Coagulation & Flocculation

Coagulation and Flocculation

PIERO M. ARMENANTE NJIT

Solutions, Colloidal Dispersions and Suspensions Solids can be dispersed in liquids under several forms. The nature of such dispersions depends on the size of the solid particles. In general, considered:

one

the

• Solutions • Colloidal dispersions • Suspensions


following

states

are

Characteristics of Solutions, Colloidal Dispersions and Suspensions System

Particle Size

Separation Method

Particle Visibility

Particle Movement

Solution

< 20 Å

Membrane

Invisible

Kinetic

Colloidal dispersion

20 - 2000 Å

Ultrafiltration

Ultramicroscope

Brownian

Suspension

> 2000 Å

Filtration

Microscope

Convective

1 Å = 10-4 µm = 10-10 m PIERO M. ARMENANTE NJIT

Motivation for the Use of Coagulation in Wastewater Treatment • Wastewaters often contain pollutants that are present is colloidal form • In such cases the colloidal suspension may contain: -

organic materials

-

metal oxides

-

insoluble toxic compounds

-

stable emulsions

-

material producing turbidity


Motivation for the Use of Coagulation in Wastewater Treatment (cont.'d) • This material must be removed prior to discharge • Because of the nature of the colloidal suspension these particles will not sediment or be separated with conventional physical methods (such as filtration or settling) unless they are agglomerated through coagulation


Coagulation • Coagulation is the process by which colloidal particles and very fine solid suspensions initially present in a wastewater are combined into larger agglomerates that can be separated via sedimentation, flocculation, filtration, centrifugation or other separation methods • Coagulation is commonly achieved by adding different types of chemicals (coagulants) to the wastewater to promote destabilization of the colloid dispersion and agglomeration of the resulting individual colloidal particles


Additional Benefits of Coagulation • The addition of some common coagulants to a wastewater not only produces coagulation of colloids but also typically results in the precipitation of soluble compounds, such as phosphates, that can be present in the wastewater. • In addition, coagulation can also produce the removal of particles larger that colloidal particles due to the entrapment of such particles in the flocs formed during coagulation


Effects of Coagulation Treatment on Wastewater Primary Effect • Agglomeration and eventual removal of colloids (primarily responsible for wastewater turbidity) Secondary Effects • Precipitation of some chemical species in solution • Agglomeration of larger particles in the floc


Coagulation vs. Flocculation • Although the words "coagulation" and "flocculation" are often used interchangeably they refer to two distinct processes • Coagulation indicates the process through which colloidal particles and very fine solid suspensions are destabilized so that they can begin to agglomerate if the conditions are appropriate • Flocculation refers to the process by which destabilized particles actually conglomerate into larger aggregates so that they can be separated from the wastewater PIERO M. ARMENANTE NJIT

Stability of Colloids in Wastewater • The colloids commonly found in wastewater are stable because of the electrical charge that they carry • The charge of colloids can be positive or negative. However, most colloidal particles in wastewater have a negative charge • Such a charge typically comes from: - lattice imperfections - ionizable groups that become ionic species in water (e.g., amino, or hydroxyl groups) - ionic species that can become adsorbed on the surface of the colloid PIERO M. ARMENANTE NJIT

Types of Colloids Colloids are commonly classified as: • hydrophilic (e.g., proteins) • hydrophobic (e.g., clays, metal oxides)


Hydrophilic Colloids • Hydrophilic colloids are typically formed by large organic molecules that become hydrated (solvated) when they are in the presence of water • These molecules are thermodynamically stable in their solvated form • The charge in such molecules originates from the presence of ionizable groups on the molecule that transform the molecule in a "macro-ion" when placed in solution


Hydrophilic Colloids (continued) • As a result of these charges hydrophilic colloidal particles are significantly hydrated when placed in solution • Agglomeration of hydrophilic colloids typically involves the addition of significant amounts of ions which compete with the colloids for water molecules thus resulting in the dehydration of the colloidal particles ("salting out" of the colloid)


Hydrophobic Colloids • Hydrophobic colloids are made of small colloidal particles having little or no affinity for water (the solvent) • Their stability in due to the presence of a charge which attracts other ionic species present in water and results in the formation of an electrically charged layer around the colloidal particles • Colloidal dispersions are thermodynamically unstable. If the charge layer is removed these particles tend to agglomerate spontaneously and can be removed from the wastewater PIERO M. ARMENANTE NJIT

Double Layer Associated with Colloidal Particles • If a colloidal particle is electrically charged it attracts ions and other colloidal particles of opposite sign • These ions are tightly attached by electrostatic forces to the colloidal particle forming a first inner layer of charges. This layer is often referred to as the Stern layer and has a thickness of a single hydrated ionic layer • The particle and the attached ions of opposite sign form an electric double layer PIERO M. ARMENANTE NJIT

Double Layer Associated with Colloidal Particles (continued) • Additional ions of opposite sign to that of the colloidal particle also accumulate next to the Stern layer. They form the diffuse layer

• Within the diffuse layer there is typically another layer of ions which are not so tightly attached to the particle as the Stern layer, but which cannot be removed by the presence of any external velocity gradient either

• These ions bound to the particle even as the particle moves delimit the shear plane, i.e., the plane of ions which are unaffected by fluid motion PIERO M. ARMENANTE NJIT

Distribution of Charges in a Double Layer Shear Plane

- + + + - + + - + + Colloidal - + Particle - + - + - + + + + + Stern Layer Diffuse Layer


Zeta Potential • The zeta potential is defined as the electric potential difference between the shear plane of a colloidal particle and the bulk of the solution • The zeta potential is an indirect measure of the electrical charge of the colloidal particle • The zeta potential can be experimentally measured (using a microscope) by determining the velocity of a particle moving under an electric potential of known intensity (Note: the actual potential between the surface of the particle and the solution cannot be measured experimentally) PIERO M. ARMENANTE NJIT

