Fouling barrier layer

Interesting progress has been made recently in chemicaUy modifying the barrier-layer surface of asymmetric polymeric gas permeation membranes by reactive gaseous or liquid treatment (e.g., fluorination) to improve membrane permselectivity or stability [42]. Such surface treatments modify the ultrathin barrier layer almost exclusively and aUow conversion of that layer into a compositionaUy difierent structure. The result may be a more permselective membrane without significant permeabUity loss, a more fouling resistant membrane. 

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

Figure 3.4 Fouling on membrane surface creates an additional barrier to permeate transport that requires additional pressure to force permeate through the fouling layer.

Membrane fouling involves the deposition of suspended solids, including bacteria, on the membrane or components within the membrane module. These foulants form a layer on the surface of the membrane that becomes an additional barrier for water to flow through to the permeate side of the membrane. Hence, if the feed pressure is held constant, the permeate flow will decrease. 

Control of fouling because of corrosion is possible by the employment of additives. For corrosion to occur, all the elements of the electrochemical circuit must be complete, so that an imposed electrical barrier in the circuit prevents the movement of ions and electrons, which is fundamental to the fouling mechanism. A thin layer of metal oxide can act as such a barrier, provided that the layer is continuous. The protective layer is itself a product of limited corrosion, its presence inhibiting further attack. 

As a result of the charges in the relationship between F and x the summed curve shows a maximum at F at a short separation distance. F is an energy barrier. At large distances the energy is a minimum (F J. If the thermal energy of the particles is smaller than F, no interaction will take place and fouling will not occur. The problem is more complex than the simple approach taking only the van der Waals forces and the double layer forces into account, since other interactions are also likely to be present. 

While this critical flux phenomena is generally accepted in MF and UF, some authors also mention limiting fluxes in NF (Levenstein et al. (1996)). Cohen and Probstein (1986) determined a characteristic permeation velocity below which no fouling was observed in the RO of colloids. Bacchin et al. (1995) defined a critical flux for the MF, UF, and RO of large colloids. The critical flux Jcot is a function of diffusion and the potential barrier between particles Vb as shown in equation (3.36), where 5 is the boundary layer thickness and D the particle diffusivity. 

UF PAN membranes. The coated membranes were immersed in isopropanol for 30 min and thereafter in a water bath. It was shown that during precipitation, the copolymer undergoes microphase separation, forming interpenetrating networks of PAN-rich and PEO-rich nanodomains. Transmission electron microscopy reveals that PEO domains act as water-permeable nanochannels and provide the size-based separation capability of the membrane. A small decline in flux (15%) was observed in a 24 h dead-end filtration experiment with 1 g/1 BSA solution using the modified membrane, while the base UF membrane lost 81% of its flux irreversibly in the same conditions. It was concluded that the PEO brush layer, formed on the membrane surface, acts as a steric barrier to protein adsorption, endowing these membranes with exceptional fouling resistance. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization, which causes membrane fouling due to deposition of retained colloidal and macromolecular material on the membrane surface. The pure water flux of ultrafiltration membranes is often very high— more than 1 cm /(cm min) [350 gal/(ft day)]. However, when membranes are used to separate macromolecular or colloidal solutions, the flux falls within seconds, typically to the 0.1 cm /(cm min) level. This immediate drop in flux is caused by the formation of a gel layer of retained solutes on the membrane surface because of the concentration polarization. The gel layer forms a secondary barrier... 

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