Water & Wastewater Treatment

Sedimentation & Overflow Rate

Surface overflow rate, settling velocity and removal efficiency in primary clarifiers.

PART 1

Topic Breakdown & Traps

The Engineering Principle

In an ideal settling tank, a particle is removed if its settling velocity exceeds the surface overflow rate (SOR) — the design flow divided by the plan (surface) area. Remarkably, ideal removal depends on surface area, not depth. Discrete particles settle by Stokes' law; the detention time sizes the tank volume.

The Core Formula Matrix

Surface overflow rate:

v_{o} = \frac{Q}{A _{s u r f a ce}}

(also the critical settling velocity)

Removal: particles with

v_{s} \geq v_{o}

are fully removed; partial removal

= v_{s} / v_{o}

.

Detention time:

t = \frac{V}{Q} = \frac{A H}{Q}

Stokes settling velocity:

v_{s} = \frac{g d ^{2} ( ρ _{p} - ρ )}{18 μ}

The ‘IIT Traps’

⚠
Ideal removal depends on surface area, not depth. A bigger footprint settles more.
⚠
SOR has velocity units (m/s or m³/m²·s), despite the 'rate' name.
⚠
Stokes' law assumes laminar (small-particle) settling; large particles deviate.

📚 Standard references

Water and Wastewater Engineering — Davis & Cornwell · Sedimentation

PART 2

Progressive 3-Tier Question Suite

Q1MEDIUM2 Marks · NAT

A clarifier treats Q = 4000 m³/day through a surface area of 200 m². Its surface overflow rate is _____ m³/m²·day.

Q2BASIC1 Mark · MCQ

In an ideal horizontal-flow settling tank, the removal efficiency of discrete particles depends primarily on:

Q3HARD2 Marks · MCQ

A discrete particle has settling velocity

v_{s}

less than the overflow rate

v_{o}

. Its fractional removal in an ideal basin is: