1. ENG421 (7c) – Coagulation and Flocculation
Flocculation – design considerations
Flocculation – design example

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. Flocculation Flocculation
- gentle mixing of stabilised colloids to speed agglomeration
some mixing brings colloids together
too much (too vigorous) mixing breaks up flocs
- forms flocs
- physical process that brings particles in contact with one another
- three mechanisms due to range of particle sizes in natural water :
Brownian diffusion
rapid, random motion of particles
aka perikinetic flocculation
particle size < 1μm
Differential settling
particles and flocs of larger size and density settle faster than
small, less dense particles
relative velocity results in particle-particle contact
→ particle size grows
→ particle settles faster, strikes more particles
Fluid shear
during slow mixing, particles follow motion of fluid
different fluid layer velocity causes particle-particle contact
aka orthokinetic flocculation
particle size > 1μm

4. Flocculation – design considerations (1 of 12)
factors to consider :
raw water characteristics
turbidity, particle size distribution, organic and algae content
pH, alkalinity, temperature
plant layout
coagulation, other treatments to follow
headloss
type of flocculator
flow variations

5. Flocculation – design considerations (2 of 12)
factors to consider (cont) :
no chemical addition
mixing intensity is lower, G = s-1
mixing time is longer, t = min
as flocculation proceeds
→ mixing must become gentler (promote floc growth)
→ tapered mixing across flocculation unit
→ flocculation unit is compartmentalised
→ decreasing velocity gradient occurs in steps
→ use of baffle walls to avoid short-circuiting
hydraulic flocculators
higher headloss
mechanical flocculators
preferred where flow variations are high (up to 50%)

6. Flocculation – design considerations (3 of 12)
Types of flocculators
Mechanical mixers
most common
very flexible
paddle flocculators
low to medium mixing energy applications
preferred in conventional treatment system
sedimentation tanks
high turbidity water
velocity gradient 10 – 50 s-1
aka horizontal shaft flocculators
power requirements
CD = 1.1 – 1.5

7. Flocculation – design considerations (4 of 12)
Types of flocculators
Mechanical mixers
propeller or power turbine flocculators
medium to high mixing energy applications
preferred in direct filtration treatment system
strong flocs of small size
velocity gradient 50 – 100 s-1
aka vertical shaft flocculators
power requirements

8. Flocculation – design considerations (5 of 12)
Types of flocculators
Baffled Channel Flocculators
baffles are
vertical (over and under)
horizontal (around the ends)

9. Flocculation – design considerations (6 of 12)
Types of flocculators
Baffled Channel Quick Turn Flocculators

10. Flocculation – design considerations (7 of 12)
Types of flocculators
Baffled Channel Flocculators
advantages
minimal short-circuit problems
inexpensive
produce good flocs
ideal for fairly constant flow conditions
disadvantages
higher headloss
inflexible mixing arrangements
to taper mixing energy
baffles adjusted to slow velocity near outlet
velocity gradient 20 – 30 s-1

11. Flocculation – design considerations (8 of 12)
Types of flocculators
Baffled Channel Flocculators - calculations

12. Flocculation – design considerations (9 of 12)
Types of flocculators
Baffled Channel Flocculators - calculations

13. Flocculation – design considerations (10 of 12)
Types of flocculators
Baffled Channel Flocculators – calculations
Manning’s number is a measure of the roughness of the channel

14. Flocculation – design considerations (10 of 12)
Types of flocculators
Contact Flocculators
water passes through a gravel bed in the contact flocculator
→ a velocity gradient
velocity gradient is a function of :
gravel size
flow rate
cross-sectional area of bed
flow may be upwards or downwards
gravel size is uniformly increased along the flow
→ velocity decreases (greater interstitial spaces)
simple and inexpensive system
used in small water supply systems (<60 L/s)

15. Flocculation – design considerations (11 of 12)
Types of flocculators (cont)
- typical design criteria
mixing time
mixing intensity
also :
number of separate basins
number of stages (tapering)
energy levels

16. Flocculation – design considerations (12 of 12)
Types of flocculators (cont)
- design criteria for different types of flocculators

17. Flocculation – design example (1 of 4)

18. Flocculation – design example (2 of 4)

19. Flocculation – design example (3 of 4)

20. Flocculation – design example (4 of 4)

21. References
Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D ), pages 384 – 415
Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H ) , pages , , 481 – 527
Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K ), pages 74 – 138
MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W ), pages 643 – 777
Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E ), pages
Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P ), pages 26 – 42
Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 324 – 330
Vigneswaran, S., and Visvanathan, C., 1995, Water Treatment process : Simple Options, CRC Press, Boca Raton