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Coagulation and Flocculation - Continuing Education at NJIT

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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
PIERO M. ARMENANTE
NJIT
following
states
are
Characteristics of Solutions,
Colloidal Dispersions and
Suspensions
System
Particle
Size
Separation
Method
Particle
Visibility
Particle
Movement
Solution
< 20 Г…
Membrane
Invisible
Kinetic
Colloidal
dispersion
20 - 2000
Г…
Ultrafiltration
Ultramicroscope
Brownian
Suspension
> 2000 Г…
Filtration
Microscope
Convective
1 Г… = 10-4 Вµm = 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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
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
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)
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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)
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
Quantitative Determination of Zeta
Potential from Experimental Data
The zeta potential is defined as:
О¶ =
4 ПЂv 4 ПЂВµ EM
=
ОµVx
Оµ
where: О¶ = zeta potential
v = particle velocity
Оµ = 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:
О¶ = 12EM
where:
О¶ = zeta potential in mV
EM = electrophoretic mobility in (Вµm/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)
PIERO M. ARMENANTE
NJIT
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:
Оµ
z = 0. 33 в‹…10
I
where: z = thickness of layer, in cm
Оµ = dielectric constant for the solution, in C/(V cm)
I = ionic strength, in moles/L
в€’2
PIERO M. ARMENANTE
NJIT
Repression of Double Layer (cont.'d.)
For water at 20 oC it is:
3.0 в‹…10 в€’ 8
z≅
I
where z is in cm and I is in moles/L.
Example:
for I = 0.001 M
в†’
z ≈100 Å
for I = 0.1 M
в†’
z ≈10 Å
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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)В·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
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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)
PIERO M. ARMENANTE
NJIT
Characteristics of Common Inorganic
Coagulants
Name
Formula
Mol.
Weight
Density
(kg/m3)
Alum
Al2(SO4)3
Al2(SO4)В·14H2O
Al2(SO4)В·18H2O
FeCl3
Fe2(SO4)3
Fe2(SO4)3В·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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
Equilibrium Composition of Solutions
in Contact with Al(OH)3
After Sundstrom and Klei, Wastewater Treatment, 1979, p. 343
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
Equilibrium Composition of Solutions
in Contact with Fe(OH)3
After Sundstrom and Klei, Wastewater Treatment, 1979, p. 344
PIERO M. ARMENANTE
NJIT
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 ↓
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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 в‹…14 H2O (alum) + 2 PO4в€’ 3 в†’
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
Examples of Polyelectrolytes
Type
Functional
Group
Example
Cationic
Amine
Polyethyleneamine
hydrochloride
Quaternary
Polydiallyldimethylammonium
Polyalcohol
Polyvinylalcohol
Amide
Polyacrylamide
Carboxylic
Polymethacrylic acid
Sulfonic
Polyvinylsulfonate
Nonionic
Anionic
PIERO M. ARMENANTE
NJIT
Examples of Coagulant
Polyelectrolytes
Cationic
[
H
CH2
CH2 N
+
H
Anionic
Nonionic
-
CH2
CH
C O
NH2
Cl
n
Polyethyleninine
PIERO M. ARMENANTE
NJIT
]
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
FeSO4В·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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
Pollutant
Residual Concentration
Pollutant
Residual Concentration
Typical Results of Jar Test
pH
PIERO M. ARMENANTE
NJIT
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
ПЂ
where Np = number of separate colloidal particles
Npo = initial number of separate colloidal particles
О· = 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
=
VВµ
where: Pave = average power consumption in the
system (e.g., a tank stirred by an
impeller)
V = volume of suspension
Вµ = viscosity
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
Shear Stress and Shear Rate
(Velocity Gradient)
The shear rate, П„, 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
П„xy = в€’ Вµ
dy
PIERO M. ARMENANTE
NJIT
Shear Stress, Velocity Gradient and
Power Dissipation
It can be shown that the relationship between the
shear rate, П„, 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
в€’ П„ G = ВµG = = Оµ
V
2
where:
Оµ = power dissipation per unit fluid volume
PIERO M. ARMENANTE
NJIT
Velocity Gradient and
Power Dissipation
The relationship:
G=
P
ВµV
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 =
ВµV
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.).
