Settling and Floatation

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Settling and Floatation Part 1
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and Flotation Settling,
Gravity separation is a physical water and
wastewater treatment processes in which suspended
and floating solids are removed from water by the
force of gravity. Suspended and floating
solids are either heavier than water or lighter
than water and accordingly there are two gravity
separation processes, sedimentation and
flotation. Some fine solid particles with
densities heavier than water get attached with
air bubbles and are also removed by
flotation. Settling or sedimentation is a unit
operation in which solids are drawn toward a
source of attraction.
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Settling and Floatation

• In gravitational settling, solids are drawn
  toward gravity in centrifugal settling, solids
  are drawn toward the sides of cyclones as a
  result of the centrifugal field and in
  electric-field settling, as in electrostatic
  precipitators, solids are drawn to charge plates.
  
• Flotation is a unit operation in which solids are
  made to float to the surface on account of their
  adhering to minute bubbles of gases (air) that
  rises to the surface.
• On account of the solids adhering to the rising
  bubbles, they are separated out from the water.
  This chapter discusses these three types of unit
  operations as applied to the physical treatment
  of water and wastewater.

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Settling or Sedimentation

• Settling has been defined as a unit operation in
  which solids are drawn toward a source of
  attraction. The particular type of settling that
  will be discussed in this section is
  gravitational settling. It should be noted that
  settling is different from sedimentation,
  although some authors consider settling the same
  as sedimentation.
• Strictly speaking, sedimentation refers to the
  condition whereby the solids are already at the
  bottom and in the process of sedimenting.
  Settling is not yet sedimenting, but the
  particles are falling down the water column in
  response to gravity. Of course, as soon as the
  solids reach the bottom, they begin sedimenting.
  In the physical treatment of water and
  wastewater, settling is normally carried out in
  settling or sedimentation basins. We will use
  these two terms interchangeably.

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Settling Tanks, Basins, or Clarifiers

• Generally, two types of sedimentation basins
  (sometimes called also tanks, or clarifiers) are
  used
• rectangular and
• circular.
• Rectangular settling, basins or clarifiers, as
  they are also called, are basins that are
  rectangular in plans and cross sections. In
  plan, the length may vary from two to four times
  the width.
• The length may also vary from ten to 20 times the
  depth. The depth of the basin may vary from 2 to
  6 M. The influent is introduced at one end and
  allowed to flow through the length of the
  clarifier toward the other end.

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Settling or Sedimentation

• Sedimentation or settling is a process in which
  water is collected in basins and given proper
  detention time during which suspended solids
  present in water is allowed to settle.
• Sedimentation is a process with low cost and low
  energy requirements, however, proper basin design
  is very important for proper operation and better
  efficiency.
• In specifying a water and wastewater
  sedimentation tank size, the major features to be
  considered are
• - tank cross sectional area,
• - tank depth,
• - and type of cleaning mechanism used.

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Settling or Sedimentation

• In specifying a design basis for water and
  wastewater sedimentation tanks three conditions
  are commonly considered
• - solid handling capacity (ton/day),
• - overflow rate (gpm/ft2),
• and detention time.
• Additional design data required to ascertain
  mechanical construction, specific gravity of
  solids, size distribution of solids, underflow
  construction, operating temperature, and
  geographical location. Typical dimensions of
  sedimentation tanks are given in Table 1 bellow.

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Table 1 Typical Dimensions of Sedimentation Tanks

• __________________________________________________
  ____
• Description
  Dimensions
• 
  Range Typical
• __________________________________________________
  ____
• Rectangular
• Depth, m 3-5
  3.5
• Length, m 15-90
  25-40
• Width, m 3-24
  6-10
• Circular
• Diameter, m 4-60
  12-45
• Depth, m 3-5
  4.5
• Bottom Slope, mm/m 60-160
  80
• __________________________________________________
  ____

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Shapes and Sizes of sedimentation Tanks

• Water and wastewater sedimentation tanks are
  mostly
• cylindrical or
• rectangular in shape (See Figures bellow).
• The ratio between width to length in rectangular
  sedimentation tanks is ranging between 14 to 16
  (see Table in slide 5).

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Classification of Suspended Particles ?????
???????? ??????? ??????

• Suspended particles in water and wastewater have
  been categorized into three general classes
• 1 - Discrete particles ???????? ????????
  Particles that will not readily flocculate,
  independent, settling rate is independent of
  concentration, and flow rate is critical (see
  Figure bellow-class 1). Examples of discrete
  particles are sand, gravel washing, and silt.
• 2 - Flocculent particles ???????? ????????
  Particles with relatively low concentration,
  possible aggloromation, and their settling is
  highly affected by detention time and flow
  rate(see Figure bellow-class 2).
• 3 - Hindered particles ???????? ????? ???????
  ?????????? Particles with high suspended
  concentrations (as in sludge thickening), their
  settling is affected by mixing and the duration
  of detention time (see Figure bellow-class 3).

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Types of Particle Settling

• Type I settling , single or discrete particle,
  applies to particles that settle with constant
  velocity -- particles will be removed if v gt vs
• If particles flocculate during settling, velocity
  generally increases Type II settling
• As particle concentration increases with depth,
  zone settling occurs
• At bottom of tank compression settling occurs

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Types Of Sedimentation ????? ???????

• Types of sedimentation are dictated by
• types of solids to be removed from water,
  therefore, for the three types of particles in
  aqueous suspensions , three separate mechanisms
  and theory of estimating settling velocities and
  removal rates
• better understanding of the process let us first
  define the different settling properties of.

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Types of Particle Settling

• Type I settling applies to particles that settle
  with constant velocity -- particles will be
  removed if v gt vs
• If particles flocculate during settling, velocity
  generally increases Type II settling
• As particle concentration increases with depth,
  zone settling occurs
• At bottom of tank compression settling occurs

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a. Discrete Particles Settling ????? ????????
????????

