Enhancing grass-fed anaerobic digestion by mechanical cotreatment

The aim of this study was to increase the accessibility and accelerate the breakdown of lignocellulosic biomass in an anaerobic fermentation system by mechanical cotreatment: milling during fermentation, as an alternative to conventional pretreatment prior to biological deconstruction. Effluent from a mesophilic anaerobic digester running with unpretreated senescent switchgrass as the predominant carbon source was subjected to ball milling for 0.5, 2, 5 and 10 minutes. A batch fermentation test was conducted with this material in triplicate for 18 days with unmilled effluent as the ‘status quo’ control. The results indicate 0.5 – 10 minutes of cotreatment increased sugar solubilization by 5– 13% when compared to the unmilled control, with greater solubilization correlated with increased milling duration. Cotreatment also increased biogas production and decreased mean particle size. Energy consumption measurements of a lab-scale mill indicate that longer durations of milling offer diminishing benefits with respect to additional methane production. Cotreatment in anaerobic digestion systems, as demonstrated in this study, provides an alternative approach to conventional pretreatments to increase biogas production from lignocellulosic grassy material.