Koagulasi Dan Flokulasi Prinsip

PIERO M. ARMENANTENJIT

Coagulation andFlocculation


Solutions, Colloidal Dispersions andSuspensions

Solids can be dispersed in liquids under severalforms. The nature of such dispersions dependson the size of the solid particles.

In general, one the following states areconsidered:

• Solutions

• Colloidal dispersions

• Suspensions


Characteristics of Solutions,Colloidal Dispersions and

Suspensions

System ParticleSize

SeparationMethod

ParticleVisibility

ParticleMovement

Solution < 20 Å Membrane Invisible Kinetic

Colloidaldispersion

20 - 2000Å

Ultra-filtration

Ultra-microscope

Brownian

Suspension > 2000 Å Filtration Microscope Convective

1 Å = 10-4 µm = 10-10 m


Motivation for the Use of Coagulationin Wastewater Treatment

• Wastewaters often contain pollutants that arepresent is colloidal form

• In such cases the colloidal suspension maycontain:

- organic materials

- metal oxides

- insoluble toxic compounds

- stable emulsions

- material producing turbidity


Motivation for the Use of Coagulationin Wastewater Treatment (cont.'d)• This material must be removed prior to

discharge

• Because of the nature of the colloidalsuspension these particles will not sedimentor be separated with conventional physicalmethods (such as filtration or settling) unlessthey are agglomerated through coagulation


Coagulation• Coagulation is the process by which colloidal

particles and very fine solid suspensionsinitially present in a wastewater are combinedinto larger agglomerates that can be separatedvia sedimentation, flocculation, filtration,centrifugation or other separation methods

• Coagulation is commonly achieved by addingdifferent types of chemicals (coagulants) to thewastewater to promote destabilization of thecolloid dispersion and agglomeration of theresulting individual colloidal particles


Additional Benefits of Coagulation• The addition of some common coagulants to a

wastewater not only produces coagulation ofcolloids but also typically results in theprecipitation of soluble compounds, such asphosphates, that can be present in thewastewater.

• In addition, coagulation can also produce theremoval of particles larger that colloidalparticles due to the entrapment of suchparticles in the flocs formed duringcoagulation


Effects of Coagulation Treatment onWastewater

Primary Effect

• Agglomeration and eventual removal ofcolloids (primarily responsible for wastewaterturbidity)

Secondary Effects

• Precipitation of some chemical species insolution

• Agglomeration of larger particles in the floc


Coagulation vs. Flocculation• Although the words "coagulation" and

"flocculation" are often used interchangeablythey refer to two distinct processes

• Coagulation indicates the process throughwhich colloidal particles and very fine solidsuspensions are destabilized so that they canbegin to agglomerate if the conditions areappropriate

• Flocculation refers to the process by whichdestabilized particles actually conglomerateinto larger aggregates so that they can beseparated from the wastewater


Stability of Colloids in Wastewater• The colloids commonly found in wastewater are

stable because of the electrical charge that theycarry

• The charge of colloids can be positive ornegative. However, most colloidal particles inwastewater have a negative charge

• Such a charge typically comes from:- lattice imperfections- ionizable groups that become ionic species in

water (e.g., amino, or hydroxyl groups)- ionic species that can become adsorbed on

the surface of the colloid


Types of ColloidsColloids are commonly classified as:

• hydrophilic (e.g., proteins)

• hydrophobic (e.g., clays, metal oxides)


Hydrophilic Colloids• Hydrophilic colloids are typically formed by

large organic molecules that become hydrated(solvated) when they are in the presence ofwater

• These molecules are thermodynamically stablein their solvated form

• The charge in such molecules originates fromthe presence of ionizable groups on themolecule that transform the molecule in a"macro-ion" when placed in solution


Hydrophilic Colloids (continued)• As a result of these charges hydrophilic

colloidal particles are significantly hydratedwhen placed in solution

• Agglomeration of hydrophilic colloids typicallyinvolves the addition of significant amounts ofions which compete with the colloids for watermolecules thus resulting in the dehydration ofthe colloidal particles ("salting out" of thecolloid)


Hydrophobic Colloids• Hydrophobic colloids are made of small

colloidal particles having little or no affinity forwater (the solvent)

• Their stability in due to the presence of acharge which attracts other ionic speciespresent in water and results in the formation ofan electrically charged layer around thecolloidal particles

• Colloidal dispersions are thermodynamicallyunstable. If the charge layer is removed theseparticles tend to agglomerate spontaneouslyand can be removed from the wastewater


Double Layer Associated withColloidal Particles

• If a colloidal particle is electrically charged itattracts ions and other colloidal particles ofopposite sign

• These ions are tightly attached by electrostaticforces to the colloidal particle forming a firstinner layer of charges. This layer is oftenreferred to as the Stern layer and has athickness of a single hydrated ionic layer

• The particle and the attached ions of oppositesign form an electric double layer


Double Layer Associated withColloidal Particles (continued)

• Additional ions of opposite sign to that of thecolloidal particle also accumulate next to the Sternlayer. They form the diffuse layer

• Within the diffuse layer there is typically anotherlayer of ions which are not so tightly attached tothe particle as the Stern layer, but which cannot beremoved by the presence of any external velocitygradient either

• These ions bound to the particle even as theparticle moves delimit the shear plane, i.e., theplane of ions which are unaffected by fluid motion


Distribution of Charges in a Double Layer

++

+++

++

--

-

-

-

-

-

+

+

+ -

-

-

-

-

-

+

+

+

+

+

- ----

--

ShearPlane

DiffuseLayer

SternLayer

ColloidalParticle


Zeta Potential• The zeta potential is defined as the electric

potential difference between the shear plane ofa colloidal particle and the bulk of the solution

• The zeta potential is an indirect measure of theelectrical charge of the colloidal particle

