Reverse osmosis is a process to separate solute and solvent

components in the solution.

Although the solvent is usually water, it is not necessarily

restricted to water.

The pore radius of the membrane is less than 1 nm.

While solvent water molecules, whose radius is about one tenth of

1 nm, can pass through the membrane freely, electrolyte solutes, such

as sodium chloride and organic solutes that contain more than one

hydrophilic functional group in the molecule (sucrose, for example),

can not pass though the membrane. These solutes are either rejected

from the membrane surface, or they are more strongly attracted to the

solvent water phase to the membrane surface.

The preferential sorption of water molecules at the solvent-

membrane interface, which is caused by the interaction force working

between the membrane-solvent-solute, is therefore responsible for the

separation.

Polymeric materials such as cellulose acetate and aromatic

polyamide are typically used for the preparation of reverse osmosis

membranes.

When a membrane is placed between pure water and an aqueous

sodium chloride solution, water flows from the chamber filled with

pure water to that filled with the sodium chloride solution, whereas

sodium chloride does not flow (Figure 1a). As water flows into the

sodium chloride solution chamber, the water level of the solution

increases until the flow of pure water stops (Figure 1b) at the steady

state.

The difference between the water level of the sodium chloride solution

and that of pure water at the steady state, when converted to hydrostatic

pressure, is called osmotic pressure.

When a pressure higher than the osmotic pressure is applied to the

sodium chloride solution, the flow of pure water is reversed: the flow

from the sodium chloride solution to the pure water begins to occur.

There is no flow of sodium chloride through the membrane. As a result,

pure water can be obtained from the sodium chloride solution. The above

separation process is called reverse osmosis.

The most successful application of the reverse osmosis process is in

the production of drinking water from seawater.

This process is known as seawater desalination and is currently

producing millions of gallons of potable water daily in the Middle East.

Fishing boats, ocean liners, and submarines also carry reverse

osmosis units to obtain potable water from the sea.

Ultrafiltration is a process based on the same principle as that of

reverse osmosis. The main difference between reverse osmosis and

ultrafiltration is that ultrafiltration membranes have larger pore sizes

than reverse osmosis membranes, ranging from 1 to 100 nm.

Ultrafiltration membranes are used for the separation and

concentration of macromolecules and colloidal particles.

Osmotic pressures of macromolecules are much smaller than those

of small solute molecules, and therefore operating pressures applied

in the ultrafiltration process are usually much lower than those

applied in the reverse osmosis process.

Membranes having pore sizes between those for reverse osmosis

and ultrafiltration membranes are sometimes called nanofiltration

membranes. The size of the solute molecules that are separated from

water, and the range of operating pressures, are also between those

for reverse osmosis and ultrafiltration.

Ultrafiltration membranes are prepared from polymeric materials

such as polysulfone, polyethersulfone, polyacrylonitrile, and

cellulosic polymers.

Inorganic materials such as alumina can also be used for

ultrafiltration membranes.

Typical applications of ultrafiltration processes are the treatment of

electroplating rinse water, the treatment of cheese whey, and the

treatment of waste water from the pulp and paper industry.

The pore sizes of microfiltration membranes are even larger than

those of ultrafiltration membranes and range from 0.1 µm (100 nm) to

several µm. The sizes of the particles separated by microfiltration

membranes are therefore even larger than those separated by

ultrafiltration membranes.