Wastewater flows from the east side of the City are transported to the Southeast Wastewater Treatment Plant (SEP), which is located on Phelps Street between Jerrold and Evans Avenues. This plant treats up to 150 million gallons per day (mgd) of sewage to a secondary level. During dry weather, the annual average flow is 65 to 70 mgd, it consists mainly of municipal and industrial sanitary sewage and wastewater and is treated to a secondary level at the SEP. The treated wastewater is then discharged to the Bay through the deep water outfall at Pier 80, immediately to the north of the Islais Creek Channel.

During wet weather, the combined sewage system collects large volumes of stormwater runoff in addition to municipal and industrial wastewater, and the combined wastewater and stormwater flow is conveyed to treatment facilities before eventual discharge to the Bay.

Depending on the amount of rainfall, wet weather flows are treated to varying levels before discharged to the Bay. Up to 150 mgd of wet weather flows receive secondary treatment at the SEP, which can also treat up to an additional 100 mgd to a primary treatment standard plus disinfection. Treated wet weather discharges from the SEP occur through the Pier 80 outfall directly to the Bay or through the Quint Street outfall to Islais Creek Channel. Only wastewater treated to a secondary level is discharged at the Quint Street outfall.

Up to an additional 100 mgd of wet weather flows receive primary treatment plus disinfection at the North Point Wet Weather Facility, located on the north side of the City at 111 Bay Street, which operates only during wet weather. Treated effluent from this facility is discharged through four deep water outfalls, approximately 800 feet from the Bay shore and 18 feet below mean lower low water. Two of the deep water outfalls terminate at the end of Pier 33 and two terminate at the end of Pier 35 on the northeastern Bay shore.

The combined sewer system includes storage and transport boxes which, during wet weather, retain the combined stormwater and sewage flows that exceed the capacities of the SEP and the North Point Wet Weather Facility for later treatment. When rainfall intensity results in combined flows that exceed the total capacity of the SEP, North Point Facility, and the storage and transport structures, the excess flows are discharged through 29 combined sewer overflows (CSO) structures located along the City’s Bayside waterfront from Fisherman’s Wharf to Candlestick Point. Discharges from the CSO structures, consisting of about 6 percent sewage and 94 percent stormwater, receive “flow-through treatment,” which is similar to primary treatment, to remove settleable solids and floatable materials. Wet weather flows are intermittent throughout the rainy season, and combined sewer overflow events vary in nature and duration depending largely on the intensity of individual rainstorms.