The Kiski Valley Water Pollution Control Authority (KVWPCA) owns and operates the combined sewer system (CSS), which includes 23 diversion chambers, 8 pump stations, 12.6 miles of interceptor sewers, and the wastewater treatment plant (WWTP). KVWPCA has selected to utilize the EPA CSO Control Policy “presumption” approach criteria ii 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.
Flow monitoring sites selection included each CSO diversion chamber inlet flow and overflow, as well as tributary flows from each municipality. A total of 102 flow meters and seven rain gages were utilized through the duration of the study. During the monitoring period, five significant rain events occurred. An InfoSWMM model was developed, calibrated and verified utilizing inflow, overflow and rain data from the flow monitoring study. RTK method was used in separate sewershed areas and non-linear reservoir method was used in combined sewershed areas to simulate the RDII and runoff. Calibration and verification were evaluated 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 year-long continuous simulation given the typical precipitation year, which was 41.24 inches, was run and analyzed. To obtain the WWTP percent capture during all rainfall events, the dry weather flow, RDII, Runoff, and wet weather flow to the WWTP during individual event need to be calculated. Traditional methods for these calculations are time consuming, so the author proposed a new method to calculate percent capture. The new method utilizes statistical tools in InfoSWMM model, 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 percent of all combined sewage is captured and conveyed to the WWTP during wet weather in a typical precipitation year. Therefore, the KVWPCA system is capable of providing conveyance of over 85 percent of all combined sewer to the WWTP on a system-wide basis.
It is clear that the KVWPCA conveyance system meets the EPA CSO Control Policy “presumption” approach criteria ii, based on existing system flows and conditions. Additionally, sewer separation projects are planned for the Borough of Apollo, Borough of East Vandergrift, and the Borough of Vandergrift. Therefore in the future, percent capture should significantly increase.