Hydraulic Analyses of a Force Main System at a Wastewater Treatment Plant

A numerical model study of steady state system capacities and transients caused by pump failures and diversions of flows between a secondary treatment plant (STP) and an outfall at a wastewater treatment plant (WWTP) is presented in the paper. The study was conducted to determine the impacts of constructing a new force main extension and upgrading pump stations. The 22.8-km-long force mains (pipelines carrying wastewater under pressure) consists of three pump stations with a combined rated capacity of 89.3 m³/s, four surge tanks, and two outfall tunnels. The model was calibrated and validated by comparing computed hydraulic grade lines (HGLs) against available field data. The study indicated that the conveyance capacity of the outfall tunnels was sufficient for discharging 29.5 m³/s of effluent without overtopping the surge tanks. The force main extension will require the incorporation of two 0.6-m-diamter vent stacks to avoid water column separations following failures of pump. Unless discharge control gates were closed or opened in less than 30 second, a diversion of 17.7 m³/s of STP flow to the outfall was not likely to pose operational problems.