The Common Types of Water-Filtration System Explained
Water accounts for between two thirds and three quarters of our body mass and it is essential to most of the biochemical and physiological processes necessary for life. One of its most important properties is that it is an exceptionally good solvent and this accounts for its usefulness in enabling the body’s chemical reactions. That same property, however, applies water that is circulating in the environment, where it tends to pick up various contaminants from the various materials with which it comes into contact. As a result, for drinking and many other purposes, it requires cleansing, for which there are a number of water-filtration systems available.
Interestingly, nature has an excellent means for removing contaminants from water. Following the evaporation of surface water by the sun, it falls as rain and proceeds to percolate through the soil until it encounters fractures in the rock beneath where it forms underground stream known as aquifers. The combination of distillation and passage through the soil serves to clarify its contents. Humans employ a similar process in which contaminated liquid is dripped through successive layers of vegetation, sand, charcoal and gravel to remove the bulk of contaminants prior to further treatment. These so-called reed beds are one of the earliest forms of water-filtration system and are still in use today.
It is, in fact, humans who are primarily responsible for the contamination of this vital resource. In the home, we make use of it for cooking, to wash our linen and dirty dishes, to bathe or take showers, and to flush our toilets. To the accumulated grey and black wastewater from domestic sources is added the effluent produced by industry, which often contains the highly toxic chemical waste that occurs as a by-product of activities such as mining, construction, and manufacturing, while farming activities also introduce animal waste into the mix. With a world population in excess of 7 billion, obtaining sufficient clean water would now be quite impossible without effective filtration systems. The method outlined above is one that employs physical filtering. Such methods, however, are generally limited to removing only the larger particles present and so act much like the small gauze filters sometimes fitted to household taps. In order to remove smaller particles such as colloids, microorganisms, or solids in solution, a more stringent type of filtering process will be required.
One of the more effective water-filtration systems makes use of a process known as reverse osmosis (RO). If that sounds familiar, it is because the process is a modification of that used by plants to transport nutrients and retain their turgid stems. Rather than a cell wall, the semi-permeable membrane used in an RO system is man-made, initially from cellulose acetate but, more recently, from more efficient thin-film composites. Sometimes described as a molecular sieve, the pore size of the membrane determines the size of the particles it will retain. Like some other water-filtration systems, reverse osmosis requires the application of external pressure. In this case, it is required to reverse the natural tendency of water to move across a semi-permeable membrane from a region of high solute concentration to one of lower concertation, and thus ensuring only the water passes through. An RO system is capable of removing many harmful contaminants, including heavy metals.
While not as effective as RO, where it is only necessary to remove chemicals like chlorine or fluoride that may mar the taste of the domestic supply or to remove any rust particles that may occur in older installations, among the simplest water-filtration systems on the market are the activated-charcoal and carbon-block filters. Their properties vary according to their precise composition and some, for example, are certified to remove protozoan cysts that could cause diarrhoea. In some regions, the presence of calcium and magnesium salts in water leads to a condition termed water hardness. This results in deposits of limescale in kettles and immersion heaters, as well as inhibiting the formation of a lather by detergents. A simple remedy is the ion-exchange process in which calcium and magnesium ions are absorbed by a resin and exchanged for sodium ions.
These are some of the more common water-filtration systems in use today and all of them are available in South Africa from WaterIcon.