Behaviour of oxytetracycline in two-phase anaerobic cattle manure digesters and its effects on microbial communities

Antibiotics used in veterinary practice may have inhibitory effects on microorganisms in anaerobic digestion of manure for biogas production as a renewable energy. Defining the effects of antibiotics on microbial diversity and activity can lead to improvements in better understanding of interactions between microbial communities and biochemical processes, thus achieving a more efficient system performance. In this study, inhibitory effects and behaviour of commonly used veterinary antibiotic, oxytetracycline (OTC) on the system performance and biogas production in a two-phase anaerobic digestion of cattle manure was investigated, as well as degree of acidification of cattle manure was improved.As preliminary studies, optimum operational conditions for acidification of cattle manure was investigated and a commercially available hydrolytic enzyme mixture was decided to be added in order to improve the acidification rate of the cattle manure in the acidogenic phase. 2 seperate two-phase anaerobic cattle manure digesters, one operated with non-medicated manure and the other operated with medicated manure, and furthermore, 2 single-phase digesters were operated for comparison in semi-continuous mode for 60 days in mesophilic conditions. Parameters such as digestion efficiency, biogas and VFA production were compared in order to determine the effects of OTC on system performance. In the presence of OTC, two-phase anaerobic digestion was found out to be superior to single-phase digestion. According to DGGE and FISH results, almost all bacterial and Archaeal species were negatively affected by OTC. During the digestion period, DGGE results showed that the bacterial diversity in acidogenic digesters and Archaeal diversity in methanogenic digesters were higher than single-phase digesters. FISH results indicated that the relation between digestion period and the activity of microorganisms were mostly effective on Firmicutes and Gammaproteobacteria. In methanogenic activity, OTC presence was comparatively more effective on Methanosarcinales. The ratios of active methanogenic cells were higher in methanogenic digesters compared to single-phase digesters, both in the absence and presence of OTC.