Physicochemical properties of Fe-containing mineral powder mediated Anammox granular sludge formation and its mechanisms

The long doubling time and low cell yield of anaerobic ammonium oxidizing bacteria (AnAOB) make the formation of Anammox granular sludge (AmGS) essential for enhancing AnAOB retention and nitrogen removal efficiency, yet its practical application is still hindered by slow granulation, loose structure, and poor settleability. In this study, a Fe-containing mineral powder (FMP) was introduced to accelerate sludge granulation, enhance physicochemical properties, and elucidate the underlying mechanisms. The results demonstrated that FMP addition markedly promoted Anammox sludge granulation. After 60 days of operation, granules larger than 200 um accounted for 56% of the sludge in the FMP-amended reactor, compared with only 2% in the control. The settleability was significantly improved, with the sludge volume index reduced to 16.5 mL/g (63.9% lower than the control), while the settling velocity and density increased to 2.60 cm/s and 1.07 g/cm3 (71.1% and 3.9% higher than the control), respectively. Meanwhile, the Anammox activity increased by 56.4%. Morphological observations revealed that FMP acted as a crystal nucleus and promoted the formation of a stable granular skeleton through EPS-mediated adhesion with filamentous bacteria. High-throughput sequencing analysis showed that the relative abundance of AnAOB increased by 58.0% in the presence of FMP. Mechanistic analysis further indicated that FMP enhanced EPS secretion, modified protein secondary structures and elevated the zeta potential. This study provides an effective strategy for accelerating AmGS granulation and improving its physicochemical properties, offering valuable insights for the practical application of the Anammox process.