# Particles Contributing to Turbidity

## Introduction

To address turbidity removal during treatment processes, an
understanding of the physical characteristics and properties of
particles in raw water is required. This chapter provides an overview of
the inorganic, organic and biotic particles, as well as particles
created during typical treatment processes that contribute to turbidity.
Because the stability of particles in water is dominated by the
electrokinetic properties, a discussion of electrokinetic properties is
included to provide information concerning how these properties affect
the removal of particle contamination during the treatment process.

## Characteristic Properties of Particles

Particles in a raw water supply may be composed of inorganic materials,
pathogens, or toxic materials. These particles may also provide sorbent
sites for pesticides and other synthetic organic chemicals and heavy
metals. Particles are undesirable not only for the cloudy appearance
they impart to finished water, but because they also have the ability to
shelter microorganisms from inactivation by disinfectants. Consequently,
a principal element in supplying quality drinking water is the maximum
removal of particles. To establish or optimize a particle removal
process, it is important to understand the physical properties of
particles.

Particles suspended in water can be categorized into three classes based
on their origin:

1.  Inorganic materials, such as silt or minerals;

2.  Living or dead organic matter; and

3.  Biotic material including algae, viruses and bacteria.

Due to the range of small sizes for common particles in water, it is
common to find sizes termed in "microns" within the water industry. A
micron, or micrometer, is equal to 1 x 10^-6^ meters, or 0.00004 inches.
Generally, particulate contaminants to be removed from a raw water
source range from the larger macro sized particles visible to the naked
eye, to the ionic particles viewed only by scanning electron
microscopes.

Figure 8-1 illustrates some common particles found in raw water sources
and indicates where, within the size range, these particles would
typically be detected.

Source: Osmonics, Inc., 1996; AWWA, 1990.

[]{#__RefHeading___Toc448538312 .anchor}Figure 8-1. Particle Size
Spectrum

### Particle Settling

Particle settling, or sedimentation, may be described for a singular
particle by the Newton equation for terminal settling velocity of a
spherical particle. A knowledge of this velocity is basic in the design
and performance of a sedimentation basin.

The rate at which discrete particles will settle in a fluid of constant
temperature is given by the equation:

V = \[(4g (ρ~s~ - ρ) d) \\ (3 C~d~ ρ)\] ^0.5^

> where V = terminal settling velocity

g = gravitational constant

ρ~s~ = mass density of the particle

ρ = mass density of the fluid

d = particle diameter

C~d~ = Coefficient of drag (dimensionless)

The terminal settling velocity is derived by equating the drag, buoyant,
and gravitational forces acting on the particle. At low settling
velocities, the equation is not dependent on the shape of the particle
and most sedimentation processes are designed so as to remove small
particles, ranging from 1.0 to 0.5 micron, which settle slowly. Larger
particles settle at higher velocity and will be removed whether or not
they follow Newton\'s law, or Stokes' law, the governing equation when
the drag coefficient is sufficiently small (0.5 or less) as is the case
for colloidal products (McGhee, 1991).

Colloids are very fine solid particles, typically between 10 and 0.001
microns in diameter, which are suspended in solution. Colloidal
particles are not visible even with the aid of high-powered microscopes
(Sawyer and McCarty,1978). Colloids will not settle out by gravitational
forces and may not be removed by conventional filtration alone. The
removal of colloidal particles is typically achieved by coagulation to
form larger particles, which then may be removed by sedimentation and/or
filtration. Coagulation, as defined by Kawamura (1991), is the
"destabilization of (the) charge on colloids and suspended solids,
including bacteria and viruses," and is further discussed in Section
8.7, "Electrokinetic Properties of Particles."

### Particle Density and Size Distribution 

Typically, a large range of particle sizes will exist in the raw water
supply. Type 1 settling is the designation given to discrete particles
of various sizes, in a dilute suspension, which settle without
flocculating. Dilute suspensions of flocculating particles, where
heavier particles overtake and coalesce with smaller and lighter
particles, are given the designation of Type 2. As there is no
mathematical equation which can be applied to the relationships of Type
1 and 2 sedimentation, statistical analysis is applied to predict the
settling velocities for particles in water having a broad range of size
and density. Particle size distribution analysis (Type 1) or
settling-column analysis (Type 1 or 2) is applied and a settling
velocity cumulative frequency curve is obtained and used in settling
basin design. An excellent resource for understanding the use of
settling column analysis, and discrete particle settling is given by
Gregory and Zabel (1990).

