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

South Camera

East Camera

Rendering of new Waste Water Treatment Facility

What Is the Wastewater Treatment Plant Upgrade and Expansion Project?

The Clean Water Program is the most significant upgrade to the City’s sewer infrastructure in its modern history. After five years of planning and design, we are starting construction on an expansion to the Wastewater Treatment Plant (WWTP). This upgrade is the largest component of the Clean Water Program, and the most significant rehabilitation work executed at the WWTP since the 1970s. Once completed, the WWTP will produce high-quality treated water that will protect human health and the environment while meeting water quality regulations. Improvements will benefit the City’s residents, future generations, and the San Francisco Bay Community. Due to the scale and complexity, the WWTP project will be done in three phases over the next five years. Phase 1 begins in August 2019.

In San Mateo, wastewater from homes and businesses is collected by a network of pipes and pump stations. The wastewater is conveyed to the wastewater treatment plant (WWTP), and then treated for discharge to the San Francisco Bay.

In summer, the WWTP treats approximately 11 million gallons per day (mgd) of wastewater. Primary treatment uses gravity to remove solids from the wastewater. Secondary treatment uses biological processes to remove more dissolved waste matter from the wastewater. The WWTP can treat up to 60 mgd per day through primary treatment and 40 mgd through secondary treatment. During heavy rains, the WWTP’s treatment capacity is regularly exceeded. This causes sewer overflows in the 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. Blending of the flows from primary treatment and secondary treatment is sometimes required to manage these high flows and protect the treatment process. 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.

The City of San Mateo is currently under a Cease and Desist Order to eliminate sewer overflows. Additionally, our discharge permit requires us to eliminate the blending that occurs under emergency conditions to manage heavy flows, as mandated by the Regional Water Quality Control Board.

The upgrade and expansion project consists of new liquids treatment process facilities, including a headworks, primary treatment, biological nutrient removal/membrane bioreactor process, biological and chemically enhanced high-rate wet weather treatment, and other plant upgrades, including odor control to serve the new facilities. The high-rate wet weather treatment process consists of a biological contact tank and high-rate clarification to provide secondary treatment of wet weather flows to eliminate blending and meet permit requirements. A new administration building for operations and maintenance staff will house the new main control room and laboratory. These facilities will be designed to provide advanced treatment to 21 mgd and allow the plant to better handle heavy storm events up to 78 mgd.

The City selected the design with the best overall value and ability to meet regulations, both now and into the future. The upgraded and expanded WWTP will provide the necessary level of service, protect the environment, and accommodate future population growth. The treated water will be available for landscape irrigation and other reuse purposes.

The plant’s design will be completed in February 2020. Construction will occur in three phases, with the first phase beginning in summer 2019. For information about the Special Use Permit application, please see the Planning Department’s project site here.

Map of project location.

Project Timeline: Phase 1, Site Preparation, August 2019-Fall 2020. Phase 2, Foundation, Summer 2020-Winter 2021. Phase 3, Facilities, Fall 2020-Summer 2024.

Rendering of Site Preparation

How Will the Plant Improvements Be Built?

The upgrade and expansion work will occur in three phases over approximately 5 years.

Phase 1: Site Preparation

During this phase, the general construction activities will focus on site preparation. The contractor will mobilize to the site to set up construction trailers and bring equipment, demolish outdated or abandoned facilities, excavate, install perimeter shoring, dewater, and implement early power upgrades and stormwater management improvements. This phase is expected to occur over 14 months.
Mitigation measures will be implemented for dust, noise, and traffic control. Equipment and processes to monitor groundwater levels will also be installed. Site preparation begins in August 2019 and includes the following activities:

Rendering of Site PreparationRepresentation of Site Preparation

Map of Construction-related Changes around the Wastewater Treatment PlantConstruction-related Changes around the Wastewater Treatment Plant

Contractor Mobilization

The contractor will mobilize to the site to install temporary fencing to secure the site and setup construction trailers in two locations: on the Detroit Drive parcel and the Dale Avenue parcel. For safety, some changes in public access to the site will be made.


Demolition of outdated facilities will occur during Phase 1. The WWTP will remain fully functional during the demolition and construction of the new facilities. The contractor will employ low-impact construction methods to minimize noise and dust during demolition.


A shoring system will safely hold the work area open during excavation. During this phase, shoring equipment will be brought to the site. A cutter soil mix shoring system was selected to address the challenging soils found at the site. This method cuts the soil while injecting a cement slurry to form soil cement panels.

Groundwater Dewatering

Once the shoring equipment is installed, groundwater will be pumped out of the work area. Water will be removed (dewatering) from inside the shoring to maintain stability and treated prior to discharge to the Bay. This dewatering method will not cause the groundwater levels to change from typical conditions beyond the property line. The figure below shows the effects of dewatering from the construction site and groundwater levels. The contractor will monitor groundwater levels outside of the excavation during construction.


Once the shoring is complete, excavation will begin and the soil inside the shoring will be removed. During this phase, the contractor will monitor groundwater levels. Traffic and dust management measures will be in place.

Illustration of technique for controlling groundwaterExample of Dewatering Method

Phase 2: Building the Foundation

During Phase 2, foundation piles will be driven to prevent uplift (floating) from the high groundwater levels, to support the structure, and to keep it in place during seismic events. Phase 2 is expected to begin in spring 2020, overlapping with Phase 1. Work in this phase is expected to occur over 18 months. Additional community updates will be provided before the start of Phase 2.

