How Water Treatment Has Been Changed By Reverse Osmosis

How Reverse Osmosis Has Transformed Water Treatment

It is no exaggeration to say that water is the foundation of all life on earth. While some microorganisms may have the ability to thrive in the absence of oxygen and can even withstand dehydration for long periods, sooner or later, even these essential anaerobes will require water to resume their active lifecycle. For the creatures that live in earth’s oceans and rivers, there is no scarcity but, despite inhabiting a planet whose surface is 71% water, that which is suitable for human consumption is dwindling. Consequently, technologies such as reverse osmosis (RO) are seen as vital tools in the global effort to reverse this increasingly worrying trend.

South Africa, like much of Africa, has been experiencing unusual weather. To which extent these are natural cyclic variations or the consequence of global warming is still under debate. Whatever the cause, the effect has been an increase in the incidence of drought that often creates the need for water restrictions as a conservative measure. To boost the supply of potable water, reverse osmosis has been adapted to extract the salts from seawater and brackish sources. Currently, desalination plants are perhaps the best prospect we have of meeting the increasing demand for freshwater. Until relatively recently, however, most plants engaged in the processing of seawater have relied on evaporative methods. Unfortunately, these require sufficient heat to boil the water, so the associated high cost of energy makes this type of technology markedly more expensive than conventional water treatments. Although still costly, reverse osmosis is proving to be a significantly cheaper option than evaporative technology for extracting potable water from our oceans. So how does RO work?

Like electricity, flight, numerous modern medicines, and so many of mankind’s other inventions, RO has its origins in nature. In this case, one need look no further than the plant kingdom to discover the basis of this latest method of water purification. Unlike animal cells, plant cells are enclosed by a cell membrane and it is this that allows water molecules to pass in either direction from regions of low salt concentration to regions of high salt concentration until equilibrium is reached. While not strictly the reverse of this process, which botanists term osmosis, RO is a modification in which the movement of water molecules is forced, by the application of external pressure, to continue beyond the point of equilibrium. In this manner, the small water molecules and the larger molecules of the dissolved salts can be completely separated.

RO is, therefore, a type of filtration that operates at a molecular level. As such, there are a number of other useful applications of reverse osmosis in addition to its role in desalination. For a start, there are several situations in which it may be desirable or even essential to remove molecules other than salt from water. In the home, for example, many consumers now choose to attach filters to their incoming domestic supply to remove residual chlorine and any metallic contaminants present that they consider might spoil its taste. Typically these are compound filters that combine layers of activated charcoal with RO membranes and other filtration media. The effect is to produce a crystal-clear, odourless liquid free of all contaminants.

Reverse osmosis also plays a role in the pharmaceutical and processed-food industries. Not only can RO be used to produce water of exceptional purity for use in the manufacture of medicines and edible products, but it is also an efficient way to prepare concentrates. Rather than traditional reduction methods that involve heating the ingredients to boil off excess water, forcibly extracting the water by applying pressure across a semi-permeable RO membrane has the same effect while making it easier to control the final concentration and requiring far less energy. Many municipal water-treatment plants now also rely on reverse osmosis in the final stages of the purification process. Often referred to as polishing, this process removes stray particles that may have escaped earlier sedimentation and filtration stages. Followed by accurate dosing with chlorine gas or solid hypochlorite, the final product can be guaranteed safe for human consumption and will remain so right to the point of delivery. For world-class products and extensive experience of reverse osmosis and its possible applications, many South African industries, businesses, and individual consumers choose to deal only with WaterIcon.

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