A 40 1 tank reactor, filled to 3 % (v·v−1) with a small size (5-25µm) carrier material and equipped with a mechanical mixer, was used to create a sludge bed system for anaerobic waste water treatment. Solids leaving the tank with the effluent were recycled to the tank from an external settler.

Two different substrates were used, fodder molasses diluted with tap water and beet sugar factory waste water. Influent concentrations were 9.3 g COD·1−1 and 4-7 g COD·1−1 respectively, and treatment was performed at 35-37 °C. With the synthetic molasses waste water, an organic load of 5-6 kg COD·m−3·d−1 could be tolerated, with the sugar industry waste water 25 kg COD·m−3·d−1.

The difference in loading capacity was ascribed to different types of sludges formed, the molasses waste water resulting in a much more bulky sludge and a lower attainable volatile suspended solids concentration. It was concluded that much care must be exercised before designing sludge bed systems for high loads with unhydrolyzed waste waters.

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