Fouling membranes

Membrane fouHng is a membrane system phenomena. The type of feed water determines the severity of folding. For example, seawater RO membranes are mainly fouled by organic and particulate matter, whereas brackish water RO membranes are primarily fouled by dissolved but sparingly soluble salts. Substances that foul membranes are Hsted 

Feed-channel spacers — spiral-wound modules and some flat-sheet modules High shear — vibrating membrane High shear — rotating the membrane High shear — rotor above the membrane Dean vortices Pulsing the feed Baffles 

Vibrational energy in a VSEP system focuses shear waves at the membrane surface, repelling solids and foulants. Cuikin, B. Armando. AD. (September/October f992) Courtesy Filtration Separation 

The causes of fouling vary depending on the nature of the solute and solute-membrane interactions. Fouling is often the result of a strong interaction between the membrane and the components in the feed stream for example, fouling by coUoids, iron and biomaterials can be especially severe. As a general rule, a reversible flux reduction is due to CP, whereas an irreversible flux reduction is due to fouHng. 

fabricating (or modifying) membranes that are more hydrophilic and, therefore, less prone to fouling with the added benefit of enhanced flux, has been a key priority for more than 30 years. 

Of particular interest in this study is the fouling of membrane processes by natural organics. Fouling depends on the characteristics of the natural organics. A detailed review of the characteristics of interest is required to highlight the factors that may influence membrane fouling. Membrane filtration 

A summary of key characteristics of natural organics and the possible implications of these characteristics on treatment is shown in Table 2.4. The effect of some of these characteristics on membrane treatment is investigated in more detail in Chapter 3. 

Definition HA adsorbed by XAD8 resin and Aquatic organics are operationally defined 

Composition Particulate (POC) and dissolved (DOC) POC and DOC are removed by different processes (MF removes POC, but not DOC) 

Concentration 0 to 50 mgL Concentration determines how much DOC needs to be removed and may impact treatment efficiency (e.g., more fouling at higher concentration) 

The following equation for temperature polarisation can be derived for this process. (It should he noted that this equation is similar to eq.VII - 61. e.xcept that the enthalpies of vaporisation and condensation are not included since no phase transitions occur). 

The heat conductivity in the membrane, appears in both eqs. VII - 64 and VII - 65. However, both values are not equal the value in eq.VII 65 (thermo-osmosis) will be greater so that this factor will have a stronger effect on the temperature polarisation. Because a convective term which mainly depends on the volume flu.x appears in eq.VII -58, the net result is that the effect of temperature polarisation is always greater in membrane distillation even when the temperature difference across the membrane is the same in both processes and when the same membrane material is used. 

The performance of membrane operations is diminished by polarisation phenomena, although the extent to which these phenomena can occur differ considerably. Thus, in microfiltration and ultrafiltration the actual flux through the membrane can be only a fraction of the pure water flux, whereas in pervaporation the effect is less severe. 

With all polarisation phenomena (concentration, temperature polarisation), the flux at a finite time is always less than the original value. When steady state conditions have been attained a further decrease in flux will not be observed, i.e. the flux will become constant as a function of time. Polarisation phenomena are reversible processes, but in practice, a continuous decline in flux decline can often be observed. This is shown schematically in figure VH - 23. 

Such continuous flux decline is the result of membrane fouling, which may be defined as the (ir)rcversible deposition of retained particles, colloids, emulsions, suspensions, macromolecuies. salts etc. on or in the membrane. This includes adsorption, pore blocking, precipitation and cake formation. Some extensive review articles have been written on fouling [18-21]. 

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Tubular Modules. Tubular modules are generally limited to ultrafiltration appHcations, for which the benefit of resistance to membrane fouling because of good fluid hydrodynamics overcomes the problem of their high capital cost. Typically, the tubes consist of a porous paper or fiber glass support with the membrane formed on the inside of the tubes, as shown in Figure 24. 

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

In reverse osmosis, most modules are of the hollow-fine fiber or spiral-wound design plate-and-frame and tubular modules are limited to a few appHcations in which membrane fouling is particularly severe, for example, food appHcations or processing of heavily contaminated industrial wastewater. 

Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

Paulson, David J., "An Overview of and Definitions for Membrane Fouling," Osmonics, Inc. Presented at 5th Annual Membrane Technology/Planning Conference, October 1987, Cambridge, MA. 

The foregoing equations assume that membrane performance is time independent. In some cases, a noticeable reduction in permeability occurs over time primarily due to membrane fouling. In such cases, design and operational provisions are used to maintain a steady performance of the system (Zhu et a ., 1997). 

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... 

Reverse osmosis requires good pretreatment to prevent membrane fouling and loss of performance. Because it is seldom better than 60 to 70% efficient, there is a relatively high cost for pumping and discharging the additional supply water consumed. Nevertheless, it is good as a bulk water roughing process for purification. 

Typically, RO plants require a RW supply with a salt density index (SDI) of below 3.0 (ideally below 1.0) to prevent excessive membrane fouling. 

Automatic periodic membrane flush to remove recent surface deposits, thus reducing the risk of membrane fouling... 

Reverse osmosis plant are always subject to an insidious and gradual loss of permeate volume output or quality deterioration due to membrane fouling. The rate of decline is strongly influenced by the input RW quality. Therefore, any and all features, such as those above, that can be employed to delay the onset and degree of fouling and extend membrane life are to be recommended. 

Pretreatment Requirements for RO In addition to CIP and other fouling control systems constructed within the main body of the RO frame, it is vital that all due consideration be given to providing the correct kinds of RO RW pretreatment, in order to further reduce risks of membrane fouling. Options for pretreatment include ... 

As with reverse osmosis, feed pretreatment can be used to minimize membrane fouling and degradation, and regular cleaning is necessary. 

The dead-end setup is by far the easiest apparatus both in construction and use. Reactor and separation unit can be combined and only one pump is needed to pump in the feed. A cross-flow setup, on the other hand, needs a separation unit next to the actual reactor and an additional pump to provide a rapid circulation across the membrane. The major disadvantage of the dead-end filtration is the possibility of concentration polarization, which is defined as an accumulation of retained material on the feed side of the membrane. This effect causes non-optimal membrane performance since losses through membrane defects, which are of course always present, will be amplified by a high surface concentration. In extreme cases concentration polarization can also lead to precipitation of material and membrane fouling. A membrane installed in a cross-flow setup, preferably applied with a turbulent flow, will suffer much less from this... 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

The concentration of iron present in the permeate wax was found to be consistently less than 35 ppm, with over 85% below the 16 ppm level. Following an active flux maintenance procedure results in short-term recovery of flux, which declines to base value within 24 h. The passive flux maintenance procedure of interrupting the permeate flow for 30 or 60 s per 30 min was effective in recovering the initial membrane fouling temporarily. Better flux stability was attained only after increasing the permeate off-cycle to 1 h per day in addition to 30 s off per half-hour cycle. Variation of flux magnitude with TMP was found to follow a linear relationship within the range studied. 

To overcome membrane scaling, the operating pH of the feed brine to the unit was lowered to a range between 4 and 7. A simple modification was made to the plant to control the pH of the plant feed brine by mixing acidic dechlorinated brine with alkaline dechlorinated brine. This modification has proven to be effective and no further membrane fouling has occurred over the last two years. 

Membrane processes such as ultrahltrahon or reverse osmosis have been proposed as oil removal processes. Laboratory tests have indicated favorable oil removal, although relatively low flux rates, membrane fouling, and membrane life problems have presented concerns for the practical applicahon of membrane processes to oil removal. 

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