The arrangement is for anaerobic purification of waste water in an (upflow) reactor (1) by passing the water (via. 2,3) through a sludge layer (4) containing anaerobic bacteria and separately removing the developed gas (via 9) and the purified water (via 10). After passing through the sludge but before removing the water, it is brought into intimate contact with a granular filter material (5) which holds back the entrained sludge particles and separates them from adhering gas bubbles after which they return to the sludge layer. The specific gravity of the filter material is pref. less than that of the water so that it floats as a layer on the top. Method is more sensitive to toxic components than aerobic methods and produces less sludge. It provides a nett energy gain in the form of methane instead of the energy consumption due to the oxygen (air)supply. The arrangement provides better gas/sludge separation thus overcoming the problem of loss of sludge with the purified water experienced with existing anaerobic upflow reactors. There is separate gas recovery.
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