Colloid milling as an energy efficient cotreatment strategy for lignocellulosic anaerobic digestion

Background: In this study, mechanical cotreatment of lignocellulosic biomass with a colloid mill enhanced degradation during anaerobic fermentation. Once-fermented slurry from a mesophilic switchgrass-fed anaerobic digester was subjected to milling (mechanical cotreatment) by recirculating through a colloid mill for various durations (0-min/no-milling, 10-sec/1-pass, 0.5-min/3-passes, 2-min/12-passes, 5-min/30-passes, 10-min/60-passes). Following this, a second fermentation was conducted in batch mode for 19 days.
Results: Biogas production during the fermentation post milling was 82.3±4.1 mL per gram volatile solids for the unmilled control and between 91.3±8 mL g-1 VS (10-sec/1-pass milling) to 102.6±5.2 mL g-1 VS (10-min/60-passes milling) for the milled treatments. Mechanical cotreatment using a colloid mill decreased the mean particle size of the milled treatments relative to the control, and this difference persisted throughout the secondary fermentation post milling. For milling durations up to 0.5 min/3-passes, the bench-scale colloid mill consumed less energy than the additional biogas energy released due to milling.

Conclusions: The results indicate that mechanical cotreatment using a colloid mill can be an efficient strategy for improved biomass solubilization. The Colloid mill also provides additional benefits of being an in-line system that may be implemented at larger scales. Theoretical scale-up calculations based on this finding indicated that the colloid mill may become more energy efficient at larger scales and with higher solids loading rates, thereby making cotreatment an energetically feasible strategy at large scale.