Page 409 - Water and wastewater engineering

10-26 WATER AND WASTEWATER ENGINEERING

TABLE 10-2
a
Typical sedimentation tank overflow rates
Application Long rectangular and circular, Upflow solids-contact,
3 2 3 2
m /d · m m /d · m
Alum or iron coagulation
Turbidity removal 40 50
Color removal 30 35
High algae 20
Lime softening
Low magnesium 70 130
High magnesium 57 105
a
These rates are guides that are applicable at moderate water temperatures—not less than 10 C. For lower
temperatures the rates should be reduced.
Source: Adapted from AWWA, 1990.

Open sedimentation tanks greater than 30 m in length are especially susceptible to wind effects.
For longer tanks, wave breakers (launders or baffles) placed at 30 m intervals are recommended.
The tank depth is usually increased by about 0.6 m to provide freeboard to act as a wind barrier.
Horizontal flow velocities must be controlled to avoid undue turbulence, backmixing, and
scour of particles from the sludge. GLUMRB (2003) recommends that the velocity not exceed
0.15 m/min. Velocities of 0.6 to 1.2 m/min have been found to be acceptable for basin depths of
2 to 4.3 m (Willis, 2005). Reynolds and Froude numbers can be used to check on turbulence and
backmixing. The Reynolds number is determined as
vR
fh
R (10-32)

where R Reynolds number, dimensionless
v f average horizontal fluid velocity in tank, m/s
R h hydraulic radius, m
A s / P w
2
A s cross sectional area, m
P w wetted perimeter, m
2
kinematic viscosity, m /s /
dynamic viscosity, Pa · s
3
density of fluid, kg/m
The Froude number is determined as
() 2
v f
Fr (10-33)
gR h

where Fr Froude number, dimensionless
2
g acceleration due to gravity, 9.81 m/s