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What is Distillation?
Distillation is one of mankind's earliest
forms of water treatment, and it is still a popular treatment
solution throughout the world today. In ancient times, the
Greeks used this process on their ships to convert sea water
into drinking water. In far-eastern cultures, water was distilled
for use in "Ranbiki" tea ceremonies.
Today, distilled water is still used to
convert sea water to drinking water on ships and in arid parts
of the world, and to treat water in other areas that is fouled
by natural and unnatural contaminants. Distillation is perhaps
the one water treatment technology that most completely reduces
the widest range of drinking water contaminants.
Not only is distillation one of the most
effective forms of treatment, but it is also one of the easiest
to understand: untreated water is converted into water vapor,
which is then condensed back into liquid form. Most of the
contaminants are left behind in the boiling chamber, with
the condensed water being virtually contaminant-free. Anyone
who has accidentally let a pot of water boil completely out
on the stove is familiar with this process, and familiar with
the crust of contaminants typically left behind after the
water is gone.
In nature, this basic process is responsible
for the hydrologic cycle. The
sun causes water to evaporate from surface sources such as
lakes, oceans, and streams. The water vapor eventually comes
in contact with cooler air, where it re-condenses to form
dew or rain. This process can be imitated artificially, and
more rapidly than in nature, using alternative sources of
heating and cooling.
Sixteenth Century Clay Porcelain Distiller
Early distillation equipment was very simple
in design: a pot of undrinkable water (or water unfit for
a ceremonial, commercial, or medical purpose) would be heated
over an open flame until it boiled, forming steam. The steam
would then condense on a cool surface suspended above the
pot. The condensed water droplets would then run off into
a storage container for future use. Alternatively, sponges
could be suspended above the pot to collect the treated water.
While such systems were relatively inefficient, it tended
to be quite adequate for the limited water treatment needs
of the time.
The efficiency of the distillation process
began to see improvements as distillation was adapted to commercially
refine many different liquids such as alcohol, perfume, petroleum,
and various solvents. Finally, population demands have strained
water resources in the 20th century to the point where efficiently
treating otherwise undrinkable sources of water for human
consumption is increasingly important.
How Distillation Systems Work
Several different types of distillation
systems are available, the system chosen generally depending
on the quantity of water required. Most households and businesses
use inexpensive and effective single-stage distillers to provide
less than a gallon, or up to 100 gallons, per day (gpd) of
treated water. More economically efficient commercial distillation
units produce up to 5,000 gallons per day, and use variations
in steam pressure to help heat the water. Converting water
into steam requires significant amounts of heat.
Most distillation units use either electricity,
or, to a lesser extent, gas to generate the heat necessary.
While it is difficult to get past the initial heating of the
water, efficiency can be gained in keeping the process going
by reusing the heated steam to preheat the incoming water.
Household Distillation
The most common
type of household and commercial distiller available is
a basic, single-effect distiller. Single-effect distillers
are simple in design, inexpensive, and effective. They are
less efficient in energy use than some of the more elaborate
processes used mainly for commercial treatment. Most single-stage
distillers are relatively compact counter top or stand-alone
units for use in the kitchen or office. These distillers can
be either batch distillers, where a measured quantity of water
is manually poured in, distilled, and collected; or plumbed
distillers that automatically treat and maintain a constant
supply of drinking water.
In a single-effect distiller, a heating
element heats the water until it boils and eventually becomes
steam. The steam is then drawn away from the boiling chamber,
where it cools, condensing into highly treated distilled water.
The contaminants in the original water are left behind in
the boiling chamber.
Modern Point-of-Use Distillation System
The condensing process is accomplished by
using air or water to cool the steam. With some designs, the
steam passes through coiled tubing which is either immersed
in cool water, or cooled by a fan. (See above). In others,
the roof of the boiling chamber is cone-shaped, with the cone
being cooled by the non-heated water stored above it. Water
droplets condense on the inside of the cone-shaped dome, and
run down for collection in a drip pan. With some water-cooled
systems, a portion of the heat lost as the steam is cooled
and condensed can be reclaimed by channelling the heated cooling
water into the boiling chamber. It is then replaced with fresh,
cool water. The pre-heated water requires less new energy
to convert it into steam. (See the WaterReview Technical Report
"Efficiencies of Distillation Equipment" for more
information on energy-conserving designs.)
While the distillation process alone is
very effective, certain pesticides and contaminants like volatile
organic compounds (VOCs) and radon convert into vapor readily,
and can travel with the steam our of the boiling chamber.
