Water quality indicators influencing the formation and morphology of hydrostatically-formed photogranules†

Hydrostatic photogranulation represents an intriguing phenomenon with potential applications in aeration-free wastewater treatment. In this process, activated sludge batches transform into photogranules, manifesting as either spherical or disk-dominated shapes. Yet, the factors contributing to this morphological diversity remain unknown. Moreover, the impact of morphology on granule structure and physical characteristics remains poorly understood, posing potential implications for photogranulation in reactors that frequently utilize these hydrostatic granules as seeding materials. This study investigates the influence of water quality parameters on hydrostatic photogranulation and its role in shaping granule morphology. Spherical photogranules exhibited lower chlorophyll a concentration (5.97–7.40 mg L⁻¹) and higher Chl a/b ratio (13−14) than disk-shaped photogranules (Chl a concentration: 8.13–11.70 mg L; a/b ratio: <10), indicating a higher cyanobacteria content in disk-shaped granules. Additionally, spherical photogranules showed significantly lower concentrations of EPS proteins and polysaccharides than disk-shaped granules, suggesting enhanced granulation under EPS limitations. Correlation analysis indicates that higher initial NO₃⁻ and total polysaccharides (TPS) increase the likelihood of producing spherical photogranules. Conversely, higher initial Ca²⁺ and Mg²⁺ concentrations were observed in cultivations predominantly producing disks. Furthermore, principal component analysis identified Cl⁻, Na⁺, NH₄⁺, and SO₄²⁻ as key initial water quality indicators and TPS, tCOD, and VSS as important sludge biomass characteristics that distinguished between different photogranule morphologies. Compared to spherical photogranules, disk-dominated photogranules exhibited higher stiffness and shear resistance, potentially due to increased cyanobacterial and EPS contents. Controlling hydrostatic photogranulation to achieve desired photogranule shapes holds potential for customizing seed granules and thus enhancing the OPG wastewater treatment performance.