Electric Potential Around a Colloidal Particle Electrical Potential

Shear Plane

Zeta Potential

Distance from Particle Surface


Quantitative Determination of Zeta Potential from Experimental Data The zeta potential is defined as: ζ =

4 πv 4 πµ EM = εVx ε

where: ζ = zeta potential v = particle velocity ε = dielectric constant of the medium Vx = applied potential per unit length EM = electrophoretic mobility PIERO M. ARMENANTE NJIT

Quantitative Determination of Zeta Potential from Experimental Data At 25 oC in water the zeta potential can be directly calculated from:

ζ = 12EM where: ζ = zeta potential in mV EM = electrophoretic mobility in (µm/s)/(V/cm) Average zeta potential for wastewater colloids: -16 to -22 mV (range: -12 to -40 mV) PIERO M. ARMENANTE NJIT

Relationship Between Zeta Potential and Coagulation 40

35 0 30 Residual Turbidity Zeta Potential

25

-10

-20

Residual Turbidity (Jackson turbidity units)

20 -30

15

-40

10

-50

5

0 0

100

200

300

400

Alum Dosage (mg/L)


500

-60 600

Zeta Potential (mV)

10

Isoelectric Point • When the colloidal particles are in the presence of enough counter ions they become electrically neutral • This point is called the isoelectric point • The zeta potential at the isoelectric point is zero • If the particles are not any more "shielded" by the double layer they can interact with each other • Hence, at the isoelectric point the particles have the highest potential for agglomeration PIERO M. ARMENANTE NJIT

Energy Forces Between Colloidal Particles

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 339 PIERO M. ARMENANTE NJIT

Energy Forces Between Colloidal Particles (continued) • If the repulsive forces produced as a result of the presence of the double layer are too significant the colloidal particles will not be able to come in close contact for the attractive forces to produce agglomeration and hence coagulation • In such a case the effects of the double layer must be neutralized (e.g., by increasing the ionic strength of the solution) for coagulation to occur PIERO M. ARMENANTE NJIT

Coagulation as a Process of Destabilization of Colloids • Coagulation of colloids occurs when a stable colloid (because of the intrinsic stability of the colloid dispersion) or a stabilized colloid (because of the presence or a double layer) is destabilized • Destabilization of colloids occurs when the colloidal particles are brought close enough to each other for agglomeration to occur • Several destabilization possible PIERO M. ARMENANTE NJIT

mechanisms

are

Destabilization Mechanisms Depending on the type of colloidal suspension that should undergo coagulation different destabilization mechanisms can be employed such as: • Repression of the double layer • Neutralization of colloid charge by adsorption of counter ions on the surface of the colloid • Bridging of colloidal particles via polymer addition • Entrapment of colloidal particles by sweeping floc PIERO M. ARMENANTE NJIT

Repression of Double Layer • The potential generated by a charged particle decreases rapidly with the distance from the particle surface • As a consequence the thickness of the electrically charged layer surrounding the particles also decreases with the distance form the particle • If the particle is surrounded by a large number of added ionic species their presence will interfere with the potential generated by the particle. Consequently, the potential will decrease even faster with the distance from the particles PIERO M. ARMENANTE NJIT

Repression of Double Layer (cont.'d) • The thickness of the electrically charged layer surrounding the particles is arbitrarily taken as that distance at which the potential falls to 37% (=1/e) of the surface value • From the Debye-Hückel theory it is possible to calculate that the thickness of this layer is given by the equation:

ε z = 0. 33 ⋅10 I where: z = thickness of layer, in cm ε = dielectric constant for the solution, in C/(V cm) I = ionic strength, in moles/L −2


Repression of Double Layer (cont.'d.) For water at 20 oC it is:

3.0 ⋅10 − 8 z≅ I where z is in cm and I is in moles/L. Example: for I = 0.001 M

→

z ≈100 Å

for I = 0.1 M

→

z ≈10 Å


Repression of Double Layer (cont.'d.) • The results from the previous equations indicate that double layer repression can be accomplished by increasing the ionic strength of the solution • This increase does not alter the charge of the colloidal particles but reduces the extent to which the same charge affects the potential around the charge


Electrical Potential

Repression of Double Layer (cont.'d.) Low Ionic Strength High Ionic Strength

Z2 Z1 Distance from Particle Surface PIERO M. ARMENANTE NJIT

Repression of Double Layer (cont.'d.) Double layer repression can be achieved by: • increasing the ionic strength of the solution by adding additional ionic species • even more effectively increasing the ionic strength of the solution by adding ions of high valence since: 1 2 I= C Z ∑ j j 2 all ionic species

where: C = concentration of ionic species j Z = charge of ionic species j PIERO M. ARMENANTE NJIT

Repression of Double Layer (cont.'d) • The typical chemicals used in double layer repression are those that produce cations with a large charge such as Al+3 and Fe+3 • Therefore chemicals such as -

Al2(SO4)·14H2O (alum)

-

FeCl3

are often used as coagulants Remark: these salts produce coagulation also because of their charge suppression and bridging capability PIERO M. ARMENANTE NJIT

Neutralization of Colloid Charge • This mechanism is based on the addition of large organic molecules containing ionizable groups to the colloidal dispersion

• The charge of the dissociated molecule must be of the opposite sign as the colloid charge

• As the organic molecules dissociate the resulting large, charged molecules can effectively interact with the Stern layer replacing the counter ions originally present in this layer

• Because of the size of the molecule and the low charge of its organic "tail" the potential around the particle is decreased making the colloidal particles interact, agglomerate and coagulate PIERO M. ARMENANTE NJIT

Bridging of Colloidal Particles Via Polymer Addition • The addition of large polymeric molecules having charged or ionizable sites (polyelectrolytes) to a colloidal dispersion results in the attachment of the polymer to the colloid (just as in the charge neutralization process examined previously)

• Because of the length of the polymeric chain the motion of the colloid-polymer particle is hindered, thus promoting contact with other particles

