MP Board · Class 12 · Biology · Microbes in Human WelfareDescribe the secondary treatment of sewage (biological treatment) in detail. Explain how activated sludge and anaerobic sludge digesters work in purifying wastewater.

Introduction to Secondary Treatment\nPrimary sewage treatment is a physical process involving the removal of floating and settled debris through sequential filtration and sedimentation. However, the primary effluent still contains large amounts of organic matter and dissolved pathogens, making it unfit for direct discharge into natural water bodies. Therefore, secondary treatment, also known as biological treatment, is essential to reduce the Biological Oxygen Demand (BOD) of the wastewater.

Aeration Tank and Activated Sludge Formation

• Transfer to Aeration Tanks: The primary effluent is continuously pumped into large aeration tanks where it is mechanically agitated and air is pumped through it.
• Growth of Aerobic Microbes: This vigorous aeration allows rapid growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
• Reduction of BOD: While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the Biochemical Oxygen Demand (BOD) of the sewage, which is a measure of the organic matter present in the water.

Sedimentation and Settling

• Once the BOD of wastewater is significantly reduced, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge.
• A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.

Anaerobic Sludge Digesters

• Processing the Remaining Sludge: The remaining major part of the activated sludge is pumped into large tanks called anaerobic sludge digesters.
• Action of Anaerobic Bacteria: Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi in the sludge.
• Biogas Production: During this digestion process, bacteria produce a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide. This gas is called biogas and can be used as a source of energy as it is inflammable.

Final Discharge\nThe effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams. This treated water is safe and does not cause pollution or depletion of dissolved oxygen in aquatic ecosystems.