The practical urgency of addressing problems of the transient pollution of receiving waters by stormwater overflows has done much to assist in dismantling the restrictive assumption of a steady state that has until recently dominated analyses of water quality management. Despite this, however, there are still misconceptions about the complexity and data requirements of dynamic mathematical models, and about the capacity to exercise operational control over the hour-by-hour, day-by-day performance of a wastewater treatment plant. The paper reviews some of these misconceptions, and draws attention to the crucial need for servicing the development of models of transient behaviour with appropriately designed, specialised, monitoring exercises. Results from a case study based on the River Cam in eastern England are presented. These illustrate the impacts of stormwater discharges on the receiving water body as a function of various treatment plant designs and operational strategies. Results are also presented for a second case study, based on the Bedford Ouse River in eastern England. A dynamic model of receiving water quality has been used for assessing the scope for seasonally varying policies for the treatment of ammonium, subject to fish toxicity and in-plant oxygenation requirements. Further studies exploring the interactions between seasonally varying conditions and transient pollution events are currently in progress.

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