Download - Coagulation Ly Thuyet Ve Keo Tu
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
1/41
Everything you want to know aboutCoagulation & Flocculation....
Zeta-Meter, Inc.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
2/41
Fourth Edition
April 199 3
Copyright
Copyright by Zeta-Meter, Inc. 1 993, 1 991, 1 990,1988 . All rights reserved. No part of this publicationmay be reproduced, t ransm it ted , t ranscr ibed, s tored ina retrieval system, or translated into any language inany form by any m eans without the wri tten perm issionof Zeta-Meter, Inc.
Your Comments
We hop e this guide will be helpfu l. If you ha ve an ysu ggestions on h ow to make i t better, or if you ha veadd it ional informa tion you th ink would help other
readers , then please drop u s a note or g ive us a cal l .Future edit ions will incorporate your comments.
Address
Zeta-Meter, Inc.765 Middlebrook Avenu ePO Box 3008Stau nton , Virginia 2 4402
Telephone (540) 886-350 3Toll Free (USA) (800) 333-022 9Fax (540) 886-372 8E-Mail [email protected]
ht tp: / / www.zeta-meter .com
Credits
Conceived a nd writ ten by Louis RavinaDesigned and i l lustrated by Nicholas Moramarco
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
3/41
Chapter 4 _____________________ 19
Using Alum and Ferric CoagulantsTime Tested Coagulants
Alum inu m S u lfate (Alum )
pH Effects
Coagu lant Aids
Chapter 5 _____________________ 25
Tools for Dosage Control
J ar Tes tZeta Potential
St reaming Cu r rent
Tu rbidity an d Par t icle Coun t
Chapter 6 _____________________ 33
Tips on MixingBasics
Rapid Mixing
Flocculation
The Zeta Potential Experts ________ 37
About Zeta-Meter
Introduction _____________________ iii
A Word About This Guide
Chapter 1 ______________________ 1
The Electrokinetic ConnectionParticle Cha rge Prevent s Coagu lation
Microscopic Electrical Forces
Balancing Opposing Forces
Lowerin g the En ergy Barrier
Chapter 2 ______________________ 9
Four Ways to FlocculateCoagulate, Then Flocculate
Double Layer Compression
Cha rge Neu tra lization
Bridging
Colloid En trapm ent
Chapter 3 _____________________ 13
Selecting PolyelectrolytesAn Aid or Su bs titut e for Traditiona l
Coagulants
Picking the Best One
Cha ra cterizing Polymers
En h an cing Polymer Effectivenes s
Polymer Packaging and Feeding
Interferences
Contents
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
4/41
A Word About This Guide
The removal of su spen ded m at ter f rom
water is on e of th e ma jor goals of water
t reatmen t . Only dis infect ion is u sed more
often or cons idered more imp ortan t. In fact,
effective clar ification is rea lly n ecess ar y for
completely reliab le disin fection becau se
microorganisms are shielded by particles in
the water .
Clarification usually involves:
coa gu la t ion
floccu la t ion
s et t lin g
filt ra t ion
This gu ide focuses on coagulat ion an d
floccu lation : th e two key steps wh ich often
determine finished water quality.
Coagu lat ion control techn iques ha ve ad-
vanced slowly. Many plan t opera tors re-
mem ber when dosa ge control was ba sed
u pon a visu al evalu ation of th e floccu lation
ba sin an d th e clarifier. If th e operators
eyebal l evaluation found a deterioration inqual ity, then h i s common sense response
was to increase the coagulan t dose. This
remedy was based u pon the ass um pt ion
th at if a l it t le did s ome good, then more
ought to do bet ter , bu t i t often did worse.
The comp etency of a plant opera tor de-
pend ed on h is years of experience with tha t
sp ecific water su pp ly. By trial , error an d
oral t radi t ion, h e would eventu al ly encoun -
ter every type of problem and learn to deal
with it.
Rel iable inst ruments now help us under-
s tan d a nd control the clar i ficat ion process .
Our abi lity to meas u re tu rbidity, par t icle
count , zeta potent ial and s t reaming current
ma kes coagulat ion an d f loccu lat ion more of
a s cience, al though ar t a nd experience s t i llhave their place.
We make ze ta m eters an d h appen to be a
little biased in favor of zeta pote n tial. In
this gu ide, h owever , we ha ve at tem pted to
give you a fair picture of all of the tools at
your d isposa l, and how you can pu t them to
work.
Introduction
iii
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
5/41
1
Particle Charge Prevents Coagulation
The key to effective coagulation and floccu-
lat ion is a n u nd erstan ding of how individ-
u al colloids in teract with each oth er. Tu r-
bidity par t icles ra nge from a bou t .01 to 100
microns in s ize. Th e larger fraction is
relatively eas y to sett le or filter. The
smaller, colloidal fraction, (from .01 to 5
microns ), pres ent s the real cha llenge. Their
settl ing times are intolerably slow and they
eas ily esca pe filtra tion.
Th e beh avior of colloids in water is str ongly
influen ced by th eir electrokinetic cha rge.
Each colloidal part icle carries a l ike ch arge,
which in na tu re is u su al ly negat ive. This
like cha rge cau ses ad jacent pa r t icles to
repel each oth er an d pr event s effective
agglomera tion an d floccu lation . As a
resu lt , cha rged col loids tend to rema in
discrete , dispersed, and in suspension.
On the other hand, if the charge is signifi-
cant ly redu ced or elimina ted, then the
colloids will gath er t ogether. First formin g
sm all grou ps, th en larger aggregates an d
fina lly in to visible floc pa rticles wh ich s ettle
rapidly and filter easily.
Chapter 1
The Electrokinetic Connection
Charged Particles repel each other
Uncharged Particles are free to collide and aggre-gate.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
6/41
2
Chapter 1 The Electrokinetic Connection
Microscopic Electrical Forces
The Double Layer
The dou ble layer m odel is u sed t o visu alizeth e ion ic environm ent in th e vicinity of a
cha rged colloid an d explains h ow electrical
repu lsive forces occu r. It is easier to u nd er-
s tan d this model as a s equen ce of s teps
tha t wou ld take place aroun d a s ingle
n egative colloid if the ions su rrou nd ing it
were sudd enly s t r ipped away.
We first look at th e effect of th e colloid on
th e positive ions , which a re often called
counter-ions . Initially, att ract ion from th e
n egative colloid cau ses som e of th e positive
ions to form a firmly at tach ed layer a roun dth e su rface of th e colloid. Th is layer of
counter- ions is known as the Stern layer.
Add itiona l positive ions are sti ll attra cted b y
th e negative colloid bu t n ow they are re-
pelled b y the positive Stern layer as well as
by other n earby posi t ive ions tha t are a lso
tryin g to approa ch th e colloid. A dyna mic
equ ilibriu m r esu lts, forming a diffu se layerof cou n ter-ion s. The diffu se positive ion
layer has a high concentrat ion near the
colloid which gra du ally decreas es with
distance u nt i l it reach es equ ilibr ium with
the n ormal coun ter- ion concentra t ion in
solut ion.
In a s imilar bu t opposi te fash ion, th ere is a
lack of negat ive ions in th e neighborh ood of
the s u rface, becaus e they are repelled by
th e n egative colloid. Negat ive ions ar e
called co-ions because they have the sam e
cha rge as th e colloid. Their concen tra tionwill gradu ally in creas e as th e repu lsive
forces of th e colloid are s creened out by th e
positive ions , u n til equ ilibriu m is again
reached with the co- ion concen trat ion in
solut ion.
Two Ways to Visualize the
Double Layer
The left view shows the
change in charge densityaround the colloid. Theright shows the distributionof positive and negativeions around the chargedcolloid.
Stern Layer
Diffuse Layer
Highly Negative
Colloid
Ions In Equilibrium
With Solution
Negative Co-Ion
Positive Counter-Ion
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
7/41
3
Double Layer Thickness
Th e diffu se layer can be visu alized as acharged a tmosph ere sur round ing the
colloid. At an y distan ce from th e su rface,
its cha rge dens ity is equa l to th e difference
in concen tra tion of positive an d n egative
ions a t th at p oint . Ch arge dens ity is great-
est n ear th e colloid a nd rapidly diminish es
towards zero as the concen trat ion of posi-
tive an d n egative ions m erge togeth er.
The at ta ched coun ter- ions in the Stern
layer an d th e charged a tm osphere in the
diffu se layer are wh at we refer to as th e
double layer.
The th icknes s of the d ouble layer depends
u pon th e concentrat ion of ions in solu t ion.
A h igher level of ion s mea n s more positive
ion s a re a vailable to neu tra lize the colloid.
The resu lt i s a th inn er doub le layer .
Decreasing the ionic concentration (bydi lu t ion, for exam ple) redu ces th e n u mb er
of positive ions an d a t h icker d oub le layer
resul t s .
The type of coun ter-ion will also influ ence
doub le layer thickness . Type refers to the
valence of th e positive coun ter-ion . For
ins tan ce, an equa l concentrat ion of alum i-
n u m (Al+3) ions will be much more effective
tha n sodium (Na+) ion s in n eu tra lizin g the
colloidal char ge and will resu lt in a th inn er
double layer.
Increas ing th e concentra t ion of ions or th eir
valence are both referred to as double layer
compression.
Variation of Ion Density in the Diffuse Layer
Increasing the level of ions in solution reduces thethickness of the diffuse layer. The shaded arearepresents the net charge density.
