Spatiotemporal Assembly and Immigration of Heterotrophic and Anammox Bacteria Allow a Robust Synergy for High-Rate Nitrogen Removal

Thisarticle revealed the significant role of bacterialaggregation and spatiotemporal immigration in a novel partial denitrification/anammoxsystem for stable nitrogen removal. The novel partial denitrification-driven anammox (PD/A)is an energy-efficientmethod for nitrogen removal from wastewater. However, its stabilityand efficiency are impeded by the competition between heterotrophicdenitrifying bacteria and relatively slow-growing anammox bacteria.In this study, a PD/A granular sludge system was developed, whichachieved a nitrogen removal efficiency of 94% with 98% anammox contribution,even as the temperature dropped to 9.6 degrees C. Analysis of bacterialactivity in aggregates of different sizes revealed that the largestgranules (>2.0 mm) exhibited the highest anammox activity, 2.8timesthat of flocs (<0.2 mm), while the flocs showed significantly highernitrite production rates of PD, more than six times that of the largestgranules. Interestingly, fluorescent in situ hybridization (FISH)combined with confocal laser scanning microscopy (CLSM) revealed anest-shaped structure of PD/A granules. The Thauera genus, a key contributor to PD, was highly enriched at the outeredge, providing substrate nitrite for anammox bacteria inside thegranules. As temperature decreased, the flocs transformed into smallgranules to efficiently retain anammox bacteria. This study providesmultidimensional insights into the spatiotemporal assembly and immigrationof heterotrophic and autotrophic bacteria for stable and high-ratenitrogen removal.