Settling and Floatation Part 1

Settling and Floatation – Settling and Floatation – Part 1Part 1

and Flotation Settling, and Flotation Settling, Gravity separation is a physical water and wastewater treatment Gravity separation is a physical water and wastewater treatment

processes in which suspended and floating solids are removed processes in which suspended and floating solids are removed from water by the force of gravity. from water by the force of gravity.

Suspended and floating solids are either heavier than water or Suspended and floating solids are either heavier than water or

lighter than water and accordingly there are two gravity lighter than water and accordingly there are two gravity separation processes, sedimentation and flotation. Some fine separation processes, sedimentation and flotation. Some fine solid particles with densities heavier than water get attached with solid particles with densities heavier than water get attached with air bubbles and are also removed by flotation.air bubbles and are also removed by flotation.

Settling or sedimentation is a unit operation in which solids are Settling or sedimentation is a unit operation in which solids are drawn toward a source of attraction. drawn toward a source of attraction.

Settling and FloatationSettling and Floatation

In gravitational settling, solids are drawn toward gravity; in In gravitational settling, solids are drawn toward gravity; in centrifugal settling, solids are drawn toward the sides of cyclones centrifugal settling, solids are drawn toward the sides of cyclones as a result of the centrifugal field; and in electric-field settling, as as a result of the centrifugal field; and in electric-field settling, as in electrostatic precipitators, solids are drawn to charge plates. in electrostatic precipitators, solids are drawn to charge plates.

Flotation is a unit operation in which solids are made to float to the 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 surface on account of their adhering to minute bubbles of gases (air) that rises to the surface. (air) that rises to the surface.

On account of the solids adhering to the rising bubbles, they are On account of the solids adhering to the rising bubbles, they are separated out from the water. This chapter discusses these three separated out from the water. This chapter discusses these three types of unit operations as applied to the physical treatment of types of unit operations as applied to the physical treatment of water and wastewater.water and wastewater.

Settling or SedimentationSettling or Sedimentation

• Settling has been defined as a unit operation in which solids Settling has been defined as a unit operation in which solids are drawn toward a source of attraction. The particular type are drawn toward a source of attraction. The particular type of settling that will be discussed in this section is of settling that will be discussed in this section is gravitational settling. It should be noted that settling is gravitational settling. It should be noted that settling is different from sedimentation, although some authors different from sedimentation, although some authors consider settling the same as sedimentation.consider settling the same as sedimentation.

• Strictly speaking, sedimentation refers to the condition Strictly speaking, sedimentation refers to the condition whereby the solids are already at the bottom and in the whereby the solids are already at the bottom and in the process of sedimenting. Settling is not yet sedimenting, but process of sedimenting. Settling is not yet sedimenting, but the particles are falling down the water column in response the particles are falling down the water column in response to gravity. Of course, as soon as the solids reach the to gravity. Of course, as soon as the solids reach the bottom, they begin sedimenting. In the physical treatment bottom, they begin sedimenting. In the physical treatment of water and wastewater, settling is normally carried out in of water and wastewater, settling is normally carried out in settling or sedimentation basins. We will use these two settling or sedimentation basins. We will use these two terms interchangeably.terms interchangeably.

Settling Tanks, Basins, or Settling Tanks, Basins, or ClarifiersClarifiersGenerally, two types of sedimentation basins (sometimes called Generally, two types of sedimentation basins (sometimes called

also tanks, or clarifiers) are used: also tanks, or clarifiers) are used:

rectangular and rectangular and circular. circular.

Rectangular settling, basins or clarifiers, as they are also called, Rectangular settling, basins or clarifiers, as they are also called, are basins that are rectangular in plans and cross sections. are basins that are rectangular in plans and cross sections. In plan, the length may vary from two to four times the In plan, the length may vary from two to four times the width. width.

The length may also vary from ten to 20 times the depth. The 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 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 introduced at one end and allowed to flow through the length of the clarifier toward the other end.of the clarifier toward the other end.


Sedimentation or settling is a process in which water is Sedimentation or settling is a process in which water is collected in basins and given proper detention time collected in basins and given proper detention time during which suspended solids present in water is during which suspended solids present in water is allowed to settle. allowed to settle.

Sedimentation is a process with low cost and low energy Sedimentation is a process with low cost and low energy

requirements, however, proper basin design is very requirements, however, proper basin design is very important for proper operation and better efficiency. important for proper operation and better efficiency.

In specifying a water and wastewater sedimentation tank In specifying a water and wastewater sedimentation tank size, the major features to be considered are:size, the major features to be considered are:

- tank cross sectional area,- tank cross sectional area,- tank depth,- tank depth,- and type of cleaning mechanism used.- and type of cleaning mechanism used.


