The fate and dynamics of iron during the transformation of activated sludge into oxygenic photogranules (OPGs) under hydrodynamic batch conditions for environmental applications

Oxygenic photogranules (OPGs) are dense, spherical structures containing filamentous cyanobacteria, micro -algae, and non-phototrophic bacteria. Besides hydrostatic batch conditions, OPGs can be produced directly from activated sludge under illuminated hydrodynamic batch conditions. The role of Fe in these hydrodynamic batch conditions is still unknown. In this study, four replicate hydrodynamic batches with two times diluted Amherst and Hadley activated sludges were operated under continuous illumination (126 & PLUSMN; 9 mu mol/m2-s) and mixing (20 rpm) for a period of 19 days. Illumination and initial anaerobic condition development led to the rapid release of Fe, especially Fe (II), into the bulk liquids. However, this Fe pool quickly declined and remained at overall steady values. The Fe linked with biomass-bound extracellular polymeric substances (bEPS-Fe) continued to decline during batches and eventually reached stable levels. Cyanobacterial growth exhibited moderate to strong negative correlations to both bEPS and bulk-liquid Fe pools whereas microalgal growth showed more depen-dence on bulk-liquid Fe compared to bEPS-Fe. The Fe distribution analysis revealed higher Fe content in the biomass pellet fraction, followed by bEPS extract and bulk liquid fraction. Increases in the level of pelletized Fe, over the course of photogranulation, are likely due to increases in intracellular Fe content. Overall, the study demonstrates that the limitation of available Fe in bEPS and bulk liquid fractions encouraged the formation of photogranules in hydrodynamic batch conditions. These observations are similar to earlier reported batch hy-drostatic cultivation, which further supports the shared photogranulation phenomena in the two batch conditions.