Re ECF Project: 2014-73

Project Title: Optimization of dechlorination to generate greener chlorinated saline sewage effluents

Applicant: Professor Xiangru ZHANG, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology

Total Approved Grant: $ 494,696 (ECF & WWGF: 50/50)

Duration: 1/8/2015 to 31/7/2017

Project Status/Remarks: Completed

This project aims to investigate the feasibility of reducing brominated/iodinated disinfection byproducts in chlorinated saline sewage effluents by excess dechlorination, and to optimize the excess dechlorination process to minimize the overall toxicity of chlorinated saline sewage effluents to the marine ecosystem.

Summary of the Findings/Outcomes:
Chlorine is the most commonly used disinfectant for wastewater disinfection, but it could pose adverse effects on aquatic organisms. Sulfur-containing chemicals are commonly used as the dechlorination agents to remove chlorine residual. In this study, factors affecting dechlorination of a chlorinated primary saline sewage effluent were investigated. Dechlorination ratio and mixing condition were found to be critical. The toxicity of the chlorinated-dechlorinated sewage effluent was mainly caused by the chlorine residual resulting from insufficient dechlorination. Dechlorination with Na2S2O3 required a more vigorous mixing condition than that with NaHSO3 or Na2SO3, as the density of the dechlorination agent might affect the mixing. Moreover, under insufficient mixing, increasing dechlorination time or decreasing dechlorination agent concentration reduced the toxicity of the effluent. The overall toxicity of DBP mixture was also evaluated, as DBPs formed in the chlorinated effluent cause chronic adverse effects on the receiving marine ecosystem. Increasing dechlorination ratio slightly decreased the toxicity of the DBP mixture in the chlorinated effluent, most likely due to the slight decrease of TOBr and TOI levels. The sulfite or thiosulfate ions from chlorination agents might act as nucleophiles to substitute halogen atoms in halogenated DBPs, leading to the decrease of TOBr and TOI levels. The study has generated five publications in leading environmental journals. Results were disseminated in two conferences and a report in the Chemical & Engineering News.