Effects of Dispersion on DO-Sag in Uniform Flow

Dispersion is found to have little effect on steady DO-sag. With periodic supply of BOD at outfall, the amplitude of fluctuations may be significantly reduced along stream by dispersion. Boundary-layer solutions are obtained for case with BOD abruptly supplied at outfall. A uniform stream has been used to reveal the effects of dispersion on the distribution of BOD and DO. The results show that the effects of dispersion on a steady DO-sag are negligible. Dispersive action is evident when there is a sharp concentration gradient. A sharp front due to an abrupt supply of BOD at an outfall has been found to spread into a transition of length (π Dt)^1/2. When the supply of BOD at an outfall fluctuates with a period (e.g., 1 day) shorter than the representative time of oxidation 1/k₁, dispersion reduces significantly the amplitudes of the fluctuations of BOD and DO along the stream.