Two microbial consortia obtained through purposive acclimatization as biological additives to relieve ammonia inhibition in anaerobic digestion

•Two microbial consortia showed different mechanisms relieving ammonia inhibition.•Syntrophic consortium (MC) improved the conversion of propionic and butyric acids.•Single microbial consortium (SS) mainly enhanced the hydrogenotrophic pathway.•Methanospirillum and Methanothermobacter contributed most at early stage.•Methanoculleus contributed most to relieve ammonia inhibition at the later stage.