On March 30, 1967, the Lake City Wastewater Treatment Plant becomes the last facility to stop discharging treated sewage into Lake Washington. Treated sewage is piped instead to treatment plants on Puget Sound. This comes nine years after Metro was formed. Within the year, lake waters begin to improve.
Metro -- the Municipality of Metropolitan Seattle -- was formed in 1958 to provide a region-wide solution to water pollution issues. Metro's first step was to redirect the flow of 10 secondary treatment plants from Lake Washington to Puget Sound. In Puget Sound, the tidal action and salt water could absorb the effluent with less damage to the environment.
Although secondary treatment (removal of 95 percent of solids from the influent and treatment with chlorine) was very high-tech at the time, the phosporous-rich wastewater stimulated the growth of algae. This deprived the lake of light and oxygen. When the algae died, it drifted ashore in odorous clumps. In 1955 and 1956, the State of Washington Pollution Control Commission reported that algae would eventually "take over the lake" if the discharge of treated effluent continued.
In 1950, an eight-inch disk was visible from the surface of Lake Washington at a depth of 12 feet. By 1966, the lake had become so polluted that the disk could not be seen more than two feet below the surface. By 1993, after all the Metro plants became operational, the disk was visible at a depth of 25 feet.