wr230 l12 sedimentation.ppt

8/11/2019 WR230 L12 Sedimentation.ppt

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FACULTY OF CIVIL ENGINEERING AND THE BUILT ENVIRONMENT

WATER RESOURCES ENGINEERING DEPARTMENT

R 450: WATER RESOURCES MANAGEMEN


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LECTURE 12

SEDIMENTATION

Introduction

Sedimentation theory

Sizing of the tanks

Tank construction Examples of sedimentation in water

treatment practice


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IntroductionEngineered Water Systems

Sedimentation


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IntroductionExample: Surface Water Treatment

Surface water

from supply

RapidMix

FlocculationBasin

Sedimentation

basin

Sludge

RapidSand Filter

Disinfection

StorageToDistribution

System

Screen


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Introduction

What is sedimentation???

Sedimentation is a physical treatment process inwhich particles with a higher mass density than

the surrounding liquid will move downwardsunder the influence of gravity.

In water treatment, sedimentation is a primarytreatment process e.g. when using Rapid SandFilters (RSF) or Slow Sand Filters (SSF) forfiltration


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Introduction

Why sedimentation is important process???

RSF require - waters with turbidity of 10 - 20 parts

per million (ppm) of suspended solids (SS) while

SSF can filter water with 2 - 5 ppm of suspended

solids directly - without pretreatment.

Maximum allowable amount of suspended solids

in RSF for double stage filtration systems is about

50 ppm.


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Introduction

In wastewater treatment, one often finds flowsheets with Grit chamber, primary sedimentation

tanks and final or secondary sedimentation tanks.


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Settling of a single particle in quiescent (still)water


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Fd= Frictional drag (due to flow resistance)

Fi= Impelling force (due to submerged weight)

S = terminal or settling velocity of the particle

S is a function of (s- w, d and V)

Discrete settling refers to Non sticking togetherof particles during settling.

V = Kinematic viscosity and d = particle

diameter


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When a discrete particle with a density s

greater than the density of a fluid (water, w) is

released in quiescent conditions, it will

accelerate downwards until the frictional drag(Fd) of the fluid equals the value of the

impelling force (Fi) after which the vertical

velocity of the particle with respect to thesurrounding fluid will remain constant as S.

Illustration required


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Let Fi= Impelling force and Fd= Frictional drag

Fi= {s-w) x g x V

Where: s, w.: Are mass density of particle and water

respectively.

V = Volume of the particle

According to Newton,

ASCF wDd2

2

ASCF wDd2

2


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Where:

CD = Dimensionless number (dragcoefficient)

A = "Projected area" in the direction

of motion. Note that a projection of aspherical particle in any plane is

circular.

ASCF wDd2

2


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Equality of both forces with uniform

movement gives the settling velocity S.

Derivation is required, For Fi = Fd and

spherical particles with diameter = d

6

3d

V

AVg

CS

w

ws

D

...2

4

2dA

Therefore


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gd

CS

wD

ws

3

4

Note:CDvaries with the ReynoldssNumber.

For example: Laminar conditions i.e. streamline flow


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dSR

e

. ,

Re

24

DC

For turbulent conditions, i.e. Re >2000, CDis almost constant and for spherical particles (up

to Re < 105), CD=0.40


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2


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Variation of particle settling velocity withtemperature

Figure overleaf shows the settling velocities ofseveral types of discrete spherical particles in

quiescent water at 10C

ASCF wDd2

2


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Discrete spherical

particles terminal

settling velocity-

diameter relationship at

10oC



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t

t SS

1010

. (5)

Where:

S10= Settling velocity at 10C

V10= Kinematic viscosity at 10C

Vt= Kinematic Viscosity at tC

For other temperatures, use the following equation (5)


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Discrete settling in

a continuous

horizontal flow

basin.

A typical horizontal

flow sedimentation

tank


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HB

Q

A

Qvo

./ and applying proportionality

L

HVS

L

H

V

SoO

o

o .;

So= The Overflow rateor

surface loadof the

sedimentation tank

On substitutionAQ

LBQ

HBLHQSo

....

A

QSO

Where: A = The plan

area of the A tank

Discrete settling in a continuous horizontal flow basin.

A typical horizontal flow sedimentation tank


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ASCF w 2


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Cumulative Frequency Distribution of SettlingVelocities

ASCF wDd2

2

ASCF w 2


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In this experiment, concentrations of impurities

(C) at (h1

) are measured at given time intervals

(t)and the same may conveniently be done for

samples from (h2). The results can be plotted in

a graph as shown below:

ASCF wDd2

ASCF wDd2

2

ASCF w 2


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Sedimentation theoryASCF Dd

2

ASCF wDd2

2

A typical cumulative frequency curve

ASCF w 2


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All particles with settling velocity greater thanSo will be removed: (1. - Po) %

Other particles traveling at a surface overflowrate lower than or equal to So will be removedpartially

The above two components can be depicted onthe cumulative frequency graph for settlingvelocities as shown below

ASCF Dd2

ASCF wDd2

2

ASCF w 2


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ASCF Dd2

ASCF wDd2

2

Derivation of the total removal ratio, r


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If, r = removal ratio; from the graph (Fig. 2.5)

oP

oo

dpS

SPr0

.1 .. (11)

and (11) can be re-arranged such that:

Total removal ratio, r = (1 - Po) + OP

oo

SdpS

1(12)

- Note that the integral part of eqn. (12) represents the shaded area on the graph

he basic idealizations which lead to the


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he basic idealizations which lead to thedetermination of the total removal ratio, r

ASCF wDd2

2

ASCF wDd2


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Sizing of tanks

The following design criteria are followed

Tank plan area (A)

Given the flow Q and the settling distribution curve,

A = Q/So

Therefore, So should be chosen according to the

outlet water quality requiredwhich is established by

laboratory tests on raw water.

