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CHE 321UNIT OPERATION 1 (3 UNITS)

Module 8: Particulate Separation Operations 2

8.1: Drying, Conveying

8.2: Sedimentation, Clarification.

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RECOMMENDED READING/TEXT

Chemical Engineering Volume 2 byCoulson & Richardson Engineering

Transport Processes & Separationprocess

Principles by Geankopolis C..

Principles o! "nit #perations by$oust %.S.

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BRIEF OVERVIEW OF THE CORSE

nit Operation: is concerned with those separations that depend on

differences in physical properties rather than chemical behaviour.Such process depend either upon a difference in composition of

phases at euilibrium or upon a difference in the rate of mass transfer

of constituents of a mi!ture e.g drying, filtration, evaporation etc.

Separation Processes: when faced with problem of separating

components out of homogenous mi!ture, differences in the properties

of the constituents of the mi!ture is reuired to effect the separation.

"he various chemical and physical properties offer the greatest

difference among components because the difference in the property

will generally permit an easier, more economical separation.

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8.1.1. !R"I#$

rying is a mass trans!er process 'hich in(ol(es the remo(al o!

'ater) or another sol(ent by e(aporation !rom a solid) semi*

solid or li+uid.

,n drying) 'ater is remo(ed as a (apor by air 'hile in

e(aporation 'ater is remo(ed as (apor at its boiling points.

The drying o! materials is o!ten the -nal operation in a

manu!acturing process) carried out immediately prior to

packaging or dispatch.

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Purpose o% !r&in'

"o reduce the cost of transportation.

"o ma#e a material more suitable for handling, for e!ample, soap

powders, dyestuffs and fertilisers.

%s a preser(ation techni+ue. ry !oods can be stored !or

etended period o! time.

"o remove moisture which may otherwise lead to corrosion e.g drying

of gaseous fuels or ben$ene prior to chlorination.

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$eneral Met(ods o% !r&in'

Drying methods and processes can be classified in several different ways.

%atch process& where the materials is inserted into the drying euipment and dryingproceeds for a given period of time

Continuous process & where the material is continuously added to the dryer and dried

material is continuously removed.

Drying processes can also be categori$ed according to the physical conditions used

to add heat and remove water vapor'

Convective or direct drying& heat is added by direct contact with heated air at

atmospheric pressure and the water vapor formed is removed by air'

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(ndirect or contact drying ) involves drum drying, vacuum drying, the evaporation

of water proceeds more rapidly at low pressures, and the heat is added indirectly by

contact with metal wall or by radiation

*ree$e&drying& is a drying method 'here the sol(ent is !ro/en prior to

drying and is then sublimed, i.e.) passed to the gas phase directly

!rom the solid phase) belo' the melting point o! the sol(ent.

Dielectric drying +radiofreuency or microwaves being absorbed inside the

material& may be used to assist air drying or vacuum drying.

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Supercritical drying +superheated steam drying& involves steam drying of

products containing water. "his process is feasible because water in the

product is boiled off, and -oined with the drying medium, increasing its flow.

atural air drying& ta#es place when materials are dried with unheated

forced air, ta#ing advantage of its natural drying potential. (t is a slow and

weather dependent process. /rains are increasingly dried with this

techniue, and the total time may last from one wee# to various months.

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Rate o% !r&in'

(n drying, it is necessary to remove free moisture from the surface and also moisture

from the interior of the material. (f the change in moisture content for a material is

determined as a function of time, a smooth curve is obtained from which the rate of

drying at any given moisture content may be evaluated. "he form of the drying rate

curve varies with the structure and type of material, and two typical curves are

shown in *igure 1.

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*igure 1: 0ate of drying of a granular material

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(n curve 1, there are two well&defined $ones: %, where the rate of

drying is constant and %C, where there is a steady fall in the rate of

drying as the moisture content is reduced. "he moisture content at theend of the constant rate period is represented by point %, and this is

#nown as the critical )oisture content. Curve 2 shows three stages, D,

* and *C. "he stage D represents a constant rate period, and * and

*C are falling rate periods. (n this case, the Section * is a straight line,however, and only the portion *C is curved. Section * is #nown as the

first falling rate period and the final stage, shown as *C, as the second

falling rate period. "he drying of soap gives rise to a curve of type 1, and

type 2 shows the drying of sand.