In specifying a design basis for water and wastewater In specifying a design basis for water and wastewater sedimentation tanks; three conditions are commonly sedimentation tanks; three conditions are commonly considered:considered:

- solid handling capacity (ton/day),- solid handling capacity (ton/day),- overflow rate (gpm/ft2),- overflow rate (gpm/ft2),- and detention time. and detention time. - Additional design data required to ascertain Additional design data required to ascertain

mechanical construction, specific gravity of solids, mechanical construction, specific gravity of solids, size distribution of solids, underflow construction, size distribution of solids, underflow construction, operating temperature, and geographical location. operating temperature, and geographical location. Typical dimensions of sedimentation tanks are given Typical dimensions of sedimentation tanks are given in Table 1 bellow.in Table 1 bellow.

Table 1 Typical Dimensions of Table 1 Typical Dimensions of Sedimentation TanksSedimentation Tanks

____________________________________________________________________________________________________________ Description DimensionsDescription Dimensions Range TypicalRange Typical____________________________________________________________________________________________________________ RectangularRectangular Depth, m Depth, m 3‑5 3.5 3‑5 3.5 Length, m Length, m 15‑90 25‑40 15‑90 25‑40 Width, m Width, m 3‑24 6‑103‑24 6‑10 CircularCircular Diameter, m Diameter, m 4‑60 12‑454‑60 12‑45 Depth, m Depth, m 3‑5 4.53‑5 4.5 Bottom Slope, mm/m Bottom Slope, mm/m 60‑160 8060‑160 80____________________________________________________________________________________________________________

Shapes and Sizes of Shapes and Sizes of sedimentation Tankssedimentation Tanks

Water and wastewater sedimentation tanks Water and wastewater sedimentation tanks are mostly are mostly

cylindrical or cylindrical or rectangular in shape (See Figures bellow). rectangular in shape (See Figures bellow).

The ratio between width to length in The ratio between width to length in rectangular sedimentation tanks is ranging rectangular sedimentation tanks is ranging between 1:4 to 1:6 (see Table in slide #5).between 1:4 to 1:6 (see Table in slide #5).

Classification of Suspended Classification of Suspended ParticlesParticles بالماء العالقة الحبيبات بالماء تصنيف العالقة الحبيبات تصنيف

Suspended particles in water and wastewater have been Suspended particles in water and wastewater have been categorized into three general classes:categorized into three general classes:

1 ‑ Discrete particles 1 ‑ Discrete particles المنفردة المنفردة الحبيبات Particles that will not : Particles that will not :الحبيباتreadily flocculate, independent, settling rate is readily flocculate, independent, settling rate is independent of concentration, and flow rate is critical (see independent of concentration, and flow rate is critical (see Figure bellow‑class 1). Examples of discrete particles are Figure bellow‑class 1). Examples of discrete particles are sand, gravel washing, and silt.sand, gravel washing, and silt.

2 ‑ Flocculent particles 2 ‑ Flocculent particles المتلبدة المتلبدة الحبيبات Particles with : Particles with :الحبيباتrelatively low concentration, possible aggloromation, and relatively low concentration, possible aggloromation, and their settling is highly affected by detention time and flow their settling is highly affected by detention time and flow rate(see Figure bellow‑class 2).rate(see Figure bellow‑class 2).

3 ‑ Hindered particles 3 ‑ Hindered particles أوالمضغوطة التركيز عالية أوالمضغوطة الحبيبات التركيز عالية : :الحبيباتParticles with high suspended concentrations (as in sludge Particles with high suspended concentrations (as in sludge thickening), their settling is affected by mixing and the thickening), their settling is affected by mixing and the duration of detention time (see Figure bellow‑class 3).duration of detention time (see Figure bellow‑class 3).

Types of Particle SettlingTypes of Particle Settling• Type I settling , single or discrete particle, Type I settling , single or discrete particle,

applies to particles that settle with constant applies to particles that settle with constant velocity -- particles will be removed if velocity -- particles will be removed if v v > > vvss

• If particles flocculate during settling, velocity If particles flocculate during settling, velocity generally increases – Type II settlinggenerally increases – Type II settling

• As particle concentration increases with As particle concentration increases with depth, zone settling occursdepth, zone settling occurs

• At bottom of tank compression settling At bottom of tank compression settling occursoccurs

Types Of SedimentationTypes Of Sedimentation أنواع أنواع الترسيبالترسيبTypes of sedimentation are dictated byTypes of sedimentation are dictated by types of solids to be removed from water, types of solids to be removed from water,

therefore, for the three types of particles therefore, for the three types of particles in aqueous suspensions , three separate in aqueous suspensions , three separate mechanisms and theory of estimating mechanisms and theory of estimating settling velocities and removal ratessettling velocities and removal rates

better understanding of the process let us better understanding of the process let us first define the different settling first define the different settling properties of.properties of.

