Coagulation & Flocculation

Everything you want to know aboutCoagulation & Flocculatio n....

Zeta-Meter, Inc .

Fourth EditionApril 1993

Copyright© Copyright by Zeta-Meter, Inc. 1993, 1991, 1990,1988. All rights reserved. No part of this publicationmay be reproduced, transmitted, transcribed, stored ina retrieval system, or translated into any language inany form by any means without the written permissionof Zeta-Meter, Inc.

Your CommentsWe hope this guide will be helpful. If you have anysuggestions on how to make it better, or if you haveadditional information you think would help otherreaders, then please drop us a note or give us a call.Future editions will incorporate your comments.

AddressZeta-Meter, Inc.765 Middlebrook AvenuePO Box 3008Staunton, Virginia 24402

Telephone (540) 886-3503Toll Free (USA) (800) 333-0229Fax (540) 886-3728E-Mail [email protected]

http://www.zeta-meter.com

CreditsConceived and written by Louis RavinaDesigned and illustrated by Nicholas Moramarco

Chapter 4 _____________________ 19

Using Alum and Ferric CoagulantsTime Tested CoagulantsAluminum Sulfate (Alum)pH EffectsCoagulant Aids

Chapter 5 _____________________ 25

Tools for Dosage ControlJar TestZeta PotentialStreaming CurrentTurbidity and Particle Count

Chapter 6 _____________________ 33

Tips on MixingBasicsRapid MixingFlocculation

The Zeta Potential Experts ________ 37

About Zeta-Meter

Introduction _____________________ iii

A Word About This Guide

Chapter 1 ______________________ 1

The Electrokinetic ConnectionParticle Charge Prevents CoagulationMicroscopic Electrical ForcesBalancing Opposing ForcesLowering the Energy Barrier

Chapter 2 ______________________ 9

Four Ways to FlocculateCoagulate, Then FlocculateDouble Layer CompressionCharge NeutralizationBridgingColloid Entrapment

Chapter 3 _____________________ 13

Selecting PolyelectrolytesAn Aid or Substitute for Traditional CoagulantsPicking the Best OneCharacterizing PolymersEnhancing Polymer EffectivenessPolymer Packaging and FeedingInterferences

Contents

A Word About This Guide

The removal of suspended matter fromwater is one of the major goals of watertreatment. Only disinfection is used moreoften or considered more important. In fact,effective clarification is really necessary forcompletely reliable disinfection becausemicroorganisms are shielded by particles inthe water.

Clarification usually involves:• coagulation• flocculation• settling• filtration

This guide focuses on coagulation andflocculation: the two key steps which oftendetermine finished water quality.

Coagulation control techniques have ad-vanced slowly. Many plant operators re-member when dosage control was basedupon a visual evaluation of the flocculationbasin and the clarifier. If the operator’seyeball evaluation found a deterioration inquality, then his common sense responsewas to increase the coagulant dose. Thisremedy was based upon the assumptionthat if a little did some good, then moreought to do better, but it often did worse.

The competency of a plant operator de-pended on his years of experience with thatspecific water supply. By trial, error andoral tradition, he would eventually encoun-ter every type of problem and learn to dealwith it.

Reliable instruments now help us under-stand and control the clarification process.Our ability to measure turbidity, particlecount, zeta potential and streaming currentmakes coagulation and flocculation more ofa science, although art and experience stillhave their place.

We make zeta meters and happen to be alittle biased in favor of zeta potential. Inthis guide, however, we have attempted togive you a fair picture of all of the tools atyour disposal, and how you can put them towork.

Introduction

iii

1

Particle Charge Prevents CoagulationThe key to effective coagulation and floccu-lation is an understanding of how individ-ual colloids interact with each other. Tur-bidity particles range from about .01 to 100microns in size. The larger fraction isrelatively easy to settle or filter. Thesmaller, colloidal fraction, (from .01 to 5microns), presents the real challenge. Theirsettling times are intolerably slow and theyeasily escape filtration.

The behavior of colloids in water is stronglyinfluenced by their electrokinetic charge.Each colloidal particle carries a like charge,which in nature is usually negative. Thislike charge causes adjacent particles torepel each other and prevents effectiveagglomeration and flocculation. As aresult, charged colloids tend to remaindiscrete, dispersed, and in suspension.

On the other hand, if the charge is signifi-cantly reduced or eliminated, then thecolloids will gather together. First formingsmall groups, then larger aggregates andfinally into visible floc particles which settlerapidly and filter easily.

Chapter 1The Electrokinetic Connection

Charged Particles repel each other

Uncharged Particles are free to collide and aggre-gate.

2

Chapter 1 The Electrokinetic Connection

Microscopic Electrical ForcesThe Double LayerThe double layer model is used to visualizethe ionic environment in the vicinity of acharged colloid and explains how electricalrepulsive forces occur. It is easier to under-stand this model as a sequence of stepsthat would take place around a singlenegative colloid if the ions surrounding itwere suddenly stripped away.

We first look at the effect of the colloid onthe positive ions, which are often calledcounter-ions. Initially, attraction from thenegative colloid causes some of the positiveions to form a firmly attached layer aroundthe surface of the colloid. This layer ofcounter-ions is known as the Stern layer.

Additional positive ions are still attracted bythe negative colloid but now they are re-pelled by the positive Stern layer as well asby other nearby positive ions that are also

trying to approach the colloid. A dynamicequilibrium results, forming a diffuse layerof counter-ions. The diffuse positive ionlayer has a high concentration near thecolloid which gradually decreases withdistance until it reaches equilibrium withthe normal counter-ion concentration insolution.

In a similar but opposite fashion, there is alack of negative ions in the neighborhood ofthe surface, because they are repelled bythe negative colloid. Negative ions arecalled co-ions because they have the samecharge as the colloid. Their concentrationwill gradually increase as the repulsiveforces of the colloid are screened out by thepositive ions, until equilibrium is againreached with the co-ion concentration insolution.

Two Ways to Visualize theDouble LayerThe left view shows thechange 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-IonPositive Counter-Ion

3

Double Layer ThicknessThe diffuse layer can be visualized as acharged atmosphere surrounding thecolloid. At any distance from the surface,its charge density is equal to the differencein concentration of positive and negativeions at that point. Charge density is great-est near the colloid and rapidly diminishestowards zero as the concentration of posi-tive and negative ions merge together.The attached counter-ions in the Sternlayer and the charged atmosphere in thediffuse layer are what we refer to as thedouble layer.

The thickness of the double layer dependsupon the concentration of ions in solution.A higher level of ions means more positiveions are available to neutralize the colloid.

The result is a thinner double layer.Decreasing the ionic concentration (bydilution, for example) reduces the numberof positive ions and a thicker double layerresults.

The type of counter-ion will also influencedouble layer thickness. Type refers to thevalence of the positive counter-ion. Forinstance, an equal concentration of alumi-num (Al+3) ions will be much more effectivethan sodium (Na+) ions in neutralizing thecolloidal charge and will result in a thinnerdouble layer.

Increasing the concentration of ions or theirvalence are both referred to as double layercompression.

Variation of Ion Density in the Diffuse LayerIncreasing the level of ions in solution reduces thethickness of the diffuse layer. The shaded arearepresents the net charge density.

