Catch Basins and Storm Drains
Stormwater enters the combined sewer system through roof drains on buildings or the thousands of catch basins along the street. Sanitary sewage flows from homes and businesses into sewer lateral pipes to sewer mains and through a network of over 1,000 pipes. We own and operate about 1,900 miles of sewer mains and laterals right under the street. End to end, it would stretch from here to Colorado (and back) and over 300 miles are more than 100 years old!
A few areas in San Francisco are serviced by a separate sanitary sewer system, which is designed to transport just sewage (and not stormwater) to the treatment plant. Storm drains in these areas lead to the bay or ocean. Pursuant to the separate sanitary sewer permit requirements, we have developed a Sewer System Management Plan (SSMP) which describes programs to provide proper and efficient management, operation, and maintenance of the City’s sanitary sewer system.
Catch Basins: Gateways into the Sewers
Catch basins are the semi-circular grids you see at almost every street corner throughout the City. San Francisco has close to 25,000 catch basins. They are the main entryway for rainwater and street runoff into our combined sewer system, where stormwater combines with wastewater from homes and businesses in the same set of pipes, and is transported to the treatment plant for treatment.
Storm drains direct stormwater directly to the Bay or Ocean with minimal treatment. Storm drains require greater public awareness due to the potential for pollutants, such as motor oil, pesticides, and trash, to be washed into the Bay. As of June, 2016 six storm drains in the SF Mission Bay neighborhood have been adorned with special educational murals created by a local artist, drawing awareness to the impact pollutants can have on the environment.
Storage/Transport Boxes: Underground Storage Tanks Around the City
The storage/transport boxes are huge underground rectangular tanks or tunnels that surround the City. They are about 50' deep and as wide as the streets running along the Embarcadero and Great Highway.
Storage/transport boxes capture combined stormwater and sewage as it overflows the sewer system, but before it reaches the shoreline of the Bay or Pacific Ocean. The boxes are able to hold approximately 200 million gallons of stormwater and sewage for later treatment at wastewater treatment plants. The storage/transport boxes provide treatment consisting of settling and screening of floatable materials, which is equivalent to primary treatment at one of our wastewater treatment plants.
Generally, only during the most prolonged intense rainstorms do the storage boxes completely fill up. Instead of allowing the excess water to backup through the sewers into homes and streets, water is discharged into either the Bay or Ocean through one of 36 discharge points. Studies have shown that discharges are 94 percent stormwater. On average, only 10 discharges happen each year. Before the storage/transport boxes were constructed, discharges happened more than 80 times a year without any treatment.
How do we make dirty water clean again? Next Step: Treating the Liquid Flows