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Methane production from solid-state anaerobic digestion (SS-AD) of empty fruit bunches (EFB) using recycling solid-anaerobic digested (S-AD) sludge and liquid-anaerobic digested (L-AD) sludge as inoculum was investigated under thermophilic (55 °C) and mesophilic conditions (40 °C). The methane production rate of L-AD sludge (2.98–3.27 L-CH4 L−1 reactor d −1) was 1–2 times higher than recycling S-AD sludge (1.36 L-CH4 L−1 reactor d −1) as inoculum. Methane production of L-AD and S-AD sludge inoculum was 53.5–54.1 and 31.2–39.0 m3 CH4 tonne-1 EFB, respectively. The SS-AD of EFB with L-AD and S-AD sludge under thermophilic condition had specific methane activity of 34–96 times higher than the mesophilic condition. Clostridium sp., Methanosaeta sp., and Methanoculleus sp. were responsible for the SS-AD of EFB with L-AD sludge inoculum. The L-AD sludge was suitable inoculum for SS-AD of EFB in terms of methane production rate, cellulose degradation efficiency, biogas production, and microbial activity.

Thermophilic solid-state anaerobic digestion (SS-AD) of biomass from palm oil mill industry, including empty fruit bunches (EFB), oil palm fronds (OPF) and oil palm trunks (OPT) were investigated under different feedstock-to-inoculum ratios (F:I), carbon-to-nitrogen ratios (C:N) and total solids contents (TS). The optimal condition for biogas production was achieved at F:I ratio of 2:1, C:N ratio of 40:1 and TS content of 16%. The highest methane yield from EFB, OPF and OPT were 223, 207 and 161 m 3 CH 4 tonne −1 VS, respectively. Bacterial community structure in thermophilic SS-AD of oil palm biomass was dominated by Thermoanaerobacterium sp. and Clostridium sp. Archaeal community structure was dominated by Methanosaeta sp. and Methanoculleus sp. The highest methane production of 45 m 3 CH 4 tonne −1 fresh biomass was achieved from EFB. Solid waste residues from palm oil mill industry have great potential for methane production via thermophilic SS-AD.

The highly heterogeneous structure of oil palm empty fruit bunches (EFB) restrict the exploitation of its potential in biogas plants. Fungal pretreatment of EFB with Trichoderma reesei TISTR 3080 and Pleurotus ostreatus DSM 11191 to improve EFB biodegradability and methane production via solid-state anaerobic digestion (SS-AD) was investigated. The pretreatment by T. reesei and P. ostreatus were effective at an EFB moisture content of 80–85 (%w/w) and incubation for 10–15 days. The methane yield was increased by up to 44–52 % compared to raw EFB. A strong linear relationship was observed between the methane yield, cellulose, hemicellulose and lignin degradation for both fungal strains. Maximum methane production from untreated EFB, T. reesei and P. ostreatus pretreated EFB was 36.2, 75.8, and 64.9 m3 CH4 tonne−1 EFB, respectively. The pretreatment of EFB with T. reesei is thus a promising strategy to achieve higher methane yields and a 60–75 % higher biodegradation efficiency without any additional energy input and undesired by-product inhibition.