Distance From Colloid

Ion

Co

nce

ntr

atio

n

Diffuse Layer


Ion

Co

nce

ntr

atio

n

Diffuse Layer

Lower Level of Ions in Solution Higher Level of Ions in Solution

Level of ions in solution

4


Zeta PotentialThe negative colloid and its positivelycharged atmosphere produce an electricalpotential across the diffuse layer. This ishighest at the surface and drops off pro-gressively with distance, approaching zeroat the outside of the diffuse layer. Thepotential curve is useful because it indi-cates the strength of the repulsive forcebetween colloids and the distance at whichthese forces come into play.

A particular point of interest on the curve isthe potential at the junction of the Sternlayer and the diffuse layer. This is knownas the zeta potential. It is an importantfeature because zeta potential can bemeasured in a fairly simple manner, while

the surface potential cannot. Zeta potentialis an effective tool for coagulation controlbecause changes in zeta potential indicatechanges in the repulsive force betweencolloids.

The ratio between zeta potential and sur-face potential depends on double layerthickness. The low dissolved solids levelusually found in water treatment results ina relatively large double layer. In this case,zeta potential is a good approximation ofsurface potential. The situation changeswith brackish or saline waters; the highlevel of ions compresses the double layerand the potential curve. Now the zetapotential is only a fraction of the surfacepotential.

Zeta Potential vs Surface PotentialThe 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.


Zeta Potential

Surface Potential

Po

ten

tial

Stern Layer

Diffuse Layer


Zeta Potential

Surface Potential

Po

ten

tial

Stern Layer

Diffuse Layer

Fresh Water Saline Water

5

Electrostatic repulsion is always shown as apositive curve.

Balancing Opposing ForcesThe DLVO Theory (named after Derjaguin,Landau, Verwery and Overbeek) is theclassic explanation of how particles inter-act. It looks at the balance between twoopposing forces - electrostatic repulsionand van der Waals attraction - to explainwhy some colloids agglomerate and floccu-late while others will not.

RepulsionElectrostatic repulsion becomes significantwhen two particles approach each otherand their electrical double layers begin tooverlap. Energy is required to overcomethis repulsion and force the particlestogether. The level of energy requiredincreases dramatically as the particles aredriven closer and closer together. Anelectrostatic repulsion curve is used toindicate the energy that must be overcomeif the particles are to be forced together.The maximum height of the curve is relatedto the surface potential.

AttractionVan der Waals attraction between twocolloids is actually the result of forcesbetween individual molecules in eachcolloid. The effect is additive; that is, onemolecule of the first colloid has a van derWaals attraction to each molecule in thesecond colloid. This is repeated for eachmolecule in the first colloid and the totalforce is the sum of all of these. An attrac-tive energy curve is used to indicate thevariation in attractive force with distancebetween particles.

Van der Waals attraction is shown as a negativecurve.

Distance Between ColloidsR

epu

lsiv

e E

ner

gy

ElectricalRepulsion

Distance Between Colloids

Att

ract

ive

En

erg

y

Van der WaalsAttraction

6


The Energy BarrierThe DLVO theory combines the van derWaals attraction curve and the electrostaticrepulsion curve to explain the tendency ofcolloids to either remain discrete or toflocculate. The combined curve is calledthe net interaction energy. At each dis-tance, the smaller energy is subtractedfrom the larger to get the net interactionenergy. The net value is then plotted -above if repulsive, below if attractive - andthe curve is formed.

The net interaction curve can shift fromattraction to repulsion and back to attrac-tion with increasing distance betweenparticles. If there is a repulsive section,then this region is called the energy barrierand its maximum height indicates howresistant the system is to effective coagula-tion.

In order to agglomerate, two particles on acollision course must have sufficient kineticenergy (due to their speed and mass) tojump over this barrier. Once the energybarrier is cleared, the net interaction energyis all attractive. No further repulsive areasare encountered and as a result the par-ticles agglomerate. This attractive region isoften referred to as an energy trap since thecolloids can be considered to be trappedtogether by the van der Waals forces.

InteractionThe net interaction curve is formed by subtracting theattraction curve from the repulsion curve.

Att

ract

ive

En

erg

yR

epu

lsiv

e E

ner

gy

ElectricalRepulsion


Net InteractionEnergy

EnergyBarrier

Energy Trap

van der WaalsAttraction

7

Lowering the Energy BarrierFor really effective coagulation, the energybarrier should be lowered or completelyremoved so that the net interaction isalways attractive. This can be accom-plished by either compressing the doublelayer or reducing the surface charge.

Compress the Double LayerDouble layer compression involves addingsalts to the system. As the ionic concentra-tion increases, the double layer and therepulsion energy curves are compresseduntil there is no longer an energy barrier.Particle agglomeration occurs rapidly underthese conditions because the colloids canjust about fall into the van der Waals “trap”without having to surmount an energybarrier.

Flocculation by double layer compression isalso called salting out the colloid. Addingmassive amounts of salt is an impracticaltechnique for water treatment, but theunderlying concept should be understood,and has application toward wastewaterflocculation in brackish waters.

CompressionDouble layer compression squeezes the repulsiveenergy curve reducing its influence. Further compres-sion would completely eliminate the energy barrier.

Att

ract

ive

En

erg

y


Rep

uls

ive

En

erg

y ElectricalRepulsion



EnergyBarrier

Energy Trap

8


Charge ReductionCoagulant addition lowers the surface charge anddrops the repulsive energy curve. More coagulantcan be added to completely eliminate the energybarrier.

Lower the Surface ChargeIn water treatment, we lower the energybarrier by adding coagulants to reduce thesurface charge and, consequently, the zetapotential. Two points are important here.

First, for all practical purposes, zeta poten-tial is a direct measure of the surface chargeand we can use zeta potential measure-ments to control charge neutralization.

Second, it is not necessary to reduce thecharge to zero. Our goal is to lower theenergy barrier to the point where the par-ticle velocity from mixing allows the colloidsto overwhelm it.

The energy barrier concept helps explainwhy larger particles will sometimes floccu-late while smaller ones in the same suspen-sion escape. At identical velocities thelarger particles have a greater mass andtherefore more energy to get them over thebarrier.

Att

ract

ive

En

erg

y


Rep

uls

ive

En

erg

y

ElectricalRepulsion



EnergyBarrier

Energy Trap

9

Coagulate, Then FlocculateIn water clarification, the terms coagulationand flocculation are sometimes used inter-changeably and ambiguously, but it isbetter to separate the two in terms offunction.

Coagulation takes place when the DLVOenergy barrier is effectively eliminated; thislowering of the energy barrier is also re-ferred to as destabilization.

Flocculation refers to the successfulcollisions that occur when the destabilizedparticles are driven toward each other bythe hydraulic shear forces in the rapid mixand flocculation basins. Agglomerates of afew colloids then quickly bridge together toform microflocs which in turn gather intovisible floc masses.

Reality is somewhere in between. The linebetween coagulation and flocculation isoften a somewhat blurry one. Most coagu-lants can perform both functions at once.Their primary job is charge neutralizationbut they often adsorb onto more than onecolloid, forming a bridge between them andhelping them to flocculate.

Coagulation and flocculation can be causedby any of the following:• double layer compression• charge neutralization• bridging• colloid entrapment

Chapter 2Four Ways to Flocculate

In the pages that follow, each of these fourtools is discussed separately, but thesolution to any specific coagulation-floccu-lation problem will almost always involvethe simultaneous use of more than one ofthese. Use these as a check list whenplanning a testing program to select anefficient and economical coagulant system.

