Semi-continuous fermentation of chopped rice straw in a solid-state anaerobic digester: Influences of the tank height-to-diameter ratio, solid-state bed concentration, and leachate recirculation ratio

Abstract

Chopped rice straw fermentation in solid-state anaerobic digesters (SS-ADs) is a viable path for biogas generation. Thus, three fermentation test series in typical anaerobic digesters were operated to investigate the effects of the external surface area, permeability, and liquid fluidity of the solid-state bed (SSB) in improving anaerobic digestion, as determined by varying the tank height-to-diameter (H:D) ratio, SSB concentration, and leachate recirculation ratio (RR), respectively. In Series A, the H:D ratio impacted the modulation of digestion progress, including biogas generation, volatile fatty acids release, and lignocellulosic degradation. In Series B, the use of an SSB concentration below the inflection threshold (10.7 %–13.3%) for biogas production decline resulted in the highest methane yield and average methane content. In Series C, a 25.0% RR increased methane production significantly (P < 0.01) in both the semi-continuous fermentation SS-AD and downward plug-flow anaerobic digester (DPAD). Accordingly, energy flow analysis revealed that the DPAD produced 19.3% more net energy than the liquid anaerobic digester.