The correct option is (b).
Explanation
Wastewater treatment involves physical, chemical, and biological processes to remove contaminants. The biological treatment of sewage sludge is a critical phase where organic matter is broken down by microorganisms, either in the presence of oxygen (aerobic) or in its absence (anaerobic).
Statement-wise Analysis:
- Statement 1 is Incorrect: The aeration phase occurs during secondary treatment (activated sludge process), where air is pumped into the tank to support aerobic bacteria. These bacteria consume organic matter and release carbon dioxide ($CO_2$) and water. Biogas is not produced in this oxygen-rich environment.
- Statement 2 is Correct: Biogas is generated in the anaerobic digester, where the sludge (settled solids) is treated. In this sealed tank, anaerobic bacteria decompose the organic material in the absence of oxygen, producing a mixture of gases, primarily methane ($CH_4$) and carbon dioxide ($CO_2$).
- Statement 3 is Incorrect: Biogas typically consists of 50–70% methane, which is highly combustible. It has sufficient calorific value to be used as a fuel source. Many treatment plants utilize this biogas to generate electricity and heat to power the facility's operations.
Key Takeaway:
Biogas is a byproduct of the anaerobic decomposition of sludge, not the aerobic aeration process. It is a valuable renewable energy source used for electricity generation.