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Constant Rate Periods

,t is the initial stage o! drying 'here 'ater is e(aporated !rom the sur!aceo! the product and the temperature o! the product remains constant.

,n order to calculate the rate o! drying under these conditions) the

relationships obtained !or di0usion o! a (apour !rom a li+uid sur!ace into agas may be used.

The simplest e+uation o! this type is1

345

'here kGis the mass trans!er coe6cient.

A is the sur!ace area)Ps is the (apour pressure o! the 'ater) and

Pw is the partial pressure o! 'ater (apour in the air stream.

Since the rate o! trans!er depends on the (elocity u o! the air stream)raised to a po'er o! about 7.8) then the mass rate o! e(aporation is1

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E+. 2 simply states that the rate o! trans!er is e+ual to the trans!er

coe6cient multiplied by the dri(ing !orce. ,t may be noted) ho'e(er) that3Ps 9 Pw5 is not only a dri(ing !orce) but it is also related to the capacity o!

the air stream to absorb moisture

The rate o! drying in the constant rate period is gi(en by1

: : . 3;5

'here1 w is the rate o! loss o! 'ater)

h is the heat trans!er coe6cient !rom air to the 'et sur!ace)

T is the temperature di0erence bet'een the air and the sur!ace)

is the latent heat o! (aporisation per unit mass)kGis the mass trans!er coe6cient !or di0usion !rom the 'et sur!ace

through the gas -lm)

. 325

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A is the area o! inter!ace !or heat and masstrans!er) and

(Ps 9 Pw) is the di0erence bet'een the (apourpressure o! 'ater at the sur!ace and the partialpressure in the air.

First f!!i"#$rt% &%ri'

The points < and E in $igure 4 represent conditions'here the sur!ace is no longer capable o! supplyingsu6cient !ree moisture to saturate the air incontact 'ith it.

S%'" f!!i"#$rt% &%ri'%t the conclusion o! the -rst !alling rate period itmay be assumed that the sur!ace is dry and thatthe plane o! separation has mo(ed into the solid. ,n

this case) e(aporation takes place !rom 'ithin the

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Ti*% f'r r+i"#

,! a material is dried by passing hot air o(er asur!ace 'hich is initially 'et) the rate o! dryingcur(e in its simplest !orm is represented by

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'here1 w is the total moisture)

we is the e+uilibrium moisture content 3point E5)

w 9 we is the !ree moisture content) and

wc is the critical moisture content 3point C5.

C'"st"t$rt% &%ri'

uring the period o! drying !rom the initial moisturecontent w4to the critical moisture content wc) the

rate o! drying is constant) and the time o! drying tcis gi(en by1

Rc is the rate o! drying per unit area in theconstant rate period)

%is the area o! eposed sur!ace.

.. 3=5

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F!!i"#$rt% &%ri'

uring this period the rate o! drying is)approimately) directly proportional to the !reemoisture content 3w 9 we5) or1

.. 3>5

3?5

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T't! ti*% 'f r+i"#

The total time t o! drying !rom w4 to w is gi(en by t: (tc @ tf ).

The rate o! drying Rco(er the constant rate periodis e+ual to the initial rate o! drying in the !allingrate period) so that

Rc: m!c.

Thus1

and the total drying time,

.. 3A5

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E,*&!% 1

% 'et solid is dried !rom 2> to 47 per cent moistureunder constant drying conditions in 4> ks 3=.4A h5.,! the critical and the e+uilibrium moisture contentsare 4> and > per cent respecti(ely. Bo' long 'ill ittake to dry the solid !rom ;7 to 8 per cent moisture

under the same conditionsS'!-ti'"

