Overflow Rate & Weirs

v_{o} = \frac{Q}{A _{s}}, t_{d} = \frac{V}{Q}, weir loading = \frac{Q}{L _{w}}

In an ideal rectangular clarifier, particles with settling velocity vs > vo are fully removed. Overflow rate has units of velocity (m³/m²·d = m/d). Primary tanks use lower vo than high-rate trickling-filter clarifiers. Detention time is a complementary check.

$v_{o}$ (m/d): Overflow rate
$A_{s}$ (m²): Surface area
$L_{w}$ (m): Weir length

Step-by-step solved example

Q = 8000 m³/d, rectangular tank 20 m × 8 m. Overflow rate?

OR = Q/A. Keep OR below the design settling velocity.

1. vo
As = 160 m². vo = 8000/160 = 50 m/d.
$v_{o} = 50 m/d$

Answer: 50 m/d

10 practice questions

0/10 correct

1.Particles with vs > vo are

2.Depth of a clarifier primarily affects

3.Weir loading has units

4.Filtration rate is analogous to

5.Grit chambers are designed around

6.Hardness is expressed as

7.CT concept in disinfection is

8.Q = 0.12 m³/s, As = 40 m². vo =

9.Alkalinity in coagulation is needed to

10.Secondary clarifier must also consider