Type 3a and 3b, or hindered settling, occur when high densities of
particles in suspension result in an interaction of particles. The
displacement of water produced by the settling of one particle affects
the relative velocities of its neighbors (McGhee, 1991). A zone is
formed in which more rapidly-settling particles act as a group with a
reduced settling velocity. However, even at fairly high concentrations,
the reduction in settling velocity is not significant. The following
equation from McGhee (1991) gives an estimate of the magnitude for
hindered settling:

V~h~/V = (1 - C~v~)^4.65^

where V~h~ = hindered settling velocity

V = free settling velocity

C~v~ = volume of particles divided by total volume of the suspension.

## Inorganic Particles

Inorganic particles in water are produced by the natural weathering of
minerals, including both suspended and dissolved materials. Inorganic
particles may consist of iron oxides, salts, sulfur, silts and clays
such as bentonite or muscovite. Depending on the concentration of
inorganic particles present in raw water sources, human health effects
can vary from beneficial to toxic.

### Naturally Occurring Minerals 

Naturally occurring minerals find their way into raw water sources
either naturally through the breakdown of minerals in rock, or through
industrial process discharges which have contaminated a raw water
source. Industrial contributors can include mining, smelting, coal
burning power producers, oil and gas companies, and electroplating
operations.

Clays, metal hydroxides, and other particles originating from mineral
sources typically vary from several nanometers to several microns in
diameter, with a continuous size distribution over this range. In
surface waters, the majority of these particles are within a 0.1 to 1
micron size range. As a result of their settling characteristics,
particles in this size range have the ability to remain in suspension in
moving water. Particles of this size range scatter visible light
efficiently, due to the larger surface areas which are created as
particles decrease in size. This scattering gives the water a turbid, or
cloudy, appearance at very low concentrations. However, Wiesner and
Klute (1998) suggest that the real threat of these particles is the
adsorptive properties. The large surface areas created by even a small
mass concentration of the colloid particles provide abundant adsorption
sites for natural and synthetic organic matter, metals, and other toxic
substances. Bacteria and viruses can also attach to these particles, and
there is some concern that inorganic particulate contamination has the
ability to shield microorganisms from inactivation by disinfectants.

Dissolved inorganics known to have adverse health effects on humans when
ingested include aluminum, arsenic, cadmium, copper, fluoride, lead, and
mercury. The EPA has established maximum contaminant levels (MCLs) for a
variety of inorganic contaminants and is in constant review of health
advisories to determine the health effects from inorganics ingested in
drinking water (Tate and Arnold, 1990). The inorganic materials for
which MCLs have been established are toxic to humans in some form.

## Organic Particles

Organic materials are compounds, natural or manmade, having a chemical
structure based upon the carbon molecule. Millions of organic compounds
containing carbon have been identified and named, including;
hydrocarbons, wood, sugars, proteins, plastics, petroleum-based
compounds, solvents, pesticides and herbicides.

Both naturally-occurring and synthetic organics are present in surface
waters and typically originate from the following sources (Tate and
Arnold, 1990):

1.  The decomposition of naturally occurring organic materials in the
    environment;

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4.  Industrial, agricultural and domestic activities; and

5.  Reactions occurring during the treatment and distribution of
    drinking water.

Organics may have adverse human health impacts, such as toxicity, or as
carcinogens when ingested. In addition, naturally occurring organics,
most widely referred to as natural organic matter (NOM), can give raw
water a characteristic color, taste, or odor. Furthermore, organics in
water can be altered by treatment processes resulting in disinfection
byproducts (DBPs). In the following sections, a description of the
organic constituents in raw water is provided.

### Synthetic Organics

Artificial organics, or synthetic organics, can infiltrate raw water
supplies through overland flow of contaminated urban and agricultural
rainwater; direct discharge from industries and wastewater treatment
plants, and, as leachate from contaminated soils. Most contaminants
found in water supplies that have adverse health effects are synthetic
organics including: herbicides and pesticides; solvents; and,
polychlorinated biphenyls commonly known as PCBs (Tate and Arnold,
1990). The EPA has set MCLs for many synthetic substances, both in
industrial waste discharge and within the primary drinking water
standards.

### Natural Organic Matter (NOM)

In the majority of raw water sources, the largest fraction of all
organic particles is due to NOM originating from the degradation of
plant or animal materials (Wiesner and Klute, 1998). NOM is undesirable
in raw water for a variety of reasons, ranging from undesirable color to
providing adsorption site for toxic substances. NOM will also adsorb to
inorganic particles present in raw water, reducing the settling
properties of those particles. Aiken and Cotsaris (1995) recognized
numerous studies supporting the importance of NOM in mobilization of
hydrophobic organic species; of metals (lead, cadmium, copper, zinc,
mercury, and chromium); and radionuclides through the treatment process.
Elevated levels of certain NOM constituents require additional
coagulation in order to destabilize the particles and remove them in
sedimentation and/or filtration basins.

