1. ENG421 (7ab) – Coagulation and Flocculation
Coagulation – design considerations
Coagulation – design example
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. Coagulation and Flocculation
Large particles may be removed from water by settling (gravity force)
their mass overcomes fluid forces
Fine colloidal particles do not settle due to gravity quickly enough
insufficient mass to overcome fluid forces
e.g. clay, silt, mineral oxides, usually < 10μm
Coagulation
- combines small particles into large aggregates
1. coagulant formation
2. particle destabilisation
3. inter-particle collisions
Flocculation
- gentle mixing of stabilised colloids to speed agglomeration
- forms flocs

4. Coagulation (1 of 10) Coagulation
- combines small particles into large aggregates
1. coagulant formation
2. particle destabilisation
3. inter-particle collisions
- coagulant adsorption on colloidal particles
→ destabilisation begins (in rapid mixing tanks)
→ inter-particle collisions (in flocculation tanks)
- water treatment plants have separate rapid mixing
tank followed by combined coagulation-
flocculation tank

5. Coagulation (2 of 10) Coagulants - used to : destabilise particles
minimise floc break-up
- other desirable characteristics :
low cost
ease of handling
availability
chemical stability during storage
highly insoluble
low residual concentration in treated water
strongly absorbed on particulate surface
- selection and dose of coagulant depends on :
coagulant characteristics
the particulates
water quality

6. Coagulation (3 of 10) Coagulants (cont) Jar Tests - used to :
laboratory, bench-scale test
simulate water treatment processes
determine correct dosage for a given raw water
determine optimal pH
time duration of rapid mixing
time duration of slow mixing
- conducted at water treatment plant regularly part of process and water quality control

7. Coagulation (4 of 10) Coagulants (cont) Jar Tests - procedure :
see procedure in Blackboard
six beakers
six paddles (0 – 100 rpm)
different coagulants
different mixing conditions (conc., rpm, time)
compare results to determine optimal coagulant and conditions

8. Coagulation (5 of 10) Coagulants (cont) Inorganic Coagulants
- salts of aluminium or ferric ions
- hydrolysable cations (i.e. may be hydrated)
- available as sulphate or chloride salts in liquid and solid form
- aluminium sulphate (aka alum) Al2(SO4)3.x(H2O)
x varies from 14 to 20
- alum is readily soluble
- pH influences coagulant-particle interactions
aluminium effective up to pH 6
ferric ion effective up to pH 4

9. Coagulation (6 of 10) Coagulants (cont) Organic Coagulants
- organic polymers
chain of 1, 2 or 3 monomers
chains may be linear or branched
- cationic polymers (positively charged) most common
destabilise colloids by
bridge formation
charge neutralisation
both
less effected by solution conditions than other polymers
can be expensive
some uncertainties regarding chemical impurities left in water

10. Coagulation (7 of 10) Coagulants (cont) Factors effecting Coagulation
- coagulant dosage
- pH
- colloidal concentration
- total organic carbon
- colour
- anions or cations in solution
- mixing effects
- particle surface charge
electrophoretic mobility, zeta potential
- temperature

11. Coagulation (8 of 10) Coagulants (cont) Destabilisation of Colloids
- removal of colloidal and suspended particulates requires reduction in particulate stability
- coagulants destabilise colloids via :
compression of the diffuse layer
adsorption to produce charge neutralisation
enmeshment in a precipitate
adsorption to permit inter-particle bridging
- most naturally occurring colloids are negatively charged
like charges repel
→ small particles remain suspended indefinitely
use divalent (2+) or trivalent (3+) cations
→ reduces negative surface charge
→ forms precipitate to trap particles
- FeCl3 Al2(SO4)3 Fe2(SO4)3 Ca(OH)2

12. Coagulation (9 of 10) Coagulants (cont) Destabilisation of Colloids
- reduction of colloidal electrostatic repulsion

13. Coagulation (10 of 10) Coagulants (cont) Destabilisation of Colloids
- agglomeration by organic polymers

14. Coagulation – design considerations (1 of 10)
factors to consider :
raw water characteristics
turbidity, particle size distribution, organic and algae content
pH, alkalinity, temperature
types of coagulants
coagulant doses
sequence of chemical addition
type of chemical feeder
plant layout
headloss
variation in the flow
some coagulants (metal salts) hydrolyse rapidly
→ rapid adsorption
→ critical to mix vigorously and almost instantaneously
known as flash mixing

15. Coagulation – design considerations (2 of 10)
Mixing devices
- hydraulic mixing
performed by inducing changes in flow and creating turbulence
more economical than mechanical mixing
little or no flexibility in mixing
- mechanical mixing
uses motors and moving parts
may be controlled depending on changes in water flow and raw water quality
- Intensity of Mixing
velocity gradient : symbol G, units s-1
combination of velocity gradient and mixing time (or hydraulic retention time, t)
Gt value depends on device, 300 – 2000
- selection criteria
plant size
flow and quality variations
plant layout
site conditions
reliability, maintenance, and cost requirements

16. Coagulation – design considerations (3 of 10)
Mixing devices (cont)
- hydraulic mixing
Parshall Flume (hydraulic jump) venturi meters
weirs, v-notch weirs orifices
throttled valves, swirl chambers

17. Coagulation – design considerations (4 of 10)
Mixing devices (cont)
- hydraulic mixing
coagulant introduce just before mixing device
rapid mixing achieved due to turbulence
turbulence is a function of flow rate
→ variations in flow affects mixing
(considerably)
ideal for relatively constant flow systems
residence times ~ 2 seconds
G 400 – 800 s-1

18. Coagulation – design considerations (5 of 10)
Mixing devices (cont)
- mechanical mixing
motionless static mixer in-line blender
pump injection mechanical mixer

19. Coagulation – design considerations (6 of 10)
Mixing devices (cont)
- mechanical mixing : power relationships
k values

20. Coagulation – design considerations (7 of 10)
Mixing devices (cont)
- advantages and disadvantages

21. Coagulation – design considerations (8 of 10)
Mixing devices (cont)
- advantages and disadvantages

22. Coagulation – design considerations (9 of 10)
Mixing devices (cont)
- advantages and disadvantages

23. Coagulation – design considerations (10 of 10)
Mixing devices (cont)
- design criteria
mixing time
mixing intensity

24. Coagulation – design example (1 of 4)
N.B. – 1.32 m3/s is equivalent to water demand of 250,000 people

25. Coagulation – design example (2 of 4)

26. Coagulation – design example (3 of 4)

27. Coagulation – design example (4 of 4)
Design Summary :
Typical values Design Value
velocity gradient, mechanical mixer (s-1) –
hydraulic retention time (s) –
Gt –
mixing chamber (cylindrical) diameter 1.2m
height (depth) 1.2m
mixing rotor housed within 3m x 3m x 4m tank (10’ x 10’ x 13.5’ in figure)

28. 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