Water Pollution and Biological Purification

Water pollution may derive from several sources, including chemical pollutants from industry, runoff of chemicals used in agriculture, or debris from geological process, but the greatest source of pollution is organic waste. Although chemical pollutants may become diluted, they can also radically alter the ecosystem to allow the overproduction of certain forms of algae and bacteria that pollute the water with respect to its use by humans.

Once in the water, the growth of microorganisms can be exacerbated by environmental factors such as the water temperature and 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.

With specific 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 acceptable levels—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 common. 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. However, if they are ingested while still living, they can cause maladies, ranging from inconvenient intestinal upset to life-threatening infections. An 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.

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. Other 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 eliminate those bacteria of intestinal origin. Recreational waters need not be pristine. But bacterial numbers need to be below whatever standard has been deemed permissible for the particular locale.

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, 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, looking at similar situations, the low concentration of such compounds might propogate the development of resistant bacterial populations.

Natural wetlands also contribute to the purification of water. Wetlands can serve as a depositional sump and provide biological filtering. Normal percolation through soil layers also provides a significant source of water purification.

See also Aquifer; Artesian; Drainage basins and drainage patterns; Estuary; Hydrologic cycle