Outfall Rehabilitation Projects

What is a Combined Sewer Discharge (CSD)?

San Francisco is the only coastal city in California with a combined sewer system that collects and treats both sewage and stormwater in the same network of pipes and treatment facilities. This combined system means San Francisco treats stormwater runoff when other coastal cities in California would have stormwater - and the debris and pollutants it picks up - flow untreated into nearby bodies of water. 

In San Francisco, the debris and pollutants in stormwater runoff are treated to the same high standards that the SFPUC uses to treat sewage from buildings before being discharged into the San Francisco Bay or Pacific Ocean. However, during intense storms the transport/storage boxes, huge underground rectangular tanks that surround the City, can reach capacity. When this occurs, it is necessary to discharge partially treated wastewater through outfall structures around the City.

Generally, only during the most prolonged intense rainstorms do the storage boxes completely fill up. 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. Studies have shown that discharges are 94 percent stormwater. 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.

These discharge events are monitored and reported in a timely manner. Learn more about our Beach Monitoring Program and see the interactive map of outfall structures.