Development of surface activated granular media for effective adsorption and filtration

Chapter 1

Chapter 1 Introduction and Research Objectives

Water or wastewater is a complex system that often contains various kinds of dissolved
substances (organic or inorganic) and suspended/colloidal particles. Organic matters
found in water or wastewater may include such diverse species as humic substances,
carbohydrates, lignin, fats, soaps, synthetic detergents, proteins and their decomposition
products, as well as various synthetic organic chemicals from the process
industries. Examples of suspended matters in water and wastewater can include various
inorganic and organic particles, such as soil or clay particles, immiscible liquids, metal
hydroxides or microorganisms, etc. Many of the dissolved or suspended matters may
cause aesthetical problems and/or be toxic or hazardous to our human beings and other
aquatic lives. Therefore, appropriate treatment of water or wastewater is necessary in
order to make water suitable for dinking or wastewater suitable for discharging or reuse.

The unit processes used in water and wastewater treatment to remove dissolved or
suspended matters include coagulation/flocculation, sedimentation, granular media
filtration, adsorption, and biological treatment, etc. Among these processes, granular
media adsorption and filtration have widely been used. While the two processes are both
concerned with separating certain species present in a fluid stream, the sizes of the species
to be separated by the two processes are often different. The molecular sizes of the
dissolved species removed in adsorption are usually in the order of angstroms or

1


Chapter 1
nanometers, whereas the sizes of particles to be separated in granular filtration are in the
range of submicrons or microns.

Nevertheless, the operation of fixed-bed adsorption is quite similar to that of granular
filtration. In both operations, granular media are placed in a bed and the fluid to be treated
is allowed to flow through the granular media. It is also believed that the same types of
interaction forces (such as the London-van der Waals and electrostatic double-layer
forces) are active in both adsorption and filtration (Tien, 1989). Thus, many similarities
exist between adsorption and granular filtration processes, in terms of equipment
configuration, mode of operation, and the respective underlying phenomena. Because of
these similarities, the words adsorption and filtration have sometimes become
interchangeable (Tien, 1989; 1994). The removal of colloidal particles from a fluid phase
to a solid phase may be described as either adsorption or filtration (Hirtzel and
Rajagopalan, 1985). In engineering practice, granular carbon columns used to remove
organic compounds in drinking water supplies are often referred to as carbon filter.

For economic reasons, sand and anthracite have been widely used as granular filter media
in almost all water and wastewater filtration in municipalities and various industries. To
remove dissolved hazardous species, granular activated carbon is often applied as the
adsorbent in an adsorption process (Clark et al., 1989). Sand and anthracite themselves
are, in fact, not particularly efficient to remove suspended particles, especially those fine
submicron particles, such as colloids, bacteria (pathogens) and viruses, etc., nor are they
effective to remove any dissolved organic and inorganic substances. However, because of

2


Chapter 1
the difficulty and high cost in regeneration, it is impractical to use activated carbon for the
purpose of suspended particle removal. Moreover, activated carbon is inefficient in an
adsorption process to remove those dissolved organic matters with larger molecular mass
and wide molecular mass distribution, e.g. natural organic matters (NOM). Nowadays,
water and wastewater treatment systems often have to apply sand filtration followed by

activated carbon adsorption in order to achieve the desired level of purification. This
approach not only complicates the treatment process but also results in significant cost
increase.

The surface properties of granular media play an important role in the removal of
dissolved substances and suspended particles. The common mechanisms of particle
removal in granular filtration include interception, sedimentation and adsorption.
Interception and sedimentation are significant for large particles. For most of the fine
particles or colloids in water and wastewater, adsorption is the key mechanism for their
removal, which relies on the surface interactions between the media grains and the
particles to be removed. A classic theory for analysis and prediction of colloid adsorption
and deposition is the Derjaguin-Landau-Verwey-Overbeek (DLVO) model, which was the
postulate of Landon-ver der Waals and electrostatic double-layer interactions. In general,
the Landon-ver der Waals force is always attractive, but the double-layer force can be
either attractive or repulsive. Extensive theoretical calculations and experimental
investigations have demonstrated that colloid adsorption/deposition efficiency can be
enhanced significantly under attractive double-layer interactions.

