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Last class we were discussing about sedimentation. We have seen that sedimentation is a unit
operation. It is utilizing the settling velocity of the particles to remove it from water and
clean the water. So we can
either go for settling or floatation for the removal of particles which is large enough.
And based upon the properties of the suspension we can divide the sedimentation into different
categories. Those are type one
settling, type two settling, zone settling and compression settling.
And in water treatment most of the time we come across this type one settling and type
two settling. Type one settling is for discrete dilute suspensions. And we have seen in detail
what a discrete particle is. A
discrete particle is the one whose size, shape, specific gravity or the properties will not
change with respect to time. But a flocculent suspension is the one whose properties will
change with respect to time.
In water treatment, most of the time in primary sedimentation tank it is type one settling
and after the coagulation flocculation in clary flocculator we have flocculent settling.
We have also seen in detail how to
calculate the efficiency of a tank when it is a discrete particle settling or if it is
for a flocculent particle settling, and we also saw how to conduct the settling column
studies etc.
Today we will see in detail what are the different types of sedimentation tanks and what are
the important components of the sedimentation tank and how to design a sedimentation tank
and how can we find out
the efficiency of a sedimentation tank in actual practice because theoretically we can
achieve hundred percentage efficiency but in actual practice, in field we will not be
able to get that much of efficiency.
Therefore, we will discuss in detail; what are the factors that affect the efficiency
of the sedimentation process and how we can find out the efficiency etc.
Coming to sedimentation tanks we can have different types of sedimentation tanks or
we can classify the sedimentation tanks into different categories the first classification
is either horizontal flow sedimentation
tanks or vertical flow sedimentation tanks.
In horizontal flow sedimentation tank as the name indicates the flow is in horizontal direction
that means water enters in one side and it flows horizontally so that is known as a horizontal
flow sedimentation tank.
And in vertical flow sedimentation tank the water flow direction is in the vertical direction
that means water enters in the tank from the bottom portion and it comes up and it distributes.
This is very common in
circular sedimentation tank.
We can classify sedimentation tanks as either long rectangular tanks or square tanks or
circular tanks. We will see them in detail. First we will talk about long rectangular
basins. These are the best basins as far as
hydraulics is concerned because these basins are hydraulically more stable. Moreover, flow
control is very very easy in such basins. In long rectangular basins usually the length
of the tank is 2 to 4 times and
sometimes even 5 times, this is important. Sometimes the height of the tank is ten to
twenty times is equal to the length of the tank. These are the parameters.
In this case the bottom is slightly a slope. The reason for this one is, if you want to
remove the settled sludge it is very easy if you provide a slope in the bottom. So the
bottom is slightly a slope. Usually the
slope is around one percentage and the sludge removal is carried out by using a slow moving
mechanical scraper.
Redwood pieces on a chain drive continuously pulls the settled material and all the settled
material will be coming to a particular portion and from there we can pump it out, that is
what is happening.
In the sludge hopper whatever sludge is accumulated is pumped out periodically. This is the property
of a long rectangular basin. The most important one is, hydraulically it is more stable and
we will see
afterwards, what are the advantages of this long rectangular basin. The most important
one is; it provides a plug flow. Plug flow is very very important to get maximum efficiency.
This shows the sketch of a rectangular sedimentation tank. This is the inlet zone the water to
be treated is entering here and this is the baffle, the inlet pipe should be facing a
baffle and this baffle is having many
openings as we can see here, these are the openings and this is the settling zone of
the sedimentation tank whatever we are designing. This is the portion we are designing , this
is the settling zone and this is the
sludge scraper, we can see that it is connected in the pulleys and these are the scrappers,
this is the wood scraper, a motor is attached to this one so this will be moving in this
direction. Therefore, what will
happen is, whatever sludge is settled in the bottom of the sedimentation tank which is
here all those things will be scrapped. Here, this is the sludge hopper, so everything will
be coming here and it will be pumped
out using a pump.
This is the outlet zone. The outlet zone is designed in such a way that minimum disturbance
will be provided for the settled sludge. If the turbulence is more here in the outlet
zone then whatever sludge is settled
in the bottom of the sedimentation tank will be getting disturbed and the particles will
be escaping through the treated water in the effluent so we will not be getting the required
treatment efficiency. So outlet zone
should be designed properly to achieve proper efficiency or to achieve the required effluent
quality.