10

Chapter 2 Four Ways to Flocculate

Double Layer CompressionDouble layer compression involves theaddition of large quantities of an indifferentelectrolyte (e.g., sodium chloride). Theindifference refers to the fact that the ionretains its identity and does not adsorb tothe colloid. This change in ionic concentra-tion compresses the double layer aroundthe colloid and is often called salting out.

The DLVO theory indicates that this resultsin a lowering or elimination of the repulsiveenergy barrier. It is important to realizethat salting out just compresses thecolloid's sphere of influence and does notnecessarily reduce its charge.

In general, double layer compression is nota practical coagulation technique for watertreatment but it can have application inindustrial wastewater treatment if wastestreams with divalent or trivalent counter-ions happen to be available.

CompressionFlocculation by double layercompression is unusual, buthas some application inindustrial wastewaters.Compare this figure to theone on page 2.

Stern Layer

Diffuse Layer

Highly Negative Colloid


11

Charge NeutralizationInorganic coagulants (such as alum) andcationic polymers often work throughcharge neutralization. It is a practical wayto lower the DLVO energy barrier and formstable flocs. Charge neutralization involvesadsorption of a positively charged coagulanton the surface of the colloid. This chargedsurface coating neutralizes the negativecharge of the colloid, resulting in a nearzero net charge. Neutralization is the key tooptimizing treatment before sedimentation,granular media filtration or air flotation.

Charge neutralization alone will not neces-sarily produce dramatic macroflocs (flocsthat can be seen with the naked eye). Thisis demonstrated by charge neutralizing withcationic polyelectrolytes in the 50,000-200,000 molecular weight range. Microflocs(which are too small to be seen) may formbut will not aggregate quickly into visibleflocs.

Charge neutralization is easily monitoredand controlled using zeta potential. This isimportant because overdosing can reversethe charge on the colloid, and redisperse itas a positive colloid. The result is a poorlyflocculated system. The detrimental effectof overdoing is especially noticeable withvery low molecular weight cationic polymersthat are ineffective at bridging.

Charge ReductionLowering the surface chargedrops the repulsive energycurve 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


Slightly Negative Colloid

12

Chapter 2 Four Ways to Flocculate

BridgingBridging occurs when a coagulant formsthreads or fibers which attach to severalcolloids, capturing and binding themtogether. Inorganic primary coagulants andorganic polyelectrolytes both have thecapability of bridging. Higher molecularweights mean longer molecules and moreeffective bridging.

Bridging is often used in conjunction withcharge neutralization to grow fast settlingand/or shear resistant flocs. For instance,alum or a low molecular weight cationicpolymer is first added under rapid mixingconditions to lower the charge and allowmicroflocs to form. Then a slight amount ofhigh molecular weight polymer, often ananionic, can be added to bridge between themicroflocs. The fact that the bridgingpolymer is negatively charged is not signifi-cant because the small colloids have al-ready been captured as microflocs.

Colloid EntrapmentColloid entrapment involves adding rela-tively large doses of coagulants, usuallyaluminum or iron salts which precipitate ashydrous metal oxides. The amount ofcoagulant used is far in excess of theamount needed to neutralize the charge onthe colloid. Some charge neutralizationmay occur but most of the colloids areliterally swept from the bulk of the water bybecoming enmeshed in the settling hydrousoxide floc. This mechanism is often calledsweep floc.

Sweep FlocColloids become enmeshed in the growing precipitate.

BridgingEach polymer chain attaches to many colloids.

13

An Aid or Substitute forTraditional Coagulants

The class of coagulants and flocculantsknown as polyelectrolytes (or polymers) isbecoming more and more popular. Aproper dosage of the right polyelectrolytecan improve finished water quality whilesignificantly reducing sludge volume andoverall operating costs.

On a price-per-pound basis they are muchmore expensive than inorganic coagulants,such as alum, but overall operating costscan be lower because of a reduced need forpH adjusting chemicals and because oflower sludge volumes and disposal costs.In some cases they are used to supplementtraditional coagulants while in others theycompletely replace them.

Polyelectrolytes are organic macromole-cules. A polyelectrolyte is a polymer; thatis, it is composed of many (poly) monomers(mer) joined together. Polyelectrolytes maybe fabricated of one or more basic mono-mers (usually two). The degree of polymeri-zation is the number of monomers (buildingblocks) linked together to form one mole-cule, and can range up to hundreds ofthousands.

Picking the Best OneBecause of the number available and theirproprietary nature, it can be a real chal-lenge to select the best polyelectrolyte for aspecific task. The following characteristicsare usually used to classify them; manufac-turers will often publish some of these, butnot always with the desired degree of detail:• type (anionic, non-ionic, or cationic)• molecular weight• basic molecular structure• charge density• suitability for potable water treatment

Preliminary bench testing of polyelectrolytesis an important part of the selection proc-ess, even when the polymer is used as aflocculant aid for an inorganic coagulant.Adding an untested polyelectrolyte withoutknowing the optimum dosage, feed concen-tration or mixing requirements can result inserious problems, including filter clogging.

It is important to note that, within the samefamily or type of polymer, there can be alarge difference in molecular weight andcharge density. For a specific application,one member of a family can have just theright combination of properties and greatlyoutperform the others.

Chapter 3Selecting Polyelectrolytes

14

Chapter 3 Selecting Polyelectrolytes

Characterizing PolymersMolecular WeightThe overall size of a polymer determines itsrelative usefulness for bridging. Size isusually measured as molecular weight.Manufacturers do not use a uniformmethod to report molecular weight. Forthis reason, two similar polymers with thesame published molecular weight mayactually be quite different.

In addition, molecular weight is only ameasure of average polymer length. Eachmolecule in a drum of polymer is not thesame size. A wide range can and will befound in the same batch. This distributionof molecular weights is an important prop-erty and can vary greatly.

Molecular GeneralWeight Range Description10,000,000 or more Very High1,000,000 to 10,000,000 High200,000 to 1,000,000 Medium100,000 to 200,000 Low50,000 to 100,000 Very LowLess than 50,000 Very, Very Low

StructureTwo similar polyelectrolytes with the samecomposition of monomers, molecularweight, and charge characteristics canperform differently because of the way themonomers are linked together. For ex-ample, a product with two monomers A andB could have a regular alternation from A toB or could have groups of A’s followed bygroups of B’s.

Charge DensityRelative charge density is controlled by theratio of charged and uncharged monomersused. The higher the residual charge, thehigher the density. In general, the relativecharge density and molecular weight can-not both be increased. As a result, theideal polyelectrolyte often involves a tradeoffbetween charge density and molecularweight.

Type of PolymerPolyelectrolytes are classified as non-ionic,anionic or cationic depending upon theresidual charge on the polymer in solution.

Non-ionic polyelectrolytes are polymerswith a very low charge density. A typicalnon-ionic is a polyacrylamide. Non-ionicsare used to flocculate solids through bridg-ing.

Anionic polyelectrolytes are negativelycharged polymers and can be manufacturedwith a variety of charge densities, frompractically non-ionic to very strongly ani-onic. Intermediate charge densities areusually the most useful. Anionics arenormally used for bridging, to flocculatesolids. The acrylamide-based anionics withvery high molecular weights are very effec-tive for this.

Negative colloids can sometimes be suc-cessfully flocculated with bridging-type longchain anionic polyelectrolytes. One pos-sible explanation is that a colloid with a netnegative charge may actually have a mosaicof positive and negative regions. Areas ofpositive charge could serve as points ofattachment for the negative polymer.

