Microfiltration backflushing

Air Backflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillaiw module with the permeate exiting the tube side. The device is rim as an intermittent deadend filter. Eveiw few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed back-... 

Cakl, J., Bauer, I., Dolecek, P., and Mikulasek, P. 2000. Effects of backflushing conditions on permeate flux in membrane cross-flow microfiltration of oil emulsion. Desalination 127 189-98. 

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. 

SPEC was essentially able to market their Zr02-based ultrafiltration membranes to an already existing market in the sense that these membranes replaced polymeric UF membranes in a number of applications. They also developed a certain number of new applications. For Ceraver, the situation was different. When the Membralox membranes were first developed, microfiltration was performed exclusively with dead-end polymeric cartridge filters. In parallel to the development of inorganic MF membranes, Ceraver initiated the development of cross-flow MF with backflushing as a new industrial process. 

In the last few years, a third type of microfiltration operating system called semi-dead-end filtration has emerged. In these systems, the membrane unit is operated as a dead-end filter until the pressure required to maintain a useful flow across the filter reaches its maximum level. At this point, the filter is operated in cross-flow mode, while concurrently backflushing with air or permeate solution. After a short period of backflushing in cross-flow mode to remove material deposited on the membrane, the system is switched back to dead-end operation. This procedure is particularly applicable in microfiltration units used as final bacterial and virus filters for municipal water treatment plants. The feed water has a very low loading of material to be removed, so in-line operation can be used for a prolonged time before backflushing and cross-flow to remove the deposited solids is needed. 

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... 

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...

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

In the current work, we employed a modified approach, with predeposition of a secondary membrane of yeast (SMY) before starting the filtration of protein. Backflushing was employed periodically to remove the deposited secondary membrane to recover the flux, and a new secondary membrane was deposited subsequently with the start of each new cycle, prior to restarting the filtration of protein. Microfiltration experiments were performed with yeast as the secondary membrane and BSA-only solutions and yeast-BSA mixtures as the feed. Ultrafiltration experiments were performed with yeast as the secondary membrane deposition medium and cellulase enzyme solutions, used in the conversion of biomass into ethanol, as the feed. In this article, we also present direct visual observation images (19) of the formation of the secondary membrane and its subsequent removal. 

Fig. 2. Direct visual observation pictures of surface of membrane (A) during first cycle of deposition ofSMY,and (B) at end of several cycles ofyeast-BSA microfiltration with a secondary membrane, with the pictures taken just after backflushing portion at end of indicated cycle.

Fig. 3. SEM micrograph of a CA microfiltration membrane fouled (A) after filtration of BSA only for 3100 s, (B) after completion of SMY deposition portion of cycle 11, and (C) after completion of backflushing portion of cycle 11. The primary feed contained 2.0 g/L of BSA, and the secondary feed contained 1.34 g/L of yeast. The cycle conditions were tf = 300 s, tsf = 15 s, and tb = 3, with an average TMP of 7.5 psi maintained during forward as well as reverse filtration.

Fig. 5. Permeate flux during microfiltration of mixture of 2.0 g/L of BSA and 1.34 g/L of yeast with deposition of SMY and with backflushing (A), without SMY but with backflushing (x), and without deposition of SMY and without backflushing ( ) permeate flux for filtration of BSA without deposition of SMY and with backflushing...

Fig. 6. Normalized microfiltration permeate flux vs time for different concentrations of yeast in secondary feed reservoir ( ) 1.36 g/L ( ) 0.68 g/L ( ) 0.34 g/L. The cycle conditions were tf = 280 s, = 10 s, and th = 10 s, with Pf=Ph = 7.5 psi. The primary feed contained a mixture of yeast (with the same concentration of yeast as in the secondary feed) and 2.0 g/L of BSA. The dashed line represents the normalized permeate flux during filtration of protein alone without deposition of SMY and without backflushing. Error bars represent SD for two to three repeats.

Fig. 7. Normalized recovered flux plotted after 600 s ( , cycle 2), 900 s ( , cycle 3), and 1200 s (O, cycle 4) of microfiltration with deposition of SMY. The SMY was deposited for tsf= 25 s, followed by forward feed filtration of 2.0 g/L of BSA for tf= 275 s and then th = 0.1,0.2,0.5,1.0,2.0, or 5.0 s of backflushing. The points are joined by straight lines for clarity. The error bars represent SD for three repeats.

The goal of ultrafiltration, in contrast to microfiltration, is to retain protein molecules by the membrane while passing smaller solutes through the membrane with the permeate. Ultrafiltration experiments were performed with polysulfone membranes (30,000-Dalton mol wt cutoff). Figure 9 shows a comparison of the permeate flux vs time obtained during ultrafiltration of cellulase in the presence and absence of SMY that was periodically removed by backflushing and then replaced with a new SMY. 

Fig. 8. Protein transmission during microfiltration of 2.0 g/L BSA only (O) and 2.0 g/L of BSA in presence of SMY and backflushing (A), and permeate flux vs time for...

The most common sort of embodiment involving a liquid phase is the membrane separation of suspended solids from liquids, denoted variously by the terms filtration, microfiltration, and ultrafiltration, depending on the particle size, and which may include colloidal suspensions and emulsions. The solid particulates, for the most part, are deposited in the interstices or pores of a membrane barrier, and accordingly will require an intermittent backflushing operation. 

Microfiltration membranes are commonly used in MBRs to separate sohds from water. The fluxes are very low, often below critical flux, and at low pressures when hollow fibers are used, backflushing is added to prevent or reduce flux decrease. MBRs are discussed in detail in Section 35.6.3.2, tubular modules with MF membranes have also been tested in the pulp and paper industry. 

A theoretical analysis has been carried out for galvanostatic and potentiostatic pulse regimes [27]. The idea that developed is a bit the same as backflushing with pressure driven-membrane operation such as microfiltration or ultrafiltration. The time dependencies of the extent of the concentration polarization near the membrane surface during the pulse are described theoretically for both pulse regimes and a qualitative discussion of the pause duration is presented. The main characteristic of the non-stationary process is the transition time between the state without polarization and the state with stationary polarization. 

Figure 9.19 Schematic drawing of the flux vs time behavior in agiven microfiltration process with and without backflushing [2]. Reproduced with kind permission of Kluwer Academic Publishers.

Of the process options considered, microfiltration (TVIF) is the membrane process with the largest pores. It is generaLl) used for waters of high turbidity, and low colour or organics content. MF can remove bacteria and turbidity . MF is also a common pretreatment process for NF and RO. The fact that MF pores are relatively large allows cleaning methods, such as air backflush or permeate backwash, which remove deposits from pores and surface. 

A Memcor microfiltration unit with automated air backflush was used for pretreatment. The membrane area of the hollow fibre module is 1 m. ... 

Microfiltration flux decreased by approximately 60 % (630 to 385 L m %" ) during the initial batch and then stabilised. No flux variation during the filtration cycle due to backflush was measurable. Since the MF served as a pretreatment step, and therefore the flux was not continuously monitored. However, serious flux decline soon became apparent which encouraged further investigations. 

Caplllary-fiber 5-50 50-100 Limited to low-pressure applications <200 psi good fouling resistance, can be backflushed. Important in ultraflltration (UP) and microfiltration (MF) applications. 

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