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ENG421 (7ab) – Coagulation and Flocculation

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Presentation on theme: "ENG421 (7ab) – Coagulation and Flocculation"— Presentation transcript:

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


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