Reverse Osmosis (RO)

Reverse Osmosis (RO) is a technique employing a membrane which is “semi permeable” that is, under the influence of pressure a larger proportion of water (the solvent) passes through the membrane than do the dissolved salts or organic molecules (the solutes).

Such semi permeable membranes are common in nature; the skin is a good example and this phenomenon, osmosis, explains why you get thirsty as you swim in the sea or “plump-up” in a long fresh water bath.

The natural process was first described by a French Scientist in 1748, who noted that water spontaneously diffused through a pig bladder membrane into alcohol. However, it was not until the 1950’s, as a result of research funded by the Office of Saline Water in the USA that practical reverse osmosis membranes capable of discriminating against small ions were developed.

The step that made reverse osmosis a truly practical proposition was the discovery of the anistropic membrane.

The Principles of Osmotic Flow

RO Membranes

Two materials make up the bulk of commercial RO membranes, cellulose acetate and an aromatic polyamide.

Reverse osmosis membranes do not have definable pores in the way that the films used in ultrafiltration do; there are only spaces between the fibres making up the film which can take up water because of the acetyl or similar groupings which form the surface. The dense layer of active surface is about 0.25 microns thick supported by a thicker porous layer.

The water in the spaces between the fibres has a short range order with each water molecule placed so that the oxygen atom occupies the vertex of a tetrahedron and the hydrogen bond connects each pair of molecules. This is an ice-like structure. In ice it is of course continuous, in liquid water at room temperature about half of the molecules are, at any one time, in such clusters.

When pressure is applied to the membrane, molecules on the high pressure side are incorporated into the ice-like structure replacing molecules which “melt” away on the other side.

Ions in water, surrounded as they are by water molecules aligned to shield the ionic charge, cannot be made to fit into the ice-like matrix.

Membranes are cast in thin sheets or extruded as hollow fibres 80 or 250 um in diameter.

The advantage of the hollow fibre configuration is that a very large surface area can be packed into a small space because the fibres, although of very small diameter, are technically thick walled and so self supporting.

Each of the two materials has advantages and disadvantages. Cellulose acetate has a higher flux and a smaller area of membrane is therefore required. It is also resistant to small concentrations of free chlorine and may therefore be kept free of bacteria and also produce a product with residual chlorine in it to prevent subsequent re-growth.

The polyamide membrane can be used at a higher temperature (35 Degrees C) than cellulose acetate (30 Degrees C), it cannot tolerate chlorine but is not attacked by bacteria whereas some bacteria which can occur in surface water in woodlands actually destroy cellulose acetate. Finally, polyamides can be used over a much wider pH range (4-11) than cellulose acetate (4-7.5).

The choice of membrane depends upon the nature of the input of water and it is essential to be able to use the most suitable one in any particular set of circumstances.

For brackish water pressures of 20 – 28 bars are required; for sea water 50 – 60 bars.

Since the reverse osmosis membrane is a plastic material rather than a sponge there is a continuous, if small, irreversible compression of the material under pressure, temperature and time. As a result the flux of product through the membrane gradually decreases and eventually becomes too low to be economic. There is no catastrophic failure, only a slow decline. The salt passage through the membrane is not significantly affected by this compaction.

Chemical changes can also take place in the membrane, for example, cellulose acetate can be hydrolysed to cellulose. This process is accelerated at high pH and this is one reason for the limitation of pH in cellulose acetate systems.

These changes are inevitable, their effect can be reduced by choosing the conditions of operation but a finite life of 3 – 5 years could be expected.

For further information or assistance please contact Reverse Osmosis Chemicals International on +44 (0) 161 877 2334 or e-mail info@rochemicals.com.