Because of stricter environmental regulations for surface water quality in Flanders, large efforts are made for the construction of sewers and water treatment plants. The present methodology to determine the necessary pollutant load reductions is based on the combination of simple mass balances, the 90-percentile discharge and neglects the self-purification capacity of the surface water. This method is very easy to apply but offers little guarantee that the goals set will be met. A sustainable river basin management requires a methodology to evaluate different options in pollution abatement strategies. Only by this, the available resources can be applied in the most advantageous, sustainable way.

In this paper, a methodology is proposed to evaluate water quality based on dynamic modelling. This methodology is illustrated by means of a case study on the river Dender. Continuous and simultaneous measurements uin 3 places along the river were used to calibrate and validate the model. Continuous simulations over a period of four years guarantee that, in contrast with traditional event type simulations, most discharge and hydrometeorologic conditions are taken into account. Attention was given to the occurrence of extreme events. These events will determine the global ecological condition of the river. CDF-curves (Concentrated-Duration-Frequency) make it possible to gain insight in the duration and the frequency of occurrence of events with inadmissible concentrations of dissolved oxygen and ammonia.

Besides simulations taking into account the present loads, different pollutant abatement strategies were analysed and compared. The applied methodology offers the possibility to evaluate the impact of different strategies and to locate present and future problems.

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