NOM is also present in raw water supplies as colloidal organic carbon in
the form of humic materials. Humic substances have generated
considerable attention due to their disinfection by-products (DBP)
formation potential (Amirtharajah and O'Melia, 1990).

### Total Organic Carbon (TOC)

TOC is a composite measure of the overall organic content, in a water
sample. TOC is measured by the amount of carbon dioxide produced when a
water sample is atomized in a combustion chamber (Standard Methods,
1985). Total organic halogen (TOX) indicates the presence of halogenated
organics, and is a proper indication of synthetic chemical
contamination. Either of these methods are more economical than testing
for any, or all, individual organic compounds likely to be in a raw
water supply.

### Organic Disinfection By-products (DBPs)

The use of oxidants for disinfection, taste and odor removal, or for
decreasing coagulant demand also produces undesirable organic
by-products. These by-products are difficult to analyze and remove from
the treatment process. Organic contaminants formed during water
treatment include trihalomethanes (THMs) and haloacetonitriles. Surveys
conducted since the mid-1970s have determined that chloroform and other
THMs are the organic chemicals occurring most consistently, and at
overall highest concentrations, of any organic contaminant in treated
drinking water (Wiesner and Klute, 1998).

THMs are formed in water when chlorine being used as a disinfectant
reacts with NOM, such as humic acids from decaying vegetation.
Chloroform is one of the most common THMs formed in this type of
reaction. The THMs include trichloromethane or chloroform;
dibromochloromethane; dichlorobromomethane; and bromoform.

Water chlorination not only produces THMs, but also a variety of other
organic compounds. Alternative disinfectants such as, chloramines,
chlorine dioxide, and ozone can also react with source water organics to
yield organic by-products. Exactly which compounds are formed, their
formation pathways, and their health effects are not well known. To
complicate matters, many of the DBPs are not susceptible to even highly
sophisticated methods of extraction and analysis.

## Particles of Biotic Origin

Four categories of waterborne microorganisms exist as particles
contributing to the turbidity of raw water:

-   Protozoans;

-   Enteric viruses;

-   Algae; and

-   Bacteria.

Microorganisms are living organisms that are invisible or barely visible
to the naked eye. While many microorganisms commonly found in source
waters do not pose health risk to humans, others such as
*Cryptosporidium* can be sources of infectious and communicable
diseases.

Isolation and identification of a specific organism such as
*Cryptosporidium* may prove difficult due to the volume and variety of
other microorganisms in the sample. Most municipal water plant labs do
not possess the equipment required for testing and identification of
specific pathogens. Indicator organisms are frequently used to assess
contamination by a biotic constituent. Total coliforms are the widely
used indicator for pathogens. While the presence of coliforms is not
proof that the water contains harmful pathogens, the absence of them is
often used as evidence that it is free of pathogens.

### Protozoans

Protozoans are organisms that can exist in colonies or as single cells.
Some protozoans are capable of producing spores, a small reproductive
body capable of reproducing the organism under favorable conditions. In
water, most spores resist adverse conditions that would readily destroy
the parent organism.

Of the tens of thousands of species of protozoa, the principal protozoan
pathogens of concern in potable water are *Cryptosporidium*, *Giardia
lamblia*, and *E. histolytica*. When these organisms are ingested by
humans, they can cause symptoms including; stomach cramps, diarrhea,
fever, vomiting, and dehydration. These parasites are typically more
resistant to traditional chlorine disinfection than coliforms.

*Cryptosporidium* is a disease-causing protozoan housed in a
hard-shelled oocyst (pronounced o-he-sist). The oocyst is typically 2 to
5 microns in diameter, round to egg shaped, colorless and nearly
transparent. Human and animal feces are sources of *Cryptosporidium* in
surface water. Normally, oocysts are found dormant in the environment.
When ingested, the oocyst splits open to release sporozoites. In this
new form, a complex reproductive cycle begins. Figure 8-2 describes the
lifecycle of *Cryptosporidium*.

The sporozoites invade the lining of the gastrointestinal tract and can
cause an illness called Cryptosporidiosis. The disease can be fatal to
people with suppressed immune systems, including persons with acquired
immune deficiency syndrome (AIDS), those undergoing chemotherapy,
children and the elderly (Current and Garcia, 1991). Human
Cryptosporidiosis was first reported in 1976, and outbreaks in public
water systems have motivated numerous studies and regulatory attention
on the effectiveness of filtration and chemical disinfection in the
removal and inactivation of these protozoans. Limited data suggests that
*Cryptosporidium* oocysts are resistant to disinfection at levels
practiced in the U.S. at the time of IESWTR promulgation. While research
is underway to identify more appropriate inactivation techniques,
removal by filtration is currently the most effective means of dealing
with *Cryptosporidium*.

