Permselective membrane fouling during azo dye wastewater treatment

A Na-montmorillonite membrane wastewater renovation prototype system was developed to specifically treat an ionic azo dye. Efficiency of this prototype system was limited to membrane fouling. Fouling rates were not consistently uniform owing to steric effects and competition for exchange sites. The decrease in solute rejection with time can be attributed to the decrease in the relative permeability of the compacted Na-montmorillonite membrane to the dye with time due to fouling. This decrease occurs probably as a two-step nucleation-growth mechanism with the nucleation part dependent in part on solvent flux, number of nucleation sites on the membrane, and sorbed mass part that controls solute flux and organic polymerization. The effect of concentration polarization was significant since the flux was higher than the mass-transfer coefficient. The low diffusion coefficient of the ionic azo dye resulted in low mass transfer coefficients. The most important macromolecular solution properties to be considered for pilot systems may include high concentration-dependent viscosity, possible non-Newtonian fluid behavior, and low and concentration-dependent self-diffusivity amongst other factors. For pilot systems, the greater the quantity of large macromolecules in the ambient water, the greater the necessity of reducing the permselectivity of the membrane to prevent significant polarization.