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

-=

Clarifier theory

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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

Clarifier

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•Circular => Sludge removed from the middle•Water enters in the middle

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

Lamella or tube settlers

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Robescu et al. 2010

Lamella settler

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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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Flotaatio (DAF)

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)

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Belt press

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Belt press

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Centrifuge

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Centrifuge