Coagulation and Flocculation · 9 Coagulation and Flocculation 543 Term Definition Inorganic metal...

9CoagulationandFlocculation9-1 Role of Coagulation and Flocculation Processes in Water

TreatmentCoagulation ProcessFlocculation ProcessPractical Design Issues

9-2 Stability of Particles in WaterParticle–Solvent InteractionsElectrical Properties of ParticlesParticle StabilityCompression of the Electrical Double Layer

9-3 Coagulation TheoryAdsorption and Charge NeutralizationAdsorption and Interparticle BridgingPrecipitation and Enmeshment

9-4 Coagulation PracticeInorganic Metallic CoagulantsPrehydrolyzed Metal SaltsOrganic PolymersCoagulant and Flocculant AidsJar Testing for Coagulant EvaluationAlternative Techniques to Reduce Coagulant Dose

9-5 Coagulation of Dissolved ConstituentsEffects of NOM on Coagulation for Turbidity RemovalEnhanced CoagulationDetermination of Coagulant Dose for DOC RemovalRemoval of Dissolved Inorganics

9-6 Flocculation TheoryMechanisms of FlocculationParticle CollisionsFlocculation of Spherical ParticlesFractal Flocculation ModelsFloc BreakupUse of Spherical Particle Models for Reactor Design

541MWH’s Water Treatment: Principles and Design, Third Edition John C. Crittenden, R. Rhodes Trussell, David W. Hand, Kerry J. Howe and George TchobanoglousCopyright © 2012 John Wiley & Sons, Inc.

542 9 Coagulation and Flocculation

9-7 Flocculation PracticeAlternative Methods of FlocculationVertical Turbine FlocculatorsHorizontal Paddle Wheel FlocculatorsHydraulic FlocculationImportant Design Features in Flocculation

Problems and Discussion TopicsReferences

Terminology for Coagulation and Flocculation

Term Definition

Coagulation Addition of a chemical to water with the objective ofdestabilizing particles so they aggregate or forminga precipitate that will sweep particles from solutionor adsorb dissolved constituents.

Coagulant aid Chemicals (typically synthentic polymers) added towater to enhance the coagulation process.

Counterions Ions of opposite charge to the surface charge ofparticles.

Critical coagulationconcentration(CCC)

Concentration of coagulant that reduces the electricdouble layer to the point where flocculation canoccur.

Destabilization Process of eliminating the surface charge on aparticle so that flocculation can occur.

Electric double layer(EDL)

Electrostatic potential surrounding a charged particlein solution, consisting of a layer of counterionsadsorbed directly to the surface and a diffuse layerof ions forming a cloud of charge around theparticle.

Enhancedcoagulation

Coagulation process with the objective of removingnatural organic matter, typically for minimizing theformation of disinfection by-products (see Sec 9-5).

Enmeshment orsweep floc

Entrapment or capture of particles by amorphousprecipitates that form when a coagulant is added towater.

Flocculation Aggregation of destabilized particles into largermasses that are easier to remove from water thanthe original particles.

Flocculant aid Organic polymers used to enhance settleability andfilterability of floc particles.

9 Coagulation and Flocculation 543

Term Definition

Inorganic metalcoagulant

Metal salts such as aluminum sulfate and ferric chloridethat will hydrolyze, forming mononuclear andpolynuclear species of varying charge. When added inexcess, metal coagulants form chemical precipitates.

Jar test Procedure to study effect of coagulant addition towater; used to determine required doses andoperating conditions for effective coagulation andflocculation.

Stable particlesuspension

Suspension of particles that will stay in solutionindefinitely; stable particles have a surface chargethat causes them to repel each other and preventaggregation into larger particles that would settle ontheir own.

Synthetic organiccoagulant

High-molecular-weight (typically 104 to 107 g/mol)organic molecules that can carry positive (cationic),negative (anionic), or neutral (nonionic) charge.

Zeta potential Measurement of the charge at the shear plane ofparticles, used as a relative measure of particlesurface charge.

Natural surface waters contain inorganic and organic particles. Inorganicparticulate constituents, including clay, silt, and mineral oxides, typicallyenter surface water by natural erosion processes. Organic particles mayinclude viruses, bacteria, algae, protozoan cysts and oocysts, as well asdetritus litter that have fallen into the water source. In addition, surfacewaters will contain very fine colloidal and dissolved organic constituentssuch as humic acids, a product of decay and leaching of organic debris.Particulate and dissolved organic matter is often identified as naturalorganic matter (NOM).

Removal of particles is required because they can (1) reduce the clarityof water to unacceptable levels (i.e., cause turbidity) as well as impart colorto water (aesthetic reasons), (2) be infectious agents (e.g., viruses, bacteria,and protozoa), and (3) have toxic compounds adsorbed to their externalsurfaces. The removal of dissolved NOM is of importance because manyof the constituents that comprise dissolved NOM are precursors to theformation of disinfection by-products (see Chap. 19) when chlorine is usedfor disinfection. NOM can also impart color to the water.

The most common method used to remove particulate matter and a por-tion of the dissolved NOM from surface waters is by sedimentation and/orfiltration following the conditioning of the water by coagulation and floc-culation, the subject of this chapter. Thus, the purpose of this chapter is topresent the chemical and physical basis for the phenomena occurring in

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the coagulation and flocculation processes. Specific topics include (1) therole of coagulation and flocculation processes in water treatment, (2) sta-bility of particles in water, (3) coagulation theory, (4) coagulation practice,(5) coagulation of dissolved and organic constituents, (6) flocculationtheory, and (7) flocculation practice.

9-1 Role of Coagulation and Flocculation Processes in Water Treatment

The importance of the coagulation and flocculation processes in watertreatment can be appreciated by reviewing the process flow diagramsillustrated on Fig. 9-1. As used in this book, coagulation involves the additionof a chemical coagulant or coagulants for the purpose of conditioningthe suspended, colloidal, and dissolved matter for subsequent processingby flocculation or to create conditions that will allow for the subsequentremoval of particulate and dissolved matter. Flocculation is the aggregationof destabilized particles (particles from which the electrical surface chargehas been reduced) and precipitation products formed by the additionof coagulants into larger particles known as flocculant particles or, morecommonly, ‘‘floc.’’ The aggregated floc can then be removed by gravitysedimentation and/or filtration. Coagulation and flocculation can also bedifferentiated on the basis of the time required for each of the processes.Coagulation typically occurs in less than 10 s, whereas flocculation occursover a period of 20 to 45 min. An overview of the coagulation and floc-culation processes is provided below.

CoagulationProcess

The objective of the coagulation process depends on the source of thewater and the nature of the suspended, colloidal, and dissolved organic

Settledsolids

SedimentationInfluent fromsurface water

Effluent todistributionsystem

Granularfiltration

Oxidant/disinfectant

Oxidant/disinfectantPolymer

Flocculation

Bypass flocculationfor contact (in-line) filtration

Bypasssedimentation

for direct filtration

Liquid processing

Residuals processingand management Waste

washwater

CoagulantFlashmix

Figure 9-1Typical water treatment process flow diagram employing coagulation (chemical mixing) with conventional treatment, directfiltration, or contact filtration.

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9-1 Role of Coagulation and Flocculation Processes in Water Treatment 545

constituents. Coagulation by the addition of the hydrolyzing chemicals suchas alum and iron salts and/or organic polymers can involve

1. Destabilization of small suspended and colloidal particulate matter

2. Adsorption and/or reaction of portions of the colloidal and dissolvedNOM to particles

3. Creation of flocculant particles that will sweep through the waterto be treated, enmeshing small suspended, colloidal, and dissolvedmaterial as they settle

Coagulants such as alum, ferric chloride, and ferric sulfate hydrolyzerapidly when mixed with the water to be treated. As these chemicalshydrolyze, they form insoluble precipitates that destabilize particles byadsorbing to the surface of the particles and neutralizing the charge(thus reducing the repulsive forces) and/or forming bridges betweenthem. Natural or synthetic organic polyelectrolytes (polymers with multiplecharge-conferring functional groups) are also used for particle destabi-lization. Because of the many competing reactions, the theory of chemicalcoagulation is complex. Thus, the simplified reactions presented in this andother textbooks to describe the various coagulation processes can only beconsidered approximations, as the reactions may not necessarily proceedas indicated (Letterman et al., 1999).

FlocculationProcess

The purpose of flocculation is to produce particles, by means of aggrega-tion, that can be removed by subsequent particle separation proceduressuch as gravity sedimentation and/or filtration. Two general types of floc-culation can be identified: (1) microflocculation (also known as perikineticflocculation) in which particle aggregation is brought about by the ran-dom thermal motion of fluid molecules (known as Brownian motion) and(2) macroflocculation (also known as orthokinetic flocculation) in whichparticle aggregation is brought about by inducing velocity gradients andmixing in the fluid containing the particles to be flocculated. Another formof macroflocculation is brought about by differential settling in which largeparticles overtake small particles to form larger particles.

Practical DesignIssues

When it comes to the practical design of coagulation and flocculationfacilities, designers must consider four process issues: (1) the type andconcentration of coagulants and flocculant aids, (2) the mixing intensityand the method used to disperse chemicals into the water for destabilization,(3) the mixing intensity and time for flocculation, and (4) the selectionof the liquid–solid separation process (e.g., sedimentation, flotation, andgranular filtration). With the exception of sedimentation and flotation(considered in Chap. 10) and filtration (considered in Chaps. 11 and 12),these subjects are addressed in the subsequent sections of this chapter.

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9-2 Stability of Particles in Water

The particles in water may, for practical purposes, be classified as suspendedand colloidal, according to particle size. Because small suspended andcolloidal particles and dissolved constituents will not settle in a reasonableperiod of time, chemicals must be used to help remove these particles. Thephysical characteristics of particles found in water including particle size,number, distribution, and shape have been discussed previously in Chap. 2,Sec 2-3.

