The Influence of SDM on the Performance of the EGSB–Two‐Stage A/O Biofilm Process for Pig Wastewater Treatment and Mechanism Investigation

ABSTRACT To address the treatment of pig wastewater containing high levels of organic matter, this study investigated the impact of sulfadimethoxine (SDM) on the performance and underlying mechanisms of an expanded granular sludge bed (EGSB) combined with a two‐stage anoxic/oxic (A/O) biofilm process. The reactor demonstrated robust organic loading tolerance (up to 12,195 mg/L COD) while maintaining stable treatment performance, achieving > 95% COD removal efficiency and 70%–80% ammonia nitrogen (NH 3 ‐N) elimination. Systematic analysis of sludge microstructure and microbial consortia was conducted through integrated techniques, including scanning electron microscopy (SEM) and high‐throughput sequencing. The introduction of SDM induced significant restructuring of microbial architecture and population dynamics, with Proteobacteria (21.73%–89.74%), Firmicutes (5.54%–33.53%), Chloroflexi (1.22%–23.07%), Bacteroidetes (2.48%–12.23%), and Synergistetes (15.64%–26.86%) emerging as dominant phyla. Notably, Proteobacteria and Firmicutes demonstrated superior resistance under the reaction process. Concurrently, microbial analysis revealed a significant enrichment of nitrogen‐phosphorus‐removing genera Acinetobacter and Pseudomonas during the reaction process, establishing that these bacterial taxa play a central role in the biodegradation of organic pollutants. Therefore, the microorganisms exhibited adaptability to high‐load antibiotic‐containing environments, substantiating the hybrid reactor's potential for livestock and poultry breeding wastewater treatment. Summary The presence of antibiotics reduces the removal of conventional pollutants by the combined process. Antibiotics cause changes in microbial population structure. EGSB–two–stage A/O combination process can effectively treat high organic load wastewater.