Natural waters are classified by several factors, the most common of them is a salt content: salt content of fresh water is up to 1 g/dm³, brackish water - 1-10 g/dm³, seawater - more than 10 g/dm³. Salt content of river and ground waters may vary from 50-200 up to 1500-2000 mg/dm³. The largest amount of dissolved impurities is contained in ocean and see waters, g/dm³: the Baltic Sea - 11, Caspian Sea - 13, Black Sea - 19, Atlantic Ocean - 36.

By the prevailing anion content waters are classified into hydrocarbonate, chloride, sulphate waters. Fresh waters usually belong to the hydrocarbonate class, as the hydrocarbonate content in such waters amounts to 60-70%.

Impurities in natural waters are divided by particle size into truly-dissolved ones (ion- or molecular-dispersed), distributed in water as separate ions and molecules; colloid-dispersed ones with particles size from 1 up to 100 nm; roughly-dispersed ones with particles size more than 100 nm (0,1 µm). This classification is conventional. Roughly-dispersed impurities of water also called as suspensions or suspended solids with particles size around several microns show the same properties as colloidal systems and they are often combined under the common name of microheterogeneous systems.

Colloidal impurities are the agglomerates consisting of the significant number of molecules with the boundary surface between solid phase and water. Due to the small size colloidal particles do not lose the ability to diffuse and have the large specific surface. For example, if to split the cube of substance with a volume of 1 cm3 into smaller cubes with 10 nm on edge, then the number of such cubes will amount to 1018 pieces with general surface area of 600 m2. The colloidal particles can not be extracted from water under the gravity force and are not retained by the common filtering material such as sand, filter paper and are detectable in the scattered light (Tyndall cone). Natural waters contain various derivatives of silicon acid and iron, organic substances - plant and animal bodies' decomposition products.

Roughly-dispersed impurities, so called suspended solids, have so large mass that actually are not able to diffuse. With the course of time the certain sedimentation equilibrium is established and impurities either precipitate or come to the surface (if the particle density is lower than water density).

Roughly-dispersed impurities in suspended state cause the water turbidity. The bigger particle size of the roughly-dispersed impurities, the faster sedimentation equilibrium is established and the easier these impurities are extracted from water during sedimentation or filtration. The sedimentation rate of sand and silt particles with size of 100 and 20 µm amounts to 7 and 0.4 mm/s at 10^o С in the still water.

Based on chemical composition impurities in natural water could be divided into two types: mineral and organic.

Mineral impurities of water include dissolved N2, O2, CO2 gases contained in the atmosphere, NH3, CH4, H2S derived from oxidation and biochemical processes, as well as gases introduced by waste water; various salts, acids, bases mostly in ionized form, i.e. in form of cations and anions constituted them.

Organic impurities of natural waters include humic substances, washed out from soils and peats, as well as organic substances of various types coming into the water along with agricultural run-off and other types of insufficiently treated waste waters.

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* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)

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