SFPUC Home    Wastewater (Sewers)    Where Does IT Go?    Treating the Solids

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". Once treated, sludge is transformed into biosolids. San Francisco produces 80,000 wet tons of biosolids every year (1 ton = 2000 pounds).

Step One: Thickening

Sludge separated from wastewater during primary and secondary treatment still contains a lot of water. Too much water makes the sludge more difficult and expensive to treat. At Oceanside and Southeast Treatment Plants we use a Gravity Belt Thickener (pictured left), to "thicken" the sludge.

Step Two: "Digestion"

"Digestion?" As in how we digest our food? Actually, the solids digesting process is more similar to that of a cow, because of the many different microorganisms utilized.  The thickened sludge is pumped into one of several enclosed tanks called digesters. Here, it’s mixed for a minimum of 15 days and cooked at a constant temperature of 95 degrees Fahrenheit (98 degrees Fahrenheit is temperature of the human body).  The combined heat and lack of oxygen excite anaerobic bacteria in the sludge. (Anaerobic bacteria are bacteria that don’t like oxygen, as opposed to the oxygen-loving "aerobic" bacteria used in the secondary treatment process.)  Anaerobic bacteria break down remaining organic material in the sludge.

15-25 days later...after a lot of heat and a lot of hungry bacteria...the fully treated sludge has become biosolids. San Francisco reuses 100% of our biosolids on land application and non-food crops. Visit our biosolids page to learn more.

Step Two-and-a-half: Feeling a little gassy?

Okay, we forgot one small, but important detail. During digestion, the anaerobic bacteria release methane gas as a byproduct. Instead of letting this great energy source go to waste, we recycle the methane to generate electricity and hot water. The hot water is used to keep the digesters heated. In all, the combined energy output accounts for 33% of the treatment plant's power needs.

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!

Back to Top