The fate of sulfamethoxazole and trimethoprim in a micro-aerated anaerobic membrane bioreactor: implications for antibiotic resistance spreading

Interest in reusing treated wastewater drives efforts to eliminate antibiotics from water sources to prevent antibiotic resistance. Micro-aerated anaerobic membrane bioreactors (MA-AnMBR) promote wastewater reuse with high organic matter conversion to biogas, under a small footprint. However, the fates of antibiotics, antibiotic-resistant bacteria (ARB), and their antibiotic-resistance genes (ARGs) are not known in these systems. We studied the effects, conversions, and resistance induction, following the addition of 150 μg·L-1 of two antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), in a laboratory-scale MA-AnMBR. TMP and SMX were removed at 97 and 86%, indicating that micro-aeration did not hamper the removal of the antibiotics. These antibiotics only affected the pH and biogas composition of the process, with a significant change in pH from 7.8 to 7.5, and a decrease in biogas CH4 content from 84 to 78%. TMP was rapidly adsorbed onto the sludge and subsequently degraded during the long retention of the solids of 27 days. SMX adsorption was minimal, but the applied hydraulic retention time of 2.6 days was sufficiently long to biodegrade SMX. The levels of three ARGs ( sul1 and sul2 for SMX, dfrA1 ) and one mobile genetic element biomarker ( intI1 ) were analysed by qPCR, in combination with ARB tracked by plating. Additions of the antibiotics increased the relative abundances of all ARGs and intI1 in the MA-AnMBR sludge, with the sul2 gene folding 15 times after 310 days of operation. The MA-AnMBR was able to reduce the concentration of ARB in the permeate by 3 log. Highlights ### Competing Interest Statement The authors have declared no competing interest.