Underground In-System Storage (ISS)

The existing wastewater system capacity is insufficient to address the wet weather flows that occur in the winter months and lead to sanitary sewer overflows (SSOs). To address this issue, a comprehensive approach to improve capacity involving an underground, in-system storage facility upstream of the WWTP is planned.

This storage facility will be capable of temporarily storing approximately 5 million gallons of wastewater during heavy rains. During these conditions, flows will divert from the sewer pipes to the storage facility to prevent overflows from occurring. After a storm subsides, and generally within 24 hours, wastewater in the facility will be pumped back into the collection system pipes and conveyed to the wastewater treatment plant.

The underground storage will provide the needed capacity and reduce the size of upgraded pipelines and pump stations downstream from the storage facility. On July 17, 2017, the City Council selected the San Mateo County Event Center for the location of the underground storage facility. The Event Center is near where the most SSOs have historically occurred, near the intersection of S. Delaware Street and 25th Ave. The storage facility is part of the overall Basin 2/3 Collection System Capacity Improvements Project.

This element of the Clean Water Program plays an integral role in meeting the RWQCB’s mandate to eliminate SSOs.

Graphic depicting sewer overflow prevention