Effect of veterinary antibiotics on methane yield from livestock manure anaerobic digestion: an analytical review of the evidence

Antibiotics are administered to livestock animals as medications and, in some jurisdictions, as growth promotors. This review examines the impact of veterinary antibiotics on methane production from manure anaerobic digestion (AD). The animals excrete about 17–90% of the administered antibiotics in manure unchanged or as metabolites, which adversely affect microorganisms catalyzing the manure AD, thereby reducing methane yields. Different antibiotics influence methane production to different extents (0–80%). The results from studies on manure artificially spiked with antibiotics differ from those on manure from antibiotic-fed animals, likely due to the effect of other bioactive substances in the manure. Over time, the microbial culture might adapt to the antibiotics, altering its composition, and further affecting the methane yield. Such adaptation indicates that short-term studies might not fully capture the antibiotic’s long-term effects on AD. Effects of oxytetracycline and chlortetracycline on methane production are debatable, with chlortetracycline generally believed to have a slightly stronger inhibition. Correlation, nonlinear modeling/simulation, and principal component analysis (PCA) reveal that the antibiotic effects on methane yield are complex and depend on various parameters such as antibiotic type, concentration, application mode, duration, specific microbial communities, and digester conditions. The PCA showed that the temperature and concentration rather than the manure origin (pigs vs cows) dictate the magnitude of methane production inhibition. Data on the kinetics of antibiotics’ impact, isomerization, and effects of operation strategies are missing. This review summarizes the main knowledge gaps concerning AD of antibiotics-containing manure and suggestions for operational strategies and future research.