The primary stage of the sewage treatment includes the removal of floating wastes. This process is done mechanically and does not involve any living organisms. Whereas the secondary stage is also known as the biological stage and uses the microorganisms in order to break down the organic matter and decrease the biochemical oxygen demand (BOD) level of the sewage.
BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water were oxidized by bacteria. If the BOD level is high, the water is said to be polluted. Thus, before the deposition of the sewage water into the water bodies, the BOD level has to be reduced. Biological treatment ensures that the deposition of the sewage water into the water body does not pollute the water body by reducing the BOD of the sewage water.