Comparison of Different Methods in Calculating CSS Percent Capture
The Kiski Valley Water Pollution Control Authority (KVWPCA) owns and operates a combined sewer system (CSS) which includes 23 diversion chambers, eight pump stations, 12.6 mi (20 km) of interceptor sewers, and a wastewater treatment plant (WWTP).
KVWPCA decided to use the United States Environmental Protection Agency (USEPA) Combined Sewer Overflow (CSO) Control Policy presumptive approach Criterion 2 through their long term control plan (LTCP) process. Criterion 2 requires "The elimination or capture for treatment of no less than 85% by volume of combined sewage collected in the CSS during precipitation events on a system-wide annual average basis." In order to assess the overflow volumes relative to total CSS conveyance on an annual average basis, KVWPCA completed a comprehensive flow monitoring, CSS hydrologic hydraulic modeling study, and evaluated several methods to calculate their system percent capture.
These calculation methods can be divided into two categories: the indirect method and the direct method. The indirect method first calculates the percent of flow loss (overflows and flooding) relative to total wet weather flow, and then deducts the percent loss from 100%. The direct method calculates the ratio of flow to the WWTP to the total wet weather flow during wet weather time.
In either method, the determination of dry weather flow and wet weather time is critical. In order to assess the effect of different wet weather flow and time assumptions, different fixed and varied dry weather flow (DWF) thresholds were used. Also, in a large system like KVWPCA, which includes combined and separate sewersheds, the selection of pure combined system flow and mixed system flow also has significant influence on the calculated percent capture estimate.
The evaluation demonstrated that the KVWPCA CSS meets the USEPA CSO Control Policy presumptive approach Criterion 2 using any calculation methods. However, it was also determined that the appropriateness of the methods varied: some were considered to be too conservative, while others were too optimistic. Further, several of the methods were most appropriate if used jointly, while others could be used individually. With any of the calculation procedures, the pure combined flow method (i.e. excluding the impact of tributary separate sanitary flows) is recommended.
Although many methods were evaluated, the authors recommended two methods that use a varied DWF threshold, with adjustments to minimize overcounting of wet weather flow (IVC2 and DVC2) as the most reasonable and efficient methods for utilities and engineers in practice.
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