High-Rate\nNitrite Production by Stable Denitrification\nGranular Sludge in a Reinforced-Mixing Up-Flow System: Shift in Spatiotemporal\nCharacteristics of Functional Bacteria and Metagenomic Mechanisms

The potential of the partial denitrification granular\nsludge (PDG)\nhas been highlighted for achieving high nitrite (NO<sub>2</sub><sup>–</sup>-N) production. However, there remain bottlenecks regarding\nsatisfactory stability and NO<sub>2</sub><sup>–</sup>-N production\nrates (NPR). In this study, we comprehensively explored the physicochemical\ncharacteristics and spatiotemporal succession of the microbial community\ninside the aggregates, cultivated under multidimensional shear stress\nwith horizontal mechanical agitation and vertical hydraulic mixing\nin a continuously up-flow reactor. High-rate NO<sub>2</sub><sup>–</sup>-N production with an NPR of 2.66 kg N/m<sup>3</sup>/day was achieved,\nfar exceeding the currently reported value, accompanied by a relatively\nstable nitrate-to-nitrite transformation ratio of 72% and satisfactory\nsettleability without sludge flotation and blocking. Interestingly,\nthe morphological structure of PDG spontaneously changed to be much\nmore pleated at the outer edge. This provided sufficient attachment-growth\nsites and surface area for functional bacteria (<i>Thauera</i>), exhibiting a capability of NO<sub>3</sub><sup>–</sup>-N\nreduction much higher than that of NO<sub>2</sub><sup>–</sup>-N at the level of the genes. They were rapidly enriched with proportion\nincreasing from 36.8 to 67.4% in 56 days. These spatiotemporal structures\nand microbial distribution provide an extra opportunity to select\nspecific bacteria for high-rate NO<sub>2</sub><sup>–</sup>-N\nproduction and treatment of high-strength nitrate-containing wastewater\nby combining them with Anammox.