Once again the important parts of a sedimentation tank are; first one is the inlet zone, second
one is the settling zone and then the outlet zone then the sludge zone and sludge removal
mechanism.
This is another picture which is showing a circular sedimentation tank. The earlier one
was a rectangular sedimentation tank; this is the circular sedimentation tank with peripheral
feeding. The inlet is here. Water is
flowing here and it is taking a path like this and this is the effluent collection weir
so this portion will be acting as the settling zone. Similarly, here also this is the inlet
zone, water takes a path like this, during this
period whatever particle is present in water will be settling down and the clear water
will be collected earlier, this is the effluent collection we have and here the slope is in
this direction so whatever sludge is settled
in the bottom of the tank everything will be coming here and from here it is taking
away. This is the way a circular tank with peripheral feed is working.
This is again another example. Here the outlet weir is situated in the center. in the previous
case we have seen that the weir is also in the periphery and inlet and outlet both are
located in the periphery, in this case
the inlet is in the periphery, the outlet is in the center so water will be taking this
type of a path and here we are providing a baffle wall in order to reduce the kinetic
energy of the incoming water. Otherwise the
turbulence will be very very high and that will be affecting the settling process so
we will not be getting much removal. Here also whatever is settling here will be coming
to this portion and from here the sludge
will be taking away.
I have already discussed that the sedimentation tank will be having various zones. The first
one is inlet zone, then the settling zone, outlet zone and the sludge zone. The inlet
zone is the one in which the baffles
intercept the incoming water and spreads the flow uniformly. or the major purpose of the
inlet zones are to uniformly distribute the incoming flow because there is only one incoming
pipe and you will be having
so much of width for the sedimentation tank so we have to distribute the flow uniformly
throughout the sedimentation tank then only the entire area or the entire volume of the
sedimentation tank will be effectively
utilized. That is the most important purpose of the inlet zone; distributing the flow uniformly
throughout the tank.
Second one is to minimize the turbulence created by the incoming flow. We know that the water
will be entering into sedimentation tank with a high velocity
So if we allow that one to enter in the settling zone with the same velocity then it will be
creating lot of turbulence in the sedimentation tank. If the turbulence is more then whatever
sludge gets settled in the bottom
of the tank will be coming out. Therefore, how should we design the inlet zone in such
a way that the turbulence should be at minimum?
The third purpose is to train the flow so that it will be equally distributing and it
will be uniformly flowing throughout the sedimentation tank. These are the purposes of inlet zone.
Always the inlet pipe will be
facing a baffle which will be reducing the kinetic energy of the incoming water. These
baffles will be having many pores on them or many holes on the baffle wall therefore
water will be uniformly escaping through
these pores to the settling zone and when water enters the pores there will be a head
loss. The pores are designed in such a way that the velocity through the pores should
be around 0.2 to 0.3 meter per second
and the head loss through the pores will be around 1.7 times the velocity head. These
are the important things we have to consider when we design the inlet zone.
Once again, the inlet pipe should be face to a baffle wall to dissipate the kinetic
energy of the incoming wall and most of the time the inlet zone will be made in such a
way that the baffle wall will be having many
pores or many holes and the water will be coming into the inlet zone and from the inlet
zone through these holes of the baffles it will be entering into the settling zone. Therefore,
the ports in the baffle walls will be
distributing water equally to the inlet zone moreover it will be reducing the kinetic energy
because there will be lot of head loss when the water passes through the holes.
Now, in the outlet zone what is happening is, water flows upwards and over the outlet
weir so it will be reducing the turbulence and it will be improving the quality of the
treated water. The sludge zone which
extends from the bottom of the tank to just above the scrapper mechanism, this is the
sludge zone and the last one is the settling zone which is responsible for the settling
process. In the inlet zone the opening
must face a baffle, this I have already explained and we have seen the purpose. The purpose
of this one is to dissipate the kinetic energy.
I will show you some of the inlet arrangements.
This is one of the inlet arrangements. Here the influent pipe is coming and this is the
inlet of the sedimentation tank and this is the baffle. Here we can see multiple openings
and this baffle wall goes up to the
bottom of the tank and the influent pipe brings the water here and the water will be stored
here and from here the water will be going to the settling zone of the sedimentation
tank. This portion will be the settling
zone through these multiple openings. Hence, the velocity of the incoming water will be
reduced and the water will be equally distributed throughout the width and depth of the tank
so the settling zone will be
completely utilized if you give this type of an arrangement for the inlet.
