Comparison of dry and wet milling pre-treatment methods for improving the anaerobic digestion performance of the Pennisetum hybrid

Two types of milling pre-treatment methods were used to study their effects on the anaerobic digestion performance of the Pennisetum hybrid. The physical structure of the Pennisetum hybrid could be significantly damaged with both milling pre-treatment methods. With an increase in treatment time, the pore volume, surface area, average pore diameter, and the crystallization index of the Pennisetum hybrid increased gradually. The anaerobic fermentation test showed that, among the dry and wet milling pre-treatments, the highest specific methane yield was obtained at 3 h dry milling pre-treated (358.07 mL g(-1) VS) and 6 h wet milling pre-treated grass (315.87 mL g(-1)), which was 41.04% and 24.42% higher than that of the untreated Pennisetum hybrid (253.88 mL g(-1) VS). The median diameter (D50) of the Pennisetum hybrid was 25.28% and 22.36% lower than that of the untreated Pennisetum hybrid after 3 h dry and 6 h wet milling pre-treatment. After wet milling pre-treatment, the fluidity of the Pennisetum hybrid was much better than conventional "smashing" pre-treatment, which may make feeding easier in large-scale biogas projects. Good fluidity and optimization of the degree of milling pre-treatment could, therefore, be applied on an industrial scale in the future