Types of Particle SettlingTypes of Particle Settling• Type I settling applies to particles Type I settling applies to particles

that settle with constant velocity -- that settle with constant velocity -- particles will be removed if particles will be removed if v v > > vvss

• If particles flocculate during settling, If particles flocculate during settling, velocity generally increases – Type II velocity generally increases – Type II settlingsettling

• As particle concentration increases As particle concentration increases with depth, zone settling occurswith depth, zone settling occurs

• At bottom of tank compression At bottom of tank compression settling occurssettling occurs

a. Discrete Particles a. Discrete Particles Settling Settling الحبيبات الحبيبات ترسيب ترسيب المنفردةالمنفردةFor discrete particles in aqueous suspensions and For discrete particles in aqueous suspensions and

which have density greater than water, it will which have density greater than water, it will accelerate downward under the force of gravity accelerate downward under the force of gravity until the resistance of the liquid equals the until the resistance of the liquid equals the effective weight of the particle. According to its effective weight of the particle. According to its weight, shape, and specific weight or density, weight, shape, and specific weight or density, discrete particle is affected by the gravity, drag, discrete particle is affected by the gravity, drag, and buoyancy force (see Figure bellow). and buoyancy force (see Figure bellow).

Settling velocity for a discrete particle is Settling velocity for a discrete particle is approximately constant and its magnitude depend approximately constant and its magnitude depend on shape, size, and density of the article, and on shape, size, and density of the article, and density and viscosity of the liquid. The force density and viscosity of the liquid. The force balance on the particle;balance on the particle;

FG = FD + FBFG = FD + FB

wherewhereννss = = settling velocitysettling velocityρρss = density of particle (kg/m = density of particle (kg/m33))ρρ = density of fluid (kg/m = density of fluid (kg/m33))gg = gravitational constant (m/s = gravitational constant (m/s2)2)

dd = particle diameter (m) = particle diameter (m)μμ = dynamic viscosity (Pa·s) = dynamic viscosity (Pa·s)

Sedimentation:Sedimentation:Particle Terminal Fall Particle Terminal Fall VelocityVelocity

Particle Terminal Fall Particle Terminal Fall Velocity (continued)Velocity (continued)

bd FWF

gVAC wppt

wPD )(2

2

wPD

wppt

AC

gV

)(2 2

dAp

p

3

2

w

wp

D

tC

gdV

3

4 2 ( )4 3

p wt

D w

gdVC

r rr-=

Force balance (zero acceleration)

3

3

4 rp 2rAp

We haven’t yet assumed a shape

Assume a _______sphere

Drag Coefficient:Drag Coefficient:EquationsEquations

Laminar flow R < 1

Transitional flow 1 < R < 104

Fully turbulent flow R > 104 4.0

34.0324

24

2/1

D

D

D

C

RRC

RC

dVR t

18

2wp

t

gdV

w

wpt

gdV

3.0

w

wp

D

tC

gdV

3

4 General Equation

Sedimentation of Small Sedimentation of Small Particles?Particles?• How could we increase the How could we increase the

sedimentation rate of small particles?sedimentation rate of small particles?

18

2

wpt

gdV

Increase d (stick particles together)

Increase g (centrifuge)

Decrease viscosity (increase temperature)

Increase density difference(dissolved air flotation)

Graphical Solution to Settling VelocityGraphical Solution to Settling Velocity

Overflow Rate, QOverflow Rate, Qovrovr = Q/A = Q/A

QQArea A

Solids Loading Rate, SLR = (Q*C)/ASolids Loading Rate, SLR = (Q*C)/A

Q * CQ * CArea A

h

l

vl

vs

vl

vs

Rectangular clarifiersRectangular clarifiers

h

l

vl vs

vl vs

In horizontal tank some percentage of particles with vs < vo will be removed P = 100 (vP = 100 (vss/v/voo) = ) = percentage of percentage of particles removed particles removed with a settling with a settling velocity of vvelocity of vss in a in a rectangular rectangular sedimentation sedimentation basin designed with basin designed with an overflow an overflow rate of vrate of voo

Removal EfficiencyRemoval Efficiency

Example (1)Example (1)

Example(2)Example(2)

Solution:Solution:

