Fouling and Nanofiltration Performance
Most municipal, industrial, and natural waters contain complex mixtures of dissolved, macromolecular colloidal, and particulate matter. Conventional processes used to pretreat NF feed waters fail to remove submicron colloids, macromolecules and dissolved matter.
This matter accumulates at the membrane surface and results in severe performance decline—a phenomenon known as fouling. Fouling of NF membranes may result in loss of both solvent flux and solute retention. A review of fouling studies reveals that the foulants of greatest concern for nanofiltration (NF) separations are colloidal matter consisting of organics, silica, clays, metal oxides (specifically iron and manganese), and microorganisms.[33-42] Fouling may be further categorized into reversible and irreversible fouling. Flux and solute retention may decline because of solution chemistry effects, concentration polarization, or colloid cake layer formation. The original membrane performance may be recovered by simply flushing the membrane with clean water, and hence these types of fouling are considered reversible. Irreversible fouling is defined as a decline in performance that can only be recovered through harsh chemical cleaning, but often only partially recovered. Frequent chemical cleaning of membranes degrades polymeric thin films, and hence reduces the life span of membranes in many applications. Furthermore, it decreases process efficiency because of the reduced flux and enhanced solute passage, requiring higher applied pressures and larger membrane area. This makes fouling a very important parameter in process design.
The trans-membrane hydraulic pressure drop is determined by rearranging Eq. (1) to obtain
Hence by analogy, the trans-cake hydraulic pressure drop may be described from
where Rc is the hydraulic resistance offered by the cake layer to the pure solvent.
The cake resistance may be described through the Carman-Kozeny equation, as a function of specific cake resistance (per unit thickness) and cake layer thickness,
or specific cake resistance (per unit mass) and cake layer mass per unit membrane area,
Pfd*3
Fouling Mechanisms and Models
Interactions between accumulated dissolved and colloidal matter at a NF membrane surface result in several potential fouling mechanisms. Fouling by dissolved organics and sparingly soluble salts is difficult to describe quantitatively, but colloidal fouling is better understood. Because colloids fall within the approximate size range of 10 nm to 10 mm and NF membrane ''pores'' are no more than a few nanometers in diameter, pore-blocking mechanisms are considered negligible. Thus the major fouling mechanisms discussed below include the hydraulic pressure drop across the foulant deposit (cake) layer and enhanced osmotic pressure effects, which were recently shown to be the predominant fouling mechanism for RO and NF membranes.[28] Most practical nanofiltra-tion processes operate with a constant flux, so this convention will be used to describe fouling mechanisms. The objective is to be able to describe the transient applied pressure required to maintain a constant flux in light of the various transient mechanisms of fouling.
The total system pressure is a combination of the trans-membrane hydraulic (Dpm), the trans-membrane osmotic (Dpm), and the trans-cake hydraulic (Dpc) pressure drops and may be expressed mathematically as
Here e is the cake layer porosity, df represents the nominal foulant diameter, and Pf is the nominal foulant particle density. In most existing models, it is assumed that the specific cake resistance is constant in time and across the cake layer thickness and only the foulant layer thickness (or mass) changes with time.
It was recently experimentally demonstrated that diffusion of rejected salt ions was hindered within colloid deposit layers formed over nanofiltration mem-branes.[28,43] In addition, it was suggested that tangential flow may be hindered within a foulant deposit layer, further reducing solute mass transfer within the deposit layer. The result was elucidation of a single mechanism—''cake-enhanced concentration polarization''—capable of describing the majority of observed flux decline, as well as the observed decline in salt rejection because of colloidal fouling of NF (and RO) membranes.
The overall mass transfer coefficient was considered the sum of two mass transfer coefficients, one describing salt back-diffusion from the membrane surface through the cake layer, and one through the remainder of the salt CP layer. Incorporating the hindered mass transfer coefficient into Eq. (13) yields
Apm = fosCbRo exp
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