The correct option is (c) 1 and 3 only.
Explanation
Activated sludge is a key component of the secondary (biological) treatment of wastewater. This process relies on aerobic microorganisms to consume organic matter, thereby reducing the Biochemical Oxygen Demand (BOD) of the effluent.
Statement 1 is Correct:
During the secondary treatment, sewage is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows for the vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures). Once the BOD of sewage is significantly reduced, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is technically referred to as activated sludge.
Statement 2 is Incorrect:
Activated sludge does not contain a low percentage of water; rather, it has a very high water content (typically 97% to 99%). It is a slurry of solids suspended in water, not a mostly solid substance. The solids must be separated from the water through thickening or dewatering processes (like drying beds or centrifuges) to become "mostly solid."
Statement 3 is Correct:
A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. This ensures that there is a sufficient population of microorganisms to begin the decomposition process for the incoming fresh sewage. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.
Key Takeaway:
Activated sludge is the sediment of aerobic microbial flocs formed during secondary sewage treatment. A small portion is recycled as an inoculum to maintain microbial density, while the majority is processed in anaerobic digesters to produce biogas.