This is another type of inlet arrangement . this is the inlet pipe and here we have
a submerged orifice so the inlet pipe brings the water here from here, it is passing through
this orifice and this orifice will be having
multiple openings because this is only a portion and throughout the cross section we can see
many openings and afterwards to reduce the kinetic energy further we provide a baffle
wall. So you will be having
uniform distribution of water because of these multiple openings and because of the baffle
wall the turbulence will be reduced or minimized to some possible extent.
This is another type of inlet arrangement it is influent channel with bottom openings.
Here what is happening is, water is coming through this pipe and this is your storage
place and it will be having a bottom
opening so what will happen is that water enters here like this and the direction will
be changed and it will be coming to the settling zone like this. Here also multiple openings
will be there throughout the width of
the tank so there will be some uniform distribution of the water as well as energy destruction.
This is another type of an inlet arrangement which is an overflow weir followed by baffle.
We have a submerged orifice here and here we have an overflow and the inlet pipe is
here, the entire water that is passing
here will be overflowing through this weir, we have a baffle wall here and afterwards
the water will be going to the settling zone. All these things are meeting the basic requirement
that is uniform distribution of
water as well as minimizing the turbulence and training the flow in the settling zone.
These are the purposes of the inlet and these are the commonly used inlet arrangements in
a sedimentation tank. So whenever
we design a sedimentation tank it is very very important to design properly the inlet
zone, outlet zone and sludge zone. Anyway we will be designing the settling zone properly
based upon the principles whatever
we have seen earlier.
Now we will see how the outlet zones are designed in a sedimentation tank. Most of the time,
the outlet zone consists of a launder and an outlet pipe. It consists of V notches attached
to single or both sides of
single or multiple troughs.
This I will explain in detail when we do the design problem.
Most of the time what will happen is the center distance of this V notch will be around 150
to 300 mm so in the outlet zone trough many V notches will be there, these V notches will
be placed at a center distance
of 150 to 300 millimeters and just before the outlet zone there will be a baffle. We
have seen that in the inlet zone after the inlet pipe we will be providing a baffle but
in the outlet zone what is happening is the
baffle will be provided first then the outlet launder or the V notches will be coming.
The purpose of this baffle is to prevent floating matter from escaping into the effluent because
there will be many floating materials as well as *** in the sedimentation tank. Whatever
amount of water is entering
in the sedimentation tank is having oil or grease or any floating material, it will not
be removed by settling so definitely it will be present in the surface of the sedimentation
tank so definitely if we do not provide
any obstruction it will be coming and escaping through the effluent so naturally the effluent
quality will be destroyed or effluent quality will be spoiled. So if you want to improve
the effluent quality we have to
remove the *** as well as floating matter from the sedimentation tank. So these baffles,
whatever is provided in the outlet zone is serving that purpose.
Another important factor which we have to consider when we design an outlet zone is
weir loading rate. If we go for a high weir loading rate that means the rate at which
the water escaping through the weir is
very high then definitely the turbulence will be very high so that will be destroying or
that will be reducing the efficiency of the tank. Hence, weir length relative to the surface
area determines the strength of the
outlet current. This point is very very important. The weir length is the one which decides the
strength of the outlet current. To get maximum efficiency we have to keep the strength the
minimum possible.
Usually whenever we design a sedimentation tank the weir loading rate we usually keep
it at this value 300 meter cube per day for per meter but if the settling is very very
good then we can go for a higher weir
loading rate up to 1500 meter cube per day per meter. But usually we keep a value of
300 meter cube per day per meter.
How can we find out the weir loading rate? It is nothing but the flow rate divided by
the weir length. The flow rate per weir length will be giving you the weir loading rate that's
why we get this unit because your flow rate unit is meter cube per day so that
much is the flow entering into sedimentation tank and your weir length will be given in
terms of meter so weir loading rate will be in terms of meter cube per day per meter or
meter cube per hour per meter.
These are typical outlet arrangements. As we discussed earlier, we can see here, this
is the baffle, this baffle will be preventing all the floating materials and *** materials
to escape into the outlet zone. From here
this is the treated effluent , and here we can see many V notches and the center to center
distance between these two V notches is usually 15 to 30 centimeters. The treated water here
will be escaping through the
effluent launder through the V notches and from here it will be going to the effluent
box and from the effluent box it will be collected through the effluent pipe. Therefore, an outlet
zone consists of many V
notches which collect water to the effluent launder and from there it is going to the
effluent box and from the effluent box the treated water is collected through the effluent
pipe.
