MP Board · Class 12 · Biology · Microbes in Human WelfareDiscuss the significance of microbes in sewage treatment. Explain the primary and secondary treatment processes with reference to biochemical oxygen demand (BOD).

Introduction to Sewage Treatment and Microbes\nMunicipal wastewater, commonly known as sewage, contains large amounts of organic matter and pathogenic microbes. Untreated sewage cannot be discharged directly into natural water bodies because it leads to severe water pollution and depletion of dissolved oxygen. The treatment of sewage is carried out in wastewater treatment plants (WWTPs) using heterotrophic microbes naturally present in the sewage, categorized mainly into primary and secondary treatment stages.

Primary Treatment (Physical Removal)\nPrimary treatment is a physical process that involves the removal of large and small particles from sewage through sedimentation and filtration.

• Sequential Steps:
  1. Floating Debris Removal: Initial filtration using wire mesh screens removes floating debris, plastic bags, and large objects.
  2. Grit Removal: The filtered wastewater passes into grit chambers where sand, pebbles, and small grit settle down by gravity.
  3. Sedimentation: The wastewater is then sent to large settling tanks where suspended solids settle to form the primary sludge, while the supernatant forms the primary effluent. The primary effluent is subsequently taken for secondary treatment.

Secondary Treatment (Biological Treatment and BOD)\nSecondary treatment is a biological process that relies heavily on aerobic and anaerobic microbes to break down dissolved and suspended organic matter present in the primary effluent.

• Aeration Tank Process:

  • The primary effluent is passed into large aeration tanks and continuously agitated mechanically while air is pumped through it.
  • This promotes vigorous growth of aerobic microbes into masses of bacteria associated with fungal filaments to form mesh-like structures called flocs.
  • As these microbes grow, they consume a major part of the organic matter in the effluent, which leads to a significant reduction in the Biochemical Oxygen Demand (BOD).
  • BOD Definition: BOD refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. Higher BOD indicates higher organic pollution potential of wastewater.

• Settling in Secondary Clarifier:

  • When the BOD of sewage is reduced significantly, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.
  • A small portion of the 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.

• Anaerobic Sludge Digestion:

  • In anaerobic sludge digesters, anaerobic bacteria digest the bacteria and fungi in the activated sludge, producing a mixture of gases such as methane, hydrogen sulfide, and carbon dioxide, which constitutes biogas.
  • The treated effluent from the secondary treatment plant is finally released into natural water bodies like rivers and streams.

Conclusion\nMicrobial action during secondary treatment transforms hazardous organic pollutants into harmless end products, drastically lowering BOD and ensuring ecological safety before wastewater disposal.