Impact of granular fragment size on photogranulation systems

Abstract

Photogranules are an emerging algal-bacterial system with potential for wastewater treatment and resource recovery. However, the mechanisms underlying their re-granulation after collapse remain poorly understood, limiting stable application. This study systematically examined the re-granulation mechanisms of photogranule fragments (0.3, 0.5 and 1.0 mm) by analysing growth dynamics, treatment performance, sludge characteristics and microbial community shifts. Results showed that 1.0-mm fragments (R3) achieved the average granular size of 3.34 ± 0.35 mm within 10 days of recovery, with 89.0 % ± 4.9 % chemical oxygen demand (COD) and 60.2 % ± 7.5 % TIN removal rates. In addition, Reactor 3 (R3) exhibited 50 % faster recovery rates and achieved a re-granulated particle size 33.07 % larger than that of smaller fragments. The extracellular polymeric substance (EPS) content in R3 was 273 mg/g VSS, with a protein (PN)/polysaccharide (PS) ratio of 4.7. In comparison, R1 and R2 showed lower Chl-a/Chl-b ratios and weaker EPS accumulation, consistent with their relatively lower pollutant removal efficiencies. Filamentous polysaccharides formed structural scaffolds enabling microbial colonization, reinforced by synergistic hydrophobic interactions and hydrogen bonding. Re-granulation drove microbial shifts from Hyphomicrobium / Lysobacter to functional genera Methylobacillus , Chthonobacter , and anammox bacterium SM1A02 . These findings establish fragment size as a key control parameter in the re-granulation process, where robust EPS and functional microorganisms enable rapid re-granulation and high-efficiency nutrient removal. This study offers an in-depth understanding of photogranule stability and a novel strategy for sustainable wastewater treatment optimisation.