Enhanced biogas generation from water hyacinth juice using an upflow anaerobic digester with effluent recirculation

In anaerobic reactors, effluent recirculation is important to enhances distribution, buffer and boost contacts. This study explores the synergistic effects of effluent recirculation and biochar addition on methane production from water hyacinth juice (WHJ) using high rate upflow anerobic reactor. Water hyacinth biomass was crushed and compressed to extract juice, while the residual solids were carbonized at 800 °C to produce biochar. The upflow anerobic reactor was operated at mesophilic conditions (37 ± 1 °C) with varying hydraulic retention times (HRT) and effluent recirculation ratios of 1:3, 1:1, and 3:1 with and without biochar addition. Among all recirculation configurations, the 3:1 recirculation ratio with biochar addition exhibited the highest methane content (79.5%) at HRT of 2 days, indicating a strong synergistic effect between recirculation and biochar’s microbial support and adsorption functions. Biochar alone also improved methane purity (up to 84.5%) by enhancing volatile suspended solid (VSS) removal and reducing CO2 content. Implementing recirculation reduced the stabilization period from 30 days to 20 days. Chemical oxygen demand (COD) and VSS removal efficiencies were also maximized under recirculation and biochar conditions, reaching up to 81.8% and 76.1%, respectively. Despite slightly elevated pH levels (up to 8.36) under recirculation scenarios, microbial activity and biogas production remained stable, suggesting the robustness of the anaerobic reactor system. Overall, integrating biochar and effluent recirculation offers a promising strategy for optimizing biogas production from WHJ while improving process stability, and methane purity at short HRT.