Influent characteristics affect biodiesel production from waste sludge in biological wastewater treatment systems

Biodiesel is a renewable and environmentally friendly energy source. The use of biological wastewater treatment systems for excess sludge in the biodiesel production can achieve both sludge reduction and resource recovery. In this study, an in situ one-step method for producing biodiesel from excess secondary sludge was investigated; then the differences in both biodiesel yield and composition produced from the five industrial wastewater sludges and two municipal wastewater sludges were studied. The results showed that ultrasonic pretreatment of sludge was not necessary for biodiesel production by in situ acid-catalyzed transesterification. The addition of n-hexane during transesterification was significant to determine the performance of biodiesel production. Significant differences were found in yield and composition of the sludge lipid from seven different types of wastewater. The sludge oil of starch wastewater and restaurant wastewater was significantly higher than those of tannery wastewater, landfill leachate, and livestock wastewater, which was mainly due to the significant differences on wastewater quality and sludge microbial structure. The higher organic content (especially sugar), COD/TN and COD/TP ratios and better biodegradability in wastewater led to the dominance of Saprospiraceae and Anaerolineaceae, finally benefited the lipid accumulation in excess secondary sludge for biodiesel production.