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).
Before the sludge reaches the anaerobic sludge digesters, the effluent of primary treatment is passed into aeration tanks, where ‘flocs’ form. What are flocs?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Masses of bacteria associated with fungal filaments forming mesh-like structures
Secondary (biological) treatment of sewage happens in large aeration tanks. The effluent from primary treatment is stirred and air is pumped into it. With plenty of oxygen, useful aerobic microbes grow into flocs. These microbes use up most of the organic matter in the effluent, so its BOD (biochemical oxygen demand) falls.
Option (1) Masses of bacteria associated with fungal filaments forming mesh-like structures — Correct. Flocs are masses of bacteria associated with fungal filaments that form mesh-like structures. They grow because air is pumped into the tank.
Option (2) Masses of methanogenic bacteria growing anaerobically on cellulosic material — Incorrect. Methanogens grow anaerobically on cellulosic material. They live in anaerobic sludge digesters and in the rumen of cattle, not in an aeration tank full of air.
Option (3) Floating mats of cyanobacteria that fix atmospheric nitrogen — Incorrect. Cyanobacteria that fix atmospheric nitrogen are important biofertilisers in paddy fields. They are not the flocs of sewage treatment.
Option (4) Grit, soil and small pebbles that settle out during primary treatment — Incorrect. Grit (soil and small pebbles) is removed earlier, in primary treatment, by sedimentation. Flocs are living microbes that form later, in secondary treatment.
NCERT Extract:
Microbes in Sewage Treatment – “… Secondary treatment or Biological treatment : The primary effluent is passed into large aeration tanks (Figure 8.6) where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent.”