What is a commonly adopted upper limit for the depth of sedimentation (clarifier) tanks in water-treatment plants?

Introduction / Context:
Primary sedimentation (clarification) removes settleable solids before filtration. Tank depth influences settling behavior, sludge storage, scraper design, and structural cost. Designers typically observe both minimum and maximum depth limits.

Given Data / Assumptions:

• Conventional horizontal-flow or circular sedimentation tanks for potable water treatment.
• Standard design practice bounds depth to aid operation and economy.

Concept / Approach:
While effective water depth is often in the 3–4 m range for potable-water clarifiers, an upper limit is generally placed to control construction cost, mixing characteristics, sludge removal mechanisms, and headloss constraints. Many references indicate that depths should not exceed about 5 m for conventional clarifiers in municipal practice.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals specify similar ranges; greater depths complicate sludge removal and offer little settling benefit compared with increased surface area.

Why Other Options Are Wrong:

• 2–4 m: Within typical design depths, but not an 'upper limit'.
• 6 m: Exceeds commonly adopted upper bounds for conventional plants.

Common Pitfalls:

• Confusing clarifier depth with detention time or surface overflow rate, which are controlled predominantly by plan area and flow.

Final Answer:
5 m