flocculation -...

FLOCCULATION NUR ISTIANAH,ST.MT.,M.Eng

INTRODUCTION

Flocculation: primary particles to approach close enough together, make contact and progressively form larger

agglomerates

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Transport Mechanisms

• Brownian motion: for relatively small particles which follow random motion and collide with other particles (perikinetic motion)

• Differential settling: Particles with different settling velocities in the vertical alignment collide when one overtakes the other (orthokinetic motion)

Design of Flocculator (Slow & Gentle mixing)

Flocculators are designed mainly to provide enough interparticle contacts to achieve particles agglomeration so that they can be effectively removed by sedimentation or flotation

Mechanism

• arises from thermal agitation (Brownian movement)

• Naturally random process

• Assumed particle in radial motion

Perikinetic flocculation

• arises from induced velocity gradients in the liquid

• Mechanical supports

Orthokinetia flocculation

Velocity gradient

• Passage around baffles

• Mechanical agitation

• Granular filter bed

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Cross flow Flocculator (sectional view)

Plan (top view)

L

H

W

Mechanical Flocculator

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Hydraulic Flocculation

• Horizontally baffled tank

Plan view (horizontal flow)

• Vertically baffled tank

L Isometric View (vertical flow)

L

W

H

The water flows horizontally.

The baffle walls help to create

turbulence and thus facilitate mixing

The water flows vertically. The baffle

walls help to create turbulence and thus

facilitate mixing

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Hydraulic flocculators: simple technology

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Hydraulic Flocculation: Pipe

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Hydraulic Flocculation: Pipe

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Mechanical flocculators

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Mecahnical flocculators

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Mechanical flocculators

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Flocculators integrated with settling

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Flocculators integrated with settling

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Flocculators both sides of settling

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Flocculator perforated wall (in background)

Velocity of flocculator

• The greater the velocity gradients induced in the liquid the more particle contacts there will be within a given time.

• However, the greater the velocity gradients the smaller will be the ultimate floe size due to a continuous breakdown of the larger flocs. Hence, for a given velocity gradient there will be a limiting flocculation time beyond which floc particles will not grow further.

• The lower the velocity gradient the longer will be the time needed to reach the optimum floe size, but the larger will be the final floc size.

Limiting time

Consideration of Flocculation

• The principal parameters governing the effective degree of flocculation are the retention time and velocity gradients applied also type of coagulant and flocculant

• These parameters influence the rate and extent of particle aggregation and the rate and extent of break-up of these aggregates

Consideration of Flocculation

• Having assessed each chemical combination alternative in terms of both destabilization

(clarification) efficiency and flocculation efficiency, some form of final choice in terms of

overall economics is required.

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Typical layout of a water treatment plant

REFERENCES

• Solid Liquid Separation Equipment Selection and Process Design

• Coagulation and floculation

• Coagulation and Flocculation in Water Treatment By J(Hans) van Leeuwen

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