Sentence:  The anaerobic conditions in the existing oxidation pond necessitated examination of other methods for supplying additional <WSD>oxygen</WSD> than by sodium nitrate . 
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