A Dissolved Oxygen (DO) is the oxygen freely available in water, vital to fish and other aquatic life and for the prevention of odors. DO levels are considered a most important indicator of a water body's ability to support desirable aquatic life. Secondary and advanced waste treatment are generally designed to ensure adequate DO in waste-receiving waters.

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Other /More definition:
A dissolved oxygen (DO) is oxygen dissolved in water and available for living organisms to use for respiration.