In nature - Heavier, solid particles settle to the bottom of the river while lighter materials float to the top and are carried away by the current.

At the treatment plant - Long, concrete settling tanks act the same as the river. The solid materials settle to the bottom and are removed along with the floating materials for additional treatment and processing. The remaining wastewater consists of suspended materials, which then are further purified in the secondary treatment process.

In nature - The remaining dissolved and suspended materials flow downstream where naturally occurring microorganisms (tiny organisms of bacteria and protozoa) in the river feed on them. As the water flows through rapids and rocky bottoms, air bubbles form which allow the organisms to continue to grow and consume suspended materials.

At the treatment plant - Wastewater from primary treatment, containing mostly dissolved organic materials, flows into aeration tanks filled with microorganisms. However in comparison to the river, the amount of air bubbled into the tanks is scaled up so the organisms can make even more bacteria and eat greater amounts of wastewater solids. Some of the bacteria are recycled in the treatment plant to treat more wastewater.

In nature - The ultraviolet rays of the sun kill the harmful bacteria, viruses, or microorganisms. Natural processes clean the water while maintaining the health of the ecosystem and surrounding environment. Then water filters through the ground beneath the river and joins the underground water supply. Some of the clear, clean water continues to flow into nearby lakes or oceans.

At the treatment plant - Filters consisting of sand and activated carbon remove any remaining suspended materials from the water. Filtered water is disinfected either with ultra-violet rays or mixed with a disinfectant to kill any harmful bacteria, viruses, or microorganisms remaining in the water. The disinfected water is treated to remove chemicals and disinfectant. The final water product is clean and free of chemicals and harmful organisms making it ready for various recycling and reuse applications.

Water quality measurements and analyses are completed at laboratories located at each Sanitation Districts' water reclamation plant (WRP). The laboratory staff provides analytical services that meet all the requirements of the Regional Water Quality Control Board for all of our 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.

At some of our facilities (such as the Warren Facility, Lancaster WRP, Palmdale WRP, and Valencia WRP), solids are processed to create energy and are reused in various applications.

In nature - Solids are decomposed by microorganisms to produce nutrient rich soils.

At the treatment plant - The process involves thickening of sludge, anaerobic digestion, and dewatering. Suspended and settled organic material from the primary settling tanks and secondary clarifiers are sent to the Dissolved Air Flotation (DAF) system. The resulting thickened sludge from the DAF system is sent to anaerobic digesters where electricity is generated from biogas. Centrifuges or filter presses are used to dry the remaining biosolids. The resulting dried and nutrient rich biosolids are sent to landfills or to compost facilities.