This is another type of an effluent outlet zone . This consists of outlet with rectangular
weir. Here we are having a weir and before this one we will be having a baffle wall to
remove the floating materials as well as
*** and the treated water is coming here which is free from floating matter and ***
and it is passing through this weir, most of the time this weir length is adjustable
and this is escaping here and what you see
here is known as the outlet fume and from here the treated water is collected through
this outlet pipe.
These are very very important. So, whenever we design a sedimentation tank we have to
design the inlet zone and outlet zone properly.
This is a sedimentation tank with all the details. This is the influent inlet arrangement
so influent is coming here and this is the baffle and this is the settling zone and we
have the sludge scraper mechanism here.
This is another important point; most of the time the sludge hopper will be situated near
the inlet zone. The reason is, if you provide the sludge hopper here near the outlet zone
then during heavy turbulence the
entire sludge present in the sludge hopper would be disturbed and it can escape through
the effluent or outlet arrangement so it is always advisable to keep the sludge hopper
in the inlet zone so that the
disturbance will be at its least and we will be getting a better effluent quality.
This is the inlet zone and this is the settling zone, this is the sludge hopper and this is
the outlet. So we are getting the outlet effluent from here and here an adjustable weir is used
as the outlet arrangement.
This is another type of outlet arrangement . Here what is happening is, this is a weir
and we are collecting the effluent here and finally whatever is collected is coming out
through this one and here we can see that
the sludge scraper is little different from the one whatever we have seen. So we can provide
any type of sludge scraper but only thing is it should be able to remove the collected
sludge effectively. We should
provide proper sludge removing mechanisms. We should not allow the sludge to accumulate
here in the sludge hopper so the pump should be operating periodically to remove the sludge
whatever is collected in
the sludge hopper.
Another important factor whenever we discuss about the sedimentation tank is surface loading
and detention periods. We have seen what surface loading is in the last class.
Removal of particle: we have seen that the time required for the removal of a particle
is depending upon the settling velocity and we have seen how to calculate settling velocity
in case of a discrete particle. We
can use the Stokes' Law to find out the settling velocity and in flocculent particles we cannot
use the Stokes' Law, the reason is, the particle size shape and the properties will be changing
with respect to time.
We have seen that the settling velocity of the discrete particle will be increasing with
respect to time. The reason is, more and more particles will be agglomerating together with
respect to time. So as the particle
size increases the settling velocity also will increase. So removal of a particle is
directly proportional to the settling velocity. And we have seen that the settling velocity
of a particle in a sedimentation tank is
numerically equal to the surface loading or surface loading rate or the removal of a particle
is a function of surface loading or surface loading rate. This is very very important
when we talk about high rate reactors
also.
I will show you how to derive this one.
The settling velocity is nothing but V0 where V0 is H by t; H is nothing but the height
of the tank and t is the settling time, this we have seen. If we have a tank this is having
a height H and here one particle is there
it is settling with a settling velocity of V0 or we have a particle, it took around t
seconds or t minutes to reach the bottom of the tank then we can find out what is the
settling velocity. So the particle removal is a
function of this settling velocity and we have seen now the settling velocity or the
particle removal is a function of surface loading or surface loading rate. Surface loading
rate is nothing but, this is the flow rate
divided by surface area.
How to show this one? We have this expression V0 the settling velocity is equal to H by
t and we have the sedimentation tank, and for any sedimentation tank we can find out
the volume by multiplying length of
the sedimentation tank by width by height so this is the volume of the sedimentation
tank and if you want to find out the height of the sedimentation tank what is that one
volume divided by surface area that will
be giving you the height. So what is the surface area? Surface area is nothing but length into
width. So we can write like this. Here we are finding out what is the height or depth
of the sedimentation tank if you
know the volume of the sedimentation tank and surface area.
So volume of the sedimentation tank is nothing but length into width into height and surface
area is length into width . That is the surface area so you will be getting the height. We
have seen that V0 is nothing but
H by t so we have the H value like this that means volume divided by surface area and t.
