Substrate selection for optimizing swine tailwater treatment in vertical subsurface flow constructed wetlands: The superior performance of a biochar-manganese ore composite

The increasing demand for pork has intensified environmental concerns related to swine farm wastewater, which contains high levels of nutrients, antibiotics, heavy metals, and antibiotic resistance genes (ARGs). This study evaluates the performance of four substrates-gravel, biochar, Mn ore, and a biochar/Mn ore mixture—in vertical subsurface flow constructed wetlands (VSFCWs) for the advanced treatment of swine tailwater. Batch adsorption experiments revealed that biochar and Mn ore exhibit superior adsorption capacities for antibiotics (17.95–106.97 μg·g −1 ) and copper (Cu, ~107 μg·g −1 ) compared to gravel (5.55–37.61 μg·g −1 ), with pseudo-second-order kinetics and Langmuir/Freundlich isotherms indicating chemisorption and multilayer adsorption mechanisms. Pilot-scale VSFCWs demonstrated excellent removal efficiencies for total organic carbon (TOC, 89.93–95.24 %), total nitrogen (TN, 90–96.98 %), ammonia nitrogen (NH₄ + -N, 93.94–98.16 %), phosphate (PO₄ 3− -P, 89.52–97.62 %), antibiotics (>94 %), Cu (>98 %), and ARGs (0.83–1.36 log units). Biochar-based systems (VSFCW-B and VSFCW-BM) outperformed others in removing TOC, TN, NH₄ + -N, antibiotics, and ARGs due to biochar's high surface area and microbial support, while gravel and Mn ore excelled in phosphorus removal. Further Microbial community analysis revealed that biochar-based systems fostered richer and more diverse bacterial communities. Environmental temperature significantly influenced pollutant removal, with low-level antibiotics (0.03–0.09 mg·L −1 ) and Cu (1.09 mg·L −1 ) enhancing nitrogen removal via co-metabolism. The biochar/Mn ore mixture is recommended for its synergistic performance in treating organic matter, sulfonamides, and ARGs, providing a cost-effective and environmentally friendly solution for swine tailwater reuse. • Biochar and Mn ore showed superior adsorption for antibiotics and Cu, outperforming gravel. • Low antibiotics/Cu aided conventional pollutants through co-metabolism. • Temperature was the primary factor governing VSFCWs performance. • VSFCW-BM recommended for swine tailwater treatment, excelling in TOC, antibiotics and ARGs removal.