Anionic polyelectrolytes may be capable offlocculating large particles, but a residualhaze of smaller colloids will almost alwaysremain. These must first have their chargeneutralized in order to flocculate.

Cationic polyelectrolytes are positivelycharged polymers and come in a wide rangeof families, charge densities and molecularweights. The variety available offers greatflexibility in solving specific coagulation andflocculation problems, but makes selectingthe right polymer more complicated.

High molecular weight cationic polyelectro-lytes can be thought of as double actingbecause they act in two ways: chargeneutralization and bridging.

15

Direct Filtration with Cationic PolymersIt is often possible to eliminate or bypass conventionalflocculation 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 itproduces 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 ScreeningThe true cost of a polymer is not its price per poundbut 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 forPolymer 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

Zet

a P

ote

nti

al,

mV

Hea

d L

oss

Aft

er 6

Ho

urs

, f

t.E

fflu

ent

Tu

rbid

ity,

NT

U

ZetaPotential

Turbidity

FilterHeadLoss

+5

+10

0

-5

-10

-15

-20

5 10 15 20 25

Zet

a P

ote

nti

al, m

V

30

Polymer Dose, mg/L

Polymer A $2.00 / lb

Polymer C

$3.50 / lb

Polymer B

$0.50 / lb

16


Enhancing Polymer EffectivenessDual Polymer SystemsTwo polymers can help if no single polymercan get the job done. Each has a specificfunction. For example, a highly chargedcationic polymer can be added first toneutralize the charge on the fine colloids,and form small microflocs. Then a highmolecular weight anionic polymer can beused to mechanically bridge the microflocsinto large, rapidly settling flocs.

In water treatment, dual polymer systemshave the disadvantage that more carefulcontrol is required to balance the counter-acting forces. Dual polymers are morecommon in sludge dewatering, whereoverdosing and the appearance of excesspolymer in the centrate or filtrate is not asimportant.

PreconditioningInorganic coagulants may be helpful as acoagulant aid when a polyelectrolyte aloneis not successful in destabilizing all theparticles. Pretreatment with inorganics canalso reduce the cationic polymer dose andmake it more stable, requiring less criticalcontrol.

Preconditioning Polymers with AlumThe effect of preconditioning can be evaluated bymaking 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

Zet

a P

ote

nti

al, m

VPolymerOnly

Polymer +20 mg/Lalum

Polymer +10 mg/Lalum

Polymer Dose, mg/L

17

Polymer Packaging and FeedingPolyelectrolytes can be purchased in pow-der, solution and emulsion form. Each typehas advantages and disadvantages. Ifpossible, feed facilities should allow anytype to be used.

Dry powder polymers are whitish granularpowders, flakes or beads. Their tendency toabsorb moisture from the air and to stick tofeed screws, containers and drums is amajor nuisance. Dry polymers are alsodifficult to wet and dissolve rather slowly.Fifteen minutes to 1 hour may be required.

Solution polymers are often preferred todry powders because they are more conven-ient. A little mixing is usually sufficient todilute liquid polymers to feed strength.Active ingredients can vary from a fewpercent to 50 percent.

Emulsion polymers are a more recentdevelopment. They allow very high molecu-lar weight polymers to be purchased inconvenient liquid form. Dilution with waterunder agitation frees the gel particles in theemulsion allowing them to dissolve in thewater. Sometimes activators are requiredfor preparation. Emulsions are usuallypackaged as 20-30% active ingredients.

Prepared batches of polymer are normallyused within 24-48 hours to prevent loss ofactivity.

In addition, polymers are almost alwaysmore effective when fed as dilute solutionsbecause they are easier to disperse anduniformly distribute. Using a higher feedstrength may mean that a higher polymerdose will be required.

Typically, maximum feed strength is be-tween 0.01 to 0.05%, but check with thepolymer manufacturer for specific recom-mendations.

Stock polymer solutions are usually madeup to 0.1 to 0.5% as a good compromisebetween storage volume, batch life, andviscosity. Diluting the stock solution byabout 10:1 with water will usually drop theconcentration to the recommended feedlevel. The polymer and dilution watershould be blended in-line with a staticmixer or an eductor.

Using Cationic Polymers in Brackish WatersIn brackish waters, the correct cationic polymer doseis often concealed by the effect of double layercompression, which drops the zeta potential but notthe 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.


Zeta PotentialAfter Dilution

Surface Potential

Po

ten

tial

Stern Layer

Diffuse Layer

Zeta PotentialBefore Dilution

18


InterferencesCationic polymers can react with negativeions in solution, forming chemical bondswhich impair their performance. Greaterdoses are then required to achieve the samedegree of charge neutralization or bridging.This is more noticeable in wastewatertreatment.

Examples of interfering substances includesulfides, hydrosulfides and phenolics, buteven chlorides can reduce polymer effective-ness.

Even pH should be considered, sinceparticular polymer families may performwell in some pH ranges and not others.

Phenol InterferenceThe effect of phenols on this cationic polymer wasevaluated by plotting zeta potential curves.

pH EffectsZeta potential curves can be used to evaluate thesensitivity 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

Zet

a P

ote

nti

al, m

V

1 2 3 4 5 6Polymer Dose, mg/L

pH 8

pH 10

+5

+10

0

-5

-10

-15

-20

Zet

a P

ote

nti

al, m

V

2 4 6 8 10 12

30 mg/LPhenol

NoPhenol

Polymer Dose, mg/L

19

Time Tested CoagulantsAluminum and ferric compounds are thetraditional coagulants for water and waste-water treatment. Both are from a familycalled metal coagulants, and both are stillwidely used today. In fact, many plants useone of these exclusively, and have noprovision for polyelectrolyte addition.

Metal coagulants offer the advantage of lowcost per pound. In addition, selection ofthe optimum coagulant is simple, sinceonly a few choices are available. A distinctdisadvantage is the large sludge volumeproduced, since sludge dewatering anddisposal can be difficult and expensive.

Aluminum and ferric coagulants are solublesalts. They are added in solution form andreact with alkalinity in the water to forminsoluble hydrous oxides that coagulate bysweep floc and charge neutralization.

Metal coagulants always require attentionto pH conditions and consideration of thealkalinity level in the raw and treated water.Reasonable dosage levels will frequentlyresult in near optimum pH conditions. Atother times, chemicals such as lime, sodaash or sodium bicarbonate must be addedto supplement natural alkalinity.

Chapter 4

Aluminum Sulfate (Alum)Alum is one of the most widely used coagu-lants and will be used as an example of thereactions that occur with a metal coagu-lant. Ferric coagulants react in a generallysimilar manner, but their optimum pHranges are different.

When aluminum sulfate is added to water,hydrous oxides of aluminum are formed.The simplest of these is aluminum hydrox-ide (Al(OH)3) which is an insoluble precipi-tate. But several, more complex, positivelycharged soluble ions are also formed,including:• Al6(OH)15

+3

• Al7(OH)17 +4

• Al8(OH)20 +4

The proportion of each will vary, dependingupon both the alum dose and the pH afteralum addition. To further complicatematters, under certain conditions thesulfate ion (SO4

-2) may also become part ofthe hydrous aluminum complex by substi-tuting for some of the hydroxide (OH-1) ions.This will tend to lower the charge of thehydroxide complex.

