In Wädenswil there is the Rietliau sewage treatment plant, which purifies the water for the municipality of Wädenswil. It was put into operation in 1947 and has since been renovated again in 2005. The treatment plant is one of the medium-sized ones in the canton of Zurich, with a population equivalent of 44,000. The purified water flows back into Lake Zurich. There is a small recycling station for glass, plastic, etc. right in front of the treatment plant.
Sources (PDF) : https://www.hw.zh.ch/ara/Waedenswil.pdf