Textofvideo.nptel.iitm.Ac.in 105106119 Lec8

8/12/2019 Textofvideo.nptel.iitm.Ac.in 105106119 Lec8

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Water and Wastewater EngineeringDr. Ligy Philip

Department of Civil EngineeringIndian Institute of Technology, adras

!edimentation Lecture " #

Last class we were discussing about whats the need of water treatment and we have alsoseen how to select the water treatment processes. We discussed how the treatment unit ortreatment operations vary depending upon the source of water. Actually we have toimprove the quality of water. So, if the source or whatever water comes to the treatmentplant or whatever raw water is available to us is not of good quality then naturally theextent of treatment required is very high. oreover, if you ta!e the water from theground or underground then we have to give some extra treatment and if you ta!e thewater source or surface water then it requires a different treatment, at least the initial

treatments.

"hus, we have seen about how to go for that treatment. #specially we have discussedabout the aeration which is essential for ground water because in the ground the oxygenavailability is limited so there are chances of it having odorous gases and elements whichis in the reduced form. So to remove these things we have we have to go for aeration. Wehave also discussed about the different types of aeration units available and how to designthem and what are the design parameters. "hen we were discussing about solidsseparation. $n water treatment itself we come across different types of solids% suspendedsolids, colloidal solids, dissolved solids which we have discussed earlier and when thesesolids are in liquid or in solution the suspension can be of various types. Sometimes the

suspension is stable and other times it is unstable. "his stability or unstability is discussedor expressed in terms of the colloidal or the solid stability.

&or example, if you ta!e a colloidal solution the colloids are so stable because of the'rownian motion and the surface characteristics so it is very difficult to remove thecolloidal particle from the suspension by only physical force. 'ut if you ta!e a solution ora suspension which is having suspended particle or particle with high density and largesi(e then it is very easy to remove them by physical force $ mean by gravity.

"oday we will be concentrating on the removal of particles which is of higher si(e whichcan be removed easily by sedimentation or by settling and this sedimentation is a unitoperation because we are ma!ing use of the gravity force and gravity is the driving forcehere.


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"his is a physical force and we can call it as unit operations. Another treatmentcommonly used for solid separation is floatation. &loatation means the separation ofsolids which is lighter than water. 'ut in water treatment most of the solids whatever wedeal with is always denser than water so most of the time we wont go for this floatationbut we always go for sedimentation.

As $ have already discussed the suspended solution can be unstable with respect to thesuspension and can be stable with respect to the suspension. So if it is a suspendedsolution then it is unstable because the si(e of the particles are high and the density is

relatively high so it will be easily settling. 'ut when you come to the colloidal anddissolved solution they are stable.

$f you want to treat this stable suspensions or if you want to remove the solids from thestable suspensions first we have to ma!e the solids unstable or ma!e the suspensionunstable with respect to the solids then we have to remove the suspension or we have toremove the solids by settling.


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)*efer Slide "ime+ 1+120

3oming to the sedimentation first we will discuss what are the different types of particles.$t is very very important when we go for the design of sedimentation systems. "heparticles can be classified basically into two categories% one is discrete particle andanother one is flocculent particle.

4iscrete particle is the one whose si(e, shape, and specific gravity do not change withrespect to time. &or example, if you have a suspension of sand or silt and if you allow itto settle in a quiescent condition then what happens is each individual particles will besettling down and if you analy(e the si(e or shape or specific gravity of that particles it

will not be changing with respect to time or all throughout the si(e, shape and densitywill be remaining as a constant.


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'ut coming to the flocculent particle those are the particles whose surface properties aresuch that they aggregate or collide with other particles upon contact, because of this onethey change their shape, si(e and specific gravity. "his often occurs in water treatmentespecially in coagulation flocculation system. We have already discussed that if thesuspension is stable with respect to the solids we have to ma!e them unstable thenremove the solids by settling.