3


Chapter 1
In most water and wastewater treatment processes, the dissolved substances and
suspended colloids and particles to be removed have negative surface charges. The
conventional filter media, such as sand, are usually negatively charged under normal
water and wastewater treatment conditions (Redman et al., 1997). Due to the force of
electrostatic repulsion, the removal of most of the fine particles, such as colloidal
particles, bacteria and viruses, has been difficult. Similarly, activated carbon also carries
negative surface charges in this pH range (Wu et al., 2001), thus decreasing the adsorption
efficiency greatly. Another important factor for effective adsorption or filtration is the

surface morphology of the granular media. Rough surfaces with micropores provide large
specific surface area for particle deposition. The heterogeneity of the surface morphology
also possibly changes surface charge distributions, with certain surface patches favorable
for particle deposition even though the overall interactions between the media grains and
the particles to be removed may be unfavorable (Tien, et al., 1979; Choo and Tien, 1995).
The rough surface will also change the hydrodynamics of colloid adsorption/deposition.
The smooth surface of the conventional filter media (e.g. sand) is another reason for their
being ineffective to the removal of suspended particles and dissolved substances.

It is therefore desirable to develop more effective granular media as both adsorption and
filtration materials for high efficient water and wastewater treatment and for
simplification of the treatment system. In the present study, the objectives are to develop
granular media with positive surface charges, which would provide attractively
electrostatic surface interactions between the granular media and the dissolved organic
substances or suspended particles to be removed in water or wastewater, thus enhance

4


Chapter 1
their removal in an adsorption or filtration process. Granular media with positive surface
charges are obtained through modifying the surface of inorganic and organic granules
with polypyrrole (PPy) or chitosan. The modified granules are then used to study the
performance and mechanisms in removing organic pollutant (use humic acid as a model
organic compound) and inorganic colloid (use clay particle as a model colloid). Attempts
are also made to understand the role of surface interactions in the removal of dissolved
and suspended substances in a granular media adsorption or filtration system.

This thesis is organized as follows. Chapter 2 presents background information on
granular adsorption or filtration in water and wastewater treatment, and the typical

pollutants in water system, and an overview of PPy and chitosan that were used in the
present study. The experimental parts of coating PPy on the surfaces of glass beads and
nylon 6,6 granules and immobilizing chitosan on the surfaces of PET and nylon 6,6
granules, as well as the adsorption and filtration experiments, are discussed in Chapter 3 –
Chapter 6. Chapter 7 discusses the qualitative and semiquantitative analyses of
electrochemical properties of PPy-water interface and presents a site-binding model for
humic acid adsorption onto PPy surface.

5


Chapter 2

Chapter 2 Literature Review

2.1 Surface Charges and Electrical Double Layer
Most solid surfaces will be charged when they are brought into contact with aqueous
solutions. The possible charging mechanisms include (1) ionization of the surface groups,
(2) specific adsorption of ions or surfactants from the solutions, and (3) lattice
imperfections at the solid surface or isomorphic substitution in the crystal lattice (Stumm,
1992; Koopal, 1993; Myers, 1999). Although the presence or absence of surface charge
may often be neglected in the macroscopic systems, in the microscopic systems of
colloids and interfaces, the surface charge is a critical factor and plays an important role in
many applications, such as coagulation, flocculation, adsorption and filtration.

The overall arrangement of the electric charge on the solid surface, together with the
balancing charge in the bulk solution, is often referred to as the electrical double layers
(EDL), or just double layers (Bockris and Reddy, 1970). The EDL can be regarded as
consisting of two regions: an inner region which may include adsorbed ions (i.e. the
adsorption layer), and a diffuse region in which ions are distributed according to the

influence of electrical forces and random thermal motion (i.e. the diffusion layer).

The Gouy-Chapman theory (Figure 2.1a) is the one-dimensional analysis of the diffusive
double layer based on Poisson’s equation and Boltzmann equation, and gives (Hunter,
1991)

6


Chapter 2

dψ
2κkT
zeψ
sinh
=−
dx
ze
2kT

(2.1)

where ψ is the electrical double layer potential at a distance x from the surface, k is the
Boltzman constant, T is the absolute temperature, z is the valency of the counterions, e is
the coulombic charge, and κ is the reciprocal of the electrical double layer thickness
which is defined as

κ=

1000 N A e 2 ∑ ci z i2

εkT

(2.2)

where NA is the Avogadro’s number, ci is the molar concentration of the counterions of
type i, zi is the valency of counterion i, ε is the solution permittivity (equal to ε0εr, ε0
being the permitivity of vacuum and εr the relative permittivity of the liquid).

