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Treating the Solids

From Sludge to Biosolids...

Let's learn how we treat solids extracted during the primary and secondary treatment processes. In the beginning, the extracted solid material is called "sludge." The treatment process transforms sludge into biosolids, a recycled product that can be used as a fertilizer replacement. San Francisco produces approximately 65,000 wet tons of biosolids every year (1 ton = 2000 pounds).


Step One:  Thickening  
Gravity Belt Thickener Sludge separated from wastewater during primary and secondary treatment still contains a lot of water. Gravity belt thickeners are used to optimize the moisture level of the sludge for the treatment process; we don’t want too much or too little water. After the sludge is “thickened” by the gravity belt thickeners, it is sent to the digesters for treatment.

Step Two: "Digestion"
egg digesters in the moonlight The solids are sent to big heated tanks called digesters for treatment. The digesters may be highly engineered, but they work in a way similar to a cow’s digestive system. Digesters at our Oceanside Treatment Plant are kept at a near constant 131 degrees Fahrenheit for 25 days, then 100 degrees Fahrenheit for another 8 days. At our Southeast Treatment Plant digesters are kept at a near constant 95 degrees¬¬ for 15 days. During this time the microorganisms are killing harmful bacteria and other pathogens, breaking down pollutants, and producing methane gas, also called biogas. Biogas is a 100% renewable alternative to fossil-fuels (see step two-and-a-half below).



The digestion process is completely transformative, and in the end we have treated biosolids. Biosolids are full of carbon and nutrients, which makes them a perfect soil amendment and replacement to synthetic fertilizer. San Francisco’s biosolids have been used by local farmers and ranchers for decades to fertilize pasture lands and hay fields. Read our biosolids fact sheet to learn more.

Our work doesn’t just end with producing the biosolids. We are audited for our Biosolids Management System (BMS) Program by a third-party, allowing us to join a leading group of utilities who are committed to continuous improvement of biosolids management. The Biosolids Management Program is intended to serve as a model for continuous improvement in the areas of environmental performance, regulatory compliance, quality management practices, and relations with those impacted by our practices of biosolids management and other interested parties and stakeholders. For more information, please read the complete Biosolids Management System Manual.

windmills in a cornfield

Plans for the future:
As part of the Sewer System Improvement Program, we will be making major upgrades and replace and relocate the biosolids digester facilities at our Southeast Treatment Plant.
Sign up for our eNewsletters to stay informed on our wastewater programs and projects. For more information or questions, contact us at or 415-554-3289.

Step Two-and-a-half: Energy

digesters at dusk
As mentioned in step two above, during digestion, the anaerobic bacteria release methane gas (also called biogas) as a byproduct. Instead of letting this great energy source go to waste, we recycle the methane as a 100% renewable fuel to generate electricity and hot water. The hot water is used to keep the digesters heated.


Congratulations! You've completed your education on how San Francisco cleans wastewater and solids!

Don't you want to see the treatment processes in person? Well, you’re in luck because we offer free monthly tours of our Oceanside and Southeast Treatment Plants. Invite your family and friends and make it a weekend field trip! Sign up to take a treatment plant tour today!

Last updated: 5/31/2019 8:30:32 AM