Various chemical disinfectants are used to kill dangerous microorganisms to eliminate waterborne disease in municipal water systems. The purification methods vary, according to the contaminants in the water source, but the process involves the use of chemicals. Drinking water, swimming pools, sewage and waste water are all treated chemically at some point in their use cycle.
Chlorine
The widest-used, best-known chemical in city water systems is chlorine. Chlorine eliminates algae, molds, slime and other bacteria to control the taste and odor of water. Unwanted chemical compounds, such as hydrogen sulfide and ammonia, are also destroyed. Chlorine is effective in controlling viruses and protozoa, including Salmonella, Shigella and Vibrio cholera. Pure chlorine is toxic, so governmental agencies jointly monitor the safety levels of chlorine.
Chlorine Dioxide
Because it is unstable in natural waters, chlorine dioxide reacts rapidly with various inorganic compounds, but oxidizes organic compounds more slowly. Primarily a disinfectant for surface water, chlorine dioxide is an effective biocide to eliminate odor and taste problems. As a disinfectant, it is a free radical, effective even at low concentrations, notes the U.S. Environmental Protection Agency.
Hydrogen Peroxide
Hydrogen peroxide takes longer to purify water than chlorine does and, if not used properly, can produce phytotoxin bacteria, capable of killing humans in a high dosage. Hydrogen peroxide also changes the pH levels of water, requiring additional chemicals to return the pH to normal.
Sodium Hypochlorite
Sodium hypochlorite is a dangerous and corrosive substance, unstable and inflammable, especially when exposed to acids, sunlight and chlorine gas. It is formed from the reaction of chlorine and sodium hydroxide. Used in agriculture, industrial and waste disposal industries, it must be monitored carefully because sodium hypochlorite can be a significant source of bromate, a chemical compound dangerous to health, according to The Chlorine Institute, Inc.
Ozone
Ozone treats water with oxidation, produced by electric discharge in oxygen. Used in the pharmaceutical industry, for treatment of drinking and ultra-pure water, and for surface disinfection, ozone is more effective against bacteria and viruses than chlorine. Although ozone is a good disinfectant, it does not afford overall protection, because of its short life span.
Dangers
In the event of accidental discharge of chlorine in concentrated amounts, there is a danger to fish and marine life in streams and rivers.
A byproduct of the chlorination process, called trihalomethane, THM, can cause liver, kidney or central nervous system problems. According to Health Canada, lab animals exposed to high levels of THM developed increased incidents of cancer. Recent studies showed people that drank chlorinated water for 35 years or more had increased risk of bladder and colon cancer.
However, the International Programme on Chemical Safety states, "risks to health from these byproducts at the level they occur in drinking water are very small with respect to the risk of inadequate disinfection."
Neither sodium hypochlorite nor chlorine will eliminate Cryptosporidium pathogens.
References
- Environmental Protection Agency: Chapter 4 Chlorine Dioxide
- The Chlorine Institute: Bromate in Sodium Hypochlorite Potable Water Treatment
- Wilkes University Center for Environmental Quality: Environmental Engineering and Earth Sciences: Ozonation in Water Treatment
- Health Canada: Drinking Water Chlorination: The Risks of Chlorination By-Products
- American Chemistry: Drinking Water Chlorination: A Review of Disinfection Practices and Issues: The Challenge of Disinfection Byproducts
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