Correct Answer - Option 2 :
(a) - 2, (b) - 3, (c) - 1, (d) – 4
Explanation:
Detention time for different types of Treatment unit
Treatment Unit
|
Detention time
|
Grit chamber
|
30 - 60 second
|
Primary sedimentation
|
2 - 2.5 hour
|
Sludge digestion
|
20 - 30 days
|
Activated sludge process
|
4 - 6 hour
|
Oxidation pond
|
2 - 6 weeks
|
Septic tank
|
12 - 36 hour
|
Note: Septic tank has high detention time whereas Grit chamber has less detention time.
Grit chambers:
(i) These are long narrow rectangular channels with an increased cross-sectional dimension that decelerate flow and allow grit to settle.
(ii) These units are provided in order to carry out the removal of the inorganic suspended solid and to pass forward the suspended solid in PST.
(iii) These chambers are designed in the form of a long channel, which may either be rectangular or parabolic in cross-section.
(iv) The grit chamber is a settling tank with a provision to control flow velocity.
Sludge digestion
i) It is a biological process in which organic solids are decomposed into stable substances.
ii) Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge. Digested sludge is inoffensive, having the appearance and characteristics of rich potting soil.
iii) It includes aerobic suspended culture.