Water pollute unnatural changes caused by human activities.
People build dams and dams, creating an artificial reservoir for drinking water supplies, irrigation and mainly for hydroelectric power generation. So, in South America with this good purpose built reservoir Suriname. During the flooding of the lands lost about 10 thousand different animals - turtles, deer, monkeys, Trees and wood residues, submerged at the bottom of the reservoir, began to rot, resulting in dramatically lower the oxygen content in the water. This is caused by the almost complete obliteration of the lake surface algae, which settled a lot Moskito. They began to destroy the chemicals. Toxic substances dissolved in the water, and the consequences of this environmental disaster is still difficult to predict.
As a result of thoughtless reclamation people have straightened the course of many small rivers, turning them almost into the gutter.
In large and small rivers dumped a huge amount of industrial wastewater and liquid waste. Poison water large cattle-breeding complexes, primarily underground. In rivers and streams fall fertilizer and plant protection from adjacent fields.
In the second half of the XX century people began to worry about the cleanliness of the surface waters of the planet. For this they had designed and built a water treatment plant.
Especially effective biological treatment plants, in which microorganisms digest and decompose harmful organic substances. In the "biofilter" water as a human, much bubbles, formed a small frothy flakes from lifeless turbidity and bacteria.
If necessary, the water is cleaned by chemical reagents. Thanks to them precipitate harmful substances, and kill pathogens.
Most frequent treatment facility, which is managed consistently and mechanical (sedimentation and filtration), biochemical and chemical wastewater treatment. After all stages of purification, the water is returned to nature. Water treatment plant are already operating in many industrial areas and large cities. Of course, unlikely to achieve recovery of nature to its natural state, but all the same it is possible to prevent the worst.