Almost all household distillers use special vents and carbon
pre- and post-filters to effectively deal with these contaminants.
Distillation units do require some maintenance,
which usually involves draining off the concentrated sediment
and other contaminants that accumulate at the bottom of the
boiling chamber. The walls of the chamber may also need to
be cleaned of hard-water scale and other sediment that can
accumulate. The required amount of cleaning depends greatly
upon the initial quality of water used. Very hard water can
produce heavy scaling in a relatively short period of time.
If soft water is used, cleaning difficulties should be minimal.
The carbon pre- and post-filters
must be changed periodically as well.
Typical household distillers cost between
$300 and $1,000 and produce water for as low as $.25 a gallon,
energy and filter costs included. Look for the
WQA Gold Seal (S-400) to find products that have been
successfully tested by WQA to industry performance standards.
Commercial Distillers
Many commercial operations use multiple-effect
distillers, to provide from 75 to millions of gallons
per day. These units typically contain a number of boiling
chambers, with the first chamber being under increased pressure,
and successive chambers having progressively decreasing pressure.
This takes advantage of the fact that the greater the steam
pressure, the higher the boiling point and temperature of
the steam produced. The steam created in the first high-pressure
chamber is "superheated" to a point well above the
temperature needed to create steam in the lower-pressure chambers.
As this superheated steam moves through tubes surrounding
each of the succeeding boiling chambers, it "flash"
vaporizes some of the cooler, lower-pressure water in each
chamber. The flash vapor is then condensed into distilled
water, as is the superheated steam when all of its heat energy
is exchanged. The self-sustaining nature of this process can
be quite efficient for large quantities of water, since only
an electric or gas heating element is required for the first
boiling chamber as an energy source.
A variation of the multiple-effect distiller
concept is the vapor-compression distiller, which is typically
used in commercial applications requiring between 25 and 5000
gallons per day. Vapor-compression water distillers also use
high-pressure, superheated steam to boil water; however, they
only use a single chamber. The water in the boiling chamber
is initially converted to steam at normal pressures and temperatures
by an electric or gas heating element. The steam then passes
through an electric compressor; the compression causes it
to become superheated. The superheated steam is then directed
through tubes back into the boiling chamber, where it eventually
takes over the boiling process, condensing into distilled
water as the heat transfer occurs. These systems are typically
more efficient than multi-stage units, since the energy required
to operate the compressor is less than that required to heat
water using a heating element. The greater efficiency usually
brings with it a greater cost, as compared to multistage units.
Both multi-stage and vapor-compression distillers
can incorporate various forms of filtration to make a broadly
effective treatment system. These systems can provide water
for such uses as commercial water bottling. Both systems also
require water that is softened to be practical, to prevent
debilitating scaling with resultant heat transfer losses and
maintenance costs.
At the municipal level, both multi-stage
and vapor-compression distillation can provide large quantities
of distilled water for drinking use, and are especially used
in distilling seawater for use in arid areas adjacent to the
oceans.
What Distiller Units Treat
Distillation is an effective process for
producing highly treated drinking water. Distillation can
significantly reduce levels of sediment, metals, and biological
contaminants, which are unable to travel out of the boiling
chamber with the steam. When combined with effective activated
carbon filtration, contaminants like VOCs and radon can
also be controlled. It is recommended that only units designed
and tested for health-contaminant reduction be used for such
purposes. These units, when combined with activated
carbon filtration, can be used to effectively treat such
contaminants as:
- Arsenic
- Asbestos
- Atracine (Herbicide/pesticides)
- Benzene
- Fluoride
- Lead
- Mercury
- Nitrate
- Trichloroethylene (TCE)
- Trihalomethanes
- Radium
- Radon
- Biological contaminants (bacteria, viruses, and water-borne
cysts like cryptosporidium)
A distillation system with activated
carbon filtration can also be quite effective for treating
aesthetic drinking water contaminants like chlorine or iron
bacteria, which lead to unpleasant tastes, odors, or colors.
Conclusion
Distillation is an effective water treatment
technology for household and commercial use. It provides water
with a distinct clarity, up to 98% free of impurities. Distillation
units are continually being improved to increase efficiency
and water output, making them increasingly popular and cost-effective
for residential and commercial users alike.
This article first appeared in the WaterReview
Technical Brief, (1995) Volume 10, No. 2; a publication of
the Water Quality Research Council; Copyright 1995 by the
WQA. All rights reserved.
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