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This article is an overview of common Point-of-Use and Point-of-Entry water treatments, including Activated Alumina; Activated Carbon; Anion and Cation Exchange; Disinfection Technologies including Chlorination, Microfiltration, Ozone, and Ultratviolet Light; Distillation, and Reverse Osmosis.
It's ironic that many areas of the world face critical
shortages of drinking water on a planet whose surface
is 3/4 covered with water. Most of the water, of
course, is seawater, which is far too saline for human
consumption. And of the little "fresh" water that
remains, most is trapped in polar ice caps where it is
difficult to harness for use by the world's
population.
Much of the natural supply of potable water that is
accessible faces stress from a growing world
population, which increases the basic demand for this
natural resource, while reducing the supply further
through biological and industrial contamination.
Major population centers in developing nations without
established waste treatment or water treatment
infrastructures often suffer from epidemics of
waterborne disease. In these areas, raw sewage often
directly contaminates the rivers and streams used for
drinking, washing, and cooking. In other cases,
unchecked industrialization leads to water
contamination through improperly disposed-of chemical
and nuclear wastes.
Some good news about this problem is that individuals
can take control of their own water quality, and treat
their water for nearly all biological and chemical
contaminants that may be encountered. These
technologies also treat for "aesthetic" contaminants
that cause potable water to have unpleasant tastes,
colors, and odors.
Point of Use (POU) and Point of Entry (POE) water
treatment equipment can effectively treat the water
used by a small community, home, or business.
POU equipment treats the water that is used at a
single tap, while the rest of the water in the
building remains untreated. POU equipment is primarily
used to treat health contaminants like lead, and
aesthetic contaminants like sulfur. These contaminants
are a concern in water used for drinking and cooking.
POE equipment treats most or all of the water before
it is distributed, either throughout a small community
or at a single building. POE equipment treats for
health contaminants like volatile organic compounds (VOC's)
that can be absorbed through the skin, or contaminants
like radon which exist as a harmful vapor suspended in
the water that can be inhaled during showering. POE is
also used to describe water softening, which inhibits
scale formation in plumbing while increasing the
efficiency and longevity of water-related appliances
like water heaters.
There are many effective technologies used to provide
POU/POE treatment solutions, and no single technology
is effective for treating all of the possible
contaminants. A specific technology or combination of
technologies is usually applied to treat the specific
problem at hand.
It should be noted that different levels of
performance can be found between products using each
technology. If a product is to be used to treat a
health contaminant, it is important that the specific
product be tested successfully for the reduction of
that contaminant. Offered below is a brief description
of the main technologies, and what they are typically
used to treat.
Activated Alumina
Activated alumina is a filter media made by treating
aluminum ore so that it becomes porous and highly
adsorptive. Activated alumina will remove a variety of
contaminants, including excessive fluoride, arsenic,
and selenium. The medium requires periodic cleaning
with an appropriate regenerant such as alum or acid in
order to remain effective.
Activated Carbon (Granular and Solid Block)
Granular activated carbon is a well-established
technology for the reduction of a wide range of
aesthetic contaminants, and is quite effective in the
reduction of some health contaminants such as volatile
organic compounds (benzene, trichloroethylene, and
other "petroleum"-based contaminants.
Because of its molecular makeup, activated carbon can
adsorb well, meaning that it can take in or collect
many organic molecules on its surface. Granular
activated carbon filters are typically inexpensive,
and maintenance involves replacing six to twelve
cartridges a year, depending on the quality of the raw
water and the filter media.
Specially designed solid block and precoat activated
carbon filters are also available, which are effective
at reducing heavy metals such as lead and mercury.
Solid block filters with a pore size smaller than 0.2
microns are often effective against biological
contaminants as well.
Anion and Cation Exchange
Anion exchange and cation exchange use the chemical
ion exchange process to exchange anions and cations on
a "resin" bed for cations and anions of the
contaminant that needs to be removed from the water.
For example, in cation exchange, a cation of hardness
mineral such as calcium is exchanged for two cations
of sodium, effectively removing most of the calcium,
and softening the water.
The anions or cations on the resin are eventually
exhausted, and replaced by the anions or cations of
the contaminant being removed. When this occurs, the
bed must be backwashed using a concentrated solution
of the base cation or anion, which recharges the bed
and flushes the built-up contaminant.
Anion exchange typically uses chloride or hydroxide
anions, and can be used to treat for mercury,
nitrates, arsenic, and various staining agents. Cation
exchange typically uses sodium or potassium chloride,
and can also treat for some forms of lead and radium.
It is also commonly used to soften water.
Disinfection Technologies
Disinfection technologies kill or screen-out
biological contaminants present in a water supply.
