine, as required by CEPA (1999)). Dechlorination kinetic analysis revealed reactions were either pseudo-first or zero order with respect to total chlorine concentration in solution at the 10x stoichiometric dosage and second order overall at 2--4x stoichiometric dosages. 48-hour aquatic toxicity tests were conducted using Daphnia magna to identify the acute (short-term) toxicity affects of the dechlorination chemicals over a range of concentrations (0--200 mg/L). Sodium sulfite and thiosulfate were found to have the least aquatic mortality effects while hydrogen peroxide and calcium thiosulfate had the most deleterious effects.;Keywords. dechlorination kinetics, dechlorination, aquatic toxicity, sulfur salts, thiosulfates...