ABSTRACT In the field of sewage sludge treatment, new technology including liquefaction and pyrolysis has been recently proposed. This review paper describes mainly the recent results of liquefaction in comparison with pyrolysis. When sewage sludge was treated at a temperature of about 300°C and at a pressure of 12MPa, oil resembling petroleum was obtained in 40-50% yieId on a dry organic basis. The optimum operational conditions were: reaction temperature 300°C, catalyst (sodium carbonate) loading 5wt%. Working pressure and holding time had no significant effects on the oil yield. For most sewage sludge, oil production proceeded satisfactorily without the addition of any catalyst. The optimum kind of sewage sludge was found to be either raw primary sludge or raw mixed sludge because average oil yield of these sludges reached 43% higher than those of other kinds of sludges. A continuous liquefaction plant of capacity of 0.5t/day was constructed and operated successfully. On the basis of data obtained, a feasibility study of a commercial plant was made. From an estimation of energy balance, the liquefaction process was evaluated as a net energy producer.
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