Distance From Colloid
IonConcentration
Diffuse Layer
Distance From Colloid
IonConcentration
Diffuse Layer
Lower Level of Ions in Solution Higher Level of Ions in Solution
Level of ions in solution
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
8/41
4
Chapter 1 The Electrokinetic Connection
Zeta Potent ial
The negative colloid and its positivelycha rged atm osph ere produce an electr ical
poten tial acros s th e diffu se layer. This is
highest at th e su rface an d drops off pro-
gressively with distance, approaching zero
at th e ou tside of th e diffu se layer. Th e
potential curve is useful because it indi-
cates the strength of the repulsive force
between colloids an d th e dis tan ce at wh ich
th ese forces come into play.
A pa rticular p oin t of interest on th e cur ve is
the p otent ial a t th e ju nct ion of the Stern
layer an d th e diffu se layer. Th is is kn ownas t he zeta potential. It is an imp ortant
featu re becaus e zeta potent ial can b e
meas u red in a fai r ly s imp le ma nn er , whi le
the su rface potent ial can not . Zeta potent ial
is an effective tool for coagulation controlbecau se cha nges in zeta p otent ial ind icate
changes in the repulsive force between
colloids.
The ra t io between zeta potent ial an d su r-
face potent ial depend s on d ouble layer
th icknes s. The low dissolved solids level
u su al ly foun d in water t reatmen t resu lts in
a r elatively large d oub le layer. In th is cas e,
zeta potential is a good approximation of
su rface potent ial . The s i tuat ion chan ges
with br ackish or sal ine waters; the h igh
level of ion s com pres ses th e dou ble layeran d the potent ial curve. Now the zeta
poten tial is on ly a fraction of th e su rface
potential .
Zeta Potential vs Surface Potential
The relationship between Zeta Potential and SurfacePotential depends on the level of ions in solution. Infresh water, the large double layer makes the zeta
potential a good approximation of the surfacepotential. This does not hold true for saline watersdue to double layer compression.
Distance From Colloid
Zeta Potential
Surface Potential
Potential
Stern Layer
Diffuse Layer
Distance From Colloid
Zeta Potential
Surface Potential
Potential
Stern Layer
Diffuse Layer
Fresh Water Saline Water
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
9/41
5
Electrostatic repulsion is always shown as apositive curve.
Balancing Opposing Forces
Th e DLVO Theory (na m ed a fter Derjagu in,
Landau, Verwery and Overbeek) is theclas sic explan ation of h ow particles inter-
act . It looks at the balan ce between two
opposin g forces - electrosta tic repu lsion
an d van der Waals at t ra ct ion - to explain
why som e colloids agglomera te an d floccu-
late while others will n ot.
Repulsion
Electrostatic repulsion becomes significant
when two par t icles app roach each other
and their electrical double layers begin to
overlap. En ergy is requ ired to overcome
this repu lsion an d force the p ar t iclestogeth er. The level of en ergy requ ired
increas es dra ma tical ly as th e par t icles are
driven closer an d closer togeth er. An
electrostat ic repu lsion curve is u sed to
ind icate the en ergy that m u st be overcome
if th e pa rticles a re to be forced togeth er.
The ma ximu m height of the cu rve is related
to the s u rface potent ial .
Attraction
Van der Waals at t ra ct ion b etween two
colloids is actu ally th e res u lt of forces
between individual molecules in eachcolloid. The effect is ad ditive; th at is, one
molecule of th e first colloid h as a van der
Waa ls a t t ract ion to each m olecule in the
second colloid. This is repea ted for each
molecule in t h e first colloid an d th e total
force is th e su m of all of th ese. An a ttra c-
tive energy curve is used to indicate the
variation in att ractive force with dista nce
between particles.
Van der Waals attraction is shown as a negativecurve.
Distance Between Colloids
RepulsiveEnergy
ElectricalRepulsion
Distance Between Colloids
AttractiveEnergy
Van der WaalsAttraction
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
10/41
6
Chapter 1 The Electrokinetic Connection
The Energy Barrier
The DLVO theory combines the van derWaals a t t ract ion curve an d th e electrostat ic
repu lsion curve to explain th e tenden cy of
colloids to either r ema in discrete or to
floccu late. Th e combined cu rve is called
th e ne t interaction e nergy . At each dis-
tan ce, the sma ller energy is su btracted
from the larger to get the net interaction
energy. The n et value is then p lot ted -
above if repulsive, below if attractive - and
the curve is formed.
The net interaction curve can shift from
att ra ct ion to repulsion an d back to at t rac-t ion with increasing dis tan ce between
pa rticles. If th ere is a repu lsive section,
then this region is cal led th e energy barrier
and it s m aximu m height indica tes how
resista n t th e system is to effective coagula-
tion.
In ord er to agglomera te, two part icles on a
collision cou rse m u st h ave su fficient kinetic
energy (du e to their speed a nd ma ss) to
jump overthis barr ier . Once the energy
barr ier is cleared, th e net intera ct ion en ergy
is all attr active. No fu rth er repu lsive area sare encoun tered and a s a resu lt the par -
ticles agglomera te. This attra ctive region is
often r eferred to a s a n energy trap since the
colloids can be considered to be t ra pped
together by the van der Waals forces.
Interaction
The net interaction curve is formed by subtracting theattraction curve from the repulsion curve.
AttractiveEnergy
RepulsiveEnergy
ElectricalRepulsion
Distance Between Colloids
Net InteractionEnergy
EnergyBarrier
Energy Trap
van der WaalsAttraction
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
11/41
7
Lowering the Energy Barrier
For really effective coagulation, the energy
barrier should be lowered or completelyremoved so th at th e net interact ion is
always at t ract ive. This can be accom-
pl ish ed by either comp ressing the d ouble
layer or redu cing th e su rface cha rge.
Com pres s t he Double Layer
Double layer compression involves adding
sal ts to the system. As the ionic concentra -
t ion increases, the dou ble layer and the
repu lsion energy cu rves a re compressed
u nt i l there is n o longer an energy barr ier .
Particle agglomeration occurs rapidly under
these condi t ions becau se th e colloids canjust about fall into the van der Waals trap
without having to surm ount an energy
barr ier .
Floccu lation b y doub le layer comp ress ion is
also called salting outth e colloid. Add ing
ma ssive amou nts of sal t is an imp ract ical
technique for water t reatmen t , but th e
underlying concept should be understood,
an d ha s app licat ion toward wastewater
flocculation in brackish waters.
Compression
Double layer compression squeezes the repulsiveenergy curve reducing its influence. Further compres-sion would completely eliminate the energy barrier.
AttractiveEnergy
van der WaalsAttraction
RepulsiveEnergy Electrical
Repulsion
Distance Between Colloids
Net InteractionEnergy
EnergyBarrier
Energy Trap
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
12/41
8
Chapter 1 The Electrokinetic Connection
Charge Reduction
Coagulant addition lowers the surface charge anddrops the repulsive energy curve. More coagulantcan be added to completely eliminate the energybarrier.
Lowe r the Surface Charge
In water t reatm ent , we lower the en ergybarr ier by adding coagulants to reduce the
su rface charge an d, consequen t ly, the zeta
potent ial. Two points a re importa nt h ere.
First , for all practical purposes, zeta poten-
tial is a direct measure of the surface charge
and we can u se zeta potent ia l measu re-
men ts to control charge neu tral izat ion.
Second, it is not necess ary to redu ce the
charge to ze ro. Ou r goal is to lower the
energy barr ier to the point wh ere the pa r-
ticle velocity from m ixin g allows th e colloidsto overwhelm it.
The energy barrier concept helps explain
why larger pa rticles will som etimes floccu-
late whi le sma l ler ones in th e sam e su spen -
sion esca pe. At iden tical velocities the
larger par t icles h ave a greater m ass an d
therefore more energy to get them over the
barr ier .
AttractiveEnergy
van der WaalsAttraction
RepulsiveEnergy
ElectricalRepulsion
Distance Between Colloids
Net InteractionEnergy
EnergyBarrier
Energy Trap
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
13/41
9
Coagulate, Then Flocculate
In wa ter clarification, th e term s coagulation
an d flocculation are sometimes used inter-
chan geably an d am bigu ous ly, bu t it is
bet ter to separate th e two in term s of
funct ion.
Coagul a t i on takes place when th e DLVO
energy barrier is effectively eliminated; this
lowerin g of th e en ergy barrier is a lso re-
ferred t o as destabilization .
Flocculat ion refers to the su ccessfu l
collisions that occur when the destabilized
part icles are d r iven toward each other by
the h ydrau l ic sh ear forces in th e rapid mix
an d floccu lation ba sins . Agglomer ates of a
few colloids th en qu ickly bridge together t o
form microflocs which in turn gather into
visible floc masses.
Real i ty is som ewhere in between. The lin e
between coagulation and flocculation is
often a somewh at blurry one. Most coagu -
lan ts can perform both fun ct ions a t once.
Th eir prim ary job is ch arge n eu tra lization
but they often ads orb onto more than one
colloid, formin g a bridge between th em a n d
helping th em to flocculate.
Coagu lat ion an d f loccu lat ion can be cau sed
by a n y of th e followin g:
dou ble layer compr ess ion
cha rge neu tral izat ion
bridging
colloid entra pm ent
Chapter 2
Four Ways to Flocculate
In t h e pages th at follow, each of these fou r
tools is discu ssed s epara tely, bu t the
solution to an y specific coagu lation -floccu -
lation problem will almost always involvethe s imul taneous u se of more than one of
these. Use these as a check lis t when
plann ing a test ing program to select an
efficient and economical coagulant system.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
14/41
10
Chapter 2 Four Ways to Flocculate
Double Layer Compression
Doub le layer comp ress ion involves th e
ad dition of large qua n tit ies of an indifferen telectrolyte (e.g., sod ium ch loride). The
indifference refers to the fact that the ion
retains i ts ident i ty an d does not ad sorb to
th e colloid. This ch an ge in ionic concen tra -
t ion compress es the dou ble layer aroun d
th e colloid an d is often called salting out.
