REDUCING SANITARY SEWER OVERFLOWS THROUGH FLOW SWAPPING WITH CSO FACILITIES — THE WALNUT LAKE PUMPING STATION PROJECT

The Walnut Lake No. 1 Pumping Station provides sanitary sewer service to the Townships of Bloomfield and West Bloomfield in Oakland County, Michigan. During wet weather events, an existing pump station was unable to handle the excess wet weather flow delivered from the upstream sewer systems. Overwhelmed, the sewers quickly surcharged and were relieved via an overflow pipe into the Franklin Branch of the Rouge River.Three Combined Sewer Overflow (CSO) Retention Treatment Basins (RTB) with permitted discharges were located just five miles away from the project site. Each RTB was served by a sewage regulator that maintained a designated flowrate to the downstream Evergreen Interceptor. During wet weather events, flows in excess of 14 cfs were diverted into the three CSO RTBs for storage and, if needed, treated discharge. Historical peak flow metering records at the Walnut No. 1 Pumping Stationindicated that relief of 14 cfs would significantly reduce Sanitary Sewer Overflows (SSO) at the project location.Working through the Administrative Consent Order (ACO) process, the Oakland County Drain Commissioner (OCDC) negotiated an agreement with the Michigan Department of Environmental Quality (MDEQ) that would reduce SSOs by transferring wet weather sanitary flow via a new pumping station and interconnection to the Evergreen Interceptor. The MDEQ agreed to allow for full transfer of up to 14 cfs with complete closure of the three regulators. To deliver excess wet weather flow from the Walnut Lake No. 1 Pumping Station, a 20-inch, SDR 11 High Density Polyethylene (HDPE) forcemain was constructed along a route 22,000 feet from the newpumping station to the Evergreen Interceptor. At the time of the construction, this forcemain wasone of the longest and largest sanitary sewer directional drilling projects of its kind in North America.Sewage regulators at three CSO RTBs were modified with remote actuator and SCADA control systems to allow for opening and closing of the regulators. This modified operational protocol allowed for increased flows to the RTBs, while providing additional capacity in the downstream interceptor during wet weather. This "flow swapping" arrangement provides an additional 14 cfs of capacity inthe Evergreen Interceptor for wet weather sanitary flow that may otherwise result in discharge ofSSOs.