Ground Truthing Oakland WWTP and Pump Station Flow Meters for Improved Peak Wet Weather Flow Management and Process Control City of Topeka, Kansas

The City of Topeka (the City) has updated their Combined Sewer Overflow (CSO) Long Term Control Plan (LTCP) and submitted the plan on June 30, 2017 to the Kansas Department of Health & Environment (KDHE) for review by that agency. The KDHE approved the City’s updated CSO LTCP on December 6, 2017.A major feature of the updated CSO LTCP is the optimization of collection system performance to maximize the conveyance of wet weather flows to the Oakland Wastewater Treatment Plant (WWTP) and achieve an average annual wet weather flow capture rate of at least 85 percent.The development of the updated CSO LTCP was unusual because the plan was developed without using a collection system computer hydraulic model. The City and Bartlett & West developed and used an empirical approach, a spreadsheet model, based on assessment of collection system flow metering data from 2010 through 2015 to develop the updated CSO LTCP.In mid-2017, review of collection system and WWTP flow data indicated that CSO discharges had decreased substantially after collection system cleaning, as projected using the spreadsheet model. However, during wet weather events, flows recorded at the WWTP did not increase commensurately with the decrease in CSO discharges.Accounting for flow and wet weather capture is critical for the City of Topeka to demonstrate and document the level of performance projected in the updated CSO LTCP. Therefore, the City developed and implemented a program of testing to assess the accuracy of critical flow meters in the collection system and at the WWTP; and to collect the data necessary to support recalibration of flow meters, as necessary.Through steady-rate flow testing, and a volumetric fill test performed using one of the WWTP primary sedimentation tanks, the City and Bartlett & West were able to establish that the one of the two WWTP influent parshall flumes, the South Flume, was under-reading flow by approximately 37 percent. The calibrations of both parshall flumes were adjusted, and work is continuing to refine the calibration of the second influent parshall flume, the North Flume.With the adjustment of the WWTP influent flow metering, the basis for future planning will be significantly more accurate, and WWTP operational control will be improved.