CSO Post-Construction Monitoring: The Theory and Practice of Achieving Performance Criteria

CWA Authority, Inc. (The Authority) in Indianapolis is currently implementing a 20-year Combined Sewer Overflow Long-Term Control Plan (CSO LTCP). In December 2011, achievement of full operation (AFO) for all CSO projects in the Upper White River (UWR) watershed was completed and the post-construction monitoring (PCM) was initiated as required by The Authority’s consent decree (CD). The 2006 consent decree required the following for CSO watershed post-construction monitoring:• Collect rainfall, flow monitoring, in-stream water quality, and effluent data for 12 months following AFO for all projects in the watershed. This was performed from December 8, 2011 - December 8, 2012.• Re-calibrate the CSO watershed hydraulic model to the 12 months of data, and apply the re-calibrated model to assess the CD performance criteria of four overflow events and 95 percent capture in a typical year. The watershed milestone report was submitted to US EPA in December 2013.The Authority’s PCM program from its 2006 consent decree, as amended, is nearly identical to the CSO Post Construction Compliance Monitoring Guidance (US EPA, 2012) released last year. The completed CSO LTCP projects consisted of consolidation of CSOs to Lift Station 507, and a satellite storage and treatment facility that can store approximately one million gallons of CSO flow with dewatering to the collection system, or discharge up to 53 million gallons per day of chlorinated effluent. Residual CSO discharges can occur at two outfalls.The Authority’s team has determined that Lift Station 507 is performing consistent with its design criteria; has successfully re-calibrated the hydraulic model to the monitoring data; and has applied the model to confirm that the UWR watershed has achieved its CD performance criteria. The team has also analyzed in-stream water quality data to determine that UWR watershed is not significantly influenced by CSO discharges.