Characterization of long-chain fatty acids from urban wastewater sludge in two-phase anaerobic digestion for biogas production

The present study describes the composition of free fatty acids (FFA) produced within each of two phases of anaerobic digestion carried out in two distinct, linked anaerobic digesters (one for acetogenesis and the other for methanogenesis) fed with municipal wastewater sludge. The FFA were characterized by using FT-TR and gas-chromatography-flame ionization detector analyses. Both techniques showed that FFA were mainly constituted by long-chain fatty acids (LCFA) with differently branched chains. In particular, saturated fatty acids (SFA) from C12:0 to C24:0 and only a few unsaturated fatty acids (UFA) (from C18:1 to C22:1) were observed in both digesters. The concentration in FFA differed between the two phases of digestion, the acetogenesis proving richer in SFA, UFA and geometrical and configurational isomers. The data are used to discuss the hypothesis that the different molecular structures are functional to the conversion of FFA to methane and that a specific bacterial activity can be revealed as adaptation to LCFA presence.