Thermochemical liquidization of anaerobically digested and dewatered sludge and anaerobic retreatment

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°C and 4 MPa with a holding time of 1 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 the 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.