The total charge σd, per unit area of surface, in the diffuse layer can be calculated from
Eq. (2.1) as (Hunter, 1991)

σd = −

zeψ d
2κkTε
sinh
2kT
ze

(2.3)

where ψd is the electrical potential at the onset of the diffuse layer.

To improve the Gouy-Chapman model, Stern proposed to divide the double layer into two
parts separated by a plane (the Stern plane) located at about a hydrated ion radius from the
surface (Figure 2.1b). Specific ion adsorption may take place at the Stern plane when
electrostatic and/or van der Waals forces are strongly enough to overcome the thermal
agitation. Across the Stern plane, the potential drops from the surface potential ψs to the

7


Chapter 2
potential at the Stern plane, ψd, and further decays from ψd to zero in the diffusion layer.
The potential drop (ψs- ψd) is related to the capacity of Stern layer, C1, as (Koopal, 1993)

ψ s −ψ d =

σs

(2.4)

C1

and C1 is defined as
C1 =

ε 0ε r
δ

(2.5)

where δ is the distance from the surface to the Stern plane.
Stern plane
Surface of shear

Diffuse layer
Stern layer

ψ0

Potential ( ψ)

Potential (ψ )

ψ0
ψd

1/κ

Distance (x)

(a) according to Gouy-Chapman model

ζ

δ

1/κ

Distance (x)

(b) according to Stern model

Figure 2.1 Schematic representation of the structure of the electrical double layers (EDL).

8

Chapter 2
ψd can be estimated from electrokinetic measurements. Electrokinetic behavior depends
on the potential at the surface of shear between the charged surface and the electrolyte
solution. This potential is called the electrokinetic or ζ (zeta) potential. The concept of the
surface of shear implies some idealization and the exact location of the shear plane is an
unknown feature of the electrical double layer (Shaw, 1970; Kohler, 1993; Kosmulski,
1995). Usually the shear plane is supposed be located at a small distance further out from
the stern plane. In general, ζ potential may not be too different from ψd (usually a little
smaller in magnitude than ψd), but ψd can often be considerably smaller than ψs. It is
customary to assume identity of ψd and ζ potential for low values of the ionic strength
(Koopal, 1993; Kosmulski, 1995).

2.2 Surface Interactions
2.2.1 Short-Range Forces and Long-Range Forces
Surface interaction forces can generally be subdivided into two types: short-range forces
and long-range forces. The action distance of short-range forces is usually no more than
0.1-0.2 nm (Garbassi et al., 1998). The typical short-range force is covalent force. When
two atoms bind to form a non-ionized molecule, the force involved in bond formation is
referred to as covalent force, and the resulting bond is covalent bond. Covalent bond has
certain characteristic bond length and bond angle which depends on the atoms involved.
Hence covalent force is directional.

Other short-range forces include hydrogen-bonding interactions and Lewis acid-base
interactions. Hydrogen bond can be formed between a proton covalently bonded to a

9


Chapter 2
highly electronegative atom (e.g. O, N, F) and another electronegative atom nearby. The

hydrogen bond energy depends, in a rather complex way, on the distance between the
participating atoms and the angle between the atoms. Fowkes (1985) defined the
interactions between electron acceptors (Lewis acids) and electron donors (Lewis bases)
as Lewis acid-base interactions. The interaction strength of a basic or acidic site depends
not only on the ability to donate or accept electrons, but also on the polarizability. Van
Oss (1994) classified hydrogen-bonding interactions and Lewis acid-base interactions as
non-covalent interactions, whilst other researchers (Drago et al., 1977) suggested that
these interactions have, at least partly, a covalent character.

As far as surface and colloidal phenomena are concerned, one often only considers the
long-range forces or physical interactions which act between discrete, non-bonded atoms
or molecules over distances significantly greater than molecular bond dimensions and are
generally nondirectional. The two fundamental long-range forces include two kinds:
coulombic or electrostatic interactions, and van der Waals forces.

For two point charges, Q1 and Q2, the free energy of electrostatic interaction, wel, may be
given by (Myers, 1999)
wel =

Q1Q2
z z e2
= 1 2
4πε 0 εr 4πε 0 εr

(2.6)

where r is the distance between the two charge, and z is the valency of each ion. For two
charges of the same sign, wel is positive, which means that the interaction is repulsive. In
contrast, wel is attractive for two unlike charges.

