The wastewater (after primary treatment) is pumped into the aeration basin, which is filled with an activated sludge/water mixture. Activated sludge are flocks of bacteria, also called biomass. In the presence of oxygen, the pollution in the wastewater will be decomposed by these bacteria. The aeration system is designed to supply optimal conditions in order to decompose the main pollutants present in the wastewater. In general the pollutants are organic compounds, comprising of carbon, hydrogen and nitrogen.