Factors Impacting the Cultivation, Structure, and Oxygen Profiles of Oxygenic Photogranules for Aeration-Free Wastewater Treatment
Caitlyn S. Butler
Oxygenic photogranules (OPGs) are novel biogranules which have successfully treated wastewater without external aeration at the bench scale. The formation of OPGs under both static and hydrodynamic conditions is a unique phenomenon which has no known counterpart. This report examines some of the granulation mechanisms and structural and functional differences between statically and hydrodynamically cultivated OPGs. As previously reported, filamentous cyanobacteria are an essential component of both types of OPGs. The addition of Oscillatoria in augmented activated sludge inoculum showed successful static OPG formation in a shorter time (28 vs 35 days) and produced more settled OPGs than native sludge (88% vs 33%) which may decrease start up time and improve settleability in reactor operation. Additionally, there are distinct mechanisms involved in forming static versus hydrodynamic OPGs that results in variations in phototrophic arrangement and the resulting oxygen gradients. Butler’s model for profiling dissolved oxygen transport in static OPGs was adapted for reactor granules taking into consideration that the oxygen concentration does not reach zero at the core of reactor OPGs (r < 2 mm). Biomass supported by oxygen diffusion is suggested to be predominantly driven by aerobic activity in smaller granules and diffusion limited in larger granules. The model also indicates there may be greater symbiosis between aerobes and phototrophs in small reactor granules (r < 1 mm) and static granules compared to greater photograph dominance in large reactor granules (r > 1 mm). Granulation conditions likely factor into aerobe and phototroph interactions that may require a thicker cyanobacteria layer to support larger granules formed under hydrodynamic shear stress. By understanding granule formation mechanisms and designing granules with desired microbial kinetics, engineers may be able to improve the efficiency of this aeration-free wastewater treatment process and continue to increase it in scale.