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Treatment process

Treatment process

South Treatment Plant

Learn about the steps we take to treat wastewater at South Plant.

FLUSH FACT: One cubic foot (about the size of a small microwave) of trash is removed from wastewater at South Plant every 2 minutes.

When you flush the toilet or turn on the sink, the water has to go somewhere. The water that travels down your drain flows into a system of pipes stretching across the region. We refer to this water as "wastewater," and it has to stop at a treatment plant to get cleaned before it can be released into regional water systems.

South Plant is a secondary treatment facility that uses an activated sludge process. This process uses oxygen and microorganisms to break down pollutants and clean wastewater. Most of the wastewater treatment process uses gravity and microorganisms. We add minimal chemicals (chlorine) to the water to meet treatment standards regulated by the Washington Department of Ecology. Effluent (water that leaves the facility) is then pumped out into the Puget Sound and returned to nature.

How we clean wastewater and recycle valuable resources at South Plant

Video (23 min,15 sec) is fully accessible and available in Spanish, Russian, Simplified Chinese, Traditional Chinese and Korean .

Overview of the treatment process

Preliminary Treatment

During preliminary treatment, bar screens pull trash from the raw sewage. Then, we squeeze excess water out of the garbage and send it on to a solid waste facility.
Watch plant tour video (starting at 5 min, 7 sec).

Primary Treatment

Once the non-biological waste is removed, primary treatment begins. The water passes through skimmers, which pull oil and grease from the surface of the water. Heavy solids fall to the bottom, where moving bars scrape them up.
Watch plant tour video (starting at 8 min, 33 sec).

Secondary Treatment

Wastewater then moves on to secondary treatment. Clarifiers have bacteria and microorganisms in them that eat the anerobic waste and large rakes scrape out any lingering solids from the bottom of the tank. Finally, the water moves on to the disinfection tanks where we add chlorine and give it time to sit and come in contact with the water and any remaining harmful germs.
Watch plant tour video (starting at 11 min, 45 sec).


The organic solids removed during primary treatment are dried and turned into Loop biosolids, a nutrient-rich organic fertilizer alternative that helps build healthy soil. We send treated water out to Puget Sound as effluent, and some of it is cleaned to an even higher standard and reused as part of our recycled water program. That benefits customers such as Starfire Sports Complex, who use recycled water on their soccer fields. Methane gas captured during treatment is sold to Puget Sound Energy, which contributes to the renewable energy market.
Watch plant tour video (starting at 16 min, 40 sec).

Contact us

24 hour emergency and odor reporting:   206-263-1760
Administration:  206-263-1810

1200 Monster Road S.W.
Renton, WA 98057

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