Correct Option
The correct option isDecomposition of sludge to produce biogas
Explanation
Sewage treatment is generally divided into primary (physical), secondary (biological), and tertiary (chemical/advanced) stages. Biological treatment utilizes microorganisms to degrade organic matter. This process involves two distinct types of bacteria: aerobic (requiring oxygen) and anaerobic (functioning in the absence of oxygen).
Option Analysis
- Consumption of waste in the aeration tank is Incorrect: The consumption of major organic waste occurs in the aeration tank during secondary treatment. This process utilizes aerobic bacteria (forming flocs with fungal filaments) which require a constant supply of oxygen to reduce the Biochemical Oxygen Demand (BOD).
- Decomposition of sludge to produce biogas is Correct: Once the BOD is reduced, the bacterial flocs settle as "activated sludge." A major part of this sludge is pumped into large tanks called anaerobic sludge digesters. Here, anaerobic bacteria (which do not require oxygen) digest the bacteria and fungi present in the sludge. During this digestion, gases such as methane, hydrogen sulphide, and carbon dioxide are produced, collectively forming biogas.
- Disinfection of the final effluent is Incorrect: Disinfection of the final effluent is typically a tertiary step involving chemical agents like chlorine, or physical methods like UV radiation, to kill pathogenic organisms before discharge. It is not a function of anaerobic bacteria.
- Filtration of suspended grit is Incorrect: The filtration of suspended grit and soil is a physical process known as primary treatment (sequential filtration and sedimentation). It does not involve biological agents.
Key Takeaway
Anaerobic bacteria are specifically employed in sludge digesters to decompose activated sludge and produce biogas (a fuel source), whereas aerobic bacteria are used in aeration tanks to consume organic matter and reduce BOD.