1. ENG421 (8ab) – Sedimentation
Theory of sedimentation
Sedimentation – design considerations
Sedimentation – settling example
Types of sedimentation tanks
- conventional sedimentation tanks
- high rate settlers
- solid contact clarifiers
Dissolved Air Flotation

2. General Water Treatment Technologies (Week 4)
Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

3. Sedimentation
- Suspended particles that are heavier than water are separated by gravitational settling
- Suspended particles :
pre-existing solids
precipitates formed by Coagulation and Flocculation
- Sedimentation tends to follow Coagulation and Flocculation
but all three can occur in same tank
- Sedimentation is aka settling, and clarification
- Sedimentation basin is aka
sedimentation tank
settling basin
settling tank
clarifier
- Smaller particles can be removed by :
increasing retention time
→ larger tank and more expense
artificially increasing size
Coagulation and Flocculation
complete removal is not possible
use filter beds after sedimentation tanks (Week 9)

4. Theory of Sedimentation (1 of 3)
Settling tanks operate using gravity only
causing aggregated particles to descend to bottom of tank
Particles settling in four ways
depends on characteristics and concentrations of particles :
- Type-I settling
aka discrete settling
negligible natural aggregation (e.g. grains of sand)
particles descend at constant rate as individual entities
- Type-II settling
aka flocculant settling
flocculation occurs due to particle collision
particles adhere to each other → particle growth → higher rate of descent
- Type-III settling
aka hindered or zone settling
particles settle as a mass (not as discrete particles)
inter-particle forces hold particles in fixed position → particles settle as a block
fluid flow through particle interstices controls descent
- Type-IV settling
aka compression settling
only occurs in bottom layers of sedimentation tank
sludge layers settle under their own weight
water is squeezed out of flocs → particle volume may decrease

5. Theory of Sedimentation (2 of 3)

6. Theory of Sedimentation (3 of 3)

7. Sedimentation – design considerations (1 of 13)
shape :
usually large and rectangular
inexpensive
easy to design
easy to add internal modifications
e.g. plates and tubes to increase settling
may add additional tanks within plant layout
create a “battery” of basins (sharing common walls)
circular and square sedimentation tanks also used
Efficiency of sedimentation system
function of available area for sedimentation
as water demand increases, must increase surface area

8. Sedimentation – design considerations (2 of 13)
Rectangular sedimentation tank – with mechanical sludge collection

9. Sedimentation – design considerations (3 of 13)
Circular sedimentation tank

10. Sedimentation – design considerations (4 of 13)
factors to consider :
nature and amount of suspended particles
variation in flow rate
method of sludge removal
settlement characteristics of particles and flocs
type and configuration of tank
need for high rate settling
selection of high rate arrangements
inlet and outlet arrangements
obtain an even distribution of flow
minimise eddy and current formation
minimise headloss
ease of scum removal
hydraulic retention time
cross-flow velocity
depth of tank
avoid :
short circuiting
resuspension of sludge in tank

11. Sedimentation – design considerations (5 of 13)
typical inlet and outlet arrangements

12. Sedimentation – design considerations (6 of 13)
Settling velocity of particles
critical in sedimentation plant design
consider vertical movement and horizontal movement
may be predicted mathematically
may used bench scale settling tests
safety factor used to cover :
type of coagulant
mixing conditions in flocculation tank
particle concentration
design parameter surface loading rate (or over flow
rate) is compared with settling velocity for effective
sedimentation

13. Sedimentation – design considerations (7 of 13)
Settling velocity of particles (cont)

14. Sedimentation – design considerations (8 of 13)
Settling velocity of particles (cont)
inlet outlet
particle X is smaller than particle Y

15. Sedimentation – design considerations (9 of 13)
Settling velocity of particles (cont)
if Re < 0.3, then CD ~ 24/Re
assuming ~
terminal velocity may be used as starting point for calculations

16. Sedimentation – design considerations (10 of 13)
Settling velocity of particles – relevant formula
if particle Y is to be settled,
L, H, and W dimensions are chosen to obtain diagonal path (critical removal)

17. Sedimentation – design considerations (11 of 13)
Settling velocity of particles – relevant formula (cont)
so all particles equal to or greater than diameter of particle Y will be removed
particles with diameter less than diameter of particle Y will only be removed partially
depends on where smaller particles are in inlet zone

18. Sedimentation – design considerations (12 of 13)
Settling velocity of particles – relevant formula (cont)
assume uniform distribution of particles across depth
→ particles with diameter dx or less will be removed
if they enter below point B
removal fraction of particles with particles with diameter dx

19. Sedimentation – design considerations (13 of 13)
Settling velocity of particles – relevant formula (cont)
allowing for the distribution of particle sizes and
settling velocities found in water, the fraction
of all particles removed is given by :

20. Sedimentation – settling example (1 of 6)

21. Sedimentation – settling example (2 of 6)

22. Sedimentation – settling example (3 of 6)
Re < 0.3, therefore settling velocity = terminal velocity

23. Sedimentation – settling example (4 of 6)
using data in table, plot
“fraction with settling velocity lower than terminal settling velocity”
against “terminal settling velocity”

24. Sedimentation – settling example (5 of 6)
Overflow rate vo = m/d
= x 1000 /(24 x 3600)
= mm/s
from cumulative distribution plot, Fy =
use steps of 0.04 on vertical axis (fraction with settling velocity less
than terminal settling velocity), and determine v.∆F

