CoagulationCoagulation

CE 547CE 547

OverviewOverviewTurbidity in surface waters is caused by Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, or organic by colloidal forms of Fe, Mn, or organic compounds.compounds.

Colloidal ParticlesColloidal Particles Difficult to settleDifficult to settle Pass through small pores of conventional filtersPass through small pores of conventional filters

How to remove colloidal particles?How to remove colloidal particles? By aggregation (making them bigger sizes)By aggregation (making them bigger sizes)

Why aggregation is difficult?Why aggregation is difficult? Small size of particlesSmall size of particles Physical and electrical forcesPhysical and electrical forces

How to aggregate?How to aggregate? Use of chemical agentsUse of chemical agents

Chemical CoagulationChemical Coagulation

It is the process of destroying the stabilizing It is the process of destroying the stabilizing forces and causing aggregationforces and causing aggregation

Aggregation occurs in two steps:Aggregation occurs in two steps:

1. 1. reduction or elimination of the inter-reduction or elimination of the inter-particle forces responsible for stability particle forces responsible for stability (by addition of chemicals)(by addition of chemicals)

2. 2. collision due tocollision due to molecular motionmolecular motion mechanical mixing (using rapid or flash mixing mechanical mixing (using rapid or flash mixing

for very short time, less than 1 minute)for very short time, less than 1 minute)

After destabilization, gentle mixing is After destabilization, gentle mixing is provided to increase the rate of particle provided to increase the rate of particle collision without breaking the aggregates collision without breaking the aggregates or flocs (this process is called flocculation)or flocs (this process is called flocculation)

Stability of ParticulatesStability of Particulates

Colloidal particulates remain in Colloidal particulates remain in suspension for very long periods due suspension for very long periods due to their stability (is it possible to give to their stability (is it possible to give such particles sufficient time to such particles sufficient time to settle?)settle?)

Particulate Particulate CharacteristicsCharacteristics

1. Size1. Size colloidal materialscolloidal materials

upper limit of ≈ 1 upper limit of ≈ 1 mm lower limit of ≈ 5 nmlower limit of ≈ 5 nm

suspended solidssuspended solids larger than ≈ 0.5 larger than ≈ 0.5 mm

Particles larger than 5 nm are in Particles larger than 5 nm are in suspensionsuspension

2. Nature of solid-water interface2. Nature of solid-water interface HydrophobicHydrophobic

Well defined interfaceWell defined interface Low affinity for water moleculesLow affinity for water molecules Thermodynamically unstable and will Thermodynamically unstable and will

aggregate over time (irreversible)aggregate over time (irreversible) Generally inorganicGenerally inorganic

Hydrophilic (Fig 12.1)Hydrophilic (Fig 12.1) Lack of clear interfaceLack of clear interface Generally organic matter such as Generally organic matter such as

proteinsproteins Can aggregate (but reversible)Can aggregate (but reversible)

Mechanisms of StabilityMechanisms of Stability

The main mechanism of particle The main mechanism of particle stability is the electrical repulsionstability is the electrical repulsion

Presence of adsorbed water molecules Presence of adsorbed water molecules (this will provide barrier to successful (this will provide barrier to successful collision)collision)

How electrical charges exist at particle How electrical charges exist at particle surfaces? There are three principle ways:surfaces? There are three principle ways:

1. Crystal Imperfections1. Crystal ImperfectionsSilicon atoms in crystals can be replaced by Silicon atoms in crystals can be replaced by atoms with lower valence (such as Al) giving atoms with lower valence (such as Al) giving excess negative charge to the crystal material.excess negative charge to the crystal material.

2. Preferential Adsorption of Specific Ions2. Preferential Adsorption of Specific IonsWhen particles are dispersed in water, soluble When particles are dispersed in water, soluble polyelectrolytes of natural origin may adsorb polyelectrolytes of natural origin may adsorb on particle surfaces (for example, fulvi acid, -on particle surfaces (for example, fulvi acid, -vely charged, can be adsorbed on CaCOvely charged, can be adsorbed on CaCO33, , +vely charged)+vely charged)

3. Specific Chemical Reactions of 3. Specific Chemical Reactions of Inorganic Groups on Particulate Inorganic Groups on Particulate SurfacesSurfaces

many particulate surfaces contain many particulate surfaces contain inorganic groups such as hydroxyl or inorganic groups such as hydroxyl or carboxyl functional groups which carboxyl functional groups which dissociate in water producing a dissociate in water producing a surface electrical charge (that surface electrical charge (that depends on the pH of the solution)depends on the pH of the solution)

Origin of the Double Origin of the Double LayerLayer

When particles are dispersed in water, ions with When particles are dispersed in water, ions with opposite charge to the particle surface opposite charge to the particle surface accumulate closer to the particle to produce accumulate closer to the particle to produce electro-neutrality.electro-neutrality.

