One of the major limitations of the membrane-based system in many applications is the decline in permeate flux. This flux decline is due to two distinct phenomena, namely, concentration polarization and fouling of the membrane. Accumulation of solute particles over the membrane surface leads to concentration polarization (Blatt et al., 1970). This results in enhancement of the solute concentration at the membrane–
solution interface. It is also to be noted that concentration polarization is reversible in nature and can be removed by backflushing, application of high cross-flow veloci- ties, or cleaning protocol. On the other hand, fouling of the membrane is an irre-versible phenomenon. The nature and extent of membrane fouling strongly depend on the physicochemical nature of the membrane and solute. Surface morphology, solute–solute, and solute–membrane interactions are important in understanding of fouling phenomena. The mechanisms involved are adsorption of solute particles over the pore mouth or inside the pore, making the pore completely or partially blocked. Pore blockage increases the membrane resistance, while cake formation over the membrane surface offers resistance to permeate flow. Thus, pore blocking and cake formation are the two mechanisms of membrane fouling. This effect is
permanent and membrane permeability cannot be recovered to its earlier value com- pletely even after washing of the membrane. Therefore, this is also termed as irre-versible fouling. Several studies have been undertaken to reduce the concentration polarization and associated fouling to maintain high permeate flux and to retrieve the native membrane rejection characteristics. These include (1) feed pretreatment, (2) membrane material, (3) use of turbulence promoter, (4) pulsatile flow, (5) gas sparging, (6) external forces, (7) backflushing, and (8) turbulent flow.
4.2.2.1 Feed Pretreatment
The performance of any membrane-based fruit juice clarification strongly depends on the presence of polysaccharides such as pectin and starch in it. Pectin makes the clari-fication process difficult as it forms a gel-type layer over the membrane surface in the presence of sugar and acid while starch causes haze in juice. Therefore, permeate flux and yield decrease significantly. In order to degrade the polysaccharides, an enzymatic treatment of raw juice is generally carried out with enzymes such as pectinases and amylases. Pectinases hydrolyze pectin and cause the pectin–protein complexes to floc-culate. Amylases decompose the starch molecules that may cause cloudiness during storage (Kilara and Van Buren, 1989). This reduces the viscosity and fouling, resulting in higher permeate flux and lower energy consumption. Alvarez et al. (1998a) reported that enzymatic depectinization of apple juice resulted in higher permeate flux of clari-fied juice with lower turbidity, viscosity, and total pectin content during clarification of apple juice using a commercial zirconium oxide UF membrane (Table 4.1).
4.2.2.2 Membrane Material
The fouling can be reduced by using hydrophilic polymer as membrane material or by applying a surface modification to make the surface more hydrophilic. Macrosolutes, which contain hydrophobic parts, are adsorbed easily on hydrophobic membrane.
The adsorption layer on hydrophobic surface offers higher resistance to perme-ate flow and is difficult to wash away compared with that on hydrophilic surface.
TABLE 4.1
Characteristics of Apple Juices Treated with Different Amounts of Pectinex 3XL before UF
Parameter
Concentration of Pectinex 3XL (FDU/g Pectin)
0 100 200 300 400
Turbidity (NTU) 440 97.0 52.3 27.9 26.0
Viscosity × 103 Pa s (at T = 50°C) 3.58 1.044 0.977 0.965 0.97 Reduction of total pectin (%) 0 43.0 63.0 80.0 80.0 Steady-state permeate flux (L/m2 h)
at T = 50°C and P = 400 kPa
38 55 75 125 125
Sources: Alvarez, S. et al., Colloids Surf. A Physicochem. Eng. Aspects, 138, 377, 1998a; Alvarez, S. et al., Sep. Purif. Technol., 14, 209, 1998b.
The surface roughness has a significant effect on fouling (Fane and Kim, 1988).
Fane and Kim observed that with increase in surface roughness, loss of permeate flux increases.
4.2.2.3 Turbulent Flow
The system performance in terms of flux can be improved either by reducing concen-tration polarization by increasing the mass transfer from the membrane surface to the bulk or by reducing fouling by increasing the wall shear rate. This is mostly done either by increasing cross-flow velocity or by changing of the channel geometry.
