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MICROBIAL PARTICLE ASSOCIATION AND COMBINED SEWER OVERFLOW DISINFECTION

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Microbial indicator concentrations, measured before and after treatment, typically are used to evaluate wastewater and combined sewer overflow (CSO) disinfection effectiveness. A concern with using the standard methods for measuring microbial indicator concentrations in sewage is that they fail to measure particle-associated microorganisms, underestimating the total concentrations present. A related concern is the effect of particle association on disinfection effectiveness. Particles and other matter in the water interfere with a disinfectant's ability to contact and, therefore, inactivate microbes. With CSO disinfection as a provision in the U.S. Environmental Protection Agency's (EPA) CSO Control Policy (U.S. EPA, 1994), EPA identified CSO disinfection research as a need in its Risk Management Research Plan for Wet Weather Flows (Field et al., 1996) and is conducting CSO disinfection research. Two disinfection research projects are described in this paper. The first is investigating the effects of particle association on measurements of microbial indicator concentrations in CSOs. The second is aiming to determine the effectiveness of four treatment trains for CSO disinfection and the effects of removing solids on the disinfection effectiveness of chlorination and ultraviolet (UV) light irradiation.

In the first project, the effects of blending samples to expose particle-associated microorganisms to the water column before measuring microbial indicator concentrations were investigated. Samples were blended in a blender at speeds from 14,500 to 22,000 rpm for 0.5 to 10 min to break apart particles and bacterial agglomerations. Prior to blending, samples were irradiated by UV light and diluted in a mixture of chemicals. Samples were irradiated to reduce indicator microorganism (fecal coliform {FC} and Enterococcus {EC}) concentrations estimated to be 106 – 108/100 mL by several orders of magnitude so that concentration changes due to blending would be more easily observed. Samples were then diluted in the mixture of chemicals identified by Camper et al. (1985) to enhance bacterial dissociation from solids. The resulting solution contained 10−6 M Zwittergent 3–12, 10−3 M EGTA, 0.01 M Tris buffer, and 0.01% peptone by weight, buffered to a pH of about 7. The results show that blending samples diluted with “Camper's reagents” increases FC and EC concentrations measured by membrane filtration (MF) under certain conditions. Both blending speed and time affected measured microbial indicator concentrations, with increased concentrations observed in samples blended at 22,000 rpm for up to 3 min. Measured FC and EC concentrations in blended samples were up to 10 times greater than FC and EC concentrations measured in the untreated samples, i.e., unblended sampled with no “Camper's reagents” added. Blending decreased mean particle size, although no correlations between increased indicator microorganism concentration and decreased particle size were observed. Blending CSO effluent samples diluted with “Camper's reagents” before measuring microbial indicator concentrations should be considered when evaluating technologies for disinfection effectiveness and developing CSO effluent requirements.

A second set of experiments was conducted to measure reductions in CSO FC and EC concentrations by four treatment trains. The processes investigated at the bench scale were filtration–chlorination, filtration–UV light irradiation, settling–chlorination, and settling–UV light irradiation. All MF measurements were preceded by diluting samples with “Camper's reagents” and blending at 22,000 rpm for 2 min. Once the data are analyzed, the results may show increased effectiveness of CSO chlorination or irradiation when solids are removed.
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Document Type: Research Article

Publication date: 2000-01-01

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