Water pollution

There are several classes of common water pollutants.

Disease-causing agents (pathogens)
These include bacteria, viruses, protozoa, and parasitic worms that enter water from domestic sewage and untreated human and animal wastes. Every day about 14,000 people, half of them children, die due to this type of water pollution worldwide.

Dangerous diseases caused by bacteria, viruses, protozoa and parasitic worms.

Oxygen-demanding waste
This term refers to organic waste matter requiring aerobic decomposition by bacteria. Large populations of bacteria supported by the presence of these wastes degrade water quality by depleting it of dissolved oxygen. This process can cause the death of fish and other forms of oxygen-consuming aquatic life.

Water-soluble inorganic chemicals
These are acids, salts and compounds of toxic metals such as mercury and lead. High levels of these chemicals can make water unfit to drink, harm fish and other aquatic life, depress crop yields and accelerate the corrosion of machinery that uses water.

Industry is the main source of water-soluble inorganic chemicals.

Inorganic plant nutrients
These are water-soluble nitrates and phosphates that can cause excessive growth of algae and other aquatic plants. As these plants decay they deplete the water of dissolved oxygen, which fish need to survive.

People who drink water with excessive levels of nitrates suffer a reduction in the oxygen-carrying capacity of their blood. Agriculture is the main source of such pollution.

Organic chemicals
Organic chemicals include oil, gasoline, plastics, pesticides, cleaning solvents, detergents and many other chemicals. They threaten human health and harm fish and other aquatic life.

The main sources of such water pollution are transport, industry, urban activities and household cleaning.

Sediment (suspended matter)
Insoluble particles of soil and other solids become suspended in water, mostly when soil is eroded from the land. By weight, this is by far the biggest water pollutant. Sediment clouds water, inhibits photosynthesis and destroys the aquatic food chain.

Water can be the subject of radioactive pollution (caused by water-soluble, radioactive isotopes), thermal pollution (after using water to cool down industrial and power plants) or genetic pollution (caused by accidental introduction of non-native species such as mussels and phytoplankton).