$or the -rst drying operation1

D4: 7.2> kgkg) ' : 7.47 kgkg) 'c : 7.4> kgkg

and 'e : 7.7> kgkg

Thus1 : 3'49 'e5 : 37.2> 9 7.7>5 : 7.27 kgkg

: 3'c 9 'e5 : 37.4> 9 7.7>5 : 7.47kgkg

! : 3' 9 'e5 : 37.47 9 7.7>5 : 7.7> kgkg

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$rom e+uation 8) the total drying time is1

m% : 7.7??A34.7 @ 7.?F;5 : 7.44; kgs

$or the second drying operation1

D4: 7.;7 kgkg) ' : 7.78 kgkg) 'c : 7.4> kgkg and'e : 7.7> kgkg

Thus) !4: 3'49 'e5 : 37.;7 9 7.7>5 : 7.2> kgkg

!c : 3'c 9 'e5 : 37.4> 9 7.7>5 : 7.47 kgkg

! : 3' 9 'e5 : 37.78 9 7.7>5 : 7.7; kgkgThe total drying time is then1

t : 347.44;537.2> 9 7.4757.47 @ ln37.477.7;5H

: 8.8>?34.> @ 4.27=5

t : 2;.F ks 3?.?> h5

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C!ssiti'" " S%!%ti'" Of Dr+%rs

Classi-cation and selection o! dryers is based onthe !ollo'ing !actors1

3a5 Temperature and pressure in the dryer)

3b5 The method o! heating)

3c5 The means by 'hich moist material istransported through the dryer)

3d5 %ny mechanical aids aimed at impro(ing drying)

3e5 The method by 'hich the air is circulated)

3!5 The 'ay in 'hich the moist material issupported)

3g5 The heating medium) and

3h5 The nature o! the 'et !eed and the 'ay it isintroduced into the dryer.

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T+&%s 'f Dr+%r

1. Tr+ 'r s%!f r+%rs

Tray or shel! dryers are commonly used !or granularmaterials and !or indi(idual articles. The material isplaced on a series o! trays 'hich may be heated!rom belo' by steam coils and drying is carried outby the circulation o! air o(er the material. %s air is

passed o(er the 'et material) both its temperatureand its humidity change.

2.T-""%! Dr+%rs

,n tunnel dryers) a series o! trays or trolleys is

mo(ed slo'ly through a long tunnel) 'hich may ormay not be heated) and drying takes place in acurrent o! 'arm air. Tunnel dryers are used !ordrying para6n 'a) gelatine) soap) pottery 'are)

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*igure 3: rrangements for tunnel dryers

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3. R'tr+ r+%rs

Rotary dryer) 'hich consists o! a relati(ely longcylindrical shell mounted on rollers and dri(en at a

lo' speed) up to 7.= B/ is suitable !or thecontinuous drying o! materials on a large scale) 7.;kgs 34 tonneh5 or greater. ,t in(ol(es either directheating or indirect heating

*igure 4: 0otary dryer, 5.67 m diameter I 4.7 m long for drying dessicated coconut

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0. Dr-* r+%rs

,! a solution or slurry is run on to a slo'ly rotatingsteam*heated drum) e(aporation takes place and

solids may be obtained in a dry !orm. This is thebasic principle used in all drum dryers) some !ormso! 'hich are illustrated in $igure >. The agitatorpre(ents settling o! particles) and the spreader is

sometimes used to produce a uni!orm coating onthe drum. The kni!e is employed !or remo(ing thedried material

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*igure 7: ethods of feeding drum dryers. +a Single drum, dip&feed. +b Single drum,pan&feed.+c Single drum, splash&feed. +d Double drum, dip&feed. +e Double drum, top&feed

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. S&r+ r+%rs

Dater may be e(aporated !rom a solution or asuspension o! solid particles by spraying the

miture into a (essel through 'hich a current o! hotgases is passed. ,n this 'ay) a large inter!acial areais produced and conse+uently a high rate o!e(aporation is obtained.

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8.1.2. MATERIA HANDING EUIPMENT

Jaterial handling includes a number o! operationsthat can be eecuted either by hand 3manual5 or by

mechanical means or de(ices to con(ey materialand to reduce the human drudgery.

S%!%ti'" 'f *t%ri! H"!i"# *i"%s "C'"4%+'rs

The selection o! proper con(eying system isimportant !or ease in operation and getting desiredcapacity !or a particular product. Principles basedon 'hich the material handling e+uipment is

selected are1 characteristics o! the products being con(eyed

Dorking and climatic conditions.

