DULUTH PUMP STATION/STORAGE BASIN MEETS SSO CHALLENGE

The Minnesota Pollution Control Agency and US EPA have mandated an end to sanitary sewer overflows(SSOs) to pristine Lake Superior waters. The City of Duluth and the Western Lake Superior Sanitary District (WLSSD) have taken joint responsibility for stopping the SSOs by virtue of their jointly-held NPDES permit. Of eighteen identified overflow locations, eleven have been corrected as of June 2006.The City and WLSSD have coordinated efforts to reduce chronic overflows from their sewer systems. This has involved a strategy combining clear water removal and system capacity enhancements. The improvements have taken the form of such activities as sewer rehabilitation, I/I reduction, pump station power supply improvements and overflow storage. The City of Duluth clear water removal program focuses on house footing drain disconnection and lateral rehabilitation. Rehabilitation of one basin, City Sewer Basin 2 in the City of Duluth, was completed in 2002. However, wet weather flows from this basin continued to cause sanitary sewer overflows at the Dodge Street Pump Station, albeit at much less frequency than before the work.Because of prior source reduction efforts through footing drain disconnection in the basin, the design storage facility volume was greatly reduced. A wastewater flow generation model was calibrated to flow meter data both before and after source reduction efforts. The model indicated that the design volume of the basin to accommodate a 25-year event could be reduced by roughly 50 percent due to source reduction. Because of this reduction in required design overflow volume from City Sewer Basin 2, the modeled storage facility was also able to accommodate excess flow from nearby City Lift Station Nos. 1 and 3, two other known City overflow locations. The total design volume of the overflow storage facility was 1.9 million gallons.Over the winter 2004 — 2005, the City constructed a storage tank at the Dodge St. location in order to effectively alleviate the chronic problem of sanitary sewer overflows at this location. A parcel of property adjacent to the existing pump station, constrained on one side by a railroad right-of-way, on two other sides by residential street rights-of-way, and on the fourth side by a residential property was selected for the storage tank. As is typical in Duluth, the site consisted of from one to eight feet of soil underlain by bedrock. Due to the residential nature of the location, the City directed that the tank be buried and the surface covered with grass. The depth of the tank necessary for the relatively small site dictated that a new pump station be constructed in association with the tank, totally replacing the existing station.The tank was subdivided into three basins to minimize the volume used for smaller storms that occur more frequently than the design event. Basin 1 is an extension of the wet well, with the bottom of the tank set above the normal maximum wet well level. Basin 1 was sized to provide treatmentequivalent to primary settling at the projected peak inflow rate, and is equipped with an automated tipping trough flushing system. Stored volume greater than the volume of Basin 1 passes into Basins 2 and 3 through sets of self-baffling weir pipes which draw flow from the middle of the fifteen-foot nominal tank depth, retaining settled and floating solids in Basin 1. If the three basins are filled, an alarm is transmitted to the City dispatcher and operations personnel are directed to open an overflow gate that discharges excess flow to a storm sewer. The pump station operates continuously during and after the high flow event, until the basins areemptied. The full 1.9 million gallons maximum storage capacity would typically be drained withintwo days if no further volume is added. Construction challenges included the site location in the residential neighborhood, with blastingand rock excavation as well as maintenance of access to surrounding properties. The period of construction was designed for breaking ground in late fall and commissioning the following spring.Overflow occurrences at the Dodge Street location during periods of snow melt and/or rain had averaged 11 per year between 1996 and 2003. There were thirteen overflows during 2004 and 2005, withthe last one happening just one week before placing the facility in operation. The facility had amajor test in early October 2005, when the City of Duluth was hit by a storm event widely considered to be larger than the 25-year design event. The excess flow incurred at the pump station was entirely captured by the facility, without overflowing into the Lake.