![](media/image1.jpeg){width="4.95in" height="4.536111111111111in"}

Source: Ewing, 1986.

[]{#__RefHeading___Toc448538313 .anchor}Figure 8-2. *Cryptosporidium*
Life Cycle

Human and animal feces are also sources of *Giardia* in surface waters.
*Giardia* exists as either a flagellated trophozite of approximately 9
to 21 microns, or ovoid cysts, approximately 10 microns long and 6
microns wide. Cysts can survive in water from 1 to 3 months. Objects of
this size are easily removed by packed bed filters, provided that
coagulation and flocculation pretreatment are properly controlled.

Unlike *Giardia* and *Cryptosporidium*, mammals are not a source of *E.
histolytica* to water supplies and potential contamination of surface
water is considered to be low (Wiesner and Klute, 1998). The size range
for the protozoan is 15 to 25 microns for a trophozite and 10 to 15
microns for the cyst, and are effectively removed by filtration.
Additional information regarding *Cryptosporidium* may be found in
*Occurrence Assessment for the Interim Enhanced Surface Water Treatment
Rule* (USEPA, 1997).

### Viruses

A virus is a parasitic, infectious microbe, composed almost entirely of
protein and nucleic acids that can cause disease in humans and other
living organisms. Viruses can reproduce only within living cells, and
typically range from 0.004 to 0.1 micron in diameter. The principal
viral pathogens of concern in potable water are the Enteric viruses:
hepatitis A, Norwalk-type viruses, rotaviruses, adenoviruses,
enteroviruses, and reoviruses. Enteric viruses infect the
gastrointestinal tracts of humans and are transmitted through public
water supplies. It appears that many viruses have an attraction for the
surfaces of larger colloidal particles and, if aggregated, may increase
the effective size of these pathogens to promote their removal (Wiesner
and Klute, 1998).

### Algae

Algae are common and normal inhabitants of surface waters and are
encountered in every water supply that is exposed to sunlight (Tarzwell,
undated). Algae typically range in size from 5 to 100 microns. Figure
8-3 presents common types of algae which can be found within source
water and in the water treatment process.

Algae are not typically a threat to public health in a drinking water
supply. Concerns in potable water treatment arising from the presence of
algae include; the ability to create large quantities of organic matter;
the production of turbidity, tastes and odors in source water, and; the
physical impact on the water treatment plant processes. Some species of
blue-green algae are known to produce endotoxins which may affect human
health. Algae can clog filters, resulting in reduced run times and an
increase in the volume of backwash water needed for cleaning. Examples
of filter clogging algae are seen in Figure 8-4. In slow sand filters
and biologically active filters, algae will produce oxygen for bacteria
that actively degrade organic compounds. They may also release
biopolymers that aid in the destabilization of fine colloidal materials
(Wiesner and Klute, 1998).

### Bacteria

Bacteria are single-celled organisms that lack well-defined nuclear
membranes and other specialized functional cell parts. Bacterial cells
typically range from 1 to 15 microns in length. They vary in shape from
simple spheres to filamentous threads. Figure 8-5 presents various
bacterial and fungal forms. Bacteria and fungi are decomposers that
break down the wastes and bodies of dead organisms to make their
components available for reuse. Bacteria can exist almost anywhere on
earth and in almost any medium. Some are beneficial to man while others
are harmful, or even fatal. The principal bacterial pathogens of concern
in water treatment are the *Salmonella, Shigella, Yersinia
enterocolitica, enteropathogenic E. coli, Campylobacter jejuni,
Legionella, Vibrio cholerae*, and *Mycobacterium*.

![](media/image2.png){width="5.6819444444444445in"
height="7.457638888888889in"}

Source: Standard Methods, 1985.

[]{#__RefHeading___Toc448538314 .anchor}Figure 8-3. Plankton and Other
Surface Water Algae

![](media/image3.png){width="5.6819444444444445in"
height="7.593055555555556in"}

Source: Standard Methods, 1985.

[]{#__RefHeading___Toc448538315 .anchor}Figure 8-4. Filter Clogging
Algae

**Bacteria Fungi**

a\) ![](media/image4.png){width="0.7736111111111111in"
height="0.5868055555555556in"}
![](media/image5.png){width="1.2486111111111111in"
height="0.4909722222222222in"}![](media/image7.wmf){width="1.2986111111111112in"
height="0.3458333333333333in"}

b\) ![](media/image9.png){width="0.6645833333333333in"
height="0.3958333333333333in"}
![](media/image10.png){width="1.1527777777777777in"
height="1.0416666666666667in"}

c\) ![](media/image11.png){width="0.4152777777777778in"
height="0.6145833333333334in"}
![](media/image12.png){width="1.3486111111111112in" height="0.7625in"}

d\) ![](media/image13.png){width="0.5715277777777777in"
height="0.8125in"} ![](media/image14.png){width="0.5326388888888889in"
height="1.104861111111111in"}

e\) ![](media/image15.png){width="0.6027777777777777in"
height="0.6979166666666666in"} ![](media/image16.png){width="0.95in"
height="0.95in"}

f\) ![](media/image17.png){width="0.2701388888888889in" height="0.75in"}
![](media/image18.png){width="0.34930555555555554in"
height="1.0555555555555556in"}

Source: Standard Methods, 1985.

