CBSE · Class 12 · Biology · Microbes in Human WelfareExplain the biological treatment of sewage wastewater in detail. Describe the primary and secondary treatment stages, highlighting the role of aerobic and anaerobic microbes in the process.

Introduction to Sewage Treatment\nLarge quantities of waste water are generated everyday in cities and towns, known as municipal waste water or sewage. It contains major amounts of organic matter and pathogenic microbes. Untreated sewage cannot be discharged directly into natural water bodies like rivers and streams as it would cause severe water pollution and depletion of dissolved oxygen. Therefore, sewage is treated in Sewage Treatment Plants (STPs) before disposal, primarily through heterotrophic microbes naturally present in sewage.

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

1. Sequential Filtration: Floating debris is removed by sequential filtration using wire mesh screens.
2. Sedimentation: The grit (soil and small pebbles) is removed by settling in sedimentation tanks. The solid matter that settles forms the primary sludge, and the supernatant forms the effluent.

Secondary Treatment (Biological Treatment)\nSecondary treatment is a biological process where primary effluent is passed into large aeration tanks.

1. Aerobic Digestion: The effluent is constantly agitated mechanically and air is pumped into it. This allows rapid growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
2. Consumption of Organic Matter: While growing, these microbes consume the major part of the organic matter in the effluent, which significantly reduces the Biochemical Oxygen Demand (BOD) of the effluent.
3. Settling of Flocs: Once the BOD 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.
4. Anaerobic Sludge Digesters: A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi in the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide (biogas).

Effluent Disposal\nThe effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams, which is now safe and environmentally sound.