sedimentation and flotation

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Solid-LiquidSeparation in

Water TreatmentSettling and Flotation

J(Hans) van Leeuwen, DEE

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Introduction

The need to clarify water -Aesthetic and health reasons

Technologies available

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Topics of Discussion

Separation by settlingSeparation by flotationDirect filtrationSoftening

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Typical surface watertreatment process

Ferric/alum sludge

Ferric/Alum

ScreenTREAt

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Typical water treatmentprocess with lime softening

ime sludge

Ca(OH) 2 Lime

OR

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Typical layout of a water treatment plant

Solid- liquid separation in water treatmen

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Archimedes’s Principle

Examination of the nature of buoyancy shows that thebuoyant force on a volume of water and a submerged objectof the same volume is the same. Since it exactly supportsthe volume of water, it follows that the buoyant force on anysubmerged object is equal to the weight of the waterdisplaced. This is the essence of Archimedes principle.

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Terminal Velocity of a ParticleAn expression for V t from the submerged weight of

the particle, W, and the fluid drag force, D.

The drag force on a particle is given by

D = C D l A p V t2 /2

The submerged weight of the particle can be expressed as

W = - l )g s

Since D = W, the above, after substituting A p and p for particle diameter d _______________

V t = / 4 - l ) gd

3 l C D

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(20-40 m 3m -2d -1)

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Activated Sludge Mass Balance over Settler

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Weir Details

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LAMELLA SETTLING TANKSShortening the settling distance

Pipe bundlesalso used

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(20 – 40 m 3m -2d -1 or 0.8 – 1.7 m/h)

(2.5 mm/s) – limits weir loading rate to100 – 200 m 3m -1d -1

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FLOTATION

Separation of low density flocs

If flocs have a density very closeto that of water, it may benecessary to decrease their

density by adding gas bubbles

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FLOTATION

Separation of low density flocs

Methods of forming gas bubblesDiffusionVacuum

ElectrolysisDissolved air

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Surface Tension and Bubbles • The surface tension of water provides the wall tension

for the formation of bubbles. To minimize the wall tensionthe bubble pulls into a spherical shape ( LaPlace's law ).• Pressure difference between the inside and outside ofa bubble depends upon the surface tension and the radiusof the bubble. Visualize the bubble as two hemispheres• Note the internal pressure which tends to push thehemispheres apart is counteracted by the surface tensionacting around the circumference of the circle.• For a bubble with two surfaces providing tension, thepressure relationship is:

P i – P o = 4T/r Pi

P o P i

T

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La Place's Law

The larger the vessel radius, the larger the wall tensionrequired to withstand a given internal fluid pressure.

For a given vessel radius and internal pressure, a sphericalvessel will have half the wall tension of a cylindrical vessel.

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Wall Tension

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Why does wall tension increase with radius?

If the upward part of the fluid pressure remains the same, then thedownward component of the wall tension must remain the same.If the curvature is less, then the total tension must be greater inorder to get that same downward component of tension.

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Attachment to floc

Ratio of area/volume increaseswith decreasing size

Forces acting on a gas bubble:Internal pressure x area= surface tension x circumference

Easier for bubble to form againstsolid

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Stokes’s law for flotation

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Essential Elements in a Flotation Unit Process

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Overview of solid-liquid separation alternatives in water

DirectFiltration

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What you need to be able to do

Be able to size settling tanks on the basisof particle settling rates and identifyimportant zones in the settling tankBe able to do a mass balance over aflotation unit to account for air usage

Be able to size a flotation unit based onparticle sizes and densities