Anna Mikola TkT D Sc (Tech)
Physical & chemical treatmentprocesses of water and waste
Lecture 5 Gravity separation –sedimentation, flotation, thickening,dewatering
WAT - E2120
Lecture content- Gravity separation
- Settling- Lamella settlers- Flotation- Thickening
- Dewatering
- Theories and practice- Juho Kaljunen: Example of
our research - Phosphorusrecovery
Vocabulary:Phase separationGravity separationSettler = Sedimentation basin(solids settle)Clarifier = solids up or down
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Size
Compounds
Mechanisms
Unitprocesses
Objective oftreatment
Particles Colloids Dissolved GasMechanical
PrecipitationOxidation
Thermal and catalytic
Adsorption
Course on Biological processes
Absorption
Degradation? Separation? Recovery?
Media filtrationCycloneFlotationElectroflotationSedimentation
CombustionGasificationPyrolysis
CoagulationElectrocoagulationFlocculation
Destabilization
OzonationUV irradiationChlorinationAOP
Membrane filtrationActivated carbonIon exchange
ExtractionGas transfer
1000 mm – 0.1 µm 0.1µm – 0.01µm 0.01 µm -
Electro-chemical
Some vocabulary
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Vocabulary:Phase separationGravity separation
Settler = Sedimentation basin(solids settle)Clarifier = solids up or down
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Adapted from Prof. Pagilla Illinois Institute of technology
DiscreteSettling of dilute suspensions, little or no
tendency to flocculate
FlocculentSettling of dilute suspensions with flocculation
taking place during the settling
Hindered (Zone settling)Particles settle as a mass; inter-particle forces
hold them in fixed position, settlement in a zone
CompressiveSettlement provided by the compacting mass
resulting from particles that are in contact
Gravity separation PhenomenaPhenomena Description Application
Discrete particle settling Low solids concentrationIndividual settling
Grit removal
Flocculent settling Quite dilute suspension,some coalescing
Settling basins
Hindered or zone settling Intermediate concentrationNeighbouring particles hinder
Settling basins
Compression settling Compression by the weight ofparticles
Wastewater settling
Ballasted flocculentsettling
Inert ballasting agent orchemical to improve settling
Polymers,microsand,
Accelerated gravitysettling
Settling in an accelerationfield
Grit and sandremoval
Flotation Particles rise up due to air orgas bubbles
Removal of greaseand oil, other lightparticles
Particle settling - theoryThe particle is influenced bygravitational force, buoyancyand frictional force dependingon fluid density, viscosity andparticle diameter
Stokes’ law
Vs = settling velocityg = gravitational constantDp = particle diameterµ = water viscosityρ = density of particle/water
Valid in laminar settling (Re < 1)
Reynolds number
Laminar NR < 1Transitional NR = 1 – 2000Turbulent NR > 2000ρp/w = density of particle/waterd = diameter of particleμ = dynamic viscosityΦ= shape factor
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mrr
18)( 2
pps
DgV
-=
Particle settling theory
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Particles that settle faster than the hydraulicretention time of the basin are removed.
Removal of particles in discretesettling
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v0 = h / t = Q / A
Where: t = V/QA = surface area of the basin
In a settling basin all the particles that have a terminal velocityequal or greater than v0 are removed.
Flocculent and zone settling
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Sludge zone
Length, L
Water LevelParticle trajectory
Settling zone
Slud
ge z
one
dept
h
Free
Boar
dSi
de W
ater
Dep
th H
0
D
HH0
0.5 m
In flocculent and zone settling the effects on settling velocity aredetermined by settling tests
How to measure settleability inbiological sludges?
