Read the given passage and answer the following question :
Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel. Microbes produce different types of gaseous end-products during growth and metabolism. The type of the gas produced depends upon the microbes and the organic substrates they utilise. In the examples cited in relation to fermentation of dough, cheese making and production of beverages, the main gas produced was CO2. However, certain bacteria, which grow anaerobically on cellulosic material, produce large amount of methane along with CO2 and H2. These bacteria are collectively called methanogens, and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment. These bacteria are also present in the rumen (a part of stomach) of cattle. A lot of cellulosic material present in the food of cattle is also present in the rumen. In rumen, these bacteria help in the breakdown of cellulose and play an important role in the nutrition of cattle. Thus, the excreta (dung) of cattle, commonly called gobar, is rich in these bacteria. Dung can be used for generation of biogas, commonly called gobar gas. The technology of biogas production was developed in India mainly due to the efforts of Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC).
In sewage treatment, the activated sludge is pumped into anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge. Which mixture of gases is produced during this digestion?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Methane, hydrogen sulphide and carbon dioxide
In secondary treatment of sewage, most of the activated sludge is pumped into anaerobic sludge digesters. There, bacteria that grow without oxygen digest the bacteria and fungi of the sludge. As they do so, they give off gases that together form biogas.
Option (1) Methane, hydrogen sulphide and carbon dioxide — Correct. The book names this mixture: methane, hydrogen sulphide and carbon dioxide. Because it burns, this biogas can be used as a source of energy.
Option (2) Methane, ammonia and carbon dioxide — Incorrect. Ammonia is not part of the gas mixture the book gives for sludge digesters. The third gas is hydrogen sulphide.
Option (3) Methane, sulphur dioxide and carbon dioxide — Incorrect. Sulphur dioxide forms when sulphur burns in oxygen. A sludge digester has no oxygen, and the sulphur gas the book names there is hydrogen sulphide.
Option (4) Methane, nitrogen and carbon dioxide — Incorrect. Nitrogen is not in the book’s list of gases from the digesters. The third gas is hydrogen sulphide.
NCERT Extract:
Microbes in Sewage Treatment – “… 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.”