• The polymer chains protruding from colloid-polymer particles can interact with similar particles forming "bridges." This results in particle agglomeration and eventual coagulation PIERO M. ARMENANTE NJIT

Colloid-Particle Interactions During Bridging of Colloidal Particles Via Polymer Addition

After Weber, Physicochemical Process for Water Quality Control, 1972, p. 74


Entrapment of Colloidal Particles by Sweeping Floc • If significant amounts of aluminum or ferric salts are added to a solution the hydroxide of these metal will precipitate • During the precipitation process the hydroxide forms large tridimensional polymeric structures (floc formation) • As these polymeric structures form the colloids can become entrapped in it and be precipitated by a "sweeping floc" mechanism • Large amounts of sludges are formed as a result PIERO M. ARMENANTE NJIT

Characteristics of Coagulants The ability of a chemical additive to produce coagulation is in general dependent on the: • electric charge of the ion or molecule used as coagulant: the larger the charge the more effective the coagulant will be; • size of the ion or molecule used as coagulant: the larger the size of the molecule the more effective the coagulant will be.


Coagulating Power of Inorganic Electrolytes Relative Power of Coagulation Electrolyte

Against Positive Colloids

Against Negative Colloids

NaCl Na2SO4 Na3PO4 BaCl2 MgSO4 AlCl3 Al2(SO4)3 * FeCl3 Fe2(SO4)3 *

1 30 1000 1 30 1 30 1 30

1 1 1 30 30 1000 >1000 1000 >1000

(*) Common coagulants After Droste, Theory and Practice of Water and Wastewater Treatment, 1997, p. 385.


Common Coagulants The most common coagulants used in wastewater treatment are: • Aluminum salts (alum) • Ferric and ferrous salts • Lime • Cationic polymers • Anionic and non-ionic polymers


Aluminum and Iron Salts as Coagulants The most important coagulants or this type are: •Al2(SO4)3·14H2O or Al2(SO4)3·18H2O (alum) •FeCl3 •FeCl3 (with lime) •Fe2(SO4)3 (with lime) •FeSO4·7H2O (copperas) (with lime)


Characteristics of Common Inorganic Coagulants Name

Formula

Mol. Weight

Density (kg/m3)

Alum

Al2(SO4)3 Al2(SO4)·14H2O Al2(SO4)·18H2O FeCl3 Fe2(SO4)3 Fe2(SO4)3·7H2O Ca(OH)2

342.1 594.3 666.7 162.1 400 278 74.1

2710

Ferric chloride Ferric sulfate Ferrous sulfate Lime PIERO M. ARMENANTE NJIT

2800 1899 3097 2200

Aluminum and Iron Salts as Coagulants (continued) • When added to a solution these salts dissociate into ions. The resulting cationic species are not simple ions (such as Al+3, Fe+3) but their hydrated forms (such as Al(H2O)6+3 or Fe(H2O)6+3) • The addition of small amounts of Al and Fe salts does not results in coagulation. This indicates that double layer suppression is one but not the main mechanism of coagulation


Aluminum and Iron Salts as Coagulants (continued) • When added in significant amounts the ions from these salts react with the OH- or bicarbonate and carbonate ions in solution to produce the corresponding insoluble hydroxides (Al(OH)3 or Fe(OH)3) • Coagulation of colloids is observed when precipitation of the hydroxides occurs (with or without the presence of lime) • The solubility of Al(OH)3 or Fe(OH)3 is a function of the pH PIERO M. ARMENANTE NJIT

Aluminum and Iron Salts as Coagulants (continued) • The precipitation of these Al and Fe hydroxides proceeds through the formation of polymeric hydrocomplexes • These complexes and the hydroxide particles are positively charged if the pH is below their isoelectric point. Hence they are adsorbed on the surface of the colloids producing charge suppression and coagulation


Aluminum and Iron Salts as Coagulants (continued) • If the pH of the solution is beyond the isoelectric point of the hydroxide precipitate then the hydroxide particles will be negatively charged and no charge suppression of the colloid particle is possible • In this range colloid coagulation can only occur if the amount of hydroxide precipitate is high enough for bridge formation and sweeping floc enmeshment


Hydrolysis of Alum in Water The addition of alum (hydrated aluminum sulfate) to a wastewater produces the hydrolysis of the sulfate with the consequent formation of insoluble aluminum hydroxide according to the reactions: Al2 (SO4 )3 ⋅18 H2O + 6 H2O ⇔

2 Al (OH )3 ↓ + 6 H + + 3SO42 − + 18 H2O

The insoluble aluminum hydroxide forms a floc precipitate responsible for colloid removal. Note that if the wastewater is not buffered the resulting H+ generation (pH increase) will prevent the reaction from proceeding any further. PIERO M. ARMENANTE NJIT

Reaction of Alum in the Presence of Calcium and Magnesium Alkalinity In the presence of calcium or magnesium bicarbonate alkalinity alum forms aluminum hydroxide that precipitates as before forming a sweeping floc responsible for colloid removal: Al2 (SO4 )3 ⋅18 H2O + 3 Ca( HCO3 )2 ⇔ 2 Al (OH )3 ↓ + 3CaSO4 + 6 CO2 + 18 H2O

If the alkalinity is insufficient lime can be added: Al2 (SO4 )3 ⋅18 H2O + 3 Ca(OH )2 ⇔

2 Al (OH )3 ↓ + 3 CaSO4 + 18 H2O


Equilibrium Composition of Solutions in Contact with Al(OH)3


Hydrolysis of Ferric Chloride in Water The addition of ferric chloride to a wastewater produces the hydrolysis of the ferric chloride with the consequent formation of insoluble ferric hydroxide, according to the reaction: FeCl3 + 3 H 2O ⇔ Fe(OH )3 ↓ + 3 H + + 3 Cl −

The insoluble ferric hydroxide forms a gelatinous sweeping floc precipitate responsible for colloid removal. Note that if the wastewater is not buffered the resulting H+ generation (pH decrease) will prevent the reaction from proceeding any further. PIERO M. ARMENANTE NJIT

