Secondary treatment or Biological treatment given to the sewage water before it can be released into natural water bodies –
• The primary effluent is passed into large aeration tanks.
• In these tanks, the primary effluent is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs.
• These microbes, while growing, consume the major part of the organic matter in the effluent. This significantly reduces the BOD of the effluent.
• Once the BOD of sewage is reduced significantly, the effluent is then passed into a settling tank. In the settling tank, the bacterial flocs are allowed to sediment. This sediment is called activated sludge.
• The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. In these tanks, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi in the sludge.
• The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
Rate this question :