ASCF Dd2

ASCF wDd2

2

ASCF wDd2


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Sizing of tanks

Length: Width ratio In order to ensure minimal or no reduction in basin

settling efficiency and to stimulate quiescent

conditions, rectangular horizontal flow plain

sedimentation tanks require that

Length: Width ratio = L: B be controlled such that:

L: B: = (6 l0):1

This ensures existence of plug flow conditions

ASCF Dd2

ASCF wDd2

2

ASCF wDd2


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Sizing of tanks

In Plug flow, there is no mixing of particlesduring flow

Complete mixing is applicable in design of

facultative waste stabilization ponds

Dd2

ASCF wDd2


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Sizing of tanks

To take care of basin instability and shortcircuiting, the Reynold's number and Froude

number should be kept such that

Re = Vo. R/< 2000 and Fr = Vo2/(g.R) > 10-5(flow stability)

Dd2

ASCF wDd2

2


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Sizing of tanks

Where R = Hydraulic radius = A/P (m)

P = Wetted perimeter (m)

Vo= Horizontal flow velocity (m/sec.)

Dd2

ASCF wDd2

2


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Sizing of tanks

Thus such effects are avoided:

If a tracer test is carried out in order to

determine hydraulic efficiency, in general the

curves indicated overleaf will be obtained

Dd2

ASCF wDd2

2


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Sizing of tanks

The efficiency of a basin may be reduced from

an ideal basin due to following cross current:

By eddy currents which are set up by the inertia of

the incoming fluid

Wind-induced currents when basins are not covered

By convection currents that are thermal in origin and

By density currents that cause cold or heavy water to

underrun a basin and warm and lighter water to flowacross its surface

All these will cause short circuiting of flow and reduce

the performance of the basin efficiency

2


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Si i f k


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Sizing of tanks

Bottom Scouring Velocity

Settled solids should not be scoured off the basin Scour begins if the scour velocity, Vsc

With = 0.05 (grain friction factor)

= 0.03 (Hydraulic friction factor)

d = particle diameter (m) Since scour will not occur if Vo < Vsc, this condition is the

standard design criterion

o

w

ws

SC VdgV

..

8

ASCF wDd2

2


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Sizing of tanks

Shape of the settling zone In practice the shape of the settling zone is a

compromise between on one hand hydrodynamic

requirements and on the other the economic

consideration

As a rule of thumb, the formula, H = 1/12. (L0.8)

gives a sufficient and acceptable depth for tanksprovided with continuous sludge removal

mechanisms

2

ASCF wDd2

2


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Sizing of tanks

For manual cleaning or otherwise, allow say a

storage depth of at least 1.0m

In addition, by considering the construction site

constraints like the position of the ground water

table and excavation, the total depth of such tanks

'H' usually lies in the range of 2-6 meters.

2

ASCF wDd2

2


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Sizing of tanks

Common values of overflow rates, So, forsedimentation tanks

2

ASCF wDd2

2


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Sizing of tanks

Detention time (Td)Td = 2 - 4 (6) hours for plain sedimentation

The detention time should be kept on the higher

side in case of:Requirements with respect to high removal efficiency

Light particles

2

ASCF wDd2

2


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Tank construction

MaterialsMain structure

Reinforced concrete, Quarry stone or masonry

Baffle walls Reinforced concrete/wood

Outlet troughs Aluminum sheeting or stainless steel

2

ASCF wDd2

2


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Tank construction2

Parts of aSedimentation

Tank

1... Inlet structure

2... Sedimentationor settling

zone

3... Outlet structure

4... Sludge storagezone

ASCF wDd2

2


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Tank construction

(1) Inlet structure: Objectives: To even out the distribution of flow over

the cross - sectional area

ASCF wDd2

2


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Tank construction

Clifford inlet

ASCF wDd2

2


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Tank construction

Stuttgarter inlet

ASCF wDd2

2


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Tank construction

In order to reduce the load of the outlet weir toacceptable levels, usually a number of troughs

can be provided either in the direction parallel to

water flow or across it.

Examples of sedimentation in water treatment practice


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p p

Flow sheet for treating heavily polluted water ("Coagulation -ASCF wDd

2

2


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Flow sheet for treating heavily polluted water ( Coagulation

clarification" process)

ASCF wDd2

2


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Tank construction

Materials

Main structure

Reinforced concrete, Quarry stone or masonry

Baffle walls

Reinforced concrete/wood

Outlet troughs

Aluminum sheeting or stainless steel


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Engineered Water Systems


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Sizes of Particles in Water


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Ground- vs. Surface Water

Groundwater

constant composition

high mineral content low turbidity

low colour

low or no D.O. high hardness

high Fe, Mn

Surface water

variable composition

low mineral content high turbidity

coloured

D.O. present low hardness

taste and odor


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Surface Water Treatment

Primary objectives are to

1. Remove suspended material (turbidity)

and colour

2. Eliminate pathogenic organisms

Treatment technologies largely based on

coagulation and flocculation


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Surface Water Treatment

Surface waterfrom supply

RapidMix

FlocculationBasin

Sedimentation

basin

Sludge

RapidSand Filter

Disinfection

StorageToDistributionSystem

Screen


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Groundwater Treatment

Primary objectives are to

1.Remove hardness and other minerals

2.Eliminate pathogenic organisms

Treatment technologies largely based on

precipitation


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Groundwater Treatment

Ground waterfrom wells

Sedimentation

basin

Sludge

Recarbo-nation

To Distri-butionSystem

RapidMix

FlocculationBasin

Disinfection

Storage

wr230 l12 sedimentation.ppt

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