Tetracycline, sulfadimethoxine, and antibiotic resistance gene dynamics during anaerobic digestion of dairy manure

Antibiotic use in animal husbandry is a potential entryway for antibiotics and antibiotic resistance genes (ARGs) to enter the environment through manure fertilizer application. The potential of anaerobic digestion (AD) to remove antibiotics and ARGs was investigated through tetracycline (TC) and sulfadimethoxine (SDM) additions into dairy manure digested for 44 d. This was the first study to evaluate antibiotics at concentrations quantified on-farm and relevant to field applications of manure. Triplicate treatments included a 1 mg L^–1 TC spike, a 10 mg L^–1 TC spike, a 1 mg L^–1 SDM spike (SDM 1), a 10 mg L^–1 SDM spike, a mixture of TC and SDM at 1 mg L^–1 each (TC+SDM 1), and a manure-only treatment. The SDM spikes were reduced by >99% reduction during the AD processing, but TC additions had variable reductions (0–96%). Molecular analyses showed that decreases in tetM gene copies correlated with declines in TC; however, reductions in SDM concentration did not correlate with decreases in sul1 gene copy concentrations. The AD reactors containing 10 mg L^–1 of TC and 10 mg L^–1 of SDM both had CH₄ production reductions of 7.8%, whereas no CH₄ reduction was observed in other treatments (1 mg L^–1 treatments). The study results were the first to confirm that AD can remove SDM when adding at concentrations observed in on-farm manure (<1 mg L^–1) without compromising energy production. Because TC adsorbs to the solid particles and transforms into isomers, the decreases in TC concentrations were more variable and should be closely monitored.