Sentence:  The slippage of the drive belts caused the aerator to slow down and reduce <WSD>oxygen</WSD> transfer as well as the mixing of the raw sewage . 
Meaning of the word marked with <WSD> tags:  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust