Effects of Antimicrobials on Methane Production in an Anaerobic Digestion Process

BACKGROUND: Anaerobic digestion process is recently adapted technology for treatment of organic waste such as animal manure because the energy embedded in the waste can be recovered from the waste while the organic waste were digested. Ever increased demand for consumption of meat resulted in the excessive use of antimicrobials to the livestocks for more food production. Most antimicrobials administered to animals are excreted through urine and feces, which might highly affect the biological treatment processes of the animal manure. The aim of this study was to investigate the effects of antimicrobials on the efficiency of anaerobic digestion process and to clarify the interactions between antimicrobials and anaerobes. METHODS AND RESULTS: The experiment was consisted of two parts 1) batch test to investigate the effects of individual antibiotic compounds on production of methane and VFAs(volatile fatty acids), and removal efficiency of organic matter, and 2) the continuous reactor test to elucidate the effects of mixed antimicrobials on the whole anaerobic digestion process. The batch test showed no inhibitions in the rate of methane and VFAs production, and the rate of organic removal were observed with treatment at 1~10 mg/L of antimicrobials while temporary inhibition was observed at 50 mg/L treatment. In contrast, treatment of 100 mg/L antimicrobials resulted in continuous decreased in the rate of methane production and organic removal efficiency. The continuous reactor test conduced to see the influence of the mixed antimicrobials showed only small declines in the methane production and organic matter removal when 1~10 mg/L of combined antimicrobials were applied but this was not significant. In contrast, with the treatment of 50 mg/L of combined antimicrobials, the rate of organic removal efficiency in effluent decreased by 2~15% and the rate of biogas production decreased by 30%. CONCLUSION(s): The antimicrobials remained in the animal manure might not be removed during the anaerobic digestion process and hence, is likely to be released to the natural ecosystem. Therefore, the efforts to decline the usage of antimicrobials for animal farming would be highly recommended.