The DLVO th eory ind icates th at this r esu lts
in a lowerin g or eliminat ion of th e repu lsive
ener gy ba rrier. It is importa n t to realize
tha t sa l t ing out jus t compresses th e
colloid's sph ere of influen ce an d does n ot
necessa r i ly redu ce its cha rge.
In general, double layer compression is not
a practical coagulation technique for water
t reatm ent bu t i t can ha ve appl icat ion in
ind u str ial wastewater t rea tmen t i f waste
st ream s with d ivalent or t r ivalent coun ter-
ions ha ppen to be avai lable.
Compression
Flocculation by double layercompression is unusual, but
has some application inindustrial wastewaters.Compare this figure to theone on page 2.
Stern Layer
Diffuse Layer
Highly NegativeColloid
Ions In EquilibriumWith Solution
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
15/41
11
Charge Neutralization
Inorganic coagulan ts (su ch a s alum ) an d
cationic polymers often work throughcharge neutralization. It is a p ractical way
to lower th e DLVO en ergy barr ier a nd form
sta ble flocs. Cha rge neu tra lization in volves
adsorption of a positively charged coagulant
on th e su rface of th e colloid. Th is ch arged
surface coating neutralizes the negative
cha rge of th e colloid, resu lting in a n ear
zero n et char ge. Neutra lization is th e key to
optimizing treatment before sedimentation,
gran u lar m edia filtra tion or air flotat ion.
Charge neutralization alone will not neces-
sar i ly produce dra ma tic macroflocs (flocstha t can be seen with th e naked eye). This
is d emons trated by cha rge neu tral izing with
cationic polyelectrolytes in th e 50 ,000 -
200,00 0 m olecular weight ran ge. Microflocs
(which a re too sm all to be seen) may form
but will not aggregate quickly into visible
flocs.
Charge neutralization is easily monitored
and controlled using zeta potential . This isimp ortant becau se overdosing can reverse
the char ge on the colloid, an d redisperse i t
as a pos itive colloid. The resu lt is a p oorly
floccu lated system . The detriment al effect
of overdoing is especially noticeable with
very low molecular weight cationic polymer s
th at are ineffective a t b ridging.
Charge Reduction
Lowering the surface chargedrops the repulsive energy
curve and allows van derWaals forces to reduce theenergy barrier. Comparethis figure with that on theopposite page and the oneon page 2.
Stern Layer
Diffuse Layer
Ions In EquilibriumWith Solution
Slightly Negative
Colloid
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
16/41
12
Chapter 2 Four Ways to Flocculate
Bridging
Bridging occurs when a coagulan t forms
threa ds or f ibers which a t tach to severalcolloids , ca ptu r ing and binding them
together . Inorganic pr imary coagulants an d
organ ic polyelectrolytes b oth h ave the
cap ab ili ty of bridgin g. High er molecular
weights m ean longer molecules and more
effective bridging.
Bridging is often u sed in conjun ction with
charge neutralization to grow fast settl ing
and / or shear res i s tan t flocs . For ins ta nce ,
alu m or a low molecular weigh t cationic
polymer is firs t add ed u nd er rap id m ixing
conditions to lower the charge and allowmicroflocs to form. Th en a slight am oun t of
h igh m olecu lar weight p olymer, often a n
an ionic, can be add ed to br idge between th e
microflocs. The fact th at th e bridging
polymer is n egatively charged is n ot s ignifi-
can t becau se th e sm al l colloids h ave al-
ready been ca ptu red as microflocs.
Colloid Entrapment
Colloid entra pm ent involves a ddin g rela-
tively large doses of coagulan ts, u su allyalum inu m or i ron s al ts which pr ecipi tate as
hydrou s metal oxides. The amou nt of
coagu lan t u sed is far in excess of the
am oun t needed to neutra lize the cha rge on
th e colloid. Some cha rge neu tra lization
ma y occur b u t m ost of the col loids are
literally swept from th e bu lk of th e water b y
becoming enm eshed in th e set t ling hydrou s
oxide floc. This mech an ism is often ca lled
sw eep floc.
Sweep Floc
Colloids become enmeshed in the growing precipitate.
Bridging
Each polymer chain attaches to many colloids.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
17/41
13
An Aid or Substitute forTraditional Coagulants
The class of coagu lan ts an d f loccu lan ts
known as polyelectrolytes (or polymers) is
becoming more an d more popu lar . A
proper dosa ge of th e right polyelectrolyte
can improve finished water qu ality while
significantly reducing sludge volume and
overall operating costs .
On a pr ice-per -poun d ba s is th ey a re mu ch
more expensive tha n inorganic coagulan ts ,su ch as alum , bu t overal l operat ing costs
can be lower becaus e of a redu ced need for
pH adjust ing chemicals and because of
lower s lud ge volu mes an d d isposa l costs .
In some cases they a re u sed to su pplement
t radi t ional coagulants whi le in others they
completely replace them.
Polyelectrolytes a re orga nic m acrom ole-
cu les. A polyelectrolyte is a polymer ; th at
is, i t is composed of many (poly) monomers
(mer) joined together. Polyelectrolytes ma y
be fabricated of one or more basic mono-
mer s (u su ally two). The degree of polymer i-
zat ion is th e nu mber of mon omers (bui lding
blocks) l ink ed togeth er to form one m ole-
cule, and can ra nge up to hun dreds of
t hous ands .
Picking the Best One
Becaus e of the n u mb er available and their
proprietary na tu re, it can be a real cha l-
lenge to select the best polyelectrolyte for a
sp ecific tas k. The followin g char acteristics
are u su al ly used to class ify them; man u fac-
tu rers will often p u bl ish some of these, bu t
n ot always with th e desired degree of deta il:
type (an ionic, non -ionic, or cat ionic)
m o le cu la r w eigh t
b a s ic m ole cu la r s t ru c t u r e
ch a r ge d en s it y su itab i lity for potable water t rea tment
Prelimin ary ben ch testing of polyelectrolytes
is a n imp ortant par t of the s elect ion proc-
ess , even when the polymer is us ed as a
flocculant aid for an inorganic coagulant.
Adding an untested polyelectrolyte without
kn owing th e opt imu m dosage, feed concen-
trat ion or mixing requ iremen ts can resu lt in
serious problems , includ ing filter clogging.
It is importan t to note that , within th e sam e
fam ily or type of polym er, th ere can be a
large differen ce in m olecu lar weight an d
cha rge den sity. For a sp ecific ap plication,
one m ember of a fam ily can h ave ju st th e
right combination of properties and greatly
outperform th e others .
Chapter 3
Selecting Polyelectrolytes
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
18/41
14
Chapter 3 Selecting Polyelectrolytes
Characterizing Polymers
Molecular Weight
The overall size of a polymer determ ines itsrelat ive u sefu lnes s for bridgin g. Size is
u su al ly meas u red as m olecular weight .
Manufacturers do not use a uniform
met hod to report molecu lar weight. For
this reas on, two similar polymers with th e
sam e pub lish ed molecular weight ma y
actu ally be qu ite differen t.
In ad dition, m olecu lar weight is on ly a
meas u re of average polymer length. Each
molecule in a d ru m of polymer is n ot the
sa me size. A wide ran ge can a nd will be
foun d in the sam e batch . This dis t r ibut ionof molecular weights is a n imp ortant prop-
er ty and can vary great ly.
Mo le c ular Ge ne ral
We ight Range De s c ript io n
10,000 ,000 or m ore Very High
1 ,0 00 ,0 00 t o 1 0,0 00 ,0 00 High
200 ,000 to 1 ,000,000 Med iu m
100 ,000 to 200 ,000 Low
50,000 to 100 ,000 Very Low
Les s th an 50 ,000 Very, Very Low
StructureTwo similar polyelectrolytes with th e sa me
composition of monomers, molecular
weight , an d ch arge chara cter is t ics can
perform different ly becau se of the way th e
mon omers are link ed together . For ex-
am ple, a produ ct with two monom ers A an d
B could have a regular alternation from A to
B or cou ld h ave grou ps of As followed by
grou ps of Bs.
Charge Density
Relative ch arge den sity is con trolled by th e
rat io of charged and uncharged monomers
u sed. The higher the residu al cha rge, the
h igher th e den sity. In genera l, th e relative
cha rge densi ty an d m olecular weight can -
not both be increas ed. As a resul t , the
ideal polyelectrolyte often involves a tradeoff
between ch arge densi ty and m olecular
weight.
Type o f Polym er
Polyelectrolytes ar e clas sified as n on-ionic,an ionic or cat ionic depending up on th e
residual cha rge on the p olymer in solu t ion.
Non-ionic polyelectrolytes are polymers
with a very low char ge dens ity. A typical
non -ionic is a polyacrylamide. Non -ionics
are used to flocculate solids through bridg-
ing.
Anionic polye lec t ro ly t es are negat ively
charged polymers and can be m anu fac tured
with a variety of cha rge den sities, from
pra ctically non -ionic to very stron gly an i-onic. Interm ediate charge densi t ies are
u su al ly the most u sefu l. Anionics are
norm ally u sed for br idging, to flocculate
solids . The acrylamide-ba sed an ionics with
very h igh molecular weights ar e very effec-
tive for th is.
Negative colloids can som etimes b e su c-
cessfully floccu lated with bridging-type lon g
cha in anionic polyelectrolytes. On e pos-
sible explan ation is th at a colloid with a n et
negat ive cha rge may actu al ly ha ve a mosa ic
of positive an d n egative regions . Areas ofpositive charge could serve as points of
at tach men t for th e n egat ive polymer.
