Assessing the occurrence of pharmaceuticals and antibiotic resistance genes during the anaerobic treatment of slaughterhouse wastewater at different temperatures.

This study investigates the effect of psychrophilic, mesophilic and thermophilic temperatures on the anaerobic treatment of slaughterhouse wastewater, in terms of biogas production, occurrence of 30 pharmaceutical compounds of veterinary use, 4 antibiotic resistance genes (ARGs) which provide resistance to tetracyclines (tetW), fluoroquinolones (qnrS), macrolide-lincosamide-streptogramin (ermB) and sulfonamides (sul1) antibiotics, as well as class I integron-integrase gene (intI1), related to horizontal gene transfer. The highest methane yield was obtained at a mesophilic temperature (35 °C) (323 mL CH4/g TCOD) followed by the yield obtained at thermophilic temperature (53 °C) (242 mL CH4/g TCOD). Regarding pharmaceuticals, chlortetracycline, oxytetracycline, tilmicosin, and lincomycin were the most abundant in the slaughterhouse wastewater, being detected predominantly in the solid phase (with median concentrations >200 μg/kg dry weight). On the other hand, ciprofloxacin, ofloxacin, norfloxacin, lincomycin and ibuprofen were the most predominant in the anaerobic digestate regardless of the treatment temperature. Psychrophilic temperatures (21 °C) exhibited moderate to low pharmaceuticals removal, while a large fraction of them were removed at a thermophilic temperature reaching 70-90% removals for tetracycline, macrolides and one sulfonamide (sulfapyridine). The highest relative abundance of the quantified ARGs was found at 53 °C, suggesting that thermophilic temperatures normally associated with better removals of pathogens do not necessarily show better removals of antibiotic resistance genes.