Nitrogen removal in aquaculture tailwater: Comparison of the performance and intrinsic mechanisms of bioflocs versus zoogloea granules

Effective nitrogen removal from aquaculture tailwater is crucial to sustainable aquaculture. Bioflocs and zoogloea granules, two distinct microbial aggregates, are known to mediate nitrogen removal under high- and low-carbon conditions, respectively. However, the differences in nitrogen removal efficiency and the underlying mechanisms remain unclear. This study provided the first comparison of the nitrogen removal performance of these two aggregates in tailwater, and combined with their physicochemical properties, extracellular polymeric substances (EPS), and metagenomic analysis, aimed to reveal their underlying mechanisms. Results showed that bioflocs had a significantly larger average particle size than zoogloea granules (224 μm vs. 80 μm, P < 0.05), with a looser structure, a higher proportion of viable cells, and inferior sedimentation performance. Bioflocs also exhibited a significantly higher 24-h TN removal rate than zoogloea granules (69.92 % vs. 45.22 %, P < 0.05). During the nitrogen removal process, bioflocs secreted more EPS to stabilize structures and store carbon, whereas zoogloea granules degraded EPS to supply energy for denitrification. Metagenomics revealed that bioflocs were dominated by Nakamurella (39.41 %) and showed a dominant heterotrophic ammonia assimilation. In contrast, zoogloea granules hosted a more diverse and stable microbial community (dominated by Candidatus Promineifilum) that facilitated nitrogen removal through a synergistic interplay of glutamate dehydrogenase (GDH)-mediated assimilation, nitrification, and denitrification. These findings provide new insights into the distinct nitrogen removal processes, offering a basis for technology selection and process optimization.