Chlorination, microfiltration, ozone, and ultraviolet
light are the four major technologies used to
disinfect water.
Chlorination
Chlorination adds a concentration of the chemical
chlorine or chloramine to the water supply, where the
oxidizing ability of this chemical "burns up" the
organic contaminants in the water. Chlorine can
effectively treat biological pathogens like coliform
bacteria and legionella, though it is ineffective
against hard-shelled cysts like those produced by
Cryptosporidium. Chlorination also treats for
organically-related taste, color, and odor problems.
Chlorine is typically fed directly into a well, or
into a retention tank where concentration and contact
time can be controlled. Chlorination is effective for
treating pathogens like coliform bacteria and
legionella, though it is ineffective against
hard-shelled cysts like Cryptosoridium and Giardia
lamblia. Other chemicals like bromine and iodine can
also be used to disinfect water through much the same
process as chlorination, though they are not as
frequently used.
Microfiltration
Microfiltration uses a filter media with a pore size
smaller than 0.2 microns to physically prevent
biological contamination from passing through. Ceramic
and solid block carbon are commonly used to provide
microfiltration. Ceramic filters have and advantage in
that they can often be cleaned and reused a number of
times before they lose effectiveness.
Carbon block media usually has to be disposed of after
each use. This media, however, provides additional
treatment for a variety of other health and aesthetic
contaminants (see activated carbon section).
Microfiltration is effective for treating the full
range of biological contaminants, including
hard-shelled cysts like Cryptosporidium.
Ozone
Ozone treatment has typically been used in large-scale
commercial and industrial applications; however, there
has been a recent growth in the number of ozone units
designed for use in a single home or business
application.
Ozone treatment oxidizes organic contaminants in much
the same way that chlorine does. An ozone generator
converts the oxygen found in air to O3, or ozone. As
with chlorination, proper concentrations and contact
time is essential for disinfection. Ozone usually
requires the use of a retention tank to accomplish
this, and can be used to provide partial treatment in
pools. Ozone is effective for treating pathogens like
coliform bacteria and legionella, but it is not
effective against hard-shelled cysts like
Cryptosporidium or Giardia lamblia without using high
contact times and concentrations.
Ultraviolet Light (UV)
Ultraviolet light has treated water since the
beginning of time through natural sunlight. Modern
ultraviolet treatment units use a UV bulb in a clear
quartz or plexiglas housing, around which flows the
untreated water. The UV light destroys the genetic
material of pathogens like coliform bacteria and
legionella, which effectively neutralizes them by
preventing them from reproducing. UV is not effective
for the treatment of hard-shelled cysts like
Cryptosporidium and Giardia lamblia.
Distillation
Distillation produces high quality, treated water by
heating the raw water until it turns to steam. The
steam travels through a condensation coil, where it is
cooled and condensed back into liquid form in a
separate section. Typically, the contaminants present
when the water is converted to steam remain in the
boiler section, with the condensed water in the second
section being substantially free of contaminants.
Maintenance of a distillation unit usualy involves
cleaning oout the built-up contaminants on the boiler
side of the unit.
Distillation typically provides a high degree of
effectiveness against a broad range of health
contaminants.
Distillation is typically not effective for treating
contaminants such as benzene and radon, which give off
harmful vapors that can move through the system with
the steam. The energy requirement of distillation and
a relatively long production time typically limits its
use to POU drinking water applications in home and
commercial markets. Some distillation untis are also
tested and approved for the reduction of biological
pathogens.
Reverse Osmosis
Reverse osmosis (RO) is a common treatment technology
that produces high quality water. The process works by
forcing water under great pressure against a
semipermeable membrane, where ion exclusion occurs.
With ion exclusion, water molecules form a barrier
that allows other water molecules to pass through
while excluding most contaminants.
Typical contaminant rejection rates range from 85% to
95%, and a gallon of highly treated water can usually
be produced from two to four gallons of raw water,
depending on the initial quality of the water.
Maintenance involves the replacement of the RO
membrane cartridge every two or three years, and the
carbon filter cartridges six to twelve times per year.
RO is effective for the reduction of a broad range of
health and aesthetic contaminants, though it is
typically not used for the reduction of biological
pathogens. RO also incorporates an activated carbon
filter, which can provide added treatment for the
volatile organic compounds (VOC's) not treated by the
membrane itself.
It should be remembered that this brief description of
water treatment technologies is only intended to
provide an overview of how each technology can be
applied to solve a water contamination problem. The
advice of a WQA Certified Water Professional of
Certified Sales Representative should be sought when
looking for a specific treatment solution; directories
of these personnel in your area are available at this
site.
With a knowledgeable application of these effective
POU/POE technologies, you can take the quality of your
water into your own hands.