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The use of cefazolin to treat mastitic cows leads to cefazolin residues in milk and manure. This is responsible for the high occurrence of cefazolin resistant bacteria (CRB) in waste and the environment. Anaerobic digestion is considered to have the potential to reduce antibiotic-resistant bacteria present in waste that results from concentrated animal feeding operations. Thus, the objective of this study was to investigate the survival of CRB and the digester performance in mesophilic co-digestion of dairy manure and waste milk. The experiment was carried out using three digester compositions: 100% slurry (slurry), 50% slurry + 50% manure (manure mixture) and 50% slurry + 45% manure + 5% waste milk (milk mixture) in batch digesters of 1 l with a working volume of 800 ml in triplicate at 37°C for 34 days. The daily biogas production in each digester, and methane (CH4) and carbon dioxide compositions in the gas were determined. The population densities of total culturable bacteria (TCB) and CRB were determined by plate counts on agar media at day 0, 10, 20 and 34 of digestion. Milk mixture produced the highest (P < 0.05) daily and cumulative total and CH4 gas. The maximum percentage reductions of TCB and CRB in manure and milk mixture was observed at day 20, the values being 96.2%, 96.0% and 99.8% and 99.8% respectively. Final volatile fatty acids (VFA) and pH values of the digesters confirmed the digester stability. Based on the findings, mesophilic anaerobic digestion can be considered a potent method to avoid the dissemination of CRB in nature.

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The effects of co-digesting different proportions of sugar beet tops (SBT) silage (40, 60, 80 and, 100 %, wet weight basis) with cow manure and duration of storage of two types of SBT silage (silage I: storage time of 120 days and silage II: storage time of 210 days) on digester performance in terms of biogas production and volatile solids (VS) reduction were investigated. The highest methane (CH 4 ) yields were obtained at the 40 % proportion treatments and were 0.422 and 0.377 l/g VS for silage I and II, respectively. The digestion systems failed at 80 and 100 % proportions of silage I and 100 % proportion of silage II. Furthermore, 40 % proportion treatments presented highest VS reductions among manure-silage mixtures (57.0 % for silage I and 57.4 % for silage II) while the control treatment (manure only) had the highest VS reduction (65.0 %) among all the treatments. Based on the results, the 40 % proportion of SBT silage was most efficient for methane production. Comparison of digester performance at 40 and 60 % proportions showed that no remarkable differences were observed between silage I and II, indicating that duration of storage of SBT did not have any appreciable effect on digester performance.