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By means of stimulus-response experiments an Li(+) tracer, models for the fluid flow in a 30-m(3) UASB reactor, used for the anaerobic treatment of wastewater, were tested. From the model with the best fit it could be derived that both the sludge bed and the sludge blanket can be described as perfectly mixed tank reactors with short-circuiting flows; the settler volume acts like a plug-flow region.Apart from the volumes of the different flow regions, two parameters are necessary and sufficient to describe the fluid flow in a well functioning UASB reactor, i.e., the short-circuiting flow over the sludge bed and the short-circuiting flow over the sludge blanket. The volumes could be measured accurately.The short-circuiting flow over the sludge bed is a linear function of the sludge bed height. When the optimal height of the sludge bed is defined as the height for which the short-circuiting flows are as small as possible, a bed-height of 3.5-4 m is sufficient (for superficial gas velocities between 1 and 1.5 m/h). This is in contradiction to the results of other authors. The short-circuiting flows over the sludge bed and the sludge blanket were also influenced by the superficial gas velocity.

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