The What, the How, and the Why of Reverse Osmosis
While it may not be a phrase that comes up too often in general conversation or that is mentioned on TV except in specialised documentaries, reverse-osmosis (RO) technology has been playing a significant role in our lives for quite some time, even though many of us might be unaware of it. What follows is an explanation of precisely what this technology is, how it works, and why it has become so important to so many aspects of our modern society.
The mechanism on which this technology is based has been around since long before the first appearance of animal life on our planet. First, green plants utilised the catalytic action of chlorophyll and sunlight to convert water and atmospheric carbon dioxide into glucose, releasing the oxygen essential for animal life into the air. Some of the water was needed to maintain rigid stems using the process from which reverse-osmosis technology was later derived and which is known simply as osmosis. The process is possible in plants because they have cell walls which act as selective filters by allowing some substances to pass through them but not others. One application controls the movement of water between the interior of plant cells and the intercellular spaces. The direction of movement is determined by the concentration of solutes on each side of the cell wall, driving water from regions of high solute concentration to those of lower concentration until equilibrium is reached. In reverse osmosis, external pressure is applied to drive the process beyond the point of equilibrium, which then permits the complete separation of water from dissolved solids.
RO, therefore, can be thought of as a type of ultra-efficient filtration process in which the materials to be separated are of molecular size. The technique, as it happens, is unsuitable for the separation of large particles for which more conventional filtration methods are more effective and are often used prior to RO. This is done to prevent the microscopic pores of the semipermeable membranes used in reverse osmosis applications from becoming clogged, which would render them ineffective. Having gained some basic insight into the nature of RO and how it works, the next question that we must answer is what it is used for. The short answer is far more than you might think. Let’s take a closer look at a few of the more common and important domestic, commercial, and industrial applications for this comparatively simple but extremely valuable filtration technology.
For those who dislike the taste of chlorinated water, reverse-osmosis filter assemblies offer a means to remove it, along with any metal that may have been leached from pipework and stray bacteria. Attached to a domestic tap, the pressure is sufficient to force the water through the pores, leaving the undesirable molecules and particles behind. Typically, these devices employ pre-filtration through carbon blocks as a means to extend the working lifespan of the RO membrane. A more compact but essentially similar setup is used by soldiers in the field. Contaminated water is placed in the device and then pumped by hand to force it through the reverse-osmosis membrane and other filtration layers. Larger versions are used both by farmers and homeowners for further purification of borehole water to supplement the domestic supply of potable water. Also, a similar process is used in municipal treatment plants for the final “polishing” of the domestic water supply prior to chlorination.
The food-and-beverage industry is one of the biggest users of this technology for commercial purposes. In practice, reverse osmosis can also be used to harvest the solid phase rather than the clear filtrate. This makes it an ideal technique for the preparation of concentrates such as fruit cordials, sauces, and pureed foods. RO is also a means to reclaim and reuse costly chemicals and minerals that might otherwise go to waste.
However, if one were to nominate just one application of this technology as possibly the most important of all, the prize should almost certainly go to desalination. Gradually replacing the more energy-hungry process of evaporation and distillation, reverse osmosis requires markedly less power to produce the same yield. While still costlier than conventional water treatment, the future of this technology looks promising and may well be our best hope to overcome the growing global water crisis.