167 O IWA Publishing 2011 Journal of Water Supply: Research and Technology—AQUA | 60.3 | 2011 Comparison of dechlorination rates and water quality impacts for sodium bisuifite, sodium thiosulfate and ascorbic acid Onita D. Basu and Nigel P. De Souza 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 bisuifite (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 LC50 of 68 mg/L with no toxicity impacts observed when the concentration was < 20 mg/L for D. magna. /VA 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. Key words | acute toxicity, ascorbic acid, Daphnia magna, sodium bisulfite, sodium thiosulfate, water dechlorination Onlta D. Basu (corresponding author) Nigel P. De souza Dept of Civil and Environmental Engineering, 1125 Colonel By Drive. Ottawa, ON. K1S 5B6, Canada E-mail: onita_basu'Scarleton.ca INTRODUCTION Disinfection using chlorine is a common treatment process employed by municipal and private industries for the disin- fection of both potable water and wastewater effluent. The drawback to chlorine disinfection is the production of disin- fection by-products (DBPs) and aquatic toxicity. It is well established that chlorine reacts with organic matter typical of raw water and wastewater to produce various DBPs (Guanghui & Reckhow 2007; Zhang et al. 2008). These DBPs include trihalomethanes and haloacetic acids, some of which are carcinogens (Singer 2002). The formation of by-products can be minimized by reducing excess residual chlorine levels in a process called dechlorination. Further- more, the presence of chlorine in wastewater effluent has been linked with the mortalities of aquatic species at dis- charge sites, signifying the toxicity of chlorine to aquatic doi: 10.2166/aqua.2011.040 environments (Heiz & Nweke 1995). Wastewater effluent with total residual chlorine values greater than 0.02 mg/L have been reported to have deleterious impacts on the receiving environment (MSSC 1993). Dechlorination can be used to remove excess residual chlorine before it enters the receiving waters thus reducing chlorine-associated water toxicity. Dechlorination is most typically achieved by the addition of chemical dechlorination agents to chlorinated waters. These agents include sulfite salts - sodium meta- bisulñte, sodium bisulfite and sodium sulfite; thiosulfates - calcium thiosulfate and sodium thiosulfate; and even ascorbic acid (Connell 2002). While the aforementioned agents are all capable of reducing residual chlorine levels, sulfite salts are commonly used due to the ease of handling