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Ecological indexes Dissolved oxygen (DO) is the oxygen present in water and is used by fish to live. When DO drops below a critical level, fish starts to die off. As oxygen decreases, anaerobic bacteria, which live in the absence of oxygen, increase, which can produce toxic gases such as methane, hydrogen sulfide and ammonia. Biochemical oxygen demand BOD is a measure of the amount of oxygen that bacteria will consume during the decomposition of organic matter under aerobic conditions. Wastewater treatment plants reduce BOD in naturally discharged water and function as bacteria farms, where bacteria are fed with oxygen and organic waste. Excess bacteria grown in the system are removed as sludge, "solid" waste which is disposed of on the ground. Chemical oxygen demand COD does not distinguish between biologically available and inorganic organic matter. It is a measure of the total amount of oxygen needed to oxidize everything into carbon dioxide and water. COD values are always greater than BOD values.