Water pollution is mainly caused by chemical substances, bacteria and other micro-organisms present in natural waters in greater numbers. Chemical substances, organic or inorganic (mineral) are in the form of solutions, colloidal solutions and dispersions. The chemical composition of impurities is shaped by natural factors, such as the breakdown of substances from soil and rocks, and the withering away of the development of aquatic organisms and anthropogenic factors. The most common anthropogenic pollution of surface water include pesticides, surfactants, petroleum hydrocarbons, phenols, chlorinated derivatives of biphenyl and heavy metals: lead (Pb), copper (Cu), chromium (Cr), cadmium (Cd), mercury (Hg) and zinc (Zn), and heated water (thermal pollution), which are particularly dangerous for surface water with little flow or stagnant water. Most of the anthropogenic pollution is toxic to aquatic organisms. Impurities very stable in aqueous media and is very difficult to undergoing chemical and biochemical processes in substances called refraction.
