Stochastic design of wastewater flow equalization basins.

Wastewater flow equalization basin volume design has been an imprecise art in which the use of safety factors has been required to allow for the lack of sophistication. A design method is proposed which takes into account the time-dependent nature of flows, the brevity of the observed data record, the facility operation, and the expected performance. Wastewater treatment plant raw inflow rates are modelled using combined deterministic-stochastic time series techniques. Long flowrate series samples generated by Monte Carlo simulation with a fitted time series model are routed through flow equalization basins to estimate their performance capabilities. The routing procedure includes forecasting of inflow rates and storage requirements and adjustment of basin outflow rates at discrete time intervals. With the inclusion of a plant effluent recycle option, the major performance variables are basin overflow occurrence, reduction of diurnal flowrate variance, and the effect of the recycle option on increasing the mean flowrate through the plant. For the Ottawa, Ontario, Green Creek wastewater treatment plant, this study indicates highest expected performance is derived from basins with volumes close to those suggested by previous design methods. The uncertainty in the flows and the limitations inherent in basin operation contribute to a reduction in attainable performance from the ideal level