SEDIMENTATION. INTRODUCTION Sedimentation, or clarification, is the process of letting suspended...

SEDIMENTATIONSEDIMENTATION

INTRODUCTIONINTRODUCTION

Sedimentation, or clarification, is the process Sedimentation, or clarification, is the process of letting suspended material settle by gravity.of letting suspended material settle by gravity.

Suspended material may be particles, such as Suspended material may be particles, such as clay or silts, originally present in the source clay or silts, originally present in the source water.water.

More commonly, suspended material or floc is More commonly, suspended material or floc is created from material in the water and thecreated from material in the water and the

chemical used in coagulation or in other chemical used in coagulation or in other treatment processes, such as lime softening. treatment processes, such as lime softening.

INTRODUCTIONINTRODUCTION

Sedimentation is accomplished by Sedimentation is accomplished by decreasing the velocity of the water decreasing the velocity of the water being treated to a pointbelow which being treated to a pointbelow which the particles will no longer remain in the particles will no longer remain in suspension. suspension.

When the velocity no longer supports When the velocity no longer supports the transport of the particles, gravity the transport of the particles, gravity will remove them from the flow.will remove them from the flow.

FACTORS AFFECTING FACTORS AFFECTING SEDIMENTATIONSEDIMENTATION

PARTICLE SIZEPARTICLE SIZE The size and type of particles to be The size and type of particles to be

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

The shape of the particle also affects The shape of the particle also affects its settling characteristics.its settling characteristics.

All particles tend to have a slight All particles tend to have a slight electrical charge.electrical charge.


WATER TEMPERATUREWATER TEMPERATURE When the temperature decreases, the rate of When the temperature decreases, the rate of

settling becomes slower. The result is that as the settling becomes slower. The result is that as the water cools, the detention time in the water cools, the detention time in the sedimentation tanks must increase. sedimentation tanks must increase.

In most cases temperature does not have a In most cases temperature does not have a significant effect on treatment. significant effect on treatment.

A water treatment plant has the highest flow A water treatment plant has the highest flow demand in the summer when the temperatures demand in the summer when the temperatures are the highest and the settling rates the best. are the highest and the settling rates the best.

When the water is colder, the flow in the plant is When the water is colder, the flow in the plant is at its lowest and, in most cases, the detention at its lowest and, in most cases, the detention time in the plant is increased so the floc has time time in the plant is increased so the floc has time to settle out in the sedimentation basins.to settle out in the sedimentation basins.


CURRENTSCURRENTS Several types of water currents may occur in the Several types of water currents may occur in the

sedimentation basin:sedimentation basin:• • Density currents caused by the weight of the solids in the Density currents caused by the weight of the solids in the tank, the tank, the concentration of solids and temperature of the water in concentration of solids and temperature of the water in the tank.the tank.• • Eddy currents produced by the flow of the water coming Eddy currents produced by the flow of the water coming into the tank and into the tank and leaving the tank.leaving the tank.

The currents can be beneficial in that they promote The currents can be beneficial in that they promote flocculation of the particles. However, watercurrents also flocculation of the particles. However, watercurrents also tend to distribute the floc unevenly throughout the tank; as tend to distribute the floc unevenly throughout the tank; as a result, it does not settle out at an even rate.a result, it does not settle out at an even rate.

Some of the water current problems can be reduced by the Some of the water current problems can be reduced by the proper design of the tank. Installation of baffles helps proper design of the tank. Installation of baffles helps prevent currents from short circuiting the tank.prevent currents from short circuiting the tank.

SEDIMENTATION BASIN SEDIMENTATION BASIN ZONESZONES

Inlet zoneInlet zone The inlet or influent zone should provide a The inlet or influent zone should provide a

smooth transition from the flocculation smooth transition from the flocculation zone and should distribute the flow zone and should distribute the flow uniformly across the inlet to the tank. uniformly across the inlet to the tank.

The normal design includes baffles that The normal design includes baffles that gently spread the flow across the total gently spread the flow across the total inlet of the tank and prevent short inlet of the tank and prevent short circuiting in the tank. circuiting in the tank.

