Solids can easily lead to blockages in plant components such as pipes, valves and fittings. In multi-stage water treatment plants the first stage is usually to remove solids by mechanical means for this reason. 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.

Industrial wastewater often contains dissolved inorganic substances (e.g. heavy metals) or organic materials which cannot be biodegraded. This is also true of a lot of landfill leachate and contaminated groundwater. The use of physical/ chemical processes is ideal in such cases.

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.