Which of the following statements is INCORRECT?

Question

Examoogle · Accepted Answer

The correct answer is Option B: Coagulation is a chemical technique that is directed towards the destabilisation of charged particles and the flocculation promotes the agglomeration of stabilised particles.. The statement in Option B is incorrect because it swaps the definitions of coagulation and flocculation. Coagulation is the chemical process of destabilizing charged colloidal particles by neutralizing their surface charges using coagulants (like Alum), whereas flocculation is the physical process of bringing these destabilized particles together through gentle mixing to form larger flocs.

Step-by-Step Solution & Concepts:
The statement in Option B is incorrect because it swaps the definitions of coagulation and flocculation. Coagulation is the chemical process of destabilizing charged colloidal particles by neutralizing their surface charges using coagulants (like Alum), whereas flocculation is the physical process of bringing these destabilized particles together through gentle mixing to form larger flocs.

Working Principle: In coagulation, coagulants like aluminum sulfate ($Al_{2}(SO_{4})_{3} \cdot 18H_{2}O$) provide positive ions to neutralize the negative zeta potential of colloidal particles. Once destabilized, flocculation occurs via collision of particles under slow velocity gradients ($G$), allowing them to agglomerate into settleable flocs.

Key Formulas: $v_{s} = \frac{g(S_{s}-1)d^{2}}{18
u}$ — Stokes' Law for settling velocity of particles; $G = \sqrt{\frac{P}{\mu V}}$ — Velocity gradient for flocculation where $P$ is power, $\mu$ is dynamic viscosity, and $V$ is volume

Key Points: Sedimentation efficiency depends on surface overflow rate ($v_{0} = \frac{Q}{A}$), making it independent of depth for ideal tanks.; Mechanical straining, sedimentation, biological action, and adsorption represent the removal mechanisms in rapid sand filters.; Activated carbon removes chlorine through surface adsorption reactions: $C + 2Cl_{2} + 2H_{2}O ightarrow CO_{2} + 4HCl$.; Coagulation involves charge neutralization; Flocculation involves particle collision.

Advantages: Coagulation-Flocculation effectively removes turbidity and color.; Activated carbon provides dual benefits of dechlorination and taste/odor removal.

Disadvantages: High chemical dosage in coagulation increases sludge volume.; Improper flocculation leads to carryover of light flocs to the filter bed.

Applications: Water treatment plants for municipal supply.; Industrial effluent treatment to remove colloidal impurities.

Additional Info: The surface overflow rate ($v_{0}$) is defined as $\frac{Q}{A}$, where $Q$ is discharge and $A$ is the plan area of the tank.. Option B is the incorrect statement as it attributes charge destabilization to flocculation instead of coagulation.

Answer: B is correct — It incorrectly defines the roles of coagulation and flocculation by interchanging their primary functions.

Exam Tip: Always remember that in sedimentation tanks, if the surface area is constant, increasing the depth does not improve the removal efficiency for ideal particles; therefore, surface overflow rate is the key design parameter.

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Option A: In plain sedimentation tanks, the removal of particles is independent of the depth of the sedimentation tank.

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Option C: Filters purify water through the following four different processes: mechanical straining, flocculation and sedimentation, biological metabolism and electrolyte charges charging.

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Option D: Activated carbon is a de-chlorinating agent.

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