Role of Hydrodynamic Shear in the Oxygenic Photogranule (OPG) Wastewater Treatment Process

The role of hydrodynamic shear in the oxygenic photogranule (OPG) wastewater treatment process was investigated in three sequencing batch reactors (SBRs) operated under different shear conditions for 250 days. Mixing in Reactor 1, Reactor 2, and Reactor 3 was set at 50, 100, and 250 rpm, which induced theoretical shear stresses (τ) of 0.015, 0.04, and 0.14 N/m2, respectively. As shear increased, we observed significant increases in the relative abundance of filamentous cyanobacteria, the key microbial group for OPG granulation. Additionally, increasing hydrodynamic shear force in OPG reactors stimulated linearly increased production of extracellular polymeric substances (EPSs). Compared to high shear, photogranules produced at low shear were more spherical and much larger in size and, thus, contributed to higher total nitrogen removal through denitrification, a potential legacy of limited oxygen transfer in their structure. On the contrary, the smaller photogranules produced under high shear promoted better oxidation processes and, thus, higher removal of tCOD and NH4+ due to their higher oxygen production capabilities compared to the larger photogranules. Hydrodynamic shear can be manipulated to drive photogranulation toward desired size distribution and, thus, achieve specific treatment goals in the OPG wastewater treatment process.