Water is an essential substance for humans and all other living beings including plants. Water is a naturally available element in earth. It is available in the form of underground streams, lakes, ponds, above ground streams and sea water. Movement of water takes place on the earth through the cycle of evaporation (1), transpiration, precipitation, condensation and ultimately running off into the sea. A good amount of water is also found absorbed in various hydrated minerals. Water purification is an essential activity and water needs to be pure when it comes to consumption.
Sources of water are many. They are ground water, surface water, water stored in ponds and lakes, water running in canals and rivers, sea water and generation of water from the atmosphere.
Water, in its purest form is a colorless, tasteless and odorless liquid. But water sourced for various purposes including consumption and irrigation in the agricultural field do contain various mineral, salts and other impurities. So water needs to be purified according to the purpose for which it is used. Consumption of contaminated water is the cause of death of about half a million people worldwide.
Mostly, water used in industries for washing/cleaning purposes, are not subjected to purification process (2). But, when it comes to water for drinking or other household purposes, the purification process needs proper purification.
Of all the purposes, drinking water needs to be standardized by removing excess salt and all impurities. Contaminated water is the major cause for most of the health disorders encountered by humans and animals. Hence purification of water for drinking as well as preparation of food is essential.
Methods of purifying water
There are various methods available for purification of water based on the purpose. Both traditional methods and modern methods are used for such purification process.
Purification of water is carried out mainly for drinking and preparation of food. The process involves removal of undesirable chemicals, biological substances and suspended solids and gases dissolved in water.
Physical, biological and chemical processes are undertaken to purify water essentially based on the source. It is the source of water which will facilitate perception about its safety and purity for its consumption.
- Physical process of purification is by filtration, distillation (3) and sedimentation.
- Biological process is by using sand filters and carbon filters.
- Chemical process is by chlorination (4), flocculation and electromagnetic radiation.
During ancient times water was consumed by selecting the source for its safety. For example river water was used as such. As the river water originate from mountains, it was considered biologically purified and subsequently as it ran through the land natural sand filtering could have taken place, Humans then did not think of purifying water before consumption. However since the late 19th century various human habitats have noticed death of humans due to water contamination. Consequently scientific methods were invented.
Separation or sedimentation, filtration, use of chemicals and oxidation are broadly some of the methods adopted for water purification based on the source of water and the level of impurities present in it in addition to the purpose for which the purified water is to be used.
Consequent to the awareness about the contamination water and its adverse effects on human health, sand filters were used for removing impurities. Intervention of science in subsequent years enabled the usage of chemical processes to purify water.
Traditional water purification methods
Ancient water purification techniques were primarily based on traditional methods without involving any scientific analysis on the quality of water. Some of the methods adopted include,
- Winnowing: water polluted by wind borne impurities like leaves, stalks and coarse substances is passed throw winnowing (5) sieve. But this method is not helpful if the water is muddy.
- Impure water is filtered through ordinary cotton cloth which filters plant debris, mud and other particles
- Highly turbid water is purified by pouring the water into a clay vessel with appropriate pores and allowed to settle. While the particles settle down, water trickles through the pores which is collected and used.
- Another method is horizontal flow coarse media filter wherein impure water is passed through coarse gravel or crushed small stones by which suspended solids are filtered. Like horizontal flow up flow gravel filter, two stage filter and up flow or down flow filter are also used.
After sand filters, chlorination was the first chemical process used in purification. Chlorination of water has been in practice for more than a hundred years.
Scientific water purification methods
All sources of water are now contaminated at the source point or during the transition. Hence to ensure consumption of safe drinking water, various scientific methods for water purification were invented and are put to use from time to time.
- Coagulation and flocculation (6) is a conventional process in which certain chemicals are added to remove organic and inorganic impurities from water.
- Coagulation converts the impurities into semi solid state and flocculation is to segregate the semisolids by sedimentation.
- After sedimentation sludge is stored and then removed.
- Then water is filtered either by rapid sand filters or by slow sand filters depending upon the convenience and need.
- The next step is filtration, which is not a very simple process as it appear to be.
There are many methods and steps involved in purifying water through filtration. Filtration do not remove all the impurities including water borne viruses which may remain in water even after the process is over. In such cases heating treatment or use of chemicals are required to further purify. Even then there is no guarantee that the water will not get re contaminated.
The effectiveness of the filtration technique is based on the filter used. Not all types of filters produce the same level of purified water. In addition to this, the problem of cleaning the filters for reuse arises. Hence chlorination needs to be done after filtration to ensure pot ability of the water.
- Membrane filtration can be done where there are no dissolved substances. If there are dissolved substances ion exchange system is used to purify.
- Disinfection is carried out by chlorination of water or by chlorine dioxide or chloramine.
- Ozone disinfection and ultraviolet disinfection are also done to achieve purity.
- Some people also conduct solar disinfection which is just exposing water to direct sun rays.
- Softening water is also carried out by ion exchange system.
Chlorination of water
The most effective and cost effective method of water purification is by boiling water. When water is subjected to rigorous rolling and boiling it eliminates all bacterial contamination. But this system is not used for large scale requirement of water for want of heating source or sufficient place. Hence chlorination process is used by corporations to supply water to households.
Chlorination is the universally most popular and accepted form of water purification for human use including drinking purpose. Chlorine is an effective disinfectant. Either chlorine or hypochlorous acid is added to water for purification. This method has found to be used since ancient times.
- Chlorine effectively removes bacteria, viruses and cysts from water. Chlorine present in the water prevents further bacterial contamination and the purified water remains potable for relatively longer period of time.
- Chlorine is available in the form of liquids, granules and tablets in different brand names. Handling of tablets needs to be with utmost care as they are flammable and excess of the same may make water unsafe for consumption.
- NaDCC tablets (sodium dichloroisocynurate) are used as they dissolve in water slowly to release hypochlorous acid (7). This tablet in contact with water produce chlorine and so considered safer than directly using chlorine tablets. Another advantage with this tablet is that it produces chlorine without the normal taste or odor associated with chlorine.
- Iodine (8) tablets are also used in water purification. Its use is much older to usage of other chemicals. Its efficacy is good but not much used as it turns the water color into light yellow and also make the taste of water unpalatable. Hence use if iodine for purification of drinking water is not generally adopted.
Conditioning the water
One of the methods adopted for water purification is conditioning, in which the effects of hard water are reduced. When water with hardness of salts is subjected to heating with soda ash, excess salts are precipitated and potable water is obtained. The precipitate is high quality sodium bicarbonate which becomes one of the raw materials for tooth pastes.
Chlorine, bromine, iodine, hydrogen peroxide, organic acids that are not toxic and other chemicals are used for treating water.
In recent times drinking water purification is done by reverse osmosis system. This system removes minerals and salt but do not eliminate bacteria (9) and other chemicals. But about four times more of water is wasted in the process which may not be a prudent method given the fact that many areas are starved of water.
Purification of water for consumption is also done by using certain herbs or herbal products. One such herb is Moringa. When moringa seed is pressed to derive oil from it, the seed cake is obtained as byproduct. The seed cake is added to impure water. Dimeric cationic proteins present in the cake absorbs and neutralizes the colloidal suspensions in water and make it potable. The impure suspensions that collect in the bottom of the container is removed as sludge. This process does not involve use of any chemical for purification and the cake is also nontoxic.
Even though fluoride is added to water in limited quantity to enhance the quality of water, primarily to protect tooth decay, excessive fluoride presence in water can be toxic and result in staining the teeth. Hence in such cases activated alumina or bone char filter is used to purify water.
Since many bacterial infections affecting the humanity is water borne, one cannot be complacent in adopting the appropriate system of purifying water. While traditional method can be helpful to some extent, it is the modern techniques which thoroughly makes water potable and prevents health hazards.