Reaction of Ferric Chloride in the Presence of Calcium and Magnesium Alkalinity In the presence of calcium or magnesium bicarbonate alkalinity ferric chloride forms ferric hydroxide, which precipitates as before forming a sweeping floc responsible for colloid removal: 2 FeCl3 + 3Ca( HCO3 )2 ⇔

2 Fe(OH )3 ↓ + 3CaCl2 + 7CO2

If the alkalinity is insufficient lime can be added: 2 FeCl3 + 3 Ca(OH )2 ⇔ 2 Fe(OH )3 ↓ + 3 CaCl2


Equilibrium Composition of Solutions in Contact with Fe(OH)3


Reaction of Ferric Sulfate in the Presence of Alkalinity or Lime In the presence of calcium or magnesium bicarbonate alkalinity ferric sulfate forms ferric hydroxide, which precipitates as before forming a sweeping floc responsible for colloid removal: Fe2 (SO4 )3 + 3 Ca( HCO3 )2 ⇔

2 Fe(OH )3 ↓ + 3 CaSO4 + 6CO2

If the alkalinity is insufficient lime can be added:

Fe2 (SO4 )3 + 3Ca(OH )2 ⇔ 2 Fe(OH )3 ↓ + 3 CaSO4 PIERO M. ARMENANTE NJIT

Reaction of Ferrous Sulfate in the Presence of Alkalinity and Lime If ferrous sulfate is used as a coagulant alkalinity and lime (or lime alone) are required to form ferrous hydroxide, which is then converted to ferric hydroxide by the oxygen in the water:

FeSO4 ⋅7H2O + Ca( HCO3 )2 ⇔ Fe( HCO3 )2 + CaSO4 + 7 H2O Fe( HCO3 )2 + 2 Ca(OH )2 ⇔ Fe(OH )2 + 2 CaCO3 + 2 H2O 4 Fe(OH )2 + O2 + 2 H2O ⇔ 4 Fe(OH )3 ↓


Strategies for Al and Fe Coagulant Addition The appropriateness of the use of Al or Fe salts as coagulants and their dosage depend primarily on three factors, i.e.: • pH of the wastewater • alkalinity of the wastewater • concentration of the colloids Several situations are possible and will be examined.


Strategies for Al and Fe Coagulant Addition Colloid concentration

Alkalinity level

Destabilization mechanism

Al and Fe Addition Strategy

High

Low

Charge neutralization

Easy coagulation

High

High

Charge neutralization

High dose or remove alkalinity and add low dose

Low

High

Sweeping floc

High dose. Easy coagulation

Low

Low

Ineffective

Increase alkalinity or use other coagulant


Effect of the Presence of Phosphates on Coagulation with Alum or Iron Salts Multivalent metal ions such as aluminum or ferric ions form very sparingly soluble precipitates in the presence of phosphate ions. The reactions involved in phosphate precipitation are: Al2 (SO4 )3 ⋅14 H2O (alum) + 2 PO4− 3 → 2 AlPO4 ↓ + 3 SO4− 2 + 14 H2O FeCl3 + PO4− 3 → FePO4 ↓ + 3 Cl − These reactions are of course in competition with the many other reactions occurring as a result of alum or ferric chloride addition to a wastewater PIERO M. ARMENANTE NJIT

Lime as a Coagulant • Lime is a generic name used to identify several combinations of calcium and oxygen, such as Ca(OH)2 or CaO (quicklime). Lime often contains magnesium • In the presence of bicarbonate ion lime will react to form a calcium carbonate precipitate that can remove colloids through the sweeping floc mechanism. The reaction involved is: Ca(OH )2 + Ca(HCO3 )2 ⇔ 2 CaCO3 ↓+ 2 H 2O • If the wastewater is acidic the lime addition will go first toward the neutralization of the acid before colloid removal can take place PIERO M. ARMENANTE NJIT

Lime as a Coagulant (continued) • If the lime addition is sufficient to raise the pH above 9.5 the magnesium content of the lime will precipitate as Mg(OH)2 • Magnesium hydroxide precipitates in a gelatinous form which results in good clarification because of its enhanced potential for colloid removal • On the other hand the gelatinous magnesium hydroxide precipitate typically makes the resulting sludge more difficult to dewater


Effect of the Presence of Phosphates on Coagulation with Lime The addition of lime to a wastewater will first result in the neutralization of the wastewater followed by the precipitation of CaCO3 if bicarbonates are present, as described above. If, as a result of Ca(OH)2 addition, the pH of wastewater goes above about 10 and if phosphates are present then the following precipitation reaction can take place: 10 Ca + 2 + 6 PO4− 3 + 2 OH − → Ca10 (PO4 )6 (OH )2 ↓

where hydroxylapatite is formed as a precipitate. PIERO M. ARMENANTE NJIT

Polyelectrolytes as Coagulants • Polyelectrolytes (also often referred to a “polymers” in coagulation processes) are synthetic or organic polymeric molecules having ionizable groups or charged groups along their chain • Polyelectrolytes can be classified according to their origin as: - natural, i.e., derived from starch products or of biological origin (e.g., alginate from algae, chitosan from the acidification of chitin in shells) - synthetic, i.e., synthetically polymerized from monomers (e.g., polyamine, sulfonate, etc.) PIERO M. ARMENANTE NJIT

