Most living organisms depend on oxygen in one for or another to maintain the metabolic processes that produce energy for growth and reproduction. Although present in large amounts in the atmosphere, water solubility of oxygen is quite low and heavily depends on temperature varying from 14.6 mg/L at 0 °C to 7 mg/L at 35 °C at 1 atm pressure. The low solubility of oxygen is the primary reason why the purification capacity of natural waters is limited, and it is necessary to treat wastes to remove pollutional matter before discharge to receiving bodies of water. The concentration of dissolved oxygen in water is the determining factor as to whether the biological processes are carried out aerobically or anaerobically. As all aerobic treatment processes depend on the presence of dissolved oxygen, monitoring DO is indispensable for controlling the rate of aeration. Dissolved oxygen based control enables operators to ensure that sufficient oxygen is supplied to maintain aerobic conditions as well as preventing excessive use of air and energy.