In the natural environment, microorganisms degrade pollution.
A wastewater treatment plant reproduces this, but continuously and in an organized way in a succession of basins.
In an "activated sludge" plant, after the primary settling which removes some of the suspended solids, the secondary settling is often biological.
Biological treatment uses bacteria to eliminate pollution. In the aeration tank, bacteria live thanks to the presence of food (waste water) and oxygen (generated by a turbine or a booster)