Question

(a) What do you mean by activated sludge in an STP?
(b) Explain the biological treatment of the major part of the sludge transferred from the large aeration tank into the anaerobic sludge digesters before its final release into the natural water bodies.

Hint:

Activated sludge contains microbes that digest sewage. In anaerobic digesters, microbes degrade organic matter and release biogas, reducing pollution.

Answer 1

Step 1 (a): Activated sludge refers to the thick, flocculent mass of bacteria and fungi formed in the aeration tank of a Sewage Treatment Plant (STP). These microbial flocs actively digest the organic matter present in sewage.
Step 2 (b): After aerobic treatment in the aeration tank, the major part of the sludge is transferred to anaerobic sludge digesters, where:

• Anaerobic bacteria break down the organic matter in the absence of oxygen.
• This process releases biogases such as methane, carbon dioxide, and hydrogen sulfide.
• The digested sludge settles at the bottom and is used as manure, while the treated water is released into water bodies after disinfection.