Why are they called catch basins?  Aren't they storm drains?

There is a big difference.  Catch basins and storm drains both capture rainwater and street runoff.  However, with storm drains, trash, debris and oil wash into storm sewer pipes which then empty into a local water body with little or no treatment.

Catch basins "catch" debris so it doesn't enter the sewer system.  Look at the catch basin diagram.  Water does not enter the City's sewer system until the water level rises above the exit pipe level.  The difference in pressure passively forces the water into the connecting sewer main.  

Heavier debris that doesn't float accumulates and eventually clogs catch basins.  Our crews will now have to "vacuum" the catch basin.   Not an easy job considering we have 23,000 catch basins in the city.  That's why not littering and cleaning your neighborhood is important.  Give us a hand and find out how to adopt-a-catch basin!



Why are there colored dots next to the catch basins?  What about mosquitos breeding in the standing water?

The water stored in catch basins provides a perfect place for mosquitos to breed.  To prevent breeding of mosquitos and the spread of the West Nile Virus, a squad of bike messengers ride around the city to apply a safe, non-toxic water-soluable chemical that stops the mosquito growth cycle.  When applications are made to any one of the 23,000 catch basins, a non-toxic, water-soluable colored dot is placed to note 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!