Using Alum and Ferric Coagulants

20

Chapter 4 Using Alum and Ferric Coagulants

How Alum WorksThe mechanism of coagulation by alumincludes both charge neutralization andsweep floc. One or the other may predomi-nate, but each is always acting to somedegree. It is probable that charge neutrali-zation takes place immediately after addi-tion of alum to water. The complex, posi-tively charged hydroxides of aluminum thatrapidly form will adsorb to the surface ofthe negative turbidity particles, neutralizingtheir charge (and zeta potential) and effec-tively lowering or removing the DLVOenergy barrier.

Simultaneously, aluminum hydroxideprecipitates will form. These additionalparticles enhance the rate of flocculation byincreasing the chances of a collision occur-ring. The precipitate also grows independ-ently of the colloid population, enmeshingcolloids in the sweep floc mode.

The type of coagulation which predominatesis dependent on both the alum dose andthe pH after alum addition. In general,sweep coagulation is thought to predomi-nate at alum doses above 30 mg/L; belowthat, the dominant form depends upon bothdose and pH.

Alkalinity is required for the alum reactionto successfully proceed. Otherwise, the pHwill be lowered to the point where solublealuminum ion (Al+3) is formed instead ofaluminum hydroxide. Dissolved aluminumion is an ineffective coagulant and cancause “dirty water” problems in the distri-bution system. The reaction between alumand alkalinity is shown by the following:

600 300 Alum AlkalinityAl2(SO4)3 + 3Ca(HCO3)2 + 6H2O ➜

Aluminum Carbonic Hydroxide Acid ➜ 3CaSO4 + 2Al(OH)3 + 6H2CO3

Estimating Alkalinity RequirementsThis equation helps us develop severalsimple rules of thumb about the relationbetweem alum and alkalinity.

Commercial alum is a crystalline material,with 14.2 water molecules (on average)bound to each aluminum sulfate molecule.The molecular weight of Al2(SO4)3•14.2H2Ois 600.

Alkalinity is a measure of the amount ofbicarbonate (HCO3

-1), carbonate (CO3-2) and

hydroxide (OH-1) ion. The reaction showsalkalinity in its bicarbonate (HCO3

-) formwhich is typical at pH's below 8, but alka-linity is always expressed in terms of theequivalent weight of calcium carbonate(CaCO3), which has a molecular weight of100. The three Ca(HCO3)2 molecules thenhave an equivalent molecular weight of 3 x100 or 300 as CaCO3.

The rules of thumb are based on the ratioof 600 (alum) to 300 (alkalinity):• 1.0 mg/L of commercial alum will

consume about 0.5 mg/L of alkalinity.• There should be at least 5-10 mg/L of

alkalinity remaining after the reactionoccurs to keep the pH near optimum.

• Raw water alkalinity should be equal tohalf the expected alum dose plus 5 to 10mg/L.

1.0 mg/L of alkalinity expressed as CaCO3

is equivalent to:• 0.66 mg/L 85% quicklime (CaO)• 0.78 mg/L 95% hydrated lime (Ca(OH)3)• 0.80 mg/L caustic soda (NaOH)• 1.08 mg/L soda ash (Na2CO3)• 1.52 mg/L sodium bicarbonate (NaHCO3)

21

When to Add AlkalinityIf natural alkalinity is insufficient then addartificial alkalinity to maintain the desiredlevel. Hydrated lime, caustic soda, sodaash or sodium bicarbonate may be used toraise the alkalinity level.

Alkalinity should always be added up-stream, before alum addition, and thechemical should be completely dissolved bythe time the alum reaction takes place.Alum reacts instantaneously and willproceed to other end products if sufficientalkalinity is not immediately available. Thisrequirement is often ignored in an effort tominimize tanks and mixers, but poorperformance is the price that is paid.

When alum reacts with natural alkalinity,the pH is decreased by two different means:the bicarbonate alkalinity of the system islowered and the carbonic acid content isincreased. This is often an advantage,since optimum pH conditions for alumcoagulation are generally in the range ofabout 5.0 to 7.0, while the pH range ofmost natural waters is from about 6.0 to7.8.

At times, some of the alum dose is actuallybeing used solely to lower the pH to itsoptimum value. In other words, a loweralum dose would coagulate as effectively ifthe pH were lowered some other way. Atlarger plants it may be more economical toadd sulfuric acid instead.

It is important to note that not all sourcesof artificial alkalinity have the same effecton pH. Some produce carbonic acid whenthey react and lower the pH. Others do not.Optimum pH conditions should be takeninto account when selecting an alkalinitysource.

Zeta Potential Control of Alum DoseThere is no single zeta potential that will guaranteegood 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

TurbidityZ

eta

Po

ten

tial

, mV

Alum Dose, mg/L

Tu

rbid

ity

of

Fin

ish

ed W

ater

, N

TU

22


pH EffectsChargeThere is a strong relation between pH andperformance, but there is no single opti-mum pH for a specific water. Rather, thereis an interrelation between pH and the typeof aluminum hydroxide formed. This inturn determines the charge on the hydrousoxide complex. Other ions come into playas well. The effect of pH on charge can beevaluated using a zeta potential curve anddirect measurement of zeta potential is abetter method of process control than pH.

Solubility of AluminumA second important aspect of pH is its effecton solubility of the aluminum (Al+3 ) ion.Insufficient alkalinity allows the pH to dropto a point where the aluminum ion becomeshighly soluble. Dissolved aluminum canthen pass right through the filters. Afterfiltration, pH is usually adjusted upward forcorrosion control. The higher pH convertsdissolved aluminum ion to insoluble alumi-num hydroxide which then flocculates inthe distribution system and almost 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

Zet

a P

ote

nti

al, m

V

pH

Res

idu

al C

olo

r

0

+10

60

80

54 6

+15

Color

0

+10

-5

-10

-15

0.0

0.2

0.4

ZetaPotential

Turbidityof Filtered

Water

Zet

a P

ote

nti

al, m

V

Alum Dose, mg/L

Tu

rbid

ity,

NT

U

+5

+15

0

0.1

0.2

0.3

0.4

0.6

0.8

20 40 60 80 100 120

Res

idu

al A

lum

inu

mResidual Aluminum

23

Cationic Coagulant AidZeta potential curves can be used to evaluate thecharge neutralizing properties of cationic polymers.

Coagulant AidsTailoring Floc CharacteristicsPolyelectrolytes which enhance the floccu-lating action of metal coagulants (such asalum), are called coagulant aids or floccu-lant aids.

They provide a means of tailoring floc size,settling characteristics and shear strength.Coagulant aids are excellent tools fordealing with seasonal problem periodswhen alum alone is ineffective. They canalso be used to increase plant capacitywithout increasing physical plant size.

Activated silica was used for this purposebefore organic polymers became popular.Activated silica produces large, dense, fastsettling alum flocs and simultaneouslytoughens it to allow higher rates of filtrationor longer filter runs. A disadvantage is thefairly precise control that is needed duringpreparation of the activated silica. Silica isprepared (activated) on-site by partiallyneutralizing a sodium silicate solution, thenaging and diluting it.

Polyelectrolyte coagulant aids have theadvantages of activated silica and aresimpler to prepare.

