The effect of mixed oil on the drying characteristics of sewage sludge in oil vacuum evaporation systems was studied. The experimental results showed that the drying rate with cooking oil was faster than that with refined oil due to the difference of thermal conductivity and composition of mixed oil. However, the heating value of all dried sludge was enhanced and the moisture content was below 1% due to penetration of oil into the microbial cells in sludge during the drying procedure. TGA analysis of sludge mixed with the refined oil, which had a higher volatility, showed the slope of the primary falling period was sharply declined. The result of DTA analysis also showed that the first peak was higher than the second peak and corresponded with the phenomena observed in the TGA analysis. In the DTA analysis, the temperature of the primary peak and the secondary peak of dried sludge were comparatively lower than those of raw sludge. Therefore, mixed oil could decrease the self-ignition temperature in an oil-sludge mixing system. In case of waste cooking oil, the TGA and DTA results showed similar results to those of raw sludge, but the DTA results showed that the secondary peaks of dried sludge were narrower and sharper than those of raw sludge. Overall, mixing oil could be a principal factor in controlling the drying efficiency and thermal properties of sludge-derived fuel in oil vacuum evaporation systems.

The drying and fuel technologies for sewage sludge have been developed due to the prohibition of ocean dumping and new renewable portfolio standard. This study was performed to enhance the quality of sludge derived fuel and compare drying characteristics for thickened and digested sewage sludge at different temperature, pressure and mixing oil conditions in oil vacuum evaporation system. In addition to investigate calorific value and characteristic analysis of dried sludge. The thickened and digested sludge used in this study were taken from municipal sewage treatment plant and coagulated using polymer (C-310P) in laboratory. The drying rate was increased with temperature and degree of vacuum and it was 25 mL/kg-sludge·min at 110oC and –450 mmHg. The moisture content of dried sludge products showed very low within 1% in the range of 0.4 ~ 0.8%. The evaporation rate of thickened sludge was lower than digested sludge and the constant evaporation period was also shorter. Compared the effect of waste cooking oil and refined waste petroleum oil on drying efficiency, the waste cooking oil showed more effective than refined oil in evaporation rate and drying time. The carbon and hydrogen contents of dried sludge with refined oil were higher than waste cooking oil. The low heating value of thickened dried sludge was higher than digested dried sludge about 400 kcal/kg and both of dried sludge showed high calorific value more than 4,000 kcal/kg.