Page 402 - Water and wastewater engineering

SEDIMENTATION 10-19

For cocurrent settling, the settling time for a particle to move between two parallel plates is
given by Equation 10-23 . The time that particles moving with the fluid spend in the plates is
L p
t p (10-28)
v fc v s sin

If Equation 10-23 is equated to Equation 10-28 , t p is equal to the settling time t. Those particles with a
settling velocity v s are removed. Those particles with larger settling velocity are also removed, that is
v fc d
v s (10-29)
L p cos d sin

For crosscurrent settling, the settling time for a particle to move between two parallel plates is
also given by Equation 10-23 . The time that particles moving with the fluid spend in the plates is
L p
t p (10-30)
v fc

If t p is equal to the settling time (equating Equation 10-23 and Equation 10-30 ), then the
particles with settling time v s are removed. Those particles with a larger settling velocity are also
removed, that is

v fc d
v s (10-31)
L p cos

Non-Ideal Behavior of Settling Tanks
Numerous factors affect settling tank performance. These include turbulence, inlet energy dissipa-
tion, density currents, wind effects, outlet currents, and sludge equipment movement. In general, for
most water treatment sedimentation basins, performance is primarily a function of turbulence, inlet
energy dissipation, density currents, and wind effects rather than outlet currents and mechanical
movement. Therefore, this discussion is focused on these effects.

Turbulence. In Camp’s development of a theoretical basis for removal of discretely settling
particles he assumed a uniform horizontal velocity in the settling zone. This assumption implies
near laminar flow conditions (Reynolds number 1). This is rarely, if ever, achieved in actual
settling tanks. However, this important assumption is considered in the design of the tank by
evaluation of the Reynolds number and the Froude number. These dimensionless ratios are
described in the next section of this chapter.
The Reynolds number is important as a measure of turbulence in flows that are influenced by
viscous effects, such as internal flows and boundary layer flows. The Froude number is important
in flows that are influenced by gravity, such as free surface flows.

Inlet Energy Dissipation. The performance of the sedimentation basin is strongly influenced
by the effectiveness of energy dissipation at the inlet. Again using Camp’s theory, the flow must