As residual sludge after lipid extraction still contains a large amount of organic matter, this lipid exhausted sludge was subjected to anaerobic digestion in order to evaluate the remnant potential for biogas generation. Fig. 5 shows the biogas production during anaerobic digestion of evaporated lipid-extracted sludge (ELES), i.e., hexane free and lipid extracted sludge (LES). The biogas measure was converted at standard conditions (0 °C and 1 atm) and is given as the volume of biogas per gram of VS fed (mLBiogas/gVS). Biogas production from ELES reached 365 ± 10 mLBiogas/gVS, whereas LES only reached 31 ± 4 mLBiogas/gVS. This huge difference, over tenfold, can be attributed to the presence of hexane in LES. In a mass balance, it was calculated that solvent still represented approximately 9% of the volume in LES. Furthermore, a VFA analysis revealed a concentration of 12.0 ± 0.1 mol/m3in the reactor with LES , while no VFAs were detected in reactors with ELES. A value over the range of 6.7–9.0 mol/m3has been reported to be toxic for methanogenic microorganisms, stopping the biogas production [15].