Treatment Process


An average of 10 million gallons per day of wastewater flows through the Ryder Street treatment plant on its way to San Francisco Bay.  During large storms, the number can hit 60 million gallons. What does that mean? Each year we treat about 4 billion gallons of wastewater so that your health and property are protected, and the Bay is safe.
We provide this service for more than 120,000 residents in the greater Vallejo area. Every day, wastewater from showers, toilets, sinks, garbage disposals, dishwashers, and washing machines flows through sewer pipes to the District’s treatment plant. Our job is to remove all the solids and suspended sediment from the wastewater and then disinfect it so that it is safe.
It is important to know that treatment plants are designed to remove human waste, toilet paper, and food particles from wastewater. Treatment plants are not able to completely remove contaminants from medicine, paint, oil, and other pollutants. These need to be recycled, put in the garbage, or disposed of at household hazardous waste facilities where appropriate.

Treatment Process

The treatment process begins as raw wastewater passes through bar screens and grit chambers to remove rags, plastic, and other large items. TreatmentProcess1.png
The water trickles over plastic honeycomb in the biotowers, where microorganisms eat the dissolved organic matter. The water becomes clearer as microorganisms settle to the bottom of the secondary clarifiers. The organisms are pumped to the blend tank or aeration basins to be reused.​
The clarified water flows through disinfection chambers, where chlorine is added to kill off remaining organisms. Sodium bisulfite is then added to neutralize the chlorine. Finally, the treated wastewater is pumped to the Carquinez Strait.
At each step in the process, we conduct chemical and biological analyses to ensure that the cleaning processes are successful.​ TreatmentProcess5.png
After extracting solids from the wastewater, they are treated in blend tanks with a lime slurry that raises the pH and destroys harmful organisms. Excess water is removed in a belt press, and the resulting biosolids are taken to Tubbs Island farm, where they are used as a soil amendment and fertilizer. TreatmentProcess6.png


Biosolids are the nutrient-rich natural by-product of wastewater treatment. They are produced by removing the organics from municipal sewage – the majority of which comes from residential homes. The District currently produces about 20,000 cubic yards of biosolids each year.

Most treatment plants in the United States pay landfills to dispose of their biosolids. Since 1978, the District has been using its biosolids as a soil amendment on Tubbs Island Farm, located north of the San Francisco Bay adjacent to intertidal marshlands and a large nature preserve. Tubbs Island provides habitat for the endangered Salt Marsh Harvest Mouse and other native wildlife. The farm primarily grows wheat and oat hay, providing feed for chickens and cattle.

Biosolids produced at the District’s wastewater treatment plant are hauled by special truck to Tubbs Island and used as fertilizer to help crops. This practice is beneficial in two ways: the project results in substantially lower costs, and the addition of biosolids has promoted better soil conditions for farming.

The U.S. EPA recognized this innovative use of biosolids in 1990, rewarding Vallejo Sanitation & Flood Control District with National First Place as the most outstanding project involving land application of biosolids in the United States.

Looking Ahead

The District has joined a coalition of wastewater treatment plants around the Bay Area that are studying the development of a regional approach to sustained energy using biosolids as a resource. Biosolids contain latent energy that can be harnessed either through the capture and combustion of methane gas or through direct combustion. For current information on this project, visit