Plain sedimentation tank design What is the usual detention period adopted for plain (unflocculated) sedimentation tanks in conventional water treatment?

Introduction / Context:
Detention period (hydraulic residence time) in sedimentation governs settling performance and tank volume. For plain sedimentation (without chemical flocculation), a moderate detention time is chosen to settle the readily settleable fraction while balancing footprint and cost.

Given Data / Assumptions:

• No coagulation/flocculation stage before the basin.
• Design flow is steady for sizing purposes.
• Temperature and particle sizes typical of raw surface water.

Concept / Approach:
Standard practice adopts 4–8 hours for plain sedimentation. Longer times can lead to septic conditions and are generally unnecessary for routine clarification goals; shorter times may underperform for typical particle sizes without floc aids.

Step-by-Step Solution:
Recall design range from manuals: 4–8 h for plain sedimentation.Select the matching option.

Verification / Alternative check:
Settling theory (surface overflow rate approach) and plant designs converge to similar volumes that translate to detention within the 4–8 h band for many municipal plants.

Why Other Options Are Wrong:

• 8–16 h, 16–24 h, and 24–36 h are excessive for plain sedimentation, increasing volume without proportional benefit.

Common Pitfalls:
Confusing plain sedimentation with flocculation–clarification units, which may target different design rates but typically still yield comparable basin residence times.

Final Answer:
4 to 8 hours