Microbial species, community structures and functional profiles were explored to explain the strong degradation capability of the combined up-flow anaerobic filter and up-flow biological aerated filter (UAF-UBAF) system with novel sludge and coal cinder ceramsite for treating tetracycline (TET) wastewater. Concentrations of different pollutants and extracellular polymeric substance (EPS) with increasing reactor height were measured. DNA extracted from biofilm at the corresponding height was sequenced on the Illumina MiSeq platform. Results showed that the main removal process of pollutants happened at the height of 0-80 cm. The highest average removal rate was in the region of 0-40 cm. EPS decreased with rising reactor height. For the same height, EPS concentration in UAF was higher than that in UBAF. Due to the analysis of dominant population, the main bacteria that degraded TET were Comamonadaceae, Thauera and Sinobacteraceae. Hydrolysis mainly took place at low reactor height while acidification and acetogenesis mainly happened at high height in UAF. For UBAF, the dominant bacteria species changed from anaerobic to aerobic species. Furthermore, microbial diversity became richer with the gradual biodegradation of TET. (C) 2015 Elsevier B.V. All rights reserved.