Impact of Wet-Weather Peak Flow Blending on Disinfection Performance

A U.S. EPA study evaluated the impact on disinfection during peak flows (wet-weather flow events) when a portion of the flow to the wastewater treatment plant (WWTP) bypasses secondary treatment prior to disinfection. The practice of bypassing secondary treatment during peak flows, referred to as "blending," takes place when the volume of primary treatment flow exceeds the capacity of the secondary treatment. The bypassed flow is only treated by primary clarification before it is recombined with the fully treated secondary effluent prior to disinfection. Blending practice prevents passing of excess flow to secondary treatment, which could result in inactivation and destruction of the vulnerable biological process. The trade-off is that during blending only a portion of the total flow receives full secondary treatment. The study was conducted at three WWTPs in New York City, ranging from 60 MGD to 275 MGD capacity. A total of four dry-weather and 12 wet-weather events were sampled and analyzed. Three samples from four sampling points of the treatment train in the WWTP were collected per event. The principal analytical parameters were fecal coliform, Enterococcus, viruses, and protozoa. Other parameters included total residual chlorine, BOD₅, and TSS.