The most common parameter used to assess the efficiency of a primary sedimentation tank is:

Question

The most common parameter used to assess the efficiency of a primary sedimentation tank is:

1. A. Removal of Suspended Solids (SS) (Correct answer)
2. B. Reduction in COD
3. C. Reduction in BOD
4. D. Change in pH

Correct Answer

Option A — Removal of Suspended Solids (SS)

Detailed Solution & Explanation

The correct answer is Removal of Suspended Solids (SS).

Key Points

• A primary sedimentation tank works by gravity settling. Its function is to remove settleable suspended solids, so its efficiency is properly judged by the percentage of SS removed.
• BOD reduction does occur — around 30-35% — but only as a by-product of removing the organic fraction of the solids, so it is a secondary measure rather than the defining one.
• COD reduction is likewise incidental, and pH change is not an objective of primary settling at all.

Additional Information

• Typical performance of a well-designed primary clarifier is 50-60% removal of suspended solids and 30-35% of BOD, and those figures are worth memorising as a pair.
• Design rests on surface overflow rate — about 20-40 m³/m²/day — with a detention time of roughly 2-2.5 hours. Depth serves sludge storage and scour prevention rather than settling efficiency.
• Adding coagulant converts the unit into chemically enhanced primary treatment, lifting SS removal above 80% and BOD removal to 50-60%, at the cost of chemical dosing and much more sludge.
• The tank also removes floating matter — oil, grease and scum — by surface skimming, a function easily overlooked when only settling is considered.