Abstract

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 U. S. Environmental Protection Agency (USEPA)’s CSO Control Policy presumption approach criterion 2 through their long term control plan (LTCP) process. In order to assess the overflow volumes relative to total CSS conveyance on an annual average basis, KVWPCA completed a comprehensive flow monitoring and CSS hydrologic hydraulic modeling study.

The RTK method was used in separate sewershed areas and a non-linear reservoir method was used in combined sewershed areas to simulate the RDII and runoff. Calibration and verification were performed using three criteria: integral square error; the percentage of model peak higher than meter peak; and the percentage of model volume higher than meter volume. To evaluate the long term system performance, a one year continuous simulation (given the typical precipitation year, which was 41.24 in., 104.74 cm) was run and analyzed (USEPA, 1995b). Traditional methods for these calculations are time consuming, so the authors proposed a new method to calculate the percentage capture. The new method utilizes statistical tools, RDII, runoff, and dry weather analysis during rainfall events. It reduced the calculation time from one week to one hour. The model predicted that 88.09% combined sewage is captured and conveyed to the WWTP during wet weather in a typical precipitation year.

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