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The City of Atlanta is in the third decade of its combined sewer overflow (CSO) control program, this time implementing court ordered CSO controls by 2007 with a price tag far exceeding previous program costs. Corresponding rate increases have caused dissension among city leaders, environmental stewards and the public. Like many combined sewer programs, the attitude of the paying public has turned from one of extreme environmental protection to a demand for cost-effective solutions. The engineered program included a billion dollar sewer separation and deep tunnel storage and treatment scheme to meet new water quality standards. The City's Value Engineering (VE) team reduced program costs to three-quarter billion dollars and is evaluating cost-effective alternative treatment technologies through pilot testing of actual CSOs from existing components of the Atlanta system.

The treatment approach being pilot tested by Atlanta is similar to the facilities implemented and tested by Columbus, GA, under a peer-reviewed project by the Water Environment Research Foundation (WERF) and the USEPA Office of Research and Development. CSO treatment includes coarse and fine screening before tunnel storage and vortex and compressed media filtration of dewatered tunnel CSO flow for the West side facilities. Treatment of dewatered tunnel flow on the East side includes chemical flocculation and sedimentation before compressed media filtration. Chemical disinfection and dechlorination will be applied to all treated and residual CSO flows prior to discharge into the receiving waters. One objective of the pilot testing was to determine benefits of chemical disinfection before and after filtration.

Pilot testing was conducted on raw CSO directly from the collection system, from the tunnel and after the sedimentation to evaluate performance on different pollutant characteristics. Characterization on solids separation processes included particle size distribution, TSS and turbidity of process influent and effluent and backwash from the filter. UV disinfection testing was eliminated early in the program due to low transmissivity and colloidal particle residuals. High rate chemical disinfection testing of screened and filtered flow was conducted throughout the CSO hydrograph representing different waste strengths and chemical demands. Disinfection testing included protocols following the WERF studies conducted in Columbus and Syracuse in which dose was normalized by different pollutants to produce dose-response curves and chemical feed schemes for design.

Results show that the pilot facility solids removal performance is associated with particle size. CSO wastewaters tested were characterized having a mean particle size between 20 and 50 micron depending upon the source of the CSO. Direct discharges of CSO from the collection system had the largest particle size distribution. A typical CSO hydrograph also showed a decreasing particle size and TSS concentration as the hydrograph progressed from the beginning to the end. The tunnel dewatering operation showed a more pronounced progression to the smaller particle size distribution and then an increase near the end of the tunnel dewatering. The sedimentation basin effluent without chemical feed had the smallest mean particle size distribution.

Vortex separation was found to be effective at removing grit and other gross pollutants and can be used as a plug flow contact tank for high rate chemical disinfection (WERF, 2003).

The compressed media filter was found to be very effective at raw CSO TSS reduction (75% to 94%) having a mean particle size of 30 to 50 micron. Filter performance removing TSS from the dewatered tunnel flow was found to be 60% to 80% having a mean particle size range of 25 to 35 micron. Filter performance of sedimentation basin effluent having a mean particle size of 20 micron was found to be about 35% TSS reduction. The compressed media filter was effective at removing particles down to 20 micron. Greater overall removal including removal of the smaller particles was found to occur when an initial filter media coating of larger particles occurred. The filter was found to process high rates of flow at high TSS concentrations. Findings demonstrate the compressed media filter as an effective technology for first flush treatment of wet weather contaminants.

High rate disinfection was correlated to wastewater strength, which confirmed previous WERF study protocols and results (WERF, 2004). WERF protocols for developing disinfection doseresponse curves were confirmed. Filtered CSO disinfection required approximately half the dose of raw CSO disinfection.

It was concluded that screening, vortex separation and compressed media filtration with disinfection and dechlorination exceeded the technology-based requirements of primary clarification and disinfection as required under the EPA CSO Policy.

Document Type: Research Article


Publication date: 2004-01-01

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