Optimization of direct liquid-liquid extraction of lipids from wet urban sewage sludge for biodiesel production

•Urban sewage sludge is a suitable low-cost feedstock for production of biodiesel.•Efficient lipid extraction from sludge is one of the main remaining challenges.•Direct liquid-liquid extraction of lipids from wet sludge was optimized.•Optimized method gives larger lipid amount (34.5 compared to 32.8%wt using hexane).•Lipid extraction from wet sludge avoids energy intensive drying steps. Efficient lipid extraction is one of the main remaining challenges for full-scale production of biodiesel from urban sewage sludge, a low-cost feedstock. The common approach is to extract lipids from dewatered or dry sludge by means of organic solvents. However, sludge dewatering and drying are energy-consuming and represent more than 50% of the total production cost. Direct liquid-liquid extraction of lipids has been poorly explored; only extraction using hexane as solvent is described in the literature. This study focuses on the optimization of the extraction of lipids from wet raw urban sewage sludge at room temperature. This optimization is conducted varying solvent, pre-treatment, number of extraction steps, contact time and ratio sludge to solvent. The optimized method gives larger lipid amount (34.5%wt dry matter (DM)) than using hexane extraction (32.8%wt DM). Finally, the potential of lipid fractions extracted from eight sludge samples for biodiesel production was evaluated through the determination of acid value and free fatty acid (FFA) content, saponification number and fatty acid composition.