Optimization of recirculation ratio for biohythane production by two-phase anaerobic co-digestion of high-solid food waste and paper waste

The recirculated two-phase anaerobic digestion (R-TPAD) system has gradually replaced the conventional TPAD configuration, emerging as a stable and efficient strategy for biohythane production. However, inappropriate recirculation ratios (R) may cause either incomplete organic conversion or insufficient phase separation. In this study, a semi-continuous R-TPAD system comprising two continuously stirred reactors was established to digest food and paper waste (total solids ≈ 17 %). The influence of R on process performance, microbial structures, metabolic pathways, and energy recovery was systematically assessed. Results showed that the buffering effect of recycled methanogenic sludge maintained the pH of acidogenic phase at 5.2 and ensured complete phase separation at R0.8, achieving optimal yields of H2 (75 L/kg-VS) and CH4 (402 L/kg-VS) with 83 % COD removal. Stoichiometric and thermodynamic analyses confirmed that R0.8 strengthened metabolic coupling between the acidogenic and methanogenic phases, thereby promoting methane harvest. 16S rRNA analysis indicated that the appropriate ratio could push forward the cooperation between functional guilds by stimulating acidogens in acidogenic phase and enriching methanogens in methanogenic phase. Energy balance analysis indicated that R0.8 would deliver superior net energy recovery. These findings provide a practical operational basis for scaling up R-TPAD technology.