Diversionary Tactics: Pilot Testing Wet and Dry Weather Flow Diversions to Sanitary Sewer as a Means of Reducing PCBs from Stormwater
Normally, municipal staff would never consider deliberately diverting stormwater into their community’s sanitary sewage treatment systems, but that is exactly what this award-winning pilot project accomplished. The motivation was a requirement established in the 2009 National Pollutant Discharge Elimination System (NPDES) Permit for Urban Stormwater Discharges issued to the Contra Costa Clean Water Program. Contra Costa County Watersheds Program (County) led the pilot project for CCCWP.
The lessons learned from this pilot project include both good news and bad news.
The Good News:
• CCCWP permittees complied with provision C.12.f of MRP 1.0 by collaborating with several partners to complete a pump station stormwater diversion pilot with a permanent, “hard-piped” diversion system installed at the NRSPS.
• WCWD experienced no overflows, sewage treatment system upsets, or other disruptions to operations as a result of the pilot diversion project.
• In addition to rehabilitating existing infrastructure, the NRSPS diversion project offers new operational flexibility to the Pump Station owners.
• Project partners gained a new understanding of the incentives and opportunities that can potentially support co-management of urban runoff with water reclamation systems originally designed for sanitary sewage.
• There is now an established partnership and relationship between the County and WCWD, and with new infrastructure now in place and the pilot successfully completed, there is an opportunity to pursue grant funding to support stormwater harvest and use projects in the future.
The Bad News:
• The wet and dry diversion pilot tests accomplished miniscule load reductions: e.g., about one milligram (0.001 grams) of PCBs, against a required Baywide PCB load reduction of 18,000 grams by the year 2028.
• Conveyance limitations of the sanitary sewage system prohibit substantial scale-up of the pilot to larger diversion flows. The diversion pump installed pumps 200 to 250 gallons per minute into the WCWD collection system. Larger flow rates risk sanitary sewer overflows. The design of the pump station provides 135,000 gallons per minute of stormwater pumping capacity, about 600 times more volume than the diversion. That might be comparable to a person sipping water from a gushing fire hydrant.
• Even if all of the stormwater from the 339 acre catchment served by the NRSPS could be captured and treated – which would require a substantial capital project - the total PCB load reduction possible is on the order of one to ten grams at best, still a tiny fraction of the overall load reduction mandate for the Bay.
• The total project cost was over $1.4 million which included some necessary upgrades to the existing Pump Station infrastructure. The cost for a "stand-alone" stormwater diversion project would be approximately $1 million.
This is an example of opportunistically combining stormwater quality enhancement and municipal infrastructure restoration into one project. Diversion is not a “silver bullet” that will make a significant difference to PCB loads; however, consideration of multiple water quality benefits, such as trash controls, water resource development, and reduction of bacteria, oil and grease, and other urban pollutants discharged to Wildcat Marsh and the Bay may motivate additional, expanded stormwater harvest and use projects in this watershed.
On February 25, 2016, the NRSPS Stormwater Diversion Project was awarded the honor of Environmental Project of the Year by the Northern California Chapter of the American Public Works Association (Appendix A). The award named CCCWP as “an essential partner in the development and construction of this innovative project.”