Impact of Wet-Weather Peak Flow Blending on Disinfection and Treatment: A Case Study at Three Wastewater Treatment Plants

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 coliformEnterococcus, viruses, and protozoa. Other parameters included total residual chlorine, BOD5 , and TSS.The results showed, that the difference between wet-weather blending and dry-weather final effluent results ranged from a half order of magnitude to one order of magnitude higher at two plants for fecal coliform and at one plant for Enterococcus. A difference of three orders of magnitude higher, which represents a significant impact, was observed at one plant for fecal coliform and at two plants for Enterococcus. During blending, the sampled WWTPs removed, on average, between 97% and 99% of coliphage and enteric viruses, approximately 71% of Cryptosporidium, and between 40% and 88% of Giardia. During blending, average effluent BOD5 and TSS concentrations remained above 30 mg/l at two out of three plants. The third plant had average results above 30 mg/l for both BOD5 and TSS parameters: however, the plant was undergoing a partial construction at the time of sampling.A maceration procedure was used for disinfected, final effluent samples prior to analyses for fecal coliform and Enterococcus. Maceration exposes the particles associated with occluded/clumped-together bacteria to the disinfectant. For the majority of samples, maceration of effluent samples resulted in some (in most cases, less than half an order of magnitude) increase in both fecal coliform and Enterococcus values during both wet- and dry-weather. At one out of three plants, in each instance, maceration of effluent samples resulted in one order of magnitude increase in fecal coliform during wet-weather and Enterococcus during dry-weather.