Representation of Building the FoundationRepresentation of Building the Foundation

Phase 3: Plant Construction

Phase 3 will involve the construction of the new treatment facilities and the administration building. Street and landscape improvements will be built, and the contractor will demobilize from the site. Phase 3 is expected to begin in fall 2020, overlapping with Phase 2. This phase is expected to take approximately 4 years. Additional community updates will be provided before the start of Phase 3.

Rendering of new Waste Water Treatment FacilityRepresentation of Completed Facility

What Is Happening during Phase 1?

During Phase 1, residents can expect parking and pedestrian/biking closures for safety purposes, additional signage in the area, and general construction activities focused on site preparation.

Site Safety and Security: Road Closures and Parking Restrictions

The following roads and biking/walking paths will be closed for construction beginning in July 2019:

  • The Dale Avenue walking path will be closed for the entire construction period of 5 years. Alternate routes are shown on the map below.
  • Public access will be permanently closed through Detroit Drive and temporarily closed through Joinville Road. Alternate routes are shown on the map below.
  • Public parking on Detroit Drive will be permanently closed.
  • Public parking on Joinville Road will be temporarily closed. Limited public parking will be reopened at the end of construction.
  • The Anchor Road bike/pedestrian path section is expected to be shared during construction Phases 2 and 3 for up to 3 years.
  • A portion of the Anchor Road parking lot (8 parking spaces, including one accessible space) adjacent to Bay Trail with paved access will be reserved for public use during construction. The entire parking lot will be reopened for public use at the end of construction.

Informational signage will be posted prior to changes in road or path accessibility.

The John Lee Dog Park will remain open during construction, but use of the Seal Point Dog Park is encouraged as an alternate because access paths near the treatment plant will be limited during construction.

Map of Route Closures and Alternate Routes during ConstructionRoute Closures and Alternate Routes during Construction

Map of Parking Restrictions and Alternate Parking Facilities during ConstructionParking Restrictions and Alternate Parking Facilities during Construction

Construction Hours

Normal work hours will be Monday through Friday 7 am – 7 pm.

Construction work not is not planned for the weekends, except in special circumstances or if emergency work is required, subject to City approval. There may be exceptions for work during weekend and holiday hours (Saturday 9 am – 5 pm; Sundays and holidays 12 pm – 4 pm).

Demolition and Vibration

Demolition activities may cause minor vibrations. The Contractor will use low-impact construction methods during demolition activities to reduce vibration.

Control of Dust, Dirt, and Noise

Dust and Dirt

Wet-sweeping, covered dump trucks, and rumble strips will be used to reduce dust from construction. The contractor will prevent dust by using an airless sprayer with water for demolition activities. The contractor will use standard dust control measures during construction. Water trucks and sweepers will be dedicated to keeping roadways clean. Trucks leaving with construction and demolition materials will be followed by with a water truck and a sweeper.


The contractor is required to use noise-reduction measures during construction, including installation of a sound wall on the western boundary of the site. Trucks and construction equipment will be adequately muffled.

Photos of dust control techniques

Map of truck traffic routesTraffic Management

A traffic management plan will be in place for the duration of the project. Construction traffic will be routed from the construction site to the nearest designated haul routes (City of San Mateo Truck Route Program).

Residents can expect approximately 130 to 175 trucks in and out of the construction site per day during Phase 1, contributing to less than 1 percent of the road capacity. Truck trips will occur during regular work hours. The contractor will use the designated City of San Mateo truck routes, shown in the figure below. The entrance off Dale Avenue to Staging Area 1 (Dale Ave Undeveloped Open Space) will be limited to placement of construction trailers only; this is expected to occur for a few days only. During specific project activities, access through the residential streets for a limited time may be needed.


California Environmental Quality Act Compliance

A Programmatic Environmental Impact Report (PEIR) was prepared and adopted by the City Council in July 2016. The PEIR included measures to minimize impacts on residents and local ecosystems.

  • The Final Programmatic Environmental Impact Report for this project is available here. Hard copies are also be available at City Hall, the San Mateo Main Library, and the Hillsdale Branch of the San Mateo Public Library.
  • A Final EIR, incorporating written responses to comments received during the public comment period, was certified by City Council on June 6, 2016. The City prepared addenda that were approved on July 16, 2018 (Addendum 1) and April 15, 2019 (Addendum 2).

Construction Hotline

If you have any construction-related questions or concerns, please call the Clean Water Program hotline at (650) 727-6870 or email

What Are the Community Benefits of these Improvements?

In addition to treating wastewater and protecting human health, the plant design incorporates features to serve as a community asset. The front edge of the plant will have improved visual aesthetics and be landscaped with a native grassland to balance the built and natural elements. The grassland will treat the site’s stormwater naturally. This natural system reflects the historical marshes that treat and filter water along the bay.

A meandering pedestrian path will allow the public to walk or bike beside the treatment plant and through the re-created natural area. Educational signs will be located along the path to inform the public of the ecological processes in the landscape and the treatment processes occurring in the plant. The path will cross the bioretention areas and continue to the new administration building, which will have an educational area and serve as the public interface for the plant. From the administration building, the path will continue south to the parks, dog park, and school beyond the plant. The landscape design will provide pedestrian access along the treatment plant with educational demonstrations and connectivity between surrounding neighborhoods and the Bay Trail.

WWTP Entry View from East 3rd Avenue

Public Interface at Administration Building

Rendering of Pedestrian and Bike Path with Landscape Features