+

+

+ +

+

--

-

-

--

--

-

--

-

-

-

This accumulation is opposed by the This accumulation is opposed by the tendency of ions to diffuse in the direction of tendency of ions to diffuse in the direction of decreasing concentrationdecreasing concentration

The result is a diffuse cloud of ions The result is a diffuse cloud of ions surrounding the particle, which is known as surrounding the particle, which is known as the the electrical double layer (Fig 12.3)electrical double layer (Fig 12.3)

-+

Diffusion

Electro-staticAttraction

As ionic strength (TDS) increases, this As ionic strength (TDS) increases, this will compress the diffuse layer.will compress the diffuse layer.

Mechanisms of Mechanisms of DestabilizationDestabilization

Removal of colloidal and suspended Removal of colloidal and suspended particulates depends on reduction in particulates depends on reduction in particulate stability. particulate stability.

Destabilization can be achieved through:Destabilization can be achieved through: compression of the electrical double layercompression of the electrical double layer electrostatic attractionelectrostatic attraction interparticle bridginginterparticle bridging enmeshment or sweep flocenmeshment or sweep floc

Double Layer Double Layer CompressionCompression

Increasing the ionic strength will compress Increasing the ionic strength will compress the double layer, causing a decrease in its the double layer, causing a decrease in its thickness. This will result in decreasing thickness. This will result in decreasing the Zeta Potentialthe Zeta Potential

The amount of dissolved ions that produce The amount of dissolved ions that produce rapid coagulation is defined as the Critical rapid coagulation is defined as the Critical Coagulation Concentration (CCC), which Coagulation Concentration (CCC), which depends on:depends on:

type of particulatetype of particulate type of dissolved ionstype of dissolved ions

for hydrophobic particles, CCC is for hydrophobic particles, CCC is inversely proportional to the sixth inversely proportional to the sixth power of the charge on the ion. So, power of the charge on the ion. So, for mono-, di-, and trivalent ions, the for mono-, di-, and trivalent ions, the CCC values are in the ratio:CCC values are in the ratio:

As an Example: 3000 mg/l NaCl is As an Example: 3000 mg/l NaCl is equivalent to 44 mg/l CaClequivalent to 44 mg/l CaCl22

14.0:6.1:100

36

1:

26

1:1

or

Electrostatic Attraction Electrostatic Attraction This occurs when surfaces are oppositely This occurs when surfaces are oppositely charged, which is promoted by the charged, which is promoted by the adsorption of specific ions on the surface of adsorption of specific ions on the surface of the particle. Particles in natural waters the particle. Particles in natural waters exhibit both +ve and –ve charges based on exhibit both +ve and –ve charges based on the pH of the water.the pH of the water.

Zero Point of Charge (ZPC): is the pH Zero Point of Charge (ZPC): is the pH corresponding to a surface charge of zero.corresponding to a surface charge of zero.

Above ZPC, surface charge is –ve Above ZPC, surface charge is –ve (anionic)(anionic)

Below ZPC, surface charge is +veBelow ZPC, surface charge is +ve

Reduction of surface charge can be Reduction of surface charge can be achieved by:achieved by:

pH alterationpH alteration addition of specifically adsorbed ionsaddition of specifically adsorbed ions

Interparticle BridgingInterparticle Bridging

Long-chain polymers carrying –ve Long-chain polymers carrying –ve charges can form bridges between charges can form bridges between particle, thus destabilizing the particle, thus destabilizing the suspension. This mechanism was suspension. This mechanism was shown to be the major mechanism shown to be the major mechanism controlling the aggregation of controlling the aggregation of bacterial and alga suspensions.bacterial and alga suspensions.

Enmeshment (Sweep Enmeshment (Sweep Floc) Floc)

This mechanism is predominant in This mechanism is predominant in water treatment where pH values are water treatment where pH values are between 6 and 8 and Al or Fe salts are between 6 and 8 and Al or Fe salts are used at concentrations exceeding used at concentrations exceeding saturation with respect to amorphous saturation with respect to amorphous metal hydroxide solid that is formed. metal hydroxide solid that is formed. In this mechanism, finely divided In this mechanism, finely divided particles are entrapped in the particles are entrapped in the amorphous precipitate formed.amorphous precipitate formed.

Chemistry of CoagulationChemistry of Coagulation

Functions of CoagulantsFunctions of Coagulants destabilizationdestabilization strengthening of flocs to reduce floc breakupstrengthening of flocs to reduce floc breakup

Selection of CoagulantsSelection of Coagulants low costlow cost availabilityavailability stability during storagestability during storage ease of handlingease of handling must form highly insoluble compounds to must form highly insoluble compounds to

minimize the concentration of soluble minimize the concentration of soluble residualsresiduals

Selection of Type and Dose of Selection of Type and Dose of Coagulants Depends on:Coagulants Depends on:

characteristics of the coagulantcharacteristics of the coagulant characteristics of the particlescharacteristics of the particles characteristics of the watercharacteristics of the water

Jar test is used to investigate:Jar test is used to investigate: what coagulant to be usedwhat coagulant to be used optimum pHoptimum pH optimum doseoptimum dose

In selection of coagulants, cost and quantity In selection of coagulants, cost and quantity and dewaterability of produced solids and dewaterability of produced solids (sludge) should be taken into consideration.(sludge) should be taken into consideration.

