Coagulation aided Sedimentation

This material deals with type-II settling (Hindered settling), mechanism involved in arresting colloidal particles. Definitons, Types of mixing devices, flash mixing, and flocculators

1. COAGULATION AIDED SEDIMENTATION
Stabilityanddestabilizationofcolloids,Coagulationtheory,
typesofcoagulants,chemicalfeeding,flashmixing,
Clariflocculators,typesofflocculatorsandtheirdesign
aspects
14/18/2020
Dr. Dayananda H S
Professor of Civil Engineering,
VVCE, Mysore, Karnataka

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Preamble
•CoagulationandFlocculationdealswithTypeIIsettling(i.e.,
Hinderedsettling)
•Particles-Silt,smallandlargecolloids
•Typesofcolloids–HydrophilicandHydrophobic
•Forcolloids,ratioofsurfaceareatomassishigh,resultingin
electrostaticrepulsionandhydrationpredominates
•Settlingvelocityvariesfromfewmeter/hrtometer/millionyear
•Hydrophiliccolloidsarereadilydispersedinwaterandits
stabilitydependsonaffinityforwaterandarenegatively
charged

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•Eg-Soaps,solublestarch&proteins,synthetic
detergents
•StabilityofHydrophobiccolloidsdependsonpossessed
electriccharge,noaffinityforwaterandpositivelycharged
Eg-metaloxides
•Particlescarrychargeduetolossofatoms,abrasion,
molecularstructure
•Vanderwal’sforceisaresistingforceandBrownian
movementfavorscoagulationprocess
•Acolloidaldispersioninaliquidisknownas‘Sol’
Eg:Bentoniteclay,Kaolinite

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Definitions
Coagulationisaunitprocess(chemicaltechnique)
adoptedforchargeneutralizationofcolloidalparticles.Ion
transferiseffectedbychemicalcoagulation
•Flocformingchemicalsareaddedtowaterforthepurpose
ofenmeshingthecolloidalparticlestoformrapidlysettling
aggregates
•Mixingreferstoblendingofcoagulatingchemicalswith
watertocreatemoreorlesshomogeneoussingleor
multiplephasesystem

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Flocculationisthestirringoragitationofwatercontaining
flocs,aslowmixingtechniquewhichpromotesthe
agglomeration(togather)ofthestabilizedparticles
•Distinctionbetweencoagulationandflocculationis
somewhatarbitrary
•However,combinationofmixing&stirring/agitationthat
enhancesaggregationisknownasFlocculation

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Stability of Colloids
•Acolloidalsuspensionissaidtobestablewhenthedispersion
donotshowanytendencytoaggregate
•i.e.,Colloidalsuspensionthatdonotagglomerateistermed
as‘stable’
•StabilityofColloidalSuspensionisduetoexcessivelylarge
surfacetovolumeratioduetoverysmallsize
•Surfacephenomenapredominatesmassphenomena
•SurfacePhenomenaistheaccumulationofelectricalcharges
@theparticlesurface
•ElectricalConductivityisduetomoleculararrangementwithin
crystals,lossofatomsduetoabrasionofthesurfaceorany
otherfactors
•Inmostsurfacewaters,thesurfaceofthecolloidalparticlesare
negativelycharged

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Destabilization of Colloids
•Twoforces responsible for destabilization of a sol are
i) Vander Waal’s force and ii) Brownian movement
•VanderWaal‘sforces-molecularcohesiveforcesofattraction
thatincreaseinintensityasparticlesapproacheachother
•Thisforcebecomesverydominantwhentwocolloidscomein
closeproximity
•Brownianmovementisrandommovementofcolloidscaused
bymolecularbombardmentofthedispersionmedia
•Thismovementhasdestabilizingeffectonasourceandresults
withaggregation
•Inwaterpurification,destabilizationofHydrophobiccolloidsis
attainedbychemicallycoagulatingusingtrivalentmetallicsalts
•Aluminumsulfate,ferricchlorideintoclusterswhicharelarge
enoughtoberemovedbygravitysettling