[]{#__RefHeading___Toc448538316 .anchor}Figure 8-5. Examples of Bacteria
and Fungi Forms

## Particles Added or Created During Treatment

Several steps in the water treatment process may contribute to
turbidity. As discussed in Section 10, water treatment is provided to
remove undesirable constituents from raw water, and many of these
processes are intended to remove suspended solids and reduce turbidity.
However, this section identifies those chemicals and practices used in
water treatment which are known to increase turbidity. Specifically, the
addition of pretreatment chemicals for
coagulation-flocculation-sedimentation or filter aids prior to
filtration can substantially increase the particulate materials loading
of sedimentation basins, filters and other processes used in water
treatment. Moreover, increases in turbidity may occur when any aspect of
the water treatment process fails.

### Coagulants

The coagulation of water generally involves the chemical addition of
either hydrolyzing electrolytes or organic polymers for the
destabilization of colloids in suspension. Some common coagulants are
those based on aluminum, such as aluminum sulfate and alum; and those
based on iron, such as ferric and ferrous sulfate. The action of
metallic coagulants is complex and is dependent on the fact that colloid
particles are charged entities in water solution. More discussion of the
electrokinetic properties of colloids is included in section 8-7.
Additionally, the use of bentonite, and activated silica for coagulation
enhancement will increase the particle loading in the treatment stream
(Wiesner and Klute, 1998).

#### Polymers {#polymers .unnumbered}

Natural and synthetic coagulant aids are known as "polyelectrolytes,"
because they have characteristics of both polymers and electrolyte.
Polyelectrolytes are long-chain, high-molecular-weight molecules which
bear a large number of charged groups. The net charge on the molecule
may be positive, negative, or neutral. The chemical groups on the
polymer are thought to combine with active sites on the colloid,
combining them into a larger particles which will then settle by
gravitational force. Both the molecular weight of the polymer and charge
density influence the effectiveness of polyelectrolytes.

Polyelectrolytes may be used alone or in tandem with metallic
coagulants. Optimal doses for polymeric coagulant are typically
determined in bench scale or pilot scale plant testing utilizing source
water. Use of quantities over the optimal dose will not increase
coagulation and instead will create unnecessary loading of particles to
be removed.

#### Lime {#lime .unnumbered}

Lime is a calcinated chemical material used in lime or lime and soda ash
water treatment processes to add alkalinity to the water and adjust the
pH. Lime treatment has the incidental benefits to remove iron, aid in
clarification of turbid waters, and minimal bactericidal benefit
(Logsdon et al., 1994). Lime has a tendency to deposit solids at changes
in directions and will precipitate out of solution at areas where
velocity decreases or where changes in velocity occur. The precipitates
formed in the lime-soda softening process consist principally of calcium
carbonate and magnesium hydroxide with size ranges from 15 to 20
microns. If lime is dosed in quantities greater than the water supply
requires, residual lime particles will increase the turbidity in treated
water effluent.

### Powdered Activated Carbon (PAC)

PAC adsorption is generally used for the removal of organics, radon,
color, and taste and odor treatment. Activated carbon is produced from
bituminous coal, or cellulose-based substances like wood or coconut
shells, by a destructive distillation process that drives off the
volatile components of the material. A highly porous, adsorbent material
is created which possesses a large surface area per unit volume.

PAC is generally less than 0.075 millimeters in size and has an
extremely high ratio of surface area to volume. Nonpolar compounds of
high molecular weight are attracted and held, or adsorbed, to this
surface. The effectiveness of organic removal by PAC is dependent on the
pH, temperature, contaminant concentration, molecular weight of the
particles to be adsorbed, type of PAC used, and the contact time of the
PAC with the water.

The relative capacity of different carbons to attract and adsorb
particles to their surfaces is best assessed by bench or pilot scale
testing of the raw water supply. Therefore, the addition of PAC for the
removal of organic materials, or to control tastes and odors, creates an
additional loading of materials to the downstream processes, as it is
slow to settle because of its small size and low density.

