Non-biodegradable organic and inorganic substances can be removed by means of physical/chemical processes. Examples of this are water softening by ion exchange and the adsorption of chlorinated hydrocarbons on activated carbon.
Removal of undissolved substances (solids) is effected by mechanical processes. In mechanical processes, the solids are not altered either physically or chemically. Only separation of the solids from the liquid phase (water) takes place.
The aim of biological wastewater treatment is the elimination of organic, biodegradable materials. This elimination is carried out by microorganisms which use organic substances as a source of food. This biodegradation also causes a conversion of materials to take place.
Water to be treated usually contains several substances with different properties. Consequently, a single basic process is not sufficient to remove these substances. Water treatment plants are therefore generally built in several stages.