Improving treatment capacity and process stability via a two-stage anaerobic digestion of food waste combining solid-state acidogenesis and leachate methanogenesis/recirculation

Food waste (FW) accounts for the largest category of municipal solid wastes in the United States and the proper disposal of FW is a unique challenge with urgent environmental concerns. Solid-state anaerobic digestion (SSAD) is a viable option to treat FW but still hindered by poor mass transfer and severe inhibition caused by the accumulation of acidic intermediates. To improve the treatment capacity and process stability using high-solid FW as the sole feedstock via AD, a bench-scale two-stage system comprising of a first-stage solid-state digester (Stage 1; 2 L) and a second-stage liquid digester (Stage 2; 1.4 L) was established and carried out in a semi-batch mode for 99 days. When one-stage SSAD started to encounter acid accumulation and methanogenesis inhibition at a FW loading of 300 g (15% of working volume), the two-stage system secured robust and stable biogas production when the FW loading continued rising to 400 g (20% of working volume), and an increase of 2.4 kJ/gVS (equivalent to 18.2% in one-stage system) in energy yield was achieved. Stage 1 served as a hydrolysis/acidogenesis reactor with a total acids level of 28.5 g/L and a pH of 5.65. A large portion of lactic acid in total acids indicated shifts of metabolic pathways. The transfer of acid-rich leachate from Stage 1 to Stage 2 ensured a high methane yield of 393.0 mL/gVS (90.2% of the biological methane potential), while the liquid effluent recirculation provided buffering to Stage 1. Further increase of FW loading triggered a system instability. The enhanced buffering capacity and increased free ammonia molecules, synergistically induced certain inhibitory effects on FW acidogenesis in Stage 1. To conclude, the two-stage system outpaced the one-stage SSAD with higher FW loading and energy yield.