Sedimentation

Question 1

What is the force balance on a particle falling through water under gravity?

Answer 1

mg = Fb + Fd

Question 2

What are the parameters in the Fd equations?

Answer 2

r = particle radius
g = acceleration due to gravity
pp = particle density
pf = fluid density
Cd = drag coefficient
V = settling velocity

Question 3

What does the drag coefficient depend on?
Explain trend

Answer 3

Reynolds number
low Re = high Cd = laminar
high Re = low Cd = turbulence

Question 4

Give equation for specific gravity of the particle

Answer 4

S = pp / pf

Question 5

What conditions are required for sedimentation?

Answer 5

Quiescent - laminar flow since turbulence will remain SS in suspension

Question 6

How are small particles with low density removed from WW?

Answer 6

Floatation

Question 7

What law is assumed in sedimentation calculations and what are some effects that need considered which the law doesn’t assume?

Answer 7

Stoke’s Law
Shape effects
Flocculation and deflocculation effects
Hindered settling effects

Question 8

Describe the shape effects on sedimentation

Answer 8

Less important since only important at high Re where shedding can produce unsteady settling

Question 9

Describe flocculation and deflocculation effects on sedimentation

Answer 9

Stokes law underestimates flocculent suspensions because the agglomeration of floc particles increases the settling velocity due to formation of larger, heavier particles.

Question 10

Describe hindered settling effects on sedimentation

Answer 10

Settling particles interact and settling velocities reduced. High concentrations of particles have a significant upward displacement of water due to settling particles which reduces the apparent settling velocity.