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Metro cleaning out emergency sewage outfall from treatment plant

King County is clearing an outfall June 13 for a pipeline used only in emergencies at Metro
King County is clearing an outfall June 13 for a pipeline used only in emergencies at Metro's South Treatment Plant in Renton.
— image credit: King County

Crews from King County’s South Treatment Plant will perform routine maintenance Thursday on the Renton plant’s emergency outfall pipe in the Green River.

Anyone in the area could see divers and equipment at work which is being done upriver and around a bend from Fort Dent Park in Tukwila.

King County has a permitted emergency outfall pipe that protects the plant and private property in the event of a major sewer emergency. In an emergency, wastewater or treated effluent from the plant could be discharged into the Green River, which would help protect the treatment plant and reduce the potential for backups into homes and businesses, according to the county.

The emergency outfall once served as the main effluent discharge point into the Green River. In the late 1980s a new discharge pipeline was constructed to redirect treated effluent to Puget Sound.

Since that time the emergency outfall has never been used, and over time river sediment builds up around the outfall and inside the outfall structure. Cleaning the outfall ensures the system is clear of sediment and debris in the unlikely event of an emergency requiring it to be placed into service by the plant, according to the county.

Thursday, plant staff will send treated effluent through the pipe to push the sediment trapped within the pipe and along its sides out into the river.  This activity is timed to minimize impacts to migrating salmonid species while ensuring sufficient flow volume to flush the river sediment out of the pipe.

Fully treated and dechlorinated water will be sent through the pipe.  Any discoloration of the water is from river sediment stirred up during cleaning.

This is what the public should expect:

• Cloudy water and sediment downriver from outfall pipe, possibly as far as Starfire Soccer fields.  Sediment may appear dark brown to black.

• Water quality sampling upstream, downstream and near pipe outfall into Green River

• Divers in river a few days after pipe flushing to confirm that the pipe is clean

• Outfall cleaning work hours 7:30 a.m. to noon

• Green River Trail will remain open

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