Dissolved oxygen balance in a stream

The course of mineralization of organic pollution by aerobic processes during self-purification is decisively influenced by molecular oxygen dissolved in water. The most important sources of oxygen in water are atmospheric oxygen and assimilation activity of phytoplankton and green plants. 

Oxygen is introduced into water through the water surface. At a constant temperature the solubility of oxygen is directly proportional to the partial pressure above the liquid (Henry s law). Oxygen dissolved in polluted water is used for mineralization of organic pollution, therefore, its concentration usually becomes lower than the equilibrium concentration [40-43]. 

Time dependence of the dissolved oxygen consumption on biochemical degradation of organic substances (expressed in terms of BOD) is shown in Fig. 3.40. 

Oxygen conditions in the stream decisively influence two simultaneously proceeding processes  

The time dependence of these two processes is evident from curves 1 and 2 in Fig. 3.41. Their resultant is the oxygen sojf curve (curve 3) expressing the time dependence of the concentration of dissolved oxygen in water. 

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