Effect of Cefazolin and Waste milk on Multi-drug Resistant Bacterial Reduction during Anaerobic Co-digestion of Dairy Manure and Waste Milk

Anaerobic digestion is one of the major biological waste treatment processes in use today. Waste milk from mastitic cows treated with cefazolin is considered a good candidate as a co-substrate in anaerobic digestion. The objective of this study was to determine the effect of cefazolin (10 mg L) and waste milk on reduction of multi-drug resistant bacteria (MDRB) resistant to three groups of antibiotics; group 1. Cefazolin, Neomycin, Vancomycin, Kanamycin, group 2. Penicillin, Oxytetracycline, Ampicillin, Streptomycin, group 3. all above antibiotics, in anaerobic digestion of dairy manure and co-digestion of dairy manure and waste milk at 37°C and 55°C for 22 days respectively. Antibiotic free manure and milk samples were mixed with slurry accordingly to form manure mixture (slurry 50% and manure 50%) and milk mixture (slurry 50%, manure 45% and milk 5%), each of which was further separated as the control and the cefazolin spiked (10 mg L ), loaded into triplicate 1 L batch digesters and anaerobically digested at 37°C and 55°C for 22 days separately. Overall reduction of more than 90% of three groups of MDRB was observed in mesophilic digestion with no significant differences (p>0.05) between each manure and milk mixture. Differences in MDRB reductions were not statistically significant (p>0.05) between control and respective cefazolin spiked digesters. Thermophilic digestion showed 100% destruction of all three groups of MDRB in both substrates. VFA accumulation and reduction of pH were not observed in all digesters at both temperatures at the end of the experiment which confirm the process stability.