Complex network analysis of slaughterhouse waste anaerobic digestion: From failure to success of long-term operation

•Solid-state digestion experienced operation instability due to ammonia inhibition.•Liquid-state digestion showed stable biogas production at 2 gVS L−1 d.•Tissierellaceae contributes to the early stage of slaughterhouse waste digestion.•Hydrogenotrophic methanogens had higher tolerance to FAN than other methanogens.•Complex network patterns differ depending on the stability of anaerobic digester.