Water treatment unit design: The usual detention period adopted for plain sedimentation tanks (coagulation not considered) is approximately:

Introduction / Context:
Plain sedimentation tanks remove settleable solids by gravity prior to filtration. The detention period is a key parameter that influences particle settling, tank dimensions, and operational performance.

Given Data / Assumptions:

• Plain (unassisted) sedimentation without flocculation.
• Typical municipal water treatment practice.
• Temperatures in a normal range and conventional tank depths.

Concept / Approach:
Design detention periods for plain sedimentation commonly fall in the range of several hours to provide sufficient time for particles with practical settling velocities to reach the bottom, considering inlet and outlet hydraulics and short-circuiting controls.

Step-by-Step Solution:
Select a detention time that balances tank size with settling efficiency.Standard range for plain sedimentation is about 4–8 hours.Verify with sedimentation theory (surface overflow rate) rather than detention time alone in final design.

Verification / Alternative check:
Surface overflow rate approach typically yields tank sizes corresponding to several hours of detention for conventional plants, validating the 4–8 hour guideline.

Why Other Options Are Wrong:
8–36 hours: excessive for plain sedimentation; impractical tankage.1–2 hours: too short for reliable removal in plain sedimentation without coagulation.

Common Pitfalls:

• Relying solely on detention time without checking surface overflow rate.
• Ignoring inlet and outlet baffles which affect effective detention.

Final Answer:
4 to 8 hours