Underground Flow Equalization System (formerly called In-System Storage)

Project Title: Underground Flow Equalization System (formerly called In-System Storage)

Project Schedule:

CEQA Public Scoping Period: September 7, 2018 through October 22, 2018

Draft EIR Public Review and Comment Period: expected December 2018 through February 2019

Final EIR Release: expected June 2019

Project Description:

The purpose of the proposed Underground Flow Equalization System Project (formerly called In-System Storage) is to help eliminate sewer overflows to San Mateo streets, creeks, the lagoon, and San Francisco Bay. These sewer overflows occur primarily during heavy rains, when additional water gets into the sewer system and overwhelms its capacity to collect and transport the combined excess water and wastewater to the wastewater treatment plant. Additionally, during peak rains, the wastewater treatment plant cannot treat the volume of combined excess water and wastewater it receives as quickly as it is entering the system.

In 2009, the San Francisco Bay Regional Water Quality Control Board (RWQCB) issued a Cease & Desist Order that requires the City to eliminate sewer overflows and provide capacity assurance. The permit that allows the City to discharge treated water to the San Francisco Bay (Environmental Protection Agency [EPA] 2013 National Pollutant Discharge Elimination System permit) also requires the City to eliminate blending and use an integrated approach to collect and treat the combined excess water and wastewater.

The proposed project will help San Mateo comply with the RWQCB’s and EPA’s regulations. The project will provide a mechanism to increase the holding capacity of the overall wastewater collection system, allowing excess water and wastewater to be adequately treated at the downstream wastewater treatment plant.  It consists of an underground, concrete holding structure, pump station, odor control equipment room, and diversion pipelines. Most of the system will be underground; however, some minor fixtures, including access hatches, an electrical building, and vents for treated air, will be at or above the ground surface.

These facilities are becoming more widely used across the entire country as a means of providing additional capacity in sewer systems during heavy rains to help prevent sewer overflows. Daly City has been operating a similar system as part of its award-winning wastewater treatment system since 1989; and the city of Pacifica is in the process of constructing a similar system.

To meet the sewer overflow reduction goals, the underground structure must allow for temporary holding of up to approximately 5.3 million gallons. Gravity diversion pipelines conveying wet weather flows from existing sewer pipes to the facility will have a total flow capacity of approximately 25 million gallons per day. During heavy rains, when sewer system overflows are common, two new diversion pipelines will temporarily route wet weather flows from the existing sewer pipelines to the underground holding structure, where it can be stored for up to 24 hours. Once the downstream collection system has available capacity, a pump station inside the holding structure will release the excess wastewater back into the collection system via a pressure pipeline, or force main. An emergency diesel generator will allow diversion and holding processes to continue during power outages.

The facility will also provide temporary wastewater diversion and holding to facilitate future sewer construction projects, as well as operations and maintenance activities, such as pipeline inspection and cleaning.

After any use, self-cleaning mechanisms will flush and clean the facility, while odor control systems will operate to prevent odors from affecting the surrounding community.

Approximate dimensions for the underground temporary holding structure are 200 feet by 150 feet. These dimensions may change as the design progresses.

Map showing the routing of the sewer system around the San Mateo County Event Center.The location of the proposed Project is the San Mateo County Event Center property between Saratoga Drive and South Delaware Street. The proposed temporary holding structure portion would be located in the east corner of the Event Center parking lot, adjacent to Saratoga Drive (Assessor’s Parcel Number 040 030 220). The northernmost point of the site is located at approximately latitude 37°32’51.13″N, longitude 122°17’53.78″W. Diversion pipelines will be installed under South Delaware Street and Saratoga Drive.


The City, as lead agency under the California Environmental Quality Act, is proceeding with an Environmental Impact Report (EIR) for the project, which will tier from the previously certified 2016 Final Programmatic EIR and provide a project-level review of environmental impacts.

Timeline of California Environmental Quality Act Process showing the seven steps: Notice of Preparation and Scoping Period; Prepare Draft Environmental Impact Report; Release Draft EIR; Prepare Final EIR; Release Final EIR; Recommendation of EIR Certification; and EIR Certification. We are currently on the first step.

During the 40-day scoping process starting in late summer 2018, interested parties will have the opportunity to provide input and ask questions. A Draft EIR is expected to be released in late 2018. There will be a 60-day public review and comment period for the Draft EIR. A Final EIR, incorporating written responses to comments received during the public comment period, will be released in late spring 2019. The Final EIR will be presented to the Planning Commission and City Council for consideration of adoption.

For frequently asked questions, please click here.

To provide formal comments on the CEQA scoping process during the formal scoping period (September 7, 2018 through October 22, 2018), please send them to:
City of San Mateo
Clean Water Program
330 W. 20th Avenue
San Mateo, CA 94403
Attention: Heather Waldrop

Formal comments on the scoping process will also be accepted via email between September 7, 2018 and October 22, 2018 at the following address:

If you have additional questions regarding the CWP, please email: