Effect of low digestate recirculation ratio on biofuel and bioenergy recovery in a two-stage anaerobic digestion process

Abstract

A relatively high (0.2–4.3) digestate recirculation ratio (RR) is typically adopted to raise the pH and provide the dark fermentation reactor (DF) with alkalinity and hydrogen-producing microorganisms in a two-stage anaerobic digestion process. This study examined the production of bio-H2 and bio-CH4 from readily biodegradable organic waste in a large scale recirculated two-stage thermophilic anaerobic system to determine the effect of low RR on biofuel and bioenergy recovery. The performance of the two-stage system was evaluated at 2 hydraulic retention times (HRT) (1.1 and 2.5 d) in DF and 4 RR (0, 0.11, 0.18 and 0.25). The pH in DF was not controlled and ranged from 3.8 to 4.2. Hydrogen yield was negatively affected by digestate recirculation, while CH4 yield, as well as H2 and CH4 production rates, first tended to increase and then decrease with increasing RR. Overall, biofuel and bioenergy were best recovered at an RR of 0.11, namely 1.48 L H2/L/d, 0.88 L CH4/L/d, 106.2 mL H2/g VSinit.,161.3 mL CH4/g VSinit., 7.7 kJ/g VSinit. and 88.2 kJ/L/d were obtained depending on HRT in DF. It has been shown that a low RR can improve the performance of the two-stage anaerobic digestion process.