• For discrete particles in aqueous suspensions and
  which have density greater than water, it will
  accelerate downward under the force of gravity
  until the resistance of the liquid equals the
  effective weight of the particle. According to
  its weight, shape, and specific weight or
  density, discrete particle is affected by the
  gravity, drag, and buoyancy force (see Figure
  bellow).
• Settling velocity for a discrete particle is
  approximately constant and its magnitude depend
  on shape, size, and density of the article, and
  density and viscosity of the liquid. The force
  balance on the particle
• FG FD FB

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SedimentationParticle Terminal Fall Velocity
where ?s settling velocity ?s density of
particle (kg/m3) ? density of fluid
(kg/m3) g gravitational constant (m/s2) d
particle diameter (m) µ dynamic viscosity
(Pas)
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Particle Terminal Fall Velocity (continued)
Force balance (zero acceleration)
We havent yet assumed a shape
sphere
Assume a _______
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Drag CoefficientEquations
General Equation
Laminar flow R lt 1
Transitional flow 1 lt R lt 104
Fully turbulent flow R gt 104
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Sedimentation of Small Particles?

• How could we increase the sedimentation rate of
  small particles?

Increase d (stick particles together)
Increase g (centrifuge)
Increase density difference (dissolved air
flotation)
Decrease viscosity (increase temperature)
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Graphical Solution to Settling Velocity
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Overflow Rate, Qovr Q/A

• Q

Area A
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Solids Loading Rate, SLR (QC)/A

• Q C

Area A
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Rectangular clarifiers
In horizontal tank some percentage of particles
with vs lt vo will be removed
P 100 (vs/vo) percentage of particles removed
with a settling velocity of vs in a rectangular
sedimentation basin designed with an overflow
rate of vo
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Removal Efficiency
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Example (1)
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Example(2)
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Solution
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Item                       Range             Typical                    Range               Typical Item                       Range             Typical                    Range               Typical Item                       Range             Typical                    Range               Typical Item                       Range             Typical                    Range               Typical Item                       Range             Typical                    Range               Typical
Detention time(hr) 1.5-2.5 2 1.5-2.5 2
Average Overflow rate(mt3/mt2 . d) 32-48 40 24-32 28
Peak hourly Overflow rate(mt3/mt2 . d) 80-120 100 49-99 59
Weir loading (mt3/mt.d) 125-500 250 125-500 250
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Rectangular
Item                 Range            Typical Range          Typical
Depth(mt) 3-4.5 3.6 -- --
Length(mt) 15-90 24-39 -- --
Width(mt) 3-24 4.8-9.6 -- --
Flight speed(mt/min) 0.6-1.2 0.9 -- --
Lengthwidth 31 41 -- --
Lengthdepth 151 -- -- --
Bottom Slope(cm/mt) 6.3-17 8.5 -- --
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Circular/Cylindrical
Item                         Range                 Typical   Range            Typical
Depth(mt) 3-4.5 3.6 -- --
Diameter(mt) 3-60 12-45 -- --
BottomSlope(cm/mt) 6.3-17 8.5 -- --
Flight travel speed(r/min) 0.02-0.05 0.03 -- --
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FLOW-THROUGH VELOCITY AND OVERFLOW RATEOF
SETTLINC BASINS
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Design Criteria for Sedimentation Tanks

• _______________________________
• _______________________________
• _______________________________
• _______________________________
• _______________________________

Minimal turbulence
Uniform velocity
No scour of settled particles
Slow moving particle collection system
Q/As must be small (to capture small particles)
This will be one of the ways you can improve the
performance of your water filtration plant.
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Non-Idealities in Settling
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Example (3)For the water quality data given in
the following table, determine the overall
removal efficiency and the change in fractional
removal ?
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Example (4)

• A settling basin is designed to have a surface
  overflowrate of 32.6 m/d. Determine the overall
  removal efficiency for a suspension with particle
  size distribution given bellow
• __________________________________________________
  __________________
• dp, mm 0.15 0.12 0.10 0.08 0.06 0.04
  0.02 0.01
• __________________________________________________
  __________________
• Mass
• Fraction 5 10 35
  60 80 90 95 100
  
• Less Than
• __________________________________________________
  __________________
• Also given the characteristics of water and solid
  particles
• 
  Particle Water
• __________________________________________________
  __________________
• Density, kg/l 1.350
  0.999
• Kin. Viscosity -
  1.027 x 10-3
• kg-sec/m2
• Temperature, oC -
  20oC
• __________________________________________________
  __________________

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Example (5)

• Settling velocity of a solid particle is 0.0044
  m/s in water at 15 oC. Compute the overflowrate
  in gpd/ft2. What is the minimum detention time
  in hours to settle out this flocc if the depth of
  the sedimentation tank is 15 feet ?

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Example (6)

• A rectangular sedimentation tank is to be
  designed for a flow of 1 mgd using a 61
  length/width ratio, an overflowrate of 0.00077
  fps, and a detention time of 3 hours. What are
  the dimensions of the basin ?

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Example (7)

• A 120 feet in diameter 15 feet deep cylindrical
  sedimentation tank has an influent flow of 10
  mgd. Compute the overflowrate and detention
  time? Is the estimated overflowrate and
  detention time are within the acceptable range?
  With influent suspended solids concentration of
  1500 mg/l. estimate the solid loading rate?

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Example (8)

• A sedimentation tank 25 m in diameter and 4.5
  deep treating 15 mgd of surface water. With
  suspended solids of 1500 mg/l. Estimate the
  detention time, the overflow rate and the solid
  loading rate?

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