The baffle could include a wall across the The baffle could include a wall across the inlet, perforated with holes across the inlet, perforated with holes across the width of the tank.width of the tank.



Settling ZoneSettling Zone The settling zone is the largest portion of The settling zone is the largest portion of

the sedimentation basin. the sedimentation basin. This zone provides the calm area necessary This zone provides the calm area necessary

for the suspended particles to settle.for the suspended particles to settle.

Sludge ZoneSludge Zone The sludge zone, located at the bottom of The sludge zone, located at the bottom of

the tank, provides a storage area for the the tank, provides a storage area for the sludge before it is removed for additional sludge before it is removed for additional treatment or disposal.treatment or disposal.


Basin inlets should be designed to Basin inlets should be designed to minimize high flow velocities near the minimize high flow velocities near the bottom of the tank. bottom of the tank.

If high flow velocities are allowed to enter If high flow velocities are allowed to enter the sludge zone, the sludge could be the sludge zone, the sludge could be swept up and out of the tank.swept up and out of the tank.

Sludge is removed for further treatment Sludge is removed for further treatment from the sludge zone by scraper or from the sludge zone by scraper or vacuum devices which move along the vacuum devices which move along the bottom.bottom.


Outlet ZoneOutlet Zone The basin outlet zone or launder should The basin outlet zone or launder should

provide a smooth transition from the provide a smooth transition from the sedimentation zone to the outlet from the sedimentation zone to the outlet from the tank. tank.

This area of the tank also controls the This area of the tank also controls the depth of water in the basin. depth of water in the basin.

Weirs set at the end of the tank control the Weirs set at the end of the tank control the overflow rate and prevent the solids from overflow rate and prevent the solids from rising to the weirs and leaving the tank rising to the weirs and leaving the tank before they settle out.before they settle out.

PARTICLE REMOVALPARTICLE REMOVAL

In sedimentation, particles are falling In sedimentation, particles are falling from rest under the force of gravity. from rest under the force of gravity. Therefore in sedimentation, eqn. Therefore in sedimentation, eqn. (1.1) takes the familiar form of (1.1) takes the familiar form of Stokes' LawStokes' Law::

vvmm = = DD22gg((pp - - ff)/18)/18µµ


Stoke's Law applies only in streamline Stoke's Law applies only in streamline flow and strictly only to spherical flow and strictly only to spherical particles. particles.

In the case of spheres the criterion for In the case of spheres the criterion for streamline flow is that (Re) = 2, and streamline flow is that (Re) = 2, and many practical cases occur in the region many practical cases occur in the region of streamline flow, or at least where of streamline flow, or at least where streamline flow is a reasonable streamline flow is a reasonable approximation approximation


Stokes' Law applies only to cases in Stokes' Law applies only to cases in which settling is free, that is where the which settling is free, that is where the motion of one particle is unaffected by motion of one particle is unaffected by the motion of other particles. the motion of other particles.

Where particles are in concentrated Where particles are in concentrated suspensions, an appreciable upward suspensions, an appreciable upward motion of the fluid accompanies the motion of the fluid accompanies the motion of particles downward. motion of particles downward.


So the particles interfere with the flow So the particles interfere with the flow patterns round one another as they fall. patterns round one another as they fall.

Stokes' Law predicts velocities proportional Stokes' Law predicts velocities proportional to the square of the particle diameters. to the square of the particle diameters.

In concentrated suspensions, it is found In concentrated suspensions, it is found that all particles appear to settle at a that all particles appear to settle at a uniform velocity once a sufficiently high uniform velocity once a sufficiently high level of concentration has been reached. level of concentration has been reached.

Where the size range of the particles is not Where the size range of the particles is not much greater than 10:1, all the particles much greater than 10:1, all the particles tend to settle at the same rate. tend to settle at the same rate.


This rate lies between the rates that This rate lies between the rates that would be expected from Stokes' Law for would be expected from Stokes' Law for the largest and for the smallest the largest and for the smallest particles.particles.

In practical cases, in which Stoke's Law In practical cases, in which Stoke's Law or simple extensions of it cannot be or simple extensions of it cannot be applied, probably the only satisfactory applied, probably the only satisfactory method of obtaining settling rates is by method of obtaining settling rates is by experiment. experiment.

