Activated sludge is the sediment formed when bacterial flocs settle in the settling tank after secondary treatment. A small part is pumped back into the aeration tank as inoculum, and the major (remaining) part is sent to large tanks called anaerobic sludge digesters, where anaerobic bacteria digest it and make biogas. Memory hook: "Small part back, big part to biogas."
After secondary treatment, flocs settle in the settling tank as activated sludge. A small part returns to the aeration tank as inoculum, while the major part goes to the anaerobic sludge digester, which produces biogas.
Your doubts, answered
Is activated sludge the same as primary sludge?
No. Primary sludge is the solid material that settles during primary (physical) treatment. Activated sludge is different: it is the sediment of bacterial flocs that settles in the settling tank after secondary (biological) treatment. So primary sludge comes from sedimentation of dirt and grit, while activated sludge is mostly living microbes.
What exactly is put into the anaerobic sludge digester?
The major part of the activated sludge is pumped into the anaerobic sludge digesters. It is NOT the primary sludge, NOT the effluent, and NOT the floating debris. NEET has directly asked this, so remember: activated sludge goes into the digester.
Why is a small part of activated sludge sent back to the aeration tank?
That small part is full of useful aerobic microbes. It is pumped back into the aeration tank to act as the inoculum (a starter of microbes). This keeps the floc-forming bacteria growing so the next batch of effluent is treated fast.
What is made inside the anaerobic sludge digester?
Anaerobic bacteria digest the bacteria and fungi in the sludge. During this digestion they produce a mixture of gases: methane, hydrogen sulphide and carbon dioxide. Together these form biogas, which is inflammable and used as a fuel.
What is the difference between flocs and activated sludge?
Flocs are mesh-like masses of aerobic bacteria associated with fungal filaments that grow in the aeration tank and consume organic matter. When these flocs are allowed to settle (sediment) in the settling tank, the sediment they form is called activated sludge. So flocs are floating microbe masses; activated sludge is the settled version of them.
⚠️ The NEET trap ✗ The major part of activated sludge is pumped back into the aeration tank as inoculum. ✓ The SMALL part is pumped back as inoculum; the MAJOR (remaining) part goes to the anaerobic sludge digesters. 🧠 Small goes back, big makes biogas. NEET swaps 'small' and 'major' to trick you.
Real NEET questions
NEET 2020
Which of the following is put into Anaerobic sludge digester for further sewage treatment?
A · Effluents of primary treatment
B · Activated sludge ✓
C · Primary sludge
D · Floating debris
Solution: After secondary treatment, bacterial flocs sediment as activated sludge. A small part is recycled to the aeration tank as inoculum, and the remaining major part is pumped into anaerobic sludge digesters, where anaerobic bacteria digest it and produce biogas. So the correct answer is activated sludge.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
