Which of the following statements are correct?
I. The solids that settle down after the primary treatment of sewage form the primary sludge, while the liquid portion is called the effluent.
II. The spent slurry from a biogas plant is utilised as fertiliser.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → I and II
Both statements deal with what is left after microbes have done their work: the sludge and effluent of sewage treatment, and the slurry of a biogas plant.
I. The solids that settle down after the primary treatment of sewage form the primary sludge, while the liquid portion is called the effluent. — True. In primary treatment, all solids that settle form the primary sludge, and the supernatant (the liquid above) forms the effluent, which goes on to secondary treatment.
II. The spent slurry from a biogas plant is utilised as fertiliser. — True. The spent slurry is removed through a separate outlet of the biogas plant and may be used as fertiliser.
Both statements are correct.
NCERT Extract:
Microbes in Sewage Treatment – “… initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.”
Microbes in Production of Biogas – “… The biogas plant has an outlet, which is connected to a pipe to supply biogas to nearby houses. The spent slurry is removed through another outlet and may be used as fertiliser.”