Read the following statements about the secondary treatment of sewage:
I. During secondary treatment, aerobic microbes proliferate rapidly and form flocs, which are clusters of bacteria intertwined with fungal filaments to create mesh-like structures.
II. During secondary treatment, the growing microbes utilise organic matter, leading to a reduction in the biochemical oxygen demand (BOD) of the sewage.
III. Once the BOD of the sewage has been decreased, the effluent is directed into a settling tank. Within this tank, the bacterial flocs settle down, forming sediment known as activated sludge.
IV. A portion of the activated sludge is used as an inoculum in the aeration tank, while the remaining sludge is transferred to anaerobic sludge digesters, which are large tanks.
V. Within the anaerobic sludge digesters, heterotrophic microbes decompose the bacteria and fungi present in the sludge through anaerobic digestion, resulting in the production of a gas mixture that includes carbon dioxide, nitrogen, and carbon monoxide. This gas mixture is commonly referred to as biogas.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → I, II, III and IV only
In secondary (biological) treatment, the primary effluent is aerated so that useful aerobic microbes grow as flocs and use up the organic matter. The flocs then settle as activated sludge, part of which goes back to the aeration tank and the rest to anaerobic sludge digesters.
Statement I: During secondary treatment, aerobic microbes proliferate rapidly and form flocs, which are clusters of bacteria intertwined with fungal filaments to create mesh-like structures. — True. In the aeration tanks the useful aerobic microbes grow vigorously into flocs, which are masses of bacteria associated with fungal filaments forming mesh-like structures.
Statement II: During secondary treatment, the growing microbes utilise organic matter, leading to a reduction in the biochemical oxygen demand (BOD) of the sewage. — True. While growing, the microbes consume the major part of the organic matter, and this lowers the BOD of the effluent.
Statement III: Once the BOD of the sewage has been decreased, the effluent is directed into a settling tank. Within this tank, the bacterial flocs settle down, forming sediment known as activated sludge. — True. The flocs are allowed to sediment in a settling tank, and this sediment is called activated sludge.
Statement IV: A portion of the activated sludge is used as an inoculum in the aeration tank, while the remaining sludge is transferred to anaerobic sludge digesters, which are large tanks. — True. A small part is pumped back as inoculum and the major part goes into the large anaerobic sludge digesters.
Statement V: Within the anaerobic sludge digesters, heterotrophic microbes decompose the bacteria and fungi present in the sludge through anaerobic digestion, resulting in the production of a gas mixture that includes carbon dioxide, nitrogen, and carbon monoxide. This gas mixture is commonly referred to as biogas. — False. The anaerobic bacteria do digest the bacteria and fungi of the sludge, but the gases formed are methane, hydrogen sulphide and carbon dioxide. Nitrogen and carbon monoxide are not the gases of biogas.
Statements I, II, III and IV are correct.
NCERT Extract:
Microbes in Sewage Treatment – “… 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. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. … 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.”