The impacts of combined sewage discharges on river water quality are studied using the MCSTR (Multiple Continuously Stirred Tank Reactor) dynamic model. The potential for applying this model in a real-time context is demonstrated as a tool to support decisions regarding treatment plant operating during storm events, when it is often not possible to sustain full treatment of the incoming sewage flow. Discharges to the River Cam of treated and untreated urban wastewaters from Cambridge and the Cambridge Sewage Works are addressed as a hypothetical case study. Alternative treatment strategies are defined for improving receiving water quality and assessed through simulated water quality downstream of the discharge; the state variables of the model include the concentrations of biochemical oxygen demand, ammoniacal- and nitrate-nitrogen, and dissolved oxygen (chlorophyll-a concentrations are also calculated, but not considered herein). Strategies are assessed and ranked according to the reduction in maximum pollutant concentration (or the increase in minimum concentration, in the case of dissolved oxygen) promoted by each alternative, relative to conventional operation. The consequences of discharging overflows at an alternative position in the river, rather than together with the treatment-plant effluent, are also evaluated. Run times fr the MCSTR model are of the order of just a few minutes (at most), thus allowing the potential for its use in real time as a decision-support aid.

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