CE 370CE 370

Coagulation Coagulation

and and

Flocculation Flocculation

DefinitionsDefinitions

Coagulation IsCoagulation IsThe addition and rapid mixing of coagulantsThe addition and rapid mixing of coagulantsThe destabilization of colloidal and fine particlesThe destabilization of colloidal and fine particlesThe initial aggregation of destabilized particlesThe initial aggregation of destabilized particles

Flocculation IsFlocculation IsThe gentle agitation to aggregate destabilized The gentle agitation to aggregate destabilized

particles to form rapid-settling flocparticles to form rapid-settling floc

Theory of CoagulationTheory of Coagulation

Colloidal CharacteristicsColloidal Characteristics

DestabilizationDestabilization

Colloidal CharacteristicsColloidal Characteristics

Nonsettleable SolidsNonsettleable SolidsHave particle size between 0.1 and 100 micronHave particle size between 0.1 and 100 micron

ColloidsColloidsHave particle size between 0.001 to 1.0 micronHave particle size between 0.001 to 1.0 micron

Most of nonsettleable solids are colloidal Most of nonsettleable solids are colloidal particulatesparticulates

Colloids do not settle by the force of Colloids do not settle by the force of gravitygravity

Are stable in suspensionsAre stable in suspensions

Colloidal CharacteristicsColloidal Characteristics

Have large surface area per unit volumeHave large surface area per unit volumeAdsorb substances from surrounding Adsorb substances from surrounding

waterwaterHave electrostatic chargeHave electrostatic chargeCan be hydrophilic (organic colloids) or Can be hydrophilic (organic colloids) or

hydrophobic (inorganic colloids)hydrophobic (inorganic colloids)Attract ions of opposite charge to its Attract ions of opposite charge to its

surface (fixed layer and diffused layer)surface (fixed layer and diffused layer)

Figure 8.3Figure 8.3

DestabilizationDestabilization

When coagulant is added, destabilization When coagulant is added, destabilization occurs due to the following interactions:occurs due to the following interactions:

Agitation results in collision of particlesAgitation results in collision of particlesAggregation of particles by interparticle bridgingAggregation of particles by interparticle bridgingEnmeshment of particles in the precipitate that is Enmeshment of particles in the precipitate that is

formedformed

Coagulant salts dissociate when added to Coagulant salts dissociate when added to water and produce positively charged water and produce positively charged hydroxo-metallic ion complexes hydroxo-metallic ion complexes (Me(Meqq(OH)(OH)pp

z+z+))These complexes tend to polymerize These complexes tend to polymerize

DestabilizationDestabilization

For aluminum saltsFor aluminum salts

For iron saltsFor iron salts

When complexes adsorb to the surface of the When complexes adsorb to the surface of the colloid, the zeta potential is reduced and particle colloid, the zeta potential is reduced and particle is destabilizedis destabilized

Destabilized particles aggregate by attraction Destabilized particles aggregate by attraction due to van der Waals forces or chemical due to van der Waals forces or chemical interactions between reactive groups available interactions between reactive groups available on the surface of the colloid on the surface of the colloid

53413

4208

4177

3156 )(,)(,)(,)( OHAlOHAlOHAlOHAl

542

422 )(,)( OHFeOHFe

DestabilizationDestabilization

Coagulants are used in excess of the Coagulants are used in excess of the amounts needed to form the amounts needed to form the hydroxometallic complexeshydroxometallic complexes

The excess complexes will form insoluble The excess complexes will form insoluble hydroxides [Al(OH)hydroxides [Al(OH)33 or Fe(OH) or Fe(OH)33] ]

While precipitating, the hydroxides While precipitating, the hydroxides enmesh the negative colloids (sweep enmesh the negative colloids (sweep coagulation)coagulation)

For dilute suspensions, the use of large For dilute suspensions, the use of large amount of coagulant may re-stabilize the amount of coagulant may re-stabilize the colloidscolloids

CoagulantsCoagulants

Mainly aluminum and iron saltsMainly aluminum and iron saltsAluminum sulfateAluminum sulfateFerrous sulfateFerrous sulfateFerric sulfateFerric sulfateFerric chlorideFerric chlorideLime [Ca(OH)Lime [Ca(OH)22] ]