"herefore, in coagulation flocculation, if you are interested in removing the colloidalparticle how can we ma!e them unstable7 We have to add some chemical agents which

can change their surface properties and ma!e them agglomerate. $n the process whathappens is the particles will be agglomerating so this agglomeration is a time dependentprocess so as more and more particles come in contact the agglomeration will be more sowith respect to time what will happen is initially a single particle will be there and whenit passes it will come in contact with the other particle so the si(e will be graduallyincreasing and with respect to time the si(e will be increasing and towards the end thesi(e will be maximum.

"herefore, what is happening, here their shape, si(e and the specific gravity of the entireagglomerate will be changing with respect to time. 8ow coming bac! to the suspensionwe can classify the suspension into two different categories% one is dilute suspension and

another one is concentrated suspension. What is this dilute suspension7

4ilute suspension is the one where the concentration of suspension is not sufficient tocause significant displacement of water as they settle or particle will not be close enoughfor velocity field interference to occur. "hat means the suspension is so dilute or theparticle density or particle concentration is so low so each particle will be settling asindependent particle or the velocity field of one particle is not being affected by the otherparticle or the concentration of the suspension is so low that it will not cause any


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significant displacement of water 9ust li!e in soil mechanics where we tal! about theconsolidation test etc.

"herefore because of the weight what happens is whatever water thats entrapped inbetween the particles will be getting displaced so that type of a thing will not be

happening in dilute suspension.

What is a concentrated suspension73oncentrated suspension is the suspension which doesnt have these properties. thatmeans the concentration is so high that each particle cannot act as independent particlesor the velocity field of one particle is being affected by the other particle and there will bedisplacement of water because of the weight of the particles, because it is so dense oneparticle will be sitting over the other particle and it will be compressing the entire systemso water will be getting displaced. So based upon the type of the particle and type of thesuspension we can classify the settling into different categories.

"his figure shows it clearly.

)*efer Slide "ime+ :+-0

"he y axis gives the percentage solid starting from to 6 percentage and this siderepresents the discrete particles and this is flocculent particles )*efer Slide "ime+ :+/0.

So if the concentration of particle is low and the nature of particle is discrete then we callthe settling as class one or type one whereas if the concentration of the suspension is lowand the particles are of flocculent nature then we call it as type two or class two settling.

When we tal! about water treatment mainly we come across these two types of settling%class one or type one settling is very common in primary sedimentation tan! where weusually remove the sand, silt and clayey particles and type two settling is common incoagulation flocculation or clary flocculator where we add alum and coagulate and


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flocculate the particles and we remove these flocs so that is an example of class two ortype two settling.

$n (one settling the concentration of the solids are relatively high so the particles will befully interfering the velocity field of other particles so the particle cannot settle as

independent particle so they will be settling as a group. $f you !eep this suspension in aquiescent condition in a cylinder then we can see that a clear (one is developing and thatclear (one will be coming down very slowly. "hats why it is !nown as (one settling.

$n environmental engineering point of view this (one settling is very common inwastewater treatment systems. &or example, in activated sludge process after the aerationtan! we have to go for sludge removal so that is ta!ing place in this (one settling mode.

And coming to compression settling either it is discrete particle or flocculent particle ifthe concentration is very high then we will be having compression settling because theweight of the particle will be displacing the water and because of that compression

settling is ta!ing place. So this type of settling is very common when we go for sludgethic!ening. these two settling we will be discussing in wastewater treatment portion andwe will go in detail about class one and class two or type one and type two settling today.

)*efer Slide "ime+ 6/+620

&irst we will discuss about type one settling. "he significance or the property of thissettling is these are discrete particles and they are in dilute suspension. So imagine if oneparticle is put into the liquid so what are the forces that will be acting on the particleimmediately7 4efinitely the gravity force will be acting on the particle because of theweight and gravity and another force is the buoyant force. According to the Archimedesprinciple it is equal to the volume of water displaced by this particle. So we can find outwhat is the total force or the net force acting on the particle. 'ut once the particle startsmoving there is yet another force coming into picture that is the drag force because of the


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viscous friction experienced by the particle. So based upon that one we can find out whatis the actual settling velocity of the particle if it is in a suspension.

