Direct Thermochemical Liquefaction of Sewage Sludge by a Continuous Plant

Direct thermochemical liquefaction of sewage sludge was studied as one economical solution to the sludge disposal problem. A continuous plant, which was capable of processing up to 500 kg/d as dewatered sludge, was operated at temperatures of 270-300°C, pressures of 6-12 MPa and holding times of 0-60 min for more than 700 hr without any trouble. Organic materials in sludge were converted into heavy oil with 40-53 %maf yields, and 11-16 %maf of the heavy oil were continuously separated from the reaction mixture by high pressure distillation. Heating value and viscosity of the heavy oil separated were 38 MJ/kg and 0.05 Pa·s, respectively. From the calculation of energy balance ona practical scale, sludge liquefaction process was evaluated as a net energy producer. The disposal of wastewater discharged from sludge liquefaction process was also discussed.