In municipal wastewater treatment, controlling the stability and efficiency of the anaerobic process is the premise and key to the high-standard discharge of organic matter and total nitrogen. In this study, two types of hydrolytic acidification, immobilized biological active fillers and traditional activated sludge, were used to treat municipal wastewater at different temperatures (13-24 degrees C) and working conditions. The degradation of organic matter and the transformation of organic nitrogen in the anaerobic process were compared, and the internal flora of the immobilized fillers was systematically analyzed by high-throughput sequencing technology. The results show that the filler exhibited higher activity at low temperatures, and the residual organic nitrogen level in the effluent (0.3-0.5 mg L-1) was significantly lower than that in the sludge reactor (0.8-1.2 mg L-1). The performance of hydrolysis-acidification of skeleton-less fillers (EB3) was higher than that of skeletoned fillers (EB2) (the hydraulic retention time was 1 h and fill rate was 8%). The organic nitrogen of fillers was completely converted, while the volatile fatty acid of EB3 effluent increased by 2-2.5 mg L-1, greater than the increase in EB2 effluent (0.9-1.3 mg L-1). Based on the results of high-throughput sequencing, the fillers contained the same functional species, albeit at different abundances. The immobilized fillers could improve the performance of traditional activated sludge to maintain biomass through reflux and achieved the dual advantages of fixing biomass and maintaining good anaerobic characteristics. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.