Pollutional Analysis of Combined Sewer Systems

Mathematical models for sewer networks and wastewater treatment plants are used to simulate the BOD levels at different points in the treatment works and to combine sewer overflows under different conditions. The network and the treatment plant were simulated as a unit during an entire year's varying precipitation. Storm-water retention basins proved to be beneficial in reducing total pollution load discharged to the receiving waters only when the degree of pollution in storm-water runoff exceeds a specific limit. However, the effect of a retention basin increased considerably when the storm overflows and the basin outlet flows were set to an optimum setting, and when the size of the secondary clarifier was compatible with the retention basin. The simulations showed that retention basins could have an adverse effect on the discharge of BOD to the receiving water if the amount of BOD is removed by the storm overflow anyway.