Formation of soluble organics and sugar degradation byproduct during hot compressed water pre-treatment of empty fruit bunch for biogas production

To assess the potential for improvements in soluble organics formation, biogas production, and inhibitory effects resulting from the concurrent generation of furan byproducts, palm oil empty fruit bunches (EFB) were subjected to hot compressed water (HCW) treatment at high temperatures and extended reaction times. The application of HCW pre-treatment at temperatures ranging from 180 °C to 220 °C for 5–60 min was found to greatly enhance the formation of soluble organic compounds and furan derivatives derived from EFB. However, only the pre-treatment carried out at 180 °C for a duration of 15 min led to a significantly greater biogas yield of 710.45 mL CH₄/gVS. Biogas yields decreased under severe pre-treatment conditions (200–220 °C), likely due to the inhibitory effects of furfural and 5-hydroxymethylfurfural (5-HMF). Then, a co-digestion system was simulated to assess the inhibitory impact of furfural and 5-HMF on biogas production. Kinetic inhibition models were used to determine the threshold at which these substances became inhibitory. Furfural and 5-HMF's inhibitory threshold was found to be 2000 mg/L. The Haldane model proved to be the most suitable for simulating the inhibitory effect of furfural, whereas the Aiba model was well-suited for simulating the impact of 5-HMF in the anaerobic co-digestion process. The current study novelty lies in identifying the limitation of HCW pre-treatment for biogas production. By elucidating the inhibitory effect of furan derivatives, it provides clear guidance for mitigating the inhibition caused by furfural and 5-HMF.