Evaluating the effects of micro-zones of granular sludge on one-stage partial nitritation-anammox nitrogen removal

The one-stage partial nitritation-anammox (PN-A) process is considered an efficient process for low-cost nitrogen removal. In this study, the nitrogen removal performance of different-sized granules in a one-stage PN-A reactor was studied. The total autotrophic nitrogen removal rate (TANRR) of the granular sludge increased as the granule size increased, and the TANRR of granular sludge with a radius larger than 500 μm reached 0.14 kgN kgVSS^-1 d^-1. High-throughput sequencing revealed that the abundance of ammonium-oxidizing bacteria and anaerobic ammonium-oxidizing (anammox) bacteria in granular sludge of different sizes differed, indicating that the bacterial community structure was affected by the granule size. The TANRR of different-sized granules was affected by the volumes of aerobic micro-zone and anaerobic micro-zone inside the granule. Appropriate micro-zone volumes inside the granules could be regulated by the dissolved oxygen (DO) concentration of the reactor, which are favourable for achieving a balance between partial nitritation and anammox and then satisfactory nitrogen removal. Small-volume variations in the range of micro-zones have a significant influence on the balance between partial nitritation and anammox. The proper DO concentration required for different-sized granules to achieve better nitrogen removal differed. This study provides a novel perspective for understanding the effect of micro-zones of granular sludge on one-stage PN-A nitrogen removal.