# COMPUTATION OF MINIMUM DO

It is important to be able to predict the minimum dissolved oxygen level in a polluted stream or river. For Example, if a new sewage treatment plant is to be discharge its effluent into a trout stream, it is possible that conventional (secondary) treatment levels will not remove enough BOD to prevent excessively low DO downstream. To determine if some form of advanced treatment is required to preserve the stream for trout spawning and survival, it is necessary to compute the minimum DO caused by the sewage effluent and to compare it to the allowable value for trout streams.

One technique used to describe and predict the behaviour of a polluted stream uses the so-called Streeter-Phelps equation. This equation is based on the assumption that the only two processes taking place are deoxygenation of BOD and the reaeration by oxygen transfer at the surface, as previously discussed. Two key formulas from the Streete-Phelps model of stream pollution and oxygen sag follow. Figure 5.11 illustrate some of the variables in these equations. The minimum DO in the stream is the difference between the saturation DO level and the critical oxygen deficit. The formulas are