(125b) The Fouling Mechanisms of Complex Fluid Filtration: Modeling the Combined Fouling Effect of Colloidal Materials and Dissolved Macromolecules

Natural water and wastewater contain both colloidal materials and dissolved organic matter. While extensive research has been done on colloidal fouling and organic fouling processes separately and their mechanisms are relatively well understood, little is known about the combined fouling process during the filtration of a complex suspension containing both types of foulants. This research explores the physiochemical mechanisms involved in the combined fouling process. The two major mechanisms investigated were hindered back diffusion due to the high concentration of each foulant near the membrane surface and changes in colloid deposition behavior due to adsorption of macromolecules onto the colloid surface. The impact on the characteristics of the membrane due to macromolecule adsorption is also considered. The model integrates the hindered back diffusion mechanism by incorporating viscosity dependence on solute concentration during the concentration polarization regime. Fouling experiments in a cross-flow filtration system using both nanofiltration and reverse osmosis membranes were conducted with colloidal silica and three model organic foulants: a non-interacting macromolecule, Dextran, an interacting wastewater macromolecule, bovine serum albumin (BSA), and an interacting natural water foulant, humic acid. The model organic foulants were chosen for their respective adsorptive properties in order to isolate the effects of hindered back diffusion from the effects of macromolecule-mediated colloid deposition. Model foulant particle sizes and surface zeta potentials were characterized using dynamic light scattering (DLS) and electrophoretic mobility measurements through phase analysis light scattering, respectively. A quartz crystal microbalance with dissipation monitoring (QCM-D) technique was employed to study the particle-particle interactions as well as membrane-particle interactions. Model predictions are compared to experimental results as well as existing concentration polarization/fouling models.