Polyelectrolytes as Coagulants • Polyelectrolytes are also classified according to the charge that they have when they are in solution as: -

cationic, i.e., forming a macro cation when placed in water

-

nonionic, i.e., having no net charge when placed in water

-

anionic, i.e., forming a macro anion when placed in water


Examples of Polyelectrolytes Type

Functional Group

Example

Cationic

Amine

Polyethyleneamine hydrochloride

Quaternary

Polydiallyldimethylammonium

Polyalcohol

Polyvinylalcohol

Amide

Polyacrylamide

Carboxylic

Polymethacrylic acid

Sulfonic

Polyvinylsulfonate

Nonionic

Anionic


Examples of Coagulant Polyelectrolytes Cationic

[

H CH2

CH2 N

+

H

Anionic

Nonionic

-

CH2

CH C O NH2

Cl

n Polyethyleninine


]

n

R CH2

CH C O O

-

n Polyacrilamide Poly(meth)acrilic acid

Polyelectrolytes as Coagulants • Cationic polyelectrolytes are typically used to coagulate colloids that are negatively charged (the most common type) • The mechanisms involved in the colloid destabilization by cationic polyelectrolytes are: charge neutralization and bridging of colloidal particles • Nonionic and anionic polyelectrolytes can also be used to destabilize negative colloids. In this case the destabilization mechanism is not too well elucidated but is believed to be due to bridging PIERO M. ARMENANTE NJIT

Typical Coagulant Dosage Coagulant

Dosage (ppm)

pH

Alum

75 - 250

4.5 - 7.0

FeCl3

35 - 150

4.5 - 7.0

FeSO4·7H2O

70 - 200

4.0 - 7.0

Lime

150 - 500

9.0 - 11.0

Cationic Electrolytes

2-5

---

Nonionic and Anionic Electrolytes

0.25 - 1.0

---

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 92 PIERO M. ARMENANTE NJIT

Coagulant Aids • Despite their names coagulant aids are additives that can be added to a destabilized colloidal suspension to promote the growth of large, rapid-settling floc which can then flocculate • Typical coagulant aids are: -

Activated silica

-

Polyelectrolytes


Activated Silica as a Coagulant Aid • Activated silica is a short chain polymer capable of binding together aluminum hydrate particles used in coagulation processes • Common dosage for activated silica is 5 - 10 ppm


Laboratory Coagulation Tests • The selection of a coagulant and its dosage cannot be made without carrying out laboratory experiments • Typically two types of tests are the most effective in the determination of the optimal dosage of coagulant and optimal pH for coagulation. The are: -

jar test

-

zeta potential


Jar Test The jar test is the most common laboratory coagulation test and consists of: • placing a sample of the wastewater in a jar: • rapidly adding the coagulant and intensely mixing the sample for a short time (coagulation/destabilization phase); • allowing the floc to form under gentle mixing conditions (flocculation phase) • allowing the floc to sediment in the quiescent sample (settling) • comparing the turbidity of the sample with the initial turbidity PIERO M. ARMENANTE NJIT

Jar Test Apparatus

After Droste, Theory and Practice of Water and Wastewater Treatment, 1997, p. 388.


Strategy to Conduct Laboratory Coagulation Tests Since two variables (coagulant dosage and pH are) are involved it is typically more convenient to fix one of the variables initially, scan for the optimal value of the other variable, and finally scan for the optimal value of the first variable


Jar Test Procedure • A wastewater sample is placed in a beaker and magnetically stirred. The pH must be adjusted to a desired valued (typically 6); • A known amount of coagulant is added and the agitation is maintained at a high value for 1 minute to promote coagulation. Then the sample is agitated slowly for 3 minutes to promote flocculation. New additions are made until a visible floc is obtained; • Using this concentration of coagulant the coagulation experiment is now repeated at different pH values using a longer flocculation time (typically about 10 to 40 minutes), followed by settling with no agitation (15-60 minutes); PIERO M. ARMENANTE NJIT

Jar Test Procedure (continued) • The amount of residual pollutant in solution is measured at the end of each pH experiment. The optimal coagulation pH is obtained;

• Using this optimal pH value a new series of experiments is conducted in which the coagulant dosage is changed. The optimal coagulant dosage is obtained;

• Plots of residual pollutant concentration vs. pH and residual pollutant concentration vs. coagulant dosage can be constructed.


Pollutant Residual Concentration

Pollutant Residual Concentration

Typical Results of Jar Test

pH


Coagulant Dosage

Zeta Potential Test • A wastewater sample is placed in a beaker and magnetically stirred. The pH must be adjusted to a desired valued (typically 6)

• A known amount of coagulant is added and the agitation is maintained at a high value for 1 minute to promote coagulation. Then the sample is agitated slowly for 3 minutes to promote flocculation

• The zeta potential is measured • The same test is repeated at different coagulant dosage

• The results are plotted and the dosage for zeta potential equal to zero is determined PIERO M. ARMENANTE NJIT

Coagulation vs. Flocculation • Although the words "coagulation" and "flocculation" are often used interchangeably they refer to two distinct processes • Coagulation indicates the process through which colloidal particles and very fine solid suspensions are destabilized so that they can begin to agglomerate if the conditions are appropriate • Flocculation refers to the process by which destabilized particles actually conglomerate into larger aggregates so that they can be separated from the wastewater PIERO M. ARMENANTE NJIT

Mechanisms of Flocculation • Perikinetic flocculation This type of flocculation is generated by the Brownian motion associated with destabilized small colloidal particles. The Brownian motion contributes to transport the particles close enough for agglomeration to occur • Orthokinetic flocculation This type of flocculation mechanism is based on the generation of velocity gradients within the wastewater to promote particle interaction. In this case a mild agitation promotes the aggregation of the particles and hence flocculation PIERO M. ARMENANTE NJIT

Model for Flocculation Process The rate of orthokinetic flocculation can be predicted (in principle) from the equation: Np 4 ln o = − η Ω Gave t Np π where Np = number of separate colloidal particles Npo = initial number of separate colloidal particles η = particle collision efficiency Ω = volume of colloidal particles per unit volume of suspension Gave = average velocity gradient t = time PIERO M. ARMENANTE NJIT

Model for Flocculation Process The average velocity gradient, Gave, can be expressed as: Gave

Pave = Vµ

where: Pave = average power consumption in the system (e.g., a tank stirred by an impeller) V = volume of suspension µ = viscosity


Example of Apparatus for Coagulation-Flocculation and Settling

After Eckenfelder, Industrial Water Pollution Control, 1989, p. 96 PIERO M. ARMENANTE NJIT

Design Approach for Coagulation and Flocculation Equipment • Coagulation depends on the rapid reaction through which the coagulant destabilizes the colloids. • Coagulation requires the rapid dispersion of the coagulant throughout the wastewater. This is accomplished through very intense agitation and mixing. • High shear rates are beneficial to coagulation.