Anionic polyelectrolytes m ay b e ca pa ble of
flocculating large particles, but a residual
haze of smaller colloids will almost always
remain. These mu st firs t ha ve their charge
neu tra lized in order to flocculate.
Cat ionic po lye lec t ro ly tes are positively
char ged polymers an d come in a wide range
of fam ilies, ch arge den sities a nd molecular
weigh ts . The var iety availab le offers great
flexibility in s olving sp ecific coagu lation a n d
floccu lat ion problems , bu t m akes select ing
the right polymer more complicated.
High molecular weight cationic polyelectro-
lytes can be th ought of as double acting
becau se th ey act in two ways: charge
neu tral izat ion an d br idging.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
19/41
15
Direct Filtration with Cationic Polymers
It is often possible to eliminate or bypass conventional
flocculation and sedimentation when raw watersupplies are low in turbidity on a year-round basis.For new plants, this can mean a significant savings incapital cost. Coagulant treated water is then feddirectly to the filters in what is known as the directfiltration process. Cationic polymers are usually veryeffective in this type of service.
In this example, polymer dosage, filter effluentturbidity and filter head loss after 6 hours of operationwere plotted together. The minimum turbidity level isproduced by a dose of 7 mg/L at a corresponding zetapotential of +10 mV. A polymer dose of 3 mg/L wasselected as a more practical optimum because it
produces almost the same turbidity at a substantialsavings in polymer and with a much lower head lossthrough the filter. The result is a target zeta potentialof -1mV.
Cationic Polymer Screening
The true cost of a polymer is not its price per pound
but the cost per million gallons of water treated. Plotsof zeta potential versus polymer dosage can be usedto determine the relative dose levels of similarpolyelectrolytes.
In this example the target zeta potential was set at-5 mV. The corresponding doses are: 3 mg/L forPolymer A, 8 mg/L for Polymer B and 21 mg/L forPolymer C. The cost per million gallons ($/MG) isestimated by converting the dosage to pounds permillion gallons and then multiplying by the price perpound.
The result is $88/MG for Polymer A, $133/MG for
Polymer B and $88/MG for Polymer C. If all otherconsiderations are equal, then Polymers A & C areboth economical choices.
2 4 6 8 10 12
0
-5
-10
-15
-20
+5
+10
+15
+20
1
3
2
4
5
0.4
0.3
0.2
0.1
0.0
Polymer Dose, mg/L
ZetaPotential,mV
HeadLossAfter6Hours,f
t.
EffluentTurbidity,N
TU
ZetaPotential
Turbidity
FilterHeadLoss
+5
+10
0
-5
-10
-15
-20
5 10 15 20 25
ZetaPo
tential,mV
30
Polymer Dose, mg/L
Polymer A $2.00 / lb
Polymer C
$3.50 / lb
Polymer B
$0.50 / lb
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
20/41
16
Chapter 3 Selecting Polyelectrolytes
Enhancing Polymer Effectiveness
Dual Polyme rSys tems
Two polymer s ca n h elp if no s ingle polymercan get the job done. Each h as a sp ecific
fu nct ion . For exam ple, a highly cha rged
cationic polymer can be added first to
n eu tra lize the cha rge on th e fine colloids,
an d form sm all microflocs. Th en a h igh
molecular weight anionic polymer can be
u sed t o mech an ically bridge th e microflocs
into large, rapidly settling flocs.
In water t reatm ent , du al polymer systems
ha ve the disadvanta ge tha t more carefu l
control is requ ired to ba lan ce the coun ter-
acting forces. Du al polymer s are morecommon in s ludge dewater ing, where
overdosing an d th e app earan ce of excess
polymer in the centrate or fi l trate is not as
impor tant .
Preconditioning
Inorgan ic coagu lan ts m ay be h elpfu l as a
coagulant aid when a polyelectrolyte alone
is not s u ccessful in d esta bilizing all th e
part icles . Pretreatment with inorganics can
also reduce th e cat ionic polymer dose an d
make it more stable, requiring less crit ical
control.
Preconditioning Polymers with AlumThe effect of preconditioning can be evaluated by
making plots of zeta potential versus polymer dosageat various levels of preconditioning chemical. In thisexample, the required dosage of cationic polymer wassubstantially reduced with 20 mg/L of alum while 10mg/l of alum was not effective.
+5
+10
0
-5
-10
-15
-20
5 15 20 25
ZetaPo
tential,mV
PolymerOnly
Polymer +20 mg/Lalum
Polymer +10 mg/Lalum
Polymer Dose, mg/L
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
21/41
17
Polymer Packaging and Feeding
Polyelectrolytes can be pu rchased in pow-
der , solu t ion an d emu lsion form. Each typehas a dvanta ges and d isadvantages . If
poss ible, feed facili t ies sh ould allow an y
type to be us ed.
Dry powderpo lymers are whit ish granular
powders , flakes or beads. Their tendency to
abs orb moisture from th e ai r and to s t ick to
feed screws, containers an d dru ms is a
ma jor nu isan ce. Dry polymers are also
difficult to wet and dissolve rather slowly.
Fifteen m inu tes to 1 hou r ma y be required.
Solut ion po lymers are often preferred todry powders becau se they are more conven-
ient. A little mixin g is us u ally su fficien t to
dilu te l iqu id polymers to feed str ength .
Active ingredients can vary from a few
percent to 50 percent .
Emuls ion polymers are a m ore recent
developm ent. They allow very high m olecu -
lar weight polymers to b e pu rcha sed in
convenient l iqu id form. Dilu tion with water
u nd er agi tat ion frees the gel par t icles in the
emu lsion a llowin g them to dissolve in t h e
water . Sometimes act ivators are requiredfor prepar at ion. Em u lsions are usu al ly
packa ged a s 2 0-30% a ct ive ingredients .
Pr epar ed ba t ches of polymer are n orma lly
u sed within 24 -48 h ours to prevent loss of
activity.
In addition, polymers are almost always
more effective when fed a s d ilu te s olu tion s
becau se they are easier to disperse an d
u niformly distribu te. Using a higher feed
s t rength may m ean th a t a h igher polymer
dose will be requ ired.
Typically, maximum feed strength is be-
tween 0 .01 to 0.05%, but ch eck with th e
polymer manufacturer for specific recom-
mendat ions .
Stock polymer solu t ions a re us u al ly made
u p to 0.1 to 0.5% as a good compromisebetween storage volume, batch life, and
viscosity. Dilu ting th e stock solution by
abou t 10:1 with water will usu al ly drop the
concentrat ion to th e recomm ended feed
level. The polymer an d dilu tion water
sh ould be blended in- l ine with a s tat ic
mixer or an edu ctor .
Using Cationic Polymers in Brackish Waters
In brackish waters, the correct cationic polymer doseis often concealed by the effect of double layercompression, which drops the zeta potential but not
the surface potential.
Diluting the treated sample with distilled water willgive an indication of whether enough polymer hasbeen added. If the surface potential is still high, thendilution will cause the zeta potential to increase andmore cationic is required.
Distance From Colloid
Zeta PotentialAfter Dilution
Surface Potential
Potential
Stern Layer
Diffuse Layer
Zeta PotentialBefore Dilution
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
22/41
18
Chapter 3 Selecting Polyelectrolytes
Interferences
Cationic polymers can react with negative
ions in solution, forming chemical bondswhich imp air their performan ce. Greater
doses are then required to ach ieve the sam e
degree of cha rge neu tra lization or bridging.
This is more noticeable in wastewater
t rea tment .
Examp les of interfer ing su bsta nces include
su lfides, h ydrosulfides a nd ph enolics , bu t
even chlorides ca n redu ce polymer effective-
nes s .
Even p H sh ould be considered, s ince
pa rticular p olymer families m ay performwell in s ome pH ran ges an d n ot others .
Phenol Interference
The effect of phenols on this cationic polymer wasevaluated by plotting zeta potential curves.
pH Effects
Zeta potential curves can be used to evaluate the
sensitivity of a cationic polymer to changes in pH. Forthis particular polymer the effect is quite large. It canbe much less for other products.
+5
+10
0
-5
-10
-15
ZetaPotential,mV
1 2 3 4 5 6
Polymer Dose, mg/L
pH 8
pH 10
+5
+10
0
-5
-10
-15
-20
ZetaPotential,mV
2 4 6 8 10 12
30 mg/LPhenol
NoPhenol
Polymer Dose, mg/L
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
23/41
19
Time Tested Coagulants
Alum inu m and fe r r ic compounds a re the
tradi t ional coagu lan ts for water a nd waste-
water t reatment . Both are from a fam ily
cal led m etal coagulan ts , an d both are s t ill
widely u sed today. In fact , man y plants u se
one of th ese exclu sively, an d h ave no
provision for polyelectrolyte addition.
Metal coagu lant s offer th e ad vant age of low
cost per pou nd . In a ddi t ion, select ion of
the opt imu m coagulan t is s imp le, s inceonly a few choices a re availab le. A distinct
disadvanta ge is the large s ludge volu me
produ ced, s ince s lud ge dewater ing an d
disposa l can be difficult an d expen sive.
Alu minu m a nd ferr ic coagu lants a re solu ble
sal ts . They are added in solut ion form an d
react with alkalinity in th e water t o form
ins olu ble hydrous oxides th at coagulate by
sweep floc and charge neutralization.
Metal coagu lan ts always r equire at ten t ion
to pH condi t ions an d cons iderat ion of the
alkalinity level in the raw and treated water.
Reason ab le dosa ge levels will frequ ent ly
resu lt in n ear opt imu m pH cond it ions. At
other t imes, ch emicals su ch as lime, soda
ash or sodiu m bicarbonate mu s t be added
to sup plement n atu ral alkal ini ty.
