Combined Sewers are designed in such a way that when their design capacity is exceeded excess water from the sewers is diverted to the rivers and streams resulting in combined sewer overflow (CSO). Now, with increased Environmental Protection Agency (EPA) monitoring and stringent new river water standards, the City of Pittsburgh finds itself having to come up with measures to combat the CSO problem. One approach investigated in this research is to capture the CSO water, treat it using polymeric microfiltration membranes, send the permeate to the nearby rivers and streams while the concentrated retentate is directed back to the Wastewater Treatment Plant. Seven polymeric microfiltration membranes ranging in size from 0.2 μm to 0.8 μm were obtained from membrane manufacturers (e.g Osmonic, PALL and Millipore) and were tested for their effectiveness in treating the combined sewer overflow. Primary effluent wastewater from Allegheny County Sanitation Authority (ALCOSAN) Wastewater Treatment Plant (WWTP) with COD values of less than 100 mg/l was assumed to resemble combined sewer overflow pretreated by a swirl separator and was used in the filtration runs. A particular membrane's effectiveness to treat CSO was measured for its ability to reduce fecal coliforms, Escherichia Coli and enterococci bacteria to levels recommended by EPA. Reduction of chemical oxygen demand and flux rate for different membranes were also investigated. Effect of membrane surface chemistry on the flux rate and on the permeate quality were investigated. The results show that flux is proportional to pore size for membranes of the same surface chemistry. Hydrophilic membranes gave higher initial flux rate than hydrophobic membranes of the same pore sizes. The steady state flux rates were about the same at all pressures and pore sizes, indicating that at steady state fouling becomes the only variable that controls flux. The membranes with pore size 0.45 μm and below were able to reduce the bacteria levels to low detection limits except for the one membrane with pore size of 0.3 μm. The AC+0.2 membrane gave at least 57% removal of COD, which was the highest removal, while the AC+0.45 and AC+0.8 membrane gave some of the lowest removals at 23%. The results show that polymeric microfiltration membranes can be used to treat combined sewer overflow.
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