Design Approach for Coagulation and Flocculation Equipment • Flocculation depends on the frequency of collision of the destabilized colloids to form larger floc particles. • The frequency of collision depends on the intensity of the agitation and the shear rate. • However, too high a value of the agitation intensity and shear rate may break up the floc just formed. • Therefore, moderate shear rates and agitation intensities are used in flocculators. PIERO M. ARMENANTE NJIT

Batch Coagulation-Flocculation Process • Two tanks in parallel are typically used. Each tank operates in a full cycle (filling, coagulation, flocculation, settling). • Each step is conducted in series and is time dependent. • After the tank has been filled, the coagulant is added while the wastewater is rapidly stirred (typically for 10 minutes). • The agitation is reduced during the flocculation phase and stopped altogether during settling (typically for a few hours). • Batch treatment can be cost effective only for flows up to 50,000 gal/day.


Continuous Coagulation-Flocculation Process A continuous coagulation-flocculation system typically includes the following: • Chemical feeding system: to continuously feed the coagulant to the wastewater; • Rapid mix tank: in this tank (or pipe, channel, etc.) high shear condition are generated (e.g., by a rapidly rotating impeller) to rapidly mix the coagulant with the wastewater; • Flocculation tank: in this tank moderate agitation (typically through impellers or paddles) is provided to promote coalescence and flow growth; • Sedimentation tank: to remove the floc particles; • Filtration system: to polish the effluent, if needed. PIERO M. ARMENANTE NJIT

Shear Rate (Velocity Gradient) In any moving fluid in which different velocities exist the rate of change of the velocity in any given direction along another direction (e.g., the rate the velocity in the x direction changes along the y direction) is called the shear rate or velocity gradient along vx=v that direction (e.g., dvx/dy), and is defined as G:

Gxy

dv x = dy

y x vx=0


Shear Stress and Shear Rate (Velocity Gradient) The shear rate, τ, is the force per unit area that is transmitted between two adjacent fluid layers as a result of viscosity. The shear rate transmitted along the y direction as a result of the velocity gradient (or velocity gradient) dvx/dy is:

dv x τxy = − µ dy


Shear Stress, Velocity Gradient and Power Dissipation It can be shown that the relationship between the shear rate, τ, the velocity gradient, G, and the power, P, dissipated in a volume V, of fluid as a result of viscous dissipation can be expressed as: P − τ G = µG = = ε V 2

where: ε = power dissipation per unit fluid volume


Velocity Gradient and Power Dissipation The relationship: G=

P µV

can be used to determine the local velocity gradient from the local power dissipation. Since this is typically very difficult, in practice the average velocity gradient, Gave, is determined from the average power dissipation, Pave: Pave Gave = µV PIERO M. ARMENANTE NJIT

Estimation of Power Dissipation in Coagulation and Flocculation Equipment • Phenomena such as coagulation and flocculation are directly or indirectly affected by the velocity gradients. • Since the average velocity gradient, G, can be estimated only if the mechanical power delivered to the system is known, it becomes essential to be able to determine the power, P, consumed by different systems (such as impellers, paddles, sparged gas, etc.).


Direct Power Input: Classification of Power Sources The power dissipated inside a tank must be supplied from the outside in some way. In principle, this external power can be supplied by means of three primary different sources, i.e.: • Mechanical agitation (e.g., impellers, paddles, turbines) • Power delivered by a compressed gas (e.g., through gas dispersers, sparged gas diffusers) • Power delivered by the liquid (e.g., venturi mixers, hydraulic jumps, water jets) PIERO M. ARMENANTE NJIT

Mechanical Agitation Systems in Coagulation and Flocculation • Submerged impellers (mixers) mounted on vertical shafts. • Paddles mounted on horizontal shafts.


Tanks with Submerged Impeller(s) Motor

H

C D T


Submerged Impellers • Many different types of impellers exist. • Impellers are classified on the basis of: ♦ shape ♦ dimensions ♦ type of flow pattern generated ♦ flow intensity ♦ power consumption under aerated and nonaerated conditions ♦ their ability to disperse a gas effectively. PIERO M. ARMENANTE NJIT

Example of Submerged Impellers

After Tatterson (1991), p. 7. PIERO M. ARMENANTE NJIT

Impeller Pumping Action • Radial (e.g., Rushton Impeller, Flat-Blade Impeller) • Axial (e.g., Propeller, Lightning A310) • Mixed-flow (45o Bladed Turbine)


Pitched-

Flow Patterns Resulting from the Presence of Baffles (Stators) Vortex

• Unbaffled cylindrical vessel

Flat intefacial area

• Baffled cylindrical vessel (or unbaffled square vessel)


Baffle

Nomenclature for Submerged Impellers C Impeller off-bottom clearance D Impeller diameter H Height of liquid in the mixing vessel N Agitation speed; revolutions per minute (rpm) P Power consumed by the impeller T Vessel diameter; m w Baffle width; m


Calculation of the Power Dissipated by a Submerged Impeller For a given impeller of known type, size, and agitation speed it is possible to calculate the power dissipated by that impeller, P, from: P = Po ρ N 3 D 5

The non-dimensional power number Po is specific for each impeller type and agitation system. Remark: in this expression the agitation speed, N, is expressed in rotations per unit time (e.g., rotations per second [rps], rotation per minute[rpm] to yield P in the appropriate units). PIERO M. ARMENANTE NJIT