• The zeta potential can be experimentallymeasured (using a microscope) bydetermining the velocity of a particle movingunder an electric potential of known intensity(Note: the actual potential between the surfaceof the particle and the solution cannot bemeasured experimentally)


Electric Potential Around a ColloidalParticle

Distance from Particle Surface

Ele

ctri

cal P

oten

tial

ZetaPotential

Shear Plane


Quantitative Determination of ZetaPotential from Experimental DataThe zeta potential is defined as:

ζ πε

πµε

= =4 4vV

EM

x

where: ζ = zeta potential

v = particle velocity

ε = dielectric constant of the medium

Vx = applied potential per unit length

EM = electrophoretic mobility


Quantitative Determination of ZetaPotential from Experimental DataAt 25 oC in water the zeta potential can be directlycalculated from:

ζ = 12EM

where:

ζ = zeta potential in mV

EM = electrophoretic mobility in (µm/s)/(V/cm)

Average zeta potential for wastewater colloids:-16 to -22 mV (range: -12 to -40 mV)


Relationship Between Zeta Potentialand Coagulation

0 100 200 300 400 500 600Alum Dosage (mg/L)

0

5

10

15

20

25

30

35

40R

esid

ual T

urbi

dity

(Jac

kson

turb

idity

uni

ts)

-60

-50

-40

-30

-20

-10

0

10 Zeta P

otential (mV

)

Residual TurbidityZeta Potential


Isoelectric Point• When the colloidal particles are in the

presence of enough counter ions they becomeelectrically neutral

• This point is called the isoelectric point

• The zeta potential at the isoelectric point iszero

• If the particles are not any more "shielded" bythe double layer they can interact with eachother

• Hence, at the isoelectric point the particleshave the highest potential for agglomeration


Energy Forces Between ColloidalParticles

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 339


Energy Forces Between ColloidalParticles (continued)

• If the repulsive forces produced as a result ofthe presence of the double layer are toosignificant the colloidal particles will not beable to come in close contact for the attractiveforces to produce agglomeration and hencecoagulation

• In such a case the effects of the double layermust be neutralized (e.g., by increasing theionic strength of the solution) for coagulationto occur


Coagulation as a Process ofDestabilization of Colloids

• Coagulation of colloids occurs when a stablecolloid (because of the intrinsic stability of thecolloid dispersion) or a stabilized colloid(because of the presence or a double layer) isdestabilized

• Destabilization of colloids occurs when thecolloidal particles are brought close enough toeach other for agglomeration to occur

• Several destabilization mechanisms arepossible


Destabilization MechanismsDepending on the type of colloidal suspensionthat should undergo coagulation differentdestabilization mechanisms can be employedsuch as:

• Repression of the double layer

• Neutralization of colloid charge by adsorptionof counter ions on the surface of the colloid

• Bridging of colloidal particles via polymeraddition

• Entrapment of colloidal particles by sweepingfloc


Repression of Double Layer• The potential generated by a charged particle

decreases rapidly with the distance from theparticle surface

• As a consequence the thickness of theelectrically charged layer surrounding theparticles also decreases with the distance formthe particle

• If the particle is surrounded by a large number ofadded ionic species their presence will interferewith the potential generated by the particle.Consequently, the potential will decrease evenfaster with the distance from the particles


Repression of Double Layer (cont.'d)• The thickness of the electrically charged layer

surrounding the particles is arbitrarily taken asthat distance at which the potential falls to37% (=1/e) of the surface value

• From the Debye-Hückel theory it is possible tocalculate that the thickness of this layer isgiven by the equation:

zI

= ⋅ −0 33 10 2.ε

where: z = thickness of layer, in cmε = dielectric constant for the solution, in C/(V cm)I = ionic strength, in moles/L


Repression of Double Layer (cont.'d.)For water at 20 oC it is:

zI

≅ ⋅ −3 0 10 8.

where z is in cm and I is in moles/L.

Example:

for I = 0.001 M → z ≈ 100 Å

for I = 0.1 M → z ≈ 10 Å


Repression of Double Layer (cont.'d.)• The results from the previous equations

indicate that double layer repression can beaccomplished by increasing the ionic strengthof the solution

• This increase does not alter the charge of thecolloidal particles but reduces the extent towhich the same charge affects the potentialaround the charge


Repression of Double Layer (cont.'d.)

Distance from Particle Surface

Ele

ctri

cal P

oten

tial

Low Ionic StrengthHigh Ionic Strength

Z1Z2


Repression of Double Layer (cont.'d.)Double layer repression can be achieved by:

• increasing the ionic strength of the solution byadding additional ionic species

• even more effectively increasing the ionicstrength of the solution by adding ions of highvalence since:

I C Zj jall ionicspecies

= ∑12

2

where: C = concentration of ionic species jZ = charge of ionic species j


Repression of Double Layer (cont.'d)• The typical chemicals used in double layer

repression are those that produce cations witha large charge such as Al+3 and Fe+3

• Therefore chemicals such as

- Al2(SO4)·14H2O (alum)

- FeCl3are often used as coagulants

Remark: these salts produce coagulation alsobecause of their charge suppression andbridging capability


Neutralization of Colloid Charge• This mechanism is based on the addition of large

organic molecules containing ionizable groups to thecolloidal dispersion

• The charge of the dissociated molecule must be ofthe opposite sign as the colloid charge

• As the organic molecules dissociate the resultinglarge, charged molecules can effectively interact withthe Stern layer replacing the counter ions originallypresent in this layer

• Because of the size of the molecule and the lowcharge of its organic "tail" the potential around theparticle is decreased making the colloidal particlesinteract, agglomerate and coagulate


Bridging of Colloidal Particles ViaPolymer Addition

• The addition of large polymeric molecules havingcharged or ionizable sites (polyelectrolytes) to acolloidal dispersion results in the attachment of thepolymer to the colloid (just as in the chargeneutralization process examined previously)

