Sedimentation ProcessFF

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CEG 507: Lectur e Seri es No 4: Treatment Process- Sedimentati on. Apr il 7, 2014

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Sedimentation Process

It is the removal of solid particles from suspension by settling under gravity.

Sedimentation process is used for the removal from water discrete particles, flocculatedmatter, and precipitation formed during various water treatment operations.

Classification of Settling Behaviour/Categories of Sedimentation Process

i.

Unhindered of discrete particles

ii.

Settling of dilute suspension of flocculent particles.

iii.

Hindered / zone settling

iv.

Compression settling or compaction settling

The above could be referred to as four classes of settling behaviour of particles.

Settling Of Discrete Particles

A discrete particle moving vertically downward through a quiescent fluid will accelerate until

the frictional resistance of drag of the fluid equals the impelling force acting upon the

particle, particle then settles at a uniform velocity.

Vp= particle settling velocity (m/s)

d = the drag force

w = submerged weight of the particle

d = the diameter of the particle (m)

Ap = projected area of particle normal at direction of motion (m2)

p = volume of particle (m3)

= density of particles (kg/m3)

f = fluid density (kg/m3)

= dynamic viscosity of fluid (N.s/m2).

Figure 1: An idealized spherical particle


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CD= drag coefficient

To find an expression for Vp (particle settling velocity);

Drag force on the particle is given:

D = CD Ap2

2

pV................ eqn 1

Taking the submerged weight of particles as:

w = (p -f)g p .................. eqn 2

Substitute for Ap and p , and re-arranging by equating equations 1 and 2

Vp= Ddf Cg /3/4

Drag coefficient CD is a function of the Reynolds number (Re). Re for spherical particle

therefore:

i. Re< 1, CD= 24/Re

ii.

For 1 < Re


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i. Particles diameter

ii. Particles density

iii. Fluid viscosity

iv. Temperature

In practical sedimentation tanks, the terminal settling velocity is reached quickly, discrete

particlesand in uniform fluid flow; the settling velocity is constant throughout the settling

time.

Sedimentation in an Ideal Tank

In ideal rectangular horizontal flow sedimentation:

i. Inlet zone where water is distributed along the cross-section.

ii.

A settling zone which removes suspended particles and is in a quiescent state.

iii. A sludge zone which collects settled particles.

iv. An outlet zone through which water along with non-settleable particles is carried

outside the tank.

The critical particle for design purpose is that one that enters at the top of the settling zone at

point A.

It settles with velocity just sufficient to reach the sludge zone at the outlet end of the tank at

point B.

Velocity components of such a particle = Vh in the horizontal direction, while Vp (theterminal settling velocity) is in the vertical direction.

Time require for the particle to settle, considering the geometry of the tank i.e. detention

time.

to=Vh

L

Vp

H

Note:Vh (velocity in the horizontal direction) = Q/WH

Vp= Q/WL (Surface Area)

Q = flow rate

L = length of tank

W= width of tank

H = depth of tank

Note: the surface area of the tank (A) = W * L

Hence Vp=Q/WL = Q/A

[Vp = Q/A]-the termi nal settli ng velocity whi ch is also the sur face loading rate or overf low rate.


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Figure 2: Types of Sedimentation tanks

For any particle to settle in an ideal sedimentation tank; a settling velocity of Q/Ais essential.


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Note: (Q/A) = the surface loading rate or overflow rate is a fundamental parameter governing

sedimentation tank performance.

Sedimentation efficiency is independent of tank depth (under the condition that the froward

velocity is low enough to ensure that settled materials is not scoured and re-suspended fromthe tank floor.

Circular Radial Flow Tank

Same relationship as affects ideal rectangular tank applies here (See figure).

Vp = Q/A, Vp= terminal settling velocity

Detention time: to= H/Vp

to= 2

1

R

R hvr

Since Vr=rH

Q

2

Then, t = 2

1

.2 R

Rdrr

Q

H

= Q

HRR 2

1

2

2

Since, r 21

2

2 RR = A, hence

Q

AH

V

H

p

Vp A

Q

Ideal Upflow Sedimentation Tank

Particle will only be removed if the settling velocity (Vp) exceeds the upflow velocity of

water.

Minimum upflow velocity is represented by (Q/A) (flowrate / surface area)

The minimum settling velocity needed equals Q/A). See Figure 1c

Particle remover occurs when Vu= Q/A

Vp= Vu

Also: Vp= Q/A


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Note:

(i) In ideal sedimentation tank, horizontal or radical flow pattern:

-

Particles with settling velocity less than: Vp= Q/A, i.e.n

Vp , will also be removed in

the proportion (1/n).

(ii) In ideal upflow tank, particle with settling velocityA

Qp will be removed.

We have just treated the first class of settling, unhindered settling of discrete particles. The

remaining three are:

i. Settling of dilute suspension of flocculent particles.

ii.

Hindered/ zone settling

iii. Compression or compaction.

Class II: Settlement of Dilute Suspension of Flocculent Particles

This class of settling behaviour is for organic waste matter, biological solids, and flocculated

impurities.

Sedimentation Process:i.

Variety of particles sizes settle at different rates, larger particles settle first,

overtaking, slower settling particles.

ii.

Colliding particles coalesce to form larger particles with increased settling

velocity

i i i . Depth should be greater enough to provide the opportunity for particle

agglomeration to occur. Hence, sedimentati on tanks for f locculent suspension

must have depth consideration.

iv. Detention time (t) =Q

WLH

Q

, note WL = Area (A)

= volume of settling zone (m3)

Q = flow rate (m3/s)

Detention time (t) =Q

AH,

AQ

Hwhere L, W, and H are length and the depth of

the tank.


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Figure 3: Settling path in an ideal settling tank

Class III: Hindered Settling, Zone Settling and Sludge Blanket Clarifiers

Class three setting behaviour occurs with increasing particles concentration in suspension.

i. There is a reduced particle-settling velocity due to display of velocity field of the

fluid by adjacent particles overlap.

ii. There is also a reduced particle settling velocity known as hindered settl ing

iii.

When the whole suspension tends to settle as a blanket under extreme particleconcentration, it is known as zone settl ing.

Class IV: Compression or Compaction Settling

This class of settling behaviour occurs due to extreme condition of high particle

concentration.

Compressed particles get settled under the weight of overlying solids, with decrease in void

spaces with a corresponding squeezing of water out of the matrix. Compression settling is

very important in gravity thickening processes.

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