The first residents of Ross Valley held large tracts of land, allowing for acceptable use of septic tank systems. Upon completion of the North Pacific Coast Railway in 1875, some of the large land holdings were subdivided allowing a rapid influx of new homeowners. As a result of the increased population and failing onsite septic tank systems, an election was held in 1899 to create Sanitary District No. 1 as a coordinated solution for sanitary sewers for the communities of Fairfax, San Anselmo, Ross and Kentfield, making Sanitary District No. 1 California's first sanitary district and Marin County's first regional agency.
The District's first project was a 10-inch trunk line from Fairfax to Greenbrae, discharging wastewater into the deep-water slough of Corte Madera Creek near what is now Bon Air Shopping Center. Continued growth in Ross Valley made it necessary to discontinue discharging untreated wastewater into the Bay. In 1922, a bond election approved $450,000 for construction of 7.5 miles of trunk sewer line and a wastewater treatment facility utilizing Imhoff reduction tanks - one of the first wastewater treatment facilities in California. The 7.5 miles of trunk sewer line remained in service until 1985 when it was replaced because of old age and lack of capacity. In 1948, the Greenbrae Pump Station was built at the site of the Imhoff reduction tanks. In 1984 the Greenbrae Pump Station was replaced at a cost of over $2 million dollars.
In 1945, the volume of wastewater being generated required construction of a trickling filter wastewater treatment facility providing secondary treatment. The City of Larkspur and Sanitary District No. 2 joined Sanitary District No. 1 in the construction of this facility which was completed in 1948 at the site now known as 2000 Larkspur Landing Circle. The Larkspur Landing treatment facility was expanded several times over the years and in 1962 the treatment capacity was increased from 3 to 4.5 Million Gallons a Day. Operations were closely monitored by the State Water Quality Control Board and engineering reports reflect that operations were excellent with biochemical oxygen demand removals averaging 90 percent.
The District continuously operated the Larkspur Landing treatment facility from 1948 through 1984 when it was decommissioned after the startup of the Central Marin Sanitation Agency (CMSA). CMSA resulted from the East Marin/Southern Sonoma Study which examined alternatives for advanced treatment and deep-water discharge for seventeen wastewater agencies in Marin and southern Sonoma Counties in response to environmental concerns and deep-water discharge requirements of the mid-1970's Clean Water Quality Act. As a result, CMSA, a joint powers authority (JPA), was formed by Sanitary District No. 1, San Rafael Sanitation District, Sanitary District No. 2 and the City of Larkspur for the purpose of constructing and operating a wastewater treatment facility with deep-water discharge to San Francisco Bay. Construction of the CMSA treatment plant, interceptors, and related facilities totaled over $84 million.
Over the years, Sanitary District No. 1 has provided wastewater treatment and/or collection service to the following districts and agencies:
- From completion of the Larkspur Landing facility in 1948 until startup of CMSA in 1985, the District, under contract, provided wastewater treatment and disposal for Sanitary District No. 2 (Corte Madera) and the City of Larkspur.
- In 1993 the Larkspur Sanitation Area was annexed to Sanitary District No. 1 at the request of the City of Larkspur, and the District took over ownership and maintenance of Larkspur's wastewater collection system and replaced Larkspur in the contract with the County of Marin for wastewater collection maintenance in Murray Park Sewer Maintenance District