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Wastewater Treatment Process

Nature purifies water in lakes, streams and rivers. The Sanitation Districts’ Water Reclamation Plants replicate nature in concrete tanks using state-of-the-art technology. Wastewater treatment achieves in approximately 12 hours what it takes nature months to accomplish.

When wastewater leaves a home or business it first flows into local sewers that are owned and maintained by the cities and/or the county. These local sewers feed into approximately 1,400 miles of large trunk sewers that are owned and maintained by the Sanitation Districts.

The trunk sewers deliver the wastewater to 11 Wastewater Treatment Plants. The entire system is set up to flow by gravity as much as possible to save pumping costs. In some low-lying areas, the wastewater must be pumped to a higher elevation so it can flow to the treatment plants. This is accomplished by pumping plants located throughout the service area.

Primary Treatment
In nature, when wastes first enter a river, the heavy materials settle to the bottom while the floatable and dissolved materials flow downstream with the current. In the treatment plant, the wastewater is first piped into primary settling tanks where the solid materials settle to the bottom, floatable materials are skimmed from the top, and both are removed for treatment.

Secondary Treatment
In nature, bacteria in the river feed on the organic materials while breathing the dissolved oxygen in the water.
In secondary treatment, microorganisms in aeration tanks feed on the remaining organic materials and air is bubbled through the water to provide dissolved oxygen. The microorganisms are removed from the water in secondary settling tanks and returned to the aeration tanks to begin the process again.

Tertiary Treatment
In nature, water filters through earth and sand, eventually soaking into the groundwater. In the third (tertiary) stage of treatment, filters made of coal, sand and gravel remove nearly all of the remaining suspended materials.

In nature, the sun's ultraviolet rays kill harmful microorganisms. Likewise, in the treatment plant, a disinfectant is used to kill any remaining bacteria, viruses or other microorganisms. The water then leaves the plant either for reuse or discharge to the river.

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Water flows from the San Jose Creek Water Reclamation Plant into the San Gabriel River.

Large trunk sewers in the Joint Outfall System.

Photograph of completely covered primary settling tanks.

Diagram shows heavy materials in primary treatment tanks settle to the bottom. (View Video)

Photograph shows open secondary treatment tanks (tanks at the Joint Water Pollution Control Plant in Carson are covered).

Diagram illustrates microorganisms (mainly protozoa and bacteria) in tanks breathe injected air and feed on organic materials. (View Video)

Photograph shows tertiary filter.

Diagram shows filtration system that removes nearly all remaining suspended materials. (View Video)

Photograph shows reclaimed water in the process of leaving the treatment plant.

The result is high quality reclaimed water that is available for groundwater recharge and reuse.