The effect of mixing intensity on the performance and microbial dynamics of a single vertical reactor integrating acidogenic and methanogenic phases in lignocellulosic biomass digestion.

The ready formation of scum in vertical reactors has been a bottleneck in the digestion of lignocellulosic materials for biogas production. This study describes a single vertical reactor that integrates the acidogenic and methanogenic phases of this process. The effects of two types of maize stover feedstock (fresh and silage) and two mixing intensities (20 and 70rpm) on methane yield were orthogonally determined. Fresh maize stover yielded approximately 14% more methane than silage maize stover. Mixing at 20rpm contributed to methane yield, while mixing at 70rpm blurred the phase boundary, resulting in accumulation of volatile fatty acids and loss of methanogens. The upper and lower phases clearly constituted a two-phase fermentation system. Clostridiales occupied the acidogenic phase, while the predominant bacteria in the methanogenic phase were Bacteroidetes, Chloroflexi, and Synergistetes. The absolute predominance of Methanosaetaceae clearly demonstrated that aceticlastic methanogenesis was the main route of methane production.