1. Sedimentation

2. Where are we? Unit processes* designed to
remove _________________________
remove __________ ___________
inactivate ____________
*Unit process: a process that is used in similar ways in many different applications
Unit Processes Designed to Remove Particulate Matter
Screening
Coagulation/flocculation
Sedimentation
Filtration
Particles and pathogens
dissolved chemicals
pathogens
Empirical design
Theories developed later

3. Sedimentation the oldest form of water treatment
uses gravity to separate particles from water
often follows coagulation and flocculation

4. Sedimentation: Effect of the particle concentration
Dilute suspensions
Particles act independently
Concentrated suspensions
Particle-particle interactions are significant
Particles may collide and stick together (form flocs)
Particle flocs may settle more quickly
At very high concentrations particle-particle forces may prevent further consolidation

5. Types of Settling Four types of sedimentation: Discrete settling
Flocculant settling
Hindered settling
Compression

6. Examples of Settling Types
Discrete
Flocculant
Hindered

7. Type I settling
During settling, a particle will accelerate until the drag force, FD, equals the impelling force, F1
Identify forces
CD (drag force) :depend on whether the flow regime around the particle is laminar or turbulent or function of NRe

8. Drag Coefficient on a Sphere
Stokes Law
Tambo, N. and Y. Watanabe (1979). "Physical characteristics of flocs--I. The floc density function and aluminium floc." Water Research 13(5):
turbulent boundary
laminar
turbulent

9. NRe < 1 : laminar flow; CD = 24/NRe
Stokes equation
1< NRe < 104 : transition flow;
NRe > 104 : turbulent flow; CD=0.4
Newton equation

10. Sedimentation Rate vp = particle settling velocity (m/s or ft/s)
p = particle density (kg/m3 or lbm/ft3)
 = fluid density (kg/m3 or lbm/ft3)
d = particle diameter (m or ft)
g = gravitational acceleration (9.81 m/s2 or 32.2 ft/s2)
μ = dynamic viscosity (kg/m·s or lbm/ft·s)

11. Applications
Stoke’s Law can be used to determine the surface area of a settling tank
Set the critical velocity equal to the settling velocity of the smallest particle
The overflow rate is equal to the flow rate into the tank divided by the surface area
Setting the overflow rate equal to the critical settling velocity allows time to capture smallest particles of interest

12. Applications OFR = over flow rate (m/s or ft/s )
vc = critical settling velocity (m/s or ft/s)
Q = the flow rate into the basin (m3/s or ft3/s)
A = the surface area of the basin (m2 or ft2)

13. t＝V/Q＝(L×W×H)/Q for rectangular settling basin
Circular basin