Standard activated sludge

The principle of this treatment involves contact between a biomass composed of aerobic bacteria (i.e. developing in the presence of oxygen), nutrients (organic substances present in the water and constituting the pollution) and the oxygen contained in the air.

In the presence of oxygen, the bacteria feed on the organic substances and reproduce in the aeration basins. Non-aerated phases may also be forecast
for advanced treatment of nitrogen and phosphorous.

The bacteria are agglomerated while also incorpo-rating a proportion of the suspended solids and the soluble organic matter. The aggregates thus formed are known as “flocs”.

Coming out of the aeration basins, the purified water and the biomass are still mixed.

The separation of these elements takes place in a clarifier (or settler) where the flocs, which are greater in density than the water, will settle constituting the biological treatment sludge.

The settled biomass is recirculated at the top of the biological treatment unit, with the excess sludge being extracted and treated.