• Because of the length of the polymeric chain themotion of the colloid-polymer particle is hindered,thus promoting contact with other particles

• The polymer chains protruding from colloid-polymerparticles can interact with similar particles forming"bridges." This results in particle agglomeration andeventual coagulation


Colloid-Particle Interactions During Bridgingof Colloidal Particles Via Polymer Addition

After Weber, Physicochemical Process for Water Quality Control, 1972, p. 74


Entrapment of Colloidal Particles bySweeping Floc

• If significant amounts of aluminum or ferric saltsare added to a solution the hydroxide of thesemetal will precipitate

• During the precipitation process the hydroxideforms large tridimensional polymeric structures(floc formation)

• As these polymeric structures form the colloidscan become entrapped in it and be precipitatedby a "sweeping floc" mechanism

• Large amounts of sludges are formed as a result


Characteristics of CoagulantsThe ability of a chemical additive to producecoagulation is in general dependent on the:

• electric charge of the ion or molecule used ascoagulant: the larger the charge the moreeffective the coagulant will be;

• size of the ion or molecule used as coagulant:the larger the size of the molecule the moreeffective the coagulant will be.


Coagulating Power of Inorganic ElectrolytesRelative Power of Coagulation

Electrolyte Against PositiveColloids

Against NegativeColloids

NaCl 1 1Na2SO4 30 1Na3PO4 1000 1BaCl2 1 30MgSO4 30 30AlCl3 1 1000Al2(SO4)3 * 30 >1000FeCl3 1 1000Fe2(SO4)3 * 30 >1000(*) Common coagulants

After Droste, Theory and Practice of Water and Wastewater Treatment, 1997, p. 385.


Common CoagulantsThe most common coagulants used in wastewatertreatment are:

• Aluminum salts (alum)

• Ferric and ferrous salts

• Lime

• Cationic polymers

• Anionic and non-ionic polymers


Aluminum and Iron Salts asCoagulants

The most important coagulants or this type are:

• Al2(SO4)3·14H2O or Al2(SO4)3·18H2O (alum)

• FeCl3• FeCl3 (with lime)

• Fe2(SO4)3 (with lime)

• FeSO4·7H2O (copperas) (with lime)


Characteristics of Common InorganicCoagulants

Name Formula Mol.Weight

Density(kg/m3)

Alum Al2(SO4)3

Al2(SO4)·14H2OAl2(SO4)·18H2O

342.1594.3666.7

2710

Ferric chloride FeCl3 162.1 2800Ferric sulfate Fe2(SO4)3 400 1899

Ferrous sulfate Fe2(SO4)3·7H2O 278 3097Lime Ca(OH)2 74.1 2200


Aluminum and Iron Salts asCoagulants (continued)

• When added to a solution these saltsdissociate into ions. The resulting cationicspecies are not simple ions (such as Al+3, Fe+3)but their hydrated forms (such as Al(H2O)6

+3 orFe(H2O)6

+3)

• The addition of small amounts of Al and Fesalts does not results in coagulation. Thisindicates that double layer suppression is onebut not the main mechanism of coagulation



• When added in significant amounts the ionsfrom these salts react with the OH- orbicarbonate and carbonate ions in solution toproduce the corresponding insolublehydroxides (Al(OH)3 or Fe(OH)3)

• Coagulation of colloids is observed whenprecipitation of the hydroxides occurs (with orwithout the presence of lime)

• The solubility of Al(OH)3 or Fe(OH)3 is afunction of the pH



• The precipitation of these Al and Fehydroxides proceeds through the formation ofpolymeric hydrocomplexes

• These complexes and the hydroxide particlesare positively charged if the pH is below theirisoelectric point. Hence they are adsorbed onthe surface of the colloids producing chargesuppression and coagulation



• If the pH of the solution is beyond theisoelectric point of the hydroxide precipitatethen the hydroxide particles will be negativelycharged and no charge suppression of thecolloid particle is possible

• In this range colloid coagulation can onlyoccur if the amount of hydroxide precipitate ishigh enough for bridge formation andsweeping floc enmeshment


Hydrolysis of Alum in WaterThe addition of alum (hydrated aluminum sulfate)to a wastewater produces the hydrolysis of thesulfate with the consequent formation of insolublealuminum hydroxide according to the reactions:

( )( )

Al SO H O H O

Al OH H SO H O2 4 3 2 2

3 42

2

18 6

2 6 3 18

⋅ + ⇔

↓+ + ++ −

The insoluble aluminum hydroxide forms a flocprecipitate responsible for colloid removal.

Note that if the wastewater is not buffered theresulting H+ generation (pH increase) will preventthe reaction from proceeding any further.


Reaction of Alum in the Presence ofCalcium and Magnesium AlkalinityIn the presence of calcium or magnesiumbicarbonate alkalinity alum forms aluminumhydroxide that precipitates as before forming asweeping floc responsible for colloid removal:

( ) ( )( )

Al SO H O Ca HCO

Al OH CaSO CO H O2 4 3 2 3 2

3 4 2 2

18 3

2 3 6 18

⋅ + ⇔↓+ + +

If the alkalinity is insufficient lime can be added:

( ) ( )( )

Al SO H O Ca OH

Al OH CaSO H O2 4 3 2 2

3 4 2

18 3

2 3 18

⋅ + ⇔↓+ +


Equilibrium Composition of Solutionsin Contact with Al(OH)3



Hydrolysis of Ferric Chloride in WaterThe addition of ferric chloride to a wastewaterproduces the hydrolysis of the ferric chloride withthe consequent formation of insoluble ferrichydroxide, according to the reaction:

( )FeCl H O Fe OH H Cl3 2 33 3 3+ ⇔ ↓+ ++ −

The insoluble ferric hydroxide forms a gelatinoussweeping floc precipitate responsible for colloidremoval.