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
Tanks with Submerged Impeller(s)
Motor
H
C
D
T
PIERO M. ARMENANTE
NJIT
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)
PIERO M. ARMENANTE
NJIT
Pitched-
Flow Patterns Resulting from the
Presence of Baffles (Stators)
Vortex
• Unbaffled cylindrical vessel
Flat intefacial area
• Baffled cylindrical vessel
(or unbaffled square
vessel)
PIERO M. ARMENANTE
NJIT
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
PIERO M. ARMENANTE
NJIT
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 ПЃ 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 =
ПЃ N3 D5
ПЃ N D2
• Impeller Reynolds Number = Re =
Вµ
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:
пЈ« ПЃ N D 2 N 2D T H C w
пЈ¶
P
Po = Ne =
,
, , , , ,impeller typeпЈ·
3
5 = fпЈ¬
g D D D T
ПЃN D
пЈ­ Вµ
пЈё
For baffled vessels, Fr is unimportant, and:
пЈ«ПЃ N D 2 T H C w
пЈ¶
P
Po = Ne =
= fпЈ¬
, , , , ,impeller typeпЈ·
ПЃ N3 D5
Вµ
D D D T
пЈ­
пЈё
For geometrically similar baffled vessels:
пЈ«ПЃ N D 2 пЈ¶
P
Po = Ne =
пЈ· = f (Re,impeller type)
3
5 = fпЈ¬
ПЃN D
пЈ­ Вµ пЈё
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
PIERO M. ARMENANTE
NJIT
10000
100000
Power Number for Submerged
Impellers
Laminar Regime:
1
Po = Ne в€ќ
Re
Transitional Regime:
Po = Ne = f (Re)
Turbulent Regime:
Po = Ne = Constant
PIERO M. ARMENANTE
NJIT
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
Flat-Blade Turbine (12 Blades)
8.5
Gate
5.5
Disc Turbine (Rushton Type)
5.0
Flat-Blade Turbine (6 Blades)
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
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
Horizontal Paddle Systems
Paddles mounted on horizontal shafts are often
encountered in flocculation tanks.
Plan View
PIERO M. ARMENANTE
NJIT
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
пЈ®2 ПЂ
пЈ№ 4
P = ПЃ CD b пЈЇ N (1 в€’ k )пЈє ( ro в€’ ri 4 )
8
пЈ°60
пЈ»
N = agitation speed in rpm
CD в‰… 1.8 for flat paddles
k = 0.25 (for tanks without baffles); 0-0.15 (for
tanks with baffles)
Range for peripheral velocity, 2ПЂN/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
PIERO M. ARMENANTE
NJIT
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
вЊ p dV = в€’ пЈµ ( po Vo ) = ( po Vo )ln
p
po
вЊЎ 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 lnпЈ¬1 +
пЈ·
t
t
po
po
пЈ­ po пЈё
PIERO M. ARMENANTE
NJIT
Power Dissipated by Gas Sparging
Recall that the pressure generated by a column of
liquid is:
p = ПЃL g h
where h is the height of the column of liquid, and
ПЃL 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 
пЈ« h (in m)пЈ¶
P = po Qo lnпЈ¬1 +
пЈ· = po Qo lnпЈ¬1 +
пЈ·
10.3 пЈё
пЈ­
пЈ­ po пЈё
пЈ« h (in ft)пЈ¶
= po Qo lnпЈ¬1 +
пЈ·
33.8 пЈё
пЈ­
PIERO M. ARMENANTE
NJIT
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 = ПЃ g Q hL
where:
ПЃ = 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).
PIERO M. ARMENANTE
NJIT
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)
GВ·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 в‹…t в‹…C 1.46 = 5.9 в‹…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.
PIERO M. ARMENANTE
NJIT
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 GaveВ·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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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.
PIERO M. ARMENANTE
NJIT
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
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