Coupled enhancement of sulfate-assisted anaerobic digestion for livestock wastewater treatment under antibiotic stress using sludge-derived biochar and zero-valent iron.

Antibiotic stress severely inhibits anaerobic digestion performance by disrupting microbial metabolism and volatile fatty acid (VFA) conversion. To address this bottleneck, a sulfate (SO42-)-assisted anaerobic digestion system co-coupled with waste sludge-derived biochar and zero-valent iron (BC/Fe0) was developed to treat livestock wastewater under antibiotic stress. Results demonstrated that the coupled system exhibited exceptional process robustness. Even under a severe shock load of 100 mg/(L·d) antibiotic, the system maintained a chemical oxygen demand (CODCr) removal efficiency of 76.5 ± 1.6% and an antibiotic removal efficiency of 53.6 ± 2.1%, alongside near-complete precipitation of Cu2+ and Zn2+. The coupled system successfully preserved microbial metabolic flux, yielding 38% more VFAs than the Blank system and maintaining a stable peak gas production of 575 ± 30 mL/cycle. System stability was further evidenced by a 58.8% reduction in pH fluctuations and mitigated biomass loss (maintaining mixed liquor volatile suspended solids at 12.2 ± 0.7 g/L). Additionally, the system optimized extracellular polymeric substance (EPS) secretion to strengthen the biological protective barrier. Mechanistic analysis suggested that this resilience may be associated with multiple complementary mechanisms, primarily involving adsorptive attenuation, Fe0-facilitated electron transfer, and the coupling of SO42- reduction with propionate oxidation. These findings provide a reliable technological foundation for stabilizing anaerobic bioprocesses treating highly antibiotic-laden livestock wastewater.