In the first steps of treatment the wastewater is cleaned mechanically. Larger pieces of debris and garbage are removed, and the flow velocity is reduced so sand, pebbles and heavier dirt particles can sink to the bottom, where they can be siphoned off. At the same time fats and oils collect on the water surface and can also be removed.
Next follows the biological treatment. In an aerated basin activated sludge consisting of microorganisms degrades harmful carbon and nitrogen compounds. The bacteria need oxygen to metabolize, which is why air is blown into the basin. Generally, the oxygen content in the aerated tank is controlled by oxygen sensors and held at about 2 mg/L, which is optimal for the microbial degradation processes. The water is then directed into a non-aerated tank. The lack of oxygen ensures that the microorganisms in the activated sludge are better able to break down even harmful phosphorus compounds. After that the activated sludge is allowed to settle and can be reused or treated further.  For the treatment of industrial wastewater, it might also be necessary to add certain chemicals to precipitate harmful substances so they can be extracted. There can be further filtration or disinfection steps but finally the treated wastewater is discharged from a secondary clarifier into the so-called receiving water (a river, lake, or the ocean) or is repurposed e. g. for use in irrigation. At this point the biochemical oxygen demand (BOD) in the water is a critical parameter. The BOD is the amount of oxygen needed by microorganisms to metabolize organic compounds in water. The BOD of the treated wastewater can influence the oxygen levels in the receiving body of water and must therefore closely be monitored.