Chapter 4
Aluminum Sulfate (Alum)
Alum is one of th e mos t widely u sed coa gu-
lan ts an d will be us ed as a n exam ple of the
react ions th at occur with a metal coagu -
lan t . Ferr ic coagulan ts react in a general ly
s imila r m ann er , but th e ir opt imu m pH
ranges are different.
When alum inu m su lfate is add ed to water ,
hydrou s oxides of alum inu m a re formed.
The s imp lest of these is alum inu m h ydrox-
ide (Al(OH)3) which is a n insolub le precipi-tat e. Bu t severa l, more comp lex, positively
cha rged solu ble ions are a lso formed,
including:
Al6(OH)
15
+3
Al7(OH)
17
+4
Al8(OH)
20
+4
The proport ion of each will vary, depen ding
up on both the a lum d ose and the pH afte r
alum add it ion. To fu r ther complicate
mat te r s , u nder cer ta in condi t ions the
su lfat e ion (SO4-2
) ma y also become p ar t of the h ydrous a lum inu m complex by subs t i-
tu ting for som e of th e h ydroxide (OH-1 ) ions .
This will ten d to lower th e ch arge of the
h ydroxide com plex.
Using Alum and Ferric Coagulants
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
24/41
20
Chapter 4 Using Alum and Ferric Coagulants
How Alum Works
The m echan ism of coagu lat ion by alumincludes b oth cha rge neu tral izat ion an d
sweep floc. One or th e other m ay predomi-
na te, but each is always act ing to some
degree. It is probab le that ch arge neutra li-
zation takes place immediately after addi-
tion of alu m to water. The com plex, posi-
t ively charged h ydroxides of alum inu m tha t
rap idly form will adsorb to th e su rface of
the negative turbidity particles, neutralizing
th eir ch arge (an d zeta p otent ial) an d effec-
tively lowerin g or r em ovin g th e DLVO
energy barrier.
Simultaneously, aluminum hydroxide
precipitates will form. These add itiona l
particles enhance the rate of flocculation by
increasing the chances of a collision occur-
ring. Th e precipitate also grows indep end -
ently of the colloid population, enmeshing
colloids in th e sweep floc m ode.
The type of coagulation which predominates
is dependent on both th e a lum dose and
the pH after alu m ad di t ion. In general ,
sweep coagulat ion is th ought to predomi-
na te at alum d oses above 30 m g/ L; belowtha t , the dominan t form depends u pon both
dose and pH.
Alkalin ity is requ ired for th e alu m reaction
to su ccessfully proceed. Oth erwise, the pH
will be lowered to th e point wh ere solu ble
aluminum ion (Al+3) is formed instea d of
alum inu m hydroxide. Dissolved alum inu m
ion is an ineffective coagulan t an d can
cause dirty water problems in the distri-
bu t ion system. The react ion between alum
an d a lkalin ity is sh own b y the following:
600 300
Alum Alkalinity
Al2(SO
4)
3+ 3Ca(HCO
3)
2+ 6H
2O
Aluminum Carbonic
Hydroxide Acid
3CaSO4
+ 2Al(OH)3
+ 6H2CO
3
Estimating Alkalinity Requirem ent s
This equ at ion helps u s d evelop severals imple rules of thu mb abou t the relat ion
betweem a lum an d alkal ini ty.
Commercial alum is a crystalline material ,
with 14.2 water molecules (on average)
bound to each aluminum sulfate molecule.
The molecular weight of Al2(SO
4)
314 .2H
2O
is 600 .
Alkal ini ty is a meas u re of the a mou nt of
bicarbonate (HCO3
-1 ), car bon ate (CO3
-2 ) an d
hydroxide (OH -1 ) ion . Th e reaction sh ows
alkalinity in i ts bicarb ona te (HCO3 -) formwhich is typical at pH's b elow 8, bu t alka-
lin ity is always express ed in term s of th e
equivalent weight of calcium carbonate
(CaCO3), which has a molecular weight of
100 . Th e th ree Ca(HCO3)
2molecules th en
ha ve an equ ivalent m olecu lar weight of 3 x
100 or 300 as CaCO3.
The ru les of thu mb a re based on th e rat io
of 600 (alum) to 300 (alkalinity):
1 .0 mg/ L of comm ercia l a lum will
consu me ab out 0.5 m g/ L of alkal ini ty.
The r e s hou l d be a t le a s t 5 -10 m g / L o f
alkalinity rem aining after th e reaction
occurs to keep the pH near opt imu m.
Raw water a lka l in i ty shou ld be equal to
ha lf the expected alum dose plus 5 to 10m g/ L.
1.0 m g/ L of alkal ini ty expressed as CaCO3
is equivalent to:
0 .66 m g / L 85% qu ick lim e (CaO)
0 .78 mg/ L 95% hydra ted lime (Ca(OH)3)
0 .80 m g / L caus t ic s oda (NaOH)
1 .0 8 m g/ L s od a a s h (Na 2CO 3) 1 .52 m g/ L s od ium b ica r bona t e (NaHCO3)
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
25/41
21
Whe n t o Add Alkalinity
If na tu ral alkalinity is ins u fficient th en a ddart ificial alkalinity to ma intain th e des ired
level. Hydrat ed lime, cau stic soda , soda
ash or sodiu m bicarbonate m ay be used to
raise the alkalinity level.
Alkal ini ty shou ld a lways be add ed u p-
st ream , before alu m a ddi t ion, an d the
chem ical sh ould be com pletely diss olved by
the t ime the alum react ion tak es place.
Alu m rea cts instan tan eously and will
proceed to other end products if sufficient
alkalinity is not imm ediately availab le. Th is
requ iremen t is often ignored in a n effort tominimize tank s an d mixers , bu t poor
performan ce is th e pr ice tha t is paid.
When alum reacts with na tu ral alkalini ty,
the p H is decreased b y two different m ean s:
the bicarbonate alkalinity of the system is
lowered an d th e carbonic acid conten t is
increas ed. This is often an advan tage,
s ince opt imu m pH condi t ions for alum
coagulation are generally in the range of
abou t 5.0 to 7.0, whi le the pH ra nge of
mos t n a tura l waters is f rom abou t 6 .0 to
7.8.
At t imes, som e of the a lu m dose is actu al ly
being u sed solely to lower the p H to its
opt imu m valu e. In other words, a lower
alum dose would coagu late as effectively if
th e pH were lowered some other way. At
larger plants i t may be more economical to
add su lfu r ic acid ins tead.
It i s imp ortant to note tha t n ot al l sour ces
of artificial alkalinity have the same effect
on pH. Some produce carbonic acid when
they react an d lower the pH. Others do not .
Opt imum pH condi t ions should be taken
into a ccou nt when select ing a n alkal ini ty
source.
Zeta Potential Control of Alum Dose
There is no single zeta potential that will guarantee
good coagulation for every treatment plant. It willusually be between 0 and -10 mV but the target valueis best set by test, using pilot plant or actual operatingexperience.
Once the target ZP is established, then thesecorrelations are no longer necessary, except forinfrequent checks on a weekly, monthly, or seasonalbasis. Control merely involves taking a sample fromthe rapid mix basin and measuring the zeta potential.If the measured value is more negative than the targetZP, then increase the coagulant dose. If it is morepositive, then decrease it.
In this example a zeta potential of -3 mV correspondsto the lowest filtered water turbidity and would beused as the target ZP.
0
+5
-5
-10
-15
-20
-2510 20 30 40 50 60
0.0
0.1
0.2
0.3
0.4
0.5
Zeta-Potential
TurbidityZetaPo
tential,mV
Alum Dose, mg/L
TurbidityofFin
ishedWater,NTU
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
26/41
22
Chapter 4 Using Alum and Ferric Coagulants
pH Effects
Charge
There is a s t rong relat ion between pH an dperforman ce, bu t th ere is n o s ingle opt i-
mu m pH for a specific water . Rather , there
is a n interrelat ion between pH a nd the type
of aluminum hydroxide formed. This in
turn de termines the charge on the hydrous
oxide complex. Oth er ions com e in to play
as well . The effect of pH on ch arge can be
evaluated u sing a zeta potent ial curve and
direct m easu remen t of zeta p otent ial is a
bet ter meth od of process control tha n p H.
Solubility of Aluminum
A secon d imp ortan t as pect of pH is i ts effecton s olu bili ty of th e alu m inu m (Al+3 ) ion .
Ins u fficient a lkalinity allows th e pH to dr op
to a point where th e alu minu m ion becomes
highly solub le. Dissolved alu min u m can
then pas s r ight th rough th e filters . After
fil t rat ion, p H is u su al ly adjus ted u pward for
corrosion con trol. The h igh er pH converts
dissolved alum inu m ion to ins olu ble alu mi-
nu m hydroxide which th en f loccu lates in
the d i s t r ibu t ion sys tem an d a lmos t guaran -
tees dirty water complaints .
Effect of pH on Alum FlocThe charge on alum floc is strongly effected by pH.This example shows the effect of pH on zeta potentialand residual color for a highly colored water. Thealum dose was held constant at 60 mg/L.
OverdosingChanges in zeta potential are good indicators ofoverdosing. This plant should operate at a zetapotential of about -5 mV. Overdosing produced azeta potential of +5 mV and is accompanied by amarked increase in residual aluminum in the finishedwater. Monitoring of turbidity is not effective forcontrol because the lag time between coagulantaddition and filtration is several hours.
-5
+5
-10
-15
0
20
40
ZetaPotential
ZetaPotential,mV
pH
ResidualColor
0
+10
60
80
54 6
+15
Color
0
+10
-5
-10
-15
0.0
0.2
0.4
ZetaPotential
Turbidityof Filtered
Water
ZetaPotential,mV
Alum Dose, mg/L
Turbidity,NTU
+5
+15
0
0.1
0.2
0.3
0.4
0.6
0.8
20 40 60 80 100 120
ResidualAluminum
ResidualAluminum
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
27/41
23
Cationic Coagulant Aid
Zeta potential curves can be used to evaluate the
charge neutralizing properties of cationic polymers.
Coagulant Aids
Tailoring Floc Charact eristic s
Polyelectrolytes which enh an ce th e floccu -lat ing act ion of metal coagulants (su ch a s
alum ), are ca lled coagulant aids or floccu-
lan t aids .
They provide a means of tailoring floc size,
set t l ing character is t ics and shear s t rength.
Coagulan t a ids ar e excellen t tools for
dealing with seasonal problem periods
when alu m a lon e is ineffective. They can
also be us ed to increas e plant capa city
without increasing physical plant size.
Ac t iva ted s i l i ca was u sed for th is pu rposebefore organic polymers became popular.
Activated silica produ ces large, den se, fast
set t ling alum flocs an d s imu ltan eously
tou ghen s it to a llow higher ra tes of fi ltra tion
or longer fi lter ru ns . A disad vanta ge is the
fairly precise control that is needed during
prep ara tion of th e activated s ilica. Silica is
prepared (activated) on-site by p art ially
neutralizing a sodium silicate solution, then
aging and diluting it .
Polyelectrolyte coagulan t aids ha ve the
advantages of activated sil ica and aresimpler to prepa re.
Cat i on ic p o l ym er s are ch arge neu tral izing
coagu lan t aids . They can redu ce the alum
or ferric dose while simultaneously increas-
ing floc size an d tou ghen ing it . Th ey also
redu ce the effect of su bsta nces th at inter-
fere with meta l coagulan ts. Long cha in,
high m olecu lar weight ca tion ics oper ate via
mecha nical br idging in add it ion to cha rge
neu tral izat ion. Cha rge neu tral izing can be
monitored using zeta potential , while the
bridging effect should be evaluated using
jar tes ting.
0
+5
-5
-10
-15
-20
-25
ZetaPo
tential,mV
Alum
Alum +CationicPolymer
10 20 30 40 50 60
Dose, mg/L
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
28/41
24
Chapter 4 Using Alum and Ferric Coagulants
Wastewater Coagulation with Ferric Chloride
These curves are for coagulation of a municipaltrickling filter effluent. In this case, the optimum zetapotential is -11mv. Overdosing by only 5 mg/Lcauses a significant deterioration in performance, andis accompanied by a large change in zeta potential.Overdosing by 10 mg/L results in almost completedeterioration.
Non-Ionic Polymer Increases Settling Capacity
Jar tests were used to evaluate the effect of a very
small amount of non-ionic polymer on settling rates.Sedimentation velocities were converted to equivalentsettling basin overflow rates to illustrate the dramaticeffect of the polymer. With no polymer the turbiditywas 8 NTU at an overflow rate of 720 gallons per dayper square foot. After the polymer was added, theflow rate could be tripled with the settled waterturbidity actually improving at the same time.
10 20 30 40 50 60
0
-5
-10
-15
-20
+5
+10
+15
+20
5
15
10
20
100
60
40
20
Fe (III) Dose, mg/L
ZetaPotential,mV
ResidualTurbidity,NTU
O
rganicCarbonasCOD,mg/L
+25
-25
COD
ZetaPotential
Turbidity
0
80
25
0
Anionic and non-ionic polym ers a re
popular becau se a sm al l amou nt canincr eas e floc size severa l times . An ionic
an d n on-ionic polyelectrolytes work b est
because their very high molecular weight
promotes growth through mechan ica l
br idgin g. It is imp orta n t to allow m icroflocs
to form before add ing th ese polymer s. In
addi t ion, excess amounts can actual ly
inh ibit floccu lation . J ar testing is the best
way to establ ish the opt imu m d osage.
0
Turbidity
ofSettledWater,NTU
Overflow Rate, gal per day / sq. ft.
2
4
6
8
10
12
14
16
18
20
720 1440 2160 2880 3600
30 mg/LAlum
30 mg/LAlum +0.1 mg/LNon - IonicPolymer
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
29/41
25
Tools for Dosage Control
Chapter 5
Jar Test
An Unde rvalued Tool
We u su al ly ass ociate th e jar tes t wi th a
tedious dosage control tool tha t seems to
take an agonizingly long time to set up, run
an d then clean u p after . As a resul t the
tru e versa tili ty of th e hu mb le jar test is
often overlooked.
The following applications for jar testing
may be m ore impor tant th an it s u se in
rout ine dosage control. It is s tu dies su chas th ese where the jar tes t h as i ts major
value, s ince the resu lts ar e worth the t ime
an d effort to do th e test p roperly:
se lec tion of pr imary coagulant
com par is on o f coagu l an t a id s
opt imizing feed poin t for pH adjus t ing
chemicals
mixing energy and mixing t ime s tu dies
(rap id mix an d flocculation)
opt imizing feed point for coagulant aids
eva lua t ing d ilu t ion requi rements for
coagulants est imat ing set t ling velocit ies for sedimen -
tat ion bas in s izing
s tu dying e ffect of rap id changes in
mixin g energy
evalua ting effect of slu dge recycling
Jar Testing
A tedious but valuable tool which allows side by sidecomparison of many operating variables.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
30/41
26
Chapter 5 Tools for Dosage Control
The Gator Jar
The original design was constructed of 1/4 inch thickplexiglas. It measures 11.5 cm square (inside) by anoverall depth of 21 cm. The sample tap is located 10cm below the water surface. Liquid volume is2000mL. The tap consists of a piece of soft tubingand a squeeze clamp, or a small plastic spigot.
Jar Test Apparatus
Many comm ercial laboratory jar tes t u ni tsare in u se today. The Phipps & Bird s ix
place gan g stirrer is very tra ditiona l. It ha s
1" x 3" flat blade pad dles a nd is u su al ly
u sed with 1.5 l iter glass beakers . A disad-
vantage is the large vortex that forms at
high s t i r r ing sp eeds.
Some provision is usually made for drawing
off a s ettled sa mp le from below th e water
su rface. Siphon type draw-offs are fre-
quen t ly used for glass beakers .
C a m p improved th e Phipps & Bird designby using 2-li ter glass beakers outfit ted with
vortex breaking s tators . These were espe-
cially suitable for mixing studies because
Cam p developed cu rves for mixin g in ten sity
(G) versu s s t i r rer speed u sing this d esign.
Gat or o r W agner j a r s are a relatively
recent imp rovement a nd are squ are plexi-
glas jars with a l iquid volu me of 2 l iters.
These a re n ow comm ercially availab le, an d
have several advantages over traditional
glas s beak ers. First , they are less fragile
and allow for easy insertion of a sample tap,thu s avoiding the need for a s iph on type
draw-off. Their squ are sha pe helps dam pen
rotat ion al velocity with out th e n eed for
stators while their plexiglas walls offer
mu ch grea ter therma l insu la t ion tha n
glass , thus minimizing temperature change
du ring test ing.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
31/41
27
Settl ing Studies
The d ata from a well thought out jar tes tcan be u sed to realistically size settl ing
bas ins a nd to evalua te th e abi lity of coagu -
lan t aids to increas e the h ydrau lic capaci ty
of existing ba sins . Su rface overflow rate
(gallons per da y per squ are foot) is m ore
important than retent ion t ime in determin-
ing the h ydrau lic capa city of a s ettl ing
ba sin, a n d is calculated from th e velocity of
th e slowest s ettl ing floc tha t we wan t to
rem ove. In pra ctical term s, a settl in g ra te
of 1 cm / minu te is equ al to an overflow rate
of 360 gallons per day per square foot.
J ar test set t l ing data is obtained by drawing
off sa mp les a t a fixed dista n ce below the
water su rface, at var iou s t ime intervals
after the en d of rap id mixing an d floccula-
tion. Sett lin g velocity an d overflow ra te are
calcu lated for each t ime interval and the
sam ple reflects th e qu al ity tha t we can
expect at th at overfow rate.
The Gator jar ha s a convenient s am ple
dra w-off located 10 cm b elow th e water
su rface. Sam ples are u su al ly withd rawn at
intervals of 1, 2, 5, an d 10 minu tes af terthe mixers are s topped. These correspon d
to settl in g velocities of 10, 5 , 2 a n d 1 cm/
minu te which ar e equivalent to set t l ing
bas in overflow rates of 3600 , 1800 , 720 an d
360 gallons per da y per squ are foot .
Th e resu lts of th e overflow rate s tu dies can
be plotted on r egular (arith met ic) graph
paper, or on semi-logarithmic paper (with
tu rbidity on th e log scale) or on logar ithm ic
graph pap er . Actua l plant operat ing resu lts
can a lso be plot ted on the sa me graph s for
comparison.
See page 24, Non-Ionic Polymer Increases
Settling Capacity, for an exam ple of a
set t ling s tu dy.
G Curves for the Gator JarThese are for the Gator Jar, with a 1x3 inch Phippsand Bird stirrer paddle. See Chapter 6, Tips onMixingfor a discussion of the G value and its impor-tance as a rational measure of mixing intensly.
Predic ting Filtered Wate r Quality
For water t reatmen t , it i s imp ortant toremem ber tha t our u lt ima te goal is to
produ ce the best fi ltered water. We often
ass u me tha t the best set t led water will
always correspond to the best fi l tered
water , but tha t is not necessa r i ly so. In
fact , su bsta nt ial savings m ay be rea lized by
optimizing filtered water qu ality instea d.
Bench sca le test ing can be u sed in conjun c-
tion with jar tests to pred ict fil tered water
qu ality. Wh atm an # 40 filter paper (or
equ ivalent) provides a good simu lation . Use
fresh fil ter paper for each jar and discardthe first portion filtered.
