ACUTE TOXICITY TESTING OF ALTERNATIVE HIGH-RATE DISINFECTION TECHNOLOGIES FOR CSO TREATMENT IN NEW YORK CITY

As part of the design upgrade for the New York City Department of Environmental Protection's (NYCDEP) Spring Creek Auxiliary Wastewater Pollution Control Plant (AWPCP), an existing combined sewer overflow (CSO) detention and treatment facility, a field pilot study was performed to evaluate alternative technologies for CSO wastewater disinfection. Previous pilot testing had identified applicable high-rate CSO disinfection technologies, however there was limited data on the marine toxicity of residuals and byproducts from CSO disinfection. This study evaluated the acute toxicity of CSO treated via ultraviolet irradiation (UV), chlorination/dechlorination, and chlorine dioxide. Other objectives of the study included optimization of disinfection performance, evaluation of mixing and contacting configurations, and testing of an innovative chlorine dioxide generator. However, this paper will address the results of the effluent toxicity analyses.Three pilot scale disinfection systems (UV, chlorination/dechlorination, and chlorine dioxide) were operated concurrently on the same wastewater feed during both dry weather and wet weather conditions to simulate the range of wastewater quality considered “typical” of CSO for the Spring Creek facility. The pilot units employed high-rate treatment processes applicable to CSO including medium pressure-high intensity UV and high-rate mixing/short detention times (i.e., 5-minutes) for chemical disinfection. The units were operated over a range of dosages (UV: 46 to 173 mWs/sq.cm.; chlorination: 18 to 28 mg/l; chlorine dioxide: 6 to 10 mg/l) and test conditions which were selected from the first phase of testing. Eight separate test events were performed, 4 during wet weather CSO conditions and 4 during dry weather conditions. Sampling was performed to characterize influent wastewater quality, unit treatment performance, effluent disinfection residuals/byproducts, and effluent toxicity.The toxicity analyses included field and laboratory toxicity measurements using bioluminescent bacteria (i.e., Microtox), and off-site laboratory analysis of 48-hour acute whole effluent toxicity (WET) using opossum shrimp and sheepshead minnow as indicator species.WET results showed no correlation to either disinfectant dose or wet/dry weather events. Toxicity effects observed in the treated effluents were associated with the untreated wastewater rather than the disinfection processes. The results showed poor correlation between Microtox analyses taken in the field versus the off-site laboratory WET analyses.