Which of the following statements is INCORRECT?
- AActivated carbon is a de-chlorinating agent.
- BFlocculation is a chemical technique that is directed towards the destabilisation of charged particles and the coagulation promotes the agglomeration of stabilised particles.Correct
- CIn plain sedimentation tanks, the removal of particles is independent of the depth of the sedimentation tank.
- DFilters purify water through the following four different processes: mechanical straining, flocculation and sedimentation, biological metabolism and electrolyte charges changing.
Solution
Concept
Coagulation destabilises charged particles. Flocculation joins the destabilised particles into big flocs.
Step-by-step solution
1. Check A: Activated carbon removes extra chlorine from water, so it is a de-chlorinating agent. A is correct. 2. Check B: Coagulation is the chemical step that destabilises charged particles. Flocculation is the slow mixing that brings these particles together into flocs. B swaps the two terms, so B is wrong. 3. Check C: In ideal plain sedimentation, removal depends on the surface overflow rate (Q/A), not on depth. C is correct. 4. Check D: Filters work by mechanical straining, flocculation and sedimentation, biological metabolism, and electrolytic changes. D is correct. 5. So the INCORRECT statement is B.
Explanation
Coagulation comes first. Chemicals like alum neutralise the charge on colloids. Flocculation comes next: gentle mixing makes the destabilised particles stick together and grow into flocs. Statement B gives the opposite meaning, so it is incorrect.
Why the other options are wrong
A: Correct, activated carbon removes residual chlorine. C: Correct, ideal settling efficiency depends on surface loading (Q/A), not depth. D: Correct, these are the four standard mechanisms of filtration.
Shortcut
Remember the order: Coagulation = Charge destabilisation; Flocculation = Floc formation.
Common mistake
Mixing up coagulation and flocculation, or thinking a deeper tank always removes more particles.
Correct answer: B) Flocculation is a chemical technique that is directed towards the destabilisation of charged particles and the coagulation promotes the agglomeration of stabilised particles.