Effect of viscosity on process stability and microbial community composition during anaerobic mesophilic digestion of Maotai-flavored distiller’s grains

[Display omitted] •Viscosity above 782 mPa·s was proposed as an early warning indicator for foaming.•High OLR or continuous effluent recirculation increased the digestate viscosity.•High viscosity caused severe foaming that blocked the biogas outlet pipe.•Candidatus_Methanofastidiosum was found only in the high viscosity system. To investigate the effects of viscosity on the mesophilic digestion of Maotai-flavored distiller’s grains, a continuous experiment was conducted in a 70 L reactor at organic loading rates of 3, 4, 5, and 6 g VS/(L·d) with and without effluent recirculation. High organic loading rates and continuous effluent recirculation increased the digestate viscosity, and high viscosity caused severe foaming, which blocked the biogas outlet pipe. Moreover, a viscosity above 782 mPa·s was proposed as an early warning indicator for foaming. A maximum volumetric biogas production rate of 1.72 L/(L·d) was accomplished by diluting the feed without effluent recirculation at a recommended organic loading rate of 5 g VS/(L·d). Proteiniphilum, Ruminococcus_2, norank_f_Synergistaceae, norank_o__DTU014, Syntrophomonas, Methanosarcina, Methanobacterium, and Methanosaeta were the dominant acidogens, syntrophic bacteria, and methanogens existed in both low and high viscosity groups. Candidatus_Methanofastidiosum capable of employing the methylated thiol reduction pathway was found only in the high viscosity system.