Common sludge drying methods decrease the yield of lipids as well as the saponifiable fraction, thus reducing the biodiesel production. In addition, they require high energy input. The proposed alternative, liquid–liquid extraction using hexane, is feasible and compares well
with those classical methods. Previous sludge acidification improves lipid and subsequently biodiesel yields. The FAMEs obtained from liquid extracted lipids are similar to those obtained by standard method. The cost of the proposed liquid–liquid extraction process and the lipid yield depends on the number of extraction stages. The scale-up of the process should allow reducing the final biodiesel price, as the cost of drying is eliminated. Finally, the lipid extracted sludge can be used to produce biogas by anaerobic digestion, avoiding the generation
of a new sludge. The biogas obtained maintains a similar composition, i.e., quality, than that coming from raw excess sludge.