Aluminum salts are cheaper but iron salts Aluminum salts are cheaper but iron salts are more effective over wider pH rangeare more effective over wider pH range

Aluminum SulfateAluminum Sulfate

To produce the hydroxide floc, enough alkalinity To produce the hydroxide floc, enough alkalinity should present in the watershould present in the water

If alkalinity is not enough, then it should be If alkalinity is not enough, then it should be added. Usually hydrated lime is used for that added. Usually hydrated lime is used for that purpose (optimum pH is 4.5 – 8)purpose (optimum pH is 4.5 – 8)

2243232342 143)(2)(314.)( COOHCaSOOHAlHCOCaOHSOAl

OHCaSOOHAlOHCaOHSOAl 24322342 143)(2)(314.)(

Ferrous SulfateFerrous Sulfate

Requires alkalinity in the form of hydroxide to Requires alkalinity in the form of hydroxide to react rapidly [Ca(OH)react rapidly [Ca(OH)22]]

The pH should be raised to about 9.5 and The pH should be raised to about 9.5 and excess lime is stabilizedexcess lime is stabilized

More expensive than alumMore expensive than alum

OHCaSOOHFeOOHCaOHFeSO 2432224 132)(22

1)(27.2

Ferric SulfateFerric Sulfate

It reacts with alkalinity presents in waterIt reacts with alkalinity presents in water

Fe(OH)Fe(OH)33 is dense and settle fast is dense and settle fast If alkalinity is not enough, hydrated lime is usedIf alkalinity is not enough, hydrated lime is used Optimum pH is between 4 and 12Optimum pH is between 4 and 12

24323342 63)(2)(3)( COCaSOOHFeHCOCaSOFe

Ferric ChlorideFerric Chloride

It reacts with natural alkalinityIt reacts with natural alkalinity

If alkalinity is insufficient, lime is addedIf alkalinity is insufficient, lime is added

Optimum pH is 4 - 12Optimum pH is 4 - 12

243233 63)(2)(32 COCaSOOHFeHCOCaFeCl

2323 3)(2)(32 CaClOHFeOHCaFeCl

LimeLime

Slaked lime or hydrated lime are usedSlaked lime or hydrated lime are usedSlaked lime [Ca(OH)Slaked lime [Ca(OH)22]]

Produced by reacting quicklime [CaO] with waterProduced by reacting quicklime [CaO] with water

Hydrated lime [Ca(OH)Hydrated lime [Ca(OH)22]]

Coagulant AidsCoagulant Aids

Are used to produce quick-forming, dense Are used to produce quick-forming, dense and rapid-settling flocsand rapid-settling flocs

PolyelectrolytesPolyelectrolytespH adjustmentpH adjustmentAlkalinity additionAlkalinity addition turbidity additionturbidity addition

PolyelectrolytesPolyelectrolytes

Anionic (-vely charged)Anionic (-vely charged)Cationic (+vely charged)Cationic (+vely charged)Polyampholites (both +vely and –vely Polyampholites (both +vely and –vely

charged groups)charged groups)Natural such as starchNatural such as starchSynthetic (more common in coagulation)Synthetic (more common in coagulation)They aid in coagulation by:They aid in coagulation by:

Chemical bridgingChemical bridgingInteraction between reactive groups on the Interaction between reactive groups on the

polyelectrolyte and the flocpolyelectrolyte and the floc

pH AdjustmentpH Adjustment

Is used if pH of water to be treated is not Is used if pH of water to be treated is not within the optimum pH of the coagulantwithin the optimum pH of the coagulant

pH is increased using limepH is increased using limepH is reduced using sulfuric acid pH is reduced using sulfuric acid

Alkalinity AdditionAlkalinity Addition

Is used when natural alkalinity is not Is used when natural alkalinity is not enough to produce good flocenough to produce good floc

Hydrated or slaked lime is usedHydrated or slaked lime is usedSoda ash (NaSoda ash (Na22COCO33) is also used ) is also used

(expensive)(expensive)

Turbidity AdditionTurbidity Addition

Is used to provide sufficient particulate Is used to provide sufficient particulate concentration to achieve rapid coagulation concentration to achieve rapid coagulation through sufficient interparticle collisionthrough sufficient interparticle collision