The capacity o! con(eying

,n a con(eying system possibility o! use o! gra(ity

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T+&%s 'f Mt%ri! H"!i"# E5-i&*%"t

The most common types o! mechanical de(ices !orgrain handling areK

1. 6%!t '"4%+'rs

% belt con(eyor is an endless belt operatingbet'een t'o pulleys 'ith its load supported onidlers. The belt may be Lat !or transporting bagged

material or V*shaped. The belt con(eyor consists o!a belt) dri(e mechanism and end pulleys) idlers andloading and discharge de(ices 3$ig. 45.

Fi'*+ !ia'ra) o% a ,elt con-e&or

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2. 6-7%t E!%4t'r

% bucket ele(ator consists o! buckets attached to achain or belt that re(ol(es around t'o pulleys one

at top and the other at bottom. The (ertical li!t o!the ele(ator may range bet'een !e' metres tomore than >7 m. Capacities o! bucket ele(atorsmay (ary !rom 2 to 4777 thr.

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Fi# 2. 6-7%t E!%4t'r

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3. Sr% C'"4%+'r

The scre' con(eyor consists o! a tubular or "*shaped trough in 'hich a sha!t 'ith spiral scre'

re(ol(es. The scre' sha!t is supported hangerbearings at ends. The rotation o! scre' pushes thegrain along the trough. The scre' con(eyor is usedin grain handling !acilities) animal !eed industries

and other installations !or con(eying o! productsgenerally !or short distances. Scre' con(eyorre+uires relati(ely high po'er and is moresusceptible to 'ear than other types o! con(eyors.

Fi'ure .* Scre/ Con-e&or

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0. Ci" C'"4%+'r

% chain is a reliable machine component) 'hichtransmits po'er by means o! tensile !orces) and is

used primarily !or po'er transmission andcon(eyance systems. The !unction and uses o!chain are similar to a belt. Chains are di(ided into-(e types based on material o! composition or

method o! construction. Cast iron chain

Cast steel chain

$orged chain

Steel chain Plastic chain

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. P"%-*ti C'"4%+'r

The pneumatic con(eyor mo(es granular materialsin a closed duct by a high (elocity air stream.

Pneumatic con(eying is a continuous and Leibletransportation method. The material is carried inpipelines either by suction or blo'ing pressure o!air stream. The granular materials because o! high

air pressure are con(eyed in dispersed condition.$or dispersion o! bulk material) air (elocities in therange o! 4>*;7 ms is necessary.

Fi'ure 0: Separation o% product particles %ro) air ,& )eans o% a %a,ric %ilter

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i*itti'"s 'f P"%-*ti C'"4%+i"#

4. Erosion o! solid sur!aces and e+uipment sur!acesby solid particles 'ith con(eying air stream.

2. ,n case o! bends or misaligned sections) theerosion problem becomes se(ere.

;. Chances o! repeated impacts bet'een theparticles and the solid sur!aces are high. ue to

such impacts) product degradation results.

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Module 8*2: Sedi)entation1 Clari%ication*

8.2.1. SEDIMENTATION

Sedimentation is the tendency of particles in suspension to settle out of the

fluid in which they are entrained and come to rest against a barrier. "his is

due to their motion through the fluid in response to the forces acting on them

which could be gravity, centrifugal acceleration or electromagnetism.

(n simple terms, sedimentation is the separation of a dilute slurry by gravity

settling into clear fluid and a slurry of higher solids content. Settling is the

falling of suspended particles through the liuid while sedimentation is the

termination of the settling process.

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*ilteration, centrifugation and sedimentation are the types of solid&fluid separation

methods

"he ob-ective of settling process is to remove particles from a stream in order to

eliminate contaminants from the fluid or to recover the particles e.g. elimination of

dust, fumes or flue gas from the air or the removal of solids form liuid waste

lso in some cases particles are deliberately suspended in fluids to obtain

separations of the particles into fractions differing in si$e or density. "he fluid is the

n recovered, sometimes for reuse, from the fractionated particles.

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Settling velocities are too low and for practical operation the particles must be agglomerated

+i.e forming a round mass or flocculated +i.e aggregating into clumps or mass that sin#s or

can be removed by filtering into large particles that possess higher settling velocity.

"he settling process can also be carried out rapidly with the aid of coagulant such as alum,

*eCl2. "his process is called Clarification through the use of Clarifier.

n e!ample of a sedimentation euipment are "hic#ners. thic#ner enhances the

concentration of the sediment separating the mi!ture into underflow and overflow through

mechanical means.

