Grit Chamber Overflow and Stokes Settling

v_{s} = \frac{g ( s - 1 ) d ^{2}}{18 ν} (Stokes), v_{o} = \frac{Q}{A} \leq v_{s}

Type-I discrete settling: overflow rate vo must be ≤ particle vs. Stokes for laminar spheres (grit sand). Horizontal velocity is limited so grit is not scoured.

$v s$: Settling velocity
$v o$: Overflow rate Q/A
$d$: Particle diameter

Step-by-step solved example

Want vs=0.025 m/s sand. Chamber Q=0.4 m³/s. Minimum plan A?

Grit chamber: overflow vo ≤ vs (Stokes).

1. vo=vs
A=Q/vs=0.4/0.025=16 m².
2. Note
Provide more A and a controlled vh (channel grit).

Answer: A ≥ 16 m²

10 practice questions

0/10 correct

1.Stokes vs ∝

2.If vo > vs the particle

3.Aerated grit chambers use air to

4.s=2.65 silica, d smaller → vs

5.Primary clarifier vo is much

6.Temperature: colder water (higher ν) makes vs

7.Q=0.5 m³/s, A=25 m²: vo=

8.Horizontal vh too high causes

9.Weir loading is

10.s−1 in Stokes is