To appreciate the role of chemical coagulants and flocculant aids, itis important to understand particle solvent interactions and the electricalproperties of the colloidal particles found in water. These subjects alongwith the nature of particle stability and the compression of the electricaldouble layer are considered in this section.

Particle–SolventInteractions

Particles in natural water can be classified as hydrophobic (water repelling)and hydrophilic (water attracting). Hydrophobic particulates have a well-defined interface between the water and solid phases and have a low affinityfor water molecules. In addition, hydrophobic particles are thermodynam-ically unstable and will aggregate irreversibly over time.

Hydrophilic particles such as clays, metal oxides, proteins, or humicacids have polar or ionized surface functional groups. Many inorganicparticulates in natural waters, including hydrated metal oxides (iron or alu-minum oxides), silica (SiO2), and asbestos fibers, are hydrophilic becausewater molecules will bind to the polar or ionized surface functional groups(Stumm and Morgan, 1996). Many organic particulates are also hydrophilicand include a wide diversity of biocolloids (humic acids, viruses) and sus-pended living or dead microorganisms (bacteria, protozoa, algae). Becausebiocolloids can adsorb on the surfaces of inorganic particulates, the par-ticles in natural waters often exhibit heterogeneous surface properties.Some particulate suspensions such as humic or fulvic acids can be reversiblyaggregated because of their hydrogen bonding tendencies.

ElectricalPropertiesof Particles

The principal electrical property of fine particulate matter suspended inwater is surface charge, which contributes to relative stability, causingparticles to remain in suspension without aggregating for long periodsof time. The particulate suspensions are thermodynamically unstable and,given sufficient time, colloids and fine particles will flocculate and settle.However, this process is not economically feasible because it is very slow.A review of the causes of particulate stability will provide an understandingof the techniques that can be used to destabilize particulates, which arediscussed in the following section.

9-2 Stability of Particles in Water 547

HO

HO

Si

Si4+

O

O

Al3+

O

O

Si

O

O

Si

OH

OH

Silicon atom displacedby aluminum atom

Figure 9-2Charge acquisition through isomorphous substitution of Al for Si.

ORIGIN OF PARTICLE SURFACE CHARGE

Most particulates have complex surface chemistry and surface charges mayarise from several sources. Surface charge arises in four principal ways, asdiscussed below (Stumm and Morgan, 1996).

Isomorphous replacement (crystal imperfections)Under geological conditions, metals in metal oxide minerals can bereplaced by metal atoms with lower valence, and this will impart a neg-ative charge to the crystal material. An example where an aluminum atomreplaced a silicon atom in a clay particle is shown on Fig. 9-2. This process,known as isomorphous replacement, produces negative charges on thesurface of clay particles (van Olphen, 1963).

Structural imperfectionsIn clay and similar mineral particles, imperfections that occur in theformation of the crystal and broken bonds on the crystal edge can lead tothe development of surface charges.

Preferential adsorption of specific ionsParticles adsorb NOM (e.g., fulvic acid), and these large macromoleculestypically have a negative charge because they contain carboxylic acidgroups:

R − COOH � R − COO− + H+ (pKa = 4 to 5) (9-1)Consequently, particle surfaces that have adsorbed NOM will be negativelycharged for pH values greater than ∼5.

Ionization of inorganic groups on particulate surfacesMany mineral surfaces contain surface functional groups (e.g., hydroxyl)and their charge depends on pH. For example, silica has hydroxyl groupson its exterior surface, and these can accept or donate protons as shownhere:

Si − OH+2 � Si − OH + H+ � Si − O− + 2H+

pH � 2 pH = 2 pH � 2 (9-2)


pH

0

2 4 6 8 10 12

Par

ticle

surf

ace

char

ge

Silica

Alumina+ψ0

−ψ0

Figure 9-3Variation in particle charge with pH.

The zero point of charge, as shown on Fig. 9-3, forsilica is at pH 2, whereas the zero point of chargefor alumina is about pH 9. The pH correspondingto a surface charge of zero is defined as the zeropoint of charge (ZPC). Above the ZPC the surfacecharge will be negative (anionic), and below theZPC the charge will be positive (cationic). TheZPC for other particles that commonly occur inwater are listed in Table 9-1. When examiningTable 9-1, it is important to realize that many ofthe measurements that are reported are in low-ionic-strength waters (i.e., distilled water); conse-quently, the reported pHzpc values are higher thanis observed in natural waters.

Table 9-1Surface characteristics of inorganic and organic particulates commonlyfound in natural waters

Zero Point of Charge,Type of Particle pHzpc

InorganicAl(OH)3 (amorphous) 7.5–8.5Al2O3 9.1CuO3 9.5Fe(OH)3 (amorphous) 8.5MgO 12.4MnO2 2–4.5SiO2 2–3.5Clays

Kaolinite 3.3–4.6Montmorillonite 2.5

AsbestosChrysotile 10–12Crocidolite 5–6

CaCO3 8–9Ca5(PO4)3OH 6–7FePO4 3AlPO4 4

OrganicAlgae 3–5Bacteria 2–4Humic acid 3Oil droplets 2–5

Source: From Parks (1967) and Stumm and Morgan (1981).


ELECTRICAL DOUBLE LAYER

In natural waters, negatively charged particulates accumulate positive coun-terions on and near the particle’s surface to satisfy electroneutrality. Asshown on Fig. 9-4, a layer of cations will bind tightly to the surface of anegatively charged particle to form a fixed adsorption layer. This adsorbedlayer of cations, bound to the particle surface by electrostatic and adsorp-tion forces, is about 5 Å thick and is known as the Helmholtz layer (alsoknown as the Stern layer after Stern, who proposed the model shown onFig. 9-4). Beyond the Helmholtz layer, a net negative charge and electricfield is present that attracts an excess of cations (over the bulk solutionconcentration) and repels anions, neither of which are in a fixed position.These cations and anions move about under the influence of diffusion(caused by collisions with solvent molecules), and the excess concentrationof cations extends out into solution until all the surface charge and electricpotential is eliminated and electroneutrality is satisfied.

Approximate shearlayer measured byelectrophoresis

Fixed charge(Stern) layer

Negatively chargedparticle surface

Diffuseion layer

Ions in equilibriumwith bulk solution

Negative ion

Positivecounterion

Ele

ctro

stat

ic p

oten

tial,

mV

Double layer

Distance from particle surface, A

0

−ψ0

−ψm

−ψζ

κ−1

Nernstpotential

Zeta(Helmholtz)

potential

Zetameasuredpotential

Figure 9-4Structure of the electricaldouble layer. The potentialmeasured at the shear planeis known as the zeta potential.The shear plane typicallyoccurs in diffuse layer.

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The layer of cations and anions that extends from the Helmholtz layer tothe bulk solution where the charge is zero and electroneutrality is satisfiedis known as the diffuse layer. Taken together the adsorbed (Helmholtz)and diffuse layer are known as the electric double layer (EDL). Dependingon the solution characteristics, the EDL can extend up to 300 Å into thesolution (Kruyt, 1952). It is interesting to note that the double-layer modelproposed by Stern (see Fig. 9-4) is a combination of the earlier modelsproposed by Helmholtz–Perrin and Gouy–Chapman. In fact, the diffuselayer is often identified as the Gouy–Chapman diffuse layer (Voyutsky, 1978).

MEASUREMENT OF SURFACE CHARGE

The electrical properties of highly dispersed particle systems having a soliddispersed phase and a liquid dispersion medium can be defined in terms offour phenomena: (1) electrophoresis, (2) electroosmosis, (3) sedimentation poten-tial (also known as the Dorn effect), and (4) streaming potential . Collectivelythese four phenomena, described in Table 9-2, are known as electrokineticphenomena because they involve the movement of particles (or a liquid)when a potential gradient is applied or the formation of the potential

Table 9-2Description and application of electrochemical phenomena

Phenomena Description Application in Water Treatment

Electrophoresis,discovered by R. Reuss,circa 1808

Refers to the movement of charged particlesrelative to a stationary liquid subject to anapplied electrical field. The particles movealong the lines of the electrical field.

Used to assess the destabilizationof particles subject to the additionof coagulant chemicals. Also usedin laboratory studies to isolate newproteins and other organicmolecules.

Electroosmosis,discovered by R. Reuss,circa 1808

Refers to the movement of liquid relative toa stationary charged surface (e.g., a porousplug) subject to an applied electrical field.

Streaming potential,discovered by G.Quincke, circa 1859

Refers to the creation of a potential gradientwhen liquid is made to flow along astationary charged surface (e.g., whenforced through a porous plug). The chargesfrom the particles are carried along with thefluid.

Used to assess the destabilizationof particles subject to the additionof coagulant chemicals. Onlineinstruments are now available thatcan be used to control chemicaladdition in water treatment.

Sedimentation potential,discovered by Dorn,circa 1878

Refers to the creation of a potential gradientwhen charged particles move (e.g., settling)relative to a stationary liquid medium.Sedimentation potential is the opposite ofelectrophoresis.

Source: Adapted from Voyutsky (1978) and Shaw (1966).

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gradient when particles (or liquid) move. It should be noted that theseaforementioned electrical phenomena are caused by the opposite chargeof the particle (solid) and liquid. Although there is no direct measure ofthe electrical field surrounding a particle or method to determine whenparticles have been destabilized from the addition of coagulants, the sur-face charge on a particle can be measured indirectly using one of the fourelectrokinetic phenomena (Voyutsky, 1978).

