Conversion of rubber wood waste to methane by ethanol organosolv pretreatment

The restricted bioavailability of structurally complex carbohydrates for digestion has hitherto resulted in a low methane potential from rubber wood waste (RW). The effects of hydrothermal (HT) and ethanol organosolv (OS) pretreatments on the methane produced by anaerobic digestion of RW were investigated in the study reported. HT with temperatures above 190 °C significantly enhanced the anaerobic digestibility of RW mainly due to the degradation of hemicellulose. On the other hand, OS with 75% ethanol provided a potential methane gas yield of 165.1 L CH₄/kg-VS, which was higher than that for HT at 210 and 230 °C by 39 and 7%, respectively. This was due to intensive delignification during OS pretreatment which led to a reduction in the non-productive adsorption of cellulolytic enzymes by lignin. A first-order kinetic model showed that OS had a higher hydrolysis rate (k = 0.073 ± 0.003 day⁻¹) resulting in a higher methane yield when lower pretreatment temperatures were applied. The anaerobic degradation of the pretreated RW in this study was a result of simultaneous CH₄ production through the symbiosis of anaerobic bacteria and methanogens using a combination of the aceticlastic and hydrogenothrophic bioconversion pathways. The recovery and use of the phenolic compounds remaining in the process water would be a way of adding value to this process and the feasibility of producing methane from RW should be further investigated.