Fig. 5
From: Enhanced economic feasibility of excess sludge treatment: acid fermentation with biogas production
The biogas yield (actual CH4 production/theoretical maximum) of various samples (S1 = anaerobic digestion of raw excess sludge, S2 = anaerobic digestion of pretreated excess sludge, and S3 = organic acids generation + anaerobic digestion of solid residue after acids generation)