Is done by recycling chemically Is done by recycling chemically precipitated sludgeprecipitated sludge

Clays are also used for that purposeClays are also used for that purpose

Jar TestJar Test

Is used to determine:Is used to determine:Proper coagulantProper coagulantProper coagulant aidProper coagulant aidProper coagulant doseProper coagulant dose

ProcedureProcedureTime for floc formationTime for floc formationFloc sizeFloc sizeSettling characteristicsSettling characteristicsTurbidity and color removal (%)Turbidity and color removal (%)pH of supernatantpH of supernatant

Chemical FeedersChemical Feeders

Solution-feed TypeSolution-feed TypeNot desirable (more labor) Not desirable (more labor) Used with ferric chlorideUsed with ferric chloride

Dry-feed TypeDry-feed Type

Rapid Mixing and FlocculationRapid Mixing and Flocculation

Rapid mixing is used to:Rapid mixing is used to: Disperse chemicals uniformly throughout the Disperse chemicals uniformly throughout the

mixing basinmixing basinAllow adequate contact between the coagulant and Allow adequate contact between the coagulant and

particlesparticlesMicroflocs are producedMicroflocs are produced

Flocculation is used to:Flocculation is used to:Agglomerate microflocs to larger onesAgglomerate microflocs to larger ones

DevicesDevices

Agitation in rapid mixing and flocculation is Agitation in rapid mixing and flocculation is performed by:performed by:

Mechanical agitators (most common)Mechanical agitators (most common)Pneumatic agitatorsPneumatic agitatorsBaffled basinsBaffled basins

DesignDesign

The degree of mixing is based on the power The degree of mixing is based on the power provided, which is measured by the velocity provided, which is measured by the velocity gradient:gradient:

G = velocity gradient, secG = velocity gradient, sec-1-1

W = power imparted per unit volume of basin, N-m/s-mW = power imparted per unit volume of basin, N-m/s-m33

P = power imparted, N-m/s P = power imparted, N-m/s V = basin volume, mV = basin volume, m33

= absolute viscosity of water ( = absolute viscosity of water ( =0.00131 N-s/m=0.00131 N-s/m22))

V

PWG

DesignDesign

The velocity gradient is:The velocity gradient is:

= specific weight of water= specific weight of water hhLL = head loss due to friction and turbulence = head loss due to friction and turbulence

T = detention timeT = detention time

T

hG L

Velocity GradientVelocity Gradient

The rate of particle collision The rate of particle collision G G

Shear force Shear force G G

Total number of particle collisions Total number of particle collisions GT GT

Rapid MixingRapid Mixing

Mixing devicesMixing devices

Retention timeRetention time

Types of impellersTypes of impellers

Mixing DevicesMixing Devices

Detention TimeDetention Time

T (seconds)T (seconds)G (sec G (sec -1-1))

202010001000

3030900900

4040790790

50 or more50 or more700700

Rotary MixingRotary Mixing

Rotary mixing devices can beRotary mixing devices can beTurbinesTurbinesPaddle impellersPaddle impellerspropellerspropellers

Basins are either circular or square in planBasins are either circular or square in planDepth of basin is 1 to 1.25 of the basin Depth of basin is 1 to 1.25 of the basin

diameter or widthdiameter or widthBaffled tanks are recommended since they Baffled tanks are recommended since they

minimize vortexing and rotational flowminimize vortexing and rotational flow

Turbine ImpellersTurbine Impellers

Flow RegimeFlow Regime

Paddle ImpellerPaddle Impeller

Flow Regime of PropellerFlow Regime of Propeller

Example – Rapid MixingExample – Rapid Mixing

A square rapid-mixing basin, with a depth of water equal to 1.25 times the width, is to be designed for a flow of 7570 m3/d. The velocity gradient is to be 790 mps/m, the detention time is 40 seconds, the operating temperature is 10 C, and the turbine shaft speed is 100 rpm. Determine:

