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Secondary & Tertiary Treatment
Secondary Treatment
Aeration
Aeration is a biological process that produces 95% clean water by pumping air into the flow. The oxygen-rich (aerobic) environment nurtures the growth of naturally-occurring 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 then sent back to the aeration tanks to repeat the process.

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 treatment plants located in South San Francisco Bay require tertiary treatment because their water discharges into shallow waters with little tidal action.

The "advanced" treatment is needed to ensure that our water meets state and federal water quality regulations. Water is 99% pure after tertiary treatment. Following filtration, liquid hypochlorite solution is used to purify the water further. Before being 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 80% 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.

The remaining 20% of the treated water is sent to South Bay Water Recycling and used to irrigate food crops, parks, schools, golf courses, street medians, and business park landscaping.