Initiation of Anammox in an Up-Flow Anaerobic Sludge Bed Reactor: Bacterial Community Structure, Nitrogen Removal Functional Genes, and Antibiotic Resistance Genes

Anaerobic ammonia oxidation (anammox) is considered an efficient and low-energy biological nitrogen removal process. However, there are limited studies addressing the changes in antibiotic resistance genes (ARGs) during the startup of an anammox reactor inoculated with activated sludge. In this study, an up-flow anaerobic sludge bed (UASB) reactor was initiated with synthetic wastewater at room temperature (20–28 °C). Metagenomic sequencing was employed to analyze the shifts in the bacterial community, nitrogen removal functional genes, and ARGs in both the seeding sludge and anammox sludge. The results show that the reactor achieved anammox activity after 122 days of cultivation, with NH4+-N and NO2−-N removal rates reaching 99.8% and 99.6%, respectively. Compared to those in inoculated sludge, the relative abundance of the anammox bacterium Candidatus kuenenia increased from 0.01% to 50.86%, while the relative abundance of denitrifying Acidovorax bacteria decreased from 8.02% to 1.77%. Meanwhile, the relative abundance of Nitrosomonas declined from 2.91% to 1.87%. The functional genes hzs, hdh, nirK, and nirS increased in relative abundance in the anammox sludge, while the ARGs decreased in relative abundance from 294.77 RPKM to 155.62 RPKM in the sludge. These findings offer valuable insights into the initiation of the anammox process using ordinary activated sludge as an inoculum and provide a scientific basis for the mitigation of ARGs through anammox technology.