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Clay, mud
Clay, mud
The smallest size of material (sediment). A clay particle
is smaller than 1 micron (a thousandth of a millimeter) across. Clay in a river bed is often called mud. It is the material that colors the water during times of high flow and flood. Mud is left behind after a flood, coating the floors and walls of flooded buildings.
Clay and mud are also the most fertile part of the material carried by a river, so that when they are deposited over a flood plain during a flood, they bring natural fertility to the land.
In the past people relied on
the fertile clay brought by floods. The Nile flood is the most famous example. Every year the river flooded the lower Nile River Valley in Egypt and brought both water and fertilizer. When the flood waters went down, the farmers grew their crops on the layer of clay deposited by it. For thousands of years this flood enabled farmers to grow enough food for all of Egypt to eat. However, since the building of the Aswan High Dam, the flood no longer occurs, and so the lower Nile no longer receives its annual supply of clay.
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Clean water
Water that is free from disease- carrying organisms, such as bacteria and viruses, harmful chemicals, and particles of dirt.
It requires enormous effort
to clean water to a standard that
is suitable for drinking. It is relatively easy to make water
look clean. Pieces of dirt can be removed by letting water stand still for a while so they settle out. Small particles are often made to clump together by adding a special chemical, often aluminum sulfate. After this the water is passed through a filter made from a bed of sand.
But although the water that comes through this process may look clean, it is not biologically or chemically clean.
Getting water chemically clean is very difficult because water is the world’s most common solvent. This means that water can dissolve a large number of
substances. Gases
dissolve in water
while it is in the
air, and fertilizers
and other chemicals
dissolve in it as it
passes through soils and along river channels.
As water stays in contact with the ground longer, more minerals will dissolve in it. That is why water pumped from underground often has a very high amount of dissolved minerals (hence the name mineral water).
To remove these minerals, the water is treated with chemicals that cause the dissolved materials to settle out (precipitate).
It takes much more complicated treatment to remove industrial pollutants such as metals and oil.
Many disease-carrying microorganisms live in water. Some of them can be killed by passing the water through a tank containing beneficial bacteria and by adding a disinfectant to the water, usually in the form of chlorine. (Compare with: Polluted water.)
(See also: Water supply and Well.)
 Clean water—An engineer (left) examines a sample of polluted water (above) before it is treated.