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Sedimentation and Sedimentation Tanks

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M.N.I.T.,JAIPUR SEDIMENTATION AND SEDIMENTATION TANKS
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Page 1: Sedimentation and Sedimentation Tanks

M.N.I.T.,JAIPUR

SEDIMENTATIONAND

SEDIMENTATION TANKS

Page 2: Sedimentation and Sedimentation Tanks

SEDIMENTATION AND SEDIMENTATION TANKS

• Removal of suspended particles by gravitational settling

• Sedimentation is accomplished by decreasing the velocity of the water below which the particles will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravity will remove them from the flow.

• Sedimentation tanks are designed to reduce the velocity of flow of water to permit to permit suspended solids to settle out of water by gravity

Page 3: Sedimentation and Sedimentation Tanks

FACTORS AFFECTING SEDIMENTATION Several factors affect the separation of settle able

solids from water. Some of the more common types of factors to consider are: PARTICLE SIZE :-The size and type of particles to be

removed have a significant effect on the operation of the sedimentation tank.

The shape of the particle also affects its settling characteristics. A round particle, for example,

will settle much more readily than a particle that has ragged or irregular edges

Page 4: Sedimentation and Sedimentation Tanks

• WATER TEMPERATURE• Another factor to consider in the operation of a sedimentation basin

is the temperature of the water being treated. When the temperature decreases, the rate of

settling becomes slower.

CURRENTS Several types of water currents may occur in the sedimentation basin: • Density currents caused by the weight of the solids in the tank, the

concentration of solids and temperature of the water in the tank. • Eddy currents produced by the flow of the water coming into the

tank and leaving the tank.

Page 5: Sedimentation and Sedimentation Tanks

TYPES OF SEDIMENTATION

• PLAIN SEDIMENTATION - when impurities are separated from fluid by action of natural forces alone

• CLARIFICATION – WHEN CHEMICALS (sedimentation with coagulation) when chemicals are added to hasten aggregation of finely suspended matter

• CHEMICAL PRECIPITATION – when chemicals are added to throw dissolved impurities out of solution than it is called chemical precipitation

Page 6: Sedimentation and Sedimentation Tanks

TYPES OF SETTLING• TYPE 1 OR DISCRETE SETTLING – also known as “FREE

SETTLING” as particles have little tendency to flocculate. It is sedimentation when particles have low concentration

• particles settles as individual entities• Example of Type I settling is the settling of silt, grit etc.,

from river water before applying to the slow sand filters.• TYPE 2 OR FLOCCULANT SETTLING – refers to dilute

suspensions which flocculate during sedimentation. This results in increase in mass and hence settling rate increase with depth

• Example of TYPE 2 SETTLING are settling in primary settlers

Page 7: Sedimentation and Sedimentation Tanks

• TYPE 3 OR HINDERED SETTLING – refers to settling of suspension of intermediate concentration. Intermediate forces hold particles together and they settle as a unit.

• In hindered settling, the velocity gradients around each particle are affected by the presence of nearby particles. So the normal drag correlations do not apply

• Example of HINDERED SETTLING IS settling in secondary clarifiers

• TYPE 4 OR COMPRESSION SETLING – refers to settling of suspension of high concentration.

• On contact particles form a structure and settling takes place by compression of structure due to weight of particles

• Settling of SLUDGE is example of compression settling

Page 8: Sedimentation and Sedimentation Tanks

As seen above, not only does the settling take more time as the slurry becomes more

concentrated but, critical sedimentation point occurs at progressively higher interface heights

(Ref 4)

Page 9: Sedimentation and Sedimentation Tanks

SETTLING OF DISCRETE PARTICLES

• When a discrete particle is placed in a fluid it will accelerate until FRICTIONAL RESISTANCE(FD) also know as DRAG FORCE on the fluid equals the DRIVING FORCE (FI) when (FD) = (FI) particles attain a uniform velocity known as TERMINAL VELOCITY and now settles down with this constant velocity

