Filtration of Complex Suspensions Using N anofiltration and Reverse Osmosis Membranes: Foulant-Foulant and FoulantMembrane Interactions

Abstract

Membrane filtration is a promising advanced treatment method that has the technological capability to treat waters containing contaminants that typically escape traditional water treatment methods, including trace micro-pollutants as well as high salt concentrations. The accurate prediction of nanofiltration and reverse osmosis membrane performance in industrial applications is dependent upon understanding the fouling behavior of representative feed solutions. Combining conventional crossflow filtration experiments and characterization of foulant-foulant and foulant-membrane interactions, three mechanisms involved in combined fouling of organic and inorganic colloidal foulants are identified: increased hydraulic resistance of the mixed cake layer structure, hindered foulant diffusion due to interactions between solute concentration polarization layers, and changes in colloid and membrane surface properties due to organic adsorption. A range of typical organic foulants that exhibited different interactions in the membrane system were studied in combination with inorganic silica on low and high salt-rejecting membranes. Autopsying of the fouled membrane using iii transmission electron microscopy (TEM) helped identify combined fouling layer structure. Direct organic adsorption of BSA onto inorganic colloids was shown to cause the greatest synergistic fouling through creation of an aggregated fouling layer structure. A stratified, active salt-rejecting layer of natural organic matter minimizes cake enhanced osmotic pressure (CEOP) and reduces fouling. The presence of divalent ions can lead to the creation of salt concentrating layers by causing aggregation of alginate molecules and enabling CEOP. The effect of membrane surface chemistry on adsorptive fouling by organics was studied using self-assembled monolayers (SAMs) with different ending functionalities. Surfaces were characterized by hydrophobicity and surface free energy incorporating van der Waals and Lewis acid-base interactions. Acid-base interactions were dominant for all model membrane surfaces tested and total interfacial energies predicted natural organic matter and polysaccharide adsorption, but do not account for protein adsorption. Specific interactions, such as hydrogen bonding and electrostatic interaction between specific functionalities, playa more important role than non-specific electrostatic and hydrophobic interactions in adsorption of and irreversible fouling by proteins. Therefore, surface modifications of NF and RO membranes that minimize -COOH and -NHz as well as other charged sites may be an effective approach to develop fouling resistant membranes.