Full-scale testing is necessary to Full-scale testing is necessary to determine:determine:

optimum doseoptimum dose optimum coagulants combination (usually optimum coagulants combination (usually

inorganic coagulant + polyelectrolyte)inorganic coagulant + polyelectrolyte)

Inorganic CoagulantsInorganic Coagulants Aluminum saltsAluminum salts Ferric saltsFerric salts Usually in forms of sulfates or chlorides and Usually in forms of sulfates or chlorides and

available in solid and liquid formsavailable in solid and liquid forms Aluminum or Ferric Ions React With OHAluminum or Ferric Ions React With OH--, SO, SO44

2-2-, , or POor PO44

3-3- To Form Soluble and Insoluble To Form Soluble and Insoluble Products.Products.

One mole of trivalent ion produces one mole of One mole of trivalent ion produces one mole of the metal hydroxide. So, 1 mg of aluminum the metal hydroxide. So, 1 mg of aluminum sulfate (alum) [Alsulfate (alum) [Al22(SO(SO44))3 3 . 14H. 14H22O] produces about O] produces about 0.26 mg of insoluble Al(OH)0.26 mg of insoluble Al(OH)33 and consumes and consumes about 0.5 mg of alkalinity expressed as CaCOabout 0.5 mg of alkalinity expressed as CaCO33..

Similarly, 1 mg of ferric sulfate [FeSimilarly, 1 mg of ferric sulfate [Fe22(SO(SO44))33] ] produces about 0.5 mg of Fe(OH)produces about 0.5 mg of Fe(OH)33 as precipitate as precipitate and consumes about 0.75 mg alkalinity (as and consumes about 0.75 mg alkalinity (as CaCOCaCO33))

Actual amount of precipitate and acidity (HActual amount of precipitate and acidity (H++) ) formed depend on system pH and concentration formed depend on system pH and concentration of reactive liquids.of reactive liquids.

HOHFeOHFe

HOHAlOHAl

S

S

33

33

)(323

323

The rate of reaction of coagulants with The rate of reaction of coagulants with water depends on:water depends on:

pH (plays a dominant role)pH (plays a dominant role) ionic species in waterionic species in water temperaturetemperature type and concentration of particlestype and concentration of particles concentration of coagulantconcentration of coagulant mixing condition at the point of mixing condition at the point of

coagulant additioncoagulant addition

AluminumAluminum

At pH values less than 6:At pH values less than 6: +vely charged Al species remain in +vely charged Al species remain in

solution long enough to interact with solution long enough to interact with particles and destabilize them by particles and destabilize them by charge neutralizationcharge neutralization

turbidity-causing particles are turbidity-causing particles are destabilized by adsorptiondestabilized by adsorption

FerricFerric

Similar to Aluminum, but at pH below 4. Similar to Aluminum, but at pH below 4.

Above pH 6 for Aluminum and pH 4 for Above pH 6 for Aluminum and pH 4 for ferric, formation of amorphous ferric, formation of amorphous precipitates occurs reapidly causing precipitates occurs reapidly causing entrapment of particles “sweep floc”. This entrapment of particles “sweep floc”. This sweep floc mechanism requires greater sweep floc mechanism requires greater quantity of coagulant than charge quantity of coagulant than charge neutralization which will result in neutralization which will result in producing more sludge.producing more sludge.

Organic CoagulantsOrganic Coagulants

Organic polymers are used as coagulants Organic polymers are used as coagulants and are termed as “ployelectrlytes”. They and are termed as “ployelectrlytes”. They are used as:are used as:

primary coagulantsprimary coagulants coagulants or filter aidscoagulants or filter aids sludge conditionerssludge conditioners

The use of polymers is restricted in water The use of polymers is restricted in water treatment due to:treatment due to:

high costhigh cost uncertainties regarding chemical impuritiesuncertainties regarding chemical impurities

PolymersPolymers

natural (sodium alginate and chitosan; natural (sodium alginate and chitosan; very high cost)very high cost)

synthetic (predominant in water synthetic (predominant in water treatment)treatment)

Functions of PolymersFunctions of Polymers destabilization of particlesdestabilization of particles form larger and more shear-resistant flocsform larger and more shear-resistant flocs

ChitosanChitosan

DestabilizationDestabilization

charge neutralization (-vely charged charge neutralization (-vely charged particles can be destabilized by particles can be destabilized by cationic polymers)cationic polymers)

polymer bridging (-vely charged polymer bridging (-vely charged particles can be destabilized by anionic particles can be destabilized by anionic polymers under appropriate conditions. polymers under appropriate conditions. The mechanism will result in: The mechanism will result in: increase floc sizeincrease floc size increase floc strengthincrease floc strength