A mathematical model of anaerobic wastewater treatment in an industrial scale fluidized bed reactor (FBR) is presented together with simulation results for reactors in lab-scale (1 l) and industrial scale (500 m3). The model was developed to study the effect of scale up on the anaerobic wastewater treatment in FBRs by examining the interactions between biological degradation and the physico-chemical environment upon the outcome of this process. Material balance equations for substrates and products in gas and liquid phase are basis of the model. Hereby, the following effects were taken into account: biological degradation steps, convection and dispersion, chemical equilibria and calcium carbonate precipitation under consideration of ion activities. Model extensions are discussed, for example the calcium carbonate precipitation and the pH-dependency of degradation reactions. Simulation results show the strong impact of reactor performance on axial pH-gradients and thus on process stability in dependence of reactor scale. In this context the crucial role of the precipitation reaction together with the substrate degradation und carbon dioxide production reactions on local pH-values in the system is illustrated.

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