Download - 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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Sedimentation theory
Settling of a single particle in quiescent (still)water
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Sedimentation theory
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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Sedimentation theory
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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Sedimentation theory
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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Sedimentation theory
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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Sedimentation theory
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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Sedimentation theory
gd
CS
wD
ws
3
4
Note:CDvaries with the ReynoldssNumber.
For example: Laminar conditions i.e. streamline flow
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Sedimentation theory
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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Sedimentation theory
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
Sedimentation theory
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Sedimentation theory
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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Sedimentation theory
Discrete settling in
a continuous
horizontal flow
basin.
A typical horizontal
flow sedimentation
tank
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Sedimentation theory
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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Sedimentation theory
Cumulative Frequency Distribution of SettlingVelocities
ASCF wDd2
2
ASCF w 2
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Sedimentation theory
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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Sedimentation theory
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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Sedimentation theory
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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d b ( )2008
Groundwater Treatment
Ground waterfrom wells
Sedimentation
basin
Sludge
Recarbo-nation
To Distri-butionSystem
RapidMix
FlocculationBasin
Disinfection
Storage