Stormwater enters the combined sewer system through building roof drains or the thousands of catch basins along the street. Since most of San Francisco is paved over, the rainwater that falls from the skies usually ends up in our sewers.
This is a waste of a pretty clean water source. That's why we have a Stormwater Management Program that develops environmentally green policies and projects for people to reduce overloading of the sewer system with stormwater, reuse stormwater for non-drinking uses, and green the city.
Catch Basins: The Stormwater Gateways into the Sewers
Only Rain Down the Drain! Catch basins 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 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 23,000 catch basins. They are the main entryway for rainwater and street runoff into our combined sewer system.
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