Acidobacteria as the dominant microorganism on the nitrogen-removal wastewater treatment with a low chemical oxygen demand/ammonium nitrogen ratio in biological contact oxidation reactor

Nitrogen-removal promotion is a significant problem when biological nitrogen removal is used to treat ammonium nitrogen (NH₄⁺-N) wastewater with a low chemical oxygen demand (COD)/NH₄⁺-N (C/N) ratio. In this work, the biological nitrogen removal capacity of the biological contact oxidation reactor (BCOR) system was enhanced through the enrichment of Acidobacteria. The system was successfully started from Day 1 to Day 50 and stably operated through temperature, pH, and dissolved oxygen (DO) regulation from Day 51 to Day 254. The average ammonium nitrogen-removal efficiency over the performance period was 96.2%, whereas the average total nitrogen-removal efficiency was 86.9%, according to the data. In the meantime, the predominant microbes in the BCOR system were initially found to belong to the phylum Acidobacteria (relative abundance of 48.39%). Additionally, it was found for the first time in this system that three different genera of unnamed microorganisms (two of which belonged to the phylum Acidobacteria and one to the phylum Chlorobi), the relative abundances of which were 30.46%, 11.45% and 16.00%, respectively. Candidatus Xiphinematobacter (belonging to the phylum Verrucomicrobia, relative abundances of 8.81%), and Pseudomonas (belonging to the phylum Proteobacteria, relative abundances of 5.59%). This new finding will be very beneficial for future research into the microbiological mechanisms and technological engineering applications of the BCOR system for treating ammonium nitrogen wastewater with a low C/N ratio.