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Principle(Mechanism)ofCoagulation
TheprocessofCoagulationandFlocculationismainlybased
ontwofactors
i)Flocformation
Whenacoagulantisaddedtowater,itdissolvesinitand
formsaspongygelatinoussubstanceformingflocwhichwill
arrestthesuspendedandcolloidalmatterinwater,while
descendingdownatthebottom.
ii)Electricalcharge
Thecolloidalparticlesinnaturalwaterswillbeusually
negativelycharged.Theflocwhichispositivelychargedis
oppositeinnatureandhenceithelpsinarrestingallthe
colloidalparticles.

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Factors affecting coagulation
1. Types of coagulant
2. Quantity of coagulant
3. Characteristics of water -Type & quantity of suspended matter,
Temperature & pH of water
4. Time, turbulence and method of mixing
Types of Coagulant
Mainly used coagulants are aluminum and iron salts
1. Aluminum sulfate
2. Chlorinated copperas
3. Ferrous sulfate and lime
4. Magnesium carbonate and lime
5. Sodium Aluminate

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1. Aluminum Sulfate (Alum)
•Aluminumsulfateischeaperanduniversallyusedcoagulant
•It contains 17% Aluminum sulfate, a dirty grey solid in form of
lumps
•Alumonadditiontowater,hydrolyzeformingainsoluble
gelatinousprecipitateofAluminumhydroxide,Al(OH)
3
•To produce the hydroxide floc, enough alkalinity should be
present in the water
•If alkalinity is not enough, then it should be added. Usually
hydrated lime is used for the purpose (optimum pH is 6.5 –
8.5)
•Under normal circumstances, dose of Alum varies from 10 to
30 mg/L of water

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Sometimes sodium carbonate (Soda ash) is added to form
alkalinity, the resulting reaction is ,
Al
2(SO
4)
3.18H
2O + 3Na
2CO
2= 2Al (OH)
2+ 3Na
2SO
4+18H
2O
+ 3CO
2 ….(c)
Al
2(SO
4)
3.18H
2O + 3Ca (HCO
3)
2= 2Al (OH)
3+ 3CaSO
4+18H
2O
+ 6CO
2 ….(a)
Al
2(SO
4)
3.18H
2O + 3Ca (OH)
2= 2Al (OH)
3+ 3CaSO
4+18H
2O
….(b)

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Advantages and disadvantages
Alum reduces taste and odor, Cheap and Easily available and
Soluble in water
The only Disadvantage isdifficult to dewater the sludge

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2. Ferrous Sulfate (Chlorinated Copperas)
• The optimum pH range is 3.5 to 6.5
• At higher pH i.e. 9.5 it removes manganese
• More expensive than alum
• Effective in colourremoval
• Low solubility in water

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6FeSO
47H
2O +3Cl
2= 2Fe
2(SO
4)
3+ 2FeCl
3+7H
2O ….(a)
•The ferric sulfate and ferric chloride produced
instantaneously are known as chlorinated copperas
•Both of these are immediately available for the formation of
ferric hydroxide floc; the resulting reactions are:
Fe
2(SO
4)
3+3Ca(OH)
2= 3CaSO
4+ 2Fe(OH)
3….(b)
2FeCl
3+3Ca(OH)
2= 3CaCl
2+ 2Fe(OH)
3….(c)

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3. Ferrous Sulfate and lime
• Ferrous sulphatecan react with natural calcium bicarbonate
alkalinity in water, but its slow process
• Hence Lime is added in water
• Ferric hydroxide is gelatinous floc, which is heavier than floc
formed by alum
• Optimum pH range is below 7
FeSO
4.7H
2O + Ca(OH)
2= Fe(OH)
2+ CaSO
4+7H
2O ….(a)
•The ferrous hydroxide, Fe(OH)2 thus formed, though an
efficient floc, is soon oxidized by dissolved oxygen in water &
ferric hydroxide is formed
4Fe(OH)
2+ O
2+ 2H
2O = 4Fe(OH)
3 ….(b)

