Utilization of subcritical water for improving methane production from oil palm empty fruit bunch by anaerobic co-digestion: Process optimization, compositional, chemical, and morphological analysis

The recalcitrance of the palm oil empty fruit bunch (EFB) lignocellulosic structure is a key hindrance to high methane production, even with co-digestion with palm oil mill effluent. This study pre-treated EFB with subcritical water (SCW) to increase methane and reduce lignin. Response surface methodology was used to examine the effects and interactions of pre-treatment temperature (120–180 °C), time (10–30 min), and liquid to solid (L/S) ratio (10–20) on cumulative methane yield (CMY), volatile solid (VS) removal, and lignin content. Experiments demonstrated that CMY increased to 803.36 mL CH₄/gVS, lignin reduced to 12.21 %, and 36.61 % VS removal at 120 °C, 10 min, and L/S ratio of 20. SCW pre-treated EFB had 55.42 % more CMY and 124.30 % greater gross energy production than untreated. Pre-treated EFB has higher sugars and volatile fatty acids, and its crystallinity index increased to 65.42 %. Fourier transform infrared spectroscopy confirmed lignocellulosic structural alterations in pre-treated EFB. Morphological analysis showed structural cracking and craters in pre-treated EFB. SCW pre-treatment efficiently improved co-digestion for high methane production.