Treating The Water We
Drink, When and Where We Drink It.
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.
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