Improving removal of combined veterinary antibiotics and mitigating their negative impacts during anaerobic digestion of swine manure using modified bentonite

This study investigated the potential of modified bentonite (MB) material in mitigating the negative effects of combined veterinary antibiotics (CVAs) on anaerobic digestion (AD) process treating swine manure. Four CVAs (i.e., Oxytetracycline (OTC), Tetracycline (TC), Norfloxacin (Norf), and Sulfadiazine (SDZ)) were selected as they are most frequently detected in swine manure; and anaerobically incubated for 30 days under mesophilic con-ditions (37 degrees C) and with addition of different MB weights (1, 5, and 10 g L-1). Results showed that 1, 5, and 10 g L-1 of MB enhanced the removal of CVAs from 61.5 (no MB addition) to 72.7, 86.4, and 91.4 %, respectively. CVAs removal was accredited to biodegradation rather than adsorption to MB. Norf was rapidly transferred to solids causing an adsorption competition and decreasing co-antibiotics removal, while it was fast removed when MB was added. When 10 g L-1 of MB were added, the CVAs' adsorption competition diminished and their in-hibition on CH4 production decreased from 27 % to 1.7 %. MB addition lowered the impact of CVAs significantly by maintaining pH stability, enhanced COD removal, decreased the VFAs accumulation, and improved the ac-tivity of bacteria and archaeal communities. The cost analysis showed that MB additions to AD would reduce cost by 52 % than applied methods for animal slurry pre-treatment or post-digestive treatment for reducing antibiotic residues. These findings suggests that using MB as an environmentally-friendly and commercial additive holds promise for reducing CVAs and eradicating their negative impacts with safe digestate disposal to land applications.