So if you talk about a sedimentation tank what is the t or what is the time the water
or any particle that is getting in the tank
that is nothing but volume of the tank divided by
the flow rate so this is nothing but the hydraulic retention time. This is the time the particle
is getting inside the tank that means volume. So we have a particle
volume and we know the flow rate so we can find out what is the time that water is going
to stay inside the tank. So we can put this one so this expression we can write like this;
V0 equal to H by t so that is equal
to L into B into H divided by L into B so that will be giving you the H value.
Now t is nothing but volume by Q so this volume we can again replace by L into B into H this
is nothing but the volume of the sedimentation tank. Now these terms will be getting cancelled
and you will be getting
Q divided by L into B so this is nothing but Q divided by surface area so that is what
we have seen in a sedimentation tank. The settling velocity or the removal of the particle
or particle removal efficiency is a
function of surface overflow rate. Surface overflow rate is nothing but the flow rate
divided by the surface area and the particle removal is independent of the height of the
tank or H. This is very very important
when we talk about the high rate sedimentation tank. We will discuss this in detail towards
the end of the class.
We have seen that the removal of a particle is a function of surface loading or surface
loading rate. Usually in a sedimentation tank we provide a depth of 2.5 meters to 3 meters
in case of discrete particle and
flocculent particle we usually give 3 to 4 meter. Though we have seen that the depth
of the tank is not an important factor what will happen is, even if the depth is too minimum
whatever sludge gets settled on the
bottom of the tank will be coming out because of the turbulence. That's why we are providing
a depth of 2.5 to 3 meter.
In case of flocculent particles what will happen as the particles travel downward direction
with respect to time the particle size will be increasing, so if you provide large depth
the removal efficiency will be high.
Usually the width of the tank is 12 meters otherwise the sludge removal is a problem.
When you install the sludge removal mechanism it is always advisable to provide a width
of 12 meters or more. The length of
the tank varies from 10 to 48 meters and the settling velocity of the particle if it is
discrete it varies from 1 to 2.5 meter per hour and for flocculent particles the settling
velocity varies from 0.6 to 1 meter per hour
and these are the typical surface loading rates and detention periods for various types
of sedimentation tanks, for a plain sedimentation tank.
That means there is no coagulation flocculation and most of the particles are discrete in
nature so the range is 0 to 6000. But typical values for design is 15 to 30 meter cube per
day. So this is the surface loading
rate usually we provide. And for horizontal flow circular sedimentation tanks the typical
values for design is 30 to 40, this is for rectangular 15 to 30 and circular, 30 to 40.
For vertical flow clarifiers here the flow direction is in the opposite from the bottom
to the top most of the time so in such cases we can provide a surface loading rate of 40
to 50 meter cube per day; and for
plain sedimentation we can give a detention period of three to four hours if the particles
are sand, silt and clay; and for horizontal flow circular sedimentation tanks the detention
period can be 2 to 2.5 hours; and
vertical flow usually the detention time of 1 to 1.5 hours is provided.
Now we will see how to design a long rectangular settling basin for type two settling. Type
two settling means it is flocculent particle settling.
This is the problem: A city must treat about 20000 meter cube per day of water. Flocculating
particles are produced by coagulation and a column analysis indicates that an overflow
rate of 20 meter cube per day
per meter square will produce satisfactory removal at a depth of 3.5 meter. Design a
sedimentation tank.
We will see what information is available. We have the Q the flow rate 20000 meter cube
per day and we have the surface loading rate that means this much meter cube per meter
squared per day and the depth of
the tank is also given 3.5 meter so we will see how to design the tank.
First we have to find out what is the surface area required because we know the depth of
the tank so how can we find out the surface area. We have the flow rate and since the
flow rate is very very high we are
providing two tanks where each tank will be taking care of 10000 meter cube per day so
Q is 10000 meter cube per day and this is equal to Q0 into As where As is the surface
area and this Q0 is nothing but the
surface overflow rate.
The surface overflow rate is already given as 20 meter cube per meter square per day
so we can find out the area required. So just divide this one by the surface overflow rate
and we will be getting 500 meter
square.
Now we have to find out how much is the length and width we have to provide. So we are assuming
the length to width ratio as 3 is to 1 so we have seen that we the length of the tank
is around two to five times
the width of the tank so we are assuming three times so we can find out the width like this
w into 3w that means this is the L with a value of 500 meter square so width is 12.9
meter so either we can go 13 or 14
meter, I have taken 13.5 meter because we have to satisfy other conditions so the length
will be around three times this one so we are providing a length of 40 meters.
