A black liquor evaporator condensate from a kraft mill was anaerobically treated in a fixed bed reactor (= an anaerobic filter), an expanded bed and a fluidized bed. The condensate treated in the fixed bed reactor and the expanded bed had an average COD concentration of 1.4 kg/m3. Before treatment in the fluidized bed the condensate was diluted to 0.42 kg/m3. Efficient purification (80 % COD-reduction or better) was obtained at volumetric loads up to 2 kg COD/m3·d in the fixed bed reactor, 10 kg COD/m3·d in the expanded bed and 13 kg COD/m3·d in the fluidized bed.

The specific methane production was 0.10-0.30 m3/kg COD added in the fixed bed reactor, 0.07-0.15 m3/kg COD added in the expanded bed and 0.07-0.20 m3/kg COD added in the fluidized bed.

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