Note that if the wastewater is not buffered theresulting H+ generation (pH decrease) will preventthe reaction from proceeding any further.


Reaction of Ferric Chloride in the Presenceof Calcium and Magnesium Alkalinity

In the presence of calcium or magnesiumbicarbonate alkalinity ferric chloride forms ferrichydroxide, which precipitates as before forming asweeping floc responsible for colloid removal:

( )( )

2 3

2 3 73 3 2

3 2 2

FeCl Ca HCO

Fe OH CaCl CO

+ ⇔↓+ +


( ) ( )2 3 2 33 2 3 2FeCl Ca OH Fe OH CaCl+ ⇔ ↓+


Equilibrium Composition of Solutionsin Contact with Fe(OH)3



Reaction of Ferric Sulfate in thePresence of Alkalinity or Lime

In the presence of calcium or magnesiumbicarbonate alkalinity ferric sulfate forms ferrichydroxide, which precipitates as before forming asweeping floc responsible for colloid removal:

( ) ( )( )

Fe SO Ca HCO

Fe OH CaSO CO2 4 3 3 2

3 4 2

3

2 3 6

+ ⇔↓+ +


( ) ( ) ( )Fe SO Ca OH Fe OH CaSO2 4 3 2 3 43 2 3+ ⇔ ↓+


Reaction of Ferrous Sulfate in thePresence of Alkalinity and Lime

If ferrous sulfate is used as a coagulant alkalinityand lime (or lime alone) are required to formferrous hydroxide, which is then converted toferric hydroxide by the oxygen in the water:

FeSO H O Ca HCOFe HCO CaSO H O

4 2 3 2

3 2 4 2

77

⋅ + ⇔+ +

( )( )

( ) ( ) ( )Fe HCO Ca OH Fe OH CaCO H O3 2 2 2 3 22 2 2+ ⇔ + +( ) ( )4 2 42 2 2 3Fe OH O H O Fe OH+ + ⇔ ↓


Strategies for Al and Fe CoagulantAddition

The appropriateness of the use of Al or Fe saltsas coagulants and their dosage depend primarilyon three factors, i.e.:

• pH of the wastewater

• alkalinity of the wastewater

• concentration of the colloids

Several situations are possible and will beexamined.


Strategies for Al and Fe CoagulantAddition

Colloidconcentration

Alkalinitylevel

Destabilizationmechanism

Al and Fe AdditionStrategy

High Low Chargeneutralization

Easy coagulation

High High Chargeneutralization

High dose or removealkalinity and add

low dose

Low High Sweeping floc High dose.Easy coagulation

Low Low Ineffective Increase alkalinity oruse other coagulant


Effect of the Presence of Phosphateson Coagulation with Alum or Iron Salts

Multivalent metal ions such as aluminum or ferricions form very sparingly soluble precipitates inthe presence of phosphate ions. The reactionsinvolved in phosphate precipitation are:

( )Al SO H O alum PO

AlPO SO H O2 4 3 2 4

3

4 42

2

14 2

2 3 14

⋅ + →↓+ +

−

−

( )

FeCl PO FePO Cl3 43

4 3+ → ↓+− −

These reactions are of course in competition withthe many other reactions occurring as a result ofalum or ferric chloride addition to a wastewater


Lime as a Coagulant• Lime is a generic name used to identify several

combinations of calcium and oxygen, such asCa(OH)2 or CaO (quicklime). Lime oftencontains magnesium

• In the presence of bicarbonate ion lime willreact to form a calcium carbonate precipitatethat can remove colloids through the sweepingfloc mechanism. The reaction involved is:

( ) ( )Ca OH Ca HCO CaCO H O2 3 2 3 22 2+ ⇔ ↓+

• If the wastewater is acidic the lime additionwill go first toward the neutralization of theacid before colloid removal can take place


Lime as a Coagulant (continued)• If the lime addition is sufficient to raise the pH

above 9.5 the magnesium content of the limewill precipitate as Mg(OH)2

• Magnesium hydroxide precipitates in agelatinous form which results in goodclarification because of its enhanced potentialfor colloid removal

• On the other hand the gelatinous magnesiumhydroxide precipitate typically makes theresulting sludge more difficult to dewater


Effect of the Presence of Phosphateson Coagulation with Lime

The addition of lime to a wastewater will firstresult in the neutralization of the wastewaterfollowed by the precipitation of CaCO3 ifbicarbonates are present, as described above.

If, as a result of Ca(OH)2 addition, the pH ofwastewater goes above about 10 and ifphosphates are present then the followingprecipitation reaction can take place:

( ) ( )10 6 224

310 4 6 2

Ca PO OH Ca PO OH+ − −+ + → ↓

where hydroxylapatite is formed as a precipitate.


Polyelectrolytes as Coagulants• Polyelectrolytes (also often referred to a

“polymers” in coagulation processes) aresynthetic or organic polymeric molecules havingionizable groups or charged groups along theirchain

• Polyelectrolytes can be classified according totheir origin as:- natural, i.e., derived from starch products or

of biological origin (e.g., alginate from algae,chitosan from the acidification of chitin inshells)

- synthetic, i.e., synthetically polymerized frommonomers (e.g., polyamine, sulfonate, etc.)


