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Welcome to Ask Tom!, a monthly column by our resident water treatment guru, Tom Keenan of
National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
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Using Zeta Potential to Optimize
Water Treatment
Guest article by Ana Morfesis & Ulf Nobbmann, Malvern
Instruments
Printer friendly PDF
 Introduction
Cryptosporidium and Giardia are
two of the most important microscopic pathogens that can be
transmitted via drinking water. Both are present in most surface
waters around the world. Infection by either organism is
particularly serious for immunocompromised individuals as no
effective drug treatment is available. The problem is becoming
ever more significant as many of the world’s highly populated
areas are experiencing diminished water supplies, due to greater
demand and an increasingly arid climate. Alternative water
sources are therefore being sought; including for example
treated municipal wastewater. Such alternatives may be more
likely to contain higher concentrations of protozoan parasites
such as Cryptosporidium and Giardia.
Water treatment programs are the
front line in the defence against all waterborne pathogens. This
article presents experimental data that demonstrate an effective
technique for maximizing the efficiency of water sanitation
methods, involving the measurement of zeta potential to enhance
contaminant coagulation.
Invisible Threat
Cryptosporidium and Giardia are
protozoan parasites commonly found in many animal species
throughout the world. Cryptosporidium oocysts and Giardia cysts
- highly infectious and environmentally robust - are excreted by
infected animals and can enter water sources, enabling
transmission to humans. Once ingested the organisms reproduce
inside the intestines, causing diarrhoea, nausea, abdominal
pains, fever and weight loss.
Patients with normal immune
systems are able to fight off the infection in a matter of
weeks. More vulnerable individuals, including those with a
defective immune response due to AIDS, cancer treatment or organ
transplant therapy, are at much greater risk. As there is no
effective treatment, an inability to clear the pathogen from the
body can result in persistent diarrhoea, eventually leading to
serious dehydration and death.
A Global Issue
Outbreaks are common throughout
the world, especially in less developed countries where the
treatment of drinking water may be inadequate or non-existent.
Although the risk is much lower, even the most sophisticated
water management systems of industrialized nations sometimes
allow microbial pathogens to reach concentrations that endanger
health. Flooding or heavy rain, for example, can cause surface
waters to mix with normally well-protected ground water
supplies, rapidly introducing contaminants.
Once parasites have entered the
water supply, they have the potential to affect large numbers of
people. The problem is exacerbated because screening techniques
are far from perfect, often only detecting a problem once
contaminated water has already reached the consumer. For
example, in 1993 a waterborne cryptosporidiosis outbreak
occurred in Milwaukee in the United States. An estimated 400,000
residents were infected by the parasite. It is believed that
over 100 immunocompromised people died prematurely as a result.
Despite this, throughout the entire episode the city's treatment
plants met all applicable water quality standards.
Importance of Filtration
Disinfectants are an important
aspect of water treatment, however, while a common disinfectant
such as chlorine can be fairly effective for removing Giardia
from water, Cryptosporidium oocysts are highly resistant to it.
Other disinfectant methods, such as UV treatment, have limited
effect on either pathogen. Furthermore, under environmental
conditions microbial pathogens can attach to colloids, reducing
disinfection efficiency.
It is filtration that forms the main barrier against the spread
of waterborne infections. It is much more effective against
protozoan parasites than disinfection. Effective filtration
relies on techniques to enhance the coagulation of microbial
parasites in water, facilitating their removal or inactivation.
While particles larger than one micron will settle out of water
in a matter of seconds, without coagulation, smaller material –
including microbial contaminants – would require a period of
days or months. The addition of chemical coagulants, also called
flocculants or flocculant additives, allows the process to take
just hours.
Coagulation is improved through
the addition of ions with the opposite charge to that of the
colloidal particles. Since colloidal particles are primarily
anionic or negatively charged, it is cationic (positively
charged) additives that are used to neutralize charged anionic
contaminants. The two main types of coagulants are aluminium
salts and iron salts, with the most common example being
aluminium sulphate.
A series of chemical reactions
occurs when a coagulant, for example aluminium sulphate, is
added to water, which result in the formation of positively
charged aluminium ions. These ions join together to form large
molecules. Once formed, these highly positive molecules move
rapidly towards the colloidal particles, where they are adsorbed
onto the negatively charged surface. This results in the
formation of a coagulated precipitate that can be easily removed
from water by sedimentation or filtration.
 Optimizing
Coagulation
Zeta potential - the degree of
repulsive interaction between particles in a dispersion - is of
particular importance as an indication of the stability of
colloids. Colloids with high zeta potential (negative or
positive) are electrically stable, while colloids with low or
zero zeta potential tend to coagulate or flocculate. For water
treatment applications, charged particles are stable and
therefore undesirable. The efficiency of contaminant removal can
be greatly enhanced by achieving a surface charge of zero, or
close to zero, in the water undergoing treatment. In these
circumstances contaminants are unstable and the conditions for
aggregation are maximized.
One way of achieving a low zeta
potential is by optimizing the amount of cationic additives. The
experimental data that follow shows how periodic measurement of
zeta potential can be used to adjust coagulant dosage levels to
reach optimum coagulation efficiency.
Instrumentation
The Zetasizer Nano was used to
monitor and optimize flocculant usage in: a) a water sample
containing bentonite, and b) a water treatment facility. All
zeta potential measurements were carried out using
reusable/disposable capillary cells, together with an MPT-2
autotitrator (Malvern Instruments). See box for more information
on the Zetasizer Nano and the MPT-2 autotitrator.
