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Impact of Wet-Weather Peak Flow Blending on Disinfection Performance
Citation:
STINSON, M. K., R. I. FIELD, AND R. B. Rukovets. Impact of Wet-Weather Peak Flow Blending on Disinfection Performance . Presented at 6th Urban Watershed Management Symposium, World Environmental and Water Resources Congress 2009, Kansas City, MO, May 17 - 21, 2009.
Impact/Purpose:
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Description:
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 exceeds the capacity of the secondary treatment. The passed flow is only treated by sedimentation before it is recombined with the fully treated secondary effluent prior to disinfection. This is considered a measure to avoid, as passing excess flow to the secondary treatment could result in inactivation or destruction of the vulnerable biological process of secondary treatment. The study was conducted at three WWTPs in New York City, ranging from 60 MGD to 275 MGD. 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 taken per event. The principal analytical parameters were fecal coliform, Entercoccus, E.Coli, viruses, and protozoa. Other parameters included total residual chlorine, BOD5 , and TSS. The results showed that when blending occurred, 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. The difference between wet-weather blending and dry-weather final effluent results was one order of magnitude higher or less than 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, effluent BOD and TSS concentrations remained above 30 mg/l at two out of three plants. The third plant had results above 30 mg/l for both BOD 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 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. Although this study was conducted at only one geographical location and may not represent a nationwide situation, it is an important first step for predicting and understanding the impact of blending on combined sewer overflow abatement and receiving water quality.