Propagated fixed-bed mixed-acid fermentation: Effect of volatile solid loading rate and agitation at near-neutral pH

To increase conversion and product concentration, mixed-acid fermentation can use a countercurrent strategy where solids and liquids pass in opposite directions through a series of fermentors. To limit the requirement for moving solids, this study employed a propagated fixed-bed fermentation, where solids were stationary and only liquid was transferred. To evaluate the role of agitation, continuous mixing was compared with periodic mixing. The periodically mixed fermentation had similar conversion, but lower yield and selectivity. Increasing volatile solid loading rate from 1.5 to 5.1 g non-acid volatile solids/(Lliq·d) and increasing liquid retention time decreased yield, conversion, selectivity, but increased product concentrations. Compared to a previous study at high pH (∼9), this study achieved higher performance at near neutral pH (∼6.5) and optimal C–N ratios. Compared to countercurrent fermentation, propagated fixed-bed fermentations have similar selectivities and produce similar proportions of acetic acid, but have lower yields, conversion, productivities, and acid concentrations.