Sentence:  The <WSD>oxygen</WSD> uptake rate in the mixed liquor averaged 0.8 mg / l / hr during the first four months of this study . 
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