### Recycle Flows

Filtration treatment processes require frequent, intermittent backwash
cycles to remove particles from the media. The backwash water is a
concentrate of particles and pretreatment chemicals added prior to the
filters. Some plants capture and return this concentrate to a location
in the treatment process as a recycled flow. The properties of the
backwash concentrate depend on the type and quantity of particles
present in the source water, and pretreatment chemicals and treatment
processes used earlier in the treatment train. The practice of returning
spent backwash water to the treatment system has become a concern due to
the potential for returning pathogens to the treatment train.

## Electrokinetic Properties of Particles

Colloidal particles comprise a large portion of the turbidity-producing
substances in waters. Examples of colloidal particles include color
compounds, clays, microscopic organisms and organic matter from decaying
vegetation or municipal wastes. Colloidal

dispersions are stable in water, as they posses a large surface area
relative to their weight. Therefore, gravitational forces alone will not
remove colloids during sedimentation. Effective removal of these
colloidal dispersions is greatly impacted by the electrokinetic
properties on the surface of the colloids.

Each colloid carries a similar electrical charge that produces a force
of mutual electrostatic repulsion between adjacent particles. If the
charge is high enough, the colloids will remain discrete and in
suspension. The addition of coagulants or polymers reduces or eliminates
this charge and colloids will begin to agglomerate and settle out of
suspension or form interconnected matrices which can then be removed
during filtration. This agglomeration causes the characteristics of the
suspension to change by creating new particle viscosity, settling rates
and effective size properties for the colloids.

Colloids are classified as hydrophobic (resistant to water bonding) or
hydrophilic (affinity for water bonding). Hydrophilic colloids are
stable because their attraction to water molecules will overcome the
slight charge characteristic they possess. This attraction makes
hydrophilic colloids difficult to remove from suspension. Examples of
hydrophilic colloids include soaps and detergents, soluble starches,
soluble proteins and blood serum. On the other hand, hydrophobic
particles are dependent on electrical charge for their stability in
suspension. The bulk of inorganic and organic matter in a turbid raw
water is of this type.

### Electrical Potential 

Most colloidal particles in water are negatively charged as a result of
differences in electrical potential between the water and the particle
phases. This charge is due to an unequal distribution of ions over the
particle surface and the surrounding solution.

The charge on a colloidal particle can be controlled by modifying
characteristics of the water which holds the particles in suspension.
Modifications include changing the liquid\'s pH or changing the ionic
species in solution. Another, more direct technique is to use
surface-active agents, such as coagulants, that directly adsorb to the
surface of the colloid and change its characteristics.

### Electrical Double Layer Theory

The double layer model explains the ionic environment surrounding a
charged colloid and explains how the repulsive forces are set up around
a colloid. Figure 8-6 illustrates the resulting colloidal state.

A single negative colloid will initially attract some of the positive
ions in the solution to form a firmly attached layer around the surface
of the colloid, known as the *Stern layer*. Additional positive ions are
still attracted by the negative colloid, but are also repelled by the
Stern layer as well as by other positively charged ions trying to get
close to the negatively charged colloid. This constant attraction and
repulsion results in the formation of a *diffuse layer* of charged ions
surrounding the colloid and Stern layers.

The diffuse layer can be visualized as a charged atmosphere surrounding
the colloid. Together, the attached positively charged ions in the Stern
layer and the charged atmosphere in the diffuse layer is referred to as
the *double layer*. The charge is a

![](media/image19.wmf)

Source: McGhee,1991.

[]{#__RefHeading___Toc448538317 .anchor}Figure 8-6. Double Layer Theory
(Guoy-Stern Colloidal Model)

maximum at the particle surface and decreases with distance from the
surface. The thickness of this layer depends on the type and
concentration of ions in solution.

The DLVO Theory (for Derjaguin, Landau, Verwey and Overbeek) is the
classic model which describes the balance of forces between charged
colloid particles. Amirtharajah and O'Melia (1990) provide a thorough
discussion of the electrostatic theory of colloidal stability from the
DLVO model and other works.

When two similar colloidal particles with similar primary charge
approach each other, their diffuse layers begin to interact. The similar
primary charges they possess result in repulsive forces. The closer the
particles approach, the stronger the repulsive forces. Repulsive forces
which keep particles from aggregating are counteracted to some degree by
an attractive force termed *van der Waals* attraction. All colloidal
particles possess this attractive force regardless of charge and
composition. As van der Waals forces tend to be relatively weak
attractions, the force decreases rapidly with an increasing distance
between particles.