)*efer Slide "ime+ 6-+10

"he force of gravity we can easily find out using this equation. "his is rhoginto g where gis the acceleration due to gravity and v of particle. "hat means volume into rho will begiving the mass and mass into acceleration will be giving you the force of gravity andbuoyant force is nothing but rhowwhich is the density of water and g acceleration due togravity and volume of particle because it is equivalent to the weight of water displacedbecause the volume of water displaced will be equal to the volume of the particle thats

why we are ta!ing ;< here.

So if that particle is in suspension as such, if it is not moving, then the net force acting onthat one is the difference between this force of gravity and buoyant force because wehave seen that both of them are acting in opposite direction% one is in the downwarddirection and another one is in the upward direction. So the net force is rho pminus rhowinto g into ;


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So drag force, this is wrong, this is drag force )*efer Slide "ime+ 6+6/0 we can find outusing this formula% 34A< rhowinto v squared by two% this A< is the exposed surface areaof the particle and v is the velocity with which it is moving.

So at steady state what will happen is, initially it was having a net force because of the

difference between the gravity force and the buoyant force so it will be moving at aparticular velocity but the drag force is acting upon that one so the acceleration will bedecreasing with respect to time and after a certain stage the system will be at steady statethat means both the forces will be equal that means the net force as well as the drag force."his is a steady state condition. "hat means gravity force minus buoyant force will beequal to the drag force. So we can write li!e this% rhopminus rhowinto g into ;


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laminar flow and it is - by *e raised to half plus .-1 in transitional and .1 in turbulent.&irst we have to see, what is the type of flow we have, then depending upon that one, wehave to find out the exact value of 34and if you can substitute that value in 34we will begetting the settling velocity or terminal settling velocity.

8ow we will see what this *eynolds number is. $t is nothing but rho ;d by mu and wehave to include a factor five it is nothing but the shape factor.

)*efer Slide "ime+ 6:+660

What is this laminar region7 $f *eynolds number is less than 6 we consider it as laminar

and if *eynolds number is greater than 6+1 it is turbulent flow. So if you considerlaminar flow and substitute 34is equal to /1 by *e and *e substituted by into ; trhowdby mu and we assume that the shape factor for spheres is 6 then we will be getting ; tequal to g rhopminus rhowd squared by 6> mu where ; tis the terminal settling velocityso we can calculate the settling velocity of particles in discrete suspension. =r this law is!nown as Sto!es law. ?ence, by using Sto!es law we can find out the settling velocityof the particle in a discrete in a dilute suspension of discrete particle or in type onesettling.

"herefore, if you !now the settling velocity and if we can find out the time required forthe particle to travel in a given distance then we can design our settling tan! in such a

way that the particle will get enough time to settle.


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)*efer Slide "ime+ 6:+120

'ut coming to reality what is happening is, whatever particles are present in water willnot be having uniform si(e, uniform density and uniform shape so how can you go for thetheoretical measurement of settling velocity, it is impossible. =r, if you calculate thesetting velocity theoretically and we use it for design you will not be getting the requiredefficiency. So what we usually do is we conduct settling column analysis in thelaboratory and use that data for the design of sedimentation tan!s for type one settling. Sohow can we conduct the settling column analysis.

"his is a very simple test. what we have to do is we have to ta!e a settling column of

around / to - meter depth and a diameter of 6 centimeter or more because if you go forvery small diameter the wall effect will be there because the particle diameter will bethere and if the particle diameter and column diameter does not meet a specific ratio thesettling will be affected by the wall, that is !nown as wall effect so we have to ta!e thesettling column in such a way that there is no wall effect and it will depict thesedimentation tan! in the field.

