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 1000+ pipes.
Catch Basins: Gateways into the Sewers
Catch basins and storm drains are designed such that most litter and leaves get "caught" and do not enter the sewer system. When too much debris builds up, the catch basins can become clogged and can flood streets and homes.
Catch basins are the semi-circular grids that 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: Drain to Bay and Ocean
Storm drains direct stormwater directly to the Bay or Ocean with minimal treatment. That's why it's especially important that only rain go down the storm drain!
Storage/Transport Boxes: Massive 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 have three functions:
- Capture. At the City’s perimeter, the storage/transport boxes catch the combined stormwater and sewage as it overflows the sewer system, but before it reaches the shoreline of the Bay or Pacific Ocean.
- Storage. The storage/transport boxes hold stormwater and sewage for later treatment at wastewater treatment plants. (Total storage capacity is approximately 200 million gallons
- Treatment. The storage/transport boxes provide treatment consisting of settling and screening of floatable materials inside the boxes. The treatment is equivalent to primary treatment at one of our wastewater treatment plants.
What happens if the amount of rainwater exceeds storage capacity?
Generally, only during the most prolonged intense rainstorms do the storage boxes completely fill up with water. 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. Three important things to remember:
- Discharges are mainly rainwater. In fact, studies have shown that it is 94 percent stormwater!
- The boxes are designed to double as treatment tanks.
- 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!
Next Step: Making Dirty Water Clean Again