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) interceptor sewers, and a Wastewater Treatment Plant (WWTP). KVWPCA decided to use the USEPA CSO Control Policy presumption approach Criterion Two through their Long Term Control Plan (LTCP) process. The Criterion Two 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 developed several methods to calculate their system percent capture.

These calculation methods can be divided into two categories: Indirect method and Direct method. The Indirect Method first calculates the percent of flow loss (overflows and flooding), and then deducts the percent loss from 100%. The Direct Method calculates the ratio of flow to the Wastewater Treatment Plant (WWTP) to the total wet weather flow during wet weather time directly. 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, different fixed and varied Dry Weather Flow (DWF) thresholds were utilized. Also, in 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 percent capture.

It was shown that the KVWPCA conveyance system meets the EPA CSO control policy presumption approach Criterion Two in any calculating methods. Methods IFM1, DFM1, IFC1, and DFC1 are too conservative to use. Methods IVM1, DVM1, IVC1, and DVC1 are too optimistic to use. Method IFM2 and DFM2 are reasonable if they are used jointly. Method IFC2 and DFC2 are reasonable if they are used jointly. Method IVM2 and DVM2 are reasonable whether they are used separately or jointly. Method IVC2 and DVC2 are reasonable whether they are used separately or jointly. In any calculation procedure, the pure combined flow method is recommended. Although many methods were presented, the authors recommended Method IVC2 and DVC2 as the most reasonable and efficient methods to scholars and engineers in practice.