People most susceptible to the harmful effects of microorganisms in water are the very young, very old and those individuals with compromised immune systems. In the last several years, microorganisms have contaminated city water supplies in several locations around the country. In Milwaukee in 1993, contamination of the municipal water supply with Cryptosporidium killed some 70 people and sickened more than 400,000. Disease-causing protozoa can sometimes be found in water in rural or developing areas, according to Wyatt. Proper well development and sanitary water storage and transport systems in those regions are the key to water safety.
Filtration is one way to remove some contaminants from water using a beam of ultraviolet light.
Ozone, a naturally occurring form of combined oxygen molecules that destroys certain microorganisms, has been used to treat water but is also expensive. Sometimes different systems are combined to treat water. Cities generally don't use filtration because it's expensive and can't be used to decontaminate large quantities of water quickly. Municipal water is regularly tested for a variety of pathogens and chemicals and must meet federal standards of the Environmental Protection Agency. Home owners are notified if drinking water becomes unsafe, and contamination sources are then located and treated or eliminated.