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Posted on: September 7, 2018

Clean Water Program - Underground Flow Equalization System

San Mateo, CA — The City of San Mateo will host a public meeting to provide information and receive public input on the Underground Flow Equalization System project – formerly called the In-System Storage Facility.

The Underground Flow Equalization System helps eliminate the potential for sanitary sewer overflows during extreme storms and is part of the City’s Clean Water Program, a 10-year effort to improve and repair the aging sewer collection and treatment system.

In compliance with the California Environmental Quality Act, the City is preparing an Environmental Impact Report and is seeking public input on what factors to study. The October 2, 2018 meeting is one way for the public to learn more about the Underground Flow Equalization System project and provide input that the City will consider during preparation of the EIR. 

During storms, rainwater enters the sewer system through cracks and other defects. The amount of rainwater can overwhelm the system’s current capacity, resulting in overflows at the wastewater treatment plant and at locations around the city, such as manholes. These overflows containing a mixture of rainwater and untreated sewage are a danger to human health and the environment because they contaminate streets, creeks, lagoons, beaches and the San Francisco Bay.

The approximately 5-million gallon underground holding structure is proposed to be located in the eastern portion of the San Mateo County Event Center parking lot off Saratoga Drive. This location was selected by the City Council in 2017 after a lengthy series of public meetings and hearings. When the sewer system becomes overwhelmed during heavy rains, the facility will serve as a temporary holding area, generally for 24 hours or less, until the mixture of rainwater and sewage can be treated at the plant. This allows the wastewater treatment plant to “catch up” with the high flows.

The structure will also include advanced odor control features, such as air-tight vaults and carbon filters, as well as mechanisms to flush and clean the structure. The design of the project is sixty percent complete, and more details about construction schedule and cost will be released when they are available.

Please visit for more information.

Upcoming Public Scoping Meeting: (comments will be taken during meeting)

When: October 2, 2018, 6:00 p.m. Where: City Hall, Council Chambers, 330 West 20th Ave., San Mateo, CA

All comments must be submitted by October 22, 2018.

Comments also can be submitted via email to or regular mail:

  • City of San Mateo
  • Clean Water Program
    Attention: Heather Waldrop
  • 330 W. 20th Avenue
    San Mateo, CA 94403


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