Page 10: Sedimentation and Sedimentation Tanks

From Newton’s second law mdV/dt = Fg – Fb – Fd • where:• m = mass• V = velocity of particle• Fg = weight of particle

• Fb = force of buoyancy

• Fd = drag force

Page 11: Sedimentation and Sedimentation Tanks

• DRIVING FORCE (FI) = Fg - Fb = (Ss- S)gV

where S = density of fluid S s = particle density

g = acceleration due to gravity Vp = volume of particle

Page 12: Sedimentation and Sedimentation Tanks

FD depends upon :-

Dynamic viscosity Mass of fluid Shape and size of fluid DRAG FORCE is given by NEWTON’s LAW for frictional drag

as FD = CDASVS

2/2

FD = DRAG FORCE

CD = Drag coefficient

A= Projected area of particle

Page 13: Sedimentation and Sedimentation Tanks

• Therefore equation 1 can be rewritten as mdV/dt = (Ss- S)gVp - CDASVS

2/2

At steady state terminal velocity will be reached hence dV/dt = 0

Therefore (Ss- S)gVp = CDASVS

2/2

Solving for VS we get

VS = √(4(Ss- S)gd/3CD S)

where d = diameter of the spherical particle

Page 14: Sedimentation and Sedimentation Tanks

CD depends upon Reynolds's Number

For R between .5 to 104

CD= 24/R+3/√(R)+.34

For high Re >103 to 104

CD = .4

For Re < .5 CD = 24/R

Page 15: Sedimentation and Sedimentation Tanks

TYPE 2 SETTLING OR FLOCCULATION

Particles that change size, shape and perhaps specific gravity over time due to agglomeration while settling

Hence the settling velocity will increase Stroke’s equation cannot be used because

particles are constantly changing shape and size

Page 16: Sedimentation and Sedimentation Tanks

• A suggested equation for the variation of floc density with particle size is

Ss – S = kd-0.7

Where Ss = floc densityS = density of waterd = diameter of particleK = coefficient dependent on characteristic of water

and chemicals involvedThe rate of flocculation is proportional to mean

velocity gradient in the system , concentration of particle and the particle size

Page 17: Sedimentation and Sedimentation Tanks

A

Page 18: Sedimentation and Sedimentation Tanks

• To develop this we have to perform a settling column test in which suspension is kept in long column and allowed to settle under quiescent conditions

• Samples are withdrawn from this cylinder, at several depths at several time interval and % removal is determined

• Numerical value of % removal is located at the co ordinates identifying the time and depth of the sample and iso concentration lines connecting points of equal removal are drawn.

Page 19: Sedimentation and Sedimentation Tanks

TYPES OF SEDIMENTATION TANK

• Fill and Draw Type• Continuous Flow Type• Fill and Draw Type• In this, sedimentation tank is first filled with incoming water

and allowed to rest for a certain period of time and the particles get settled at the bottom

• Generally a detention period of 24 hours is allowed • At the end clear water is taken out through the outlet • Cleaning of tank may take another 6 to 12 hours • Hence operation takes place in about 36 hours• Minimum 3 such units are required for constant supply

Page 20: Sedimentation and Sedimentation Tanks

• Continuous Flow Type• velocity of particles is such a parameter which

can be easily controlled • In this, particles continue to flow with a small

velocity and the settable solids settle a the bottom before they reach the outlet

• They are of two types • (a)Horizontal Flow (b)Vertical Flow

Page 21: Sedimentation and Sedimentation Tanks

HORIZONTAL FLOW SEDIMENTATION TANK

• Horizontal flow tanks is based on following assumptions1)Within the settling zone the particles settle in similar

manner as they do in a QUIESCENT TANK of equal depth2)Flow is horizontal and steady and velocity is uniform in all

parts of settling zone3)Concentration of suspended particles of each size is same at

all points of vertical cross section at the inlet end4)Particles are removed when it reaches the bottom of

settling zone

Page 22: Sedimentation and Sedimentation Tanks
Page 23: Sedimentation and Sedimentation Tanks

