High-Rate Nitrite Production by Stable Denitrification Granular Sludge in a Reinforced-Mixing Up-Flow System: Shift in Spatiotemporal Characteristics of Functional Bacteria and Metagenomic Mechanisms

The potential of the partial denitrification granular sludge (PDG) has been highlighted for achieving high nitrite (NO2–-N) production. However, there remain bottlenecks regarding satisfactory stability and NO2–-N production rates (NPR). In this study, we comprehensively explored the physicochemical characteristics and spatiotemporal succession of the microbial community inside the aggregates, cultivated under multidimensional shear stress with horizontal mechanical agitation and vertical hydraulic mixing in a continuously up-flow reactor. High-rate NO2–-N production with an NPR of 2.66 kg N/m3/day was achieved, far exceeding the currently reported value, accompanied by a relatively stable nitrate-to-nitrite transformation ratio of 72% and satisfactory settleability without sludge flotation and blocking. Interestingly, the morphological structure of PDG spontaneously changed to be much more pleated at the outer edge. This provided sufficient attachment-growth sites and surface area for functional bacteria (Thauera), exhibiting a capability of NO3–-N reduction much higher than that of NO2–-N at the level of the genes. They were rapidly enriched with proportion increasing from 36.8 to 67.4% in 56 days. These spatiotemporal structures and microbial distribution provide an extra opportunity to select specific bacteria for high-rate NO2–-N production and treatment of high-strength nitrate-containing wastewater by combining them with Anammox.