So what we have to do here, this is your suspension )*efer Slide "ime+ /6+--0 which wehave to clarify and we assume that the liquid level in the column is @ that means theheight of the liquid column in the settling column is @ so we are assuming that a particleis here in the top where @equal to and it has to come up to this region then the particle

is removed from the system, now, if we are giving a time of " then we can find out whatis the velocity7 ;elocity is nothing but the distance traveled by time of travel so we canfind out that one. "his is @because @is the distance it has to travel to get removedbecause this is the (one. We assumed that once it reaches in this (one the particle is notgoing to come bac! to the solution again or the particle is literally removed from thesystem. So the velocity is nothing but @by tu where tu is the time ta!en to travel whichis nothing but the @distance so this ;is nothing but the average settling velocity.


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Again we will consider another particle, the particle is somewhere here and we are givingthe same time as "to this particle also @< so what will be the settling velocity of thatparticle% whether it will be higher than the particle whatever we have considered here orthe settling velocity of this particle is less than that. the first particle whatever was here atthe top of the settling column it too! "time to travel @distance but the second particleis ta!ing the same time "to travel a distance of @


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removed before "and if it is somewhere here it will not be getting removed before orwithin ". So those are the things concluded here.

All the particles with diameter greater than or equal to d and settling velocity greaterthan or equal to ;will be removed from the system because the time required for their

settling will be lower than d. And if dp is less than d or ;


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)*efer Slide "ime+ /2+-0

"his is the pictorial representation of that one, when a particle is there in a sedimentationtan! and when water will be flowing continuously to the tan! it will be having ahori(ontal component of the velocity same as the hori(ontal velocity of the tan! and itwill be having a vertical velocity that is the settling velocity. So the particle will be ta!inga part li!e this )*efer Slide "ime+ /2+6/0 this is the resultant of the ; hwhich is thehori(ontal velocity and ;which is the vertical velocity. So, if the particle is here and it ishaving a vertical velocity of ;and the hori(ontal velocity that is the same throughout thetan! so that particle will be getting removed at this point. So we are assuming that anyparticle which comes up to this point is getting removed.

We are considering another particle here, it has to travel only this much distancevertically so it ta!es the same time% the ; h is the same and it is having the settlingvelocity ;


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)*efer Slide "ime+ /:+0

We have a homogenous suspension with an initial concentration of 3and what we do is,we allow that suspension to settle, then at different time intervals we ta!e theconcentration whatever is remaining in the solution, whatever is settled is alreadyremoved from the system and whatever is remaining will be there in the suspension. So att6 the concentration of suspension is 36% at t/ the concentration is 3/ and at t- theremaining concentration is 3- so these are the remaining particles in the suspension."herefore, maximum fraction of particle with ;6that is @by t6equal to is equal to 36by 3that much particle is removed in the system and the fraction of particle with ; isnothing but @by "that is having a fraction equal to .

=r we can represent li!e% this is the settling velocity )*efer Slide "ime+ -+-0 that isnothing but ;t equal to @by time. At different time intervals it is having differentsettling velocity and here we are finding out the fraction of particles which is remainingin the system. *emaining in the system at time t6is nothing but 36by 3% at time t/ isnothing but 3/by 3and at t-it is 3-by 3but at time ; tis equal to the design settlingvelocity what will happen is we are giving enough time for the fraction of particleswhatever is present and that will be getting removed completely and this region )*eferSlide "ime+ -+-0 the particle that is not in the top is in the in between portion of thesedimentation tan! so a fraction of that one will be getting removed. ?ence, we can findout what is the total removal efficiency that is nothing but up to here this is our design

settling velocity so is the fraction of particles remaining at that tan! so the particleremoved from the system is nothing but 6 minus so 6 minus particles arecompletely removed from the system and this portion up to a portion of that one willbe removed depending upon the position of the particle that was initially there.