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pplications o% Settlin' and Sedi)entation

0emoval of solids from liuid sewage wastes

Settling of crystals from the mother liuor

Separation of liuid&liuid mi!ture from a solvent&e!traction stage in a

settler

Settling of solid food particles from a liuid food

9ater treatment

"he particles can be solid particles or liuid drops. "he fluid could be liuid

or gas and it may be at rest or in motion.

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SEDIMENTATION OF FINE PARTICES

!perimental study

"he sedimentation of metallurgical slimes has been studied by C

and C;

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T&pes o% Settlin'

Depending on the particles concentration and the interaction

between particles, 4 types of settling can occur,

"ype (: *ree=Discrete particle settling :

"ype ((: *locculent settling

"ype (((: >indered settling +?one Settling

Type ,V1 Compression settling

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T&pe I Settlin'3 Free4!iscrete particle settlin':

"his is the removal of discrete particles in such low concentration

that each particle settles freely without interference from ad-acent

particles +that is, unhindered settling. "he particles settle as

individual particles and do not flocculate or stic# to other during

settling. typical occurrence is the removal of sand particles.

9hen a particle settles in a fluid it accelerates until the drag force

due to its motion is eual to the submerged weight of the particle.

t this point, the particle will have reached its ter)inal -elocit&*

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9hen the concentration of particles is very small, each particle settles

discretely, as if it is alone, unhindered by the presence of other

particles. Starting from rest, the velocity of a single particle settlingunder gravity in water will increase where the density of the particle is

greater than the density of the water. cceleration continues until the

resistance to flow through the water, or drag, euals the effective

weight of the particle. "hereafter the settling velocity remains constant.

"his velocity is called the ter)inal settlin' -elocit&1 Vt

*or a rigid particle moving in a fluid, there are 3 forces acting on the

body: gravity +*g acting downward, buoyant force +*b acting upward,

and resistance or drag force +*d acting in opposite direction to the

particle motion.

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"he buoyant force *b in ewton on the particle is

*b @ @

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*D @ CD AAAAAAAAAA. +3

9here CD is the drag coefficient= proportionality constant

"he resultant force on the body euals the force due to acceleration

m @ */) *%) *D AAAAAAAA.. +4

Substituting all the resultant forces from en. 1, 2 and 3 into en 4, it

becomes:

m : mg * * C

3>5

The terminal (elocity is the period o! constant (elocity)

hence : 7

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then,

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Vt :

.3F5

MRe :

,n turbulent Lo' region 'here MRe is about 4777 to

2.7 N 47>

C&: 7.==

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*igure 3: Drag Coefficient for a rigid sphere

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E5a)ple +

il droplets having a diameter of 25 m +5.525 mm are to be

settled from air at temperature of 36.8oC +311 and 151.3#Ea

pressure. "he density of the oil is F55 #g=m3. Calculate the

terminal settling velocity of the droplets.

Density of air @ 1.136 #g=m3

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"he droplet will be assumed to be a rigid sphere

Vt : .

3A5

Vt :

MRe:

: 4.4FA Vt

$or the -rst trial) assume that Vt: 7.;7>ms.

MRe

: 4.4FA 37.;7>5 : 7.;?>

Sol(ing !or Cin e+n A

CD@ :

C

:

2.22

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*or the 1sttrial, assuming

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lternatively, the

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T&pe II: Flocculent settlin'6

"his is defined as a condition where particles initially settle

independently but flocculate in the depth of the clarification unit thusthey increase in si$e and settle at a faster velocity.

*locculation is a process of aggregation and attrition. ggregation can

occur by %rownian diffusion, differential settling, and velocity gradients

caused by fluid shear. ttrition is caused mainly by e!cessive velocity

gradients.

E5a)ples o% Flocculent Settlin':

H Erimary settling of wastewater

H Settling of chemically coagulated water and wastewater

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T&pe III: Hindered settlin' 7one Settlin'9:

9hen the particles are crowded, they settle at a lower rate. s

the concentration of particles in a suspension is increased, a point

is reached where particles are so close together that they no

longer settle independently of one another and the velocity fields

of the fluid displaced by ad-acent particles, overlap. "here is

also a net upward flow of liuid displaced by the settling

particles. "his results in a reduced particle&settling velocity and

the effect is #nown as (indered settlin'.

