Arrange the given steps for sewage treatment plant in correct sequence?
(A) Formation of flocs
(B) Pumping of sludge into anaerobic sludge digester
(C) Filtration and sedimentation
(D) Reduction in BOD by biological treatment
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (A), (D), (B)
(C) Filtration and sedimentation — first. This is the primary treatment, a physical step. Floating debris is removed by filtration and grit by sedimentation. The settled solids form the primary sludge, and the liquid above (the effluent) goes on for secondary treatment.
(A) Formation of flocs — second. The primary effluent is passed into large aeration tanks, where it is agitated and air is pumped in. Useful aerobic microbes grow vigorously into flocs, masses of bacteria held in a mesh of fungal filaments.
(D) Reduction in BOD by biological treatment — third. While growing, the microbes in the flocs consume most of the organic matter of the effluent. This lowers the BOD (biochemical oxygen demand), and the water is treated till its BOD is low.
(B) Pumping of sludge into anaerobic sludge digester — last. The flocs are then allowed to settle as activated sludge. A small part goes back to the aeration tank, and the major part is pumped into anaerobic sludge digesters, where other bacteria digest it and produce biogas.