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Wastewater Treatment Process

Nature purifies water in lakes, streams and rivers. The Sanitation Districts’ Water Reclamation Plants replicate nature using technology to speed up the process. What usually takes nature months to accomplish, the Sanitation Districts can achieve in 10-14 hours.

After leaving a home or business, wastewater (sewage) first flows into local sewers that are owned and maintained by the local city and/or the county. These local sewers feed into approximately 1,400 miles of larger “trunk sewers” that are owned and maintained by the Sanitation Districts.

The trunk sewers deliver the wastewater to 11 wastewater treatment plants. The sewer system is designed to flow by gravity as much as possible to save pumping costs. In some low-lying areas, pumps are used to raise wastewater so that the wastewater can flow by gravity the rest of the way to a treatment plant.

Primary Treatment
In nature, when wastes 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 and floatable materials are skimmed from the top. Both settled and skimmed materials are removed for further treatment.

Secondary Treatment
In nature, bacteria in a river feed on organic materials while “breathing” the dissolved oxygen in the water. In “secondary treatment tanks,” microorganisms feed on the organic materials remaining after primary treatment and “breathe” oxygen provided by air bubblers at the bottom of the tank. The microorganisms are removed from the water in secondary settling tanks and a portion is returned to the start of secondary treatment to consume more organic material. The remaining microorganisms, referred to as “waste activated sludge,” are routed for further treatment.

Tertiary Treatment
In nature, water is filtered by soil as water percolates downward towards groundwater aquifers. In the third (tertiary) stage of treatment, filters are used to remove nearly all remaining suspended material. Most of our filters are made of coal, sand and gravel.

In nature, the sun's ultraviolet rays can kill harmful microorganisms. In treatment plants, a disinfectant such as chlorine (like the bleach used at home) is used to kill any bacteria and viruses remaining after filtration.

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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.