Plain sedimentation performance: Under normal operating conditions (without coagulation), plain sedimentation tanks typically remove what percentage of suspended impurities from raw water?

Introduction / Context:
Plain sedimentation exploits gravity settling of suspended particles without chemical coagulation. It is often used as a preliminary step to reduce turbidity load on downstream treatment units.

Given Data / Assumptions:

• Raw water with moderate turbidity and a typical particle size distribution.
• Properly designed detention time and surface overflow rate.

Concept / Approach:
In the absence of coagulation, only naturally settleable solids are removed. Textbook ranges commonly cite 50–70% removal, with 60% as a representative value under normal conditions. Higher removals (e.g., 80–90%) usually require coagulation and flocculation.

Step-by-Step Solution:

Verification / Alternative check:
Pilot tests can confirm actual removal; performance depends on particle characteristics and hydraulics.

Why Other Options Are Wrong:
70–90% imply optimized coagulation/flocculation or unusually settleable raw water; 50% is low end and not the standard representative figure.

Common Pitfalls:
Assuming high removal without coagulant addition; neglecting short-circuiting and inlet/outlet design which reduce efficiency.

Final Answer:
60%