Now we have to check for the retention time. How to find out the retention time? it is
nothing by V by Q that is a retention time. So we can find out the volume because we know
the width, we know the length,
we know the height so this is the volume divided by the flow rate so it is coming around 0.189
days or 4.5 hours so we have seen that in the discrete settling we can go up to four
hours this is higher than
Now we will check for the horizontal velocity because if the horizontal velocity is high
more turbulence will be generated and that will be affecting the efficiency of the system.
The horizontal flow velocity is
nothing but Q divided by the cross sectional area which we can find out using this expression;
10000 divided by width and depth so you are getting 8.82 meter per hour and for discrete
particles it should be less
than 9 meter per hour so it is satisfying that condition.
Now we have to see the weir overflow rate. The weir overflow rate should be less than
300 meter cube per meter square per hour. Here we are getting around 30.8 meter cube
per hour per meter and this is not
satisfying that condition so how can we achieve the condition. What we can do is, instead
of proving only one layer of weir in the end we can provide multiple weirs. I will show
you this.
This is a sedimentation tank and this is your inlet and this is your outlet and this is
the settling zone. Usually what we give is we will be having a weir here and this is
the outlet zone. So instead of providing one
weir here what we can do is we can provide multiple weir as something like this so water
will be coming to this one from both sides like this and this one we can connect through
a pipe as something like this. So
whatever amount of water that is collected here will be entering here and it will be
going to the outlet pipe. Therefore, instead of providing a single row or single weir length
we can provide two or three weirs here
so that will be taking care of the weir loading rate. This is what we usually provide in the
rectangular tanks. But when we talk about the circular tanks this weir loading rate
will not be a problem.
This is the cross section of a circular tank so the inlet will be most of the time in the
center so it will be coming like this, this is the outlet so entire periphery of the tank
will be available for this weir so the weir
loading rate will not be a problem in circular tanks. But in rectangular tank if you provide
only one line of weir it may not be meeting the weir loading rate requirement so in such
cases we have to provide parallel
weirs. This point is very very important.
Hence, if you want to draw the picture whatever we have designed so for it comes like this.
So whatever dimension we have got, that 40 by 13.5 is for the settling zone and we have
to provide the inlet zone, we
have to provide the outlet zone depending upon the weir loading rate and we have to
provide the sludge zone. Usually we provide a slope of one percentage and this is the
sludge hopper which is situated in the
inlet zone.
Then the total height of the tank will be coming around 4.5 meter the reason is, 3.5
is the depth of the settling zone and we have to provide around 0.5 meter for the sludge
zone and 0.5 meter as the free Bourne
so that's why it is coming as 4.5 meter.
In circular basins the flow regions are different from rectangular basins. In most of the cases
the flow direction is from the center to radially towards the perimeter. Here what is happening
is the horizontal will
velocity will not be a constant it decreases as distance increases from the center. The
reason is, if you take any section the cross sectional area will be increasing as we go
towards the perimeter of the circle that's
why the horizontal velocity will be keep on decreasing so you will not be getting a linear
velocity change it will be in parabolic. So V0 is not a constant it will be continuously
changing and if you find out the
particle path it will be following a parabola.
And here if you go for circular tanks the major advantages are; sludge removal mechanism
is simpler and weir overflow rate will not be a problem as I have already mentioned because
entire perimeter is available
for the weir and usually when we go for tanks in circular shape the d should be less than
30 meters. It is easy for construction and all those purposes that's why we are restricting
the diameter as 30 meter.
This is the same problem if you design a circular settling basin . Here what I have done is
I have taken three tanks instead of two tanks so we will be getting the cross sectional
area in the same way as 333.33
meter squared so we can find out the diameter. The diameter is coming as 20.6 meters or we
can provide as 22 meters and it should also have the inlet and outlet arrangements and
sludge collection systems.
This is the settling zone, this is the inlet zone and this is the outlet zone so if you
provide the inlet zone and outlet zone along with the settling zone then you will be getting
a total diameter of 34 meter though the
inlet zone itself requires only 22 meters.
Now we were discussing about the discrete particle settling. We have seen that the depth
of the tank is not very important in discrete particle settling. Based upon this principle
high rate sedimentation tanks are
being designed. Tube settlers or inclined plate settlers are examples of high rate settlers.
