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.

We present a model to simulate the increase in sludge temperature during batch-type thermal pretreatment of sewage sludge. The semi-theoretical model is based on energy balance as a function of operating conditions, including non-ideal factors determined by fitting. The model was verified by comparison with the results of bench-scale runs. It predicted the relationship between the operating conditions and steam input with sufficient accuracy. The test plant needed more energy input than the ideal during operation owing to the influence of the heat capacity of the apparatus. To optimize the scale of the apparatus, we simulated the treatment of 10 t of sludge. The energy input was minimized with 10 runs of a 1-t apparatus if the heat capacity of the ancillary apparatus exceeds a certain threshold, and 5 runs with a 2-t apparatus if the heat capacity is below the threshold. The influence of the boiler's performance on energy input is small, but its effect on the heat-up rate of the sludge is large. A boiler with sufficient equivalent evaporation and rated pressure will shorten the operating time.

To evaluate malodor reduction by means of the hydrothermal torrefaction, concentrations of the main malodorous compounds in odor samples from solid products before and after treatment were measured using gas chromatography. The concentrations of the sulfur-containing compounds were decreased by this treatment. To elucidate the mechanisms of this malodor reduction, odor distributions were analyzed with an odor-measuring device. The odor intensities in the odor sample from the solid product were lower than those from the dewatered sewage sludge (DSS), whereas the odor intensities in samples from liquid and gaseous products were as high as or higher than those from the DSS. Therefore, we conclude that one of the main malodor reduction mechanisms is reactive distillation: malodorous compounds are separated from the solid product by the inherent thermal effect of the hydrothermal torrefaction. In addition, the masking effect of odorous compounds produced as by-products in this treatment process and the sterilization of microbes in DSS probably play important roles in malodor reduction as well.

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