25. Sedimentation – settling example (6 of 6)=
Therefore, total mass fraction removed is 89.7%

26. Types of Sedimentation Tanks
three main types of sedimentation tanks
- conventional sedimentation tanks
horizontal flow long rectangular tanks
horizontal flow centre feed circular tanks
horizontal flow peripheral feed circular tanks
horizontal flow centre feed square tanks
- high rate settlers
multistoried trays
tube and plate settling
- solid contact clarifiers
reactor clarifiers
sludge blanket clarifiers

27. Conventional Sedimentation Tanks (1 of 5)
- horizontal flow long rectangular tanks
flocculated water uniformly distributed across one end of tank
water flow horizontally
effluent (out flowing water) overflows into long finger launders (troughs)
launders often have V-notch weirs
typical retention time 3 hours
diffuser or baffle walls may be installed
achieve uniform flow
avoid short circuiting
walls may contain holes
length : width > 4:1
length : water depth 15:1
minimum depth 2 m
tanks with mechanical sludge collection : minimum depth 3.5 m
sludge removal system : operating velocity ~ 1 m/min

28. Conventional Sedimentation Tanks (2 of 5)
- horizontal flow centre feed circular tanks
inlet of tank is at the centre
flow directed through a baffle towards tank periphery
cylindrical baffles : diameters 10 – 20 % of basin diameter
extend 1 – 2 m below water surface
outlets usually along entire circumference of tank
may be weirs or holes
weir length may be increased by radial finger launders
tank side depth 2 – 4 m
tank bottom slope 1/12 – 1/24

29. Conventional Sedimentation Tanks (3 of 5)
- horizontal flow peripheral feed circular tanks
inflow is distributed at periphery by a ring inlet
inflow released near bottom of peripheral wall
potential for short circuiting
flocs may rise vertically from inlet direct to outlet

30. Conventional Sedimentation Tanks (4 of 5)
- horizontal flow centre feed square tanks
designed to perform in similar way to circular tanks
may be difficult to collect sludge from corners with mechanical sweepers
fit corners with corner sweeping mechanism
construct base in corner region with adequate slope for sludge collection

31. Conventional Sedimentation Tanks (5 of 6)
- design criteria

32. High Rate Settlers (1 of 4)
- multistoried trays
considered as a modification to existing plants
extra trays improve efficiency as particles have less distance to fall
surfaces may be connected in parallel
flow divided between them
for same flow : cross-flow velocity is not changed
effective surface area is increased
poor flow distribution
difficult sludge removal
rarely recommended

33. High Rate Settlers (2 of 4)
- tube and plate settling
inserting tubes and plates improves efficiency
particles have less distance to fall
“horizontal” tubes or plates
5o to horizontal
sludge accumulates at base of tube (backwash to sludge storage)
requires regular maintenance
not feasible for high demand
systems (> 30 million L/day)
inclined tube settlers
o to horizontal
sludge slides out at > 40o
cross-section of tube 50 – 100 mm
dynamic sludge removal
→ good for high demand

34. High Rate Settlers (3 of 4)
- tube and plate settling
inclined tube settlers (cont)
direction of flow relative to sedimentation may be
countercurrent
flocs move in tube against direction of flow
beneficial as flocs may coalesce (Type-II)
cocurrent
cross-flow
challenges :
achieving settling efficiencies close to theoretical
equal flow in each tube
advantage :
current tank efficiency
greatly improved

35. High Rate Settlers (4 of 4)
- design criteria

36. Solid Contact Clarifiers (1 of 3)
- reactor clarifiers
incorporate flocculation and sedimentation in one unit
flocculation zone in central compartment
20 – 30 minute contact time (minimum)
settling zone in outer compartment
slurry circulated (up to five times raw water flow)
improves efficiency
disadvantage :
severe hydraulic or water quality shock causes problems

37. Solid Contact Clarifiers (2 of 3)
- sludge blanket clarifiers
similar to reactor clarifiers
flocculation zone in central compartment
in lower portion of settling zone :
rising particles and flocs contact suspended sludge layer (blanket)
agglomeration occurs within blanket
sludge blanket 2 – 3 m below water surface
(appropriate fluidising velocity)
advantages
ease of operation
reduced space requirement
(no flocculation tank)
best for constant flow rate with high tubidity
disadvantage
start-up of unit is time consuming

38. Solid Contact Clarifiers (3 of 3)
- design criteria

39. Sedimentation – useful design data

40. Sedimentation – design example (1 of 5)

41. Sedimentation – design example (2 of 5)

42. Sedimentation – design example (3 of 5)

43. Sedimentation – design example (4 of 5)

44. Sedimentation – design example (5 of 5)
Froude number indicates backmixing may occur if too low as flow not dominated by horizontal flow (should be > 10-5)
R is hydraulic radius (= area/perimeter), Re should be less than 10,000 (some suggest <20,000 is acceptable)

45. References
Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D ), pages 291 – 319
Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H ) , pages 139 – 160
Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K ), pages 139 – 194
Metcalf & Eddy Inc., 1991, Wastewater Engineering : Treatment, Disposal, and Reuse, 3rd ed., McGraw-Hill, New York
MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W ), pages 779 – 865
Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 330 – 343