Results and Discussion
Analysis of bentonite in water
Bentonite, an aluminium-silicate
clay material, was added to water to simulate pre-filtration
conditions at a water purification plant. It provides a good
model for monitoring the effect of flocculant dosage on zeta
potential because, analogous to most environmental contaminants
including Cryptosporidium oocysts and Giardia cysts, when
dispersed into water it becomes negatively charged. Zeta
potential and turbidity were recorded as incremental amounts of
a typical alum flocculant were introduced.
Figure 1 shows that at low
dosages of the flocculating agent, turbidity is high and the
zeta potential is approximately -16mV. The addition of
flocculating agent causes an increase in zeta potential and a
decrease of turbidity. Turbidity decreases as suspended
bentonite is removed from the water through coagulation and
sedimentation. Figure 1 shows that at a zeta potential of +2mV,
turbidity has reached an absolute minimum. From the graph it can
be ascertained that a flocculant concentration of 10 ppm was
required to achieve this point. Further addition of flocculating
agent causes an increase in zeta potential and turbidity –
corresponding to a re-stabilization of bentonite in the water.
Figure 1: Zeta
Potential and Turbidity vs. Alum (Flocculant)
Concentration of Bentonite Particles in Water
This investigation shows that the
measurement of zeta potential is a critical factor for the
determination of optimum flocculant dosage for maximum
coagulation efficiency.
Analysis at a water treatment
facility
Measurements were conducted at a
city’s water treatment facility in the United States.
Concentration of alum flocculating agent, zeta potential and
turbidity were monitored and adjusted during a period of one
year. Figures 2 and 3 show the facility’s control charts: figure
2 shows zeta potential against alum concentration in four
settling tanks; and figure 3 indicates zeta potential and
settling turbidity of the water produced in one tank (or train).
Figure 2: Water
treatment production control chart in 2002
monitoring zeta potential results and Alum concentration across
four trains or settling tanks in a US water treatment facility
Figure 3: Water
treatment control chart during 2002.
Monitoring the zeta potential and turbidity results in a
single train (settling tank)
The aim of the facility is to
maintain a zeta potential around zero, specifically between the
values of +5 and -5mV. The graphs show that this is achieved for
the vast majority of time, providing an indication of the
relationship between alum dosage and zeta potential.
Alum or coagulant dose has
historically been selected by a number of factors including zeta
potential, jar test (sedimentation) results, streaming current,
Total Organic Carbon (TOC), color, turbidity and various other
indicators. However, based on natural weather conditions during
specific times of the year, rapid decisions about coagulant
dosages must be made in water treatment facilities.
Specifically, tests like 2hr jar
tests are often not representative of the water requirements
during the day. Therefore, in this US water treatment facility,
it is the frequent use of zeta potential measurements that
provide optimal monitoring results for monitoring coagulant
dosage, particularly during times of rapidly changing water
conditions.
Figure 2, shows data for zeta
potential and alum concentration during a one year time period
across four settling tanks producing 20milliion gallons of water
per day. In this specific example, seasonal changes occurred in
April and May that affected the process control of this
manufacturing facility. These seasonal changes occurred
naturally and were due to the raw incoming water supply at this
location.
The figure 2 results indicate
that during this April to May time, frame water production did
operate more on the negative side of the zeta potential control
range, however the zeta potential specifications (-5 to +5mV)
were maintained and this produced the best floc and best water
quality during this time period.
Using the Zetasizer Nano, the
facility can monitor the zeta potential of water to optimize
alum dosage. Consequently, the plant achieves the long-term
maintenance of the most effective filtration conditions, while
also minimizing treatment costs by avoiding excessive additive
use.
 Conclusion
Purification is an essential
process for the removal of a wide range of contaminants from
water, including suspended organic material, toxic metals and
minerals, and disease causing organisms. Cryptosporidium and
Giardia are two waterborne parasites that have gained particular
notoriety as a threat to public health. They are highly
persistent, resistant to many disinfectants and very small, thus
presenting a challenge for even the most advanced water
treatment systems.
Filtration is the most effective
method for the removal of contaminants such as Cryptosporidium
and Giardia from drinking water. The highest removal rates are
achieved when coagulants (flocculants) are applied to the water
before filtration, increasing the rate of sedimentation.
Controlling the effective charge
of suspended particles, which changes with the addition of
flocculating agents, can optimize the coagulation process and
rates of sedimentation. The measurement of zeta potential
provides an accurate and precise method of quantifying the
effective charge. At close-to-neutral charge, particles
flocculate and sediment more easily.
The Malvern Zetasizer Nano,
together with the MPT-2 autotitrator, was used to monitor zeta
potential and optimize flocculant usage. The data showed that
measurement of zeta potential is a precise and accurate method
for the enhancement of water filtration, providing an invaluable
tool for water treatment facilities.
 About
the Zetasizer Nano
The Zetasizer Nano system from
Malvern Instruments was the first commercial instrument to
include hardware and software for combined dynamic, static and
electrophoretic light scattering (zeta potential) measurements.
Its unique, disposable cell design consists of electrodes and a
folded capillary, which are moulded into a single precision
measurement chamber. The Zetasizer Nano enables the measurement
of both particle size and zeta potential.
For water treatment applications,
zeta potential is often studied as a function of additive (flocculant)
concentration or pH. The MPT-2 autotitrator can be used with the
Zetasizer Nano to facilitate such comparative measurements.
For more information contact
Malvern Instruments Inc.
117 Flanders Road
Westborough, MA 01581
Telephone: 508 768 6400
Fax: 508 768 6403
Web site: http://www.malvern.com/
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