ZETA POTENTIAL

When a charged particle is subjected to an electric field between twoelectrodes, a negatively charged particle will migrate toward the positiveelectrode, as shown on Fig. 9-5, and vice versa. This movement is termedelectrophoresis. It should be noted that when a particle moves in an electricalfield some portion of the water near the surface of the particle moves withit, which gives rise to the shear plane, as shown on Fig. 9-4. Typically, asshown on Fig. 9-4, the actual shear plane lies in the diffuse layer to the rightof the theoretical fixed shear plane defined by the Helmholtz layer. Theelectrical potential between the actual shear plane and the bulk solution iswhat is measured by electrophoretic measurements. This potential is calledthe zeta potential or the electrical potential and is given by the expression

Z = v0kzμεε0

(9-3)

where Z = zeta potential, Vv0 = electrophoretic mobility, (μm/s)/(V/cm)

= νE/EνE = electrophoretic velocity of migrating particle, μm/s (also

reported as nm/s and mm/s)

Diffuse ion cloudtravels with particle

Negativelycharged ion

Negativepole

Positivepole

Positively chargedcounterions attracted

to negative pole

Particle with high negativesurface charge moves toward

positive pole

Figure 9-5Schematic illustration ofelectrophoresis in which a chargedparticle moves in an electrical field,dragging with it a cloud of ions.

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E = electrical field at particle, V/cmkz = constant that is 4π or 6πμ = dynamic viscosity of water, N · s/m2ε = permitivity relative to a vacuum (ε for water is 78.54, unitless)

ε0 = permitivity in a vacuum, 8.854188 × 10−12 C2/J · m (notethat C2/J · m is equivalent to N/V2)

Typical values for the electrophoretic mobility for particles in naturalwaters vary from about −2 to +2 (μm/s)/(V/cm). The constant kz is usedto account for the shape of the particle. The value of 4π appears in thederivation put forth by Smoluchowski and applies if the extent of the diffuselayer is small relative to the curvature of the particle. The value of 6π isused when the particle is much smaller than the thickness of the doublelayer (Kissa, 1999).

For example, if the value of the constant is 4π and the electrical mobilityis 0.5 (μm/s)/(V/cm), the value of the zeta potential at 25◦C is 80.4 mV asgiven below:

Z = (0.5 μm·cm/s·V)(4π)(0.890×10−3N·s/m2)(1 m/106μm)(1 m/102cm)(

78.54)(

8.854188 × 10−12 C2/J · m)= 80.4 mV

Empirically, when the absolute value of the zeta potential is reduced belowapproximately 20 mV, rapid flocculation occurs (Kruyt, 1952). The zetapotential will vary with the size and shape of the particle, with the numberof charges on the particle, with the strength of the electric field, and withthe nature of the ions in the diffuse layer.

Particle Stability The stability of particles in natural waters depends on a balance between(1) the repulsive electrostatic force and (2) the attractive force known asthe van der Waals force.

REPULSIVE ELECTROSTATIC FORCES

The principal mechanism controlling the stability of hydrophobic andhydrophilic particles is electrostatic repulsion. Electrostatic repulsionoccurs, as discussed above, because particles in water have a net negativesurface charge. The magnitude of the electrostatic force will depend onthe charge of the particle and the solution characteristics.

VAN DER WAALS ATTRACTIVE FORCE

Van der Waals forces originate from magnetic and electronic resonancethat occurs when two particles approach one another. This resonance iscaused by electrons in atoms on the particle surface, which develop astrong attractive force between the particles when these electrons orientthemselves in such a way as to induce synergistic electric and magnetic

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fields. Van der Waals forces are proportional to the polarizability of theparticle surfaces. Van der Waals attractive forces (


condition 1 contains a repulsive maximum that must be overcome if theparticles are to be held together by the van der Waals force of attraction.Although floc particles can form at long distances as shown by the netenergy curve for case 1, the net force holding these particles together isweak and the floc particles that are formed can be ruptured easily. In case2, there is no energy barrier to overcome. Clearly, if colloidal particles areto be flocculated by microflocculation, the repulsive force must be reducedas shown in case 2. With the addition of a coagulant, which reduces theextent of the electrical double layer, rapid flocculation can occur.

Compressionof the ElectricalDouble Layer

It has been observed that, if the electrical double layer is compressed,particles in water will come together as a result of Brownian motion andremain attached due to van der Waals forces of attraction, as discussedabove. As the ionic strength of a solution is increased, the extent ofthe double layer decreases, which in turn reduces the zeta potential. Thethickness of the double layer and the effects of ionic strength and electrolyteaddition on the compression of the double layer are described below.

DOUBLE-LAYER THICKNESS

The thickness of the electrical diffuse layer as a function of the ionicstrength and electrolyte is given in Table 9-3. The thickness of the diffuselayer may be calculated using the following equation (Gouy, 1910):

κ−1 = 1010[(2) (1000) e2NAI

εε0 kT

]−1/2(9-4)

where κ−1 = double-layer thickness, Å1010 = length conversion, Å /m1000 = volume conversion, L/m3

Table 9-3Thickness of electrical double layer (EDL) as function of ionic strength andvalence at 25◦C

Molarity z+ : z− I, mol/L κ, cm−1 1/κ, Å

0.001 1:1 0.001 1.04 × 106 96.22:2 0.004 2.08 × 106 48.13:3 0.009 3.12 × 106 32.1

0.01 1:1 0.01 3.29 × 106 30.42:2 0.04 6.57 × 106 15.23:3 0.09 9.86 × 106 10.1

0.1 1:1 0.1 1.04 × 107 9.62:2 0.4 2.08 × 107 4.83:3 0.9 3.12 × 107 3.2

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e = electron charge, 1.60219 × 10−19 CNA = Avagadro’s number, 6.02205 × 1023/mol

I = ionic strength, 12∑

z2M , mol/Lz = magnitude of positive or negative charge on ion

M = molar concentration of cationic or anionic species, mol/Lε = permittivity relative to a vacuum (ε for water is 78.54,

unitless)ε0 = permittivity in a vacuum, 8.854188 × 10−12 C2/J · mk = Boltzmann constant, 1.38066 × 10−23 J/K

T = absolute temperature, K (273 + ◦C)The relationship given in Eq. 9-4 is not actually the double-layer thicknessbut is related to how far out into the solution the repulsive force will reach.It is approximately equal to the distance at which the electrical potential is37 percent of the value at the particle surface. However, it is still importantto know the EDL thickness because it provides insight into the particlestability and the coagulation process.

EFFECT OF IONIC STRENGTH

Of the many factors that affect double-layer thickness, ionic strength isperhaps the most important. As reported in Table 9-3, the EDL thicknessshrinks dramatically with increasing ionic strength and valance. Accordingto the DLVO theory, van der Waals forces extend out into solution about10 Å; consequently, if the double layer is smaller than this, a rapidly floc-culating suspension is formed. While it is possible to reduce the thicknessof the EDL by increasing the ionic strength, this is not a practical methodfor destabilizing particles in drinking water treatment because the requiredionic strengths are greater than are considered acceptable in potable water.It is interesting to note that ionic strength can be used to explain whyparticles are stable in freshwater (low ionic strength but high electricalrepulsive forces) and flocculate rapidly in salt water (high ionic strengthbut low electrical repulsive forces). Determination of the thickness of thedouble layer as function of the ionic strength is illustrated in Example 9-1.

EFFECT OF COUNTERIONS

If the charge on the counterions in solution is altered, the thickness of theEDL will be reduced, as illustrated in Table 9-3. The ionic concentrationthat results in the reduction of the EDL to the point where flocculationoccurs is defined as the critical coagulation concentration (CCC) and willdepend on the type of particulate as well as the dissolved ions. Accordingto the DLVO theory, the CCC is inversely proportional to the sixth powerof the charge on the ion. Thus, the CCC values for mono-, di-, and trivalentions are in the ratio of 1: 12

6: 13

6, or 100: 1.6: 0.14 percent, assuming that the

electrolytes do not adsorb or precipitate. The above relationship is knownas the Schultz–Hardy rule, which was originally observed in the 1880s

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Example 9-1 Determination of thickness of electricaldouble layer

Verify that the values in Table 9-3 are correct for 0.001 M solutions ofmonovalent and divalent ions using Eq. 9-4.

Solution1. Determine the ionic strength I for a molarity of 0.001 for chemical

constituents with a charge of 1 and 2.a. Determine the ionic strength for Z = +1 and −1:

1 = 12

∑Z2M = 1

2

∑ (+1)2 (0.001) + (−1)2 (0.001) = 0.001 mol/Lb. Determine the ionic strength for Z = +2 and −2:

1 = 12

∑Z2M = 1

2

∑ (+2)2 (0.001) + (−2)2 (0.001) = 0.004 mol/L2. Substitute known terms in Eq. 9-4 and solve for 1/κ:

a. For M = 0.001, Z = +1, −1, and I = 0.001 mol/L,κ−1 =

(1010 Å/m

)

×

⎡⎢⎣ (2)

(1000 L/m3

)(1.60219 × 10−19 C

)2(6.02205 × 1023 mol−1

)(0.001 mol/L

)(78.54)

(8.854188 × 10−12 C2/J · m

)(1.38066 × 10−23 J/K)(273 + 25 K)

⎤⎥⎦

−1/2

= 96.2 Åb. For M = 0.001, Z = +2, −2, and I = 0.004 mol/L,

κ−1 =(1010 Å/m

)

×

⎡⎢⎣ (2)

(1000 L/m3

)(1.60219 × 10−19 C

)2(6.02205 × 1023 mol−1

)(0.004 mol/L

)(78.54)

(8.854188 × 10−12 C2/J · m

)(1.38066 × 10−23 J/K)(273 + 25 K)

⎤⎥⎦

−1/2

= 48.1 Å

Comment

The above computation illustrates the importance of the charge of the ionicspecies, as reported in Table 9-3.

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9-3 Coagulation Theory 557

Kruyt, 1952). Thus, if 3000 mg/L of NaCl will produce rapid flocculation ofhydrophobic particulates, then 47 mg/L of CaCl2 will achieve similar results.It should also be noted that if multivalent ions comprise the fixed layernext to a negatively charged particle, the EDL will be reduced significantlyand the CCC value would be much lower than predicted by the theory (forthe Schultz–Hardy rule).

