Potential Aquatic Health Impacts of Selected Dechlorination Chemicals

Municipal wastewater effluents are one of the largest single effluent discharges in Canada. Chlorination of wastewater effluents is a widespread practice throughout Canada (excluding Quebec), the United States and parts of Europe. As chlorine in wastewater effluents is toxic to aquatic biota, dechlorination chemicals may be used to reduce residual chlorine concentrations to below 0.02 mg/L (as Cl2) as mandated by Canadian law. However, the potential aquatic health impacts of residual dechlorination chemicals must also be determined. Seven dechlorination agents (ascorbic acid, hydrogen peroxide, calcium thiosulfate, sodium sulfite, sodium thiosulfate, sodium metabisulfite, sodium bisulfite) were evaluated with regards to their 48 hour acute toxicity. Tests were conducted using Daphnia magna to identify the acute (48 h) 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.