Microfiltration fouling

Thomassen JK, Faraday DBF, Underwood BO, and Cleaver JAS. The effect of varying transmembrane pressure and crossflow velocity on the microfiltration fouling of a model beer. Separat. Purif. TechnoL, 2005 41(1) 91-100. 

Bedwell W.B., Yates S.F., Brubaker I.M. (1988), Crossflow microfiltration-fouling mechanisms studies,... 

Field RW, Wu D, Howell JA, Gupta BB. Critical flux concept for microfiltration fouling. J. Membr. Sci. 1995 100 259. 

Eield, R., Wu, D., Howell, J., Critical flux concept for microfiltration fouling. Journal of Membrane Science 1995, 100, 259-272. 

Victor Starov (influence of surface forces on membrane separation nano-filtration, ultra- and microfiltration (fouling and/or gel layers formation)). Department of Chemical Engineering, Loughborough University, Loughborough... 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

Ceraver s entry into the microfiltration and ultrafiltration field followed a completely different approach. In 1980, it became apparent that the type of product made by Ceraver for uranium enrichment, which was a tubular support and an intermediate layer with a pore diameter in the microfiltration range, might be declassified. Ceraver therefore developed a range of a-AljOj microfiltration membranes on an a-AljOs support with two key features first, the multichannel support and second, the possibility to backflush the filtrate in order to slow down fouling. 

After extraction, the solute-laden CLAs need to be separated from the mother liquor so that they can be back stripped. Hence attempts were made to filter the solute-rich CLAs from the aqueous phase using cross-flow microfiltration [70]. The filtration characteristics of the CLAs as indicated by the flux, CLA size, and concentration showed that they are completely retained by the membrane and do not foul the membrane surface. Using this system, the CLAs could easily be concentrated up to 30% w/v at low pressures, and the permeate stream remained totally clear. The CLAs appear to maintain their structural integrity because only 3 mg dm of SDS was... 

Nanofiltration Compared to microfiltration and ultrafiltration, nanofiltration and reverse osmosis are more expensive and susceptible to fouling (Shih, 2005, 95). Most of the expenses result from the high densities of the membranes, which require high pressures (0.34-6.9 MPa) and a considerable... 

S. T. Kelly and A. L. Zydney, Effects of intermolecular thiol-disulfide interchange reactions on BSA fouling during microfiltration, Biotech. Bioeng. 1994, 44(8), 972-982. 

The key innovation that has led to the increased use of cross-flow microfiltration membrane modules in the last few years has been the development of back-pulsing or backflushing to control membrane fouling [9-11]. In this procedure, the water flux through the membrane is reversed to remove any particulate and fouling material that may have formed on the membrane surface. In microfiltration several types of backflushing can be used. Short, relatively frequent flow reversal lasting a few seconds and applied once every few minutes is called... 

Microfiltration cross-flow systems are often operated at a constant applied transmembrane pressure in the same way as the reverse osmosis and ultrafiltration systems described in Chapters 5 and 6. However, microfiltration membranes tend to foul and lose flux much more quickly than ultrafiltration and reverse osmosis membranes. The rapid decline in flux makes it difficult to control system operation. For this reason, microfiltration systems are often operated as constant flux systems, and the transmembrane pressure across the membrane is slowly increased to maintain the flow as the membrane fouls. Most commonly the feed pressure is fixed at some high value and the permeate pressure... 

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

Figure 7.17 Experiments showing the rate of fouling of 0.22-p.m microfiltration membranes used to treat dilute biomass solutions. The membranes were operated at the fluxes shown, by increasing transmembrane pressure over time to maintain this flux as the membranes fouled [12]. Reprinted from J. Membr. Sci. 209, B.D. Cho and A.G. Fane, Fouling Transients in Nominally Sub-critical Flux Operation of a Membrane Bioreactor, p. 391, Copyright 2002, with permission from Elsevier...

Figure 10.1. Experimental procedure for the determination of the colloidal index (Cl). The Cl or the silt density index (SDI) test is used to predict and prevent particulate fouling on the membrane surface. It measures the time required to filter a fixed volume of water through a standard 0.45- xm pore-size microfiltration membrane with a pressure of 2.07 bar. The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt was built up) represents the Cl or SDI value.

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

Huang, L. and Morissey, M. (1998) Fouling of membranes during microfiltration of surimi wash water Roles of pore blocking and surface cake formation. Journal of Membrane Science, 144, 113-123. 

Nuengjamnong, C., Cho, J., Polprasert, C. and Ahn, K.H. (2006) Extracellular polymeric substances s influence on membrane fouling and cleaning during microfiltration process. Water Science and Technology Water Supply, 6, 141-148. 

Wen, X., Bu, Q. and Huang, X. (7-10 June 2004) Study on fouling characteristic of axial hollow fibers cross-flow microfiltration under different flux operations, in Proc of the IWA Specialty Conference -WEMT 2004, Seoul, Korea. 

Index Entries Secondary membrane backflushing microfiltration ultrafiltration direct visual observation fouling. 

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