Removal of disease-causing organisms from water has come a long way since Dr. John Snow discovered in 1854 that cholera was transmitted through water. Snow outlined measures to contain the disease, which gave rise to the fundamental principles of epidemiology.
The goal of water disinfection is to remove or inactivate waterborne pathogens. After the water leaves the treatment facility and enters the distribution system, biological material can regrow. This is because some organic matter remains in treated water, providing a source of energy for pathogens to reactivate and multiply.
One way to prevent regrowth or contamination is to ensure that some of the chemical disinfectant remains dissolved in the water at a residual concentration when it leaves the treatment facility. Not all disinfectants are stable in treated water for a long enough time to have this effect; only chlorine, chlorine dioxide and monochloramine provide residual disinfection in the distribution system.
When a disinfectant is introduced into the water treatment process, its ability to destroy bacteria and viruses can be expressed as a “CT value.” The CT value depends on the disinfectant concentration (C) in mg/L and the time (T) in minutes that the disinfectant is in contact with the water. Studies have shown the most difficult pathogens to inactivate are Giardia cysts and viruses.
Unfortunately, these diseases still exist in many underdeveloped countries. The table below provides information on the more common waterborne diseases and causal organisms found in the water of poorer countries.
Learn more about bacteria, viruses and protozoa -- the three groups of pathogens targeted in water treatment.
Brief timeline of water disinfection research:
1774 Discovery of chlorine by Carl William Scheele. Sir Humphry Davy proposed and confirmed chlorine to be an element in 1810 and named it chlorine.
1814 Discovery of chlorine dioxide by Sir Humphrey Davy.
1854 Dr. Snow showed cholera was transmitted through water and developed initial principles of epidemiology.
1860s Slow sand filtration used to clean water from disease.
1870s Pasteur and Koch developed germ theory leading to innovations in microbiology.
1881 Koch demonstrated chlorine kills bacteria.
1890 First electrolytic chlorine generation plant built in West Germany.
1905 Continuous chlorination used for the first time in England, which prevented a typhoid epidemic.
1908 First regular use of disinfection of water in the United States at the Bubbley Creek Filtration Plant in Chicago. Also, Dr. Harriette Chick advanced her theory of disinfection* (see note below).
1910 First use of ozone for disinfection in France. Disinfection became an accepted water supply practice worldwide. Chlorination dominated but ozonation was widely used, especially in France, Germany, Canada and the Soviet Union.
1980s Chlorination practice challenged because of the byproducts chlorine creates when it bonds with organic material in water (total organic carbon). The chemical reaction with total organic carbon forms trace levels of trihalomethanes, haloacetic acids and other disinfection byproducts suspected as carcinogens. Chlorine is the standard disinfectant used in the United States because it is effective, inexpensive and familiar to most people working with public water systems.
Ozone and chlorine dioxide are two alternatives to chlorine used as a disinfectant in the United States today. These two products must be manufactured on site because ozone decomposes rapidly and chlorine dioxide is dangerous to store. Other methods used to disinfect water include ultraviolet (UV) light, iodine, bromine, bromine chloride and gamma radiation.
* Chick-Watson theory of disinfection
The principle theory of disinfection is the Chick-Watson theory developed by Dr. Harriette Chick and refined by H.E. Watson in 1908. The theory states that the rate of destruction of pathogens by a disinfectant is proportional to the number of pathogens and the concentration of the disinfectant. Although research continues on modeling disinfection, the Chick-Watson theory remains the standard.
