Read the passage and answer the following questions:
Sewage, which is a major component of urban wastewater, contains a significant amount of human excreta, organic matter, and pathogenic microbes. Disposing of such a large quantity of sewage directly into natural water bodies like rivers and streams is not feasible due to its polluting nature. Therefore, sewage undergoes treatment in sewage treatment plants (STPs) to reduce its pollution levels before disposal. The treatment process involves two main steps. The primary treatment focuses on physically removing both large and small particles from the sewage through processes like filtration and sedimentation. Initially, floating debris is removed through sequential filtration. The secondary treatment, also known as biological treatment, involves passing the primary effluent into large aeration tanks. These tanks are mechanically agitated and air is pumped into them, creating an environment suitable for the growth of beneficial aerobic microbes. These microbes form mesh-like structures called flocs, which consist of bacteria associated with fungal filaments. As these microbes grow, they consume a significant portion of the organic matter present in the effluent. This process leads to a reduction in the Biochemical Oxygen Demand (BOD) of the effluent. BOD is a measure of the organic matter content in the water and indicates its pollution potential. The sewage water is treated until the BOD is sufficiently reduced. Overall, sewage treatment plants play a crucial role in reducing the pollution level of sewage by removing particles and organic matter, thereby ensuring that the treated water can be safely disposed of without causing harm to the environment or public health.
Which among the following is not a component of biogas?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Oxygen
Biogas is a mixture of gases, mainly methane, produced by bacteria that grow anaerobically. In the anaerobic sludge digesters of a sewage plant, these bacteria digest the bacteria and fungi of the sludge and give out methane, hydrogen sulphide and carbon dioxide.
Option (1) Methane — Incorrect. Methane is the main gas of biogas. It is what makes biogas inflammable and useful as a fuel.
Option (2) Hydrogen sulphide — Incorrect. Hydrogen sulphide is one of the gases produced during anaerobic digestion of the sludge, so it is part of biogas.
Option (3) Carbon dioxide — Incorrect. Carbon dioxide is also present in biogas, along with methane.
Option (4) Oxygen — Correct. Oxygen is not a component of biogas. Biogas is formed by anaerobic bacteria in tanks kept away from air, and the gases they release are methane, hydrogen sulphide and carbon dioxide.
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.”
Microbes in Production of Biogas – “Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which may be used as fuel.”