Wastewater Treatment and Water Reclamation

The Sanitation Districts have a proud tradition of providing low-cost, high-quality wastewater management for Los Angeles County. This includes operating one of the largest, engineered wastewater recycling programs in the world.

Water recycling is very important in our arid Southern California environment where water must be imported from other parts of the state. The goal of the Sanitation Districts is to recycle as much water as possible from its Water Reclamation Plants (WRPs), therefore, the WRPs play a major role in meeting our water needs. This recycling significantly reduces the Los Angeles Basin's dependence on costly imported water and helps to replenish a large percentage of the groundwater used by the region.

The WRPs literally replicate what happens in nature. They just speed up the process and do it in modern, high-tech facilities. They provide what is known as primary, secondary, and tertiary treatment for approximately 165 million gallons of wastewater per day. The following describes the wastewater treatment and water reclamation processes.


A treatment plant is just like a natural river but in a concrete box. First, materials settle to the bottom of the primary settling tanks (Primary Treatment). Second, microbes use air to breath while they eat up organic material in the aeration tanks, then the microbes settle out in the secondary settling tanks (Secondary Treatment). Third, sand and coal filter out leftover particles in the filters (Tertiary Treatment) like sand in the bottom of a river.

Typical Flow Schematic of a Water Reclamation Plant

Just as in nature, when runoff first enters a river, heavier solid particles settle to the bottom while lighter materials float to the top and are carried away. At the water reclamation plant, long concrete tanks replace the river. The heavier solids which settle to the bottom and the lighter materials, like plastic and grease, which float to the top are called primary sludge and are removed and returned to the sewers for further treatment at the Joint Water Pollution Control Plant. The remaining wastewater, containing dissolved and suspended materials (mostly organic), moves to the second phase of treatment in aeration tanks and secondary settling tanks.


As dirty water in a river flows downstream, naturally occurring microorganisms feed on the dissolved organic materials. As the river flows downstream, oxygen naturally enters the water so the organisms can breathe. In the secondary treatment aeration tanks, air is bubbled through the water to supply oxygen. The same microorganisms in the wastewater grow as they feed on the organic materials in these tanks. In the secondary treatment settling tanks, the microorganisms clump together and settle to the bottom, where they are removed and recycled back into the treatment process.


Finally, in a natural river, the clean water percolates into the ground beneath the river and joins the underground water supply. At the treatment plant, the ground is replaced by filters, which remove any remaining suspended materials from the water. The filters contain layers of anthracite coal, sand, and gravel. The reclaimed water is then disinfected. It is now free of harmful bacteria and viruses and is safe for human contact, recharging groundwater, and for a wide variety of other uses. Following disinfection, any remaining chlorine in the reclaimed water is removed to protect aquatic life in the receiving environment.

To learn more about the Sanitation Districts' recycled water programs, browse the
 Sanitation Districts' reclaimed water programs and the recycled water resources and information.

Reclaimed Water discharged into San Jose Creek; Chemist performing lab tests; Reclaimed Water


Water quality measurements and analyses are completed at laboratories located at each WRP. The laboratory staff provides analytical services that meet all the requirements of the Regional Water Quality Control Board for all of the Sanitation Districts' treatment facilities. A full spectrum of analytical services are available, including chemical, biological, and microbiological testing. Sanitation Districts' scientists employ a combination of classical techniques and modern, sophisticated instrumentation to provide assessments of inorganic and organic chemicals, bacteria, viruses, and toxicity. More than 300,000 tests on 20,000 samples are performed annually. In addition, new and improved analytical methods are continually being evaluated and developed.

Colonial stalked ciliates found in activated sludge
as part of the secondary treatment process.

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