4.2.2.4 Use of Turbulence Promoter
Many hydraulic approaches, either increasing the fluid velocity or creating the tur-bulent behavior, have been developed for cross-flow UF to suppress the increase in concentration polarization and progressive fouling along the flow path. Metal grill, static rods, spiral wound (Geraldes et al., 2002), and disc- and doughnut-shaped inserts (Howell et al., 1993) are some of the various types of turbulence promot-ers used. Due to obstruction in flow path, turbulent promoter creates localized turbulence in its neighborhood on the membrane surface, and the generated tur-bulent eddies enhance mixing on the membrane surface. Therefore, concentration polarization and fouling decrease, leading to an enhanced permeate flux. During cross-flow UF of simulated fruit juice (a mixture of pectin and sucrose), by incor-poration of cylindrical promoters placed perpendicular to the flow path, permeate flux improvement of about 65% as compared with the no-promoter condition was achieved (Pal et al., 2008).
4.2.2.5 Pulsatile Flow
Unsteady flows and oscillation flows can be generated by pulsations into the feed or permeate channels. Pulsations reduce the formation of a fouling layer of rejected particles on the membrane by exerting a “scouring effect.” (Su et al., 1993). A three- fold increase in permeate flux was observed using periodically spaced, doughnut-shaped baffles in UF tubes together with pulsed flows with an oscillation frequency up to 2.5 Hz (Finnigan and Howell, 1989). During MF of apple juice, using a 1 Hz pulsed flow and a ceramic membrane, flux improvements of about 45% and 140%
have been reported (Gupta et al., 1992, 1993). Amar et al. (1990) reported that using the superimposition of pulsation on the inlet flow, the permeate flux was increased from 200 to 250 L/m2 h for 200 min, with a ceramic membrane (Figure 4.1) during apple juice clarification. Significant improvement in permeate flux is also observed in the case of intermittent operation. This may be explained by the fact that during shut off of UF system, the sudden release of pressure on the deposited layer results in part of the fouling materials rebounding and disassociating from the membrane. On restart-up operation, the cross-flow wave flushes the loosened fouling materials away from the membrane surface resulting in an enhancement of permeate flux. Chiang and Yu (1987) reported that during UF of passion fruit juice with intermittent on–off operation, permeate flux increased about 40% compared with that without intermit-tent on–off operation. Unsteady flows can also be obtained using the intermittent jet of the feed in the channel. This causes the feed velocity to abruptly change, resulting
in the formation of large vortices (Arroyo and Fonade, 1993). Kim and Chang (1991) used periodic back-pulsing at a frequency of 0.67 min−1 during filtration of a mixed hemoglobin (62.5 kDa) and dextran (10 kDa) solution through a 30 kDa molecular weight cut-off membrane and obtained an almost threefold enhancement in perme-ate flux. Ding et al. (1991) found a twofold increase in the permeate flux of plasma when being filtered through a MF membrane by periodically squeezing the tubing conveying the retentate, thereby pulsating the flow and pressure.
4.2.2.6 Backflushing, Pulsing, and Shocking
The life of membrane can be increased by periodic backwashing (BW). The per-meate itself is often used as the backwash fluid. Periodic BW is carried out by pumping permeate back into to the feed chamber to remove the particles deposited in pores and also on the membrane surface. The backwash pressure is generally higher than the normal operating inlet pressure. Sometimes, backflushing is done by pressurized air instead of permeate. Most backflushing is done with permeate for 1–5 s at a frequency of 1–10 times per minutes at a pressure of 0.1–1 MPa. Amar et al. (1990) investigated the effect of BW for 2–5 s every 2–5 min with compressed nitrogen gas (backwash pressure almost twice the transmembrane pressure [TMP]) pectinase using a ceramic MF membrane. They observed that the permeate flux increased about 166% with BW for pectinase-treated apple juice when compared with permeate flux without backwashing (Figure 4.2).
Su et al. (1993) reported that the improvement of permeate flux ranged between 100–110 and 70–80 L/m2 h for vacuum-filtered apple juice and pre-vacuum-filtered juices, respectively, for most of the experimental time using nitrogen backwash
Ceraflo 0.2 µm
Tmp = 3.5 bar f = 1.2 Hz ∆V = 84 cm3 V = 3 m/s
Pulsating flow
0 100 200
Time (min)
Permeate flux (L/h m2)
300 400
500
400
300
200
100
0
Steady flow
FIGURE 4.1 Effect of pulsating inlet flow on fouling of membrane (apple juice treated with pectinase); Tmp, transmembrane pressure; f, frequency of pulsations; ΔV, displaced volume;
V, inlet velocity. (Reprinted from Amar, R.B. et al., J. Food Sci., 55, 1620, 1990. With permission.)
every 15 min. In both cases, backwash improved the permeate flux about 11 L/m2 h.