C i i % i i

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C(aracteristics o% Hindered settlin'6

(t is the settling of an intermediate concentration of particles

"he particles are close to each other

(nter&particle forces hinder settling of neighboring particles

Earticles remain in fi!ed position relative to each other

ass of particles settle as a $one

"he higher effective viscosity of the mi!ture is eual to the

actual viscosity of the liuid itself , divided by an empiricalcorrection factor, , which depends upon , the volume fraction of

the slurry mi!ture occupied by the liuid.

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.... 3475

.. 3445Dhere is dimensionless

The density o! the Luid phase e0ecti(ely becomes the bulkdensity o! the slurry

.

3425

Dhere is density o! slurry in solid3kg5 @ li+uid 3m;5. Thedensity di0erence is no'

: .

34;5

Substitute !or in e+n. F) !or !rom e+n 4; andmultiplying the result by !or the relati(e*(elocity e0ect)

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. F becomes for laminar settling.

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E5a)ple 2

Calculate the settling velocity of glass spheres having a diameter

of 1.774 ! 15&4m in water at 2F3.2 +25oC. "he slurry contain

B5 wt I solids. "he density of the glass spheres is @ 24B6 #g=m3.

Density of water @ FF8 #g=m3.

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:7.?22

The bulk density

: 7.?22 3FF85 @ 34* 7.?225 32=?A5

: 4>>; kgm;

:

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Settling

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Then MRe :

MRe:

0e

@ 5.121

Since MRe 4) the settling is in a laminar range

T+&% I9: C'*&r%ssi'" s%tt!i"#

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T+&% I9: C'*&r%ssi'" s%tt!i"#1

"his occurs when the particle concentration is so high that

particles at one level are mechanically influenced by particles on

lower levels. "he settling velocity then drastically reduces to low

solids concentration. typical occurrence of this type of settling

is the removal of sand particles. (n this settling, a particle will

accelerate until the drag force, *D, euals the impelling +due to

weight force, *(' then settling occurs at a constant velocity,

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C(aracteristics o% Co)pression settlin':

Settling of particles that are of high concentration

Earticles touch each other

Settling occurs by compression of the compacting mass

(t occurs in the lower depths of final clarifiers of activated sludge

Euip)ent %or Settlin' and Sedi)entation

1. Simple gravity settling tan#' simple gravity is used for

removing a dispersed liuid phase from another phase. "hevelocity hori$ontally to the right must be slow enough to allow

time for the smallest droplets to rise from the bottom to the right

"o the interface or from the top down to the interface and

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"o the interface or from the top down to the interface and

coalesce. Dust&laden air enters at one end of the chamber.

Earticles settle towards the floor at their terminal velocities.

2. Spit$#asten Classifier: (t consists of a series of conical vessels of

increasing diameter in the direction of flow. "he slurry enters the

first vessel, where the largest and fastest&settling particles are

separated. "he slurry enters the first vessel, where the largest and

fastest& settling particles are separation. "he overflow goes to the

ne!t vessel, where another separation occurs.

3 Sedimentation thic#ener: (ndustrially sedimentation operations

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3. Sedimentation thic#ener: (ndustrially, sedimentation operations

are often carried out continuously in euipment called

thic#eners. (n the thic#ener the entering slurry spreads radially

through the cross section of the thic#ner and the liuid flows

upward and out the overflow.

8 2 2C;RIFICTIO#

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8*2*2C;RIFICTIO#

Clarification is the general term used to describe the way suspended solids are

separated from a liuid +water.

Clarification is the process of settling. *or a settling process, the velocity of water is

lowered below the suspension velocity and suspended particles settle out of water

due to gravity.

Suspended matter in raw water supplies is removed by various methods to provide

water suitable for domestic purposes and most industrial reuirements. "he

suspended matter can consist of large solids, settable by gravity alone without any

e!ternal aids, and nonsettleable material, often colloidal in nature. 0emoval is

generally accomplished by coagulation, flocculation, and sedimentation.