The balance of the two opposing forces, electrostatic repulsion and van
der Waals attraction, explains why some colloidal systems agglomerate
while others do not. As particles with similar charge approach one
another, the repulsive electrostatic forces increase to keep them
separated. However, if they can be brought sufficiently close together
to get past this energy barrier, the attractive van der Waals force will
predominate, and the particles will remain together. The random motion
of colloids caused by the constant collisions with water molecules,
termed Brownian Movement, will bring particles in close proximity and
aggregation may occur. However, the addition of coagulant and polymers
is typically used to lower the energy barriers between particles and
provide efficient agglomerations for settling.

#### Zeta Potential {#zeta-potential .unnumbered}

The Stern layer is considered to be rigidly attached to the colloid,
while the diffuse layer is a dynamic layer of charged particles. The
*Nernst Potential* is the measurement of voltage (in the order of
millivolts) in the diffuse layer. The potential is a maximum at the
Stern layer and drops exponentially through the diffuse layer. The z*eta
potential* is the electrical potential representing the difference in
voltage between the surface of the diffuse layer and the water. It is
important to know the magnitude of the zeta potential, as it represents
the strength of the repulsion between colloid particles and the distance
which must be overcome to bring the particles together.

The primary charge on a colloid cannot be measured directly. However,
the zeta potential can be computed from measurements of particle
movement within an electrical field (electrophoretic mobility).
Therefore, the zeta potential, ζ, is defined by the equation:

ζ = [4πδq]{.underline}

D

where q = charge of the particle

δ = thickness of the zone of influence of the charge on the particle

D = dielectric constant of the liquid

Zeta potential measurements can be made using a high-quality
stereoscopic microscope to observe colloidal particles inside an
electrophoresis cell (Zeta-Meter 1998). An electric field is created
across the cell and charged particles move within the field. Their
velocity and direction are then related to the zeta potential.
Measurements of zeta potential can give an indication of the
effectiveness of added electrolytes in lowering the energy barrier
between colloids, and can direct the optimization of coagulant dose in
water treatment.

The destabilization of colloids is accomplished by the reduction of the
zeta potential with coagulants such as alum, ferric chloride and/or
cationic polymers. Once the charge is reduced or eliminated, no
repulsive forces exist. Gentle agitation in a flocculation basin will
cause numerous, successful colloid collisions. Chapter 10 further
discusses the mechanics of coagulation and flocculation in the water
treatment process.

#### Streaming Current {#streaming-current .unnumbered}

As discussed in the previous section, a charged particle will move with
fixed velocity through a voltage field under the physical phenomenon
known as electrophoresis. *Streaming current* is a measurable electric
current that is generated when particles in water are temporarily
immobilized and the bulk liquid is forced to flow past the particles. A
streaming current monitor is a continuous, online sampling instrument
which measures the charge on particles. A streaming current detector, or
monitor, is a cylinder and piston. The up and downward motion of the
piston draws a sample of water into the annular space between the piston
and cylinder. An alternating current is read by the electrodes attached
to the ends of the cylinder (Amirtharajah and O'Melia, 1990). Charged
particles are temporarily immobilized by the piston and cylinder, and
the motion of charged particles in the double layer passing these
immobilized particles creates the streaming current (ChemTrac, 1997).

## References 

1.  Aiken, G. and C. Evangelo. "1995. Soil and Hydrology: their effect
    on NOM." *J. AWWA*. 1:36-37.

2.  Amirtharajah, A. and C.R. O'Melia. 1990. "Coagulation Processes:
    Destabilization, Mixing, and Flocculation." *Water Quality and
    Treatment, A Handbook of Community Water Supplies.* Fourth Edition.
    AWWA. F.W. Pontius, editor. McGraw-Hill, New York.

3.  AWWA. 1990. *Water Quality and Treatment*. Fourth Edition.
    McGraw-Hill, Inc., New York.

4.  Standard Methods. 1985. *Standard Methods for the Examination of
    Water and Wastewater,* Sixteenth Edition. Franson, M.H., Eaton,
    A.D., Clesceri, L.S., and Greenberg, A.E., (editors). American
    Public Health Association, AWWA, and Water Environment Federation.
    Port City Press, Baltimore, MD.

5.  ChemTrac Systems Inc. 1997. Optimizing Particle Removal with
    Streaming Current Monitors and Particle Counters. Atlanta, GA.

6.  Current, W.L. and L.S. Garcia. 1991. Cryptosporidiosis. *Clinical
    Microbiological Reviews*. 4(3):325.

7.  Ewing, R.B. 1986. Microbiological Reviews. *American Society of
    Microbiology*. 50:458.

8.  Gregory, R. and T.F. Zabel. 1990. "Sedimentation and Flotation. "
    *Water Quality and Treatment, A Handbook of Community Water
    Supplies.* Fourth Edition. AWWA. Ed. F.W. Pontius, editor.
    McGraw-Hill, New York.

