The pretreatment effect of thermochemical liquidization for the anaerobic retreatment of anaerobically digested and dewatered sludge was studied. The digested sludge (dry matter; 15.7%) was thermochemically liquidized at 175 degrees C and 4 MPa with a holding time of I h. The liquidized sludge was separated by centrifugation to produce a supernatant of 44.7% (w/w) and precipitate of 52.3%. The liquidized sludge and its supernatant were successfully anaerobically digested at the added VS concentrations of 1.6-2.2 g/l. The biogas yield from tile supernatant during 8 days of incubation was 339 ml/g-added VS and the digestion ratio was 61% (w/w). The digestion of the supernatant from the liquidized sludge showed a high biogas yield and high digestion ratio. Moreover, the precipitate of the liquidized sludge could be incinerated using a much smaller amount of supplementary fuel than that of the digested sludge. Copyright (C) 1996 Elsevier Science Ltd.