Gravitational Gravitational Sedimentation of Particles Sedimentation of Particles

in a Liquidin a Liquid Solids will settle in a liquid whose Solids will settle in a liquid whose

density is less than their own. density is less than their own. At low concentration, Stokes' Law will At low concentration, Stokes' Law will

apply but in many practical instances apply but in many practical instances the concentrations are too high.the concentrations are too high.


in a Liquidin a Liquid In a cylinder in which a uniform suspension is In a cylinder in which a uniform suspension is

allowed to settle, various quite well-defined allowed to settle, various quite well-defined zoneszones appear as the settling proceeds. appear as the settling proceeds.

At the top is a zone of clear liquid. At the top is a zone of clear liquid. Below this is a zone of more or less constant Below this is a zone of more or less constant

composition, constant because of the uniform composition, constant because of the uniform settling velocity of all sizes of particles. settling velocity of all sizes of particles.

At the bottom of the cylinder is a zone of At the bottom of the cylinder is a zone of sediment, with the larger particles lower sediment, with the larger particles lower down. down.

If the size range of the particles is wide, the If the size range of the particles is wide, the zone of constant composition near the top zone of constant composition near the top will not occur and an extended zone of will not occur and an extended zone of variable composition will replace it.variable composition will replace it.


in a Liquidin a Liquid In a continuous thickener, with settling In a continuous thickener, with settling

proceeding as the material flows through, proceeding as the material flows through, and in which clarified liquid is being taken and in which clarified liquid is being taken from the top and sludge from the bottom, from the top and sludge from the bottom, these same zones occur. these same zones occur.

The minimum area necessary for a The minimum area necessary for a continuous thickener can be calculated by continuous thickener can be calculated by equating the rate of sedimentation in a equating the rate of sedimentation in a particular zone to the counter-flow particular zone to the counter-flow velocity velocity of the rising fluidof the rising fluid. In this case we have:. In this case we have:


in a Liquidin a Liquidvvuu = ( = (F - LF - L)(d)(dww/d/dtt)/)/AA

where where

vvuu is the upward velocity of the flow of the is the upward velocity of the flow of the liquid, liquid, FF is the mass ratio of liquid to solid in the is the mass ratio of liquid to solid in the feed, feed, LL is the mass ratio of liquid to solid in the is the mass ratio of liquid to solid in the underflow liquid, underflow liquid, ddww/d/dtt is the mass rate of feed of the solids, is the mass rate of feed of the solids, r is the density of the liquid and r is the density of the liquid and AA is the settling area in the tank. is the settling area in the tank.


in a Liquidin a Liquid If the settling velocity of the particles is If the settling velocity of the particles is vv, ,

then then vvuu = = vv and, therefore: and, therefore:

AA = ( = (F - LF - L)(d)(dww/d/dtt)/)/vv

The same analysis applies to particles The same analysis applies to particles (droplets) of an immiscible liquid as to solid (droplets) of an immiscible liquid as to solid particles. particles.

Motion between particles and fluid is Motion between particles and fluid is relative, and some particles may in fact rise.relative, and some particles may in fact rise.

DISCRETE (TYPE I) SETTLINGDISCRETE (TYPE I) SETTLING
























FLOCCULATED (TYPE II) FLOCCULATED (TYPE II) SETTLINGSETTLING








ZONE (TYPE III) SETTLINGZONE (TYPE III) SETTLING


The zone (or hindered) settling, which The zone (or hindered) settling, which occurs when the particles do not settle occurs when the particles do not settle independently, is also studied by batch independently, is also studied by batch tests. tests.

In this case an effluent that is initially In this case an effluent that is initially uniform in solids concentration, if allowed to uniform in solids concentration, if allowed to settle, will do so in zones, the first of which settle, will do so in zones, the first of which is that of clarified water (1), below is the is that of clarified water (1), below is the interfacial zone (2) in which the solids interfacial zone (2) in which the solids concentration is considered uniform. concentration is considered uniform.


