A public drinking water supply is defined as a system that provides piped water for human consumption to at least 15 service connections or regularly serves at least 25 individuals. Water supplied by a public water system is regulated by the U.S. Environmental Protection Agency (EPA) under the Federal Safe Drinking Water Act.
Public water suppliers most often use chlorine gas and liquid forms of this chemical to disinfect water. Other less common forms of water disinfection done by water utilities include the use of chloramines, ozone gas (ozonation), potassium permanganate, ozone/peroxide, and UV light treatment (U.S. EPA 1999).
Chlorination effectively destroys or inactivates most pathogenic organisms and helps control microbiological growth in the distribution system. Although, at normal dosage rates, chlorine may not kill all cysts such as Cryptosporidium. Particle filtration followed by disinfection (primarily chlorination) effectively manages pathogenic organisms in drinking water supplies. The practice of chemical disinfection of public drinking water started in 1909 and resulted in the elimination of typhoid fever deaths in the U.S. by 1950 (U.S. Center For Disease Control, Summary of Notifiable Diseases, 1997). There are numerous other diseases such as cholera, gastroenteritis, and hepatitis and diarrhea that are prevented/controlled by the removal of pathogenic bacteria and viruses (U.S. EPA 1999) from drinking water.
However, all chemical oxidants, including chlorine and chlorine-derived chemicals, are toxic. Therefore, the EPA has established a maximum residual disinfectant level (MRDL) for chlorine of 4 parts per million (ppm). The EPA also regulates the levels of other potentially toxic water disinfectants like chlorine dioxide and chloramines in drinking water (see EPA National Primary Drinking Water Standards at http://water.epa.gov/drink/contaminants). The EPA has established an MRDL for chloramines of 4 parts per million (ppm) as chlorine gas equivalent. Ozone, which is used as a primary disinfection and not secondary (residual) disinfectant, is not found in drinking water. Residual levels of manganese (Mn) that remain in the water following potassium permanganate disinfection are regulated by the National Secondary Drinking Water Standard (NSDWS), which recommends a maximum level of 0.05 mg/L of Mn in drinking water.
An additional downside of the chemical disinfection process occurs when strong chemical oxidants like chlorine combine with naturally occurring organic and inorganic chemicals, such a plant residues and bromide ions, to form a group of chemical compounds known as disinfection byproducts (DBPs). The EPA regulates four types of DBPs: trihalomethanes (TTHMs), haloacetic acids (HAA5s), chlorite and bromate. However, other DBPs have been identified in water, including aldehydes, carboxylic acids and chlorinated phenols (U.S. EPA 1999). Up to now, EPA tests determined TTHMs, HAA5s, and bromate pose a potential health concern since they were determined to have carcinogenic properties in laboratory animal studies. The EPA Stage 1 Disinfection Byproduct Rule established drinking water standards of 80, 60 and 10 parts per billion (ppb) as the maximum levels (MCLs), for these three chemicals, respectively. The chlorite DBP also poses a health risk (not associated with cancer); its MCL is 1 mg/L (see U.S. EPA Drinking Water Contaminants at the link above).
Chloramines are a family of disinfectants formed combining chlorine with ammonia. The three most common chloramines are monochloramine, dichloramine and trichloramine. Monochloramine is preferred for water residual disinfection because of its biocidal properties and minimal taste and odor. Secondary (residual) chloramine water disinfection tends to form much smaller concentrations of DBPs such as TTHMs, as compared with chlorine when reacted with DPB precursors found in water, such as organic matter and bromide. However, recent research identified trace amounts of a new DBP, n-nitrosodimethylamine, created by using chloramines. Research is being conducted to learn if this DBP is harmful to humans in the concentrations found in drinking water. Currently, scientific knowledge shows chloramines DBPs pose less health risk than chlorine DBPs or water that is not disinfected.
Disinfection of drinking water is a necessary precaution that must be taken to minimize exposure to water borne pathogens. The addition of low levels of chemical disinfectants is necessary in a modern society that demands water on tap. The initial (primary) disinfection to kill/inactivate pathogens in water followed by the secondary (addition of residual disinfectants) provide protection against possible pathogenic re-contamination of drinking water that often has to travel through pipes several miles over long periods time before it is consumed. Therefore, safe water will reach the consumer with residual levels of disinfectants and possibly disinfection byproducts, as described in the previous sections. Methods to reduce exposure (from drinking water) to these chemicals are listed in next paragraph.
However, TTHMs and HAA5s are considered volatile chemicals. For example, chloroform, a trihalomethane that can form during chlorination, has a boiling point about 40 percent lower than water. Therefore, heating and/or aerating water can transfer a portion of these chemicals into the air. Besides drinking water ingestion, inhalation exposure to volatile DBPs can occur while showering, as these chemicals efficiently move (partition) from water (droplets) into the air.
In general, the unpleasant taste of chlorine and residual levels of chloramines, TTHMs, and HAA5s may be lowered at the tap using a granular-activated carbon filter. Reverse osmosis units may also lower the levels of DBPs, including chlorite and bromate ions.
U.S. EPA Disinfection Byproducts Information: http://www.epa.gov/enviro/html/icr/dbp.html
U.S. EPA Drinking Water Contaminants: http://water.epa.gov/drink/contaminants/index.cfm
U.S. EPA. 1999. Alternative Disinfectants and Oxidants Guidance Manual. EPA 815-R-99.014: http://www.epa.gov/ogwdw/mdbp/alternative_disinfectants_guidance.pdf