To develop a technology that can efficiently convert dewatered sewage sludge cake into solid fuel, we experimentally investigated the effects of the hydrothermal treatment on dewaterability and carbon distributions. Dewatered sewage sludge cake with a water content of about 80 wt% could not be dewatered by increasing the pressure of a mechanical press. However, after hydrothermal treatment, the sewage sludge cake could be dewatered with a mechanical press. Moisture content was negatively correlated with treatment temperature at temperatures below 473K but stayed about the same at temperatures above 473K in the range studied. Carbon contents in the residual solid after treatment with a mechanical press were negatively correlated with the reaction temperature of the hydrothermal treatment owing to the solubilization of part of the volatile carbons in the dewatered sewage sludge cake. On the basis of the above results, we proposed a novel solid fuel production system composed of the following processes: hydrothermal treatment, mechanical press treatment, concentration, and drying. The optimum operating temperature of hydrothermal treatment was 473K in the range studied, and at this temperature, we estimated that the energy input to obtain dry sewage sludge using the proposed system would be about two-thirds that of a conventional drying process.
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