In order to develop a technology which can efficiently extract energy from sewage sludge, the effects of hydrothermal treatment on dehydration performance and solid substance recovery ratio of sewage sludge were investigated. When sewage sludge was hydrothermally pretreated at the temperature of 473K and at the pressure of 300MPa, the water content in the solid product can be reduced significantly by mechanical pressing, and the dewatering ratio increased with increasing the pressure. In contrast, when sewage sludge was hydrothermally pretreated at lower temperatures: 423K and 443K, the dewatering ratio didn't correlate with pressure. The test results show that the hydrothermal pretreatment at 473K was suitable for improvement of the dehydration performance of sewage sludge. The solid substance recovery ratio was about 80% after this treatment at the temperatures of 473K due to solubilization of part of sewage sludge. In this process, the required energy for removal of water was about half compared with conventional drying process. The principal reason for this low energy requirement is that the water in hydrothermally-treated sludge can be reduced easily by the mechanical pressing.
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