To treat gibberellin (GA) wastewater, a full-scale, multi-stage combined contact process was developed. This whole process employs three anaerobic reactors followed by micro-aerobic, anoxic/aeration and biological oxidation treatment. Pollutant removal results showed that the combined process could remove more than 98% of the chemical oxygen demand (COD), NH3-N, and SO42- pollutants because of the different microbial communities in each reactor. 16S rRNA gene sequencing was used to examine the microbial communities in the internal circulation (IC) and in the two up flow anaerobic sludge blanket (UASB) reactors, as well as to investigate the effect of sampling elevation on the microbial community. The results showed that Firmicutes and Euryarchaeota were the most dominant phyla at the bacterial and archaeal levels, respectively. High levels of Synergistaceae_uncultured were detected in IC and UASB1. Chloro-flexi_uncultured was the dominant genus of bacterial communities within UASB2, and Methanosaeta was the dominant genus of archaeal communities. Principal coordinates analysis (PCoA) revealed variations among the microbial communities in 9 samples, and Venn analysis showed different operational taxonomic units (OTUs) among samples collected at various elevations within the three anaerobic reactors. However, partial Mantel tests indicated no significant correlation between the microbial community structure and elevation in the three anaerobic reactors. (C) 2018, The Society for Biotechnology, Japan. All rights reserved.