10


Chapter 2
Forces between molecules caused by permanent and induced dipoles and other multipoles
are collectively known as van der Waals forces. The fluctuating dipole-induced dipole
interactions, described by London, is called London-van der Waals (dispersion)
interactions, which often makes the most important contribution to the total van der Waals
interactions, especially in the aqueous media containing electrolytes (Chaudhury, 1984).
Hamaker (1937) calculated the London-van der Waals force between individual atoms of
spherical particles. His treatment was rather coarse, but the concept of a Hamaker
constant has still been used. A more accurate calculation was developed by Lifshitz and
co-workers (Dzyaloshinskii et al., 1961), who described the van der Waals force as
originating from spontaneous electromagnetic fluctuations at interfaces.

The London-van der Waals forces are characterized as being universal and almost always
attractive over relatively long distances. The simplest situation in analyzing London-van
der Waals force is of two hard, flat, and infinite surfaces separated by a distance, d, in a
vacuum. The free energy of attraction per unit area, wvdw, in such a case is given by
(Myers, 1999)

wvdw = −

A
12πd 2

(2.7)

where A is the Hamaker constant, which depends on the dielectric properties of the two
interacting plates and the intervening medium and typically amounts to about 10-20 J.

Although short-range forces are regarded as inferior factors to long-range physical forces
in the systems of colloids and surfaces, short-range forces may have indirect effects on
longer-range interactions. For example, solvation layer is created through the short-range

11


Chapter 2
interactions such as hydrogen bonds or chemical bonds between the surface and the
solvent molecules, and the solvation layer influences the long-range forces between two
approaching surfaces greatly. Moreover, adsorbed macromolecule layer strongly affects
the interaction between surfaces (Fleer et al., 1993a; Claesson, 1998).

2.2.2 DLVO Theory

The combined action of van der Waals forces and electrostatic or electrical double layer
forces in aqueous systems is described by the DLVO theory, which was developed by two
groups of researchers, Derjaguin and Landau, and Venwey and Overbeek, independently
in the late 1940s (Shaw, 1970). For two symmetrically charged plates in an electrolyte
solution, the total free energy of interactions in the DLVO theory is expressed by the sum
of the van der Waals and electrostatic contributions (Kohler, 1993; Garbassi et al., 1998):
w = wel + wvdw =

64c0 kT

κ

[tanh(

eψ s 2 −κd
A
)] e −
4kT
12πd 2

(2.8)

where c0 is the bulk concentration of the electrolyte solution, and ψs is the surface
potential.

In Eq. (2.8), the term of van der Waals forces tends to infinity as separation distance, d,
approaches to zero, while the electrostatic term remains finite. Therefore, the van der
Waals attractive interactions always prevail at small distances. On the other hand, the van
der Waals’ term is insensitive to the change of ionic concentration or solution pH, which
greatly affects the electrostatic term. For a given colloid suspension system, van der
Waals attraction outweighs electrostatic repulsion at small separations, while at
intermediate separations electrostatic repulsion predominates and the high energy barrier

12


Chapter 2
prevents the colloid surfaces from contacting each other. As the ionic concentration
increases, the thickness of the double layers, κ-1, decreases (see Eq. (2.2)), and thus the
repulsion between the colloid surfaces are reduced (decrease of e-κd outweighs the
increase of c0/κ). Hence, increasing ionic concentration favors flocculation and
coagulation of colloids.

DLVO theory represents the pillar of colloid science. Ninham (1999) asserted that DLVO

stands at the same level of importance for colloid science as does Darwin’s theory of the
origin of species in biology. However, DLVO theory failed at short distance from the
surface (Sposito, 1984). Non-DLVO forces, or so-called “extra-DLVO” forces (Pashley,
1981; Israelachvili, 1992), such as solvation, hydration, hydrophobic, oscillator, capillary
and water structure forces may become operative at separations below 5 nm. Moreover,
the agreement between DLVO theory and experimental measurement is less satisfactory
for biological regime or for any system where the ionic concentration is in the order of 0.1
M or higher. Boström et al. (2001a, 2001b) pointed out that this might be, in part, due to
the reason that the dispersion forces of specific ion effects are ignored.

