Ultrafiltration And Reverse Osmosis Background Information

Ultrafiltration (UF) and reverse osmosis (RO) are unit operations in which water or some solutes in a solution are selectively removed through a semipermeable membrane. Both processes are similar in that the driving force for the transport of water and solutes across the membrane is the pressure applied to the feed solution. However, the membrane and pressure used and the separation achieved are quite different. Membranes used in both UF and RO have a very thin skin layer or an asymmetric structure formed by phase-inversion processes. In RO, the thin skin layer consists of a very dense film; thus, virtually all suspended matter and most dissolved species are retained by the membrane, and only water passes through the membrane. The operating pressure is in the order of 700 kPa or higher to overcome the high osmotic pressure of the low molecular weight solutes. UF membranes have a thin skin layer that is microporous and, hence, they separate species based on molecular size. In addition to water, some dissolved low molecular weight species can pass through the membrane. The driving force for the separation is the pressure difference across the membrane, typically in the range of 70 to 700 kPa. Another important difference between UF and RO is the effect of the chemical nature of the membrane polymer. In UF, the chemical nature of the membrane polymer has only a small effect on the separation process since separation is based on a sieving mechanism. By contrast, RO separations are based on preferential sorption or solution/diffusion phenomena and are greatly influenced by the chemical nature of the polymer membrane.

The advantages of UF and RO, comparing to other separation or dewatering processes, have been summarized by Porter and Nelson (1). First, they do not require a phase change in solvent to effect the separation or dewatering processes. Second, UF and RO can be conducted at ambient or other selected temperatures. These can result in considerable savings in processing energy, elimination of thermal or oxidation degradation problems associated with evaporation and collapse of gels, breaking of emulsions, and mechanical damages associated with freezing. For UF processes, in addition, changes in ionic strength and pH that occur during other methods of separation or dewatering are also avoided because low molecular weight acids, bases, or salts are not retained and are freely permeable to the UF membranes.

The main limitations of UF and RO are (1) fouling of the membranes, which reduces the operating time between periods of cleaning; (2) the maximum concentration is limited to 30% total solids; (3) higher capital costs than evaporation; and (4) variation in the product flow rate when changes occur in the concentration of feed solution (2).

The history of UF and RO has been reviewed by Sou-rirajan (3), Matsuura and Sourirajan (4), and Cheryan (5), and the costs of their operations has been reviewed by Cheryan (5).

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