Cationic polymers are charge neutralizingcoagulant aids. They can reduce the alumor ferric dose while simultaneously increas-ing floc size and toughening it. They alsoreduce the effect of substances that inter-fere with metal coagulants. Long chain,high molecular weight cationics operate viamechanical bridging in addition to chargeneutralization. Charge neutralizing can bemonitored using zeta potential, while thebridging effect should be evaluated usingjar testing.

0

+5

-5

-10

-15

-20

-25

Zet

a P

ote

nti

al, m

V

Alum

Alum +CationicPolymer

10 20 30 40 50 60

Dose, mg/L

24


Wastewater Coagulation with Ferric ChlorideThese 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 CapacityJar tests were used to evaluate the effect of a verysmall 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

Zet

a P

ote

nti

al,

mV

Res

idu

al T

urb

idit

y, N

TU

Org

anic

Car

bo

n a

s C

OD

, m

g/L

+25

-25

COD

ZetaPotential

Turbidity

0

80

25

0

Anionic and non-ionic polymers arepopular because a small amount canincrease floc size several times. Anionicand non-ionic polyelectrolytes work bestbecause their very high molecular weightpromotes growth through mechanicalbridging. It is important to allow microflocsto form before adding these polymers. Inaddition, excess amounts can actuallyinhibit flocculation. Jar testing is the bestway to establish the optimum dosage.

0

Tu

rbid

ity

of

Set

tled

Wat

er, N

TU

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

25

Tools for Dosage ControlChapter 5

Jar TestAn Undervalued ToolWe usually associate the jar test with atedious dosage control tool that seems totake an agonizingly long time to set up, runand then clean up after. As a result thetrue versatility of the humble jar test isoften overlooked.

The following applications for jar testingmay be more important than its use inroutine dosage control. It is studies suchas these where the jar test has its majorvalue, since the results are worth the timeand effort to do the test properly:• selection of primary coagulant• comparison of coagulant aids• optimizing feed point for pH adjusting

chemicals• mixing energy and mixing time studies

(rapid mix and flocculation)• optimizing feed point for coagulant aids• evaluating dilution requirements for

coagulants• estimating settling velocities for sedimen-

tation basin sizing• studying effect of rapid changes in

mixing energy• evaluating effect of sludge recycling

Jar TestingA tedious but valuable tool which allows side by sidecomparison of many operating variables.

26

Chapter 5 Tools for Dosage Control

The Gator JarThe 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 ApparatusMany commercial laboratory jar test unitsare in use today. The Phipps & Bird sixplace gang stirrer is very traditional. It has1" x 3" flat blade paddles and is usuallyused with 1.5 liter glass beakers. A disad-vantage is the large vortex that forms athigh stirring speeds.

Some provision is usually made for drawingoff a settled sample from below the watersurface. Siphon type draw-offs are fre-quently used for glass beakers.

Camp improved the Phipps & Bird designby using 2-liter glass beakers outfitted withvortex breaking stators. These were espe-cially suitable for mixing studies becauseCamp developed curves for mixing intensity(G) versus stirrer speed using this design.

Gator or Wagner jars are a relativelyrecent improvement and are square plexi-glas jars with a liquid volume of 2 liters.These are now commercially available, andhave several advantages over traditionalglass beakers. First, they are less fragileand allow for easy insertion of a sample tap,thus avoiding the need for a siphon typedraw-off. Their square shape helps dampenrotational velocity without the need forstators while their plexiglas walls offermuch greater thermal insulation thanglass, thus minimizing temperature changeduring testing.

27

Settling StudiesThe data from a well thought out jar testcan be used to realistically size settlingbasins and to evaluate the ability of coagu-lant aids to increase the hydraulic capacityof existing basins. Surface overflow rate(gallons per day per square foot) is moreimportant than retention time in determin-ing the hydraulic capacity of a settlingbasin, and is calculated from the velocity ofthe slowest settling floc that we want toremove. In practical terms, a settling rateof 1 cm/minute is equal to an overflow rateof 360 gallons per day per square foot.

Jar test settling data is obtained by drawingoff samples at a fixed distance below thewater surface, at various time intervalsafter the end of rapid mixing and floccula-tion. Settling velocity and overflow rate arecalculated for each time interval and thesample reflects the quality that we canexpect at that overfow rate.

The Gator jar has a convenient sampledraw-off located 10 cm below the watersurface. Samples are usually withdrawn atintervals of 1, 2, 5, and 10 minutes afterthe mixers are stopped. These correspondto settling velocities of 10, 5, 2 and 1 cm/minute which are equivalent to settlingbasin overflow rates of 3600, 1800, 720 and360 gallons per day per square foot.

The results of the overflow rate studies canbe plotted on regular (arithmetic) graphpaper, or on semi-logarithmic paper (withturbidity on the log scale) or on logarithmicgraph paper. Actual plant operating resultscan also be plotted on the same graphs forcomparison.

See page 24, Non-Ionic Polymer IncreasesSettling Capacity, for an example of asettling study.

G Curves for the Gator JarThese are for the Gator Jar, with a 1x3 inch Phippsand Bird stirrer paddle. See Chapter 6, Tips onMixing for a discussion of the G value and its impor-tance as a rational measure of mixing intensly.

Predicting Filtered Water QualityFor water treatment, it is important toremember that our ultimate goal is toproduce the best filtered water. We oftenassume that the best settled water willalways correspond to the best filteredwater, but that is not necessarily so. Infact, substantial savings may be realized byoptimizing filtered water quality instead.

Bench scale testing can be used in conjunc-tion with jar tests to predict filtered waterquality. Whatman #40 filter paper (orequivalent) provides a good simulation. Usefresh filter 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-v

alu

e, (

sec

-1)

23 Co 3 Co

28


Jar Test ChecklistRoutine jar test procedures should betailored to closely match actual conditionsat the plant. This may take some experi-mentation since full scale mixing conditionsare only approximated in the jar test andtreatment is on a batch instead of a flow-through basis.

Conditions to match up include:• sequence of chemical addition• rapid mixing intensity and time• flocculation intensity and time

Visually evaluated variables include:• time for first floc formation• floc size• floc quality• settling rate

Immediately after rapid mixing the followingcan be determined:• zeta potential• pH

Later, after slow mixing and settling,samples can be carefully drawn off andanalyzed for:• turbidity• color• filterability number• particle count• residual coagulant• filtered water turbidity

Limitations of Visual EvaluationRoutine 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


Water

Zet

a P

ote

nti

al, m

V

Alum Dose, mg/L

Tu

rbid

ity,

NT

U

+5

+15

+20

Poor

PoorFair

Good

Best

Fair

29

Zeta PotentialEasily UnderstoodZeta potential is an excellent tool for coagu-lant dosage control, and we are proud tohave pioneered the use of our instrument inwater treatment over 25 years ago. Opera-tion of a Zeta-Meter is relatively simple andonly a short time is required for each test.The results are objective and repeatableand, most importantly, the techniques canbe easily learned.

The principle of operation is easy to under-stand. A high quality stereoscopic micro-scope is used to comfortably observe tur-bidity particles inside a chamber called anelectrophoresis cell. Electrodes placed ineach end of the cell create an electric fieldacross it. If the turbidity particles have acharge, then they move in the field with aspeed and direction which is easily relatedto their zeta potential.

Zeta-Meter System 3.0A standard parallel printercan be directly connectedfor a "hard copy" of yourtest run.