We can find out the efficiency by summing up this one% 6 minus plus integral of tointo ;tby ;in to dx. ;tis the settling velocity of that particle and ;is the designsettling velocity and dx is this incremental length here. So if you ta!e the area of this


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portion this will be giving you the fraction of particles which is having a settling velocitylower than the settling velocity.8ow, coming to the discrete settling now another thing is that, though we provide a tan!depth of /m or -m actually spea!ing in discrete particle settling the depth of the tan! is

not at all important. ?ow can we show that one7 "he velocity or the settling velocity atany time or any particle is nothing but tan! depth divided by the detention time. Anddetention time if you want to define you have a sedimentation tan! and you !now what isthe volume of the sedimentation tan! and you have to treat a particular volume of waterand the water will be flowing into the sedimentation tan! at a constant rate so we assumethat the inflow rate is B. "hat means this much of meter cube water per second or per dayor per hour is coming into the tan!. So what is the detention time available in the tan! itis nothing but the volume of the tan! divided by the flow rate. $f the volume is hundredmeter and we are pumping water at 6 meter per cube hour then he detention time it isnothing but 6 divided by 6 means ten hours. Similarly here we can find out what isthe detention time, it is nothing but depth divided by tan! volume by flow rate so it is

again depth.

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And tan! volume is nothing but area into the depth the flow rate. So we can find out li!ethis% depth and depth will be getting cancelled so you will be getting B by area. So the

settling velocity of the sedimentation tan! is a function of the flow rate and surface areaof the tan!, this area is nothing but the surface area of the tan! not the depth.

"his is very important in the design of a sedimentation tan! especially when we deal withtype one settling. $n type one settling the design parameter is not the detention time it isthis parameter B by A it is !nown as surface overflow rate.And the settling velocity of any particle is numerically equal to B by A or the surfaceoverloading rate.


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)*efer Slide "ime+ -1+120

"his is the thing we were explaining earlier% what is the fraction of particle remaining andat "is remaining at "is remaining so the removal is 6 minus which we havealready discussed. So whenever we go for discrete particle settling we can even increasethe efficiency of the tan! by reducing the depth.

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Csing this concept only the high rate sedimentation tan!s li!e tube settlers, plate settlersetc are being constructed. We are assuming that if you have a tan! of two meter depthand if you provide many plates parallely the surface area will be increasing depending


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upon the number of plates so you will be getting B by As a very small number sonaturally the tan! efficiency will be increasing.

8owadays many high rate sedimentation tan!s which is used for the discrete particles andsettling have come up. We will be discussing about them towards the end of this lecture.

We have discussed about type one settling, discrete particle, how to conduct the columnstudy and get the settling velocity and if you want to get the real efficiency of that systemhow can we find out what is the total efficiency of particle removal in type one settling.

)*efer Slide "ime+ -2+/:0

8ow we will come to type two settling. "ype two settling is dealing with dilute flocculentsettling. $n water treatment this type of settling is coming in clary flocculators. After theaddition of coagulants the colloidal particles are destabili(ing and because of theflocculation or because of the velocity gradient we provide in flocculation these particlescome in contact with each other and they agglomerate and as a result the particle si(e willbe increasing with respect to time and they will be settling. So naturally in type twosettling the depth of the tan! is an important criteria because if you give more time moreand more particles will be agglomerating provided there is a velocity gradient available.So we cannot predict the removal efficiency in case of type two settling by theoreticalmeans. We have to conduct laboratory experiments using the same type of suspension

and find out what is the settling velocity and what is the removal efficiency. And if youwant to find out that one again we have to go for column settling analysis.

?ere the column settling analysis is slightly different. $n the column itself we have toma!e some modifications. $n the earlier case we had only one port to collect the sample.'ut here what is happening is we have to collect the samples at different depths. Whatwill happen with respect to depth the particle velocity will be increasing particle si(e willbe increasing so we want to get a clear picture of what is the rate of particle or what is the


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amount of particle removed at different heights and at different times that is what is beingdone in type two settling.

