Stormwater enters the sewer system through building roof drains or the thousands of catch basins along the street. Since most of San Francisco is paved over most of the rainwater that falls from the skies ends up in our sewers. This is a waste of a pretty clean water source. This is why the SFPUC's has a Stormwater Management Program developing environmentally green policies and projects for people to reuse stormwater. Together we can reduce overloading the sewer system with stormwater, reuse water for non-potable uses like garden irrigation and toliet flushing, and green the city.
What is a catch basin?
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.
Keep your catch basins litter free! Catch basins are designed so that Most of the litter and leaves that get into the catch basins do not enter the sewer system. Instead they build up and clog the catch basins that during rainstorms may result in flooding.
Click here for a diagram of what a catch basin looks like from the inside.
Why are there colored dots next to the catch basins?
The water stored in catch basins make a great place for mosquitos to breed. To prevent breeding and the spread of the West Nile Virus, a squad of bike messengers kill the mosquito eggs by applying a safe, non-toxic water-soluable chemical to all 23,000 catch basins. When applications are made, a non-toxic, water-soluable colored dot is placed to mark the application.
Why do building roof drains connect to the sewers?
In the past San Francisco building codes made all roof drains connect to the City's sewer system. Not anymore. In 2005, the code was updated to allow for roof drain disconnection. This simple change allows someone to divert stormwater to a storage device like a rainbarrel or cistern reducing the burden on the City's sewer system. Even more, the rainwater can be used for irrigation of gardens, flushing toliets - saving water and money. Learn more about the SFPUC rainwater harvesting program here!
Storage/Transport Boxes
The storage/transports are huge underground rectangular tanks or tunnels that ring the City like a moat (see system map). 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 the runoff. At the City’s perimeter, the storage/transports catch the combined storm water and sewage as it overflows the sewer system but before it reaches the shoreline of the Bay or ocean.
- Storage. The storage/transports hold storm water and sewage for later treatment at wastewater treatment plants. (Total storage capacity is approximately 193 million gallons)
- Treatment. The storage/transports 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 fill up with water. Instead of allowing the excess water to flood homes and streets, the accumulated water is discharged into either the Bay or Ocean. Two important things to remember:
1) Discharges are mainly rainwater. In fact, studies have shown that it is 94 percent stormwater!
2) Don't forget that there is treatment in the boxes too! Read above for more information.
Any excess water discharged during extreme rain events are treated to the equivalent of primary treatment.
- Such exceedances occur an average of one to ten times per year depending on location. These discharges consist of approximately 6% sewage and 94% storm water, and have received primary-level treatment. Most rainstorms do not fill the storage/transports and all the wastewater is held for later treatment. The solids retained in the storage/transports are flushed to the treatment plants after storms.
What are storage boxes?