Power Number • The power number, Po (or Ne or Np) is a nondimensional number used to calculate the power consumption of an impeller. • In general, the power number is a function of the type of impeller, type of agitation system (e.g., tank, baffles), dynamic agitation regimes (e.g., turbulent flow, laminar flow), geometric dimensions of impeller and tank, location of the impeller, and the presence of other impellers. In other words, Po is a function of any variable affecting the agitation flow. • Po is typically determined experimentally. PIERO M. ARMENANTE NJIT

Non-Dimensional Groups Used in Power Consumption Calculations P • Power Number = Po = Ne = ρ N3 D5 ρ N D2 • Impeller Reynolds Number = Re = µ

N 2D • Froude Number = Fr = (important only in g unbaffled vessels) • Geometric Ratios, such as T ; H ; C ; w D D D T PIERO M. ARMENANTE NJIT

Power Consumed by Submerged Impellers In general, Po can be expressed as:  ρ N D 2 N 2D T H C w  P Po = Ne = , , , , , ,impeller type 3 5 = f g D D D T ρN D  µ 

For baffled vessels, Fr is unimportant, and: ρ N D 2 T H C w  P Po = Ne = = f , , , , ,impeller type ρ N3 D5 µ D D D T  

For geometrically similar baffled vessels: ρ N D 2  P Po = Ne =  = f (Re,impeller type) 3 5 = f ρN D  µ  PIERO M. ARMENANTE NJIT

Typical Power Number Curve for a Submerged Impeller

Power Number

100

10

Laminar

Turbulent

Transitional 1 1

10

100

1000

Reynolds Number


10000

100000

Power Number for Submerged Impellers Laminar Regime: 1 Po = Ne ∝ Re

Transitional Regime: Po = Ne = f (Re) Turbulent Regime: Po = Ne = Constant


Power Number for Submerged Impellers

After Bates, Fondy, and Corpstein (1963). PIERO M. ARMENANTE NJIT

Turbulent Power Number for Different Impellers Impeller Type

Po or Ne

Flat-Blade Turbine (24 Blades)

9.8


8.5

Gate

5.5

Disc Turbine (Rushton Type)

5.0


4.0

45o Pitched-Blade Turbine

1.3

Prochem

1.0

Glass-Lined Impeller (Pfaudler Type)

0.75

MIG Impeller

0.65

Lightnin A310

0.45

Propeller

0.35


Effect of Tank Size on Power Consumption • In general, the shape of the tank can have an effect on the power consumed by the impeller; however, typically this effect is somewhat limited. • The power consumed by an impeller in a baffled cylindrical tank (i.e., having a circular cross section) is typically very similar to the power consumed in a tank of square cross section.


Effect of Tank Size on Power Consumption • The power dissipation is lower in tanks having a rectangular (as opposed to circular or square) cross section. • For most of the cases encountered in wastewater treatment the power consumed by a turbine in an unbaffled tank having a circular or square cross section can be taken to be 75% of that dissipated in a baffled system having the same dimensions.


Horizontal Paddle Systems Paddles mounted on horizontal shafts are often encountered in flocculation tanks.

Plan View


Side View

Geometry of a Horizontal Paddle Paddle

b Shaft

ri ro PIERO M. ARMENANTE NJIT

Power Dissipated by a Horizontal Paddle The power dissipated by a single paddle can be obtained from the equation: 3 1 2 π  4 P = ρ CD b  N (1 − k ) ( ro − ri 4 ) 8 60  N = agitation speed in rpm CD ≅ 1.8 for flat paddles k = 0.25 (for tanks without baffles); 0-0.15 (for tanks with baffles) Range for peripheral velocity, 2πN/60 = 0.09-0.9 m/s. PIERO M. ARMENANTE NJIT

Power Dissipated by Gas Sparging When a single gas bubble sparged from the bottom of a tank rises to the top the gas expansion energy, W, released from the gas to the liquid can be obtained from: Vo

W = ∫ p dV Vi

where: p = pressure in the gas bubble V = bubble volume “o” subscript: at the top of the tank “i” subscript: at the bottom of tank PIERO M. ARMENANTE NJIT

Power Dissipated by Gas Sparging Air


Power Dissipated by Gas Sparging From the ideal gas law it is:

pV = po Vo = pi Vi = nRT Then: Vo

W =∫ Vi

po

dp pi ⌠ p dV = −  ( po Vo ) = ( po Vo )ln p po ⌡ pi

For m bubbles sparged in the tank during the time t it is (since the gas flow rate is Qo = m Vo/t):  ∆p  mW mVo pi pi P= = po ln = po Qo ln = po Qo ln1 +  t t po po  po  PIERO M. ARMENANTE NJIT

Power Dissipated by Gas Sparging Recall that the pressure generated by a column of liquid is:

p = ρL g h where h is the height of the column of liquid, and ρL is the density of the liquid. Since 1 atm = 101,325 Pascals the height of a column of water that produces a hydrostatic pressure of 1 atm is:

101325 , Pa h= = 10.3 m 3 2 1000 kg / m 9.81m / s PIERO M. ARMENANTE NJIT

Power Dissipated by Gas Sparging Hence, the power dissipated by a gas sparged at a distance h below the liquid level (exposed to the atmospheric pressure, po) is given by:

 ∆p   h (in m) P = po Qo ln1 +  = po Qo ln1 +  10.3    po   h (in ft) = po Qo ln1 +  33.8  


Power Dissipated by Hydraulic Devices Power can be dissipated also when water flows through hydraulic jumps. In general, the power dissipated this way can be obtained from:

P = ρ g Q hL where: ρ = liquid density g = acceleration of gravity hL = headloss in the device PIERO M. ARMENANTE NJIT

Average Velocity Gradient and Residence Time in Rapid Mix Coagulation Equipment • During coagulation the average velocity gradient is typically quite high, with G values up to 5000 s-1 (more commonly around 1000 s-1). • Residence times are between 10 s and 6 minutes, although much faster dispersion times are needed for optimal charge neutralization effects (t < 0.1 s).