S lid t t l ifi l # fl lid t t l ifi

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Solids&contact clarifiers, also #nown asupflow solids&contact clarifiers or

upflow sludge&blan#et clarifiers combine coagulation, flocculation, and

sedimentation within a single basin. Solids&contact clarifiers are often found

in pac#aged plants and in cold climates where sedimentation must occur

indoors.

Coa'ulation is the process of destabili$ation by charge neutrali$ation. nce

neutrali$ed, particles no longer repel each other and can be brought together.

Coagulation is necessary for the removal of the colloidal&si$ed suspended

matter.

Flocculation is the process of bringing together the destabili$ed, or

Jcoagulated,J particles to form a larger agglomeration, or floc.

Sedi)entation refers to the physical removal from suspension

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Sedi)entation refers to the physical removal from suspension,

or settling, that occurs once the particles have been coagulated

and flocculated. Sedimentation or subsidence alone, without

prior coagulation, results in the removal of only relatively coarse

suspended solids.

Settled solids are removed as slud'e, and floating solids are

removed as scu).

(n most municipal wastewater treatment plants, the treatment unit

which immediately follows the grit channel is the sedimentation

and flotation unit, which is also #nown as the primary.

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*igure 1: "ypical Clarifier

typical plant may have clarifiers located at two different

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typical plant may have clarifiers located at two different

points. "he one which immediately follows the bar screen, grit

channel is called the Erimary Clarifier, because it is the first

clarifier in the plant. "he other, which follows other types of

treatment units, is called the Secondary or final clarifier. "he two

types of clarifiers operate almost e!actly the same way.

(n primary clarifier only large particles settle at the bottom while

smaller particles remain in the suspension. >ence the need for a

secondary clarifier and other processes to achieve a complete

settlement.

"he main difference between primary and secondary clarifiers is

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"he main difference between primary and secondary clarifiers is

in the density of the sludge handled. Erimary sludges are usually

denser than secondary sludges. ffluent from a secondary

clarifier is normally clearer than primary effluent.

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Fi'ure 2: T&pical clari%ication process

"o calculate the efficiency of any wastewater treatment process

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"o calculate the efficiency of any wastewater treatment process,

you need to collect samples of the influent and the effluent of the

process, preferably composite samples for a 24& hour period.

e!t, measure the particular water uality indicators. *or

e!ample, %iochemical !ygen Demand +%D, suspended

solids you are interested in and calculate the treatment efficiency.

Calculations of treatment efficiency are for process control

purposes. Kour main concern must be the uality of the plant

effluent, regardless of percent of wastes removed.

E5a)ple:

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p

"he influent %D to a primary clarifier is 255 mg=l, and the effluent %D is

145 mg=l. 9hat is the efficiency of the primary clarifier in removing %DL

S'!-ti'"

Gi(en1 ,nLuent

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T&pical Clari%ier E%%iciencies

"he following is a list of some typical percentages for primary

clarifier efficiencies:

9ater Muality (ndicator !pected 0emoval fficiencySettleable Solids F7I to FFI

Suspended Solids 45I to B5I

"otal Solids 15I to 17I

%iochemical !ygen

Demand +%D 25I to 75I

%acteria 27I to 67I

p> generally will not be affected significantly by a clarifier.

Kou can e!pect wastewater to have a p> of about B.7 to 8.5

depending on the region, water supply and wastes

discharged into the collection system.

Clarifier efficiencies are affected by many factors, including:

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Clarifier efficiencies are affected by many factors, including:

1. "ypes of solids in the wastewater, especially if there is a

significant amount of industrial waste.

2. ge +time in collection system of wastewater when it reaches the

plant. lder wastewater becomes stale' solids do not settle

properly because gas bubbles cling on the particles and tend to

hold them in suspension.

3. 0ate of wastewater flow as compared to design flow. "his is

called the Nhydraulic loading.O

4. echanical conditions and cleanliness of clarifier.

7. Eroper sludge withdrawal. (f sludge is allowed to remain n the

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7. Eroper sludge withdrawal. (f sludge is allowed to remain n the

tan# it tends to gasify and the entire sludge blan#et +depth may

rise to the water surface in the clarifier.

B. Suspended solids, which are returned to the primary clarifier

from other treatment processes, may not settle completely.

Sources of these solids include waste activated sludge,

supernatant and sludge dewatering facilities +concentrate from

centrifuges and filtrate from filters.