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.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In a sewage treatment plant, a small part of the activated sludge is pumped back into the aeration tank.
Reason (R): The bacterial flocs in the activated sludge serve as the inoculum for the aeration tank.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A): In a sewage treatment plant, a small part of the activated sludge is pumped back into the aeration tank. — True. After secondary treatment, the bacterial flocs settle in a settling tank as activated sludge. A small part of it is returned to the aeration tank, and the major part goes to the anaerobic sludge digesters.
Reason (R): The bacterial flocs in the activated sludge serve as the inoculum for the aeration tank. — True. The returned sludge is full of the useful aerobic microbes (flocs), and NCERT says it serves as the inoculum.
A tells us what is done with part of the activated sludge. R tells us the purpose of that step.
Why R explains A? The aeration tank needs a large population of aerobic microbes to consume the organic matter of fresh effluent. Returning some activated sludge seeds the tank with these flocs, just as a starter of curd seeds fresh milk. That is the very reason the sludge is pumped back, so R is the correct explanation of A.
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.”