9-3 Coagulation Theory

The electrical properties of particles were considered in the previoussection. Coagulation, as described in Sec. 9-1, is the process used to destabi-lize the particles found in waters so that they may be removed by subsequentseparation processes. The purpose of this section is to introduce the prin-cipal coagulation mechanisms responsible for particle destabilization andremoval. Coagulation practice including the principal chemicals used forcoagulation in water treatment and jar testing is presented and discussedin Sec. 9-4.

Mechanisms that can be exploited to achieve particulate destabilizationinclude (1) compression of the electrical double layer, (2) adsorptionand charge neutralization, (3) adsorption and interparticle bridging, and(4) enmeshment in a precipitate, or ‘‘sweep floc.’’ While these mechanismsare discussed separately here, it will become apparent that each onehas certain pitfalls, and this is the reason that destabilization strategiesexploit several mechanisms simultaneously. It should also be noted thatcompression of the electrical double layer, discussed in the previous section,is also considered a coagulation mechanism but is not discussed herebecause increasing the ionic strength is not practiced in water treatment.

Adsorptionand Charge

Neutralization

Particulates can be destabilized by adsorption of oppositely charged ions orpolymers. Most particulates in natural waters are negatively charged (clays,humic acids, bacteria) in the neutral pH range (pH 6 to 8); consequently,hydrolyzed metal salts, prehydrolyzed metal salts, and cationic organicpolymers can be used to destabilize particles through charge neutralization.Cationic organic polymers can be used as primary coagulants, but they aremost often used in conjunction with inorganic coagulants to form particlebridges, as discussed below. Generally, the optimum coagulant dose occurswhen the particle surface is only partially covered (less than 50 percent).Polymers of high charge density and low to moderate molecular weights(10,000 to 100,000) are believed to be adsorbed on negatively chargedparticles as a patch on the surface and do not extend much from thesurface. The optimum dose appears to increase in proportion to thesurface area concentration of the particulates.

When the proper amount of polymer has adsorbed, the charge isneutralized and the particle will flocculate. When too much polymer has

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Figure 9-7Destabilization of a kaolinite claysuspension with cationic polymer No. 4.Initial clay concentration = 73.2 mg/L.(Adapted from Black et al., 1966.)

0 0.2 0.4 0.6 0.8 1.0

Polymer dosage, mg/L

Res

idua

ltu

rbid

ity, N

TU

Ele

ctro

phor

etic

mob

ility

,(μ

m/s

)/(V

/cm

)

+2

+1

0

−2

−1

0

0.2

0.1

0.3

0.4

0.5

120

100

80

60

40

200

Pol

ymer

adso

rbed

, mg/

L

been added, the particles will attain a positive charge and become stableonce again. This phenomenon is demonstrated by the classical experimentsof Black et al. (1966), which are shown on Fig. 9-7. For polymer dosagesup to 0.7 mg/L, the electrophoretic mobility becomes more positive andthe amount adsorbed increases. Higher dosages cause charge reversal,particle stability, and a higher residual turbidity. Cationic polymers andpolyaluminum chloride (PACl) are said to exhibit stoichiometry becausea certain amount of charge exists on the particle suspension surface, andwhen the precise amount of coagulant is added, a rapidly flocculatingsuspension is created.

Adsorptionand InterparticleBridging

Polymer bridging is complex and has not been adequately describedanalytically. Schematically, polymer chains adsorb on particle surfaces atone or more sites along the polymer chain as a result of (1) coulom-bic (charge–charge) interactions, (2) dipole interaction, (3) hydrogenbonding, and (4) van der Waals forces of attraction (Hunter, 2001). Therest of the polymer may remain extended into the solution and adsorbon available surface sites of other particles, thus creating a ‘‘bridge’’between particle surfaces. If the extended polymer cannot find vacant siteson the surface of other particulates, no bridging will occur. Thus, there

9-3 Coagulation Theory 559

is an optimum degree of coverage or extent of polymer adsorption atwhich the rate of aggregation will be a maximum. Polymer bridging isan adsorption phenomenon; consequently, the optimum coagulant dosewill generally be proportional to the concentration of particulates present.Adsorption and interparticle bridging occur with nonionic polymers andhigh-molecular-weight (MW 105 to 107), low-surface-charge polymers. High-molecular-weight cationic polymers have a high charge density to neutralizesurface charge.

REACTION MECHANISMS FOR POLYMERS

A schematic of the reaction mechanisms for polymers is shown on Fig. 9-8.At the optimum dosage of polymer shown in reaction (a), the particles aredestabilized and can subsequently flocculate, as shown in reaction (b). Ifthe particle concentration is very low or if adequate mixing does not allowflocculation, then nonadsorbed ends of the polymers will eventually adsorbon the destabilized particle, causing it to restabilize, as shown in reaction(c). If too much polymer is added, all adsorption sites will be taken up andthe particle will not flocculate, as shown in reaction (d). If the particlesare mixed for too long or too intensively, they will break up, as shown inreaction (e).

POLYMER SELECTION

Because polymer–solution interactions are complex, polymer selection isbased on empirical testing. In general, though, anionic polymers havebeen shown to be effective flocculant aids, while nonionic polymers havebeen effective as filter aids. Polymer selection for sludge conditioningis dependent on sludge properties, polymer properties, and the mixingenvironment (O’Brien and Novak, 1977). Polymer bridging is the dominantmechanism in sludge conditioning, and thus polymer molecular weight isthe dominant property of interest. For each system, the optimum polymerdose, mixing conditions, and pH must be determined empirically.

Precipitationand Enmeshment

When high enough dosages are used, aluminum and iron form insolubleprecipitates and particulate matter becomes entrapped in the amorphousprecipitates. This type of destabilization has been described as precipitationand enmeshment or sweep floc (Packham, 1965; Stumm and O’Melia, 1968).Although the molecular events leading to sweep floc have not been definedclearly, the steps for iron and aluminum salts at lower coagulant dosages areas follows: (1) hydrolysis and polymerization of metal ions, (2) adsorption ofhydrolysis products at the interface, and (3) charge neutralization. At highdosages, it is likely that nucleation of the precipitate occurs on the surfaceof particulates, leading to the growth of an amorphous precipitate withthe entrapment of particles in this amorphous structure. This mechanismpredominates in water treatment applications where pH values are generallymaintained between pH 6 and 8, and aluminum or iron salts are used at


Particle destabilizationresults from polymer

bonding

Particles and polymerflocculate due to perikinetic

and orthokinetic forces

Polymer added toparticulate suspension

at correct dosage

StableparticlesPolymer

Excessive dosageof polymer added

Insufficient mixing conditionsresults in particle restabilization

and poor floc formation

Correctmixing

Excessivemixing

Destabilizedparticles Floc

particle

Particles and polymerflocculate due to perikinetic

and orthokinetic forces

Floc ruptures dueto high or prolonged

mixing conditions

Flocparticle

(a) (b)

(c) (d)

(e)

Secondaryadsorption

Inactive particles

Flocfragments

Particlesenmeshedin polymer

matrix

Figure 9-8Schematic representation of the bridging model for the destabilization of particles by polymers. (Adapted from O’Melia, 1972.)

concentrations exceeding saturation with respect to the amorphous metalhydroxide solid that is formed.

One interesting finding regarding sweep floc is that, in general, thesweep floc mechanism does not depend on the type of particle, and,thus, the same dosage of coagulant is required for sweep floc formationregardless of the type of particles that may be present (in the absence of

9-4 Coagulation Practice 561

54

Al 2

(SO

4)3

.14H

2O, m

g/L

6 7 8 9 10

pH

0

50

100

150

200

Various claysOrganics

Legend

Figure 9-9Coagulation of various types of clays and organics,which supports hypothesis that sweep floc is notinfluenced by type of particles present: (©) clays and(�) organics. (Adapted from Packman, 1962.)

NOM). The dosage of alum required to reduce the turbidity of a variety ofparticles is displayed on Fig. 9-9. Although the dosage does not depend onthe type of particles, it does depend on the pH, as expected. However, acaveat that should be mentioned is that the coagulant demand exerted byNOM is not reflected on Fig. 9-9. Thus, the concentration of hydrolyzingmetal salts that is required for sweep floc will depend on the type andconcentration of NOM, which unfortunately is site specific. The effects ofNOM on coagulation practice are considered in Sec. 9-5.

9-4 Coagulation Practice

Selection of the type and dose of coagulant depends on the characteristicsof the coagulant, the concentration and type of particulates, concentrationand characteristics of NOM, water temperature, and water quality. Atpresent, the interdependence of these five elements is only understoodqualitatively, and prediction of the optimum coagulant combination fromcharacteristics of the particulates and the water quality is not yet possible.The purpose of this section is to introduce coagulation practice, includingthe types of inorganic and organic coagulants and coagulant aids used, andalternative techniques used to reduce coagulant dosages.