Here what we are doing is we are providing excess surface so that the surface overflow
rate will be decreased. We have
sedimentation tank like this say some 3.5 meters depth so if you provide many plates
parallelly in the settling zone so what will happen is the effective surface area will
be increasing because many surfaces are
available for the settling or many planes are available for settling. So, effectively
what will happen is it will be considerably reducing the surface overflow rate or in other
words it will be increasing the efficiency of
the tank. That is the principle of these tube settlers.
Here the design is based upon the surface overflow rate so here we are providing many
tube or inclined plates at an angle of 45 to 60 degree above the horizontal. Why we
are providing this 45 to 60 degree angle
is because the particles will be settling on the plates or the tubes so if you provide
an angle 45 to 60 degree then whatever the particle that is settled on the tubes or the
plates will be sliding back to the system with
the self weight so cleaning will be very very easy.
Therefore, people have shown that if you provide an angle of 45 to 60 degree the efficiency
of the system will be at maximum. And in such cases the spacing of the plates will be around
five centimeter and
length of the plates will be around 1 to 2 meters. When we talk about these tube settlers
it can work in three different modes either counter current mode or co current mode or
cross flow.
This shows how the three systems are functioning. This is an example of counter current mode.
Here what is happening is, the plates or the tubes are placed at an angle theta degree
to the horizontal and it is an
inclined plate so what will happen is the liquid will be entering through the plate
like this. If it is a tube what will happen is, water or the particle will be going and
touching the top portion of the tube and afterwards
it will be coming down and it will get settled and if it is a plate the same thing will be
happening.
So what will happen is, here the liquid flow is in this direction we can see that the velocity
V0 is going in this direction and particle settling will be in the opposite direction,
it will be settling and particle will be
coming back to this system because the plates are inclined like this. That is what is known
as counter current. That means the flow is in one direction and particle settling and
removal is in the opposite direction
that's why it is known counter current. this is co current so in co current what is happening
is, this is the liquid flow direction and this is the solid removal direction so both
are in the same direction. That means
water is entering like this and as it enters the particles will be following this trajectory
and it will be coming and settling here and it will be getting removed and clear water
will be going like this. Hence, both are in
the same direction, this is co current and this is cross flow. That means here the flow
is in this direction the plates are placed parallelly and flow is in this direction and
solids will be settling in this direction that is
why it is known as cross flow.
This is a typical cross section of a plate settler. Here we can see many plates are provided
so all these things are plates and this is a coagulation flocculation and clarification
so this is the flocculation tank and
from here the water is coming here and this is the underflow sludge the water flow direction
is like this, you can see that one, and as it comes here we will be getting the clean
clear water. This is the overflow box
and from here we can see that the clear effluent is collected. This is the feed box and this
is the sludge hopper and from here the sludge is collected through this pipe. This is how
a tube settler is functioning.
We are having many parallel plates. So here what we are doing is by utilizing a small
area we are increasing the efficiency of the tank so that is the major principle of tube settlers. In counter
current settling how
can we find out the time required? The time required is nothing but what is the time taken
by the particle to travel the perpendicular distance between the plates. That is nothing
but w by V cos theta this is the
velocity component; t is the time for a particle to settle the vertical distance between two
inclined plate and w is the perpendicular distance between the plate and V is the velocity
and theta is the angle of the
surface inclination from the horizontal.
Similarly we can find out what is the length of the plate required. It is equal to w into
V0 minus V sin theta by V cos theta where Lp is the length of the plate or length of
the surface and V0 is the liquid velocity
between the surfaces and n is the number of plates; b is the dimension of the surface
perpendicular to w and Q and we can find out what is the V theta it is equal to Q the flow
rate divided by number of plates
divided by this is the perpendicular distance and this is the width so the flow rate divided
by the cross total cross sectional area will be giving you the velocity.
In co current also similarly we can find out what is the length of the plate that is equal
to w by V0 minus V sin theta by V cos theta and V is equal to the settling velocity should
be greater than or equal to V theta
into w by Lp cos theta minus w sin theta. These things we can get by just analyzing
the velocity components, it is nothing difficult. And in cross flow settling the length of the
plate required is wV0 by V cos theta
where V0 is the flow velocity and settling velocity should be greater than or equal to
V theta into w by Lp cos theta. So depending upon the flow regime or depending upon the
arrangement the length of the plate
required and the settling velocity will be varying.
Till now we were discussing about the settling tank efficiency theoretically. but in practical
condition the settling tank efficiency will be varying because of the inertia of the flow,
because always the flow will be
having a tendency or the particles will be having a tendency to continue the same flow
regime so that is the inertia.
