Sewage belongs to the first group soda, sulphatic, nitrogen- plants, concentrating factories of lead, zinc, nickel ores, etc. which contain acids, alkalis, ions of heavy metals, etc. Sewage of this group generally changes physical properties of water.
In the rivers and other reservoirs there is a natural process of self-cleaning of water. However it proceeds slowly. While industrial household dumpings were small, the rivers coped with them. In our industrial century in connection with sharp increase in waste reservoirs do not cope with so considerable pollution any more. There was a need to neutralize, clear sewage and to utilize them.
A lot of waters consume chemical and pulp and paper industry, ferrous and nonferrous metallurgy. Development of power also leads to sharp increase in need for water. The considerable amount of water is spent for requirements of branch of animal husbandry, and also for household requirements of the population. The most part of water after its use for economic domestic needs comes back to the rivers in the form of sewage.
The sewage containing vegetable fibers, animal and vegetable fats, fecal weight, the remains of fruits and vegetables, waste tanning and pulp and paper industry, sugar and breweries, the enterprises of the meat-and-milk, canning and confectionery industry is the reason of organic pollution of reservoirs.