Impact of Aeration on Mixed Liquor in Submerged-Membrane Bioreactors for Wastewater Treatment

In submerged-membrane bioreactors (SMBRs) for wastewater treatment, aeration with coarse bubbles is applied to limit fouling. The understanding of the different mechanisms between aeration and fouling helps to manage the aeration policy. The impact of aeration (macro scale) on shear stress and mixed-liquor properties (local scale) is evaluated. Experimental characterization of gas-liquid flow in membrane modules, computational fluid dynamics simulation, and controlled breakdown of SMBR mixed-liquor samples are reported. Mean bubble velocities were significantly lower in mixed liquor than in water and the shear stress was one order of magnitude higher in mixed liquor than in water. The floc size decreased and soluble protein concentrations increased with higher shear stress values. Considering the known impacts of these mixed-liquor properties on fouling, the obtained local results explain stronger fouling when coarse bubble aeration increases.