Fouling-resistant membranes

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. 

DJ. MiUer, X. Huang, H. Li, S. Kasemset, A. Lee, D. Agnihotri, et al., Fouling-resistant membranes for the treatment of flowback water from hydraulic shale fracturing a pilot study,. Memb. Sci. 437 (2013) 265-275. 

Hollow-fiber designs are being displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. Also, thin-film interfacial composite membranes, the best reverse osmosis membranes available, have not been fabricated in the form of hoUow-fine fibers. 

Equation (20-80) requires a mass transfer coefficient k to calculate Cu, and a relation between protein concentration and osmotic pressure. Pure water flux obtained from a plot of flux versus pressure is used to calculate membrane resistance (t ically small). The LMH/psi slope is referred to as the NWP (normal water permeability). The membrane plus fouling resistances are determined after removing the reversible polarization layer through a buffer flush. To illustrate the components of the osmotic flux model. Fig. 20-63 shows flux versus TMP curves corresponding to just the membrane in buffer (Rfouimg = 0, = 0),... 

Most of today s ultrafiltration membranes are made by variations of the Loeb-Sourirajan process. A limited number of materials are used, primarily polyacrylonitrile, poly(vinyl chloride)-polyacrylonitrile copolymers, polysulfone, poly(ether sulfone), poly(vinylidene fluoride), some aromatic polyamides, and cellulose acetate. In general, the more hydrophilic membranes are more fouling-resistant than the completely hydrophobic materials. For this reason water-soluble... 

Hester, J.F. and Mayes, A.M. (2002) Design and performance of foul-resistant poly(vinylidene fluoride) membranes prepared in a single step by surface... 

Like all other membranes, dynamic membranes experience flux decline over time to low steady state values. The permeate flux can be restored to its initial high value by applying a 500 to 1,000 ppm of sodium hypochlorite solution to wash out the fouled dynamic membrane. Porous ceramic supports were used instead of organic polymer membranes due to their resistance to chemicals and heat during the cleaning cycle. 

Studies are required on effective removal of biofilms without damaging the membrane. Additional work needs to be done to find out what happens to the fouling resistance of chemically modihed membranes over the long term (i.e., after initial biofilm formation). Membrane resistance to humic acids is another area for further study. Furthemore, the molecular tools needed for exploring the biochemical details of the microbial adhesion process to membranes are now available. 

JenkinsM. and Tanner M.B., Operational experience with anew fouling resistant reverse osmosis membrane. Deja/inahon 119 1998 243-250. 

Maartens et al. [15] characterized adsorbed foulants calorimetrically and concluded that foulants in pulp and paper mill effluents are phenolic and hydrophobic in nature, a result corroborated by Puro et al. [9]. Maartens et al. also modified their membranes to make them more hydrophilic and achieved less fouling and better cleaning efficiency. Thus, pretreatment or modification of a membrane can improve its fouling resistance, its flux or retention, and can enhance its usability in pulp and paper industry applications [16]. 

The importance of biopolymer deposition on the membrane has been highlighted by Chu and Li [34] who noted that such material may accelerate bacterial attachment to the membrane in addition to its contribution to fouling resistance. The formation of a slime layer on the membrane, comprising an EPS matrix with embedded bacteria, may be analogous to a biofilm, the significance of which is the difficulty of its removal by nonchemical methods. The question of whether biofilm formation plays a significant role in MBRs is yet to be addressed. Indeed, a comprehensive model of fouling in MBRs is not yet available. 

In order to enhance the overall performance of the membrane, it is necessary to modify the membrane material or the structure (41). The objectives for modification of the existing membranes are to increase flux, selectivity, and chemical resistance (solvent resistance, swelling resistance, and fouling resistance). Some of the most commonly practiced membrane modification methods are listed in Table 3. 

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