In its never-ending circuit of water that captures and transfers the set of dissolved or suspended substances, it is cleared from them. Many of the impurities in the water are natural and come back with the rain or groundwater. The same way and pass some of the pollutants associated with human activities. Smoke ash and industrial gases with the rain deposited on the ground, chemicals and sewage, soil with fertilizers into rivers with groundwater. Some of the waste followed by the artificially created waterways – drainage ditches and sewers. MSCO takes a slightly different approach. These substances are usually more toxic, but their relief is easier to control than those that are carried in the natural water cycle. World total water use economic and household needs of about 9% of the total river flow. Therefore, no direct water consumption of water resources is a shortage of fresh water in certain regions of the globe, and their qualitative exhaustion.
For recent decades an increasingly significant part of the cycle of fresh water began to make industrial and municipal effluents. For industrial and domestic needs consumed about 600 700kub.km water per year. Of this amount, forever consumes 130 150kub. km, and about 500kub. miles of waste, so-called sewage is dumped into rivers, lakes and seas.
Important place in the prevention of water resources from depletion of quality owned wastewater treatment facilities. Purification buildings are of different types depending on the primary method of sewage disposal. If mechanical methods of insoluble impurities are removed from wastewater through septic tanks and all sorts of traps.