Effects Of Operating Parameters

The performance of a UF system is quantified by permeate flux. There are four important operating parameters that affect the flux: pressure, temperature, feed concentration, and flow rate. The pressure-flux behavior in typical UF processes is pressure dependent at low pressures and pressure independent at high pressures (5); increasing pressure increases permeate flux when the pressure is low, but permeate flux becomes independent of pressure when the pressure is high. Both the resistance model and the osmotic pressure model describe correctly the observed pressure-flux behavior of the UF processes (13). In the resistance model, the increase in pressure increases the flux and compacts the boundary layer. The compaction of the boundary layer increases its hydraulic resistance and thus opposes further increases in flux. On the other hand, the osmotic model does not consider the hydraulic resistance of the boundary layer. The increase in pressure increases the concentration of solute at the membrane surface and, therefore, the osmotic pressure. The increase in the osmotic pressure opposes the increase in flux. The mathematical expressions for the resistance model and the osmotic pressure model are shown in the following equations (5,13). The resistance model:

The osmotic pressure model:

rm where J is the permeate flux, AP is the pressure difference, rm is the membrane resistance, rb is the resistance due to the boundary layer and gel-polarized layer, q> is the proportional coefficient between rb and AP, and An is the osmotic pressure.

Higher temperatures generally favors the increase in flux of the UF processes. This is because increasing temperature reduces the solution viscosity. Cheryan (5) suggested that it is best to operate UF at the highest possible temperature that is within the limits of the feed solution and the membrane. Increasing feed concentration increases the viscosity and density of the solution, but reduces the solute diffusivity; the net results are increased concentration polarization and increased thickness of the boundary layer. Observed experimental results, in general, show that the flux decreases exponentially with increasing feed concentration (5). On the other hand, increasing flow rate or turbulence tends to remove the accumulated solids near the membrane surface and reduce concentration polarization near the membrane surface and the thickness of the boundary layer. Therefore, the permeate flux increases.

In addition to permeate flux, the performance of a reverse osmosis (RO) system is also quantified by solute rejection. Similar to a UF system, pressure, temperature, feed concentration, and flow rate are four major influencing processing parameters. The effect of pressure on permeate flux and solute rejection is complex. Although permeate flux increases with pressure, the osmotic pressure at the membrane surface also increases because of increased solute concentration. The latter tends to reduce the increase of permeate flux. Because the solute flux is not driven by pressure, it is not affected by increasing pressure. Therefore, increasing pressure increases solute rejection. However, increased concentration at the membrane surface indirectly increases the solute flux, which tends to moderate the increase in solute rejection.

Temperature increases both permeate flux and solute flux. This is because increasing temperature reduces viscosity of the permeate and increases diffusivity and solubility of the solute in the membrane (13). Because the increase in solute flux with temperature is usually faster than the increase in permeate flux, solute rejection decreases with increasing temperature.

Increasing feed concentration reduces permeate flux by increasing osmotic pressure at the membrane surface because of increased solute concentration. Because solute flux is concentration driven, increased solute concentration or concentration polarization at the membrane surface increases solute flux. Both increased solute flux and reduced permeate flux cause rapid reduction in solute rejection.

The effect of flow rate on permeate flux and solute rejection is similar to that of pressure (13). Increasing flow rate reduces concentration polarization near the membrane surface and the thickness of the boundary layer. Thus, permeate flux increases, causing the osmotic pressure to increase; this tends to reduce or moderate the permeate flux. The initial reduction in the concentration polarization caused by increase in flow rate decreases solute flux and hence increases solute rejection. However, this is also reduced or moderated by the osmotic pressure increases since solute flux is concentration driven.


UF and RO are finding increased applications in the dairy products. Milk and skim milk are concentrated for the manufacture of soft and hard cheeses (14-16), dried whole milk and skim milk (17), ice cream (18), milk beverages (19), condensed milk (20), and yogurt (14,21,22). Proteins recovered from sweet and acid wheys are by-products from the manufacture of the various kinds of cheeses. Lactose can be separated from wheys for uses in alcohol or beer production (23).

Cereal and Oilseeds

For cereal and oilseeds, UF and RO are used for (1) recovery of proteins from ground coconut meat (24), cottonseed and soy flour (25-27), and rapeseed meal (28); (2) removal of undesirable components such as raffinose, stachyose, and phytic acid from soy milk (29); (3) treatment and processing of cottonseed and soy whey (30,31); (4) protein modification (32); and (5) degumming of edible oils (33).

Agricultural Raw Materials and By-products

For agricultural raw materials and by-products, notable applications for UF and RO are (1) recovery of meat proteins from animal blood and meat waste (1); (2) recovery of useful by-products from potato, sweet potato, and wheat starch processes (34-36); and (3) removal of glucose while concentrating egg white (37-39).


UF and RO have found many other applications. Examples include concentration fractionation, or purification of alfalfa juice (40), apple juice (41-44), beet juice (45,46), cane juice (47), citrus juices (48), maple sap (49), onion juice (50), orange juice (51), passion fruit juice (52), perilla an-thocyanins (53), strawberry juice (54), tomato juices (55), and vegetable juices (56). UF and RO are also finding many applications in biotechnology (5,57) and removal of toxic components from rapeseed meal extracts (58). The discussion is beyond the scope of this article, however.

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