Pretreatment: where, with a set of railings and rotary drum screens, eliminating bulky substances suspended (plastics, rags, leaves, etc...).  Sand, fats and oils that can cause blockages or damage to pipelines and electromechanical equipment and interfere in the proper operation of downstream processes. At this stage also are developed processes of homogenization and / or neutralization of the wastewater.

Primary treatment: stage which removes suspended solids, sand, grease and oils that have not been removed at the pretreatment (usually a primary clarifier). If the wastewater is high in colloids, dosages of coagulants and flocculants techniques are used to allow its settling.

Secondary treatment: main stage that defines a treatment system, where a biological reactor removes biodegradable organic matter not removed in previous treatments, through the development of micro-organisms, with appropriate conditions of oxygen and nutrients, use the organic matter like food for microorganisms to synthesize new microorganism. In the process of assimilation of organic matter are generated settleable flocs (sludge) that are separated from the treated water in a secondary clarifier. Some of this sludge is recirculated to the biological reactor and another part is vented to the sludge treatment system.
Aerobic biological processes are divided in suspension culture (microorganisms remain suspended in the wastewater) and fixed culture (microorganisms adhering to a support, creating a biofilm).

Tertiary treatment: additional stage at wastewater treatment process to reduce levels of non-biodegradable organic matter and inorganic salts (nitrogen and phosphorus) to the required discharge limits (usually with anoxic biological processes).
As tertiary treatment is also considered reuse treatments, through a filtration and disinfection treatment of treat water is obtained reclaimed water suitable for further use.