• The basin is divided into four zones• INLET ZONE – from where the water enters the tank• SETTLING ZONE – where the suspended particles

get settled under gravity• SLUDGE ZONE – where the suspended particles get

collected after settling• OUTLET ZONE –from where water and left over

suspended particles assemble and get carried away outside the tank

Page 24: Sedimentation and Sedimentation Tanks

HORIZONTAL DISCHARGE VELOCITYAND

TIME OF FLOW

• Vd = Q/BH

• to = L/ Vd • Where • L = length • B = breath • H = height• Q = discharge• V d = horizontal discharge velocity

• to = time of horizontal flow

Page 25: Sedimentation and Sedimentation Tanks

• Consider a particle entering the tank to have a vertical falling speed of vs

• Time of falling through a distance H will be H/v s • For particles to reach the bottom before the

water leaves the tank the falling speed should be equal to the time of horizontal flow

• H/vs = LBH/Q

• vs = Q/LB• Hence it defines the surface overflow rate as flow

divided by plan area of the basin • Particles with speed greater than Q/A will get

settled

Page 26: Sedimentation and Sedimentation Tanks

DETENTION PERIOD• to = LBH/Q

• to is known as detention period

• If a particle with a falling speed less than vs’<Q/A enters the tank it will settle only a distane h during the detention time which is given as

• h/vs =to =LBH/Q

• h= LBHvs’ /Q

• But LBH/Q is vs

• Hence h=(vs’/vs)H• Ratio of removal of this size particle to that of settling

value vs is given by

Page 27: Sedimentation and Sedimentation Tanks

• xr = h/H = vs’/vs

• For example a tank of 400m2 surface area and rate of inflow is 1.6m3/sec than Q/A = .4cm/sec

• Hence all particle having speed vs = .4cm/sec or greater will be completely removed

• Only 50% particles having speed vs = .2cm/sec will be removed

• And 25% of particles having speed vs =.1cm/sec will be removed

Page 28: Sedimentation and Sedimentation Tanks

DESIGN ELEMENTS

Following are the important elements of design1)Detention Period and Displacement EfficiencyDetention period :- it is theoretical time taken by a

particle in water to pass between entry and exit of a settling tank

to = LBH/QFlow through period :-it is average time required by

a batch of water to pass through the settling tank and it is always less than the detention period due to short circuit effects

Page 29: Sedimentation and Sedimentation Tanks

• Displacement efficiency :- it is defined as the ratio of flow through period and the detention period

• ή = td/to

• 2)OVERFLOW RATE AND SURFACE LOADING :-• Quantity of water passing per hour per unit

horizontal area is known as overflow rate or surface loading

• vs = Q/A

Page 30: Sedimentation and Sedimentation Tanks

• 3)BASIN DIMENSIONS :- surface area of the basin is determined on the basis of surface loading rate or overflow rate

• surface area = (Volume of water in liters hours)/(surface loading rate)

• Length to width ratio should be from 3:1 to 5:1• Depth of the basin should be kept from 2.5 to

5m• Slope should be 1% in case of rectangular tanks

and 8% in case of circular tanks

Page 31: Sedimentation and Sedimentation Tanks

• 4) INLET and OUTLET ARRANGEMENTS• Ideal inlet structure should be such that• Uniformly distribute water across width and

depth of the tank• Mix it with water already in the tank to prevent

density currents• Minimize large scale turbulence• Initiate longitudinal and radial flow to so as to

achieve high removal efficiency • Outlet arrangements consists of weirs, notches or

orifices• Lauder• Outlet pipe

Page 32: Sedimentation and Sedimentation Tanks

SLUDGE REMOVAL• The particle settled in the basin consist of sludge

which should be removed either manually or mechanically

• Where the quantity of suspended matter is small than clearance is done once in 2 – 4 months

