The principle of the treatment consists in putting in contact a biomass made of aerobic bacteriae (i.e. developing in the presence of oxygen), nutritive substances (organic substances that are present in water and accounting for pollution) and air.

In the presence of air, the bacteriae nourish themselves from the organic substances and they multiply in aeration tanks in which wastewater is sent and from the air that is meant to oxygenize bacteriae.

Bacteriae agglomerate also incorporating a part of the suspended matter. Soluble organic matter is thus transformed into flakes, big particles that can be decanted more easily.

When leaving aeration tanks, the purified water and the biomass that was formed are not separated.

The separation of these elements is performed in a clarifier (or settling tank) where the biomass, which density is higher as water’s density, will settle at the bottom of the works.

The decanted biomass is sent back to the beginning of the biological treatment. The sludge that is produced in excess is extracted to sludge management. Purified water is sent back to the natural environment.