Our Zeta-Meter 3.0 - Simple & ReliableThe Zeta-Meter 3.0 is a microprocessor-based version of our popular instrument.The sample is poured into the cell, then theelectrodes are inserted and connected tothe Zeta-Meter 3.0 unit. First, the instru-ment determines specific conductance andhelps select the appropriate voltage toapply. Then, when the electrodes areenergized the particles begin to move andare tracked using a grid in the microscopeeyepiece.

Tracking simply involves pressing a buttonand holding it down while a colloid trav-erses the grid. When the track button isreleased the instrument instantly calculatesand displays the zeta potential. A singletracking takes a few seconds, and a com-plete run takes only minutes.

30


A Practical DesignThe Zeta-Meter 3.0 is designed to be mis-take-proof. It recognizes impractical resultsand tracking times that are too short. A“clear” button allows these or other incon-sistent results to be deleted without losingthe rest of your data.

Statistics are also maintained by the Zeta-Meter 3.0, and can be reviewed at any time.Pressing a “status” button causes the unitto display the total number of colloidstracked, their average zeta potential andstandard deviation, a statistical measure ofthe spread of the individual data values.

A printer can be connected to the Zeta-Meter 3.0 unit to record the entire run. Theoutput shows the value obtained for eachcolloid as well as a statistical summary ofthe entire test.

Simplified Coagulant Dose ControlThe zeta potential that corresponds tooptimum coagulation will vary from plant toplant. The optimal value is often called thetarget zeta potential and is best establishedfirst by correlation with jar tests or pilotunits, and then with actual plant perform-ance.

Once a target value is set, routine control isrelatively simple and merely involves meas-uring the zeta potential of a sample fromthe flash mix. If the measured value ismore negative than the target value, justincrease the primary coagulant dose. If it ismore positive, then lower the dose.

Other Applications of Zeta PotentialYou can also use zeta potential analysis forthe following useful applications:• screening of primary coagulants• cost comparison of cationic polyelectro-

lytes• optimum pH determinations• checking quality of delivered cationic

polyelectrolyte• optimizing feed point for pH adjusting

chemicals• evaluating dilution/mixing requirements

for cationic polyelectrolytes

31

Streaming CurrentOn-Line Zeta-Potential . . . . AlmostA limitation on the zeta potential techniqueis that it is not a continuous on-line meas-uring instrument. The streaming currentdetector was developed in response to thisneed. Its main advantage is rapid detectionof plant upsets.

Streaming current is really nothing morethan another way to measure zeta poten-tial, but it is actually related to the zetapotential of a solid surface, such as thewalls of a cylindrical tube, and not the zetapotential of the turbidity or floc particles.

Forcing a flow of water through a tubeinduces an electric current (called thestreaming current) and voltage difference(called the streaming potential) between theends of the tube. Some of the particles inthe liquid will loosely adhere to the walls ofthe cylinder and will affect the zeta poten-tial of the wall. As a result the streamingcurrent or streaming potential of the wallwill reflect to some degree the zeta potentialof the particles in suspension.

Commercial InstrumentsMost commercial streaming current devicesuse an oscillating flow to eliminate back-ground electrical signals. A piston in acylinder (called a boot) is very common.The piston oscillates up and down at arelatively low frequency (about 4 cycles persecond) causing the sample to flow in analternating fashion through the spacebetween the piston and cylinder. The flowof water creates an AC streaming current,due to the zeta potential of the cylinder andpiston surface. It is this current which ismeasured and amplified by the detector.

A Useful MonitorStreaming current is useful as an on-linemonitor of zeta potential. However, it isonly an indication because the value is notscaled. That is, a change in 10 streamingcurrent units does not correspond directlyto a zeta potential change of 10 mV. Inaddition, the zero position is often shiftedsignificantly from true zero and is notstable.

Streaming current and streaming potentialare both excellent ways to keep track of on-line conditions, but should be calibratedwith a zeta meter. A change in the stream-ing current tells you that it is time to take asample and measure the zeta potential.

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

ACStreaming

Current

32


Turbidity and Particle CountFinished water turbidity is often used togauge the effectiveness of a water treatmentplant, but it is important to remember thatthere is no direct relation between theamount of suspended matter, or number ofparticles, and the turbidity of a sample.

Most turbidity measurements are basedupon nephelometry. A light beam is passedthrough the water sample and the particlesin the water scatter the light. The turbidi-meter measures the intensity of scatteredlight, usually at an angle of 90° to the lightbeam and the intensity is expressed instandard turbidity units.

The angle of peak scatter and the amountof light scattered are both influenced by thesize of the particles. Other factors, such asthe nature and concentration of the par-ticles will also effect the turbidity measure-ment.

Particle size analyzers quantify the actualparticle count and the distribution withsize. Particle counters are much moreexpensive than turbidimeters and are notcommonly found in water treatment plants.

Particle Count vs. TurbiditySimultaneous 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 theturbidity standard.

0

+10

-5

-10

0.0

0.2

0.4

ZetaPotential


Water

Zet

a P

ote

nti

al, m

V

Alum Dose, mg/L

Tu

rbid

ity,

NT

U

+5

+15

0.6

0.8

20 40 60 80 100 120

1.0+20

0

+10

-5

-10

0

20

40

ZetaPotential

Zet

a P

ote

nti

al, m

V

Alum Dose, mg/L

Rel

ativ

e P

arti

cle

Co

un

t, p

erce

nt

+5

+15

60

80

20 40 60 80 100 120

100

ParticleCount

+20

33

BasicsMixing patterns are often complex and aredifficult to describe, so we usually classifymixers based upon how closely they ap-proximate one of two idealized flow pat-terns: complete mixing or plug flow.

Ideal plug flow means that each volume ofwater remains in the reactor for exactly thesame amount of time. If a slug of dye wereinjected into the flow as a tracer, then allthe dye would appear at the outlet at thesame time. Ideal plug flow is very difficultto achieve. In general, the reaction basinmust be very long in comparison to itswidth or diameter before the mixing patternbegins to approximate plug flow.

Complete mixing basins are always in-stantaneously blended throughout theirentire volume. As a result, an incomingvolume of water immediately loses itsidentity and is intermixed with the waterthat entered previously. A complete-mixreactor is also called a back-mix reactorbecause its contents are always blendedbackwards with the incoming flow. If a slugof dye tracer was injected into the basin,then some of it would immediately appearin the outgoing flow. The concentration ofdye would drop steadily as the dye in thebasin backmixed with the clear incomingwater and was diluted by it.

Chapter 6Tips on Mixing

Retention Time CurvesWhen 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

Rel

ativ

e C

on

cen

trat

ion

Fraction of Average Retention Time

One Complete Mix Basin

Two

Three

Four

A PracticalApproximationof Plug Flow

34

Chapter 6 Tips on Mixing

Rapid MixingPlug Flow versus Complete MixRecommendations for the best type of rapidmixing are confusing at best. The ex-tremely fast times for the reaction of alumwith alkalinity (less than 1 second) and forthe rapid formation of the aluminum hy-droxide microfloc (less than 10 seconds)have produced advocates of in-line instan-taneous blenders (plug flow) as well astubular plug flow reactors, all with reactiontimes of a few seconds. A plug flow reactorinsures that an equal amount of coagulantis available to each particle for an equalamount of time. This is important if thereaction time is truly only a few secondslong.

Others have found that the traditionalcomplete-mix-type basin with turbine- orpropeller-type impellers is entirely adequateand have even recommended extending thereaction time to several minutes (instead ofthe standard 30-60 seconds suggested insome state standards) in order to enhancethe initial stages of flocculation.

