In this paper a simple modelling approach is presented that allows fast computation of benthic activity in rivers. The approach extends the half-order reaction concept used in biofilm models for use in a multiple substrate/multiple bacterial species system. Moreover, it is compatible with the IAWQ Activated Sludge Model no. 1 format and has closed mass balances. The conversion of carbonaceous organic matter under aerobic and anoxic conditions and nitrification are represented in the model. The case study to which it was applied revealed that benthic activity is highly influenced by the eutrophic state of the river (presence/absence of algae) leading to diurnal oxygen fluctuations. More specifically the spatial distribution of species along the river was significantly different, resulting in postponed (downstream) nitrification in the eutrophied river. Also, oxygen depletion is found to be more severe and its spatial extension is larger.

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