So here instead of finding out the amount of particle left over in the system what we do iswe find out what is the amount of particle removed from the system at a given depth and

at a given time. "hat is what this gives% xi9, xi9 is the fractional percent removed from thesystem that is nothing but 6 minus ci9 by 3into 6. ci9 is the concentration of particleleft in the column at the port i and time 9. $t is a time dependent and position dependentvalue. So we can find out xi9 and what is the fractional removal at different ports.

"herefore, if you want to conduct the experiment first you fill the column with thesuspension whose particles we want to remove and we want to design a sedimentationtan!. $nitially we mix it homogeneously so that in all the parts of the column you will behaving uniform concentration and find out what is the 3, 3 is nothing but the initialconcentration of the particles present in the system.

)*efer Slide "ime+ -:+-0

"hese ports are located at .m distance. So starting from to - you have five differentports and what you have to do is you have to ta!e the samples at different time intervals,say / minutes or - minutes so every thirty minutes you have to ta!e the samples fromall the five ports then you have to find out what is the ci9 and corresponding xi9 you can

find out and you can tabulate the results li!e this depth and time so you will be gettingdifferent concentrations. ?ence the concentration ci9 whatever you are getting is theconcentration whatever is remaining in the system. "hen using this formula we can findout what is the fraction removed at a particular time.

"herefore, using this data we can draw this curve, this is !nown as $S= removal linesfrom the settling analysis. "hus, you have different depths li!e ., 6, 6., /., /., - andso on and different time so you will be getting different percentages. So you 9ust find


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different percentages corresponding to the depth and the next step what you have to do isdraw a line passing through the same percentage removal so at different depths you willbe getting the same percentage say 6 percentage so you will be getting different valuesof 6. 8ow you 9oin all those points. Similarly - percentage, 1 percentage, 5percentage, 2 percentage etc depending upon how DoEE16+ siveF unit you can draw

the $S= concentration line for all ten percentage removal efficiency, twenty percentage,thirty percentage etc.

=nce these $S= removal lines are there how can we find out the settling tan! efficiency7$t is very easy to find that out. "he significance of these $S= removal lines, these $S=removal lines gives the settling velocity of that fraction of particle. "hat means this is thefraction of particles or the type of particles which gives 6 percentage removal )*eferSlide "ime+ 16+1/0 and this is the fraction of particle which gives - percentage removaland this is 1 percentage and this is 5 percentage and so on.


So, at any point of this $S= removal line if you ta!e the tangent that will give the velocityof that particular fraction of suspension or that particular fraction of solids. So you cansee the top, if you ta!e the tangent the slope is low, as it comes down the slope isincreasing and towards the bottom the slope is very steep or the settling velocity is verysteep. &rom this one it is very clear that the depth of the tan! is very very important in

case of flocculent suspension. Why the velocity is increasing with respect to depth7 As $have already discussed the particle will be growing in si(e with respect to time so initiallythe velocity is low and with respect to time the velocity is increasing and finally you willbe getting very high velocity. "his is true for all these lines. As we go )*efer Slide "ime+1/+0 the velocity is increasing from the top to bottom.

?ow to find out the efficiency7 Already we will be having an idea% this is the maximumdetention time we can give for the sedimentation tan!. We can mar! that point here


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because here we have the time in minutes. So say you are planning to give only :minutes in your sedimentation tan! that means only 6. hours for the sedimentation soyou mar! this point here so what does it mean so this 1 percentage line is touching atthat point. "hat means if you give ninety minutes of time you will be getting 1percentage removal and some fraction of this 5 percentage, 2 percentage file etc will be

also getting removed at that time so what you have to do is draw a perpendicular to thatpoint which is the design time then we can find out the efficiency as * is equal to r risthe fractional removal at that point plus sigma delta r into @ iby @. "hat means we !nowthe removal efficiency is depending upon the velocity. As we have seen in discreteparticle settling it is depending upon the velocity ; percentage, : percentage something li!e that soyou find out all the fraction and sum it up so you will be getting the removal efficiency.

