High-load and Stable co-digestion of Food Waste and Chicken Manure in a Modified Continuous Stirred Tank Reactor...

High-load anaerobic digestion can enhance both economic benefits and energy efficiency, but stable operation remains challenging. The study employed a modified continuous stirred tank reactor (CSTR) to investigate operational stability and metabolic shift during long-term co-digestion of food waste and chicken manure under stepwise increasing organic loading rates (OLRs) from 2 to 8 g-VS·L^- 1·d^- 1 (VS refer to volatile solid). The system could be stably operated at OLRs up to 7 g-VS·L^- 1·d^- 1, reaching the highest energy yield of 14.0 kJ·g^- 1-VS. However, instability occurred at 8 g-VS·L^- 1·d^- 1, accompanied by volatile fatty acids (VFAs) accumulation and pH decline. The biogas and methane production were positively correlated to the OLR at stable period. High VS removal efficiency of 87.05%-91.13% can be ascribed to the sludge settling within the modified CSTR reactor. The sludge recirculation played significant roles in promoting methane yield and system stability at 6-7 g-VS·L^- 1·d^- 1 through the effects of reducing VFAs, raising pH value and adjusting microbial community. The acetoclastic Methanosaeta was predominant in stable period. Without sludge recirculation at 5 g-VS·L^- 1·d^- 1, the metabolic pathway of syntrophic acetate oxidation (SAO) combined with hydrogenotrophic methanogenesis (HM) was enhanced by the high VFAs. However, acetoclastic methanogens Methanosaeta predominated again when the bottom sludge was recirculated at 6-7 g-VS·L^- 1·d^- 1. With the extremely high VFAs content at OLR of 8 g-VS·L^- 1·d^- 1, the SAO-HM pathway regained dominance to overcome VFAs accumulation.