Our Combined Sewer

San Francisco is the only coastal city in California with a combined sewer system that collects and treats both wastewater and stormwater in the same network of pipes. Water flows through most of the sewers using gravity. Our hilly geography comes in handy reducing the energy and maintenance costs associated with mechanical pumping.

Stormwater enters the combined sewer system through building roof drains or the catch basins along the street and gets treated at our plants just like the wastewater that goes down your drain. One of the City’s challenges is that the watershed area is mostly paved or has hard surfaces, so rain has no place to go other than the City’s combined sewer system before being discharged to the Bay or Ocean.

Two ways we are managing stormwater

As part of the Sewer System Improvement Program, we will build innovative green infrastructure projects that use sustainable methods to help slow down or reduce the amount of stormwater going into our sewer system during rainstorms. These are stormwater management strategies that take advantage of natural ecological processes to treat stormwater runoff at its source and keep it from entering the sewer system. 

There is also the Stormwater Management Program to develop environmentally green policies and projects for people to reduce overloading of the sewer system with stormwater, reuse stormwater for non-drinking uses, and to green the City.

Why do we need to treat stormwater?

Cleaning stormwater is important because of the street pollutants that wash into the sewer system. Imagine all the motor oil, pesticides, metals, and other street litter that you see on the street. They all go into the sewer system when it rains. The SFPUC is able to treat a higher percentage of flows than many other municipalities. The benefits include cleaner discharges to the Bay and Ocean, improved compliance with regulatory requirements, and most importantly, the continued protection of public health.

Get involved and learn how to harvest or capture rainwater in barrels. You’ll save money, conserve our drinking water supply and reduce the burden on our sewers. Find out more about our rainwater harvesting programs. 

Learn how we capture and store the wastewater. Next Step: Wastewater Collection System.