Rapid BW seems to be more effective, and it is commonly termed as “back pulsing”
or “backshocking.” The backpulses are of short duration (0.1 s or less) and are oper-ated continuously or periodically.
4.2.2.7 Gas Sparging
The technique of gas–liquid two-phase cross-flow filtration, via the injection of gas bubbles into the feed stream, was found to be effective in enhancing the perfor-mance of membrane process. The introduction of gas slugs into the liquid stream can increase turbulence on membrane surface and restrict the growth of concentra-tion boundary layer, leading to enhancement in the flux of the filtration process. It has been observed that the enhancement of gas sparging, in terms of the increase in permeate flux, is dependent on membrane modules. As high as a threefold flux increase was found when ultrafiltering dextran solutions in tubular membranes (Cui and Wright, 1994), whereas flux enhancement by gas sparging for membrane modules such as flat sheet (Li et al., 1998), hollow fiber (Bellara et al., 1996), and spiral wound (Cui et al., 1996) membranes was less pronounced, with an observed flux increase from 7% to 60%. Bellara et al. (1996) reported flux enhancements of 20%–50% for dextran and 10%–60% for albumin using gas–liquid two-phase cross-flow in hollow fiber membranes. Youravong et al. (2010) investigated the effect of gas sparging on permeate flux, fouling, and quality of clarified wine during clarification of pineapple wine using a tubular ceramic MF membrane. They found that the per-meate flux increased up to 138%. However, further increase in the gas sparging rate
00 50 100 150 200 250 300 350
Ceraflo 0.2 µm
Tmp = 3 bar Pc = 6.5 bar V = 2.77 m/s
50 100
Without BW With BW
150 200 250
Time (min) Permeate flux (L/h m2)
300
FIGURE 4.2 Effect of BW on fouling of membrane (apple juice treated with pectinase);
Tmp, transmembrane pressure; V, inlet velocity; Pc, backwash pressure. (Reprinted with per-mission from Amar, R.B. et al., J. Food Sci., 55, 1620, 1990.)
did not improve permeate flux compared with that without gas sparging. They also observed the negative effect of gas sparging, which caused a loss of alcohol content in the wine.
4.2.2.8 External Force
The use of additional forces such as electrical, sonic, and magnetic fields to improve the performance of filtration has gained increasing attention in recent years.
4.2.2.8.1 Ultrasonic Field
The passage of ultrasound waves through a suspension can cause many phenom-ena, including particle dispersion, cavitation, viscosity reduction, and changes in particle surface properties. Cavitation, which is observed as the rapid formation and collapse of gaseous microbubbles at the membrane surface, causes trapped particulates at the pore entry regions to be loosened. The cross-flow stream is then able to carry the particle away from the membrane surface, resulting in a decrease in concentration polarization (Kobayashi et al., 1999; Lamminen et al., 2004).
This leads to an increase in permeate flux. However, ultrasound is not effective at removing the fouling material trapped inside pores (Kokugan et al., 1995). There are several factors that influence the effectiveness of the ultrasound treatment such as frequency, power intensity, etc. Lower ultrasound frequencies are found to have higher particle-removing efficiencies than higher frequencies (Kobayashi et al., 1999; Lamminen et al., 2004). In general, an increase in power intensity will result in an increase in the sonochemical effects. With increasing power intensity to the system, both the number of cavitation bubbles formed and the size of the cavitating zone increase. In addition, the hydrodynamic turbulence increases with increased power intensity (Lamminen et al., 2004). The higher the power intensity during ultrasonic irradiation, the better the membrane cleaning and the greater the flux obtained. However, application of high-power intensity sometimes causes membrane erosion though different membrane materials have different durabil-ity in ultrasonic treatment. A continuous use of ultrasonic waves from the start of filtration has been found very effective in many investigations (Tarleton and Wakeman, 1990; Matsumoto et al., 1996). However, the continuous use of ultra-sound is undesirable in terms of energy consumption. The use of an intermittent ultrasonic field was found effective in terms of both cost and flux enhancement (Matsumoto et al., 1996; Muthukumaran et al., 2004). Sabri et al. (1997) reported that flux enhancements were up to 400% higher with intermittent ultrasound at a frequency of 45 kHz than in the reference cases during filtration of wastewater effluents from pulp and paper mills and brackish water.
