ST. HELENA, CA, UNITED STATES, February 18, 2026 /EINPresswire.com/ — Thompson Builders Corporation (TBC) has successfully completed construction of the City of St. Helena’s new $18.8 million wastewater treatment facility, delivering a modern, state-of-the-art membrane bioreactor (MBR) system that replaces the City’s original 1960s-era plant and brings the facility into full regulatory compliance.
Constructed for the City of St. Helena, the project was mandated by the San Francisco Bay Regional Water Quality Control Board following a 2016 order requiring the City to address discharge deficiencies impacting the Napa River.
A primary feature of the project delivered by Thompson Builders was the installation of an advanced membrane bioreactor (MBR) treatment system. This pre-manufactured, packaged system has an average dry weather flow capacity of 0.5 million gallons per day and treats approximately 500,000 gallons of wastewater daily. The MBR system integrates biological treatment and membrane filtration into a single, compact process, producing significantly higher-quality effluent than conventional treatment systems.
After wastewater is processed through the MBR system, treated effluent is conveyed to a membrane filtration clearwell and an above-ground 20,000-gallon effluent storage tank. The water is then disinfected through a closed-vessel ultraviolet (UV) disinfection system prior to discharge or reuse.
High-quality tertiary-treated effluent is discharged to the existing Napa River outfall or reused for irrigation, including distribution to Napa Valley vineyards. Treated water is also reused on site for plant operations, allowing the facility to meet stringent treatment standards within a compact footprint while reducing reliance on groundwater.
“This project transforms the way the City of St. Helena treats its wastewater, brings it into full compliance with applicable regulations, and produces water beneficial to the environment,” said St. Helena Mayor Paul Dohring.
The additional scope of work performed by Thompson Builders included furnishing all labor, materials, equipment, and services required to construct, test, and commission the new wastewater treatment facility. The work included installation of the packaged MBR treatment plant; packaged sludge dewatering facilities; influent pump station; UV disinfection system; membrane filtration clearwell; effluent storage tank; pond retrofits; yard piping; earthwork; site paving and drainage; structural concrete slabs; chemical distribution and storage improvements; domestic water systems; electrical service upgrades; instrumentation and controls; SCADA integration; flow bypassing; and general site improvements.
Thompson Builders is proud to deliver water and wastewater infrastructure projects that support environmental protection and regulatory compliance throughout California and looks forward to continuing work on complex and innovative projects such as this.
Taylor Harrison
Thompson Builders Corporation
+1 415-203-0333
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