Two-stage anaerobic digestion of aerobic pre-treated sisal leaf decortications residues: hydrolases activities and biogas production profile

Abstract

A two-stage system was investigated for anaerobic digestion (AD) of aerobically pre-treated sisal leaf decortication residue (SLDR) with regard to hydrolytic enzymes and biogas production. The system consisted of a solid-bed bioreactor for hydrolysis connected to methanogenic bioreactor packed with sisal fibre decortication residues (SFDR) as biofilm carriers. Some of the enzymes produced by microorganisms to hydrolyse SLDR were found to be pectinase, filter paper cellulase, amylase, beta-glucosidase, carboxylmethyl cellulase, xylanase and protease. Enzyme activities observed in the acidogenic bioreactor were much higher than those in the methanogenic bioreactor. The hydrolysis and the methanogenic stages were well separated, as indicated by the high carbon dioxide production, high volatile fatty acids (VFAs) concentration and low pH in the acidogenic bioreactor compared with high methane production, low VFAs concentration and above neutral pH in the effluent of the methanogenic bioreactor. Digestion of SLDR gave energy yields of 2.45 kWh/kg volatile solids added in the form of methane. The integrity of the methane filter was maintained throughout the period of operation producing biogas with 51-70% methane content. A stable effluent pH showed that the methanogenic bioreactor had good ability to withstand the variations in load and VFAs concentrations that occurred in the two-stage process. In conclusion, the results of this study showed that the two-stage system was suitable for effective stabilization and biomethanation of SLDR.