Inorganic MetallicCoagulants

Inorganic coagulants, coagulant aids, and the chemicals used for alkalinityand pH adjustment are summarized in Table 9-4. Of the chemicals listedin Table 9-4, the principal inorganic coagulants used in water treatmentare salts of aluminum and ferric ions and prehydrolyzed salts of thesemetals. These hydrolyzable metal cations are readily available as sulfate or


Table 9-4Common inorganic coagulants, coagulant aids, and pH and alkalinity adjusting chemicals used inwater treatment

MolecularClassification Chemical Formula Weight, g/mol Application

CoagulantsAluminum sulfate Al2(SO4)3 · 14H2O 594.4 Primary coagulantSodium aluminate Na2Al2O4 163.9 Used with alum; provides

alkalinity and pH controlAluminum chloride AlCl3 160.3 Used in blends with organic

polymersPolyaluminum chloride(PACl)a

Ala(OH)b(Cl)c(SO4)d Variable Primary coagulant

Polyaluminum sulfate (PAS)b Ala(OH)b(Cl)c(SO4)d Variable Primary coagulant, producedonsite

Polyiron chloridec Fea(OH)b(Cl)c(SO4)d Variable Primary coagulant, producedonsite

Ferric chloride FeCl3 162.2 Primary coagulantFerric sulfate Fe2(SO4)3 400.0 Primary coagulant

Coagulant aidsActivated silica SiO2 60.0 Coagulant aid used with alum

during cold winter monthsSodium silicate Na2O(SiO2)3−25 242–1562 Coagulant aid, produced onsiteBentonite Al2Si2O5(OH)4 258 Used to provide nucleation

sites for enhanced coagulation

Alkalinity and pH adjustmentCalcium hydroxide Ca(OH)2 56.1 as CaO Used to provide alkalinity and

adjust pHSodium hydroxide NaOH 40.0 Used to provide alkalinity and

adjust pHSoda ash Na2CO3 106.0 Used to provide alkalinity and

adjust pH

aPrehydrolyzed metal salts made from aluminum chloride.bPrehydrolyzed metal salts made from aluminum sulfate.cPrehydrolyzed metal salts made from iron chloride.

chloride salts in both liquid and solid (dry) forms. In the United States, thepredominant water treatment coagulant is aluminum sulfate, or ‘‘alum,’’sold in a hydrated form as Al2(SO4)3 · xH2O, where x is usually 14 becauseit is the least expensive coagulant. The action, solubility, and application ofthese coagulants are considered in the following discussion.

ACTION OF ALUM AND IRON SALTS

When ferric or aluminum ions are added to water, a number of paralleland sequential reactions occur. Initially, when a salt of Al(III) and Fe(III)

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is added to water, it will dissociate to yield trivalent Al3+ and Fe3+ ions, asgiven below:

Al2 (SO4)3 � 2Al3+ + 3SO42− (9-5)FeCl3 � Fe3+ + 3Cl− (9-6)

The trivalent ions of Al3+ and Fe3+ then hydrate to form the aquometalcomplexes Al(H2O)63+ and Fe(H2O)63+, as shown on the left-hand side ofEq. 9-7. As shown, the metal ion (aluminum in this case) has a coordinationnumber of 6 and six water molecules orient themselves around themetal ion:⎡

⎣H2O OH2H2O − Al − OH2H2O OH2

⎤⎦

3+

�

⎡⎣H2O OHH2O − Al − OH2

H2O OH2

⎤⎦

2+

+ H+ (9-7)

These aquometal complexes then pass through a series of hydrolytic reac-tions, as illustrated on the right-hand side of Eq. 9-8, which give riseto the formation of a variety of soluble mononuclear (one aluminumion) and polynuclear (several aluminum ions) species, as illustrated onFig. 9-10. The mononuclear species—Al(H2O)5(OH)2+ [or just Al(OH)2+]and Al(H2O)4(OH)2+ [or just Al(OH)2+]—are among the many speciesformed. Similarly, iron forms a variety of soluble species, including mononu-clear species (one iron ion) such as Fe(H2O)5(OH)2+ [or just Fe(OH)2+]and Fe(H2O)4(OH)2+ [or just Fe(OH)2+].

Al(H2O)63+

Al(OH)(H2O)52+

Al(OH)3(s)

Al(OH)4−

Hydrogen ion

Hydrogen ion

Hydrogen ion

Hydrogen ion

Aquo Al ion

Mononuclear species

Polynuclear species

Precipitate

Aluminate ion

Al13O4(OH)247+

Figure 9-10Aluminum hydrolysis products. The dashed lines are used todenote an unknown sequence of reactions. (Adapted fromLetterman, 1981)


Polynuclear species such as Al18(OH)204+ form via hydroxyl bridges. Forexample, a hydroxyl bridge for two aluminum atoms is shown below:

2 Al(H2O)5(OH)2+ [(H2O)4

OH

Al Al

OH

(H2O)4]4+ + 2H2O (9-8)

It should be noted that all of these mononuclear and polynuclear speciescan interact with the particles in water, depending on the characteristics ofthe water and the number of particles. Unfortunately, it is difficult to controland know which mononuclear and polynuclear species are operative. Aswill be discussed later, this uncertainty gave rise to the development ofprehydrolyzed metal salt coagulants.

SOLUBILITY OF METAL SALTS

The solubility of the various alum [Al(III)] and iron [Fe(III)] species areillustrated on Figs. 9-11a and 9-11b, respectively, in which the log molar con-centrations have been plotted versus pH. The equilibrium diagrams shownon Figs. 9-11a and 9-11b were created using equilibrium constants for themajor hydrolysis reactions that have been estimated after approximately 1 hof reaction time (upper limit of coagulation/flocculation detention times).Accordingly, the composition of aluminum and iron species in contact withthe freshly precipitated hydroxide (amorphous) is illustrated on Figs. 9-11aand 9-11b. In preparing these diagrams, only the mononuclear species for

−8

−7

−6

−5

−4

−3

−2

1

10

100300

30

3

0.3

0 2 4 6 8 10 12 14

log[

Al],

mol

/L

pH of mixed solution pH of mixed solution

−12

−10

−8

−6

−4

−2

0

0.27

2.727270

0 2 4 6 8 10 12 14

log[

Fe]

, mol

/L

(a) (b)

Fer

ric a

s F

eCl 3

. 6 H

2O, m

g/L

Alu

m a

s A

l 2(S

O4)

3. 1

4.3

H2O

, mg/

L

Adsorptiondestabilization

Sweepcoagulation

Fe3+

Fetotal

Fe(OH)2

+

Fe(OH)4−

(AM) Fe(OH) (s)3

Fe(OH)2+

Restabilization zone(changes with colloidor surface area)

Charge neutralizationto zero zeta potentialwith Al(OH)3 (s)

Charge neutralization tozero zeta potential withAlx(OH)y

n+/Al(OH)3 (s)

Sweepcoagulation Al(OH)4

-

Al(OH)2+

Altotal

Adsorptiondestabilization

Restabil-ization zone

(boundarychanges

with colloid)

Al3+

Combination(sweep andadsorption)

Optimal sweep

(AM) Al(OH)3 (s)

Figure 9-11Solubility diagram for (a) Al(III) and (b) Fe(III) at 25◦C. Only the mononuclear species have been plotted. The metal species areassumed to be in equilibrium with the amorphous precipitated solid phase. Typical operating ranges for coagulants: (a) alumand (b) iron. (Adapted from Amirtharajah and Mills, 1982)


Table 9-5Reactions and associated equilibrium constants for aluminum and iron species in equilibriumwith amorphous aluminum hydroxide and ferric hydroxide

Acid Equilibrium Constants

Equilibrium Range, Used forReaction Constant log K Fig. 9-11

Aluminum, Al(III)Al(OH)3(s) + 3H+ → Al3+ + 3H2O Ks0 9.0–10.8 10.8Al(OH)3(s) + 2H+ → Al(OH)2+ + 2H2O Ks1 4.0–5.8 5.8Al(OH)3(s) + H+ → Al(OH)2+ + H2O Ks2 0.7–1.5 0.7Al(OH)3(s) → Al(OH)03 Ks3 −4.2 to −6.1 −6.1Al(OH)3(s) + H2O → Al(OH)4− + H+ Ks4 −7.7 to −12.5 −11.9Species not considered: Al2(OH)24+ ,

Al8(OH)204+, Al13O4(OH)247+ , Al14(OH)3210+

Iron, Fe(III)Fe(OH)3(s) + 3H+ → Fe3+ + 3H2O Ks0 3.2–4.891 3.2Fe(OH)3(s) + 2H+ → Fe(OH)2+ + 2H2O Ks1 0.91–2.701 1.0Fe(OH)3(s) + H+ → Fe(OH)2+ + H2O Ks2 −0.779 to −2.5 −2.5Fe(OH)3(s) → Fe(OH)30 Ks3 −8.709 to −12.0 −12.0Fe(OH)3(s) + H2O → Fe(OH)4− + H+ Ks4 −16.709 to −19 −18.4Species not considered: Fe2(OH)24+ ,

Fe3(OH)45+

Source: Benefield et al. (1982), McMurry and Fay (2003), Morel and Hering (1993), Nordstrom and May (1989a, b), Pankow(1991), Snoeyink and Jenkins (1980), Sawyer et al. (2002), and Stumm and Morgan (1981).

alum and iron have been plotted. The various mononuclear species foralum and iron are given in Table 9-5, along with the corresponding rangeof acid solubility products reported in the literature and the values usedto prepare Figs. 9-11a and 9-11b. The approximate total concentration ofresidual soluble alum (see Fig. 9-11a) and iron (see Fig. 9-11b) in solutionafter precipitation is identified by the solid line. Aluminum hydroxide andferric hydroxide are precipitated within the shaded areas, and polynuclearand polymeric species are formed outside of the shaded areas at higher andlower pH values. It should also be noted that the structure of the precip-itated iron is far more compact and inert as compared to the amorphousnature of precipitated aluminum.

In most water treatment applications for removal of turbidity, disinfec-tion by-product precursors (NOM), and color, the pH during coagulationranges between 6 and 8. The lower limit is imposed to prevent acceleratedcorrosion rates that occur at pH values below pH 6. The regions shown onFigs. 9-11a and 9-11b correspond to the operating pH and dosage rangesthat are normally used in water treatment when alum and iron are used


in the sweep floc mode of operation. The operating region for aluminumhydroxide precipitation is in a pH range of 5.5 to about 7.7, with minimumsolubility occurring at a pH of about 6.2 at 25◦C, and from about 5 to 8.5for iron precipitation, with minimum solubility occurring at a pH of 8.0.The importance of pH in controlling the concentration of soluble metalspecies that will pass through the treatment process is illustrated on Figs.9-11a and 9-11b. The effect of temperature on the solubility products foraluminum is also illustrated on Fig. 9-11a. As shown, the point of minimumsolubility for alum shifts with temperature, which has a significant impacton the operation of water treatment plants where alum is used as thecoagulant. Comparing the solubility of alum and ferric species, the ferricspecies are more insoluble than aluminum species and are also insolubleover a wider pH range. Thus, ferric ion is often the coagulant of choiceto aid destabilization in the lime-softening process, which is carried out athigher pH values (pH 9).

