Activated Sludge & Anaerobic Digestion — NEET Biology
✅ Asked in NEET 2020Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial 'flocs' are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams. An aerial view of such a plant
NTA tests whether students understand that activated sludge from secondary treatment is divided into two parts: a small portion recycled back to the aeration tank as inoculum, while the majority goes to anaerobic digesters. The common mistake is thinking all sludge is recycled or that aerobic bacteria digest the sludge—both are wrong. In anaerobic digesters, anaerobic bacteria break down the biomass to produce biogas (methane, H₂S, CO₂), which is used as renewable energy. Remember: only a fraction recycles; the rest is anaerobically digested for biogas production. This concept links microbial processes with environmental sustainability, making it a consistent NEET favorite.
This paragraph was tested 2 times in NEET.
Which of the following is put into Anaerobic sludge digester for further sewage treatment? (NEET 2020)
What gases are produced in anaerobic sludge digesters? [NEET 2014 / AIPMT 2014]
Through deep analysis of NEET and NTA, 88 of 90 questions from the NEET 2026 paper were matched straight from the MedicNEET Biology question bank.
88/90MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.