Water for Hospitals
Tower Recirculation Loops
Sterilization Loops for electronics/semiconductors)
& Offshore Platforms
of Other Applications
in-line UV system with intensity monitor and alarm)
INFORMATION ON ULTRAVIOLET TECHNOLOGY
the technical discussions - take me directly to the equipment and specification
many years chlorination has been the standard method of water disinfection.
Recent studies have shown that water chlorination causes several environmental
problems. Residuals and byproducts can be toxic to aquatic life in receiving
waters. Some by-products of chlorination may be carcinogenic, and may
require removal in a drinking water treatment; plant. Also, it has been
discovered that chlorination is much less effective in virus destruction
than in killing bacteria .
the onset of the "energy crisis" of the 1970's, chlorination
was the most cost-effective disinfection technique. Because chlorination
production is energy intensive and because energy costs have -,increased
freight rates, the price of chlorine at the plant site has risen substantially
in recent years. This trend is likely to continue. Potential gaseous
chlorination safety problems have caused come communities, e.g., New
York City and San Francisco, to choose hypochlorite which is much more
costly than gaseous chlorine.
above problems with chlorination have caused consulting engineers, users,
and regulatory agencies to-actively pursue alternatives for water disinfection.
Ultraviolet light is currently the leading candidate for water disinfection.
It has the following inherent advantages over all other disinfection
No chemical consumption-eliminates large scale storage, transportation
and handling, and potential safety hazards.
Low contact time-no contact basin is necessary and space requirements
No harmful by-products are formed.
A minimum of, or no, moving parts - high reliability.
Low energy requirements
disinfection, thus, solves the environmental and safety problems, and
is cost-effective as well.
disinfection of water employs low-pressure mercury lamps. They generate
short-wave ultraviolet in the region of 2537 Angstroms which is lethal
to microorganisms including bacteria, protozoa, viruses, molds, yeasts,
fungi, nemotode eggs, and algae. The mechanism of microorganism destruction
is currently believed to be that ultraviolet causes molecular rearrangements
in DNA and RHA, which in turn blocks replication.
micro-organisms are subjected to ultraviolet light, a constant fraction
of the number present die in each time increment (4). The fraction of
the initial number of micro-organisms present at a given time is called
the survival ratio. The fraction killed is one minus the survival ratio.
The mathematical expression of these facts is shown below:
Ratio = Nt/No = e-KIt
= The number initially present
= The number surviving at time
= The time of exposure
= the intensity (more correctly, scalar irradiance of ultraviolet light
impinging on the microorganisms)
= A constant which depends upon the type of micro-organisms and wavelength
of ultraviolet light.
above equation indicates that for each given micro-organism and UY wavelength,
the fraction killed depends upon the product of UV light intensity and
exposure time. This product is known as the "dosage". It is
the single most important parameter for rating UV disinfection equipment.
validity of this mathematical expression has been proven over a thousand
fold range in intensity. The expression fails at only low values of
intensity, which would not probably be found in a properly designed
dosage requirement varies with the type of micro-organism. Dosage requirements
for bacteria range from 2,500 to 22,000 microwatt-second/cm2. Yeast
dosage requirements range from 6,600 to 17,600 microwatt-second/ cm2.
Mold spore, fungi, and algae dosage renuirements range from 11,000 to
with the exception of the tobacco mosaic (not normally found in water)
have dosage requirements in the same range as bacteria. Protozoa and
nematode egss have extremely high dosage requirements.
is no universally accepted minimum dosage requirement for ultraviolet
In 1966, the U.S. Public Health Service published a policy statement
which contained a drinking water disinfection dosage requirement of
16,000 microwatt-second/cm2 (5). This statement has formed the basis
for several standards published throughout the world.
dosage requirement has not yet been developed for wastewater. However,
fecal coliform bacteria are the customary indicator micro-organisms
used in judging the efficiency of water disinfection. The destruction
dosage for fecal coliform is 6,600 microwatt-second/cm2 .
basic design problem in any ultraviolet system to efficiently and reliably
deliver the required dosage to micro-organisms suspended in the fluid.
There are essentially two basic design concepts in current use to accomplish
this task. One employs flow over a submerged bank of germicidal lamps
with quartz sleeves. Fluid flows through Teflon tubes surrounded by
germicidal lamps in the other design concept.
Tube Systems with Shellside flow
substances are not penetrated by short-wave ultraviolet rays. Water
is one of the few liquids which allows a significant penetration. Quartz
is one of the few solid materials that is virtually transparent to short-wave
ultraviolet. It is used in the manufacture of germicidal lamps and as
a material of construction in many commercial ultraviolet systems.
tubes tend to be brittle, fragile and difficult to seal. Complicated
in-place cleaning systems must usually be installed to keep the quartz
surface free from fouling materials.
a conventional quartz ultraviolet disinfection unit, water flows over
a bank of quartz sleeves similar to flow in the shell-side of a shell-and-tube
heat exchanger. Inside each quartz sleeve is a germicidal lamp. A separate
o-ring seal is made at the end of each quartz sleeve. The outer shell
is usually constructed of either stainless steel andized aluminum, or
polyvinyl chloride. Quartz UV systems are designed for either pressurized
or gravity water flow.
the quartz ultraviolet system is disinfecting an ultra-pure water source,
the quartz sleeves readily foul with suspended and dissolved matter
in the water. Therefore, it is necessary to employ a technique to remove
the fouling matter on an almost continuous basis to preserve the high
UV transmittance of quartz and the disinfection capability of the system.
non-chemical, cleaning methods have been used with limited success.
