Secondary treatment or biological treatment: The primary effluent is passed into large tanks where air is pumped into it. This allows vigorous growth of aerobic microbes. While growing, these microbes consume the major part of the organic matter in the effluent. This significantly change the BOD (biochemical oxygen demand) of the effluent.
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 sludge digesters. Here, other kinds of bacteria grow anaerobically in the sludge. The biogas produced can be used 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.
The biogas is a mixture of the following gases
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Methane, Carbon dioxide and Hydrogen sulphide
Option (1) Methane, Carbon dioxide and Oxygen — Incorrect. Oxygen is not part of biogas; it is made by anaerobic bacteria.
Option (2) Methane, Carbon dioxide and Hydrogen sulphide — Correct. Biogas is a mixture of gases, predominantly methane, made by microbes growing anaerobically on cellulosic material. It contains methane, carbon dioxide and hydrogen sulphide; there is no oxygen in it, as it is made anaerobically.
Option (3) Carbon dioxide, Nitrogen and Hydrogen sulphide — Incorrect. Methane, the main gas, is missing, and nitrogen is not named.
Option (4) Methane, Hydrogen sulphide and Oxygen — Incorrect. Carbon dioxide is left out and oxygen is not part of biogas.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
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.”