The Technologies Employed by Water-Treatment Plants
In keeping with the methodology employed by Mother Nature, the first projects that might reasonably be seen as attempts to create water-treatment plants relied on the principle of physical filtration. In this case, the act of passing the untreated source liquid through a bed of sand served to trap most of the solid contaminants present to produce a clear filtrate. While methods similar to this were also employed by the ancient Greeks and Romans, its use became largely forgotten during the Dark Ages. As a result, this principle was first applied on a commercial scale during the early years of the 19th century.
Despite its simplicity, the technique is surprisingly effective, so sand filtration has continued to play an important role as one of the preliminary processes commonly employed by the much later and more sophisticated municipal water-treatment plants of today. To make filtration more effective, particularly for removing tiny solid particles in colloidal suspension, the addition of a chemical flocculant to promote preliminary sedimentation in tanks often forms a part of the pretreatment process. In this case, separation of the solid and liquid phases depends upon the action of gravity so, like filtration, it is primarily a physical process but one which may sometimes require a little chemical assistance. As mentioned, these are pretreatments and it will be necessary to apply various additional techniques to remove dissolved substances and microorganisms to arrive at a final product of the standard typically expected from water-treatment plants. However, the number and types of treatment needed is also a variable and depends upon the intended use for the end-product and the degree of purity that this will necessitate.
One of the most cost-effective and efficient of these is reverse osmosis (RO). The technology is based on the manner in which water travels across the cell walls of plants to maintain the equilibrium between the concentration of intracellular and extracellular dissolved solids. For the purpose under discussion, a man-made, semi-permeable membrane made from a material such as cellulose acetate takes the place of the cell wall. Then, because the goal of water-treatment plants is to remove dissolved solids, external pressure is applied to the contaminated source. This is necessary to overcome the natural tendency of osmosis to cease at the point of equilibrium and, instead, to provide a filtrate free of all dissolved solids.
RO is a highly scalable technology and one that has found uses in a number of diverse scenarios. For example, on a small scale, it has been used to provide retail outlets with the means to produce bottled water on site, rather than purchasing it from wholesalers. At the other end of the scale, RO is rapidly becoming the technology of choice for those specialised water-treatment plants whose task it is to render seawater and brackish sources suitable for human consumption. The people of Cape Town will be especially grateful for the temporary desalination facility that proved so effective in helping to prevent the city’s taps from running dry, following the recent drought in the Western Cape.
Another technology used to separate dissolved solids from liquids is the chemical process known as ion exchange. It makes use of resins, typically in the form of beads that, for example, are able to release their hydrogen and hydroxyl ions in exchange for calcium and sulphate ions in a solution. Water-treatment plants of this type are able to remove most dissolved inorganic and organic compounds with the use of varying combinations of cationic and anionic resins. The technology is often to be found in laboratories where it is used to prepare demineralised water for use in preparing reagents.
While RO can be effective even for the removal of harmful bacteria, it is common practice to employ additional methods to ensure that the product of a municipal water-treatment plant remains free of any potential pathogens during distribution and when accessed by the consumer. Chlorination with gaseous chlorine or ozone treatments, together with exposure to ultra-violet radiation are all effective methods of sterilisation although, unlike chlorination, ozone and UV treatments leave no chemical residue and provide no guarantee that sterility will be maintained. Interestingly, many consumers dislike the tase of residual chlorine and employ RO filters to remove it.
The technologies employed in water-treatment plants are as varied as they are vital. Choose WaterIcon as your supplier and guide.