Colloidal fouling mechanisms in reverse osmosis and nanofiltration

Colloidal fouling of reverse osmosis and nanofiltration membranes is defined as the accumulation of rejected colloidal matter on the surface of a membrane, which leads to a decrease in solvent flux (also permeation) at a given applied pressure. This flux decline can be overcome by increasing applied pressure and/or by harsh cleaning methods, both of which increase operating costs and reduce membrane life. A substantial amount of research has been performed on colloidal fouling of microfiltration (MF) and ultrafiltration (UF) membranes, and the mechanisms of flux decline in these processes are fundamentally understood. However, much less is known about the actual mechanisms of flux decline in reverse osmosis (RO) and nanofiltration (NF) separations. It has historically been assumed that the MF/UF “cake filtration model” provides a mechanistically accurate description of colloidal fouling in RO/NF processes.; The objective of this investigation was to elucidate the fundamental mechanisms of colloidal fouling in RO and NF separations. The hypothesis is based on the presumption that the fundamental mechanism of flux decline in RO/NF membranes is similar to that of UF/MF membranes—a hydraulic pressure drop due to permeation drag across a colloid deposit layer. However, it was speculated that because RO/NF permeation velocities are nearly an order of magnitude less than UF/MF permeation rates, colloid-membrane interactions would be more important in determining particle deposition, cake formation, and flux decline.; An extensive literature review of colloidal fouling mechanisms in crossflow membrane filtration processes including MF, UF, NF, and RO was performed and results are summarized in Chapter l. The physical and chemical properties of four commercial RO/NF membranes were extensively characterized and a multitude of colloidal fouling experiments were performed to test the relative colloidal fouling behavior of these four membranes in Chapter 2. The use of scanning electron microscopy (SEM) and atomic force microscopy (AFM) aided in the characterization of clean and fouled RO/NF membranes.; Numerical simulations revealed that the initial rate of particle deposition may be influenced by membrane properties such as surface roughness and zeta potential in Chapter 4. However, in Chapter 4 the hydraulic pressure drop across relatively thick colloid deposit layers was found negligible, and it was proposed that overall flux decline was related to membrane salt rejection and elevated osmotic pressure due to the presence of colloid deposit layers. The cake-enhanced osmotic pressure was shown to result from hindered back-diffusion of rejected salt ions trapped within the colloid-cake layer. The hindered diffusion of salt is successfully described by coupling a modified “cake filtration” model and the classic “film theory” model.