Despite these limitations, the classical DLVO theory has appeared to work reasonably
well at separations of intermediate distance and for low ionic concentrations (<5×10-2 M).
A recent review on the application of DLVO theory for colloid adsorption and deposition
at solid/liquid interfaces was given by Adamczyk and Weroński (1999). It was
demonstrated in this review that the electrostatic interactions played the most important

13


Chapter 2
role in both adsorption (especially at the initial adsorption stages) and deposition of
colloid particles.

2.3 Adsorption and Granular Filtration

Separation processes like adsorption and filtration refer to the operations that transform a
mixture of substances into two or more products (King, 1980).

2.3.1 Adsorption
2.3.1.1 Definition and Applications

Adsorption is a term to describe the existence of a solute concentration (dissolved
substance) at the interface between a fluid and a solid higher than that present in the fluid
(Masschelein, 1992). Generally, adsorption is classified as physical adsorption (i.e.
physisorption) in which the van der Waals interactions are involved and chemical
adsorption (i.e. chemisorption) in which the adsorbed molecules are attached by chemical
bonding. Adsorption is of great technological importance in separation process, industrial
catalysis and pollution control. Adsorption phenomena also play a vital role in many
solid-state reactions and biological mechanisms (Rouquerol et al., 1999). For colloid and
surface science, adsorption affects the surface charge of suspended particles and colloids,
and so influences their aggregation and transport, which is important in water and soil
science (Stumm, 1992).

As a separation process, most of the adsorption-based processes have a single fluid feed
stream (liquid or gas) containing one or more species to be removed, or so-called

14


Chapter 2
adsorbates. The separating agent is the adsorbent pellets or powder, i.e. adsorbents, used
in the process. The products included the fluid steam with the adsorbate species removed
or depleted and the adsorbents saturated with the adsorbates initially present in the feed
steam (King, 1980; Tien, 1994).

One of the earliest adsorption applications is purification, such as the removal of H2S and
obnoxious fumes from air and the removal of organic compounds from liquid water (Al
Duri, 1996). Adsorption has also been applied to dyestuffs removal from textile industry,
removal of odor and color from edible oils, decolorization in the sugar industry, and
removal of unwanted hydrocarbons in oil refining (Pollard, 1993; Nogueira, 1996;

Ahmedna et al., 2000). Some biological materials and precious metals, e.g. gold, are also
recovered by adsorption (Nakajima and Sakaguchi, 1993).

Adsorption has been demonstrated to be an especially efficient and economically feasible
unit process in water and wastewater treatment. Adsorption by granular activated carbon
(GAC) is a widely used water and wastewater treatment operation to remove both natural
and man-made micro-pollutants such as natural organic matters (NOM), pesticides,
industrial chemicals, tastes and odors and algal toxins (Newcombe, 1999; Shawwa et al.,
2001). The aluminum hydroxide and ferric hydroxide solids formed during coagulation
process may adsorb organic compounds (Julien et al., 1994) or remove microorganisms
(Bitton, 1994). Adsorption processes for high-level treatment of metal-bearing tap water
or wastewater using low-coast adsorbents such as bark (Martin-Dupont et al., 2002),
lignin (Lalvani et al., 1997), fungus (Kapoor and Viraraghavan, 1997), algae (Hamdy,

15


Chapter 2
2000), and chitosan (Guibal et al., 1998; Inoue et al., 1999) are becoming increasingly
attractive in recent years.

2.3.1.2 Adsorption Isotherms

Adsorption is often described in terms of isotherms which show the relationship between
the bulk concentration of adsorbate and the adsorbed amount at constant temperature.
There are a few equations or models that are available to describe the adsorption
isotherms. Yu and Neretnieks (1990) thoroughly reviewed model isotherms for singlecomponent adsorption. Only two of the more common equations, the Langmuir and
Freundlich equations, are presented here because of their simplicity and wide utility.

Adsorption of molecules can be represented as a chemical reaction:

A+B

A .B

(2.9)

where A represents the adsorbate, B the adsorbent, and A·B the adsorbed compounds.
Adsorbates are attached to the surface of the adsorbent by various types of chemical
forces such as hydrogen bonds, dipole-dipole interactions, as well as van der Waals
forces. Based on the assumptions that (1) adsorption is restricted to monolayer coverage,
(2) adsorption is localized, and (3) the heat of adsorption is independent of the amount of
adsorbate adsorbed, Langmuir isotherm equation can be obtained by applying the
equilibrium equation and mass law to Eq. (2.9) as:
q=

qm K a C
1 + K aC

(2.10)

