Today's treatment plant is on the site of the very first treatment system for the area. A large septic tank served the early population - which grew drastically as survivors of the 1906 San Francisco earthquake and fire sought refuge in Daly City. In 1907, the Spring Valley Water Company, fearing pollution of its water supply, permitted the septic tank overflow to be diverted to the Vista Grande Tunnel for disposal to the Pacific Ocean.
The Vista Grande Tunnel was built in 1895 to handle storm water overflow from Lake Merced. The brick, egg-shaped tunnel is still used today to transport treated wastewater from the City's effluent pipeline to the ocean outfall at Thornton Beach.
The area's population took another sharp jump after World War II, as returning veterans bought homes for their families in Broadmoor Village and Westlake subdivisions. To serve the wastewater collection and treatment needs of this growing area, the San Mateo County Board of Supervisors created the North San Mateo County Sanitation District on January 16, 1951. In 1953, voters approved a $600,000 bond issue to finance construction of a primary wastewater treatment plant at the southerly end of Westlake Park. The plant went into operation in August of 1955, serving a population of 30,000.
Artichoke fields and hog ranches continued to give way to houses as St. Francis Heights and Serramonte were completed. In 1975, with funding from the Clean Water Act Grant Program, the plant facilities were upgraded to full secondary treatment, using a pure oxygen and activated sludge process.
On July 1, 1985, the North San Mateo County Sanitation District became a subsidiary district of the City of Daly City. At that time, the plant was nearing its treatment capacity. As interim measures, the City formalized a Water Conservation Program and instituted a moratorium on new sewer connections. Meanwhile, planning began for Daly City's most ambitious public works project ever undertaken.
In 1989 the plant was expanded to equalize the peak flows and provide additional capacity. The expansion featured a unique underground primary treatment and flow equalization system. The new underground facilities increase the plant's secondary capacity from 8 to 10.3 million gallons per day.
The City's collection, treatment and disposal systems serve the majority of the residents of Daly City, along with the Broadmoor Village, a portion of the Town of Colma, the Westborough County Water District in South San Francisco, and the San Francisco County jail in San Bruno.