Civil Engineering Reference
In-Depth Information
Density currents, due to the influent water having a different density from the
water in the basin (because of the effects of temperature or the suspension load)
•
Eddy currents, produced by the incoming water
•
These currents distort the flow pattern from ideal conditions. Particle interactions cause
flocculation that result in the formation of irregular nondiscrete particles.
Because of currents, particle interactions, and other factors, sedimentation basin
overflow rates are not based solely on settling velocities. Typical overflow rates in
rectangular and circular sedimentation basins with alum as the primary coagulant are
600 to 1,000 gpd / ft
2
(1.0 to 1.7 m / h). The higher rates are used in warm waters, the
lower rates in cold waters. Higher overflow rates with alum coagulation may be pos-
sible by using polymers to aid flocculation and settling. Ferric chloride and lime co-
agulants will form dense, fast-settling floc in many waters permitting the use of higher
overflow rates. Typical overflow rates for lime floc are 1,400 to 2,100 gpd / ft
2
(2.4 to
3.6 m / h).
Inlet and Outlet Conditions
Poor sedimentation basin performance can result from
uneven influent flow distribution, inadequate dissipation of inlet energy, nonuniform
collection of effluent flow, and the associated hydraulic short-circuiting within a basin.
An ideal inlet would distribute the water uniformly over the full cross section of
the tank. The effects of density and inertial currents have been found to be more critical
at the inlet than at the outlet.
2
Because of the relatively fragile nature of chemical floc, the velocity in the influent
channels or pipelines to a sedimentation basin must be kept low (0.5 to 1.0 ft / sec
[0.15 to 0.3 m / s]). Also, low velocities are needed through any inlet ports to minimize
floc breakup.
In rectangular basins, the flow may enter through an inlet channel across the head
end of the tank (Fig. 11-3). The water passes through a number of inlet ports across
A
Inlet Flume
Plan View
Section A
Fig. 11-3.
Rectangular basin inlet channel (From Culp, Gordon, and Williams, Robert,
Handbook
of Public Water Systems.
Copyright
1986 by John Wiley & Sons, Inc. Reprinted by permission
of John Wiley & Sons, Inc.)