STOICHIOMETRY OF METAL ION COAGULANTS

The overall stoichiometric reactions for aluminum and ferric ion in theformation of hydroxide precipitates are given by Eqs. 9-9 and 9-10. As shown,each mole of trivalent ion will produce 1 mole of the metal hydroxide and3 moles of hydrogen ions:

Al3+ + 3H2O � Al (OH)3, am↓ + 3H+ (9-9)Fe3+ + 3H2O � Fe (OH)3,am↓ + 3H+ (9-10)

The ‘‘am’’ subscripts in Eqs. 9-9 and 9-10 refer to amorphous solids(hours old), which have a much higher solubility product than crystallineprecipitates.

When alum is added to water and aluminum hydroxide precipitates, theoverall reaction is

Al2(SO4)3 · 14H2O → 2Al(OH)3↓ + 6H+ + 3SO42− + 8H2O (9-11)Similarly, the overall reactions for ferric chloride and ferric sulfate are asfollows:

Ferric chloride:

FeCl3 + 3H2O → Fe(OH)3↓ + 3H+ + 3Cl− (9-12)Ferric sulfate:

Fe2(SO4)3 · 9H2O → 2Fe(OH)3↓ + 6H+ + 3SO42− + 3H2O (9-13)After Al(OH)3 or Fe(OH)3 precipitates, the species remaining in water arethe same as if H2SO4 or HCl had been added to the water. Thus, addingalum or ferric is like adding a strong acid. A strong acid will lower the pHand consume alkalinity. Alkalinity is the acid-neutralizing capacity of waterand is consumed on an equivalent basis; that is, 1 meq/L of alum or ferric

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will consume 1 meq/L of alkalinity. Since alkalinity buffers water againstchanges in pH, the change in pH following coagulant addition depends onthe initial alkalinity. If the natural alkalinity of the water is not sufficient tobuffer the pH, it may be necessary to add alkalinity to the water to keepthe pH from dropping too low. Alkalinity can be added in the form ofcaustic soda (NaOH), lime [Ca(OH)2], or soda ash (Na2CO3). In manywater plants, caustic soda is often used because it is easy to handle andthe required dosage is relatively small. The reaction for alum with causticsoda is

Al2 (SO4)3 · 14H2O + 6NaOH → 2Al (OH)3,am↓ + 3Na2SO4 + 14H2O(9-14)

The corresponding reaction for lime is given by the expression

Al2 (SO4)3 · 14H2O + 3Ca (OH)2 → 2Al (OH)3,am↓ + 3CaSO4 + 14H2O(9-15)

Coagulants are typically purchased in a concentrated liquid form. Cal-culating coagulant doses can be confusing because the stock chemicalconcentration is often reported in percent by weight and the density ofthe stock solution will be significantly heavier than water. In addition, theextent of hydration of the alum or ferric will vary or be unknown in the stocksolution, which affects the formula weight of the chemical. To get aroundthis issue, chemical manufacturers will sometimes report the concentrationof the coagulant as a different formula entirely, for example, stock alumconcentration is often reported as percent as Al2O3, even though the chem-ical present is Al2(SO4)3 · xH2O. Ferric chloride may be reported with orwithout waters of hydration (i.e., FeCl3 · 6H2O or FeCl3) or as soluble iron(Fe3+). To calculate doses accurately, the density and chemical formulaused by the chemical manufacturer to report the concentration must beknown. The application of these principles and the above equations is illus-trated in Example 9-2. Note that the sludge produced during coagulationconsists of both the precipitate formed in the reactions shown above andthe solids that were present in the source water. Example 21-2 in Chap. 21demonstrates the procedure for calculating the amount of sludge producedconsidering both components.

Example 9-2 Calculation of coagulant doses, alkalinityconsumption, and precipitate formation

A chemical supplier reports the concentration of stock alum chemical as8.37 percent as Al2O3 with a specific gravity of 1.32. For the stock chemical,calculate (a) the molarity of Al3+ and (b) the alum concentration if reportedas g/L Al2(SO4)3 · 14H2O. Also, for a 30-mg/L alum dose applied to a

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treatment plant with a capacity of 43,200 m3/d (0.5 m3/s), calculate (c) thechemical feed rate in L/min, (d) the alkalinity consumed (expressed as mg/Las CaCO3), (e) the amount of precipitate produced in mg/L and kg/day,and (f) the amount of NaOH that would need to be added to counteract theconsumption of alkalinity by alum.

Solution1. Calculate the formula weights (FW) for Al2O3, Al2(SO4)3 · 14H2O,

Al(OH)3, and NaOH, given molecular weights: Al = 27, O = 16, H = 1,S = 32, and Na = 23 g/mol.FW: Al2O3 = 2(27) + 3(16) = 102 g/molFW: Al2(SO4)3·14H2O = 2(27) + 3(32) + 26(16) + 28(1) = 594 g/molFW: Al(OH)3 = 27 + 3(16) + 3(1) = 78 g/molFW: NaOH = 23 + 16 + 1 = 40 g/mol

2. Calculate the molar concentration of Al3+ in the stock alum chemical.a. Calculate the density of stock chemical:

ρstock = 1.32(1 kg/L

) = 1.32 kg/Lb. Calculate the concentration of alum in the stock chemical as mg/L

Al2O3:

Cstock = 0.0837(1.32 kg/L

) (103 g/kg

)= 110.5 g/L Al2O3

c. Calculate the molar concentration of Al3+ in the stock alumchemical:[

Al3+]

= 110.5 g/L Al2O3(

mol Al2O3102 g Al2O3

)(2 mol Al3+

mol Al2O3

)= 2.17 mol/L

3. Calculate the stock alum concentration if reported as g/L Al2(SO4)3 ·14H2O.

Cstock = 110.5 g/L Al2O3(

594 g/mol alum102 g/mol Al2O3

)= 643.5 g/L alum

4. Calculate the chemical feed rate.By mass balance:

CstockQfeed = CprocessQprocess

Qfeed =CprocessQprocess

Cstock=

(30 mg/L

) (43,200 m3/d

) (103 L/m3

)643.5 g/L

(103 mg/g

) (1440 min/d

) = 1.40 L/min


5. Calculate the alkalinity consumed using Eq. 9-11:

Alk = [30 mg/L alum] ( 1 mmol alum594 mg alum

) (3 mmol SO42−

mmol alum

)(2 meq SO42−

mmol SO42−

)

×(

1 meq alkmeq SO42−

) (50 mg CaCO3

meq alk

)= 15 mg/L as CaCO3

6. Calculate the precipitate formed using Eq. 9-11:[Al(OH)3

] = [30 mg/L alum] ( 1 mmol alum594 mg alum

) [2 mmol Al(OH)3

mmol alum

] [78 mg Al(OH)3mmol Al(OH)3

]

= 7.88 mg/L Al(OH)3

[Al(OH)3

] =(7.88 mg/L

) (43,200 m3/d

) (103 L/m3

)(106 mg/kg

) = 340 kg/d7. Calculate the NaOH dose required to counteract the alkalinity con-

sumption using Eq. 9-14:[NaOH

] = [30 mg/L alum] ( 1 mmol alum594 mg alum

) (6 mmol NaOH

mmol alum

) (40 mg NaOHmmol NaOH

)

= 12.1 mg/L NaOH

CommentThe sludge produced by coagulation has two components the precipitateformed by the reactions shown above and the particles from the raw water.Calculation of the total amount of sludge produced during coagulationconsidering both components is illustrated in Example 21-2.

APPLICATION OF METAL SALTS IN WATER TREATMENT

Because of the sequence of reactions that follow the addition of alum oriron salts, as discussed above and illustrated on Fig. 9-10, it is not possible topredict a priori the performance of a coagulation process. Consequently,jar testing is typically used for coagulant/coagulant aid screening, andthese results must be evaluated in the full-scale operation. Nevertheless, itis useful to review some general aspects of coagulation practice, including(1) the operating regions for the alum and iron, (2) interactions with otherconstituents in water, (3) typical dosages, and (4) the importance of initialblending when using metal salts. As noted in Chap. 6, blending is a mixingprocess to combine two liquid streams to achieve a specified level of unifor-mity. Guidance on the use of coagulants is provided in Table 9-6. Additionaleffects of NOM on the coagulation process are considered in Sec. 9-5.

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Tabl

e9-

6Ap

plic

atio

ngu

idan

cefo

rAl

(III)

and

Fe(II

I)as

coag

ulan

tsan

dpr

ehyd

roly

zed

met

alco

agul

ants

used

inw

ater

trea

tmen

t

Wat

erQ

ualit

yC

oagu

lant

Para

met

erAl

um(II

I)Fe

(III)

PAC

I

Turb

idity

For

low

-turb

idity

wat

ers

(i.e.

,low

part

icle

conc

entr

atio

n),s

wee

pflo

cw

illbe

requ

ired.

For

low

-turb

idity

wat

ers

(i.e.

,lo

wpa

rtic

leco

ncen

trat

ion)

,sw

eep

floc

will

bere

quire

d.

For

low

-turb

idity

wat

ers

(i.e.

,low

part

icle

conc

entr

atio

n),s

wee

pflo

cw

illbe

requ

ired.

