Mesophilic aerobic digestion: An efficient and inexpensive biological pretreatment to improve biogas production from highly-recalcitrant pinewood

Biological pretreatments and their combination with chemical and physical pretreatments are among the inexpensive methods for the improvement of biogas production from lignocellulosic biomass. In this study, mesophilic aerobic digestion was studied as a biological pretreatment to improve biogas production from pinewood. Its effects in combination with other pretreatments, i.e., liquid hot water (LHW), sodium hydroxide (NaOH), and ultrasonic pretreatments, were also investigated. LHW and Alkali (using 8% w/w NaOH) pretreatments were performed at less severe conditions (100 °C for 10 min) to evaluate the synergistic effect of these thermochemical pretreatments on biogas production. The ultrasonic pretreatment was performed at 40 KHz for 30 min at 40 °C. Biological pretreatment was carried out using aerobic sludge at 37 °C for 10 days. The aerobic digestion was the only pretreatment that could individually improve the methane yield from pinewood by 7.3 folds. However, the highest methane yield (57.7 mL/g VS of pretreated pinewood) was obtained by the combination of NaOH, biological, and ultrasonic pretreatments, which was 11.2 folds higher than that of the untreated substrate. It was concluded that the pretreatment with aerobic digestion could significantly improve the biogas production yield, especially in combination with mild thermo-chemical pretreatments.