1. Flocculation and Coagulation
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2. Flocculation and Coagulation
Water Treatment
Agglomeration of solids prior to sedimentation and rapid sand filtration
Wastewater Treatment
Physico-chemical treatment of primary and secondary effluents and also raw wastewater
Treatment of individual wastewater where it is necessary to coalesce solids when they are present in appreciable quantities
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3. Theory of Aggregation of Colloidal Particles
Basic properties
<10 m
Almost non-settling
Dispersions sols or emulsions
Large surface area
Electrostatic charge
Hydrophobic (tendency to repel water
Hydrophilic (affinity for water)
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4. Coagulation of Colloids
(Destabilisation)
Forces acting on colloidal particles (<10 m)
Some cause in attraction between particles and cause repulsion
electrostatic
van der Waals
Hydrodynamic lubrication force
Length scale of action is up to say 100 nm
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5. Ways of reducing the repulsion between particles
reduce the zeta potential ( ), charge neutralisation
induce aggregation by inter-particle bridging using polyelectrolytes
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6. Theory of Inter-particle Interaction
DLVO Theory(after Derjaguin, Landau, Verwey and Overbeek)
The total energy of interaction,
, is considered to be the sum
where
= energy of repulsion
= energy of attraction
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7. DLVO Theory(after Derjaguin, Landau, Verwey and Overbeek)
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9. Electrical Double Layers
When colloidal particles are dispersed in aqueous electrolyte, they acquire an electrical charge this leads to the electrostatic repulsion between particles. The most water based system most natural particles have a negative charge around neutral pH.
The charged particles are surrounded by ions.
Ions having the same charge are called co-ions
Ions having the opposite charge are called counter-ions
There is electrostatic repulsion between the particles and co-ions and attraction between the particles and counter-ions.
The result is that each particle is surrounded by a diffuse electrical double layer.
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10. Electrical Double Layers
Depending on the valency (charge number) of the counter-ions some will be adsorbed onto the particle surface.
These form the innermost layer of ions, which is known as the fixed layer or Stern layer.
Adsorbed ions cause charge neutralisation.
Outside the fixed layer is the diffuse layer or Gouy layer of mobile co- and counter-ions.
Overall the electrical neutrality of the bulk electrolyte is maintained as there are equal numbers of positive and negative charges.
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11. Electrical Double Layers
The Diffuse Double Layer
The diffuse double layer consists of an outer part where there are mainly co-ions and an inner part where the are mainly counter ions..
The layers do not consist entirely of either co-ions or counter-ions as there is always some Brownian diffusion of ions through the double layer, although on average the double layer is thought to be in the steady state.
The diffuse double layer gives rise to an electrokinetic property known as the zeta potential.
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12. Electrical Double Layers
The Zeta Potential
If a colloidal particle has an electrical surface charge it will have a corresponding electrical surface potential. Typically the magnitude of this surface potential is between 20 – 100 mV.
The potential decays with distance from the particle surface until it is zero in the bulk electrolyte.
The rate of decay depends on the concentration of the electrolyte. (higher concentration more rapid decay)
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13. Electrical Double Layers
The zeta potential is related to the stability of colloids.
STABLE – No tendency to coagulate
UNSTABLE – Coagulation can take place
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14. Coagulation and Flocculation
These terms describe the addition of a floc-forming chemical to water containing suspended solids to induce aggregation of the colloidal material that is present. Note that if excess chemical is used then a precipitate is formed and this can enmesh or combine with both the non-settling and slow settling solids to produce a rapid-settling floc.
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15. Coagulation and Flocculation
Coagulation refers to the addition and rapid mixing of a coagulant, the resultant destabilisation of the colloid and fine suspended solids and the initial aggregation of the destabilised particles.
Flocculation refers to the slow stirring or gentle agitation of the treated suspension together with the subsequent grow in aggregate size to from fast settling flocs.
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16. Mechanisms Addition of inorganic chemicals (Aluminium and Iron salts)
- coagulant salt dissolves in water and dissolved ions are formed
- metal ions undergo hydrolysis and polymerise forming positively charged hydroxometallic complexes
e.g. Al2(SO4)3  
Al6(OH)135+
and
forms
Al7(OH)174+
etc.
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17. Mechanisms
These complexes are:
polyvalent
highly positively charged
adsorbed onto the surfaces of negatively charged colloids
The effect is to reduce the zeta potential by neutralising the surface charge and consequently removing the electrostatic repulsion from between the particles – the particles are destabilised.
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18. Mechanisms
If more coagulant is added then the polymerisation is stopped and the metal hydroxide is precipitated. This can enhance the process as the precipitate can enmesh the suspended particles.
In some wastewater treatments a significant excess is used.
In both water and wastewater treatment the optimum dose of chemical should be sought. This is usually achieved by jar testing.
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19. Inter-particle Bridging
Mechanisms
Addition of organic polymers (polyelectrolytes, e.g. CTAB, SDS, Magna Floc)
Inter-particle Bridging
Charge neutralisation
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20. Mechanisms Inter-particle bridging
Polymers – very high molecular weight chain molecules
Polymers have ionisable groups ( e.g. amino, sulphonic etc.) which find reactive sites on the surface of the colloidal particles
Several particles may be bound by a single polymer molecule
Optimum conditions for bridging occur when the colloid is half covered by segments of polymer
Excess polymer should be avoided as this will restabilise the colloidal particles (i.e. flocculation is destroyed)
Jar testing is required to determine the optimum dose
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32. Initial adsorption at the optimum polymer dose
BRIDGING MECHANISM +
Polymer
Particle
Destabilised Particle
Initial adsorption at the optimum polymer dose
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33. Destabilsed particles
BRIDGING MECHANISM
Floc Formed
Destabilsed particles
Floc formation
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34. Destabilised Particle Restabilised Particle
BRIDGING MECHANISM
Destabilised Particle
Restabilised Particle
Secondary adsorption of polymer
No contact with vacant sites on another particle
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35. Excess polymer dose uses up all available adsorption sites
BRIDGING MECHANISM
Stable particle
Excess polymer dose uses up all available adsorption sites
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36. Aggregate, large flocculated particle
BRIDGING MECHANISM
Floc fragments
Aggregate, large flocculated particle
Prolonged or over-intense agitation leads to floc rupture
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37. BRIDGING MECHANISM
Secondary adsorption of polymer following floc rupture leads to restabilisation.
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38. END
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