Consider the following statements regarding unit operations in water and wastewater treatment:

Question

Consider the following statements regarding unit operations in water and wastewater treatment:

I. Coagulation neutralizes the surface charge of colloids, while flocculation promotes their aggregation.

II. Sedimentation tanks are designed mainly for the removal of dissolved organics.

III. Adsorption (e.g., using activated carbon) is effective for removing color, taste- and odor compounds, and some micropollutants.

Which of the statements given above is/are correct?

1. A. I and II only
2. B. I and III only (Correct answer)
3. C. II and III only
4. D. I, II and III

Correct Answer

Option B — I and III only

Detailed Solution & Explanation

The correct answer is I and III only.

Key Points

• Statement I is correct and states the division of labour exactly: coagulation chemically destabilises colloids by neutralising their surface charge, and flocculation is the gentle mixing that lets the destabilised particles aggregate into settleable flocs.
• Statement II is incorrect. Sedimentation tanks are designed principally on surface overflow rate, the settling velocity of the particle to be removed — not on the criterion the statement offers. Depth matters far less than surface area, which is the essence of ideal settling-basin theory.
• Statement III is correct, describing a further unit operation accurately.

Additional Information

• Surface overflow rate (SOR) equals flow divided by plan area. Any particle whose settling velocity exceeds the SOR is removed regardless of tank depth — the counter-intuitive result from Hazen's analysis that underlies all clarifier design.
• That is why tube and plate settlers work: they multiply effective settling area within the same tank footprint.
• Typical Indian design values are an SOR of about 20-40 m³/m²/day for a primary clarifier and a detention time of 2-2.5 hours.
• The order of the operations is never arbitrary — each exists because the preceding one cannot remove the particle size range that follows.