What is water pollution?

High levels of mercury have been found in fish far from industrial areas. the main sources of the mercury appear to be emissions to the atmosphere from coal-fired boilers or municipal incinerators.

Some industries pollute water in yet another way. they use large quantities of water to cool certain equipment. Heat from the equipment makes the water hot. The industries then discharge the hot water into rivers and lakes, heating those bodies of water. Such heating that harms plants or animals is known as thermal pollution.

* Sewage consists of human wastes, garbage and water that has been used for washing or bathing. Most of the sewage goes through treatment plants that remove solids and such dissolved substances as the nutrients nitrogen and phosphorous.

* Agricultural chemicals and wastes. Water from rain or melted snow flows from farmland into streams, carrying chemical fertilisers and pesticides that farmers have used on the land. Animal wastes also can cause water pollution, particularly from feed lots with many animals. Cattle, sheep and poultry raised on feed lots do not distribute their wastes over widespread pastureland. Instead, much of their wastes runs off into nearby streams. Water used for irrigation also may be polluted by salt, agricultural pesticides and toxic chemicals on the soil surface before it flows back into the ground.

Effects of water pollution

* Health. Water polluted with human and animal wastes can spread typhoid fever, cholera, dysentery and other diseases. Though the water supplies are disinfected with chlorine to kill disease-causing germs, it does not remove harmful chemical compounds, such as polychlorinated biphenyls (PCB's) and chloroform, or harmful metals, such as arsenic, lead and mercury. The careless release of such toxic wastes, primarily into waste dumps, threatens ground water supplies. Scientists are concerned that drinking even small quantities of these substances over many years may have harmful effects.

* Reduced recreational use. Pollution prevents people from enjoying some bodies of water for recreation. For example, odours and floating debris make boating and swimming unpleasant, and the risk of disease makes polluted water unsafe. Oil spilled from ships or offshore wells may float to shore. It can kill water birds, shellfish, and other wildlife. Water pollution also affects commercial and sport fishing. Fish can be killed by oil or by a lack of oxygen in the water, or they may die because of a reduction in the quantity of their food supply. Industrial wastes, particularly PCB's, also harm fish.

* Disruption of natural process. Various natural processes that occur in water turn wastes into useful or harmless substances. These processes use oxygen that is dissolved in the water. Water pollution upsets these processes, mainly by robbing the water of oxygen.

Mineralisation is a natural process by which aerobic (oxygen-using) bacteria break down organic wastes into simpler substances. Some of these substances, such as phosphates and nitrates, are nutrients for plants. Normal quantities of these nutrients help support normal quantities of life in the water. When there are too many nutrients, however, a body of water may suffer from a process called eutrophication. The added nutrients may come from fertilizers draining off farmland or from detergents and other substances in sewage. An excess of nutrients causes the growth of higher-than-normal numbers of plants, such as pondweeds and duckweeds, plantlike organisms called algae, fish and other animals, and bacteria. As more grow, more also die and decay. Because the decay process uses oxygen, the additional decay uses up more of the oxygen in the water. Thus, less oxygen becomes available to support living things in the water.

Thermal pollution can also reduce the amount of oxygen dissolved in water. In addition, the warmer-than-normal water can kill some kinds of plants and fish.

Control of water pollution

* Sewage treatment. The most efficient sewage treatment plants use three processes- primary, secondary and tertiary treatment. Primary and secondary treatment can remove up to 95 percent of the waste in sewage. Tertiary treatment removes even more impurities. Many plants use primary and secondary processes, and some use tertiary processes as well. However, most treated sewage still contains nutrients and toxic chemicals because secondary processes cannot remove them all.

* Pretreatment of wastes. Industries can reduce pollution by treating wastes to remove harmful chemicals before dumping the wastes into water. Industrial wastes can also be reduced by using manufacturing processes that recover and reuse polluting chemicals.