• And where it is large clearance is done quite often

• Also removal of sludge manually is very expensive

Page 33: Sedimentation and Sedimentation Tanks

CLARIFICATIONSEDIMENTATION WITH COAGULATION

• The efficiency of plain sedimentation is very low especially when it contains finely suspended matter and colloidal matter

• The coagulant neutralize the negative protective charge an the colloidal particles and allow them to coagulate

• This chemically assisted sedimentation is termed as CLARIFICATION

Page 34: Sedimentation and Sedimentation Tanks

Claification is achieved in 3 stages 1) addition of measured amount of chemicals known as

coagulants2)formation of precipitate which coagulates and form a floc3) sedimentation• Characteristics of Coagulants The ability of a chemical additive to produce coagulation is in general dependent on the

1)ELECTRIC CHARGE of the ion or molecule used as coagulant , the larger the charge the more effective the coagulant will be 2) SIZE OF ION or molecule used as coagulant the larger

the size of the molecule the more effective the coagulant will be.

Page 35: Sedimentation and Sedimentation Tanks

WORKING OF COAGULANTS

Page 36: Sedimentation and Sedimentation Tanks
Page 37: Sedimentation and Sedimentation Tanks

COMMON COAGULANT

1)Alum• One of the earliest, and still the most extensively

used coagulant, is aluminum sulfate (Al/S04)3 ·18 H2O), also known as alum.

• It is readily soluble in water. • When dissolved in water following hydrolysis reaction

takes place• Al2(SO4)318H2O +3Ca(HCO3)2Al(OH)3+

3CaSO4+18H2O+6CO2

• Aluminum hydroxide is insoluble in water and forms a floc

Page 38: Sedimentation and Sedimentation Tanks

• DOSAGE is 75 to 250 ppm in a pH range of 4.5 to 7

• RELATIVE COAGULANT POWER against negative colloids is greater than 1000

• (Ref 3)

DISADVANTAGES• It is effective only at certain pH range, and• good flocculation may not be possible with alum

in some waters.

Page 39: Sedimentation and Sedimentation Tanks

• Ferrous Sulphate:-Ferrous sulphate, ordinarily known as copperas, is fed usually in solution form with strength of 4 to 8 %. The alkalinity and pH value of natural water are too low to react with copperas to form the desired ferric hydroxide floc, It is necessary, therefore, to add lime with copperas to secure coagulation.

• The following reaction takes place• FeSO47H2O+Ca(OH)2Fe(OH)2+CaSO4 +7H2O• Ferrous hydroxide thus formed gets oxidized by

dissolved oxygen in water to form ferric hydroxide• DOSAGE is in between 70 to 200 ppm in a pH

range of 4 to 7

Page 40: Sedimentation and Sedimentation Tanks

LIME Lime is a generic name used to identify several combinations of calcium and oxygen, such as or CaO (quicklime). Lime often contains magnesium In the presence of bicarbonate ion lime will react to form a calcium carbonate precipitate That can remove colloids through the sweeping floc mechanism. The reaction involved is

Ca(OH)2+ Ca(HCO3)2 CaCO3 + 2H2O

DOSAGE should be in between 150 to 500 ppm and pH range may vary from 9 to 11 (Ref 2)

Page 41: Sedimentation and Sedimentation Tanks

COAGULANT AIDS

coagulant aids are additives that can be added to a destabilized colloidal suspension and

hence they tend to promote the growth oflarge, rapid-settling floc which can thenflocculate· Typical coagulant aids are:• Activated silica• Polyelectrolyte’s

Page 42: Sedimentation and Sedimentation Tanks

REFRENCES

1) WATER SUPPLY ENGNEERING By B.C.PUNMIA2) http://cpe.njit.edu/dlnotes/che685/cls07-1.pdf3) http://www.mrwa.com/OP-Sedimentation.pdf4)http://www.filtrationandseparation.com/thickener/sld006.htm5)http://lorien.ncl.ac.uk/ming/particle/cpe124p2.html

Page 43: Sedimentation and Sedimentation Tanks

THANK YOU


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