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4. Magnesium carbonate and lime
•When magnesium carbonate and lime are dissolved in
water, magnesium hydroxide and calcium carbonate are
formed as
•MgCO
3+Ca(OH)
2= Mg(OH)
2+CaCO
3
•Byproducts of above reaction forms soluble sludge, so not
commonly used

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5. Sodium Aluminate
•Sodium aluminate (Na
2Al
2O
4)is alkaline in nature and is used
very much less often than alum because of its cost. It reacts
with the salts of calcium and magnesium as follows;
Na
2Al
2O
4 + Ca(HCO
3)
2= CaAl
2O
4 +Na
2CO
3 +CO +H
2O …(a)
Na
2Al
2O
4+ CaCl
2 = CaAl
2O
4 + 2NaCl …..(b)
Na
2Al
2O
4+ CaSO
4= CaAl
2O
4+ Na
2SO
4…..(c)
•The coagulant remove both temporary and permanent
hardness
•It is effective for a pH range of 6 to 8.50 naturally available in
water

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Comparison of Alum and Iron salt
1.Iron salts forms heavy floc as compared to alum, hence
more Solids are removed
2.Time of reaction and floc formation is less for iron salts,
hence ‘t’ reduces
3.Iron salts can work efficiently over wider pH range
4.Iron salts can remove taste and odour
5.Less mud ball formation as compared to alum
6.Under some cases iron salts are more economical

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7. Iron salts cause staining and promotes growth of iron
bacteria
8.Iron salts make water more corrosive as compared to alum
9.Handling of iron salts requires skill
10. More CO
2is formed so water becomes corrosive
11.Alum Coagulation may not be proper, if K or Na are
present in water

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Chemical feeding
•Types of Feeding device -dry feeding (Powder form)
-Wet feeding (Solution form)
Fig: Dry feed devices a) Helical screw type b) Toothed wheel type

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Fig: Solution feeding device a)Conical plug type b) Adjustable weir type

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Fig: Hydraulic mixing in water flow
a)Channel with baffles
b) Overflow weir
c) Hydraulic jump mixing

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Flash Mixing (PerikineticFlocculation)
•Flashmixingisaprocessofaddingcoagulantchemicalsto
waterandthewaterismixedquickly
•Thepurposeofflashmixingistouniformlydispersethe
chemicalsthroughoutthevolumeofwater
•Thishelpsintheformationofmicroflocsandresultsin
properutilizationofchemicalpreventinglocalizationof
concentrationandprematureformationofhydroxides
•Flashmixingtypicallylastsforaminuteorless
•Ifthewaterismixedforlessthanthirtyseconds,thenthe
chemicalswillnotbeproperlymixedintothewater
•Ontheotherhand,ifthewaterismixedbeyondoneminute,
themixerbladeswillshearthenewlyformingflocintosmall
particles
•Thecoagulantisintroducedatapointofhighturbulence
•Therapidmixingofcoagulantsiscarriedoutinabasin
knownas“FlashMixer”

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Hydraulic jump: Hydraulic jump creates
Turbulance& thus helps in better mixing
Mechanical Mixing
Fig: a) Back Mix Impeller b) Flat-blade Impeller

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i)Flash mixer (Mixing basin)
•The power input required for mixing is expressed
in terms of temporal mean velocity gradient, G
’
•G
’
= [ P/µ.V]
1/2
G
’
= Velocity gradient (per second)
µ = Absolute or dynamic viscosity of water in Ns/m
2
V = Volume of water to which P is applied, in m
3
The unit of G’ will evidently be 1/s (i.e., per second)

