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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–1TC spike, a 10 mg L–1TC spike, a 1 mg L–1SDM spike (SDM 1), a 10 mg L–1SDM spike, a mixture of TC and SDM at 1 mg L–1each (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 tetMgene copies correlated with declines in TC; however, reductions in SDM concentration did not correlate with decreases in sul1gene copy concentrations. The AD reactors containing 10 mg L–1of TC and 10 mg L–1of SDM both had CH4production reductions of 7.8%, whereas no CH4reduction was observed in other treatments (1 mg L–1treatments). 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. Tetracycline (TC) and sulfadimethoxine (SDM) were added to dairy manure digesters.The 10 mg L−1TC and SDM treatments produced 8% less CH4than dairy manure only.The 1 mg L−1TC and SDM treatments had no significant reduction in CH4production.SDM was removed by >99% and TC by >85% during the digestion process.tetMgenes copies were reduced during digestion, but sul1copies were not.

The use of antimicrobials by the livestock industry can lead to the release of unmetabolized antimicrobials and antimicrobial resistance genes (ARG) into the environment. However, the relationship between antimicrobial use, residual antimicrobials, and ARG prevalence within manure is not well understood, specifically across temporal and location-based scales. The current study determined ARG abundance in untreated manure blend pits and long-term storage systems from 11 conventional and one antimicrobial-free dairy farms in the Northeastern U.S. at six times over one-year. Thirteen ARGs corresponding to resistance mechanisms for tetracyclines, macrolides-lincosamides, sulfonamides, aminoglycosides, and β-lactams were quantified using a Custom qPCR Array or targeted qPCR. ARG abundance differed between locations, suggesting farm specific microbial resistomes. ARG abundance also varied temporally. Manure collected during the winter contained lower ARG abundances. Overall, normalized ARG concentrations did not correlate to average antimicrobial usage or tetracycline concentrations across farms and collection dates. Of the 13 ARGs analyzed, only four genes showed a higher abundance in samples from conventional farms and eight ARGs exhibited similar normalized concentrations in the conventional and antimicrobial-free farm samples. No clear trends were observed in ARG abundance between dairy manure obtained from blend pits and long-term storage collected during two drawdown periods (fall and spring), although higher ARG abundances were generally observed in spring compared to fall. This comprehensive study informs future studies needed to determine the contributions of ARGs from dairy manure to the environment.

Hydrogen sulfide (H2S) in biogas is produced during anaerobic digestion of organic wastes and can lead to corrosion of generators used for energy production. Recently, there has been interest in utilizing biochar as a substitute for activated carbon (AC) to adsorb and remove H2S from biogas. This study was the first to test iron (Fe)-impregnation of biochar to increase H2S removal from biogas using a biogas scrubbing column. Two biochar substrates, corn stover biochar (CSB) and maple wood biochar (MB), with and without Fe-impregnation were evaluated for H2S adsorption from biogas. The H2S adsorption capacity was quantified using dynamic breakthrough experiments. Iron-impregnated maple biochar (MB-Fe) had the highest H2S saturation adsorption capacity (23.9 mg H2S g−1biochar), which was 3.9 times higher than the unmodified MB (6.1 mg H2S g−1biochar). The Fe-impregnated corn stover biochar (CSB-Fe) saturation H2S sorption capacity (8.2 mg H2S g−1biochar) was 2.5 times higher than the unmodified CSB (3.3 mg H2S g−1biochar). The presence of iron oxide (Fe3O4) was observed through X-ray diffraction and scanning electron microscopy in the Fe-impregnated biochar substrates, with ferrous sulfate (FeSO4) as one of the end products in the saturated biochar surface and elemental sulfur in the pores. The study showed how Fe-impregnation can result in a multifold increase in H2S adsorption from biogas compared to unmodified biochar.