This apparent conflict can be explained byconsidering the two possible types of coagu-lation: charge neutralization and sweepfloc. For sweep coagulation, extremelyshort mix times are not required since mostof the colloids are captured by becomingenmeshed in the growing precipitate. Foralum, dosages above about 30 mg/L willproduce sweep coagulation. Lower dosagesresult in a combination of sweep andcharge neutralization or just charge neu-tralization, depending on the pH and thealum dose. Clearly, the choice of mixingregimes is not easy.

Mixing IntensityThe type and intensity of agitation is alsoimportant. Turbulent mixing, character-ized by high velocity gradients, is desirablein order to provide sufficient energy forinterparticle collisions. Propeller mixersare not as well suited as turbine mixers.Propellers put more energy into circulatingthe basin contents, while turbine mixersshear the water, inducing the highervelocity gradients and the small highmultidirectional velocity currents thatpromote particle collisions.

High intensity throughout the rapid mixbasin may not be as important as the scaleand intensity of turbulence at the point ofcoagulant addition. In a turbine-typecomplete-mix basin, the mixing intensitywill not be uniform throughout the basin.The impeller discharge zone occupiesabout 10 per cent of the total volume andhas a shear intensity approximately 2.5times the average.

There is an upper limit to mixing intensitybecause high shear conditions can breakup microflocs and delay or prevent visiblefloc formation. Coagulants which actthrough charge neutralization can some-times recover from this during flocculation(examples include alum and low molecularweight cationic polyelectrolytes). Bridging-type flocculants are not as resilient andmay not recover. This may be because thelong polymer chains are cut, breaking theirparticle-to-particle bridges. The charge onthe polymer fragments may even interferewith attempts to re-coagulate the system,although a polymer of opposite charge mayhelp.

35

Rules of Thumb for Rapid MixingThe purpose of rapid mixing is two-fold.First, it should efficiently disperse thecoagulating chemicals. Second, and just asimportant, it should provide conditions thatfavor the formation of microfloc particles.The characteristics of this microfloc willdetermine the success of the flocculationstep.

It is difficult to offer hard and fast rules forrapid mixers. In fact, many rapid mixalternatives provide satisfactory results.

Back-mix reactors generally work best ifthey follow these guidelines:• Stators and square tanks funnel more

energy into mixing than into circulation.• Flat blade turbines provide more shear

intensity than propeller types.• Chemicals should be introduced at the

agitator blade level.• For sweep coagulation using alum or

ferric compounds, G-values of between300 and 1200 sec-1 in combination withretention times of 20 seconds to severalminutes have produced satisfactoryresults.

• When cationic polyelectrolytes are usedas a primary coagulant, then a G-valuerange of 400-700 is more appropriate.Overly long retention times (longer than1-2 minutes), rapid changes in G value,and G values above 1000 can all bedetrimental.

• Consider tapered velocity gradients toprovide a transition from rapid mixing toflocculation. This may be important ifthe primary coagulant is a polymer.

Plug flow reactors may be more appropri-ate if charge neutralization predominatesover sweep floc. G-values of 3000-5000with retention times of about 1/2 secondare usually recommended.

G-ValueThe G-value concept is a rough approxima-tion of mixing intensity. It is based uponinput power, basin volume and viscosity.For water, at 10OC, the G value can beexpressed in terms of horsepower per 1000gallons of basin volume:

G = 387 x (HP per 1000 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 beused 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

Inp

ut

Ho

rsep

ow

er p

er M

GD

Retention Time - seconds30 60 90

1

2

3

4

5

G =

100

0G

= 8

00

G = 600

G = 400

G = 200

36

Chapter 6 Tips on Mixing

FlocculationRate of FlocculationIn general the rate of flocculation is con-trolled by two factors:• concentration of particles (both free

colloids and floc)• velocity gradient

A greater number of particles provides moreopportunity for collisions to occur betweencolloids or between floc particles and col-loids. This increases the rate of capture.Higher velocity gradients also provide moreopportunities for collisions, but the shearforces from too high a velocity gradient canbreak up larger flocs and will limit themaximum floc size.

Creating CollisionsThe velocity gradients in a full scale floccu-lation basin can be created by a variety ofmechanisms, including baffled chambers,rotating paddles, reciprocating blades andturbine-type mixers. Baffled chambers arelimited because the velocity gradient iscontrolled only by the flow rate, whilemechanical systems offer the ability toindependently control shear intensity.

The efficiency of a mechanical systemshould be judged by its ability to produce auniform distribution of eddy currentsthroughout the basin. Some of the energyin a mechanical flocculator will unavoidablygo into rotating the water rather thancreating velocity gradients. This is particu-larly true in horizontally rotating units withlarge paddles. Stators can be used todampen this tendency. A wire-mesh-typepaddle provides more shear with less rota-tion of the basin contents than a solidpaddle. Turbine-type mixers suffer from alarge variation in velocity gradients whichare high near the mixer and low near thebasin walls.

Tapered FlocculationYou can reduce the time required for com-plete flocculation by using a number offlocculation basins in series. If possibleeach basin should approximate plug flowconditions. The advantage of several basinsis two-fold. First, the overall flow patternbegins to approximate plug flow, even ifeach basin is well mixed. In other words, itis unlikely that a specific particle will spenda short time period in each basin. With plugflow, the average retention time and overallbasin volume can be reduced by more thanhalf as compared to a single well-mixedbasin.

Second, the velocity gradient can be variedfrom basin to basin. This is sometimescalled “tapered flocculation”. The purpose ofthe first one or two basins is to capturestray colloids. High shear intensities favorthis, since the high velocities maximize thenumber of collisions. High shear conditionswill also limit floc size, thereby creatingmore particles and again increasing theprobability of collision. To achieve this, theflocculator speed in the first one or twobasins should be adjusted to limit floc size,with the first basin at a higher speed thanthe second.

The third and fourth basins are the flocgrowing stages. They should produce alarge fast setting floc. Velocity gradients areprogressively lowered to avoid floc breakup.A long chain anionic polyelectrolyte can beadded here if necessary to mechanicallybridge and toughen the growing flocs.

37

Affordable Zeta PotentialWe are a small specialty company, and havebeen developing and manufacturing zetameters for over twenty five years. Ourinstruments are reasonably priced andfeature low maintenance costs. In addition,careful quality control has enabled us tomaintain an excellent reliability record.

Technical HelpWe welcome your questions regardingspecific applications of zeta potential,streaming current or streaming potential.Demonstrations can be arranged and lim-ited ZP tests can be made on your ownsamples.

We can be a technical back-up to yourengineering or operating staff, and canprovide personnel training in the operationof the Zeta-Meter and its application towater treatment. We are not experts in allareas of water treatment, but we will provideall of the support that we can.

Just tell us something about your ownplant, and we will help you evaluate thebenefits of zeta potential control. And if it'snot for you, then we will tell you so.

Order a Zeta-Meter CatalogPlease call or drop us a short note to orderour Catalog. It will help you select theoptions and accessories that best meet yourneeds and budget.

Zeta-Meter, Inc.PO Box 3008Staunton, Virginia 24402USA

Telephone................. (540) 886-3503Toll Free (USA) ......... (800) 333-0229Fax ........................... (540) 886-3728E-mail ...................... [email protected]

http://www.zeta-meter.com

About Zeta-Meter

Documents

Coagulation & Flocculation