16


Chapter 2
where q is the amount of adsorbate adsorbed per unit weight of adsorbent at equilibrium
concentration, C is the equilibrium or final concentration of the adsorbate in the solution,
qm is the maximum adsorption at monolayer coverage, and Ka is the adsorption
equilibrium constant. The values of qm and Ka can be determined from a plot of 1/q versus
1/C in accordance with a linearized form of Eq. (2.10):
1

1
1
=
+
q qm K a qm C

(2.11)

Like many classic approaches such as DLVO theory, Langmuir equation has its
fundamental weaknesses. The assumptions of Langmuir equation mentioned above are
almost never met in practice. However, Langmuir equation has been found to be widely
applicable in many systems when such conformity doses not imply the requirement of the
Langmuir assumptions. Langmuir model accurately describes a significant amount of
adsorption data in a mathematically simple method, and this makes it invaluable as a basis
for adsorption studies (Mayers, 1999).

The Freundlich equation is another classic adsorption isotherm that is very useful at
describing adsorption. This equation is an empirical equation which has the form
1

q = KfCn

(2.12)

and can be transformed to the log-linearized form as follows
log q =

1
log C + log K f
n

(2.13)

where q and C have the same definitions as in the Langmuir equation, Kf is a constant and
represents the adsorption capacity, and n is a constant and represents the adsorption

17


Chapter 2
intensity. When 1/n < 1, the adsorption is said to be "favorable", and 1/n > 1 is called
"unfavorable". Although the Freundlich equation was developed empirically, it can also
be derived theoretically using a model in which it is assumed that the heat of adsorption is
not constant but varies exponentially with the extent of surface coverage (Halsey and
Taylor, 1947; Halsey, 1948), which is more reasonable than the Langmuir assumption in
most cases.

The Freundlich equation applies very well for solids with heterogeneous surface
properties. However, this equation cannot be applied to all values of C. As C increases, q
increases (according to Eq. (2.12)), only until the adsorbents reach saturation. At
saturation, q is a constant and is independent of further increase in C, thus the Freundlich
equation no longer applies. In addition, there is no assurance that adsorption data will be
in a good agreement with the Freundlich equation over all concentrations less than
saturation.

2.3.2 Granular Filtration
2.3.2.1 Definition and Applications

Granular filtration is a fluid-solid separation process commonly applied to remove minute
quantities of small particles (liquid or gas suspensions) from various kinds of fluid by

passing through granular media (Aitken, 1969; Tien, 1989). As the fluid or suspension is
forced through the voids or pores of the granular media, the solid particles are retained on
the medium’s surface or, in some cases, on the walls of the pores, while the fluid (i.e.
filtrate) passes through (Cheremisinoff, 1998).

18


Chapter 2

Both Sanskrit medical lore and Egyptian inscriptions give clear evidence that granular
filtration was used for water treatment as early as 200 B.C. The versatility of granular
filtration is evident from its scope of application as well as from the manner in which it is
carried out. Besides water or air, systems that may be treated by granular filtration include
such diverse substances as flue gas, molten metal, petrochemical feedstocks, polymers,
and alcoholic or nonalcoholic beverages. Although granular filtration is frequently carried
out in the fixed-bed mode, it may also be conducted in a moving-bed or fluidized-bed
mode so that the operation is continuous.

The most commonly known application of granular filtration is for water and wastewater
treatment to remove solids, including bacteria present in surface waters, suspended clay
particles, precipitated hardness from lime-softened waters, and precipitated iron and
manganese (Uchrin, 1983). It has been reported that various forms of sand filtration have
been used to purify water for centuries (Montiel et al., 1988). Slow sand filtration was
first developed to purify surface water for drinking purposes. Since the mid-19th century,
slow sand filtration has been widely employed in treating community water supplies in
many countries against water-borne disease. A significant improvement to water
treatment in the 1880s and 1890s was the development of rapid sand filters, which could
handle considerably larger volumes of water.

At present, granular filtration of water and wastewater is inevitably applied in conjunction
with sedimentation and/or coagulation. Granular filtration is widely used to remove

19


Chapter 2
residual biological floc in settled effluents from secondary treatment by trickling filters or
activated sludge process, to remove residual chemical-biological floc after alum, iron, or
lime precipitation of phosphates in secondary settling tanks of biological treatment
processes, and to remove solids remaining after the chemical coagulation of wastewaters
in tertiary or independent physical-chemical waste treatment (Cheremisinoff, 1995).

