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The main sources of pollution and contamination of reservoirs is insufficiently cleared sewage of the industrial and municipal enterprises, large livestock complexes, production wastes when developing ore minerals; waters of mines, mines, processing and alloy of forest products; dumpings of water and railway transport; flax preprocessing waste, pesticides, etc. The polluting substances, getting to natural reservoirs, lead to high-quality changes of water which are generally shown in change of physical properties of water, in particular, emergence of unpleasant smells, smacks, etc.); in change of a chemical composition of water, in particular, emergence in it harmful substances, available floating substances on a water surface and their postponement at the bottom of reservoirs.

The biological method yields big results when cleaning household drains. It is applied as well when cleaning waste of the enterprises oil processing, pulp and paper industry, production of artificial fiber.

Needs for water are huge and annually increase. The annual consumption of water on the globe by all types of water supply makes 3300-3500 km thus of 70% of all water consumption is used in agriculture.

Methods of sewage treatment can be divided on mechanical, chemical, physical and chemical and biological when they are applied together, the method of cleaning and neutralization of sewage is called combined. Application of this or that method in each case is defined by nature of pollution and degree of harm of impurity.

Growth of the cities, rapid development of the industry, agriculture intensification, considerable expansion of the areas of the irrigated lands, improvement of cultural and community conditions and some other factors complicates problems of providing with water more and more.

Deficiency of fresh water already now becomes a world problem. More and more increasing requirements of the industry and agriculture for water cram all countries, a scientific world to look for various means for the solution of this problem.

Sewage of the second group dumps oil, petrochemical refineries, the enterprises of organic synthesis, coke-chemical, etc. Drains contain different oil products, ammonia, aldehydes, pitches, phenols and other harmful substances. Harmful effect of sewage of this group consists mainly in oxidizing processes owing to which the content in oxygen water decreases, the biochemical need for it increases, organoleptic indicators of water worsen.

Growth of the population, expansion old and emergence of the new cities considerably increased receipt of household drains in internal reservoirs. These drains became a source of pollution of the rivers and lakes pathogenic bacteria and helminths. In even bigger degree pollute reservoirs the washing synthetic means which are widely used in life. They find broad application also in the industry and agriculture. The chemicals containing in them, coming with sewage to the rivers and lakes, have considerable impact on the biological and physical mode of reservoirs. As a result ability of waters to saturation by oxygen decreases, activity of the bacteria mineralizing organic substances will be paralyzed.

In biofilters sewage is passed through a layer of the coarse-grained material covered with a thin bacterial film. Thanks to this film processes of biological oxidation intensively proceed. It serves as the operating beginning in biofilters.

Quite harmful pollutant of industrial waters is phenol. He contains in sewage of many petrochemical enterprises. Thus biological processes of reservoirs, process of their self-cleaning sharply decrease, water gets a specific smell of a carbolic acid.

Pollution of reservoirs pesticides and mineral fertilizers which get from fields together with streams of rain and thawed snow causes serious concern. As a result of researches, for example, it is proved that the insecticides containing in water in the form of suspensions are dissolved in oil products by which the rivers and lakes are polluted. This interaction leads to considerable weakening of oxidizing functions of water plants. Getting to reservoirs, pesticides collect in a plankton, a benthos, fish, and on a chain of food get to a human body, affecting negatively both separate bodies, and an organism in general.