Types of Water Filtration System and Why They Are Important
It has been centuries since one could rely on the rivers and lakes near our towns and cities to provide their citizens with water that is guaranteed safe to drink. Innumerable outbreaks of water-borne diseases have led to the deaths of hundreds of thousands as a direct result of the unsanitary conditions under which the average urban dweller once lived. Today, though we may no longer pump live sewage and industrial waste directly into rivers and streams, the damage has already been done. Consequently, effective water filtration systems have become a necessity and appear destined to remain so.
Although the problem of water pollution may have started to peak during the Dark Ages and continued to worsen until as recently as the late 19th century, it was not a new phenomenon. While Roman engineers constructed vast aqueducts to transport water from the rivers to the city’s fountains and public baths, and even directly into the homes of its wealthier citizens, they failed to prevent the exposed water from picking up dust and other contaminants en route. It was, however, a Greek who appears to have introduced the first known water filtration system when, around 500 BC, the physician Hippocrates used a simple cloth filter to cleanse the water in which he bathed his patients. This simple device later became known as the Hippocratic Sleeve and, while its inventor may not have been aware of the existence of microorganisms, he did believe that removing the visible contaminants from water was important to health.
Today water filtrations systems that clean the water in swimming pools, fish farms, and similar applications sometimes rely on the use of bag filters. Even though these may be made from more sophisticated synthetic materials, their basic operating principle parallels that of the far older Hippocratic Sleeve. As is often the case, nature had already perfected its own method for dealing with the contaminants present in water. It was not, however, until the third or fourth century AD that a reference to the use of sand as a medium for water filtration systems first appears in ancient Sanskrit and Egyptian texts. This is a method which, though forgotten during the Dark Ages, only to re-emerge in the late 19th century, has since been refined and is still used in municipal water-treatment plants today. Just as rainwater is cleansed of contaminants during its passage through the soil before joining underground aquifers, passing household wastewater through sand and gravel filters has much the same effect and has become an essential preliminary step in the commercial production of potable water.
Municipalities that operate water filtration systems may use woven nylon bags and sand filter media. These two methods are both physical in that they depend on the dimensions of the particles to be removed being larger than those of the pores or gaps in the filter medium. By contrast, the method known as ion exchange is a chemical process that is indifferent to particle size but depends, instead, on the interaction between the medium and the charges on the dissociated ions in solution. The medium used in this type of water filtration system consists of beads of resin that have been chemically treated so that they contain an excess of either positively or negatively charged ions.
Their exact composition varies according to the task required of them, which includes everything from the demineralisation of hard water to the removal of specific metals from industrial wastewater. In each case, the process involves the absorption of undesirable ions from the water in exchange for harmless ions of the same charge released by the resin. For example, calcium ions might be replaced by hydrogen ions and sulphate ions by hydroxyl ions. In this example of an ion-exchange water filtration system, the calcium sulphate responsible for hardness has been replaced by ions that, together, form more water.
Another filtration technology which has captured the headlines lately is reverse osmosis (RO). The heart of the process is a synthetic permeable membrane that acts much like the cell walls found in plants. The membrane, commonly a cellulose compound, allows the passage of water molecules under pressure but nothing larger. Widely used by the bottled-water industry, RO water filtration systems are, currently, the best hope for economical desalination.