Combined Sewer Systems

Combined Sewer Systems

San Francisco (like Detroit and New York City) has a combined sewer system.

This means that the stormwater (rainfall) that flows into the storm drains is combined with wastewater from our toilets, sinks and showers.

This combination of stormwater and wastewater can be extreme.  And as the climate changes and storms are measurably increasing in strength, this extreme weather is becoming the norm.  Combined sewer systems are effective during steady rainstorms, but can be easily overwhelmed by extreme precipitation events.

Removing pavement supports the combined sewer system because the stormwater is captured into the soil before it reaches the stormdrain.  Urban gardens support more than just local wildlife.  Urban gardens also restore local watersheds and reduce pressure on the City of San Francisco’s combined sewer system.

The SF Public Utilities Commission has a volunteer program called Adopt a Drain, where residents in SF can adopt a stormdrain near their home and help to keep it free from debris so that stormwater can flow easily in the sewer.  Check out the program and support the SF Bay by being a good neighbor (and remember to bring your broom).

Capturing Stormwater

When large portions of urban areas are heavily paved-over the combined sewer system can become stressed, especially during large storms.  And as we continue to increase carbon dioxide and other greenhouse gases into the atmosphere, storms and most weather systems are becoming more and more extreme.

In California’s Mediterranean climate, precipitation is heaviest in the winter months.  When these winter storms become extreme, this can create a huge flow of water that the City’s combined sewer system cannot effectively clean.  It is then, during heavy storms, that the stressed sewer system releases untreated (contaminated) water into local waterways.

These large winter storms in San Francisco sometimes force the SF wastewater treatment plant staff to discharge untreated water into the SF Bay or the Pacific Ocean.  By removing pavement we capture stormwater in the ground, reducing  pressure on San Francisco’s combined sewer system.

There are two wastewater treatment plants in San Francisco.  80% of all the waste from all of San Francisco’s nearly 1 million residents heads to the SFPUC’s Southeast Treatment Plant located in the Bayview neighborhood.  The other 20% flows to the Oceanside Treatment plant adjacent to the SF Zoo.

As the City’s population grows, important green infrastructure improvements to the sewer system are happening currently (and will continue for many years to come) all the while improving services and environmental quality in San Francisco.

What is a Watershed?

A watershed is a basin-like landform that captures stormwater and is divided by peaks and ridges which collect into streams, rivers, lakes and aquifers eventually leading to the ocean.

In San Francisco all watersheds eventually lead to the SF Bay or to the Pacific Ocean.  Climate Action Now! has removed pavement in almost every watershed in the City!  Be a part of the progress and remove pavement at your home or school today to support your local watersheds.

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