Distillery slops from a multicrop ethanol plant are difficult to treat anaerobically, because the composition and load changes throughout the year due to processing of different sugar-and starch-containing crops. For achieving high COD removal efficiencies and good process stabilities at medium strength loading rates, a two-phase anaerobic treatment process with a fixed-bed reactor for biomethanation was developed and tested in pilot-scale. On the basis of these experiments a full-scale unit with 1,800 m3 reactor volume was erected, which has been in operation for more than one year.

The fixed-bed reactor is operated with COD loads up to 10 kg COD/m3d at hydraulic retention times of about 5 days. A variety of highly concentrated slops can be treated successfully with COD removals from 85 % to over 90 %. The fixed-bed reactor withstood sudden changes in wastewater strength and composition and is able to handle hydraulic overloads. Operational data from the industrial unit show that the performance and reliability of the fixed-bed reactor is much better than a conventional contact system, which was operated in a parallel run in the same scale. The results show, that the application of fixed-bed reactors can be recommended for a wide variety of high-strength wastewaters with low suspended solids contents.

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