Insight into the granular characteristics and microbial communities of SNDPR in an innovative continuous flow reactor across varying COD concentrations

The purpose of this study was to investigate the effects of COD interference on biological nutrient removal, granule characteristics, and microbial community dynamics in continuous-flow Simultaneous Nitrification, Denitrification, and phosphorus Removal (SNDPR) granular sludge under low aeration energy consumption conditions. The experiment employed an innovative Automatic Internal Circulation Continuous Flow Reactor (AIC-CFR) at an aeration rate of 0.8 L/min, maintaining the dissolved oxygen level below 0.5 mg/L, and the COD concentration increased from 300 to 500 mg/L in steps of 100 mg/L. The results demonstrated that increasing the COD concentration to 400 mg/L significantly enhanced the removal efficiencies of total phosphorus and total nitrogen, while simultaneously optimizing the settling properties of the granules. However, when the COD concentration reached 500 mg/L, the settling ability and stability of the granules deteriorated. As the COD concentration increased, the population of the filamentous archaea <i>Methanothrix</i> significantly increased, whereas the abundance of the filamentous bacteria <i>Thiothrix</i> gradually decreased. The abundance of these filamentous microorganisms was closely correlated with the sludge volume index, granular integrity coefficient, and extracellular polymeric substances. High-throughput sequencing results revealed that DPAOs-Pseudomonas have consistently been the absolute dominant genus in the system. It is AOA rather than AOB that undertakes the task of oxidizing ammonia nitrogen to nitrous nitrogen. Finally, a granular ecological conceptual model is proposed to elucidate the underlying mechanisms of the AIC-CFR system. This study elucidated the stability mechanism of SNDPR granules, providing technical support for the low-carbon engineering operation of granular sludge.