Med

ium

-bas

icity

PACl

s(4

0–5

0%)a

resu

itabl

efo

rco

ldw

ater

sw

ithlo

wtu

rbid

ity.

Alka

linity

High

alka

linity

valu

esm

ake

pHad

just

men

tfor

optim

umco

agul

atio

nm

ore

diffi

cult.

Ifsu

ffici

enta

lkal

inity

isno

tpre

sent

,sol

uble

alum

inum

isfo

rmed

,whi

chca

nre

sult

inpo

stflo

ccul

atio

nin

dow

nstr

eam

proc

esse

s.Su

pple

men

tala

lkal

inity

shou

ldbe

adde

dbe

fore

coag

ulan

t.

Alth

ough

high

alka

linity

valu

esm

ake

pHad

just

men

tfor

optim

umco

agul

atio

nm

ore

diffi

cult,

itsim

pact

onco

agul

atio

nus

ing

Feis

less

than

Al.

pHTh

eop

timum

pHra

nge

isbe

twee

n5.

5an

d7.

7bu

twill

fluct

uate

seas

onal

ly(s

eeFi

g.9-

11).

Typi

cally

,the

optim

umpH

will

bene

arer

6in

the

sum

mer

and

7in

the

cold

erw

inte

rm

onth

s.Hi

gher

pHle

vels

ofte

nco

rres

pond

tope

riods

ofal

galg

row

th,w

hich

intu

rnw

illaf

fect

the

coag

ulan

tdos

e.

The

optim

umpH

rang

eis

from

5to

8.5

orm

ore

(see

Fig.

9-11

).

PACl

sar

ele

ssse

nsiti

veto

pH.C

anbe

used

over

the

pHra

nge

of4.

5–9

.5.

570

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NO

MTh

ere

mov

alof

NO

Mw

illno

rmal

lyco

ntro

lthe

coag

ulan

tdos

e.Re

mov

alof

NO

Mte

nds

toin

crea

seas

pHis

redu

ced.

Rem

oval

ofup

to70

%ha

sbe

enac

hiev

ed.

The

rem

oval

ofN

OM

will

norm

ally

cont

rolt

heco

agul

ant

dose

.Rem

oval

ofN

OM

tend

sto

incr

ease

aspH

isre

duce

d.Re

mov

alof

upto

80%

has

been

achi

eved

.

The

rem

oval

ofN

OM

will

norm

ally

cont

rolt

heco

agul

antd

ose.

Rem

oval

ofN

OM

tend

sto

incr

ease

asth

epH

isre

duce

d.Re

mov

als

ofup

to70

%ha

vebe

enac

hiev

ed.L

ow-b

asic

ityPA

Cls

(up

to20

%)a

resu

itabl

efo

rw

ater

shi

ghin

colo

ran

dto

talo

rgan

icca

rbon

.

Tem

pera

ture

Tem

pera

ture

affe

cts

solu

bilit

ypr

oduc

ts.F

loc

form

edin

cold

erw

ater

tend

sto

bew

eake

r.

Floc

form

edin

cold

erw

ater

tend

sto

bew

eake

r.

Mix

ing

Hydr

olys

isre

actio

nsar

eve

ryfa

st.

Mix

ing

times

shou

ldbe

less

than

1s

and

pref

erab

lyle

ssth

an0.

5s.

Hydr

olys

isre

actio

nsar

eve

ryfa

st.M

ixin

gtim

essh

ould

bele

ssth

an1

san

dpr

efer

ably

less

than

0.5

s.

Beca

use

the

PACl

ispr

ehyd

roly

zed,

the

initi

albl

endi

ngtim

eis

som

ewha

tle

sscr

itica

l.

571


OPERATING REGIONS FOR METAL SALTS

Because the chemistry of the various reactions discussed above is so complex,there is no complete theory to explain the action of hydrolyzed metal ions.To qualitatively describe the application of alum as a function of pH, takinginto account the action of alum as described above, Amirtharajah and Mills(1982) developed the diagrams shown on Fig. 9-11. It is important to notethat the generalized information represented on Fig. 9-11 does not reflectthe effects of NOM on the dosages of coagulant required. The approximateregions in which the different phenomena associated with particle removalin conventional sedimentation and filtration applications are plotted as afunction of the alum dose and the pH of the treated effluent after alumhas been added. For example, optimum particle removal by sweep flococcurs in the pH range of 7 to 8 with an alum dose of 20 to 60 mg/L.With proper pH control it is possible to operate with extremely low alumdosages.

Interactions with other constituents in waterAs with all cations in water, hydrolysis products of aluminum and iron reactwith various ligands (e.g., SO42−, NOM, F−, PO43−) forming both solubleand insoluble products that will influence the quantity or dose of thecoagulant required to achieve a desired level of particle destabilization.Thus, the optimum dose of a coagulant depends strongly on the particularwater chemistry and the types of particles.

Typical dosagesA typical dosage of alum ranges from 10 to 150 mg/L, depending on raw-water quality and turbidity. Typical dosages of ferric sulfate [Fe2(SO4)3 ·9H2O] and ferric chloride (FeCl3 · 6H2O) range from 10 to 250 mg/L and5 to 150 mg/L, respectively, depending on raw-water quality and turbidity.Ferric chloride is more commonly used than ferric sulfate and comes as aliquid.

Importance of initial mixing with metal saltsThe rapid initial mixing (known as blending) of the metal salts in watertreatment is extremely important. The sequence of reactions shown onFig. 9-10 occurs rather rapidly (Rubin and Kovac, 1974). For example, ata pH of 4, half of the Al3+ hydrolyzes to Al(OH)2+ within 10−5 s (Baseand Mesmer, 1976). Hudson and Wolfner (1967) noted that ‘‘coagulantshydrolyze and begin to polymerize in a fraction of second after beingadded to water.’’ Hahn and Stumm (1968), studying the coagulation ofsilica dispersions with Al(III), reported that the time required for theformation of mono- and polynuclear hydroxide species was on the order of10−3 s, and the time of formation for the polymer species was on the orderof 10−2 s. The importance of initial and rapid mixing is also discussed byAmirtharajah and Mills (1982) and Vrale and Jorden (1971).


Clearly, based on the literature and actual field evaluations, the instan-taneous rapid and intense mixing of metal salts is of critical importance,especially where the metal salts are to be used as coagulants to lower thesurface charge of the colloidal particles. It should be noted that, althoughachieving extremely low blending times in large treatment plants is oftendifficult, low blending times can be achieved by using multiple mixers.Typical blending times for various chemicals are reported in Table 6-10 inSec. 6-10, where the subject of mixing is considered in detail.

PrehydrolyzedMetal Salts

From the previous discussion of the use of alum and iron salts, it is clearthat it is difficult to control the metal species formed, especially at lowdosages. The unpredictability associated with alum and iron salts led to thedevelopment of prehydrolyzed metal salts. Prehydrolyzed metal salts areprepared by reacting alum or ferric with various salts (e.g., chloride, sulfate)and water and hydroxide under controlled mixing conditions. Severaladvantages of preformed aluminum metal salts include the following:(1) lower dosages may be required for effective coagulation (on the basisof Al3+) for cases where NOM does not dictate the coagulant dosage atneutral or slightly acidic conditions, (2) flocs tend to be tougher and denser(although flocculation aids are still necessary in many cases), and (3) theperformance of prehydrolyzed alum salts is less temperature dependent ascompared to unmodified alum salts. General guidance on the applicationof prehydrolyzed metal salts is given in Table 9-6.

CHEMICAL COMPOSITION

The commercial prehydrolyzed alum salts, commonly known as PACl, havethe following overall formula: Ala(OH)b(Cl)c(SO4)d . Although many for-mulations do not contain any sulfate; the presence of sulfate ions helps tostabilize the aluminum polymers and keep them from precipitating. Thesepolymers can be more effective than those formed by simply adding alu-minum salts to solution because the larger cationic polymers can be formedby increasing the hydroxide-to-aluminum ratio (R = OH/Al, see followingbasicity discussion), which can lead to enhanced charge neutralization.Another benefit is that, as the polymer becomes larger, it becomes morecrystalline, compact, and dense. However, as the value of R increases, thepolymers become less stable and may begin to precipitate, which can causea problem in the storage of PACl.

BASICITY

As given by Eqs. 9-9 and 9-10, when metal salts such as alum and ironhydrolyze, hydrogen ions are released, which will react with the alkalinityof the water. In the formulation of PACl coagulants, some of the acidthat would have been released is neutralized with base (OH−) when thecoagulant is manufactured. The degree to which the hydrogen ions thatwould be released by hydrolysis are preneutralized is known as the basicity


of the product and is given by the following relationship for prehydrolyzedmetal salts that do not contain oxygen:

Basicity, % = [OH][M] ZM

× 100 (9-16)

where [OH]/[M] = molar ratio of hydroxide bound to metal ionZM = charge on metal species

For example, the basicity of the PACl Al2(OH)4Cl2 is 66.7 percent{[4/(3 × 2)] × 100}. It should be noted that, if oxygen is included inthe formulation, the basicity of the compound will increase by the effect ofthe oxygen. For example, the basicity for the compound Al13O4(OH)24 is82.1 percent {[24 + (4 × 2)]/(13 × 3)] × 100}. In effect, each mole of oxy-gen will neutralize 2 moles of hydrogen. Most prehydrolyzed alum productshave an OH/Al ratio of 0.45 to 2.5, which corresponds to basicity values of15 [(0.45/3) × 100] and 83.3 [(2.5/3) × 100] percent.