)*efer Slide "ime+ 1+-0

"hese are the important things. $S= concentration lines represent the maximum settlingpath for the indicated removal slope gives the settling velocity. As the timer increasessi(e increases so settling velocity also increases. "his is the most important thing )*eferSlide "ime+ 1+1>0 how to find out the settling efficiency. $nitially we decided to go for: minutes and you are getting only 5 percentage of removal but you are intended to get> percentage of the removal% how can you achieve that7 "he only thing is you have tochange your detention time. Say you go for 6/ minutes so two hours then you see what


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is the removal efficiency you can get. Li!e that you can find out what is the efficiencymaximum you can get and based upon this data you can design your sedimentation tan!.

As $ have already mentioned, (one settling and compression settling because it is notcoming in water treatment because in water treatment we are dealing only with type one

settling and type two settling. So this (one settling and compression settling we willdiscuss in detail when we tal! about wastewater treatment.

8ow we will see the different types of sedimentation tan! which are commonlyemployed. We can go for either long rectangular tan!s or circular tan!s or solidGcontactclarifiers. "he most commonly used sedimentation tan!s are long rectangular ones andcircular tan!s are also being used.

)*efer Slide "ime+ 15+/:0


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)*efer Slide "ime+ 12+/-0

What are the important parts of a sedimentation tan!7

$t will be having an inlet region and an outlet region and this is the settling (one )*eferSlide "ime+ 12+/50 and whatever is settled in the sedimentation tan! we cannot allow it toaccumulate there, we have to remove the solids from the sedimentation tan! otherwisewhat will happen is whenever there is a flow there will be disturbance and there is also achance where whatever settled sludge is present in the sedimentation tan! will come upso we have to remove the settled sludge periodically.

Csually this is the sludge removing mechanism and the removed sludge will be coming toa hopper and from here it will be pumped out. So these are the common features of asedimentation tan!. We will see each part of this sedimentation tan! in detail. 'efore thatone we will see the common dimensions of a sedimentation tan!.As $ have explained we can go for long rectangular tan!s circular ones or solidGcontactorsthough solidGcontactors are not commonly used in our country. 'ut in hydraulic point ofview long rectangular basins are always advisable because they are more stable and flowcontroller control is easier. We also !now about the velocity profile, it is constant becausethe particle will be moving with a resultant velocity of the hori(ontal velocity and thevertical velocity so we can find that out.


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)*efer Slide "ime+ 1>+-0

"he length of the tan! usually varies from two to three times the width of the tan!.Csually the length is 6 to / times the height of the tan! and always the bottom of thesedimentation tan! will be slope slightly because it will facilitate the removal of thesludge. As we discussed it will be equipped with a slow moving mechanical scrapper toremove the sludge and usually redwood slats on a chain drive is usually used for theremoval of the sludge. What will happen here is, a chain is there and this wooden plan!sare there and this one will be continuously moving so whatever sludge is settled here willbe scrapped by the scrapper and it will be moved and it will be coming here )*efer Slide"ime+ +0 and from here we can pump it out easily. "his is the sludge hopper.

)*efer Slide "ime+ +>0


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"he inlet (one is another important thing because when we allow the water to enter intosedimentation tan!, if lot of turbulence is there the sedimentation tan! will not befunctioning properly and whatever sludge that is settled in the bottom of thesedimentation tan! will be disturbed and it will be coming up so your sedimentation tan!

efficiency will be drastically decreasing. "herefore, we have to design the inlet (oneproperly. "he ma9or ob9ective of the inlet (one is allowing water to enter uniformlywithout much disturbance into the sedimentation tan!.

