Larger hydroxyapatite aggregation from Ca²⁺ adhesion in ANAMMOX granular sludge caused by high dissolved oxygen

Anaerobic ammonia oxidation (ANAMMOX), a sustainable biological process, is promising to remove NH₄⁺-N from municipal sewage. In this study, results showed that the anammox granular sludge morphology changes with the alternation of dissolved oxygen (DO), mainly attributing to the adhesion of calcium ions (Ca²⁺) to the surface of sludge particles. Diverse characterization methods revealed that gray adhesions in the form of hydroxyapatite covered the original holes on the anammox granular sludge surface, including scanning Electron Microscopy (SEM), digital camera images, Energy Dispersive Spectrometer (EDS), and X-ray diffraction (XRD). Ex-situ degradation of NH₄⁺-N and NO₂^--N yielded diverse outcomes. The protein to polysaccharide ratio (PN/PS) in the total extracellular polymeric substances (EPS) across 4 size groups demonstrated a decrease under O₂ exposure. Microbial community analysis indicated norank_f_A4b and Nitrolancea being the most abundant genus under O₂ exposure at day 1 and day 100, respectively. These findings offer an effective strategy to prevent size-larger granular sludge from deteriorating through changing DO and Ca²⁺ in municipal wastewater in ANAMMOX.