Biochar-mediated regulation of anaerobic digestion of swine wastewater at low temperature: Performance and microbial mechanism

Anaerobic digestion (AD) is a promising technology for livestock waste treatment and renewable biogas production, yet its cold-region application is limited by high heating energy consumption and low biogas yield at low temperatures. Biochar improves mesophilic/thermophilic AD performance, while its regulatory effects and mechanisms on 20°C swine wastewater AD remain unclear. Here, rice husk biochar (0, 0.5, 1.0, 1.5 wt%) was added to 20°C swine wastewater AD systems to investigate biogas production, organic matter degradation, physicochemical properties, and microbial/metabolic succession. The 1.5 wt% biochar group increased cumulative biogas yield by 30.88%, reduced organic matter concentration by 20.96%, and saved ∼18% heating energy via lower system specific heat capacity. Biochar enhanced AD stability, as shown by milder volatile fatty acid (VFA) fluctuations. Bacteroides, Streptococcus, Proteiniphilum, Advenella, Sedimentibacter were core organic-converting bacteria, and Methanobrevibacter, Methanocorpusculum, Methanobacterium were psychrophilic methanogenic marker archaea. Biochar simultaneously enhanced hydrogenotrophic and methylotrophic methanogenesis, with the former remaining dominant. Although limited to batch tests and a single biochar type, these findings offer a solid foundation for validating the strategy in continuous reactors and with other biochar feedstocks. This study clarifies biochar's regulatory effect and microbial mechanism on low-temperature swine wastewater AD, providing theoretical and technical support for cold-region AD project operation.