4.2.3 eFFeCts oF diFFerent ProCess ParaMeters on PerMeate Flux
4.2.3.1 Effects of Flow Rate on Permeate Flux
The feed flow rate is an important parameter for the performance of the fruit juice clarification process. The effects of cross-flow rate on the permeate flux are observed during clarification of various fruit juices such as kiwifruit, carrot, apple, etc.
(Vladisavljevic et al., 2003; Cassano et al., 2004). It is observed that flux increases
with cross-flow velocity. This can be explained by the fact that at a higher flow rate, development of concentration polarization and subsequent gel-type layer formation on the membrane surface are restricted due to shearing force imposed by cross-flow rate. Clarification of depectinized apple juice using ceramic tubular membranes with 300 kDa (M7), 50 kDa (M8), and 30 kDa (M9) molecular weight cut-off was investigated by Vladisavljevic et al. (2003). They observed that fouling resistance decreased with feed flow rate, and higher permeate flux was obtained in the case of a less-resistive M9 membrane (Figure 4.3).
4.2.3.2 Effects of Transmembrane Pressure on Permeate Flux
Cassano et al. (2003) have described the variation of steady-state permeate flux with applied TMP during UF of carrot juice, as illustrated in Figure 4.4. For small pressures, the solvent flux is proportional to the applied pressure. At low pressure range, significant increase in permeate flux is observed with pressure due to enhanced driving force, and a weak pressure dependence is observed at higher values of operating pressure. This is due to the direct consequence of the gel-type layer formation over the membrane surface. The enhanced driving force due to increase in pressure is compensated by the increasing gel layer thickness. Hence, the permeate flux increases up to a limiting value, and on further pressure increase, no significant increase in flux is observed.
4.2.3.3 Effects of Operating Temperature on Permeate Flux
In general, a higher operating temperature leads to a higher permeate flux, keeping other operating conditions unchanged. The effect of temperature on the permeate flux can be observed in Figure 4.5 during clarification of apple juice. Higher flux at higher temperature is observed due to decrease in juice viscosity and increase in mass transfer coefficient according to the film model (Fane and Fell, 1987). According to
20°C100 kPa
Carbosep M7 Carbosep M8 Carbosep M9 100
Feed flow rate, Qf/mL/min 1 1000
10
Steady-state permeate flux, Js/L/(m2 h)
FIGURE 4.3 UF of apple juice. Effect of feed flow rate on steady-state permeate flux for all three membranes. (Reprinted from Vladisavljevic et al., J. Food Sci., 60, 241, 2003. With permission.)
Figure 4.5, an increase in temperature from 20°C to 55°C enhanced the permeation flux irrespective of the membrane molecular weight cut-off. It is in dissimilarity to what Jiraratananon and Chanachai (1996) observed in UF of passion fruit juice.
They observed the decrease in permeate flux by an increase in temperature from 40°C to 50°C due to the effect of increased bulk viscosity and gel-type layer forma-tion on the membrane surface caused by cross-linking between deposited entangled pectin and starch molecules, resulting in an increase in the resistance to flow.
Carbosep M7 Carbosep M8 Carbosep M9
20 25 30 35
Temperature (°C)
40 45 50 55
5 10 15 20 25 30
Steady-state permeate flux, Js/L/(m2 h) 200 kPa, 800 mL/min
FIGURE 4.5 UF of apple juice. Effect of temperature on steady-state permeate flux for all three membranes. (Reprinted from Vladisavljevic et al., J. Food Sci., 60, 241, 2003. With permission.)
30 25 20 15 10 5
00 0.2 0.4 0.6 0.8
TMP (bar)
1 1.2 1.4
Jp (I/h–1 m–2)
FIGURE 4.4 UF of carrot juice. Effect of the TMP on the permeate flux, operating condi-tions: Temperature = 23.5°C; Velocity = 0.14 m/s. (Reprinted from Cassano, A. et al., J. Food Eng., 57, 153, 2003. With permission.)