Organic Polymers Organic polymers are long-chain molecules consisting of repeating chem-ical units with a structure designed to provide distinctive physicochemicalproperties. The chemical units usually have an ionic functional groupthat imparts an electrical charge to the polymer chain. Hence, organicpolymers are often termed polyelectrolytes. Organic polymers have two prin-cipal uses in water treatment: (1) as a coagulant for the destabilization ofparticles and (2) as a filter aid to promote the formation of larger andmore shear-resistant flocs. While destabilization occurs primarily throughcharge neutralization, nonionic and anionic polymers can be used to forma bridge between particles. Organic polymers are not generally used asprimary coagulants and are often used after the particles have been destabi-lized to some degree with metal coagulants. Polymers are broadly classifiedas being natural or synthetic in origin. Because of their greater use in watertreatment, the synthetic polymers are discussed first.

SYNTHETIC ORGANIC POLYMERS

Generally, synthetic organic polymers are much cheaper than those madefrom natural sources and consequently are used more often in the UnitedStates than natural organic polymers. The principal synthetic organic poly-mers used for water treatment are summarized in Table 9-7. Syntheticorganic polymers are made either by homopolymerization of the monomeror by copolymerization of two monomers. Polymer synthesis can be manip-ulated to produce polymers of varying size (molecular weight), chargegroups, number of charge groups per polymer chain (charge density), andvarying structure (linear or branched). A typical example is the productionof polyacrylamide in which the monomer, acrylamide, homopolymerizesunder appropriate conditions to form the polymer. Polyacrylamide carriesno ionic charge and is referred to as a nonionic polymer. Subsequent

Tabl

e9-

7Ty

pica

lorg

anic

coag

ulan

tsus

edin

wat

ertr

eatm

ent

Mol

ecul

arW

eigh

t,C

omm

onTy

peC

harg

eg/

mol

eAp

plic

atio

nsTy

pica

lExa

mpl

esa

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ampl

es

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nic

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ativ

e10

4−

107

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ulan

taid

,filte

rai

d,flo

ccul

anta

id,s

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eco

nditi

onin

g

Hydr

olyz

edpo

lyac

ryla

mid

esHy

drol

yzed

poly

acry

lam

ides

,po

lyac

ryla

tes,

poly

acry

licac

id,p

olys

tyre

nesu

lfona

te

CH

2

NH

2

CH

CO

x

CH

2

Na+

CH

CO

Oy

Catio

nic

Posi

tive

104

−10

6Pr

imar

yco

agul

ant,

turb

idity

and

colo

rre

mov

al

Epic

hlor

ohyd

rindi

met

hyla

min

e(e

pi-D

MA)

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omet

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poly

acry

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ide,

poly

alky

lene

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yam

ines

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lyet

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ine

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2C

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CH

3

CH

3

OH

x

Slud

geco

nditi

onin

gPo

lydi

ally

ldim

ethy

lam

mon

ium

chlo

ride

(pol

y-DA

DMAC

)Po

lydi

met

hyl

amin

omet

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poly

acry

lam

ide,

poly

viny

lben

zyl,

trim

ethy

lam

mon

ium

chlo

ride

N+

Cl−

CH

2

CH

CH

2

CH

3C

H3

CH

x

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Cl−

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2

CH

CH

CH

2CH

3

CH

2C

H3

y

Non

ioni

cN

eutr

al10

5−

107

Coag

ulan

taid

,filte

rai

d,fil

ter

cond

ition

ing

Poly

acry

lam

ides

Poly

acry

lam

ides

,po

lyet

hyle

neox

ide

CH

2

NH

2

CH

CO

x

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ers

Varia

ble

Varia

ble

—So

dium

algi

nate

Algi

nic

acid

,dex

tran

,gu

argu

m,s

tarc

hde

rivat

ives

a Num

ber

ofm

onom

erm

olec

ules

inpo

lym

erde

sign

ated

byx

and

y.

575

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hydrolysis of polyacrylamide under basic conditions produces a polymerwith anionic charges. Thus, the number of anionic groups, in this case acarboxyl group, can be controlled, providing anionic polymers of differentmolecular weights and charge density. The third type of polymer has acationic or positive charge group incorporated in the polymer chain, usuallyby a copolymerization process.

APPLICATION OF POLYMERS

Since their introduction in the early 1950s, the use of organic polyelec-trolytes such as poly diallyl-dimethyl ammonium chloride (poly-DADMAC)and epichorohydrin dimethylamine (epi-DMA) (see Table 9-7) has gainedwidespread use for water treatment in the United States. The MW rangesfrom 104 to 105 and the basic polymer units are shown in Table 9-7.

Cationic polymersIn water treatment applications, cationic organic polymers are generallydesigned to be water soluble, to adsorb on or react rapidly with particulates,and to possess a chemical structure suitable for the intended use. When usedas primary coagulants, cationic polymers, in contrast to aluminum or ferricions, do not produce large floc volumes because organic coagulants canbe effective at much lower coagulant dosages than inorganic coagulants.However, sludge from organic coagulants is usually denser and stickierthan sludge from inorganic coagulants. Consequently, cationic organiccoagulants are not suitable for every type of separation process.

It should be noted that, because organic coagulants do not always pro-duce the same water quality as is obtained with metallic ion coagulants,cationic organic polymers are rarely used alone except for direct filtration.Furthermore, if cationic organic polymers are used alone, they are ineffec-tive in removing dissolved substances (e.g., NOM, As, F). It is common touse cationic organic polymers and metallic ion coagulants together. Themain advantage of the combined usage is that the dosage of metallic ioncoagulants can be reduced by 40 to 80 percent. The lower metallic ioncoagulant dosage in turn reduces sludge and alkalinity consumption. Withlower alkalinity consumption, the pH will not be depressed as much, whichcan improve metallic ion coagulation.

Polymer dosagesBecause of the complex interactions between polymers and particulatesand the uncertain influence of water quality on these interactions, polymerselection is empirical. The typical dosage rates for sedimentation are onthe order of 1 to 10 mg/L for DADMAC and epi-DMA. Low dosages ofhigh-molecular-weight nonionic polymers (0.005 to 0.05 mg/L) are oftenadded before granular filtration and to the backwash water to improve filterperformance. Incorrect dosing can cause mudball formation in the filters,which are not usually broken apart during normal backwashing operations.


Impact of solution parametersSolution parameters will also impact polymer dose. If the polymer chargedensity depends on pH, as with nonquaternized polyamines (see Table 9-7),then the optimum polymer dose will vary with pH, generally decreasing asthe pH decreases [the charge on secondary and tertiary amines dependson pH because the amine group will tend to protonate at lower pH values(less than 6) and will remain uncharged at neutral pH]. The charge densityof quaternized polymers such as poly-DADMAC are only slightly affectedby pH. Changes in ionic strength and composition do not appear to affectpolymer dose strongly over typical ranges encountered in water treatment(TDS between 50 and 500 mg/L).

MIXING OF POLYMERS

Most polymers are available in liquid form and can be used without apreparation stage, but they must be injected directly following in-linedilution. Successful use of polymers in water treatment requires adequatedispersion of the polymer to promote more uniform polymer adsorption.Jar testing, as described in the previous section, may be used to assess theeffect of mixing.

NATURAL POLYMERS

Sodium alginate is a natural organic polymer extracted from brown seaweed.The polymeric structure of sodium alginate is comprised of mannuronicacid and glucuronic acid and the molecular weight is on the order of104 to 2 × 105 (Degrémont, 2007). Sodium alginate is particularly effectiveas a flocculant aid with ferric salts, and good results have also beenobtained with aluminum salts, with typical dosages ranging from 0.5 and2 mg/L. Chitosan, another natural organic, is obtained from chitin shells(crab, lobster, etc.). Natural starches are also classified as natural polymers.Starches can be obtained from a number of sources, including potatoes,tapioca, or plant seed extracts. Starches are branched and nonlinearglucopyranose polymers, which are sometimes partially broken down withOH− or derivatized to form carboxy-ethyl-dextrose. Starches are used inconcentrations of 1 to 10 mg/L, preferably together with aluminum salts.

Coagulantand Flocculant

Aids

A variety of chemicals and additives known as coagulant aids and flocculantaids, used to enhance coagulation and flocculation processes, are describedbelow. Some of the commonly used inorganic coagulant aids are given inTable 9-4. Polymers used as flocculant aids are given in Table 9-7.

COAGULANT AIDS

Coagulant aids, typically insoluble particulate materials, are added toenhance the coagulation process. Clay (bentonite, kaolinite), sodium sili-cate, pure precipitated calcium carbonate, diatomite, powdered activated

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carbon (used as an adsorbent), and fine sand have all been used as coag-ulation aids. Coagulant aids are often added to waters that contain lowconcentrations of particles to form nucleating sites for the formation oflarger flocs. Coagulant aids are used in conjunction with inorganic coagu-lants, organic polyelectolytes, or both. Because the density of these particlesis higher than that of most floc particles, the settling velocities of flocculatedparticles is increased.

FLOCCULANT AIDS

Uncharged and negatively charged organic polymers that were discussedin the previous section are used as flocculant aids as opposed to primarycoagulants. As previously discussed, the main advantage of using flocculantaids is that a stronger floc is formed. Flocculant aids are added after thecoagulants are added and the particles are already destabilized. The timerequired for destabilization depends on water temperature and the typeof particles; consequently, jar tests have to be conducted. The importantfactors that need to be evaluated in jars and full-scale implementation arefloc strength, size, and settling rate. It should be noted that improperdosing of flocculant aids can cause mudballs to form in gravity filters thatare not easily eliminated by backwashing.

Activated silica is an important inorganic flocculant aid that is used incombination with alum and can be effective in cold water. It is usually storedas sodium silicate, which is soluble at high pH. Usually, the concentratedsodium silicate is partially neutralized (usually with sulfuric acid) priorto use and then added immediately to the water. In some instances,aluminosilicate is used where alum is the primary

Coagulation and Flocculation · 9 Coagulation and Flocculation 543 Term Definition Inorganic metal...

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Transcript of Coagulation and Flocculation · 9 Coagulation and Flocculation 543 Term Definition Inorganic metal...