•The basin dimensions•The power required

SolutionSolutionFind the volume of the basin, Find the volume of the basin,

The dimensions areThe dimensions are(W)(W)(1.25W) = 3.50 m3 (W)(W)(1.25W) = 3.50 m3 W = 1.41 mW = 1.41 mThe depth of the basin, H = (1.25)(1.41 m) = 1.76 mThe depth of the basin, H = (1.25)(1.41 m) = 1.76 mUse W = 1.41 m; H = 1.76Use W = 1.41 m; H = 1.76Using the velocity gradient equationUsing the velocity gradient equation

sec40sec60

min

min1440

7570 3

m

V

smNmmsNVGP /2863)76.141.141.1sec)(/790)(/0013.0( 322

smNmmsNVGP /2863)76.141.141.1sec)(/790)(/0013.0( 322

FlocculationFlocculation

Agitation is provided by:Agitation is provided by:Mechanical agitation (most common) ORMechanical agitation (most common) ORPneumatic agitationPneumatic agitation

Mechanical agitation is provided using:Mechanical agitation is provided using: Paddle wheels (most common)Paddle wheels (most common)TurbinesTurbinesPropellersPropellers

Baffles are not used since G and GT are Baffles are not used since G and GT are limitedlimited

Complete flocculation depends on:Complete flocculation depends on:EaseEaseAggregation rateAggregation rateNumber of particle collisionsNumber of particle collisions

OR in other words, it depends on:OR in other words, it depends on:Floc characteristicsFloc characteristicsGGGTGT

Fragile flocs require low G values (<5/sec)Fragile flocs require low G values (<5/sec)High-strength flocs require high G values High-strength flocs require high G values

((10/sec)10/sec)

Flocculation BasinsFlocculation Basins

Designed to provide tapered flocculation Designed to provide tapered flocculation [decreasing G values (high 50 to low 20 to [decreasing G values (high 50 to low 20 to lower 10/sec)]lower 10/sec)]

Horizontal and vertical shafts are used to Horizontal and vertical shafts are used to mount the paddle wheelmount the paddle wheel

Flocculation basins are composed of Flocculation basins are composed of minimum 3 compartments to:minimum 3 compartments to:

Minimize short circuitingMinimize short circuitingFacilitate tapered flocculationFacilitate tapered flocculation

Flocculation BasinsFlocculation Basins

For cross-flow, tapered flocculation can be For cross-flow, tapered flocculation can be provided by:provided by:

Varying the paddle sizeVarying the paddle sizeVarying the number of paddlesVarying the number of paddlesVarying the diameter of the paddle wheelsVarying the diameter of the paddle wheelsVarying the rotational speed of the various shaftsVarying the rotational speed of the various shafts

For axial-flow, tapered flocculation can be For axial-flow, tapered flocculation can be provided by:provided by:

Varying the paddle sizeVarying the paddle sizeVarying the number of paddlesVarying the number of paddles

Vertical ShaftsVertical Shafts

Compartments are arranged to:Compartments are arranged to:Minimize short circuitingMinimize short circuitingFacilitate tapered flocculationFacilitate tapered flocculation

Example on FlocculationExample on Flocculation

A cross-flow, horizontal shaft, paddle wheel flocculation basin is to be designed for a A cross-flow, horizontal shaft, paddle wheel flocculation basin is to be designed for a flow of 25,000mflow of 25,000m33/d, a mean velocity gradient of 26.7/sec (at 10/d, a mean velocity gradient of 26.7/sec (at 10 C), and a detention C), and a detention time of 45 minutes. The GT value should be from 50,000 to 100,000. Tapered time of 45 minutes. The GT value should be from 50,000 to 100,000. Tapered flocculation is to be provided, and the three compartments of equal depth in series are flocculation is to be provided, and the three compartments of equal depth in series are to be used. The G values determined from laboratory tests for the three compartments to be used. The G values determined from laboratory tests for the three compartments are G1 = 50/sec, G2 = 20/sec, and G3 = 10/sec. These give an average G value of are G1 = 50/sec, G2 = 20/sec, and G3 = 10/sec. These give an average G value of 26.7/sec. The compartments are to be separated by slotted, redwood baffle fences, and 26.7/sec. The compartments are to be separated by slotted, redwood baffle fences, and the floor of the basin is level. The basin should be 1.5 m in width to adjoin the settling the floor of the basin is level. The basin should be 1.5 m in width to adjoin the settling tank. Determine:tank. Determine:

1. The GT value1. The GT value

2. The basin dimensions2. The basin dimensions

3. The power to be imparted to the water in each compartment 3. The power to be imparted to the water in each compartment

Example on FlocculationExample on Flocculation

SolutionSolution

The GT value = (26.7/sec)(45 min)(60 sec/min) = 72,100The GT value = (26.7/sec)(45 min)(60 sec/min) = 72,100

Since GT value is between 50,000 and 100,000, the detention time is satisfactory.Since GT value is between 50,000 and 100,000, the detention time is satisfactory.