Average Velocity Gradient and Residence Time in Rapid Mix Coagulation Equipment Recommended velocity gradients and residence times during coagulation in rapid mix basins: t (s)

20

30

40

>40

1000

900

790

700

20,000

27,000

31,600

28,000

G (1/s) G·t

The following empirical equation relating G, t and the concentration, C (in mg/L), of alum as the coagulant can also be used for rapid mix devices: G ⋅t ⋅C 1.46 = 5.9 ⋅10 6 PIERO M. ARMENANTE NJIT

Average Velocity Gradient and Residence Time in Flocculators • During flocculation the average velocity gradient is typically much lower than in rapid mix devices, with G values in the range 5-100 s-1 (more commonly in the range 10-60 s-1). • Residence times are between 15 and 45 minutes. • G·t values are typically in the range 104-105.


Generalized Approach to the Design of Coagulation and Flocculation Devices • Carry out jar tests to determine dosage of coagulant. • Assume values for G and the residence time. • Size the tank for the residence time selected. • Choose the type of equipment appropriate for the process (e.g., high speed impeller for rapid mix or paddle agitator for flocculation). • Size the equipment on the basis of the power dissipation that will result in the desired G value. PIERO M. ARMENANTE NJIT

Flocculator Design Guidelines Type of Flocculator Vertical shaft

Horizontal-shaft paddle

Baffled channel

Diffused air and water jets

Basic Design Criteria Gave value up to 100 s-1, maximum tip speed of 2 m/s, approximately 5m x 5m to 10m x 10m basin surface area per unit, downward flow pattern preferable for propeller unit, stator baffles should be provided for turbine units Gave value up to 50 s-1, maximum tip speed of 1 m/s, number of paddles adjusted for tapered mixing, paddle area should not exceed 20% of tank section area Tapered mixing by adjusting baffles, maximum flow velocity of approximately 0.75 m/s, end-around baffle used when total head loss across tank is limited Gave = 95-20 s-1 or Gave·t = 105-106, may be used for auxiliary mixing when plant is overloaded

James M. Montgomery Consulting Engineering, Inc., Water Treatment Principles and Design, 1985, p. 516.


Flocculator Design Guidelines Type of Flocculator Vertical shaft

Horizontal-shaft Paddle

Baffled channel

Diffused air and water jets

Advantages and Disadvantages Easy maintenance and few breakdowns. Suitable for high-energy input. Suitable for direct filtration and conventional treatment. Many units required for a large plant. High capital cost for variable-speed reducers and support slabs. Generally produces a large-size floc. Simple mixing unit. Suitable for conventional treatment. Need for precise installation and maintenance. Difficult to increase energy input. Problems with leakage and shaft alignment. Performs well if the plant flow rate is reasonably constant. Little maintenance. A lack of flexibility for mixing intensity. High head loss for the over-and-under baffle. Simple installation and less capital cost. Limited amount of operational data available. High local velocities for water jet flocculators. High operational cost for air diffuser flocculators.

James M. Montgomery Consulting Engineering, Inc., Water Treatment Principles and Design, 1985, p. 516.


Examples of Coagulation-Flocculation Processes for Industrial Wastewaters Wastewater Source

Contaminant

Type of Coagulant

Dosage (ppm)

Petroleum refinery

Oil

Alum

25-75

Petroleum refinery

Foam

Polyamine

1-3

Steel mill

Oil

Anionic

1.5-2

Steel mill

Suspended solids

Anionic

0.3

Paper mill

Suspended solids

Weak cationic

2-5

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 352. PIERO M. ARMENANTE NJIT

Efficiency of Coagulation Processes to Remove Pollutants Phosphorus removal (%)

Suspended solid removal (%)

BOD Removal (%)

Without

With

Without

With

Without

With

5-10

70-90

40-70

60-75

25-40

40-50

Trickling filters

10-20

80-95

70-92

85-95

80-90

85-95

Activated Sludge

10-20

80-95

85-95

85-95

85-95

85-95

Primary treatment Secondary treatment

After Sundstrom and Klei, Wastewater Treatment, 1979, p. 352. PIERO M. ARMENANTE NJIT

Additional Information and Examples on Coagulation and Flocculation Additional information and examples can be found in the following references: • Corbitt, R. A. 1990, The Standard Handbook of Environmental Engineering, McGraw-Hill, New York, pp. 6.92; 9.25. • Droste, R. L., Theory and Practice of Water and Wastewater Treatment, John Wiley & Sons, New York, 1997, pp. 384-415. • Eckenfelder, W. W., Jr., 1989, Industrial Water Pollution Control, McGraw-Hill, New York, pp. 84-110.


Additional Information and Examples on Coagulation and Flocculation • Freeman, H. M. (ed.), 1989, Standard Handbook of Hazardous Waste Treatment and Disposal, McGrawHill, New York, pp. 7.21-7.31. • Haas, C. N. and Vamos, R. J., 1995, Hazardous and Industrial Waste Treatment, Prentice Hall, Englewood Cliffs, NJ, pp. 144-145. • James M. Montgomery Consulting Engineering, Inc., 1985, Water Treatment Principles and Design, WileyInterscience, John Wiley & Sons, New York, pp. 116134; 504-519.


Additional Information and Examples on Coagulation and Flocculation • Metcalf & Eddy, 1991, Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw-Hill, New York, pp. 302-314; 470-472. • Sundstrom, D. W. and Klei, H. E., 1979, Wastewater Treatment, Prentice Hall, Englewood Cliffs, NJ, pp. 235-255. • Weber, W. J., Jr., 1972, Physicochemical Process for Water Quality Control, Wiley-Interscience, John Wiley & Sons, New York, pp. 61-109. • Wentz, C. W., 1995, Hazardous Waste Management, Second Edition, McGraw-Hill, New York, pp. 157-161. PIERO M. ARMENANTE NJIT

Coagulation & Flocculation

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