2.3.2.2 Filtration Theory

Although granular filtration is one of the most widely used processes in a variety of
applications, the design and operation of filters is still carried out on an almost entirely
empirical basis. The reason for this is that a filter itself may be a relatively simple device,
but the process of filtration is quite dynamic and extremely complex. In general, filtration
process involves two sequential steps, i.e. transportation and attachment (O’Melia, 1985).
Particles in the suspension to be filtered are first transported from the bulk of the fluid to
the vicinity of the stationary surface of the filter media by physical forces. Then
attachment of the particles to the collectors (filter media grains) occurs through various
physical and chemical interactions (Amirtharajah and Westein, 1980). The major surface
interaction forces between the collector grains and the suspended or colloidal particles
include the London-van der Waals force and the double layer force. The former is
attractive and remains essentially constant during particle deposition. The double layer
force can be either attractive or repulsive and may change during particle deposition (Bai
and Tien, 1997; 2000a). While much progress has been made in the area of filtration
modeling, no generally applicable models for the process have been developed yet.

20


Chapter 2
Until about 1970, most of the modeling work in filtration had followed what is known as
the phenomenological approach. In its simplest form, the process is described by two
equations (Iwasaki, 1937)
∂c
= −λc
∂Z

(2.14)

∂σ
∂c
+u
=0
∂t
∂Z

(2.15)

where c is the concentration of particles, Z the depth of filter bed, λ the filter coefficient, σ
the (absolute) specific deposit and u the approach velocity. Eq. (2.14) basically describes
the rate of removal and Eq. (2.15) is a mass balance equation. The solution of Eqs. (2.14)
and (2.15) will depend on the filter coefficient which is usually taken to be a function of σ.

Various attempts have been made to find a correlation between the initial filter coefficient
λ0 and the various system variables such as media grain size, approach velocity, etc., and

to determine a relationship between λ and σ. From about 1970 onwards, the trajectory
analysis approach has been used extensively for the study of λ0 and received much success
(Yao et al., 1971; Tien et al., 1979). Predicting the development of removal (λ against σ)
is however a much harder task. O’Melia and Ali (1978) recognized that deposited
particles can act as additional collectors, and thus enhance particle removal in a filter
(increase in removal efficiency or so-called ripening). Chang and Tien (1985) developed a
dendrite model to quantify the increase in removal efficiency due to the presence of
deposited particles. As deposition progresses, the process becomes increasingly complex.
Some pores in the filter may be blocked off, and therefore become unavailable for
deposition (Tien et al., 1979). The increase of interstitial velocity in the filter pores can

21


Chapter 2
also result in detachment of particles that deposited previously (Bai and Tien, 1999). In
addition, Choo and Tien (1995) have considered that the deposit may be porous for the
flow, whereas most others have assumed that the deposit layer is impermeable.

More recently, Bai and Tien (2000b) have proposed the rate expressions that distinguish
different types of deposition in filtration. They assume that the media grain surface may
be divided into two parts: one covered with deposited particles, and the other without
particles. The nature of surface interactions between the particles and either of those two
parts of grain surfaces are therefore characterized as particle-particle and collector-particle
types. The overall filter coefficient is given as

λ = (λ ) c − p (1 − f ) + (λ ) p − p f

(2.16)

where (λ)p-p is the filter coefficient between grains covered with deposited particles and
particles to be collected, (λ)c-p the filter coefficient between clean grains and particles to
be collected, and f is the fraction of the grain surface covered with deposited particles. In
many cases, (λ)p-p and (λ)c-p can be expected not to vary with time since the condition
determining the surface interactions remains the same. Bai and Tien (2000b) also divide
the deposition process into two types: deposition on grains and deposition on previously
deposited particles. The first type is named as the monolayer deposition while the latter as
multilayer deposition. They demonstrated that Eq. (2.16), although simple, together with
Eq. (2.14) and (2.15), is quite versatile to describe the various filtration behaviors.

