Mechanistic Study on Photosynthetic Bacteria Granulation under Synergistic Hydraulic and Organic Loading
Photosynthetic bacteria (PSB) wastewater treatment technology is promising for simultaneous pollutant removal and resource recovery (e.g., single-cell protein, hydrogen). However, poor cell hydrophobicity and aggregation lead to low biomass retention and short sludge retention time, hindering engineering application. This study investigated the driving role and mechanism of upflow velocity as a key hydraulic selection pressure on PSB granulation under stepwise increasing organic loading rate (OLR). In laboratory up-flow photobioreactors (UPBR), comparative experiments were conducted with macro-index monitoring and micro-mechanism analysis. Results showed that under high upflow velocities of 3.00–6.30 m·h−1, PSB granular sludge with an average diameter of 285.58 μm and excellent settleability (sludge volume index, SVI = 22.73 mL·g−1) was successfully formed within approximately 60 days. Compared to the control, the granules in the experimental group were larger, with clear boundaries and compact structure, and significant enrichment of filamentous bacteria was observed. Mechanism analysis indicated that OLR provided nutritional driving force for microbial growth, while upflow velocity supplied high hydraulic shear force, physically screening and enriching settleable aggregates, and specifically inducing secretion of hydrophobic tryptophan-like proteins and humic acids (key extracellular polymeric substances, EPS). Additionally, core genera such as Xanthobacteraceae, possessing stress tolerance and EPS secretion functions, were enriched. This study reveals a chain mechanism of 'physical selection–biological response' centered on hydraulic selection, demonstrating that upflow velocity is a key controllable factor for PSB granulation, providing theoretical basis and technical pathway for solving PSB biomass washout and promoting resource-oriented treatment of high-strength organic wastewater.