Polyelectrolytes as Coagulants• Polyelectrolytes are also classified according

to the charge that they have when they are insolution as:

- cationic, i.e., forming a macro cation whenplaced in water

- nonionic, i.e., having no net charge whenplaced in water

- anionic, i.e., forming a macro anion whenplaced in water


Examples of Polyelectrolytes

Type FunctionalGroup

Example

Cationic Amine

Quaternary

Polyethyleneaminehydrochloride

Polydiallyldimethylammonium

Nonionic Polyalcohol

Amide

Polyvinylalcohol

Polyacrylamide

Anionic Carboxylic

Sulfonic

Polymethacrylic acid

Polyvinylsulfonate


Examples of CoagulantPolyelectrolytes

Cationic

n

Polyethyleninine

CH2 CH2 N

H

H+

Cl-

Nonionic

[ ]n

Polyacrilamide

CH2 CH

C O

NH2

Anionic

n

Poly(meth)acrilic acid

CH2 CH

C O

O-

R


Polyelectrolytes as Coagulants• Cationic polyelectrolytes are typically used to

coagulate colloids that are negatively charged(the most common type)

• The mechanisms involved in the colloiddestabilization by cationic polyelectrolytes are:charge neutralization and bridging of colloidalparticles

• Nonionic and anionic polyelectrolytes can alsobe used to destabilize negative colloids. Inthis case the destabilization mechanism is nottoo well elucidated but is believed to be due tobridging


Typical Coagulant Dosage

Coagulant Dosage (ppm) pH

Alum 75 - 250 4.5 - 7.0

FeCl3 35 - 150 4.5 - 7.0

FeSO4·7H2O 70 - 200 4.0 - 7.0

Lime 150 - 500 9.0 - 11.0

Cationic Electrolytes 2 - 5 ---

Nonionic and AnionicElectrolytes

0.25 - 1.0 ---

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 92


Coagulant Aids• Despite their names coagulant aids are

additives that can be added to a destabilizedcolloidal suspension to promote the growth oflarge, rapid-settling floc which can thenflocculate

• Typical coagulant aids are:

- Activated silica

- Polyelectrolytes


Activated Silica as a Coagulant Aid• Activated silica is a short chain polymer

capable of binding together aluminum hydrateparticles used in coagulation processes

• Common dosage for activated silica is 5 - 10ppm


Laboratory Coagulation Tests• The selection of a coagulant and its dosage

cannot be made without carrying outlaboratory experiments

• Typically two types of tests are the mosteffective in the determination of the optimaldosage of coagulant and optimal pH forcoagulation. The are:

- jar test

- zeta potential


Jar TestThe jar test is the most common laboratorycoagulation test and consists of:• placing a sample of the wastewater in a jar:• rapidly adding the coagulant and intensely

mixing the sample for a short time(coagulation/destabilization phase);

• allowing the floc to form under gentle mixingconditions (flocculation phase)

• allowing the floc to sediment in the quiescentsample (settling)

• comparing the turbidity of the sample with theinitial turbidity


Jar Test Apparatus

After Droste, Theory and Practice of Water and Wastewater Treatment, 1997, p. 388.


Strategy to Conduct LaboratoryCoagulation Tests

Since two variables (coagulant dosage and pHare) are involved it is typically more convenient tofix one of the variables initially, scan for theoptimal value of the other variable, and finallyscan for the optimal value of the first variable


Jar Test Procedure• A wastewater sample is placed in a beaker and

magnetically stirred. The pH must be adjusted to adesired valued (typically 6);

• A known amount of coagulant is added and theagitation is maintained at a high value for 1 minuteto promote coagulation. Then the sample isagitated slowly for 3 minutes to promoteflocculation. New additions are made until avisible floc is obtained;

• Using this concentration of coagulant thecoagulation experiment is now repeated atdifferent pH values using a longer flocculation time(typically about 10 to 40 minutes), followed bysettling with no agitation (15-60 minutes);


Jar Test Procedure (continued)• The amount of residual pollutant in solution is

measured at the end of each pH experiment. Theoptimal coagulation pH is obtained;

• Using this optimal pH value a new series ofexperiments is conducted in which the coagulantdosage is changed. The optimal coagulant dosageis obtained;

• Plots of residual pollutant concentration vs. pHand residual pollutant concentration vs. coagulantdosage can be constructed.


Typical Results of Jar Test

pH

Pol

luta

ntR

esid

ual C

once

ntra

tion

Coagulant DosageP

ollu

tant

Res

idua

l Con

cent

ratio

n


Zeta Potential Test• A wastewater sample is placed in a beaker and

magnetically stirred. The pH must be adjusted to adesired valued (typically 6)

• A known amount of coagulant is added and theagitation is maintained at a high value for 1 minute topromote coagulation. Then the sample is agitatedslowly for 3 minutes to promote flocculation

• The zeta potential is measured

• The same test is repeated at different coagulantdosage

• The results are plotted and the dosage for zetapotential equal to zero is determined


Coagulation vs. Flocculation• Although the words "coagulation" and

"flocculation" are often used interchangeablythey refer to two distinct processes

• Coagulation indicates the process throughwhich colloidal particles and very fine solidsuspensions are destabilized so that they canbegin to agglomerate if the conditions areappropriate

• Flocculation refers to the process by whichdestabilized particles actually conglomerateinto larger aggregates so that they can beseparated from the wastewater


Mechanisms of Flocculation• Perikinetic flocculation

This type of flocculation is generated by theBrownian motion associated with destabilizedsmall colloidal particles. The Brownian motioncontributes to transport the particles closeenough for agglomeration to occur

• Orthokinetic flocculationThis type of flocculation mechanism is based onthe generation of velocity gradients within thewastewater to promote particle interaction. Inthis case a mild agitation promotes theaggregation of the particles and henceflocculation


Model for Flocculation ProcessThe rate of orthokinetic flocculation can bepredicted (in principle) from the equation:

lnNN

G tp

po ave= − 4

πηΩ

where Np = number of separate colloidal particlesNpo = initial number of separate colloidal particlesη = particle collision efficiencyΩ = volume of colloidal particles per unit volumeof suspensionGave = average velocity gradientt = time


Model for Flocculation ProcessThe average velocity gradient, Gave, can beexpressed as:

GPVave

ave= µ

where: Pave = average power consumption in thesystem (e.g., a tank stirred by animpeller)

V = volume of suspension

µ = viscosity


Example of Apparatus forCoagulation-Flocculation and Settling

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 96


Design Approach for Coagulation andFlocculation Equipment

• Coagulation depends on the rapid reactionthrough which the coagulant destabilizes thecolloids.

