The Expanding Role of Reverse Osmosis (RO) Systems
It’s unlikely that humans would have ever aspired to fly if they had not observed the ability of birds and insects to do so. As has been the case with so many of mankind’s scientific advances, nature invented it first. This is equally true of reverse osmosis (RO) systems that, in recent years, have grown to become an invaluable asset for many of our important industries. A form of filtration, the phenomenon on which this technology is based has its origin in the cells of plants. Unlike animal cells, they possess a cell wall that acts as a semi-permeable membrane, allowing water to pass through in either direction – but not dissolved solids. Where the internal concentration of salts is higher than that outside the cell, water is drawn in and vice versa. Transfer continues until the solute concentrations on either side of the cell wall are equal.
In reverse osmosis systems, external pressure is applied to force this osmotic activity to continue beyond the point of equilibrium. By applying that pressure, the process can be made to act as an ultra-fine filtration mechanism that can separate molecular-sized solutes from their solvents. RO has, therefore, become an important tool for use in a growing range of applications where the need is for water of exceptional purity. The pharmaceutical industry, for instance, relies on water purified by reverse osmosis systems, as do the producers of processed foods and beverages. Interestingly, the latter industry sometimes applies RO in a different manner. Instead of harvesting the pure water, the manufactures of the fruit and other concentrates often used to prepare juices, harvest the concentrated solute. Metallurgical and chemical companies may also utilise this variation of technique as a means to collect valuable trace metals or to recover costly reagents for reuse.
Boilers are susceptible to damage unless fed pure water and so reverse osmosis systems have become invaluable to many heavy industries, including the nation’s power stations, where the exceptional purity is able to improve boiler performance by as much as 10%, while enabling longer periods of continuous operation between servicing. It has been estimated that, globally, the power-generation industry now accounts for 30% of all RO use – and its use is increasing. Even more important than electricity, water is vital for life as well as for productivity and many water-treatment plants now employ reverse osmosis systems in their efforts to maintain the supply of potable water to households and businesses. However, the combination of a steadily increasing demand by consumers and a semi-arid climate, further exacerbated by global warming, has led to a growing need for an alternative source. The result has been a marked increase in the number of desalination plants, many of which now employ RO.
While desalination is still not the most economical way to produce pure water, the introduction of reverse osmosis systems as an alternative to the former use of distillation has significantly reduced the operating costs of these plants. As the technology continues to advance, those costs could fall even further. Unlike distillation, which requires a lot of power during the evaporation process, RO requires relatively little electricity to drive the pumps that create the pressure. This also holds true for the many other applications of this technology. However, the use of reverse osmosis is not restricted to industrial systems. The technology is scalable and, in the home, for example, the pressure of a domestic tap in combination with a suitable RO membrane filter provide an effective means to remove that slight taste of chlorine that some consumers find unpleasant. In practice, you don’t even need a tap to create the pressure. A portable device, widely used by the military, makes use of a similar membrane filter but relies, instead, on pressure applied by hand to force contaminated water through it in order to produce water that is safe for drinking, irrigating wounds, and similar purposes.
In terms of water purity, only ion-exchange media provide comparable performance. However, unlike reverse osmosis systems, anionic and cationic resin beds require regeneration once their exchange capacity has been reached. This, unfortunately, involves interrupting the purification process, as well as the added cost of the acidic and caustic solutions required to regenerate them. By contrast, RO costs are unaffected by the level of dissolved solids, being based purely on flow rate. Consult WaterIcon for more about this remarkable technology.