Second one is the wind effect, third one is because of the turbulence created in the system
because of temperature variations all these things affect the efficiency of the tank.
So we will see one by one in detail.
The factors that affect the efficiency are inertia of the incoming water because the
water will always be having the tendency to follow the same condition; the second one
is wind effect; third one is because of the
turbulence. Though we are trying to minimize the turbulence there will be still some turbulence
and because of that one the efficiency will be reducing.
The last one is density and temperature gradients. How the temperature gradients effect the flow
conditions? We will take two cases. For example, you have a sedimentation tank, the sedimentation
tank water
temperature is higher than the influent water so what will happen is, though the water is
getting distributed equally in the inlet zone what will happen is, since the density of
water whatever is present in the
sedimentation tank will be less compared to the incoming water. Thus, the incoming water
will be always flowing to the bottom of the tank by which your settling zone will not
be effectively utilized. That is
happening during summer time.
But during winter if we are having an open sedimentation tank, the sedimentation tank
temperature will be lower so naturally the density of the water will be more but if the
incoming water temperature is more
compared to the existing water, the water will be flowing in the top portion of the
tank and the bottom portion of the tank will not be utilized completely so we will not
be getting a complete plug flow regime in the
sedimentation tank. So naturally if the settling zone is not completely utilized then definitely
your efficiency will be decreasing.
Similarly, it is with the wind effect. Most of the time you will be having a wide rectangular
tank as the sedimentation tank, so if the tank length is larger then there will be wind
effect and because of that wind effect
there will be turbulence in the top portion of the tank and because of the turbulence
your settling will not be proper because for settling quiescent condition is a must. So
if lot of turbulence is there the settling
efficiency will be decreasing. These are the factors that affect the sedimentation tank
efficiency.
So if you want to find out what is the actual efficiency we can conduct tracer studies.
What we are doing in a tracer study is we are injecting a color, we are injecting a
dye or some chemicals which can be
analyzed easily so we find out the concentration of the dye or the chemical at the effluent
with respect to time and we can plot the concentration and from that one we can find out the flow
regime in the
sedimentation tank and how much tank volume is utilized and what is the dead volume in
the tank. So based upon that one we can find out the efficiency of the system.
This is the formula usually we use to find out the efficiency of the tank that is equal
to y by y0 which is equal to 1 minus 1 plus n into V0 by Q by A raised to 1 by n. Once
again the efficiency of the tank is y by
y0 which is equal to 1 minus 1 plus n into V0 by Q by A raised to 1 by n where y by y0
is efficiency of removal of suspended particles n a coefficient that identifies basin performance
and V0 is the surf surface
over flow rate for ideal settling basin and Q by A is the required surface overflow rate
for a real basin to achieve an efficiency of y by y0 for a given basin performance.
This formula is very very important.
We will see how to find out this n value. for finding out this n value or to determine
the short circuiting we can go for tracer study and we can make the frequency distribution
chart; from the mode, median and
mean flow through periods we can find out the central tendency of the time concentration
distribution. And using this one we can find out the efficiency of the tank and a well
designed tank usually gives an
efficiency of 70 percentage and if it is a complete plug flow regime that means a very
narrow long tank we will be getting maximum efficiency.
This shows the tracer study. Here we are floating the relative concentration with respective
time. If it is a plug flow regime we will be getting the concentration like this and
if it is a completely mixed tank we will
be getting a flow regime like this. So most of the time the sedimentation tanks will be
in between a CSTR and a plug flow regime so as it flows to the plug flow we will be getting
more efficiency. it is the best one.
It is almost approaching a plug flow regime; b is more close to CSTR; c is less; then d
is approaching plug flow and e is almost a plug flow.
For best possible the n value is 0 that means it is ideal and for very good performance
the n value is 1 by 8; good performance it is 1 by 4; average performance it is half
and for very poor performance it is 1, 1
means it is completely it is almost acting as a CSTR reactor.
We will see the things we have discussed today. We have seen what are the important components
of a sedimentation tank. Those are inlet zone, outlet zone, sludge zone and the settling
zone. And we have seen
in detail how to design the inlet zone, outlet zone and the settling zone and we have seen
about high rate reactors that means the tube settlers or plate settlers which will be increasing
the efficiency of the settling.
We have also seen that in ideal case we may be able to get 100 percentage efficiency but
in actual case the efficiency will be less.