&or outlet (one also we have to be careful. 'ecause if you ta!e out the water at a high rateagain it will be creating turbulence in the tan! and the quiescent condition will not bemaintained in the tan! so that will be influencing or that will be deteriorating theefficiency of the tan!.

Sludge (one is extended from the bottom of the tan! to 9ust above the scraper mechanismand the last one is the settling (one. We have seen surface loading ad detention periods.

&or sedimentation tan! design, especially for discrete particle or type one settling, wehave to design the tan! for surface loading not for the detention period because surfaceloading rate is nothing but the settling velocity. We have to give enough time for theparticle to settle properly so the design parameter is surface loading.

)*efer Slide "ime+ 6+-10

&or discrete particles settling we usually give a depth of /. to -m in the sedimentationtan!. "hough we say that the depth of the tan! is not so important we usually give a depthof /. to -m and if it is flocculent settling we give a depth of - to 1m because depth isalso important and usually the width of the tan! we give around 6/m because if youreduce the width tool to a too low value then the sludge removal becomes a problem. "helength is up to 1>m which we usually go for and settling velocity for discrete particle thevalue ranges from 6 to /m per hour and whereas the flocculent settling it is .5 to 6m per


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hour because for discrete particle usually it is sand or silt or clay so the specific gravitywill be very high so naturally they will be having a high settling velocity whereas forflocculent particle the density will be lower so the settling velocity will be less.

"he remaining portion of the sedimentation process we will discuss in the next class.

8ow we will see the things we have discussed in todays class. We were discussing aboutthe different types of solids and we have seen that the solids can be divided into discreteand flocculent and this division is important when we design the sedimentation tan!because the property of the solids are so different so unless we !now the property of thesolids the design will not be proper. then we were discussing the different types ofsuspension and there also we have seen that basically we can classify it into twocategories either dilute suspension or concentrated suspension and based upon theparticle nature and the suspension we can classify the settling into four categories thatmeans type one settling, type two settling, (one settling and compression settling. As faras water treatment is concerned we are dealing only with type one settling and type twosettling.

3ompression settling and (one settling we mainly use in wastewater treatment so thatportion we will be discussing in wastewater treatment. "hen we have seen that in discreteparticle we can find out the settling velocity using Sto!es Law so we will be getting thetheoretical settling velocity. but in practical conditions or if you want to design asedimentation tan! for a particular type of solids we cannot directly use Sto!es Lawbecause the diameter of the particles will be varying and the shape of the particle will bevarying because Sto!es Law is developed for uniform si(e and uniform shape particlesand we are assuming that it is spheres so we have to conduct laboratory studies. "hisstudy is !nown as settling column analysis. Csing the settling column we can find outwhat is the removal efficiency.

So if you design a sedimentation tan! for a particular settling velocity all the particleswith that settling velocity will be removed completely and some particles whose settlingvelocity is lower than the design settling velocity also will be removed because all theparticles will be homogeneously mixed the tan! from the top to the bottom. So if thelocation of the particle is lower than the top level then the time required will be less toreach the sludge (one. =nce it reaches the sludge (one we assume that it is settled.

8ow, coming to the flocculent settling the depth of the tan! is important and the columnsettling or settling column analysis is entirely different. ?ere what we have to do is wehave to find out what is the concentration of particles being removed at each height atdifferent time intervals and by analy(ing the data we can draw the $S= concentrationlines and using this one we can find out what is the efficiency of the system. We haveseen the different types of sedimentation tan!s we usually employ. "he most preferredone is long rectangular tan! because hydraulically this is the most stable one and circularones are also being used commonly. "he most important parts of a sedimentation tan!are% inlet (one, outlet (one, settling (one and sludge (one. "he bottom of the tan! will bealways incline inclined to remove the sludge and the sludge will be coming to a sludge


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hopper from there we can remove it easily. "hus, the details of the design and the valuesof surface overflow rate, weir loading etc we will be discussing in the next class.

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