With respect to microorganisms, water pollution refers to the presence in water of microbes that originated from the intestinal tract of humans and other warm-blooded animals. Water pollution can also refer to the presence of compounds that promote the growth of the microbes. The remediation of polluted water—the removal of the potentially harmful microorganisms—or the reduction of their numbers to levels considered to be acceptable for whatever purpose the water is used, represents the purification of water.

Microorganisms that reside in the intestinal tract find their way into fresh and marine water when feces contaminate the water. Examples of bacteria that can pollute water in this way are Escherichia coli, Salmonella, Shigella, and Vibrio cholerae. Warm-blooded animals other than humans can also contribute protozoan parasites to the water via their feces. The two prominent examples of health relevance to humans are Cryptosporidium parvum and Giardia lamblia. The latter two species are becoming more prominent. They are also resistant to chlorine, the most popular purification chemical.

Normally, the intestinal bacteria do not survive long in the inhospitable world of the water. But, if they are ingested while still living, they can cause maladies, ranging from inconvenient intestinal upset to life-threatening infections. A prominent example of the latter is Escherichia coli O157:H7. Pollution of the water with this strain can cause severe intestinal damage, life long damage to organs such as the kidney and—especially in the young, elderly and those whose immune systems are compromised—death.

There are several common ways in which microorganisms can pollute water. Runoff from agricultural establishments, particularly where livestock is raised, is one route of contamination. Seasonal runoff can occur, especially in the springtime when rainfall is more pronounced. The feeding of birds (e.g., ducks) is now recognized as a contributing factor. For example, a large numbers of ducks that congregate can contribute large quantities of fecal material to localized ponds and lakes.

Once in the water, the growth of microorganisms can be exacerbated by environmental factors such as the water temperature, and by the chemical composition of the water. For example, runoff of fertilizers from suburban properties can infuse watercourses with nitrogen, potassium, and phosphorus. All these are desirable nutrients for bacterial growth.

Water purification seeks to convert the polluted water into water that is acceptable for drinking, for recreation, or for some other purpose. Techniques such as filtration and exposure to agents or chemicals that will kill the microorganisms in the water are common means of purification. The use of chlorination remains the most widely used purification option. Others approaches are the use of ultraviolet radiation, filters of extremely small pore size (such that even viruses are excluded), and the use of a chemical known as ozone. Depending on the situation and the intended use of the finished water, combinations of these techniques can be used.

Purification of drinking water aims to remove as many bacteria as possible, and to completely eliminate those bacteria of intestinal origin. Recreational waters need not be that pristine. But bacterial numbers need to be below whatever standard has been deemed permissible for the particular local.

Another microbiological aspect of water pollution that has become recognized only within the past several years has been the presence in water of agents used to treat bacteria in other environments. For example in the household a number of disinfectant compounds are routinely employed in the cleaning of household surfaces. In the hospital, the use of antibiotics to kill bacteria is an everyday occurrence. Such materials have been detected in water both before and after municipal wastewater treatment. The health effect of these compounds is not known at the present time. However, by analogy with other systems, the low concentration of such compounds might provide selective pressure for the development of resistant bacterial populations.

See also Chlorination; Waste water treatment; Water quality