5 10 20 50 100 200 5001
2
5
10
20
50
100
200
500
1000
Impeller Speed, RPM
G-value,
(sec-1
)
23 Co 3 Co
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
32/41
28
Chapter 5 Tools for Dosage Control
Jar Test Chec klist
Rou tine jar tes t procedu res sh ould betailored to closely match actual conditions
at the plant . This may take some experi-
men tat ion since full sca le mixing cond itions
are only app roxima ted in th e jar tes t a nd
treatm ent is on a b atch ins tead of a flow-
through bas i s .
Condi t ions to ma tch u p include:
sequence of chemica l addit ion
rapid mixing in tens i ty and t ime floccu lat ion intensi ty an d t ime
Visua lly evalu ated variables includ e: time for first floc forma tion
floc s ize
floc qua l ity s e t tling r a t e
Imm ediately after r ap id m ixing th e followin g
can be de te rmined:
ze ta pot en t ia l pH
Later , after slow m ixing an d s ettl ing,
sam ples can be carefu lly drawn off an d
an alyzed for:
t u r bid it y
c olor
filt e rability n um ber
pa r t ic le coun t
r e s idua l coagu l an t
filtered water turbidity
Limitations of Visual Evaluation
Routine jar testing is time consuming, and plantoperators often fall back on visual evaluation as abasis of comparison. Unfortunately, visual judgmentscan be surprisingly misleading. In the exampleabove, the visual evaluation (Poor, Fair, etc.) was ofabsolutely no help in selecting an economical alumdose.
0
+10
-5
-10
-15
-206 8 10 12 14 16
0.0
0.1
0.2
0.3
0.4
ZetaPotential
Turbidityof Filtered
Water
ZetaPotential,mV
Alum Dose, mg/L
Turbidity,NTU
+5
+15
+20
Poor
PoorFair
Good
Best
Fair
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
33/41
29
Zeta Potential
Easily Understood
Zeta poten tial is a n excellent tool for coagu -lan t dosage control , and we are proud to
ha ve pioneered the u se of our inst ru men t in
water t reatm ent over 25 years ago. Opera-
tion of a Zeta-Meter is relatively simp le an d
only a s hort t ime is requ ired for each test .
The resu lts a re object ive an d repeata ble
and, most important ly, the techniques can
be eas ily learn ed.
Th e principle of opera tion is ea sy to u n der-
sta nd . A high qua lity stereoscop ic micro-
scope is u sed to comfortably observe tu r-
bidity par t icles inside a cha mb er cal led a nelectroph oresis cell . Electrodes placed in
each end of th e cell create an electric field
acros s i t . If th e tur bidity pa rticles ha ve a
cha rge, then th ey move in th e field with a
speed and direction which is easily related
to their zeta potential .
Zeta-Meter System 3.0
A standard parallel printercan be directly connectedfor a "hard copy" of your
test run.
Our Zeta-Meter 3.0 - Simple & Reliable
The Zeta-Meter 3.0 is a microprocessor-bas ed version of our popu lar inst ru men t .
The s am ple is poured into the cell, then the
electrodes a re ins er ted an d conn ected to
the Zeta-Meter 3.0 u ni t . Firs t , the ins t ru -
men t determines specific cond u ctan ce and
helps select the appropriate voltage to
app ly. Then , when th e electrodes are
energized the particles begin to move and
are t racked u sing a gr id in th e microscope
eyepiece.
Tracking simply involves pressing a button
an d h olding it down while a colloid trav-erses the gr id. When th e t rack but ton is
released th e ins t ru men t ins tan t ly calculates
an d displays th e zeta potential . A single
tracking takes a few second s, an d a com-
plete run takes only minutes .
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
34/41
30
Chapter 5 Tools for Dosage Control
A Practical De sign
The Zeta-Meter 3.0 is d esign ed to be m is-tak e-proof. It recogn izes imp ra ctical resu lts
an d t racking t imes tha t are too sh ort . A
clear butt on a llows th ese or oth er incon-
sis tent resu lts to b e deleted without losing
the rest of your d ata.
Stat is t ics are a lso ma inta ined b y the Zeta-
Meter 3.0, an d can be reviewed a t an y t ime.
Press ing a s ta tu s but ton causes the u ni t
to display th e total nu mb er of colloids
tracked, th eir average zeta poten t ial an d
stan dard deviat ion, a s tat is t ical measu re of
the s pread of the individu al data valu es.
A pr inter can be conn ected to the Zeta-
Meter 3.0 un it to record the ent i re ru n. The
outpu t sh ows the value obtained for each
colloid as well as a statistical summary of
the en t ire test .
Simplif ied Coagulant Dose Cont rol
The zeta potent ial tha t corresponds to
optimu m coa gulation will vary from p lant t o
plan t. The optim al valu e is often ca lled the
target zeta potent ial and is b est estab lish ed
first by correlation with jar tests or pilotun it s , an d th en with a c tua l p lan t per form-
ance .
Once a target value is set, routine control is
relatively simp le an d m erely in volves m eas -
uring the zeta potential of a sample from
th e flash m ix. If th e mea su red valu e is
more n egat ive tha n th e target value, just
increas e the primar y coagu lant dose. If i t is
more positive, then lower the dose.
Other Applications of Zeta Potential
You can also use zeta potential analysis forth e followin g u seful ap plications :
s c r een ing o f p r im ar y coagu lan t s
cost compa rison of cat ionic polyelectro-
lytes
op t im u m p H d et er m in a t ion s
checking qu al ity of delivered cat ionic
polyelectrolyte
opt imizing feed poin t for pH adjus t ing
chemicals
eva lu a t ing d ilu t ion/ mixing requirements
for ca tionic p olyelectrolytes
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
35/41
31
Streaming Current
On-Line Zet a-Pote nt ial . . . . Alm os t
A limitation on the zeta potential techniqueis th at i t is n ot a cont inu ous on-line m eas-
ur ing ins t ru ment . The s t reaming cur rent
detector was developed in resp onse to th is
need. Its ma in advanta ge is rapid detect ion
of plant u psets .
Stream ing cu rrent is real ly nothing more
than an other way to measure zeta poten-
tial , bu t i t is a ctu ally related to the zeta
potent ial of a s olid su rface, su ch a s th e
walls of a cylindrical tu be, an d n ot th e zeta
poten tial of th e tu rbidity or floc pa rticles.
Forcing a flow of water through a tube
ind u ces an electr ic cu rrent (cal led th e
stream ing current) an d voltage differen ce
(called the stream ing p otential) between the
end s of th e tub e. Some of th e particles in
th e liqu id will loosely adh ere to th e walls of
th e cylinder a n d will affect th e zeta poten -
tial of th e wall . As a resu lt th e stream ing
curren t or s t r eaming potent ial of the wa ll
will reflect to s ome d egree the zeta poten tial
of the p ar t icles in su spen sion.
Commercial InstrumentsMost commercial s t reaming cu rrent devices
u se a n oscillating flow to eliminat e ba ck-
groun d electrical signals. A piston in a
cylind er (called a boot) is very comm on.
The p is ton osci llates u p an d down a t a
relatively low frequency (about 4 cycles per
second) causing the sample to flow in an
al ternat ing fashion through the space
between th e piston an d cylin der. The flow
of water creates an AC stream ing cu rrent ,
du e to the zeta p otent ial of the cylind er an d
piston su rface. It is this curren t which is
meas u red an d am plified by the detector .
A Use ful Mon itor
Stream ing curren t is u sefu l as a n on-l inem on itor of zeta poten tial. However, it is
only an indicat ion becau se th e value is not
scaled. Tha t is , a cha nge in 10 s t ream ing
curren t u ni ts does not correspond direct ly
to a zeta poten tial cha nge of 10 m V. In
ad dition, t he zero position is often sh ifted
significan tly from t ru e zero and is not
s table.
St reaming cur rent an d s t reaming potent ia l
are both excellent ways to keep track of on-
line condi t ions, b u t sh ould be cal ibrated
with a zeta meter . A cha nge in th e s t ream-ing cu rrent tel ls you th at i t is t ime to tak e a
sam ple an d m easu re the ze ta p otent ia l.
Oscillating Piston Streaming Current DetectorThe AC signal is electrically amplified and conditionedto produce a signal that is proportional to the zetapotential.
SampleOut
SampleIn
Electrode
Piston
Boot
Electrode
AC
StreamingCurrent
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
36/41
32
Chapter 5 Tools for Dosage Control
Turbidity and Particle Count
Finish ed water tu rbidity is often u sed to
gauge the effectiveness of a water treatmentplant , bu t it is importan t to remem ber tha t
there is n o direct relat ion between the
amou nt of su spended ma t te r , o r num ber of
par t icles , an d th e tu rbidity of a s am ple.
Most turb id ity measu rements a re ba sed
u pon n ephelometry. A light beam is pass ed
through the water sample and the par t icles
in th e water scat ter th e light . The tu rbidi-
meter mea su res the intens ity of scat tered
light, usually at an angle of 90 to th e light
beam an d th e inten si ty is expressed in
s tan dard tu rb id ity uni t s .
The an gle of peak s cat ter and the am oun t
of l ight scattered are both influenced by thesize of th e pa rticles. Oth er factors , su ch as
the n a tu re and concent ra t ion of the par -
ticles will also effect th e tu rbidity mea su re-
m en t .
Particle size analyzers quantify the actual
par t icle coun t an d th e dis t r ibu t ion with
size. Par t icle coun ters are mu ch more
expensive than turbidimeters and are not
common ly foun d in water t reatm ent plants .
