NCERT · Class 12 · Biology · Microbes in Human WelfareExplain how Secondary Treatment (Biological Treatment) of sewage reduces the Biochemical Oxygen Demand (BOD) before wastewater is released into natural water bodies.

Primary treatment of sewage primarily involves physical removal of floating and suspended solids through filtration and sedimentation, leaving behind primary effluent which contains a high amount of dissolved organic matter and microbes. This effluent is subjected to Secondary Treatment, also known as biological treatment, to drastically reduce its Biochemical Oxygen Demand (BOD), which measures the amount of dissolved oxygen consumed by aerobic microorganisms to oxidize organic matter present in a water sample. \nThe primary effluent is pumped into large aeration tanks where it is constantly agitated mechanically and air is pumped through it. This creates an environment that promotes rapid, vigorous growth of aerobic microorganisms into masses of bacteria associated with fungal filaments to form mesh-like structures called flocs. As these microbes grow, they consume a major portion of the organic matter in the effluent as their food source. \nBecause the aerobic microbes consume the organic pollutants, the Biochemical Oxygen Demand (BOD) of the wastewater drops significantly. Once the BOD is reduced to a safe level, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment, forming what is known as activated sludge. A small portion of this activated sludge is pumped back into the aeration tank to serve as the inoculum, while the remaining major part is transferred into anaerobic sludge digesters. \nIn the anaerobic sludge digesters, specialized anaerobic bacteria digest the aerobic bacteria and fungi present in the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide, which constitutes biogas. After this comprehensive biological treatment, the effluent is safe to be released into natural water bodies like rivers and streams without causing oxygen depletion or aquatic pollution.