While having its origins in biology, the term osmosis has gained entry into the general vocabulary as a metaphor implying that knowledge may sometimes be gained purely as the result of proximity to those who are more knowledgeable. In fact, this is not a bad analogy, as in nature, the process involves the movement of water between compartments, which in this case, are the cells of a living plant or animal, under the influence of some unseen force. In practice, not only is that force now thoroughly understood, but it has since been modified by developers to create a highly efficient separation technology known as reverse osmosis.
Just as it sounds, it is a technology that works in a manner that is the opposite of the natural process. In the latter process, water molecules are driven to migrate from a region of low solute concentration, through a semi-permeable cell membrane, to a region of higher solute concentration on the other side. The process continues until the point of equilibrium is reached. The force driving the movement of water between two compartments of differing molarity has been termed osmotic pressure.
When applying sufficient external force to counteract the natural internal pressure, this changes the direction of flow to form the basis of a man-made process with numerous industrial, commercial, and domestic applications. That process is reverse osmosis, often abbreviated to RO, and for those who may have never heard of it, it is a process that is helping to resolve the global shortage of potable water.
Although already a well-established option in many countries, desalination is a subject that has recently become more popular in South Africa. The importance of this technology as a means to render seawater potable was highlighted by the Cape Town drought situation, earlier in the year, that came perilously close to allowing the taps in the Mother City to run dry. While the conventional use of heat distillation tends to make production costs high, the more recent introduction of molecular filtration techniques utilising reverse osmosis membranes is proving to be a more cost-effective option.
Among the main benefits of RO technology is that, with the development of cellulose acetate and other polymer membranes, it has become both a cost-effective option and a highly-scalable one. Consequently, its cost-effectiveness seems likely to provide the way forward for the construction of large-scale desalination plants with the capacity to meet the demands of a major population centre, and it is already being used in many conventional municipal water treatment plants. In parallel, its scalability means there is never likely to be a shortage of small to medium-scale applications for this increasingly-popular, membrane-based, reverse osmosis technology.
In research laboratories and pharmaceutical plants where there is often a need for water of exceptionally high quality for the preparation of ultra-pure reagents or medications, RO will frequently be the preferred method with which to produce it. While, though not quite as obvious, exceptional purity is also an important requirement for water used in the manufacture of the semiconductors employed in innumerable electronic devices. Once again, it is water obtained by reverse osmosis that will normally be used to guarantee the necessary standard of purity.
As mentioned earlier, the key to RO lies in the application of sufficient force to counter the osmotic pressure that would otherwise inhibit it. It may require no more than a hand-pumped device with a suitable membrane filter to achieve this – a fact that has led the manufacturers to develop compact, portable units for use by military personnel in regions where the natural sources are frequently contaminated by chemicals, parasites, and other microbes. A couple of depressions of a lever creates sufficient pressure to force the dirty water through a reverse osmosis membrane to provide clear and clean potable liquid, free of harmful contaminants.
In addition to these and the many other academic, commercial, and industrial uses for this technology, it has also found a place in the homes of many of the more health-conscious citizens both in South Africa, and in many countries overseas. Locally, growing concerns about the ability of municipalities to ensure the purity of their product have led to increased sales of bottled water. In the home, for drinking and cooking purposes, many consumers are electing to install domestic reverse osmosis units for similar reasons. In such units, pressure from the incoming supply is sufficient to drive the process, forcing the water through multiple filters to a separate tap.