Following more than five years of planning and construction, the San Mateo Wastewater Treatment Plant, on the shoreline of San Francisco Bay, has undergone a $552 million upgrade and expansion, becoming the largest public works investment in the city's history. The facility is now one of the most advanced and sustainable wastewater treatment plants in the nation.

The comprehensive five-year project, carried out in three phases, has significantly increased the plant's capacity to handle major storm events and prevent sewer system overflows that threaten both public health and San Francisco Bay. The upgraded facility now produces high-quality treated wastewater that fully meets state and federal standards. Located beside the original 1933 facility along the shoreline, the upgraded plant serves approximately 170,000 residents in San Mateo, Foster City, Hillsborough, Belmont, and surrounding unincorporated areas.

The project is a cornerstone of San Mateo's $1 billion Clean Water Program, launched in 2015 after a cease and desist order required improvements to prevent overflows into the Bay. The upgrades include new primary treatment facilities, expanded hydraulic capacity to manage wet weather flows, advanced biological nutrient removal systems to reduce nitrogen and phosphorus discharges, and new headworks, primary clarifiers, odour control systems, and biological nutrient reactors.

Advanced treatment technology installed as part of the project will also cut nutrient discharges which are key contributors to harmful algae blooms observed in the Bay in recent years.

"Upgrading our wastewater infrastructure to withstand extreme weather is a critical part of adapting to climate change for most towns and cities," said State Water Board Member Sean Maguire. "Infrastructure improvements are a major undertaking that require years of planning and extensive financing. We at the board are proud to partner with cities like San Mateo to provide grants and loans that make large-scale projects more affordable and, ultimately, achievable for municipalities and their customers."

The State Water Board supported the project with more than $137 million from the California Clean Water State Revolving Fund, a low-interest financing mechanism that will save ratepayers an estimated $22 million over time. Nearly half of the total cost -- $277 million -- came from the U.S. EPA's Water Infrastructure Finance and Innovation Act (WIFIA) program.

"Protecting our Bay and providing clean water for our community is at the heart of this project," said San Mateo City Manager Alex Khojikian. "We built one of the most sustainable treatment plants in the nation -- a facility that not only safeguards public health and prepares us for climate change, but also welcomes residents with new walking paths, landscaping and educational features. This is an investment in both our environment and our quality of life for generations to come."

Completed $552M San Mateo WWTP upgrade will prevent sewer overflows