Effects of organic loading rate on hydrogen and methane production in a novel two-stage reactor system: performance, enzyme activity and microbial structure

Two-stage anaerobic digestion (AD) system is an efficient technology; however, organic loading rate (OLR), a critical design and process parameter, influences the performance and capacity of digesters. This study investigates the impact of various OLRs on biogas production during co-digestion of food waste, chicken manure and corn straw in an innovatively designed two-stage reactor system, including variations in the enzyme and electron transfer system (ETS) activities and microbial community. The production of hydrogen and methane increased gradually with increasing OLR, with maximum production at 8 g VS/L d (690 mLH2/L d and 690 mLCH4/L d), which is 30–97 % higher than OLR of 1.5 to 6 g VS/L d. However, the maximum methane yield of 535 mL/g VSadded was attained at OLR of 4 g VS/L d consistent with the highest coenzyme F420 activity. Although no substantial ETS activity was seen during the acidogenic stage, the ETS activity decreased with increasing OLR in the methanogenesis stage, indicating that high substrate loading results in low syntrophic methanogenesis. Microbial analysis revealed a more diverse enrichment of acidogens and acetogens at high OLR with a wider range of substrate consumption, resulting in balanced acetoclastic and hydrogenotrophic methanogens (Methanosarcina and Methanothermobacter) in the biomethane process. These findings suggest the strengthening of syntrophic methanogenesis in high substrate-loading AD systems.