• Coagulation requires the rapid dispersion ofthe coagulant throughout the wastewater.This is accomplished through very intenseagitation and mixing.

• High shear rates are beneficial to coagulation.


Design Approach for Coagulation andFlocculation Equipment

• Flocculation depends on the frequency ofcollision of the destabilized colloids to formlarger floc particles.

• The frequency of collision depends on theintensity of the agitation and the shear rate.

• However, too high a value of the agitationintensity and shear rate may break up the flocjust formed.

• Therefore, moderate shear rates and agitationintensities are used in flocculators.


Batch Coagulation-Flocculation Process• Two tanks in parallel are typically used. Each tank

operates in a full cycle (filling, coagulation, flocculation,settling).

• Each step is conducted in series and is time dependent.

• After the tank has been filled, the coagulant is addedwhile the wastewater is rapidly stirred (typically for 10minutes).

• The agitation is reduced during the flocculation phaseand stopped altogether during settling (typically for afew hours).

• Batch treatment can be cost effective only for flows upto 50,000 gal/day.


Continuous Coagulation-Flocculation ProcessA continuous coagulation-flocculation systemtypically includes the following:• Chemical feeding system: to continuously feed the

coagulant to the wastewater;• Rapid mix tank: in this tank (or pipe, channel, etc.)

high shear condition are generated (e.g., by arapidly rotating impeller) to rapidly mix thecoagulant with the wastewater;

• Flocculation tank: in this tank moderate agitation(typically through impellers or paddles) is providedto promote coalescence and flow growth;

• Sedimentation tank: to remove the floc particles;• Filtration system: to polish the effluent, if needed.


Shear Rate (Velocity Gradient)In any moving fluid in which different velocitiesexist the rate of change of the velocity in anygiven direction along another direction (e.g., therate the velocity in the x direction changes alongthe y direction) is called the shear rate or velocitygradient alongthat direction(e.g., dvx/dy), andis defined as G:

Gdvdyxy

x=

vx=0

x

y

vx=v


Shear Stress and Shear Rate(Velocity Gradient)

The shear rate, τ, is the force per unit area that istransmitted between two adjacent fluid layers as aresult of viscosity. The shear rate transmittedalong the y direction as a result of the velocitygradient (or velocity gradient) dvx/dy is:

τ µxyxdv

dy= −


Shear Stress, Velocity Gradient andPower Dissipation

It can be shown that the relationship between theshear rate, τ, the velocity gradient, G, and thepower, P, dissipated in a volume V, of fluid as aresult of viscous dissipation can be expressed as:

− = = =τ µ εG GPV

2

where:

ε = power dissipation per unit fluid volume


Velocity Gradient andPower Dissipation

The relationship:

GPV

= µcan be used to determine the local velocitygradient from the local power dissipation. Sincethis is typically very difficult, in practice theaverage velocity gradient, Gave, is determined fromthe average power dissipation, Pave:

GP

Vaveave= µ


Estimation of Power Dissipation inCoagulation and Flocculation Equipment

• Phenomena such as coagulation andflocculation are directly or indirectly affectedby the velocity gradients.

• Since the average velocity gradient, G, can beestimated only if the mechanical powerdelivered to the system is known, it becomesessential to be able to determine the power, P,consumed by different systems (such asimpellers, paddles, sparged gas, etc.).


Direct Power Input: Classification ofPower Sources

The power dissipated inside a tank must besupplied from the outside in some way. In principle,this external power can be supplied by means ofthree primary different sources, i.e.:

• Mechanical agitation (e.g., impellers, paddles,turbines)

• Power delivered by a compressed gas (e.g.,through gas dispersers, sparged gas diffusers)

• Power delivered by the liquid (e.g., venturimixers, hydraulic jumps, water jets)


Mechanical Agitation Systems inCoagulation and Flocculation

• Submerged impellers (mixers) mounted onvertical shafts.

• Paddles mounted on horizontal shafts.


Tanks with Submerged Impeller(s)Motor

H

DT

C


Submerged Impellers• Many different types of impellers exist.

• Impellers are classified on the basis of:

♦ shape

♦ dimensions

♦ type of flow pattern generated

♦ flow intensity

♦ power consumption under aerated and non-aerated conditions

♦ their ability to disperse a gas effectively.


Example of Submerged Impellers

After Tatterson (1991), p. 7.


Impeller Pumping Action• Radial (e.g., Rushton

Impeller, Flat-BladeImpeller)

• Axial (e.g., Propeller, LightningA310)

• Mixed-flow (45o Pitched-Bladed Turbine)


Flow Patterns Resulting from thePresence of Baffles (Stators)

• Unbaffled cylindrical vessel

• Baffled cylindrical vessel(or unbaffled squarevessel)

Vortex

Baffle

Flat intefacial area


Nomenclature for Submerged ImpellersC Impeller off-bottom clearance

D Impeller diameter

H Height of liquid in the mixing vessel

N Agitation speed; revolutions per minute (rpm)

P Power consumed by the impeller

T Vessel diameter; m

w Baffle width; m


Calculation of the Power Dissipatedby a Submerged Impeller

For a given impeller of known type, size, andagitation speed it is possible to calculate thepower dissipated by that impeller, P, from:

P N D= Poρ 3 5

The non-dimensional power number Po is specificfor each impeller type and agitation system.

Remark: in this expression the agitation speed,N, is expressed in rotations per unit time (e.g.,rotations per second [rps], rotation perminute[rpm] to yield P in the appropriate units).


