The operating data and problems of four large-scale anaerobic-aerobic wastewater treatment plants are described. Three of these plants, treating wastewaters from a molasses distillery, a wheat-starch factory, and a potato-starch factory, use fixed-film methane reactors as the anaerobic stage. Two of these reactors are filled with packed blocks of plastic material (well known from aerobic trickling filters) with a specific surface area of 150 m2/m3. The fixed-film methane reactors in the potato-starch factory use lava slag as the support media. The companies concerned produced wastewaters which fluctuate greatly in quality and quantity. To guarantee stable degradation and high COD removal, equalizing tanks and low design rates are very important. Some of the wastewaters need additional dosing of trace elements to achieve satisfactory COD removal. All full-scale experience has shown that external anaerobic sludge retention, even when using the fixed-film reactors, is important for the anaerobic system.

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