Basin volume, V = (flow) Basin volume, V = (flow) (detention time) = (25,000 m (detention time) = (25,000 m33/d)(45 min)(hr/60 min) = 781 m/d)(45 min)(hr/60 min) = 781 m33

Profile area = (volume / width) = (781 mProfile area = (volume / width) = (781 m33 / 15 m) = 52.1 m / 15 m) = 52.1 m22

Assume compartments are square in profile, and x is the compartment width and depth.Assume compartments are square in profile, and x is the compartment width and depth.

Thus, (3x)(x) = 52.1Thus, (3x)(x) = 52.1 xx22 = 17.37 = 17.37 x = 4.17 mx = 4.17 m 3x = 3(4.17) = 12.51m3x = 3(4.17) = 12.51m

Then, Then, width = depth = 4.17 mwidth = depth = 4.17 m

length = 12.51 mlength = 12.51 m

volume = (4.17)(12.51)(15.0) = 783 mvolume = (4.17)(12.51)(15.0) = 783 m33

The Power, P = The Power, P = GG22VV (at 10(at 10 C, C, = 0.00131 N-s/m = 0.00131 N-s/m22))

P (for first compartment) = (0.00131 N-s/mP (for first compartment) = (0.00131 N-s/m22)(50)(5022/s/s22)(783 m)(783 m33/3) = 855 N-m/s = 855 J/s = 855 W/3) = 855 N-m/s = 855 J/s = 855 W

P (for second compartment) = (0.00131)(20P (for second compartment) = (0.00131)(2022)(783/3) = 137 W)(783/3) = 137 W

P (for third compartment) = (0.00131)(10P (for third compartment) = (0.00131)(1022)(783/3) = 34.2 W)(783/3) = 34.2 W

Coagulation & Flocculation in Coagulation & Flocculation in Wastewater TreatmentWastewater Treatment

The same aluminum and iron salts are The same aluminum and iron salts are used in wastewaterused in wastewater

Wastewater requires higher dosages (Wastewater requires higher dosages ( 300 mg/l) and coagulates faster than 300 mg/l) and coagulates faster than surface watersurface water

Beside coagulation, lime and iron salts Beside coagulation, lime and iron salts remove phosphorousremove phosphorous

Coagulant aids include polyelectrolytes, Coagulant aids include polyelectrolytes, addition of turbidity and lime additionaddition of turbidity and lime addition

Coagulation & Flocculation in Coagulation & Flocculation in Wastewater TreatmentWastewater Treatment

Rapid-mixing basins have detention time Rapid-mixing basins have detention time of 1 to 2 minutes (due to high SS and of 1 to 2 minutes (due to high SS and large coagulant dosage)large coagulant dosage)

Velocity gradients in rapid-mixing basins Velocity gradients in rapid-mixing basins are about 300/sec, which are lower than are about 300/sec, which are lower than those for water (due to nature of organic those for water (due to nature of organic solid)solid)

GT and T are lower than those used with GT and T are lower than those used with waterwater

Coagulation & Flocculation in Coagulation & Flocculation in Wastewater TreatmentWastewater Treatment

For alum and iron saltsFor alum and iron saltsT is typically 15 to 30 minT is typically 15 to 30 minG is typically 20 to 75/secG is typically 20 to 75/secGT is typically 10,000 to 100,000GT is typically 10,000 to 100,000

For limeFor limeT is typically 1 to 2 min in rapid-mixing basinsT is typically 1 to 2 min in rapid-mixing basinsT is typically 5 to 10 min in flocculation basinsT is typically 5 to 10 min in flocculation basinsG is typically G is typically 100/sec 100/sec