Efficient and sustainable recovery of lipids from sewage sludge using ethyl esters of volatile fatty acids as sustainable extracting solvent

This work focuses on the recovery of resources from urban sewage sludge. Sewage sludge is generated by water treatment plants and contains a significant amount of lipids that can be used in a number of applications in fine chemistry. This study reports the extraction of lipids from sewage sludge using ethyl esters of volatile fatty acids as solvents. A thermodynamic study was first performed using synthetic primary sludge in order to identify the best operating conditions to apply on real samples of urban primary sludge. Ethyl butyrate showed the best separation performance. It was capable of efficiently recovering the lipid component from primary sewage sludge, which typically resulted in 13–23% of total solids. Highly pure calcium soaps can be extracted in high amounts (yield up to 93%), while the exhausted residual aqueous phase resulted as not significantly contaminated by the solvent and could be anaerobically digested easily. Finally, calcium soaps can be directly used as biolubricant or efficiently converted into biodiesel using aluminum chloride hexahydrate as a catalyst (yield 85%).