The Sewerage Agency of Southern Marin collects and treats sewage for the city of Mill Valley, Tamalpais Community Services District, Almonte Sanitary District, Alto Sanitary District, Homestead Valley Sanitary District and Richardson Bay Sanitary District. It is a Joint Powers Agency formed in 1979 to consolidate the wastewater collection, treatment, water reclamation and disposal needs of about 28,000 residents in Southern Marin County.
Each entity owns, operates sand maintains its own sanitary sewer system. These sewer systems connect to the Sewerage Agency of Southern Marin's main sewer lines, carrying wastewater to the wastewater treatment plant. Each entity also has one seat on the agency's governing board of commissioners, which meets on the third Thursday of January, March, April, May, July, September and November at 7pm at 450 Sycamore Avenue.