The Evolution of Water-Filtration Systems
Although, sadly, not everyone has access to this basic human need, the importance of a clean water supply to ensure that it has no ill effects on the health of those who drink it is almost universally understood. Today, most people are aware of the harmful bacteria and parasites that thrive in stagnant ponds and of the toxic chemicals that can find their way into rivers as a result of industrial activities. It is, therefore, surprising that the first water-filtration systems can be traced back to around 500 BC – long before any knowledge of microbes or large-scale industrial activity.
At this time, it is believed that the famed Greek scientist, Hippocrates, reasoned that, by filtering water through a cloth bag, the characteristic taste and odour resulting from sediment accumulated during passage along an aqueduct, might be removed. His device did, indeed, produce cleaner and better-tasting drinking water and his primitive but effective water-filtration system eventually became known as the “Hippocratic Sleeve”. Despite this primitive but insightful step, nothing much changed apart from the type of fabric used as the filter medium and the mechanisms for supporting it for more than a millennium. In 1832, an outbreak of cholera, a disease encountered most often in tropical climates, swept through London killing hundreds. While current beliefs cited its cause as a miasma or bad smell in the air, a Dr John Snow believed it to be waterborne. His theory was later confirmed by microscopy and led, in time, to the development of sewers and water-filtration systems to clean up the evil-smelling Thames River, which had long served as an open sewer.
Once water purity became a government responsibility, money was made available to develop new filtration methods. Interestingly, one of the first to be adopted and which is still widely used today is filtration through a bed of sand. It is a process copied directly from nature in which rainwater percolates through successive layers of earth and rock to become largely free of an of contaminants by the time it flows into an underground aquifer. Sand filters are one of the primary water-filtration systems used in municipal treatment plants. Despite the use of sand filters, pre-filtration steps are often needed. Coarse screens may be required to remove large pieces of detritus such as branches and items of refuse. Next, a sedimentation stage is used to allow dense particulate matter to settle out under gravity before the liquid is passed through the sand bed. The use of chemical flocculants is sometimes needed to aggregate smaller particles before they can sediment out.
Bio-digestion and other methods are then used to remove any residual organic substances, at which point one of the latest water-filtration systems is now often applied as the penultimate stage of the municipal treatment process. Known as reverse osmosis or RO, it is a technique that mimics plant physiology to an extent, but which, instead, utilises synthetic membranes with microscopic pores as a means to filter out any residual bacteria, viruses, and unwanted chemical molecules in a process that is often referred to as “polishing”. That same technology also has several commercial applications. For example, of the various water-filtration systems available today, RO is the technology of choice for the bottled-water industry and vendors can even purchase equipment to prepare the purified water in-store for their customers to see. A preferred methodology when high levels of purity are required, RO is also used by food manufacturers, although to remove unwanted water in order to harvest the solid phase. It is used to produce concentrates such as sauces and cordials.
Another of the popular forms of water-filtration systems relies on a process known as ion exchange in which beads of resin form the filter bed and act to release acceptable ions into the filtrate in exchange for capturing and retaining unwanted ones. For example cations such as sodium (Na+) and calcium (Ca++) can be exchanged for hydrogen ions (H+), while anions like chlorides (Cl–) and sulphates (SO4=) are exchanged for hydroxyl ions (OH–), removing troublesome salts and forming a little extra water in the process. Depending on the type of resin employed, different anions and cations can be removed, making this a selective process with applications in many industries. For value, however, the first prize must go to RO, as the water-filtration system adopted by desalination plants intent on overcoming the potable-water shortage.