On July 22, 1965, Metro's large Renton Treatment Plant is dedicated. This plant is designed to treat 24 million gallons of raw sewage a day from South King County before discharge into the Duwamish River, with an ultimate capacity of 144 million gallons a day. It is dedicated in memory of Harold E. Miller (d. 1964), Metro's first executive director.
Construction on the 53-acre project began on July 20, 1961, next to Longacres Racetrack on land purchased from the Great Northern Railroad and the Earlington golf course. Metro promised that if the effluent caused any problems for the Duwamish River, the discharge would be moved to Puget Sound. By 1980, along with rising population in the service area, ammonia and chlorine levels had risen in the river.
Metro planned a pipe for treated effluent through deep tunnels to Point Pully near Seahurst or to Alki Point. The Point Pully option was five miles shorter and cheaper, but it met with opposition from residents. The route of the 10-foot-diameter, six-mile-long tunnel was geologically uncertain and people worried that the effluent would just wash around Vashon Island rather than out to sea. In early 1983, the Metro Council decided on an 11-mile pipeline along West Marginal Way South that discharged deep into Puget Sound. The pump station, force mains, tunnel, and outfall were completed in March 1987 at a cost of approximately $202 million, less than earlier estimates. Ammonia nearly disappeared from the river and oxygen levels improved.
As part of the King County Department of Natural Resources, the facility was renamed the South Treatment Plant. A $230 million expansion program began in 1991 to expand the facility to 108 million gallons a day.