Item Range Typical Range Typical

Detention time(hr)1.5-2.521.5-2.52

Average Overflow rate(mt3/mt2 . d)32-484024-3228

Peak hourly Overflow rate(mt3/mt2 . d)80-12010049-9959

Weir loading (mt3/mt.d)125-500250125-500250


Depth(mt)3-4.53.6----

Length(mt)15-9024-39----

Width(mt)3-244.8-9.6----

Flight speed(mt/

min)0.6-1.20.9----

Length:width3:14:1----

Length:depth15:1------

Bottom Slope(cm/

mt)6.3-178.5----

RectangularRectangular


Depth(mt)3-4.53.6----

Diameter(mt)3-6012-45----

BottomSlope(cm/mt)6.3-178.5----

Flight travel speed(r/m

in)0.02-0.050.03----

Circular/CylindricalCircular/Cylindrical

FLOW-THROUGH VELOCITY AND FLOW-THROUGH VELOCITY AND OVERFLOW RATEOF SETTLINC OVERFLOW RATEOF SETTLINC BASINSBASINS

Design Criteria Design Criteria for for Sedimentation Sedimentation TanksTanks• ______________________________________________________________

• ______________________________________________________________• ______________________________________________________________• ______________________________________________________________• ______________________________________________________________

Minimal turbulence

Uniform velocity

No scour of settled particles

Slow moving particle collection system

Q/As must be small (to capture small particles)

Settling zone

Sludge zoneInle

t zo

ne Outle

t zo

ne

This will be one of the ways you can improve the performance of your water filtration plant.

Non-Idealities in SettlingNon-Idealities in Settling

Example (3)Example (3)For the water For the water quality data given quality data given in the following in the following table, determine table, determine the overall removal the overall removal efficiency and the efficiency and the change in change in fractional fractional removal ?removal ?

Example (4)Example (4)

A settling basin is designed to have a surface overflowrate of 32.6 m/d. A settling basin is designed to have a surface overflowrate of 32.6 m/d. Determine the overall removal efficiency for a suspension with Determine the overall removal efficiency for a suspension with particle size distribution given bellow:particle size distribution given bellow:

________________________________________________________________________________________________________________________________________ dp, mm dp, mm 0.15 0.15 0.12 0.12 0.10 0.10 0.080.08 0.060.06 0.040.04 0.020.02

0.01 0.01 ________________________________________________________________________________________________________________________________________MassMassFraction Fraction 5 10 35 60 80 90 5 10 35 60 80 90

9595 100 100 Less ThanLess Than________________________________________________________________________________________________________________________________________Also given the characteristics of water and solid particles:Also given the characteristics of water and solid particles: Particle WaterParticle Water________________________________________________________________________________________________________________________________________Density, kg/l 1.350 0.999Density, kg/l 1.350 0.999Kin. Viscosity ‑ 1.027 x 10‑3Kin. Viscosity ‑ 1.027 x 10‑3 kg‑sec/m2kg‑sec/m2Temperature, oC ‑ 20oCTemperature, oC ‑ 20oC________________________________________________________________________________________________________________________________________

Example (5)Example (5)Settling velocity of a solid particle Settling velocity of a solid particle

is 0.0044 m/s in water at 15 oC. is 0.0044 m/s in water at 15 oC. Compute the overflowrate in Compute the overflowrate in

gpd/ft2. What is the minimum gpd/ft2. What is the minimum detention time in hours to detention time in hours to

settle out this flocc if the depth settle out this flocc if the depth of the sedimentation tank is 15 of the sedimentation tank is 15

feet ?feet ?

Example (6)Example (6)A rectangular sedimentation tank A rectangular sedimentation tank

is to be designed for a flow of 1 is to be designed for a flow of 1 mgd using a 6:1 length/width mgd using a 6:1 length/width

ratio, an overflowrate of ratio, an overflowrate of 0.00077 fps, and a detention 0.00077 fps, and a detention

time of 3 hours. What are the time of 3 hours. What are the dimensions of the basin ?dimensions of the basin ?

Example (7)Example (7)A 120 feet in diameter 15 feet deep A 120 feet in diameter 15 feet deep cylindrical sedimentation tank has cylindrical sedimentation tank has

an influent flow of 10 mgd. an influent flow of 10 mgd. Compute the overflowrate and Compute the overflowrate and

detention time? Is the estimated detention time? Is the estimated overflowrate and detention time overflowrate and detention time are within the acceptable range? are within the acceptable range?

With influent suspended solids With influent suspended solids concentration of 1500 mg/l. concentration of 1500 mg/l.

estimate the solid loading rate?estimate the solid loading rate?

Example (8)Example (8)A sedimentation tank 25 m in A sedimentation tank 25 m in

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

Settling and Floatation Part 1

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