A river water quality model is presented for the prediction of the concentration fields of the water quality variables in an oxidation ditch performing carbon oxidation, nitrification and denitrification. The model involves the one-dimensional convection-dispersion equations for all the variables, which are described in the IAWPRC Activated Sludge Model No 1. The model is applied to a typical oxidation ditch to show that all variables, with the exception of DO, are uniformly distributed in the ditch; thus permitting the characterisation of the oxidation ditch as a completely mixed reactor. The DO concentration, which controls the reaction rates of almost all the processes, shows a plug flow behaviour. The potential of the present model to be used to improve the DO control system of the ditch and to perform real time predictions is also demonstrated. All calculations have been performed with a PC.

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