Water used in the agricultural, industrial and civil sectors often contains substances which will alter the ecosystem and hence must not be discharged directly into river flows. The most common polluting agents are the following.
Fecal pollutants: materials of fecal origin that reach water bodies through sewage discharges or introduction of zootechnical manure that hasn’t been adequately treated. If there is a high fecal type pollution, can be observed the presence of pathogen microorganisms in water that can cause diseases as cholera, typhus fever, viral hepatitis, etc.
Toxic inorganic substances: these are constituted by heavy metal ions that can poison or kill living organisms. Industries that employ these heavy metals during processing, must sanitize them to eliminate any heavy metal leftover before discharging water.
Inorganic harmful substances: there are substances constituted by phosphates and polyphosphates existing in detergents, fertilizers, compounds of nitrogen and phosphorus and in some industrial discharges. These substances cause eutrophication (see paragraph on eutrophication).
Unnatural organic substances: in this category are included weed killers, pesticides, insecticides, etc. These substances are convenient for agriculture but can pollute both water and soil. Among these substances are included also organic solvents used by industries, such as trichlorethylene, acetone, benzene, etc., which must be eliminated before water is discharged.
Free oils and emulsifiers: these are insoluble, low density substances which for this reason form superficial oily film layers that prevent oxygen dissolution in water. It’s not a rare phenomenon and it can cause real ecological disasters. Suspended solids: creating a mixture of various kinds of substances that make water murky and prevent solar light from passing through. When they deposit deep on the bottom of a water body they obstruct vegetation growth.
Heat, acids and strong bases: originating especially from industrial discharges. They reduce the solubility of oxygen, modify the temperature and pH of the environment causing pathological alterations or the disappearance of living organisms or on the contrary the comparison of others.