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Membrane Fouling and Transport Models in Water Treatment Applications

Shih-Chieh Tu, Varadarajan Ravindran, and Massoud Pirbazari. University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, CA 90089-2531

Membrane processes in water treatment often experience permeate flux decline due to organic fouling attributed to natural organic matter (NOM). Membrane transport models were developed for prediction and simulation of permeate flux. Two different models were compared for ultrafiltration membranes. The first model essentially considers surface fouling due to deposition of organic foulants. The second model involves both surface sorption and pore diffusion to address organic fouling due to surface deposition and internal pore blocking. In these studies, tannic acid and dextran are used as model compounds to simulate organic fouling. Additionally, a real source water is employed for model calibration and verification.

The membrane transport models considered resistances due to membrane film, concentration polarization, gel formation, and internal pore fouling. Membrane filtration tests were conducted to evaluate permeate flux decline patterns due to organic fouling, as well as solute concentration profiles in the permeate and concentrate streams. The pore diffusion model exhibited good predictive capability for permeate fluxes and solute rejections for both nanofiltration and ultrafiltration membranes. The pore diffusion model (second model) provided better predictions was superior to the surface fouling model (first model) in predicting the permeate fluxes for the ultrafiltration membranes. However, there were no significant differences in permeate flux predictions between the two models for nanofiltration membranes.

Model simulation and sensitivity studies were conducted to evaluate the effects of various input parameters pertaining to operating conditions and fluid-dynamic regimes. These parameters include the mass transfer coefficient, solution dynamic viscosity, membrane resistance, and resistance per unit gel layer thickness. These studies provide a well-founded theory based on the dependence of transport resistance components and membrane performance on input parameters. The studies illustrate the boundary layer effect on gel layer formation and the overall mass transfer resistance.

Membrane filtration tests were conducted by varying operating parameters such as solute concentrations, trans-membrane pressures, and reject flow rates. These tests show that the nanofiltration membrane composed of polypiperazine amide and the ultrafiltration membrane composed of polyether sulfone were more susceptible to organic fouling by tannic acid than the nanofiltration membrane made of cross-linked aromatic polyamide. These observations are well supported by surface potential measurements that demonstrated higher negative surface charges and greater hydrophilic property for the aromatic polyamide membrane.