2.4 Granular Media for Adsorption and Filtration
2.4.1 Conventional Granular Adsorbents and Filter Media

22


Chapter 2
Adsorbents and filter media represent the hearts of adsorption and filtration devices. All
practical adsorbents have large specific surface areas and are therefore highly porous or
composed of fine particles. The old types of industrial adsorbents (e.g. activated carbons
and silica gels) are generally non-crystalline and their surface and pore structure therefore
tend to be ill-defined and difficult to characterize. Many new adsorbents possessing
intracrystalline pore structures have been developed over the past 20 years, including
carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model
mesoporous solids (Rouquerol et al., 1999). In addition, various modern spectroscopic
and microscopic techniques can now be employed for studying the surface state and the
microstructure of the adsorbents (Dobiás et al., 1999). Although several materials may be
applied in the adsorption process for water and wastewater treatment, including alumina,
silica gel, Fuller’s earth and diatomaceous earth, granular activated carbon (GAC) has
been by far the most widely used adsorbent which provides tertiary treatment for water

contaminated with organic matters.

By the structure of materials, filter media may be classified into two types: flexible and
rigid media (Cheremisinoff, 1995). Rigid filter media are commonly used for granular
filtration. Ceramic filter media for example are widely used in gas filtration, and in
separation of dust and liquid droplets from gases (Loff, 1981). The relatively uniform
particle size of diatomaceous media achieves high efficiency of filtration in retaining solid
particles of sizes less than 1 µm, as well as certain types of bacteria. Plastic granules (e.g.
polyvinyl chloride and nylon) have gained growing attention as filter media in recent
years because of their relatively low cost (Driscoll, 1977; Loff, 1981). In water and

23


Chapter 2
wastewater treatment, the most commonly used filter media include silica sand (specific
gravity =2.65), garnet sand (specific gravity = 4.0-4.4) and anthracite coal (specific
gravity = 1.35-1.75).

The effectiveness of granular adsorption and filtration processes largely relies on the
surface interactions between the substances to be removed and the granular media. As
discussed early, DLVO forces (London-van der Waals force and electrostatic double layer
force) play the most important role in colloid adsorption and deposition processes. In
general, the London-van der Waals force is always attractive, but the double layer force
can be either attractive or repulsive. Hence, the surface charges of impurities or pollutants
in water systems and those on the granular media have an impact on the efficiency of
water and wastewater treatment. In face, most impurities or pollutants in waters carry
negative surface charges in the pH range of natural waters (Ives, 1990; Chen et al., 1998).
On the other hand, the activated carbon, sand and anthracite that are used to remove these
impurities or pollutants also carry negative surface charges in that pH range. Therefore,

the conventional filter media or adsorbents are not particularly efficient to remove
suspended particles, nor are they effective to remove dissolved organic and inorganic
substances. This problem may be solved by surface modification of the traditional
granular media to obtain the desired surface properties, i.e., positive surface charges.

2.4.2 Surface Modification of Granular Media

Several studies have examined the possibility of modifying granular media to improve
their ability to remove dissolved matters and suspended particles. These modifications

24


Chapter 2
included impregnation of coal with metallic hydroxides (Chaudhuri and Sattar, 1986;
Lukasik et al., 1999), addition of positive charges to silica using organosilane derivative
(Zerda et al., 1985), incorporation of metallic hydroxides onto the surfaces of sand using
in situ precipitation of metallic hydroxides (Farrah and Preston, 1985; Lukasik et al.,
1996), adsorption of metallic flocs onto the surfaces of sand (Edwards and Benjamin,
1989), and modification of diatomaceous earth by precipitation of metallic peroxides
(Farrah et al, 1988).

Among these developments, coating sand with ferric and aluminum oxide or hydroxide
has received much attention. Because the surface of granules became electrically positive,
it facilitated the attachment of the negatively charged particles or other pollutants found in
water and wastewater (Farrah and Preston, 1985). The coated sand was also found to
effectively adsorb microorganisms (Lukasik et al., 1996; Mansoor and Chaudhuri, 1996).
The experimental and theoretical studies by Truesdail et al. (1998) demonstrated the
benefit of granule surfaces coated with metal oxides, or hydroxide rich oxide/hydroxide
mixtures in increasing the efficiencies of commercial filtration systems. The

electropositive surface coatings from aluminum or mixed (hydr)oxides, had similar
average kinetic rate constants and were five times greater than the rate constants for the
uncoated sand. In particular, the adsorption of natural organic matters (NOM) onto iron
oxide coated sand has been actively studied recently. A new granular adsorbent based on
β-FeOOH was developed by Teermann and Jekel (1999), and was shown to have high
adsorption capacities for both smaller and higher molecular weight humic substances. It
was reported that coating iron oxide particles with cationic polymer significantly

25