||Interstate Environmental Commission, New York, NY.; National Risk Management Research Lab., Edison, NJ. Water Supply and Water Resources Div.; National Risk Management Research Lab., Cincinnati, OH. Office of Research and Development.
Blending is the practice of diverting a part of peak wet-weather flows at wastewater treatment plants (WWTPs), after primary treatment, around biological treatment units and combining effluent from all processes prior to disinfection and subsequent discharge from a permitted outfall. For combined sewer systems, EPA's 1994 Combined Sewer Overflow (CSO) Policy encourages delivery of maximum flows to WWTPs, while ensuring that bypasses do not result in National Pollution Discharge Elimination System (NPDES) permit exceedences. Consistent with that principle, blending of flows at WWTPs serving combined sewer systems presents one of the more technically practicable and economically feasible alternatives. In addition, in December 2005, the EPA proposed, for public comment, a new policy for addressing peak flow events at municipal WWTPs served by separate sewer systems, also through flow maximization. This project's intent was to determine the microbiological impact of blending primary effluent flows that are in excess of secondary treatment capacity with the secondary effluent prior to disinfection at large municipal WWTPs. This approach is typically used by a number of municipal WWTPs within the Interstate Environmental Commission's (IEC) jurisdiction during wet weather to maximize the flow to the WWTP and reduce CSO events. The primary objective of the study was to evaluate the effect of wet-weather blending on the concentration of fecal coliform and Enterococcus indicator bacteria, total residual chlorine, protozoa and viruses in the WWTP final effluent. Three New York City WWTPs were monitored for this project. The project was important for better predicting and understanding the impact of blending on CSO pollution control and receiving water quality.