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Everything
You Need to Know about Ultraviolet Water Purification
by John Mone
A
number of factors combine to make ultraviolet radiation
a superior means of water purification for rainwater harvesting
systems. Ultraviolet radiation is capable of destroying [UV does not actually destroy, but inactivate bacteria, see note] all types
of bacteria. Additionally, ultraviolet radiation disinfects rapidly
without the use of heat or chemical additives which may undesirably
alter the composition of water.
The most familiar part of the spectrum is a narrow band of wavelengths
visible to the human eye. Another band with wavelengths shorter
than those of visible light, and not visible to the eye, is the
ultraviolet part of the spectrum. Ultraviolet radiation can cause
changes in living matter. The sun's rays cause sunburn. Rays from
a welder's torch burn the unprotected eyes of an observer. The
ultraviolet spectrum includes wavelengths from 2000 to 3900 Angstrom
units (Å). One unit is one ten billionth of a meter. The
2000 to 3900 Å range may be divided into three segments:
Long-wave
ultraviolet - The wavelength range is 3250 to 3900 Å.
These rays occur naturally in sunlight. They have little germicidal
value.
Middle-wave
ultraviolet - The wavelength range is 2950 to 3250 Å,
also found in sunlight. Middle-wave UV is best known for its sun-tanning
effect; it provides some germicidal action, with sufficient exposure.
Short-wave
ultraviolet - The wavelength range is 2000 to 2950 Å.
This segment possesses by far the greatest germicidal effectiveness
of all ultraviolet wavelengths. It is employed extensively to
destroy bacteria, virus, mold, spores, etc., both air- and water-borne.
Short-wave
ultraviolet does not occur naturally at the earth's surfaces,
because the atmosphere screens out sunlight radiation below 2950
Å. In order to take practical advantage of the germ-killing
potential of short-wave ultraviolet, it is necessary to produce
this form of energy through the conversion of electrical energy.
The conversion of electrical energy to short-wave radiant ultraviolet
is accomplished in a mercury vapor lamp.
Mercury
Vapor or Germicidal Lamps
The
low-pressure variety of mercury vapor lamp, which can be referred
to as a germicidal lamp, provides the most cost-effective and
efficient source of short-wave ultraviolet energy. Germicidal
lamps are made of special quartz glass that will allow 70 to 90
percent of the short ultraviolet rays to pass. Ordinary glass
is not transparent to wavelengths below 3200 Å. The low
pressure mercury vapor lamp emits radiation that is predominately
at 2537 Å. This is in the region of maximum germicidal effectiveness.
The
germicidal lamp works on the following principle: An electric
arc is struck through an inert gas carrier, in a sealed special
glass tube. Heat from the arc causes vaporization of the small
amount of mercury contained in the sealed tube. The mercury, when
vaporized, becomes ionized and in the electric arc gives off UV
radiation.
Required
Germicidal Energy
Bacteria
withstand considerably more ultraviolet irradiation in water than
in dry air. E.coli, for example, (common in the water of unprotected
catchment systems), requires more UV exposure for their destruction
in water than in dry air. In either case, the germicidal radiation
must strike a microorganism to destroy it. This requires
that the water be clear enough to allow transmission of an adequate
quantity of UV energy. The degree of microbial destruction is
a function of both the time and intensity of the radiation to
which a given microorganism is exposed. A short exposure time
at high intensity is as effective as a long exposure time at low
intensity, provided the product of the time and intensity remains
the same.
Any
turbidity in the water reduces the range of transmission to UV
radiation. Water that is naturally turbid, or that has become
turbid from corrosion products formed during storage in steel
tanks and liners, should be filtered before UV purification. A
5-micron filter prior to the UV lamp is recommended.
Purifier
Design
Several
design features are combined to determine the dosage delivered:
1.
Wavelength output of the lamp.
2. Length of the lamp - when the lamp is mounted parallel to the
direction of water flow, the exposure time is proportional to
the length of the lamp.
3. Design water flow rate - exposure time is inversely related
to the linear flow rate.
4. Diameter of the purification chamber - since the water itself
absorbs UV energy, the delivered dosage diminishes logarithmically
with the distance from the lamp.
In
a typical operation, water enters the inlet of a UV lamp and flows
through the annular space between the quartz sleeve (which contains
the germicidal lamp) and the outside chamber wall. The irradiated
water leaves through the outlet nozzle.
Features
to look for:
1.
Expandable system - parts should be as uniform and as interchangeable
as possible to permit easy expansion later.
2. Sight port - enables visual monitoring of lamp operation;
also permits later adaptation to electronic monitor device using
the same port.
3. Single lamp per chamber - provides greater safety through
more accurate monitoring than does a multi-lamp /single-chamber
system.
4. Quartz protection sleeve - cold water moving past an
unshielded lamp will reduce the lamp temperature and the radiation
yield. A protective quartz sleeve will allow the higher lamp temperature
required for optimum output of 2537 Å radiation.
5. Mechanical wiper - for cleaning the sleeve surface without
shutdown or disassembly of the unit.
6. Optional accessories - Flow controls, UV light bulb
monitor, electronic water shut-off valves and alarms, should be
available to provide fail-safe operation without operator attendance.
A
single lamp purifier can be designed to handle any flow rate up
to approximately 2400 gallons per hour (gph). By multiplying purifier
units, in series and in parallel, higher flow rates can be achieved.
The
versatility of UV purification includes:
1.
UV purification produces germ-free potable water for home, institutional
and municipal use.
· Application to water wells: bacterial contamination of
wells is unpredictable and may occur from seepage of surface water
or sewage.
· Installation on outlet side of rainwater harvesting cisterns,
most cisterns foster the proliferation of bacteria in untreated
water.
2. It provides bacteria-free food process water without the use
of germicides, oxidants, algaecides or chemical precipitants;
particularly applicable where chlorine adversely affects flavor.
Chlorine
Versus Ultraviolet Purification
As
a tertiary treatment for water, chlorination offers the advantage
of continued disinfection after initial treatment, since some
chlorine remains in the water with residual germ fighting action.
The ultraviolet method, however, has none of the following disadvantages
of chlorine:
1.
Chlorine treatment requires operation attention.
2. In small installations, when chlorine gas is liberated from
a chlorine cylinder or moistened crystals or pellets, the fumes
are extremely dangerous and may even be lethal.
3. Chlorine itself is a highly corrosive and toxic chemical.
4. Chlorine is an additive material which may impart an undesirable
taste to the water and a decrease in pH.
5. Chlorine is chemically active and can react with foreign ingredients
(e.g., as found in industrial waste-waters) to form toxic compounds,
a matter of increasing concern to the Federal Government and to
many states and municipalities.
· It may combine with ammonia to form "chloramine"
which is acutely toxic to fish even at low concentration.
· It may combine with phenol to form "clorophenols",
another dangerously toxic compound.
This
article is an excerpt from a technical article courtesy of Atlantic
UltraViolet Corporation. Sign in for a complete copy of this
original technical paper as well as other technical papers and
FAQs on UV lamps.
Note: Per the MEDRIX UV handbook, "the ultraviolet energy emitted by a UV-C germicidal lamp has the capacity to alter the nucleic acid (DNA) of viruses, bacteria, molds and parasites so they cannot reproduce and are thereby considered inactivated."
Related
Topics:
Article
on Floating Filter
Rainwater Harvesting
Vendors [by State] - HarvestH2o.com
NSF
on Water Treatment
Water Quality Handbook
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