Effect of hydrothermal and acidified food waste pretreatment on anaerobic digestion of rice straw

To address the low efficiency of anaerobic digestion (AD) of rice straw caused by its dense lignocellulosic structure and high carbon-to-nitrogen ratio, there is an urgent need to develop an efficient and environmentally friendly pretreatment method and to elucidate its underlying mechanisms. Based on the “waste-treat-waste” concept, this study prepared acidified liquid from food waste via fermentation and combined it with hydrothermal pretreatment. The effects of different pretreatments on lignocellulose degradation of rice straw and subsequent methane production were investigated. Microbial community responses were analyzed using both 16S rRNA gene relative abundance profiling and absolute quantitative sequencing.In the pretreatment phase, five groups were compared: raw acidified liquid (FW), acidified liquid adjusted to pH 2.4 with hydrochloric acid (AFW), hydrothermal treatment with water alone (Hydro), hydrothermal treatment with hydrochloric acid as control (CG), and untreated straw (RS). After pretreatment, the solid residues were characterized for lignocellulose composition, and the supernatants were analyzed for reducing sugar concentration. The AFW pretreatment showed the best performance, achieving a reducing sugar concentration of 11.73 g/L and a 87.0% reduction in absolute hemicellulose content of rice straw. In comparison, FW and Hydro achieved lower reducing sugar yields, while CG produced high SCOD but low reducing sugars. During AD phase, batch experiments were conducted at 37  ℃ for 21 days with an inoculum-to-substrate ratio of approximately 2:1 (based on volatile solids). The AD performance was evaluated by cumulative methane yield, as well as by monitoring pH, volatile fatty acids (VFAs), and SCOD over time. Six experimental groups were included: straw pretreated with AFW (EX_AFW), straw pretreated with FW (EX_FW), straw pretreated with Hydro (Hydro), untreated straw (RS), and two controls with acidified liquid only (CK_FW and FW without straw). The EX_AFW group achieved the highest cumulative methane yield of 77.06 mL/g, which was 140.0% higher than that of RS and 18.1% higher than that of Hydro. The EX_FW group also outperformed Hydro, with a 113% increase over RS. The EX_AFW group showed the fastest methane production, with most of the biogas generated within 9 days, and its acetate was nearly depleted by day 6, whereas EX_FW consumed acetate more slowly. The pH in all reactors remained between 7.7 and 8.2 after an initial slight decrease, and SCOD dynamics were consistent with methane production patterns. Microbial community analysis revealed that EX_AFW significantly altered both bacterial and archaeal community structures. At the phylum level, the relative abundances of Chloroflexi and Spirochaetota were higher in the EX_AFW compared with RS. These phyla have been previously reported to be involved in VFAs metabolism and lignocellulose degradation. At the genus level, the absolute abundance of the acetoclastic methanogen Methanosaeta reached 2.69 × 10⁸ copies/g in EX_AFW, a 50% increase over RS. The absolute abundances of the saccharolytic bacterium SBR1031 and the hydrolytic bacterium Trichococcus were also markedly higher in EX_AFW than in RS, which correlated with the high reducing sugar yield observed in the pretreatment phase. Additionally, the absolute abundance of Methanosarcina, a versatile methanogen capable of both acetoclastic and hydrogenotrophic pathways, increased nearly six-fold in EX_AFW. In conclusion, the synergistic pretreatment combining acidified food waste liquid with mild hydrothermal conditions (AFW) effectively disrupted the lignocellulosic structure of rice straw and enhanced methane production. The improvement in AD performance was associated with the enrichment of specific hydrolytic and acetoclastic microbial populations, particularly Methanosaeta and SBR1031. This study demonstrates that food waste-derived acidified liquid can serve as a low-cost and eco-friendly alternative to pure organic acids for straw pretreatment, and the “waste-treat-waste” approach provides a promising strategy for the synergistic valorization of food waste and agricultural residues.