Optimization and microbial diversity of anaerobic co-digestion of swine manure with waste kitchen oil at high organic loading rates

•Co-digesting swine manure and waste kitchen oil allowed OLR up to 6 g VS/L/d.•A reasonable amount of oil increased biogas yield and bacterial alpha-diversity.•System imbalance occurred when oil was added beyond certain threshold.•“Inhibited steady state” phenomenon was observed at high OLRs.•Up to three months were needed to confirm system failure for middle to high OLRs.