Weir Loading Rate

W L = \frac{Q}{L _{weir}}

Clarifier weir loading is effluent flow per unit length of effluent weir. High WL raises the approach velocity near the weir, pulling floc over the weir (end-wall effect). Primary tanks often allow ~125–250 m³/m·d; secondary weirs are designed lower or with finger weirs to add length. Distinct from overflow rate Q/A.

$W L$ (m³/m·d): Weir loading
$Q$ (m³/d): Effluent flow
$L_{weir}$ (m): Total weir length

Step-by-step solved example

Q = 8000 m³/d, weir length = 32 m. Find WL.

Clarifier weir loading = Q / weir length.

1. Divide
WL = Q / L = 8000/32 = 250 m³/m·d.
$W L = 250 m^{3} / m \cdot d$

Answer: WL = 250 m³/m·d

10 practice questions

0/10 correct

1.Q = 12,000 m³/d, L = 40 m. WL =

2.Weir loading has units of

3.Adding finger (inboard) weirs is done to

4.Q = 0.050 m³/s, L = 20 m. WL in m³/m·d is nearest

5.Excessive weir loading tends to

6.Overflow rate uses area A; weir loading uses

7.If Q doubles and L is unchanged, WL

8.A circular clarifier with a peripheral weir: L is about

9.Q = 5000 m³/d on a 25 m weir. WL =

10.V-notch effluent weirs are used because they