Treatment Process

Headworks
Primary Treatment
Digesters
Secondary Treatment
Tertiary Treatment
Outfall Channel

After wastewater enters the San Jose/Santa Clara Water Pollution Control Plant (Plant), it undergoes a three-step treatment process to remove solids, pollutants, and pathogenic bacteria.

Machinery and gravity separate solids from the wastewater. Added bacteria clean the water pollutants before the flow enters the advanced filter process. The treatment process produces 95 percent clean water that is discharged into the Bay.

Headworks
Original structure
The original headworks structure has operated continuously for over 50 years. Large bar screens remove rags, sticks, rocks and other debris that could otherwise clog Plant machinery. The debris is trucked to a landfill.

The original structure system has never been taken out of service for maintenance. If this system failed, it could result in a raw sewage spill onto Plant grounds. Berms and other protections surround the system to prevent sewage from entering the Bay.

New structure
The new headworks structure is in its final testing phase. The system was built to avoid accidental sewage spills due to mechanical malfunction, earthquakes or other unexpected events.

Primary Treatment
This 24-hour physical process removes about 50 percent of wastewater contaminants. In large tanks, the flow is slowed to allow gravity to separate large particles. This process mimics creeks and rivers where sediments settle to the bottom.

Fiberglass bars, or flights, move across the tank surface to skim off fats, oils and grease. Flights gradually rotate from the top to the bottom and the settled solid particles are moved into the digesters.

Digesters
The pollutants and solid material removed during all three treatment steps are separated from the liquid flows in digester tanks.

It takes 25 to 30 days for anaerobic bacteria to stabilize unwanted material and reduce the pathogens and other disease-causing organisms in the solids. Anaerobic bacteria operate best without oxygen and at about 98 degrees.

Digesters produce methane gas, which meets 35 percent of the Plant's energy needs.

Secondary Treatment
Aeration
Aeration is a biological process that produces 95 percent clean water by pumping air into the flow. The oxygen-rich (aerobic) environment nurtures growth of naturally-occuring aerobic bacteria.

Clarifiers
After aeration, the flow is piped into clarifiers where the aerobic bacteria settle to the bottom.

Treated water remains in the clarifiers for one to three hours. Mechanical arms move slowly around the tank to collect scum and bacteria for the digesters. Some bacteria are sent back to the aeration tanks to begin the process again.

Tertiary Treatment
Tertiary treatment is the third and final process. During tertiary treatment, wastewater flows through several filter beds composed of gravel, sand and anthracite coal.

This step is also known as "advanced" treatment because few communities undertake it. The three South San Francisco Bay treatment plants need tertiary treatment because their water discharges into shallow waters with little tidal action.

The extra "advanced" treatment cost is needed to ensure that our water meets state and federal water quality regulations. Water is 99 percent pure after tertiary treatment.

Following filtration, gaseous chlorine is used to further purify the water. Before discharged into the Bay, a second chemical is added to neutralize the chlorine, which could otherwise harm aquatic life.

Outfall Channel
After tertiary treatment, about 90 percent of the treated water is piped to the outfall channel. From here, it flows to Artesian Slough, through Coyote Creek, and eventually into the Bay.

Many birds and fish are found at the outfall channel, including stripers, black bass, and salmon.