In a conventional activated sludge process, the correct sequence of unit operations after preliminary treatment is
- APrimary sedimentation → Aeration tank → Secondary sedimentation → DisinfectionCorrect
- BAeration tank → Primary sedimentation → Secondary sedimentation → Disinfection
- CSecondary sedimentation → Aeration tank → Primary sedimentation → Disinfection
- DPrimary sedimentation → Secondary sedimentation → Aeration tank → Disinfection
Solution
Concept
Flow diagram sequence of conventional wastewater treatment plant
Formula
NONE
Step-by-step solution
1. Preliminary treatment removes floating debris and heavy grit. 2. Primary sedimentation removes settleable suspended solids. 3. Aeration basin provides oxygen for aerobic microorganisms to digest dissolved organics. 4. Secondary clarifier settles microbial biomass (activated sludge). 5. Disinfection (chlorination/UV) kills pathogens before discharge. Answer: A
Explanation
In a conventional wastewater treatment plant, wastewater first undergoes preliminary treatment (screens and grit chambers) to remove large debris and grit. The sequence of subsequent unit processes is: Primary Sedimentation (settles settleable organic solids) → Aeration Tank (biological oxidation of dissolved organics) → Secondary Sedimentation (separates activated biomass from treated water) → Disinfection (pathogen destruction).
Why the other options are wrong
B) Aeration before primary settling introduces heavy raw settleable solids into the biological reactor. C) Secondary settling cannot precede biological aeration. D) Clarification steps cannot occur back-to-back before aeration.
Shortcut
Standard ASP flow: Primary Clarifier -> Aeration Basin -> Secondary Clarifier -> Disinfection.
Common mistake
Placing secondary sedimentation before aeration or omitting primary clarification.
Correct answer: A) Primary sedimentation → Aeration tank → Secondary sedimentation → Disinfection