Sludge settleability (ml/1000 ml):measured in 1000 ml column during½ hSludge volume index SVI (ml/g)
Settleability is divided by MLSS(g/l)DSVI (ml/g) diluted SVI (dilution toobtain less than 250 ml settleability)
Good settling sludge:•SVI < 100 ml/gBad settling sludge:•SVI > 250 ml/g
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Applications for gravity settling- Flocculation can be enhancedby chemicals- Used widely in drinking water
treatment to separate theflocs and in wastewatertreatment to settle andthicken the sludge
- Low energy demand, quitehigh space requirement
- Horizontal (rectangular orcircular) or vertical
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Clarifier
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JScraper moves the sludge to asludge pocketJWater flows into overflow weirs atthe end
JSurface load 2-3 m/h (primary clar)JRetention time 1-2 h
Sludge
Water
Horizontal
Simple Solids/Liquid Separators• Percent solids removal
– Primary ≈ 65%– Final ≈ 99.8 %– Membrane ≈ 100%– Thickener ≈ 85 – 95 %– Centrifuge solids capture ≈ 90%
• Calculated on mass flow basis• Difference provides effluent solids
Settler design – engineering approach
Design parameters used inpractice:- the surface overflow rate(hydraulic loading);
- the solids loading rate(sludge mass loading);
- the volumetric loading rate(sludge volume loading);
- the weir loading rate(hydraulic loading of theoverflow weir).
Hydraulic loading rate ν = Qe/A(m3·m−2·h−1)
Solids loading rate is thehydraulic loading ratemultiplied by the incomingsolids concentration NA = νXa(kg·m-2·h-1) For biologicalsludge NOTE: should be <6
Volumetric loading rate is thesolids loading rate multipliedby the sludge volume indexSVI. If the units of SVI are in l/g,then the units of the volumetricloading are l/m2 h.NOTE: should be <1.5
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DEMO 1Design a secondary clarifier for awastewater treatment with thedesign flow rate of 1577 m3/h usingsurface loading rate of 0,8 m/h.Verify the design with solids loadingand volumetric loading.MLSS is 3 g/l and SVI in worst case250 ml/g. Use a max flow coefficientof 1.43.
Surface loading rate 0,8 m/hàClarifier area = 1971 m2 (à 2000 m2)
Solids loading = 1,43 x 1577 m3/h x 3g/l / 2000 m2 = 3,38 kgSS/m2/d OK
Volumetric loading rate(SVI 250)= 0,85 m3/m2/d OK
Note: 1,43 for max flow
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Lamella settler theory
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- Inclined plates are added in the clarifier basin- Distance between the plates usually 5 – 10 cm- Reduces the vertical distance a floc must settle
before agglomerating to form larger particles
Lamella settling
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In countercurrent settingtime to settle isdetermined:
=
w = distance betweenplatesΘ = angle of plates fromhorizontalV = settling velocity
Ballasted settling
- Heavy particlesare added toinhanceflocculation andsettingE.g. Actiflo®Densadeg®
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Gravity separation in accelerated flowfieldRotating flow creates a freevortexIn free vortex the tangentialvelocity times radius is aconstantVelocity’s in the middle arehigher resulting in separationof particles
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Flotation
• Separation is obtained by introducing fine gas bubblesinto the liquid
• The bubbles attach to the particulate matter• The buoyant force of the combined particle causes the
particle to rise to the surface• Surface is skimmed to remove the particles• Usually dissolved-air flotation DAF, also dispersed-air
flotation
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Working principle of flotation• A part of the effluent into
which air is dissolved underpressure is recycled in theprocess.
• When this recycle flowenters the process, pressuredrops and air forms micro-bubbles in the wastewater
• These bubbles attach orform within solids or flocscausing them to float
• Solids are removed with askimmer from the surface
Demands energy, but savesspace
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Flotation calculation• The performance of a DAF systems
depends on the ratio of air to mass ofsolids (A/S)
• = , ( )
sa is air solubility ml/l (solubility of oxygenand nitrogen)f is fraction of air dissolved at pressure PP is pressure (atm)Sa is influent suspended solids (mg/l) 22/01/2020
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Dewatering
• Separation ofwater fromsolids
• Usually withbiologicalsludge
• up to 20 – 40%
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Water thatcan beremoved bythickening
Capillarywater; can beremoved bymechanicaldewatering
Water insidethe microbialcells andadsorbed onthe solids;can beremoved bythermalmethods
Thickening Mechanical dewatering Thermal drying
Water content (%)
Dry solid content (%)
Volu
me
(l)