In the bottom a compact sludge In the bottom a compact sludge develops in the so called compaction develops in the so called compaction zone (4). Between (2) and (4), a zone (4). Between (2) and (4), a transition zone (3) generally exists. transition zone (3) generally exists.

As time proceeds, the clarified effluent As time proceeds, the clarified effluent and compaction zones will increase in and compaction zones will increase in size while the two intermediates will size while the two intermediates will eventually disappear. eventually disappear.


Diagram of a zone settling process Diagram of a zone settling process

ZONE (TYPE III) SETTLINGZONE (TYPE III) SETTLING In some cases, further compaction may In some cases, further compaction may

occur. occur. The actual configuration of a The actual configuration of a

sedimentation tank can be either sedimentation tank can be either rectangular or circular. rectangular or circular.

Rectangular settling tanks are generally Rectangular settling tanks are generally used when several tanks are required and used when several tanks are required and there is space constraint, since they there is space constraint, since they occupy less space than several circular occupy less space than several circular tanks. tanks.

COMPRESSION (TYPE IV) SETTLINGCOMPRESSION (TYPE IV) SETTLING

SELECTION OF BASINSELECTION OF BASIN

Rectangular BasinsRectangular Basins Rectangular basins are commonly found in Rectangular basins are commonly found in

large-scale water treatment plants. large-scale water treatment plants. Rectangular tanksare popular as they tend to Rectangular tanksare popular as they tend to have:have:• • High tolerance to shock overloadHigh tolerance to shock overload• • Predictable performancePredictable performance• • Cost effectiveness due to lower construction Cost effectiveness due to lower construction

costcost • • Lower maintenanceLower maintenance

• • Minimal short circuitingMinimal short circuiting

RECTANGULAR BASINSRECTANGULAR BASINS




Circular and Square BasinsCircular and Square Basins Circular basins are frequently referred to Circular basins are frequently referred to

as clarifiers.as clarifiers. These basins share some of the These basins share some of the

performance advantages of the performance advantages of the rectangular basins, but are generally more rectangular basins, but are generally more prone to short circuiting and particle prone to short circuiting and particle removalremoval

problems. problems. For square tanks the design engineer must For square tanks the design engineer must

be certain that some type of sludge be certain that some type of sludge removal equipment forremoval equipment forthe corners is installed.the corners is installed.

CIRCULAR BASINSCIRCULAR BASINS



SELECTION OF BASINSELECTION OF BASINHIGH RATE SETTLERSHIGH RATE SETTLERS High rate tube settlers are designed to improve High rate tube settlers are designed to improve

the characteristics of the rectangular basin and to the characteristics of the rectangular basin and to increase flow through the tank. increase flow through the tank.

The tube settlers consist of a series of tubes that The tube settlers consist of a series of tubes that are installed at aare installed at a

60 degree angle to the surface of the tank. 60 degree angle to the surface of the tank. The flow is directed up through the settlers. The flow is directed up through the settlers. Particle have a tendency to flow at a angle Particle have a tendency to flow at a angle

different than the water and to contact the tube different than the water and to contact the tube at some point before reaching the top of the tube. at some point before reaching the top of the tube.

After particles have been removed from the flow After particles have been removed from the flow and collected on the tubes, they tend to slide and collected on the tubes, they tend to slide down the tube and back into the sludge zone.down the tube and back into the sludge zone.

HIGH RATE SETTLERSHIGH RATE SETTLERS






SOLIDS CONTACT UNITSSOLIDS CONTACT UNITS A solids contact unit combines the A solids contact unit combines the

coagulation, flocculation, and coagulation, flocculation, and sedimentation basin in one unit.sedimentation basin in one unit.

These units are also called upflow clarifiers These units are also called upflow clarifiers or sludge-blanket clarifiers. or sludge-blanket clarifiers.

The solids contact unit is used primarily in The solids contact unit is used primarily in the lime-soda ash process to settle out the the lime-soda ash process to settle out the floc formed during water softening.floc formed during water softening.

Flow is usually in an upward direction Flow is usually in an upward direction through a sludge blanket or slurry of through a sludge blanket or slurry of flocculated suspended solids.flocculated suspended solids.

SOLID CONTACT UNITSSOLID CONTACT UNITS

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