Three-stage integrated co-digestion of pig manure and rice straw combined with biogas slurry reflux: Performance and...

To address the challenges associated with the large-scale production and difficult disposal of biogas slurry in conventional high-solid anaerobic digestion, this study developed a three-stage integrated co-digestion (TSAD) process integrated with biogas slurry reflux (BSR), and assessed its feasibility for stable semi-continuous operation over a 120-day period. Results demonstrate that TSAD enables targeted enrichment among functional microbial communities-achieved through spatially segregated operational stages: aerobic hydrolysis, microaerobic acidification, and anaerobic methanogenesis. Under an appropriate hydraulic retention time (HRT) of 20 days, the system achieved sustained high biogas production stability (with a final methane yield of 218 mL/g COD). Crucially, relying on the comprehensive strategy of high-solid feedstock, staged separation and BSR, it enhanced overall treatment efficiency by approximately 2.8-fold relative to conventional single-stage system, while reducing daily water consumption per unit substrate by 83.3 %, thereby substantially mitigating slurry emission. In contrast, reducing the HRT to 10 days boosted short-term biogas production, but also enhanced the mobilization of copper and zinc into the liquid phase and caused rapid accumulation of ammonia nitrogen (up to 4855.1 mg/L) during prolonged operation, ultimately increasing the risk of process instability and potential system failure. These findings provide a theoretical basis for achieving process optimization through staged separation and BSR, and underscore the necessity of rational HRT selection and coupling the targeted removal technologies of ammonia nitrogen and heavy metals, so as to ensure sustainable and long-term system performance.