Power Number• The power number, Po (or Ne or Np) is a non-

dimensional number used to calculate thepower consumption of an impeller.

• In general, the power number is a function ofthe type of impeller, type of agitation system(e.g., tank, baffles), dynamic agitation regimes(e.g., turbulent flow, laminar flow), geometricdimensions of impeller and tank, location ofthe impeller, and the presence of otherimpellers. In other words, Po is a function ofany variable affecting the agitation flow.

• Po is typically determined experimentally.


Non-Dimensional Groups Used inPower Consumption Calculations

• Power Number = Po NeP

N D= = ρ 3 5

• Impeller Reynolds Number = Re = ρµ

N D2

• Froude Number = Fr = N Dg

2

(important only in

unbaffled vessels)

• Geometric Ratios, such as TD

HD

CD

wT

; ; ;


Power Consumed by SubmergedImpellers

In general, Po can be expressed as:

Po NeP

N Df

ND N Dg

TD

HD

CD

wT

= = =

ρ

ρµ3 5

2 2

, , , , , ,impeller type

For baffled vessels, Fr is unimportant, and:

Po NeP

N Df

ND TD

HD

CD

wT

= = =

ρ

ρµ3 5

2

, , , , ,impeller type

For geometrically similar baffled vessels:

( )Po NeP

N Df

NDf= = =

=ρρ

µ3 5

2

Re,impeller type


Typical Power Number Curve for aSubmerged Impeller

1 10 100 1000 10000 100000Reynolds Number

1

10

100P

ower

Num

ber

TurbulentTransitional

Laminar


Power Number for SubmergedImpellers

Laminar Regime:

Po Ne= ∝ 1Re

Transitional Regime:

( )Po Ne f= = Re

Turbulent Regime:

Po Ne= = Constant


Power Number for SubmergedImpellers

After Bates, Fondy, and Corpstein (1963).


Turbulent Power Number for Different ImpellersImpeller Type Po or Ne

Flat-Blade Turbine (24 Blades) 9.8


Gate 5.5

Disc Turbine (Rushton Type) 5.0


45o Pitched-Blade Turbine 1.3

Prochem 1.0

Glass-Lined Impeller (Pfaudler Type) 0.75

MIG Impeller 0.65

Lightnin A310 0.45

Propeller 0.35


Effect of Tank Size on Power Consumption• In general, the shape of the tank can have an effect

on the power consumed by the impeller; however,typically this effect is somewhat limited.

• The power consumed by an impeller in a baffledcylindrical tank (i.e., having a circular cross section)is typically very similar to the power consumed in atank of square cross section.


Effect of Tank Size on Power Consumption• The power dissipation is lower in tanks having a

rectangular (as opposed to circular or square) crosssection.

• For most of the cases encountered in wastewatertreatment the power consumed by a turbine in anunbaffled tank having a circular or square crosssection can be taken to be 75% of that dissipated in abaffled system having the same dimensions.


Horizontal Paddle SystemsPaddles mounted on horizontal shafts are oftenencountered in flocculation tanks.

Plan View Side View


Geometry of a Horizontal Paddle

Shaft

Paddle

ro

ri

b


Power Dissipated by a HorizontalPaddle

The power dissipated by a single paddle can beobtained from the equation:

( ) ( )P C b N k r rD o i= −

−18

260

13

4 4ρ π

N = agitation speed in rpmCD ≅ 1.8 for flat paddlesk = 0.25 (for tanks without baffles); 0-0.15 (fortanks with baffles)Range for peripheral velocity, 2πN/60 = 0.09-0.9m/s.


Power Dissipated by Gas SpargingWhen a single gas bubble sparged from thebottom of a tank rises to the top the gasexpansion energy, W, released from the gas to theliquid can be obtained from:

W pdVV

V

i

o= ∫where:

p = pressure in the gas bubble

V = bubble volume

“o” subscript: at the top of the tank“i” subscript: at the bottom of tank


Power Dissipated by Gas Sparging

Air


Power Dissipated by Gas SpargingFrom the ideal gas law it is:

pV p V p V nRTo o i i= = =

Then:

( ) ( )W pdV p Vdpp

p VppV

V

o op

p

o oi

oi

o

i

o

= = − ⌠⌡

=∫ ln

For m bubbles sparged in the tank during the timet it is (since the gas flow rate is Qo = m Vo/t):

PmW

tp

mVt

pp

p Qpp

p Qp

poo i

oo o

i

oo o

o

= = = = +

ln ln ln 1

∆


Power Dissipated by Gas SpargingRecall that the pressure generated by a column ofliquid is:

p g hL= ρ

where h is the height of the column of liquid, andρL is the density of the liquid.

Since 1 atm = 101,325 Pascals the height of acolumn of water that produces a hydrostaticpressure of 1 atm is:

hPa

kg m m sm= =101325

1000 9 8110 33 2

,/ . /

.


Power Dissipated by Gas SpargingHence, the power dissipated by a gas sparged ata distance h below the liquid level (exposed to theatmospheric pressure, po) is given by:

( )

( )

P p Qp

pp Q

h

p Qh

o oo

o o

o o

= +

= +

= +

ln ln.

ln.

1 110 3

133 8

∆ in m

in ft


Power Dissipated by HydraulicDevices

Power can be dissipated also when water flowsthrough hydraulic jumps. In general, the powerdissipated this way can be obtained from:

P gQ hL= ρ

where:

ρ = liquid density

g = acceleration of gravity

hL = headloss in the device


Average Velocity Gradient and ResidenceTime in Rapid Mix Coagulation Equipment

• During coagulation the average velocitygradient is typically quite high, with G valuesup to 5000 s-1 (more commonly around 1000s-1).

