Wastewater Treatment Plant Nutrient Removal and Wet Weather Flow Management Upgrade and Expansion Project

Project Background and Description

In San Mateo, wastewater from homes and businesses is collected from individual parcels, via a network of pipelines/trunk lines and pump stations. The wastewater is conveyed to the wastewater treatment plant (WWTP), and then treated and the plant effluent is discharged to the San Francisco Bay.

In the summer months, the WWTP treats approximately 11 million gallons per day (mgd). The WWTP can treat up to 60 mgd per day for primary treatment and 40 mgd for secondary treatment. Primary treatment uses basic processes to remove solid waste present in the wastewater. Secondary treatment uses biological processes to remove more dissolved waste matter from the water.

During heavy rains (peak wed weather conditions), the WWTP’s capacity is regularly exceeded. This causes sanitary sewer overflows (SSOs) at the plant and collection system. These overflows are a danger to human health and the environment, because they contaminate City streets, creeks, lagoons, beaches, and the San Francisco Bay.

In addition to the lack of capacity, many of the existing WWTP facilities and components are more than 40 to 75 years old. Nearly half of the system is reaching the end of its useful life.

To respond to these growing challenges, the City established the Clean Water Program (CWP) in 2014. The CWP is a comprehensive 10-year plan to upgrade the aging wastewater collection system and WWTP. The goals of the CWP are to:

  • enhance the reliability of the wastewater collection and treatment system,
  • increase capacity to manage heavy flows to eliminate SSOs,
  • comply with regulatory requirements,
  • produce better-quality treated water that meets current and future permit requirements and that can be used as recycled water in the future, and
  • align with the City’s sustainability goals.

The upgrade and expansion of the WWTP is the largest project under the Clean Water Program. It is also the largest rehabilitation and expansion of the plant since the 1970s. This project consists of new liquids treatment process facilities, including headworks, primary treatment, five-stage biological nutrient removal/membrane bioreactor (BNR/MBR) process, biological and chemically enhanced treatment (BioCET) process, and other general plant upgrades, including odor control to serve the new facilities. The innovative BioCET process involves dual-use clarifiers and a biological contact tank to provide secondary treatment of wet weather flows to eliminate blending and meet NPDES permit requirements. The new facilities will be designed to handle influent flows of 21 mgd (maximum month) and 78 mgd (peak wet weather flows with both in-system and onsite storage). In addition, a new administration building will be provided for WWTP operations and maintenance staff, and will house the new main control room and a new laboratory.These new facilities will be designed to provide advanced treatment to 21 million mgd and allow the plant to better handle wet weather events up to 78 mgd.

The project began with conceptual planning, which completed in November 2016, and continued with the engineering design phase in December 2016. 

A Programmatic Environmental Impact Report (PEIR) was prepared and adopted by the City Council in July 2016. It found no anticipated violations of federal or state environmental requirements. Additionally, it included measures to minimize impacts on residents and local ecosystems. Further permitting will be needed as the engineering design phase progresses.

The City selected the project with best overall value and ability to meet regulations, both now and in the future. The new WWTP will provide the necessary level of service, protect the environment, and accommodate future population growth. The treated water can also be re-used for landscape irrigation and other purposes.

As of February 2019, the design of the WWTP is nearly complete. Construction is planned to occur in three phases, with the first phase scheduled to begin in summer 2019. For information about the Special Use Permit application, please see the Planning Department’s project site here.