Our wastewater system includes very large transport/ storage boxes along the perimeter of the City, a unique feature designed to help collect and treat storm water. Water transported in these storage boxes goes through a process similar to the first stage of what happens at our treatment plants – solids settle to the bottom, while floating materials are screened through a baffle. The largest boxes can store almost 50 million gallons of water.
When the boxes are completely filled in heavy rainstorms, the excess water is discharged into the Bay or Ocean. This is referred to as a “combined sewer overflow.” The discharge is about 90 percent rain water and 10 percent sewage. Beaches are posted immediately following any overflow event as a safety precaution while bacteria samples are analyzed.
Before the 1970s, the City averaged almost 80 overflows a year; now we are down to 10 or fewer. While the SFPUC is in compliance with all regulations, the master plan will consider further reducing the total number of overflows.