The first is a mechanical wiper system, in which a wiper periodically
scrapes fouling deposits off of the outer surface of the quartz sleeves.
For this technique to work effectively, very close toler ances are required
on quartz sleeve outer diameter and alignment. Close tolerances are
also required on the wiper system as well. These severe tolerance requirements
add to manufacturing expense and tend to be difficult to achieve with
large tube bundles.
Tube Flow Systems
the early 1970's it was discovered that FEP Teflon was also an excellent
transmitter of 253.7 nm ultraviolet light . Data obtained over fifteen
years of continuous testing by DuPont indicated that Teflon was also
very stable to solar ultraviolet (minimum 290 nm). Shorter term tests
(five years) indicated that Teflon is virtually unaffected by the shorter
germicidal wave length light. Teflon, an deal material to contain a
wastewater during disinfection, has the following advantages:
It has a high transmission of 253.7 nm ultraviolet light - approximately
80 percent transmission with wall thicknesses used in disinfection systems.
Teflon is chemically inert. Teflon tubes are not attacked by substances
present in water or wastewater.
It is non-wetting and has an extremely low-friction co-efficient. Tefton
tubes are usually not fouled by substances present in water or wastewater.
If fouling should occur, chemical cleaning can be used to easily remove
Teflon is an approved material by the U.S. Food and Drug Administration
for use with food and beverages.
It is virtually unaffected by ultraviolet rays.
the Teflon tube flow system, the fluid to be disinfected flows through
Teflon tubes. To achieve large flow capacities, these tubes can be connected
in parallel to large diameter headers. Systems of several million gallons
per day capacity can then be built as a single unit.
of germicidal lamps are placed in between the tubes so that each tube
is exposed to ultraviolet light from alt sides. The lamps are mounted
on a frame which can be slid out for easy lamp replacement.
which is an excellent reflector for ultraviolet, forms the outer casing.
Unabsorbed ultraviolet, which strikes the enclosure walls, is mostly
re-reflected and is eventually absorbed by the water. This design is
extremely efficient in the utilization of ultraviolet energy emitted
by the germicidal lamps.
the preceding, it is obvious that an essential aspect of effective application
of ultraviolet irradiation involves maintaining an optimal dose of radiant
energy. Manufacturers of ultraviolet irradiation equipment provide ratings
related to the maximum water flow rates which may be attempted, above
which the radiant dose will be inadequate.
there will be some variations among different makes and models, most
ultraviolet disinfection equipment of the type used for hemodialysis
and other purified water systems are designed to provide a radiant dose
of 30 000 microwatt-sec/cm2, which is well in excess of that needed
for the destruction of most, but not all, types of water-born bacteria.
this regard, it should be recognized that an inadequate dose of radiant
energy may result if the mercury vapor lamps are not periodically replaced,
if the water contains materials which absorb or prevent the light from
reaching the bacteria, or if the quartz sleeve becomes coated.
should also be pointed out that, for many commercial units, a continuous
flow of water is required to prevent overheating of the ultraviolet
lamps. If such overheating is allowed to occur, the wavelength of ultraviolet
light emitted may change to a point at which it is no longer bactericidal
and, when flow is resumed, initial volumes of effluent water may be
has been reported that certain bacterial species are resistant to ultraviolet
irradiation and, unless controlled by other means, may proliferate to
excessive levels. A further disadvantage of ultraviolet irradiators
is their ineffectiveness for the removal of endotoxin. The inability
of ultraviolet irradiation to remove endotoxin may preclude its use
in some medical situations, or other appicatiors in whch more stringent
control of these contaminants is necessary.
irradiation equipment is convenient to operate, routine use requiring
only that they be electrically switched "on", and may be helpful
in suppressing bacterial growth in purified water distribution systems
in which upstream equipment, such as reverse osmosis, provides the primary
means for their removal.
filtration is also frequently used, as a secondary means of bacterial
control. The capabilities of ultraviolet irradiation and submicron filtration
are both limited in that neither remove endotoxin and only destroy or
eliminate bacteria within the equipment itself. Ultraviolet irradiation
may actually cause endotoxin levels to increase as a result of bacterial
since ultraviolet irradiation has no downstream bactericidal effects,
it should be used in combination with regular chemical disinfection
procedures. In this manner organisms are chemically eliminated from
the entire system, after which ultraviolet irradiation prevents rapid
multiplication of bacteria which may penetrate the reverse osmosis equipment.
of ultraviolet irradiation equipment should recognize the need for regular
maintenance, including lamp replacement and cleaning. Ultraviolet lamps
are typically designed to operate for one year, during which time radiant
output will progressively decrease.
at a minimum, the lamps must be replaced annually. Most manufacturers
also provide, at additional cost, monitors to detect loss of lamp radiant
energy output and these provide a more accurate indication of the need
for lamp replacement than reliance on calendar-based replacement schedules.
trust that this short overview of Ultraviolet Technology is useful to
you as an end user or as a water dealer. Your comments and suggestions
are always appreciated.
VOLUME ULTRAVIOLET SYSTEM TECHNICAL SPECIFICATIONS*
Capacity Clear Fresh Water
enclosures are NEMA 3R; Operating Pressure is 125 psi maximum
NOW - SOME TYPICAL APPLICATIONS
SYSTEM WITH PRE-TREATMENT SYSTEM AND
HIGH VOLUME REVERSE OSMOSIS
VOLUME UV SYSTEM INTEGRATED WITH OZONE AND RECIRCULATION EQUIPMENT
ON PORTABLE, DISINFECTION SKID - FOR WATER STORES OR SMALL BOTTLING
details on this item,
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