Comparison of dechlorination rates and water quality impacts for sodium bisulfite, sodium thiosulfate and ascorbic acid

Abstract

The impact of water quality parameters such as organic and inorganic matter as well as chlorine species (free chlorine and monochloramine) on the rate of dechlorination by sodium bisulfite (SBS), sodium thiosulfate (STS) and ascorbic acid (AA) were studied. Reaction rate constants determined for the various dechlorination reactions showed that SBS and AA achieved dechlorination at a faster rate than STS. Organic matter present in the test solution increased the rate of dechlorination by STS but not SBS and AA. AA was found to be ineffective for the removal of monochloramine. The effect of dechlorination chemicals on water quality with respect to pH, turbidity and total organic carbon (TOC) was investigated along with the acute toxicity of the chemicals on the aquatic indicator species Daphnia magna . SBS was determined to have an LC 50 of 68 mg/L with no toxicity impacts observed when the concentration was ≤ 20 mg/L for D. magna. AA increased the TOC levels in the treated water and resulted in some D. magna mortality at higher levels. STS had the least impact on daphnia mortality rates, but the use of STS for dechlorination resulted in the largest pH change of test waters compared to the other dechlorination chemicals.