• Residence times are between 10 s and 6minutes, although much faster dispersiontimes are needed for optimal chargeneutralization effects (t < 0.1 s).


Average Velocity Gradient and ResidenceTime in Rapid Mix Coagulation Equipment

Recommended velocity gradients and residencetimes during coagulation in rapid mix basins:

t (s) 20 30 40 >40

G (1/s) 1000 900 790 700

G·t 20,000 27,000 31,600 28,000

The following empirical equation relating G, t andthe concentration, C (in mg/L), of alum as thecoagulant can also be used for rapid mix devices:

G t C⋅ ⋅ = ⋅1 46 65 9 10. .


Average Velocity Gradient and ResidenceTime in Flocculators

• During flocculation the average velocitygradient is typically much lower than in rapidmix devices, with G values in the range 5-100s-1 (more commonly in the range 10-60 s-1).

• Residence times are between 15 and 45minutes.

• G·t values are typically in the range 104-105.


Generalized Approach to the Design ofCoagulation and Flocculation Devices

• Carry out jar tests to determine dosage ofcoagulant.

• Assume values for G and the residence time.

• Size the tank for the residence time selected.

• Choose the type of equipment appropriate forthe process (e.g., high speed impeller for rapidmix or paddle agitator for flocculation).

• Size the equipment on the basis of the powerdissipation that will result in the desired G value.


Flocculator Design GuidelinesType of Flocculator Basic Design CriteriaVertical shaft Gave value up to 100 s-1, maximum tip speed of 2

m/s, approximately 5m x 5m to 10m x 10m basinsurface area per unit, downward flow patternpreferable for propeller unit, stator baffles shouldbe provided for turbine units

Horizontal-shaftpaddle

Gave value up to 50 s-1, maximum tip speed of 1 m/s,number of paddles adjusted for tapered mixing,paddle area should not exceed 20% of tank sectionarea

Baffled channel Tapered mixing by adjusting baffles, maximum flowvelocity of approximately 0.75 m/s, end-aroundbaffle used when total head loss across tank islimited

Diffused air andwater jets

Gave = 95-20 s-1 or Gave·t = 105-106, may be used forauxiliary mixing when plant is overloaded

James M. Montgomery Consulting Engineering, Inc., Water Treatment Principles and Design, 1985, p. 516.


Flocculator Design GuidelinesType of Flocculator Advantages and DisadvantagesVertical shaft Easy maintenance and few breakdowns. Suitable for

high-energy input. Suitable for direct filtration andconventional treatment. Many units required for a largeplant. High capital cost for variable-speed reducers andsupport slabs.

Horizontal-shaftPaddle

Generally produces a large-size floc. Simple mixing unit.Suitable for conventional treatment. Need for preciseinstallation and maintenance. Difficult to increase energyinput. Problems with leakage and shaft alignment.

Baffled channel Performs well if the plant flow rate is reasonablyconstant. Little maintenance. A lack of flexibility formixing intensity. High head loss for the over-and-underbaffle.

Diffused air andwater jets

Simple installation and less capital cost. Limited amountof operational data available. High local velocities forwater jet flocculators. High operational cost for airdiffuser flocculators.

James M. Montgomery Consulting Engineering, Inc., Water Treatment Principles and Design, 1985, p. 516.


Examples of Coagulation-FlocculationProcesses for Industrial WastewatersWastewater

SourceContaminant Type of

CoagulantDosage(ppm)

Petroleumrefinery

Oil Alum 25-75

Petroleumrefinery

Foam Polyamine 1-3

Steel mill Oil Anionic 1.5-2

Steel mill Suspended solids Anionic 0.3

Paper mill Suspended solids Weak cationic 2-5

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 352.


Efficiency of Coagulation Processesto Remove Pollutants

Phosphorusremoval (%)

Suspended solidremoval (%)

BOD Removal (%)

Without With Without With Without With

Primarytreatment

5-10 70-90 40-70 60-75 25-40 40-50

Secondarytreatment

Tricklingfilters

ActivatedSludge

10-20

10-20

80-95

80-95

70-92

85-95

85-95

85-95

80-90

85-95

85-95

85-95

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 352.


Additional Information and Examples onCoagulation and Flocculation

Additional information and examples can be found in thefollowing references:

• Corbitt, R. A. 1990, The Standard Handbook ofEnvironmental Engineering, McGraw-Hill, New York,pp. 6.92; 9.25.

• Droste, R. L., Theory and Practice of Water andWastewater Treatment, John Wiley & Sons, New York,1997, pp. 384-415.

• Eckenfelder, W. W., Jr., 1989, Industrial WaterPollution Control, McGraw-Hill, New York, pp. 84-110.



• Freeman, H. M. (ed.), 1989, Standard Handbook ofHazardous Waste Treatment and Disposal, McGraw-Hill, New York, pp. 7.21-7.31.

• Haas, C. N. and Vamos, R. J., 1995, Hazardous andIndustrial Waste Treatment, Prentice Hall, EnglewoodCliffs, NJ, pp. 144-145.

• James M. Montgomery Consulting Engineering, Inc.,1985, Water Treatment Principles and Design, Wiley-Interscience, John Wiley & Sons, New York, pp. 116-134; 504-519.



• Metcalf & Eddy, 1991, Wastewater Engineering:Treatment, Disposal, and Reuse, McGraw-Hill, NewYork, pp. 302-314; 470-472.

• Sundstrom, D. W. and Klei, H. E., 1979, WastewaterTreatment, Prentice Hall, Englewood Cliffs, NJ, pp.235-255.

• Weber, W. J., Jr., 1972, Physicochemical Process forWater Quality Control, Wiley-Interscience, John Wiley& Sons, New York, pp. 61-109.

• Wentz, C. W., 1995, Hazardous Waste Management,Second Edition, McGraw-Hill, New York, pp. 157-161.

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