Response of partial nitritation/anammox granules to biodegradable organic input in bacterial community and functions

Abstract

Biodegradable organic input is considered as a critical factor influencing the stability of the partial nitritation/anammox (PN/A) process for autotrophic nitrogen removal. In this study, the response of PN/A granules to stepwise increasing the organic loading rate (OLR) was investigated in a continuous-flow reactor, according to the nitrogen removal activity, sludge morphology, bacterial community composition and nitrogen cycle pathways. Results show that the increased OLR from 0.47 to 3.36 kg·m−3·d−1 with different influent organic and nitrogen (COD/N) ratios from 0.25 to 1.0 caused the specific nitrogen removal rate of the granules to decrease by 60.9 %, which was accompanied by a looser granular structure and poor settling property. Inorganic feeding was then used to recover the performance of the granules, and the removal of total inorganic nitrogen rose to 78.4 % at a nitrogen loading rate of 3.36 kg·m−3·d−1. Based on high-throughput sequencing, biodegradable organics shaped the bacterial community structure in the granules by enriching symbiotic heterotrophs (such as Chloroflexi, Proteobacteria and Bacteroidetes) and washing out the anammox biomass. Candidatus Kuenenia dominated the anammox biomass, while Candidatus Brocadia was also found, which coordinated with aerobic ammonium-oxidizing bacteria (Nitrosomonas) in the PN/A process. Metagenomic analysis indicates that biodegradable organic stress and performance recovery significantly changed the composition of nitrogen cycle-related genes in the granules, in terms of a lower capacity of anammox and higher risk of nitrous oxide emission. These findings provide insight into the self-adaption strategy of PN/A granules in response to biodegradable organic input into wastewater treatment engineering applications.