Effects of dechlorination conditions on the developmental toxicity of a chlorinated wastewater effluent

Abstract

To protect people from water borne diseases during aquatic recreational activities, a wastewater effluent is usually disinfected before being discharged to a receiving water body. Chlorine is the most commonly used disinfectant for wastewater disinfection, but chlorine residual could pose adverse effects on aquatic organisms. Thus, dechlorination is a necessary process to remove chlorine residual. Sulfur-containing chemicals (e.g., sodium bisulfite, sodium sulfite and sodium thiosulfate) are commonly used as the dechlorination agents. The dechlorination efficiency depends on many factors. In this study, factors affecting dechlorination of a chlorinated primary saline wastewater effluent were investigated. Dechlorination ratio and mixing condition were found to be critical factors in influencing the developmental toxicity of the chlorinated-dechlorinated wastewater effluent. Dechlorination time and dechlorination agent concentration also had a slight influence on the toxicity of the chlorinated-dechlorinated wastewater effluent. The toxicity of the chlorinated-dechlorinated wastewater effluent was mainly caused by the chlorine residual resulting from insufficient dechlorination. Although the three dechlorination agents performed similarly, dechlorination with sodium thiosulfate required a more vigorous mixing condition than that with sodium bisulfite or sodium sulfite, as the density of the dechlorination agent might affect the mixing. Moreover, under insufficient mixing, a prolonged dechlorination time was beneficial in achieving complete dechlorination and thus reducing the toxicity of the effluent. In addition, increase of dechlorination ratio decreased the toxicity of DBP mixture in the wastewater effluent, most likely due to the decrease of TOBr and TOI. The dechlorination agent acted as a nucleophile in the nucleophilic substitution and caused the TOBr and TOI reduction as well as the toxicity reduction of the DBP mixture. The results from this study may aid in the design and operation of dechlorination units in wastewater treatment plants.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology