The use of chlorine (in various forms such as liquid/solid/gaseous forms) in a drinking water treatment system to prevent waterborne diseases is one of the greatest public health advances in history. In order to understand the popularity of using chlorine in a water treatment one must have some depth of knowledge about water treatment.
Water treatment consists of various physical operations/chemical processes in a definite sequence (the sequence depends upon the source of water such as ground water, surface water etc.) to remove existing contaminants or to reduce the amount of contaminants to such a level that the water becomes fit for its desired end-use. It includes aeration, removal of suspended as well as dissolved solids, hardness removal, removal of biodegradable or some chemical substances causing bad taste and odor, toxic metal removal, filtration, disinfection etc.
Disinfection is the end process of water treatment to make water safe from pathogenic bacteria which are responsible for waterborne diseases. The prime objective of the disinfection in drinking water treatment is to kill pathogenic microorganisms, unlike sterilization (example: boiling of water) which means complete destruction of all living organisms. Although the other unit processes/operations of drinking water treatment, such as coagulation, clarification, and filtration, also help in reducing the load of microbial pathogens, disinfection is considered as the final barrier to the entry of pathogens into the finished product water. Few microorganisms are tabulated here with their health effects and sources:
Organism | Major Disease | Primary Source |
Salmonella typhi | typhoid fever | human feces |
Salmonella paratyphi | paratyphoid fever | human feces |
Salmonella typhimurium | gastroenteritis | human/animal feces |
Other Salmonella sp. | gastroenteritis (salmonellosis) | human/animal feces |
Shigella | bacillary dysentery | human feces |
Vibrio cholerae | cholera | Human feces, coastal |
Enterovirulent E. coli | gastroenteritis | human feces |
Yersinia enterocolitica | gastroenteritis | human/animal feces |
Campylobacter jejuni | gastroenteritis | human/animal feces |
Legionella pneumophila | Legionnaires Disease, Pontiac fever | warm water |
Helicobacter pylori | peptic ulcers | saliva, human feces |
A good disinfectant must be toxic to microorganisms at concentrations below the toxic thresholds to humans. In addition it should have fast rate of killing bacteria, should be persistent enough to prevent regrowth of microorganisms in distribution system, easily available, easy to handle, safe and inexpensive etc. sodium hypochlorite, chloramines, chlorine dioxide, ozone, other halogens, gamma rays, UV radiation are some common disinfectants used for drinking water disinfection. Among all the disinfectants, chlorine is considered as the most easily available disinfectant, cheaper and easy to handle when compared with others.
Generally, bacteria present in the water and on surfaces are harmless. But they are at the base of a food-chain for other free-living organisms such as fungi, protozoa, worms etc. A drinking water distribution system provides habitat for microorganisms, which are sustained by organic and inorganic nutrients present on the pipe and in the conveyed water. A primary concern is therefore to prevent contamination from fecal material that might build up near pipes or contaminate surface or soil water.
There is one more advantage of using chlorine extensively over all other disinfectants is that it can be present in distribution system or storage as residual chlorine for a comparatively long duration. Hence, it enhances the safety measures by suppressing the survival as well as regrowth of pathogenic bacteria in a distribution system/storage tank.
However, recent research has also shown its dark side which includes the ineffectiveness of chlorine as disinfectant as well as health hazards while using for a very long time.
People generally assume that if chlorine is present in water, the water is free from pathogens. In actual it may not be true for every condition. Recent research has proved that bacteria may get resistant to chlorine when exposed for a long duration by developing or changing its cell behavior. This also led to think upon using another sources of disinfecting agent.
Another problem associated with chlorine is the health hazards to its consumers. Chlorine alone is not harmful. But when it combines with other chemical substances like NOM (natural organic matters), it results in formation of various chlorinated disinfection by-products (DBPs).
Chloroform, a well-known anaesthetic, was discovered in chlorinated water by Rook in 1974 and is a known carcinogen. This discovery and subsequent identification of other carcinogenic DBPs led to concerns about the potential risks to human health due to chlorination. A number of alternatives to chlorine as a disinfectant are being used in different parts of the world, but elevated risks associated with their by-products are less well established than for chlorine.
Various groups of DBPs have been identified in drinking waters and are now regulated in many countries. DBPs include trihalomethanes (THMs) like chloroform, bromoform, bromodichloromethane and chlorodibromomethane, haloacetic acids (HAAs) like monochloroacetic acid, dichloroacetic acid and trichloroacetic acid and haloacetonitriles (HANs) like dichloroacetonitriles and dibromoacetonitriles. Some inorganic compounds that are also regulated as DBPs include chlorite, chlorate, bromate, and cyanogen chloride. Among these THMs and HAAs are the predominant DBPs groups in drinking water.
Some epidemiological studies show that long exposure to these DBPs caused many diseases like cancer (brain, pancreas, rectum, breast, lung, liver, bladder, kidney etc.), miscarriages etc.
Toxicity assessments for these harmful chemicals are also related with the exposure routes of DBPs. Possible routes of exposure to DBPs can include ingestion of water and food cooked or washed in tap water, inhalation of indoor air and dermal contact with chlorinated water during water use activities like cleaning, washing, showering or bathing. Water-based activities like swimming are also another major route of DBP exposure.
The factors discussed above make us to rethink upon the frequent use of chlorine as a disinfectant. Risks associated with other disinfectants has not been documented so well as compared to chlorine till today. Further research has been still going on in this direction by several developed and developing countries. Therefore, this does not mean that other alternatives are less hazardous. The decision to use chlorine in our drinking water sometimes becomes difficult, especially when it contains two utmost criteria: safety of drinking water from pathogens and safety of public health from chlorine exposure. Therefore, some countries has started to make strict regulations for DBPs in order to minimize the health hazards from these chemical substances. We need to find out good alternatives for disinfectant/chlorine. On the other hand, regulations should be strictly reinforced in a water treatment system. The advanced research work should be encouraged to find out ways to overcome these problems.
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