During sewage treatment, the anaerobic sludge digesters produce the biogas. The components of biogas are -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( CH_4, H_2S \) and \( CO_2 \)
Biogas is made when anaerobic bacteria (methanogens) digest organic matter in the sludge digesters. It is mostly methane, with hydrogen sulphide and carbon dioxide; methane makes it burn.
Option (1) \( H_2, H_2S \) and \( CO_2 \) — Incorrect. It leaves out methane, the main gas of biogas.
Option (2) \( CH_4, H_2S \) and \( CO_2 \) — Correct. Bacteria in the digesters produce methane, hydrogen sulphide and carbon dioxide.
Option (3) \( CH_4, H_2S \) and \( O_2 \) — Incorrect. The digestion is anaerobic, so no oxygen is given out.
Option (4) \( CH_4, SO_2 \) and \( CO_2 \) — Incorrect. Sulphur dioxide forms when sulphur-containing fuels burn; bacteria working without oxygen give hydrogen sulphide.
NCERT Extract:
Microbes in Sewage Treatment – “… Once 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.”