9.  Jancangelo, J.G., et al. 1992. *Low Pressure Membrane Filtration for
    Particle Removal*. AWWARF, Denver, CO.

10. Kawamura, S. 1991. *Integrated Design of Water Treatment
    Facilities*. John Wiley & Sons, New York.

11. Logsdon, G., M.M. Frey, T.D. Stefanich, S.L. Johnson, D.E. Feely,
    J.B. Rose, and M. Sobsey. 1994. "The Removal and Disinfection
    Efficiency of Lime Softening Processes for Giardia and Viruses."
    AWWARF, Denver, CO.

12. McGhee, T.J. 1991. *Water Resources and Environmental Engineering*.
    Sixth Edition. McGraw-Hill, New York.

13. Sawyer, C.N. and P.L. McCarty. 1978. *Chemistry for Environmental
    Engineering.* Third Edition. McGraw-Hill, New York.

14. Tarzwell, C.M, editor. undated. "Algae - Taste and Odor Control.
    WT-138." Robert A. Taft Sanitary Engineering Center, Cincinnati, OH.

15. Tate, C.H. and K.F. Arnold. 1990. Health and Aesthetic Aspects of
    Water Quality. *Water Quality and Treatment, A Handbook of Community
    Water Supplies.* Fourth Edition*.* F.W. Pontius, editor. AWWA,
    McGraw-Hill, New York.

16. USEPA. 1997. *Occurrence Assessment for the Interim Enhanced Surface
    Water Treatment Rule, Final Draft*. Office of Ground Water and
    Drinking Water, Washington, D.C.

17. Wiesner, M.R. and R. Klute. 1998. Properties and Measurements of
    Particulate Contaminants in Water. *Treatment and Process Selection
    for Particle Removal*. J.B. McEwen, editor. AWWARF and International
    Water Supply Association, Denver, CO.

18. Zeta-Meter Inc., 1998. Zeta Potential: A Complete Course. Internet
    Access: [www.zeta-meter.com](http://www.zeta-meter.com/).

######## THIS PAGE INTENTIONALLY LEFT BLANK  {#this-page-intentionally-left-blank .unnumbered}

8\. Particles Contributing to Turbidity
[8-1](#particles-contributing-to-turbidity)

8.1 Introduction [8-1](#introduction)

8.2 Characteristic Properties of Particles
[8-1](#characteristic-properties-of-particles)

8.2.1 Particle Settling [8-2](#particle-settling)

8.2.2 Particle Density and Size Distribution
[8-3](#particle-density-and-size-distribution)

8.3 Inorganic Particles [8-4](#inorganic-particles)

8.3.1 Naturally Occurring Minerals [8-4](#naturally-occurring-minerals)

8.4 Organic Particles [8-5](#organic-particles)

8.4.1 Synthetic Organics [8-5](#synthetic-organics)

8.4.2 Natural Organic Matter (NOM) [8-5](#natural-organic-matter-nom)

8.4.3 Total Organic Carbon (TOC) [8-6](#total-organic-carbon-toc)

8.4.4 Organic Disinfection By-products (DBPs)
[8-6](#organic-disinfection-by-products-dbps)

8.5 Particles of Biotic Origin [8-6](#particles-of-biotic-origin)

8.5.1 Protozoans [8-7](#protozoans)

8.5.2 Viruses [8-8](#viruses)

8.5.3 Algae [8-9](#algae)

8.5.4 Bacteria [8-9](#bacteria)

8.6 Particles Added or Created During Treatment
[8-13](#particles-added-or-created-during-treatment)

8.6.1 Coagulants [8-13](#coagulants)

8.6.2 Powdered Activated Carbon (PAC)
[8-14](#powdered-activated-carbon-pac)

8.6.3 Recycle Flows [8-14](#recycle-flows)

8.7 Electrokinetic Properties of Particles
[8-14](#electrokinetic-properties-of-particles)

8.7.1 Electrical Potential [8-15](#electrical-potential)

8.7.2 Electrical Double Layer Theory
[8-15](#electrical-double-layer-theory)

8.8 References [8-18](#references)

Figure 8-1. Particle Size Spectrum [8-2](#__RefHeading___Toc448538312)

Figure 8-2. *Cryptosporidium* Life Cycle
[8-8](#__RefHeading___Toc448538313)

Figure 8-3. Plankton and Other Surface Water Algae
[8-10](#__RefHeading___Toc448538314)

Figure 8-4. Filter Clogging Algae [8-11](#__RefHeading___Toc448538315)

Figure 8-5. Examples of Bacteria and Fungi Forms
[8-12](#__RefHeading___Toc448538316)

Figure 8-6. Double Layer Theory (Guoy-Stern Colloidal Model)
[8-16](#__RefHeading___Toc448538317)