Systematic analysis of photogranulation characteristics during start-up of a photogranular sequencing batch bioreactor for mariculture wastewater treatment

Photogranulation, as an innovative approach combining granulation technology with photoautotrophs, has attracted wide attention in the field of mariculture wastewater treatment. However, the photogranulation process and underlying mechanisms during system start-up under high-salinity conditions remain unclear. In this study, a photogranular sequencing batch bioreactor (PSBR) was operated to cultivate photogranules using inoculum from a turbot breeding base, under conditions of 35‰ salinity, 5000 lx light intensity, and a 48-h cycle. When mature photogranules dominated, the removal efficiencies of NH₄⁺-N, TN and PO₄³⁻-P stabilized at 97.5%, 61.8% and 50.8%, respectively. The photogranulation process during the start-up was divided into three stages: (1) the adaptation period (1–13 d), characterized by microbial aggregation into nucleating bioflocs; (2) the formation period (13–40 d), marked by the transition of bioflocs to initial photogranules with distinct cores and boundaries; (3) the maturation period (40–65 d), dominated by photogranules exhibiting high mechanical strength and robust treatment performance. Notably, mature photogranules displayed a stratified structure with tightly coiled filamentous cyanobacteria forming a structural network in the outermost layer, which maintained structural stability. Thermodynamic analysis based on the XDLVO theory revealed that a significant reduction in the interaction energy barrier during the photogranulation process facilitated photogranule formation and structural stability maintenance. These findings provide a novel, efficient, and environmentally friendly option for mariculture wastewater treatment.