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Design criteria
•Impeller speed: 100 to 120 rpm
•Detention period: 30 –60sec
•Power requirement: 2 to 5 kW/m
3
/minute
• (0.041kW/1000cum.day)
•Tank shape: circular or square
•Velocity of flow: 0.9m/sec
•Velocity gradient, G
’
: 300 S
-1
•Depth of tank: 1-3m
•Height to diameter (sides) ratio : 1:1 to 3:1
•Product G
‘
T (unit less): 30 000 -60 000
(T is the detention period)
•Impeller diameter to tank diameter ratio: 0.2 to 0.4
•Minimum Tanks: 2 Tanks

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Flocculation(FormingFlocs)
•Afterdestabilization(i.e.,Coagulation),particleswillbe
readytoatractandagglomerateandformflocs
•Butthisagglomerationisslowandtheyneedhelpto
acceleratethisagglomeration
•ThishelpiscalledFlocculation“whichistheslowstirringor
gentleagitationtoaggregatethedestabilizedparticlesand
formarapidsettlingfloc”
•Thisgentlemixingincreasesthecollisionsbetweenthe
particlesandhelpthemtoagglomerate.
(rapidmixingwilldestroytheflocs,henceneedgentle
mixing)
•FlocculationoccursinatankcalledaFlocculatoror
FlocculationBasinequippedwithamethodforSlowMixing.

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Fig: Flocculation process through Brownian movement

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Types of Flocculators(Slow stirring motion)
The most common types of Flocculatorsused,
a)Hydraulic Mixing Floculator:Hydraulic jump mixing
Horizontal baffled Channel
Vertically baffled channel
b) Mechanical Mixing Floculator:
i) Vertical shaft with impeller (turbine or propeller type blades)
ii) Paddle type with either horizontal or vertical shafts
iii) Walking Beam Flocculator
c) Pneumatic methods: Compressed air agitation,
centrifugal pump

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Fig: Rapid Mixing of coagulant in Venturi/ ParshalFlume (Hydraulic Jump)

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Fig: Hydraulic Mixing Floculators
(b) Vertically baffled channel
(a) Horizontal baffled Channels

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Fig: Mechanical Flocculator(Paddle type with vertical shafts)

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Fig: Paddle type with horizontal shafts
Fig: Vertical shaft with impeller
(propeller type blades)

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Clariflocculator
•Clarifier and flocculatorprovided in single unit is known as
Clariflocculator
•The shapes of clariflocculators-Rectangular or Circular
•In case of circular clariflocculator, the flocculating chamber is
provided at the centreand clarifier component is formed by
the peripheral space
•Velocity of flow of water around 0.3m/min
•Mechanical scrapper provided to remove sludge

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Fig: Rectangular Clari-flocculator

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Effluent
Fig: Circular Clari-flocculator

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Design criteria of Flocculators
•Depth of tank : 3 -4.5m
•Detention period: 10 -40 minute (normally 30)
•Velocity of flow:0.2 -0.8m/s, (usual 0.4)
•Total paddle area :10-25% of surface area of tank,
(usual 15%)
•Peripheral velocity of blades:0.2-0.6m/s,
(usual 0.3 to 0.4)
•Factor G
’
T : 10
4
to 10
5
•Power consumption:10 -36 kW/MLD
•Outlet flow velocity:0.15 -0.25m/s

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FAQ
1. Explain the theory of coagulation.
2. Differentiate between coagulation and flocculation
3. List out coagulants used in water treatment and explain
any one with the help of chemical reactions.
4. Write detailed note on ‘design of flocculator’.
5. Compare Alum and iron salts as coagulants.
6. Sketch and name the components of : i) Flash mixer ii)
Floculationchamber.
7. What is coagulation? What factors affect it? Describe the Jar
test.
8. With the aid of a neat sketch, explain the different functional
components of sedimentation with coagulation unit.
9. Explain suitable device to i) Feed the coagulant ii) Mix the coagulant
10. Derive an expression for “Mean temporal Velocity” gradient in
flocculation of water.
11. With the help of chemical equation, explain the action of alum in water.
Explain with sketch, suitable device to feed the chemical to water.