Particle Count vs. Turbidity
Simultaneous plots of zeta potential and turbidity areoften used to determine the target zeta potential andalum dose. The curve can shift if particle counts areused to judge performance instead. In this example,the target zeta potential was -5 millivolts usingturbidity as a criteria. This results in an alum dose of
45 mg/L. When particle count was used, the targetzeta potential is 0 millivolts, and the correspondingalum dose is 70 mg/L. Surprisingly, the particle countis almost triple at the lower dose determined by the
turbidity standard.
0
+10
-5
-10
0.0
0.2
0.4
ZetaPotential
Turbidityof Filtered
Water
ZetaPotential,mV
Alum Dose, mg/L
Tu
rbidity,
NTU
+5
+15
0.6
0.8
20 40 60 80 100 120
1.0+20
0
+10
-5
-10
0
20
40
ZetaPotential
ZetaPotential,mV
Alum Dose, mg/L
RelativeParticleCount,percent
+5
+15
60
80
20 40 60 80 100 120
100
ParticleCount
+20
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
37/41
33
Basics
Mixing pa t terns are often complex an d a re
difficult to d escribe, s o we u su ally classify
mixers b ased u pon h ow closely they ap-
proximate one of two idealized flow pat-
tern s: comp lete mixing or plug flow.
Ideal plug f low mean s th a t each volum e of
water rema ins in th e reactor for exact ly the
sa me am oun t of t ime. If a slu g of dye were
injected into the flow as a tracer, then all
the dye would appear a t the out let at th esa m e time. Ideal plu g flow is very difficult
to ach ieve. In genera l, th e reaction bas in
mu st b e very long in comparison to i ts
width or diameter before the mixing pattern
begin s to a ppr oxima te plu g flow.
Com pl e t e m i x i ng bas ins are always in-
s tan ta neous ly b lended th roughout th e ir
ent i re volu me. As a resu lt , an incoming
volum e of water imm ediately loses i ts
ident i ty and is interm ixed with the water
th at entered previou sly. A complete-mix
reactor is also called a back-mix reactor
becau se its contents are always blended
ba ckwar ds with th e incom ing flow. If a slu g
of dye tracer was injected into the basin,
then some of i t would immediately appear
in the out goin g flow. The con centr ation of
dye would drop s teadi ly as th e dye in the
bas in backm ixed with the clear incoming
water an d was di lu ted by it .
Chapter 6
Tips on Mixing
Retention Time Curves
When several complete mix reactors are placed in aseries then the chance of the same particle quicklyexiting each basin is very small. The overall retentiontime pattern then changes and becomes moreregularly distributed around the average.
0 1.0 1.5 2.0 2.5 3.00.5
20
40
60
80
100
RelativeConcentration
Fraction of Average Retention Time
One Complete Mix Basin
Two
Three
Four
A PracticalApproximationof Plug Flow
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
38/41
34
Chapter 6 Tips on Mixing
Rapid Mixing
Plug Flow versus Complete Mix
Recomm enda t ions for th e best type of rapidmixing are confus ing at best . Th e ex-
trem ely fas t t imes for th e reaction of alum
with alkalinity (less th an 1 secon d) an d for
the ra pid format ion of the alu minu m h y-
droxide microfloc (less than 10 seconds)
ha ve produced a dvocates of in- l ine ins tan -
tan eous blend ers (plu g flow) as well as
tu bu lar plug flow reactors , all with reaction
times of a few secon ds . A plu g flow rea ctor
insu res tha t a n equa l amou nt of coagulant
is availab le to each pa rticle for an equ al
am oun t of t ime. This is imp ortant if the
reaction time is tru ly only a few second slong.
Others have found that the t radi t ional
complete-mix-type basin with turbine- or
propeller-type impellers is en tirely adequ ate
and have even recommended extending the
reaction time to several minutes (instead of
the s tan dard 3 0-60 seconds su gges ted in
some s ta te s t an dards) in order to enha nce
th e initial stages of floccu lation .
This a ppa rent conflict can be explained b y
cons iderin g the two poss ible types of coagu-lat ion: ch arge n eutra lizat ion a nd sweep
floc. For sweep coagu lation, extrem ely
sh ort mix t imes a re not required s ince most
of the colloids are captured by becoming
enm esh ed in th e growing precipi tate . For
alum , dosages above abou t 30 m g/ L wil l
produ ce sweep coagu lation. Lower dosa ges
resu lt in a combina t ion of sweep an d
cha rge neu tral izat ion or ju st cha rge neu -
t ral izat ion, depend ing on th e pH an d th e
alu m dos e. Clearly, th e choice of mixing
regimes is not easy.
Mixing Inten sity
Th e type an d inten sity of agitation is alsoimp ortant . Tu rbu lent mixing, char acter-
ized b y high velocity gradient s, is des irable
in order to provide sufficient energy for
inter pa rticle collisions . Propeller m ixers
are n ot as well su ited as tu rbine mixers .
Propellers put more energy into circulating
the ba sin contents , whi le tur bine mixers
sh ear the water , ind u cing the higher
velocity gradients and the small high
multidirectional velocity currents that
prom ote p art icle collisions .
High intensi ty throughou t th e rapid mixbas in ma y not be as impor tant a s th e sca le
an d intens ity of tu rbu lence at th e point of
coagu lan t addi t ion. In a turb ine- type
complete-mix basin, the mixing intensity
will not be un iform th roughou t th e basin.
Th e impeller disch arge zone occu pies
abou t 10 p er cent of the total volu me a nd
ha s a s hear inten si ty app roxima tely 2.5
t imes the average.
There is an upper l imit to mixing intensity
because high shear condi t ions can break
u p m icroflocs a n d d elay or pr event visiblefloc forma tion . Coagulan ts which act
through charge neutral izat ion can some-
times r ecover from th is du ring floccu lation
(examples inclu de a lu m an d low molecular
weigh t ca tionic polyelectrolytes). Bridging-
type floccu lan ts a re n ot as resi lient a nd
ma y not recover . This may be becau se the
long polymer cha ins are cu t , breaking th eir
pa rticle-to-particle bridges. The ch arge on
the polymer fragments may even interfere
with a t tempts to re-coagulate the system,
al though a polymer of opposite cha rge may
help.
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
39/41
35
Rules of Thum b for Rapid Mixing
The pu rpos e of rap id mixin g is t wo-fold.First , i t sh ould efficient ly dispers e th e
coagu lat ing chem icals . Second, and ju st as
imp ortant , i t sh ould provide condi t ions tha t
favor the formation of microfloc particles.
The ch ar acteristics of this m icrofloc will
determine the success of the flocculation
s tep .
It is difficult to offer hard and fast rules for
rapid mixers . In fact , man y rapid mix
alternatives provide satisfactory results.
Back -mix rea c tors genera lly work bes t if th ey follow th ese gu idelines:
Sta tors and squ are t anks fu nn el more
energy into mixing than into circulation.
Flat blade tu rbines provide more sh ear
inten si ty than propeller types.
Chemicals should be introduced at the
agitator blade level.
For sweep coagulat ion u sing alum or
ferric comp oun ds , G-valu es of between
300 and 1200 s ec-1 in combination with
retent ion t imes of 20 seconds to several
minu tes ha ve produ ced sat isfactory
resul ts .
When cationic polyelectrolytes a re u sed
as a p r imary coagulan t , then a G-valu e
ran ge of 400-700 is m ore appropriate .
Overly long reten tion times (lon ger tha n
1-2 minu tes), rapid cha nges in G valu e,
an d G values a bove 1000 ca n a ll be
detr imental .
Consider tapered velocity gradients to
provide a transition from rapid mixing to
flocculation. This m ay be importa nt if the p r ima ry coagulan t is a polymer.
Plug flo w reac tors ma y be more appropri-
ate i f cha rge neu tral izat ion p redominates
over sweep floc. G-valu es of 30 00-5 000
with retent ion t imes of abou t 1/ 2 second
are usual ly recommended.
G-Value
The G-valu e concept is a rou gh ap proxima -tion of mixin g inten sity. It is ba sed u pon
inp u t power, bas in volu me a nd viscosity.
For water , a t 1 0 OC, the G value can be
expressed in terms of horsepower per 1000
gallons of ba sin volu m e:
G = 387 x (HP per 1 000 gal)1/ 2
Practical G-Value RelationshipsThe relation between retention time and inputhorsepower per million gallons per day (MGD) of flowis linear for any selected G-value. This figure can be
used to estimate the G-value or the power require-ment to produce a specific G-value. It is calculated at10O C. The input horsepower will be a percentage ofthe mixer motor power, in the range of 70 - 85%.
0
InputHorsepowerperMGD
Retention Time - seconds30 60 90
1
2
3
4
5
G=1000
G=800
G=600
G=400
G=200
-
8/7/2019 Coagulation Ly Thuyet Ve Keo Tu
40/41
36
Chapter 6 Tips on Mixing
Flocculation
Rate of Flocculation
In genera l the ra te of floccu lation is con-trolled by two factors:
concen tra tion of pa rticles (both free
colloids a n d floc) velocity grad ient
A greater number of particles provides more
opportu nity for collisions to occur between
colloids or between floc p art icles an d col-
loids . This increas es the rate of captu re.
High er velocity gradient s also p rovide m ore
opportun it ies for collis ions , bu t th e sh ear
forces from too h igh a velocity gradient can
brea k u p larger flocs a n d will limit thema xim u m floc size.
Creat ing Collisions
The velocity gradients in a full scale floccu-
lat ion bas in can be created by a var iety of
mech an ism s, including baffled cham bers ,
rotat ing paddles , reciprocat ing blades an d
tu rbine- type mixers . Baffled cham bers are
lim ited becau se t he velocity gradient is
controlled only by the flow rate, while
mech an ical system s offer th e ab ili ty to
ind epende