In this study, two types of sewage sludge (primary sludge and waste activated sludge) were hydrothermally treated at 125–250 °C to enhance the lipid extraction efficiency and obtain a higher biodiesel yield. The enhanced efficiency of the lipid extraction method was compared with the efficiency of the organic solvent extraction method. The results confirmed that a hydrothermal reaction could be an appropriate option for disrupting sludge cell walls and increasing the lipid extraction from sewage sludge. The highest lipid recovery efficiency was observed at 200 °C, and the lipid recovery efficiency of primary sludge and waste activated sludge increased from 7.56% and 5.35% to 14.01% and 11.55% by weight, respectively. Furthermore, transesterified lipids, such as biodiesel from sewage sludge, mostly consist of C16 and C18 methyl esters, and have features similar to those of jatropha oil-based biodiesel. During the hydrothermal treatment, the carbon content in the sludge decreased as the carbon transformed into lipids and the lipids were extracted. The volatile matter and fixed carbon content in the solid residue decreased and increased, respectively, through chemical dehydration and decarboxylation reactions under hydrothermal reaction conditions.

This study purposed to characterize the sewage sludge from various sewage treatment plants (STPs) as a biodiesel feedstock. Crude biodiesel was produced from each dried primary sludge (PS) and waste activated sludge (WAS) via in situ transesterification process. The average yield of transesterifiable lipid (TL) was 77.8% and 60.4% of the total lipid content from PS and WAS, respectively. The TL yield had a greater margin among WAS than PS samples due to differences in the biological processes adopted in each treatment plant. The TL recovered from PS and WAS contained 54.2% and 40.1% fatty acid methyl esters (FAMEs), respectively, which were mostly made up of palmitic acid (C16:0) and stearic acid (C18:0). The FAME composition of the biodiesel in the WAS sample was highly associated with a microbial community that grows otherwise, depending on the purpose of the biological treatment process. In particular, the increase in the proportion of nitrifying bacteria that grow predominantly under a relatively longer solid retention time (SRT) contributed significantly to the improvement in FAME content.