N₂‐fixation can sustain wastewater treatment performance of photogranules under nitrogen‐limiting conditions

Abstract

Wastewater characteristics can vary significantly, and in some municipal wastewaters the N:P ratio is as low as 5 resulting in nitrogen-limiting conditions. In this study, the microbial community, function, and morphology of photogranules under nitrogen-replete (N+) and limiting (N−) conditions was assessed in sequencing batch reactors. Photogranules under N− condition were nitrogen deprived 2/3 of a batch cycle duration. Surprisingly, this nitrogen limitation had no adverse effect on biomass productivity. Moreover, phosphorus and chemical oxygen demand removal were similar to their removal under N+ conditions. Although performance was similar, the difference in granule morphology was obvious. While N+ photogranules were dense and structurally confined, N− photogranules showed loose structures with occasional voids. Microbial community analysis revealed high abundance of cyanobacteria capable of N 2 -fixation. These were higher at N− (38%) than N+ (29%) treatments, showing that photogranules could adjust and maintain treatment performance and high biomass productivity by means of N 2 -fixation.

Research progress and prospect of low-carbon biological technology for nitrate removal in wastewater treatment

“…The flexible nitrogen source requirement allows them to adapt to wastewater with various nitrogen forms. In addition, some Cyanobacteria are able to fix nitrogen to sustain treatment performance when the nitrogen source is depleted (Trebuch et al, 2023b). Ammonium as the most abundant form of inorganic nitrogen in municipal wastewater can be directly assimilated by microalgae without requiring conversion steps.…”