Sentence:  One of the hidden glories of Victorian engineering is proper drains. Isolating a city's <WSD>effluent</WSD> and shipping it away in underground sewers has probably saved more lives than any medical procedure except vaccination. 
Meaning of the word marked with <WSD> tags:  Sewage water that has been (partially) treated, and is released into a natural body of water; a flow of any liquid waste.