The science, selection and applications
Disinfection—the killing of potentially harmful microorganisms—in our water and air is critical to public health and industrial process reliability. Historically, industry has relied on hazardous chemicals such as chlorine and expensive processes such as pasteurization to rid its water supplies of pathogens. Currently, though, the use of ultraviolet (UV) light, the same as the portion of the electromagnetic spectrum of sunlight responsible for killing microorganisms, is providing a safe, reliable and highly effective method of getting the job done.
Traditionally, the primary industrial markets for UV
technology have been food, beverage, pharmaceutical, aquaculture and wastewater
treatment. However, sales are expanding into more than three-dozen new industries
concerned with microbiological control. This rapid expansion in UV application
continues due to regulatory and quality requirements for reducing chemical use,
lowering costs and improving processes and water quality.
UV is a highly effective way to address bacteria, viruses,
yeast and mold including pathogens resistant to chemical treatment. It is used
often to reduce microbiological levels by greater than 99.99 percent.
This rest of this article will cover
science of UV technology,
of common systems and typical options,
applications within industry, and
to address when selecting a system.
UV light is part of the electromagnetic spectrum. It is a
“color” just as blue, yellow and even infrared are colors.
UV is the portion of the spectrum at higher energy (shorter
wavelength) than visible light. It most commonly is experienced as the energy from the sun that causes tanning. At even higher energy, the UV-C and
vacuum UV wavelengths arise. It is these wavelengths of light between 180nm and
300nm that have the desired effects.
living organism contains DNA within its cells. Single cell organisms such as
bacteria, protozoa, viruses and molds can survive and become pathogenic only if
they can reproduce. Wavelengths from 200nm to 320nm will react with all
cellular DNA and prevent replication, hence killing a single-cell organism.
This germicidal effectiveness peaks around 265nm and covers the output of
germicidal UV lamps. Generally, the larger the organism, the more
“protected” the cell nucleus, the more UV is required to inactivate
the cell. All cellular life can be affected by UV, but size matters, and spores
of yeasts and molds require much more energy to disinfect.
UV Dose. There are
widely accepted charts showing the amount of UV required to inactivate a
variety of organisms. These “D-10” charts show the UV dose required
for a single-log or 90 percent reduction in the number of colony-forming units.
Of course, application of two times this dose would result in a 99 percent
reduction and so on.
Dose is a function of the UV intensity reaching the organism
multiplied by the exposure time. Factors affecting dose include energy of the
UV lamp, clarity or transmission of the water or fluid being treated, flow
rates and residence time in the treatment chamber. Typically, UV systems are
rated at different flow rates based on fluid UV transmission to deliver doses
of 30 mW-s/cm2 (which is equal to 30,000µW-s/cm2 or 30 mJ/cm2). This dose
delivers a 5 to 6 log reduction in many common pathogens but will deliver at
least 99.9 percent reduction to most organisms of concern.
Photolysis and advanced oxidation. In addition to disinfection, UV light is very
effective in breaking down certain chemical compounds through processes know as
photolysis and UV advanced oxidation. Both vacuum and UV-C are effective in
breaking down a variety of chemicals. UV systems can remove ozone, chlorine,
chloramines and organic compounds by directly breaking the chemical bonds
(photolysis) or by creating OH– radicals directly from water or another
oxidant such as peroxide. These applications typically require UV doses of
three to four times up to as much as 20 to 40 times the dose required for
UV Treatment Systems
A typical piece of equipment used for UV treatment of water
is made of three main components: a treatment chamber, the UV lamps and a power
and control cabinet. In addition, there are a number of specialty features and
options that enhance the system’s performance and capabilities.
UV lamps. There are
two primary types UV lamp
technologies used in these applications: low pressure (LP) and medium pressure
(MP). The “pressure” refers to that of the lamp’s internal
gas but the main difference lies in the power of the lamp. All of these lamps
excite a mercury vapor inside of a quartz tube. Mercury emits UV light, and
quartz—unlike regular glass—is transparent to UV wavelengths. LP
lamps range from 40 watts to the new 160- or 180-watt bulbs. They essentially
are identical to the fluorescent lights found in most offices. LP lamps emit UV
wavelengths at 185nm and 254nm, which have become the standard nomenclature for
UV emission. MP lamps emit wavelengths across the entire UV spectrum and range
from 500 watts to more than 7 kilowatts. Due to this energy difference, the
less-costly LP lamps can treat far less fluid than a single MP lamp. MP lamps
can deliver very high doses, treat very large or poor quality flows and operate
at temperature extremes of hot and cold fluids. LP systems are broadly accepted
and are relatively less expensive.
The treatment chamber consists of a 316L stainless steel, pressurized vessel
with a single or multiple UV lamps mounted either parallel to or perpendicular
to flow. Lamps are located inside the chamber and are isolated from the process
fluid by a quartz sleeve.
Power and control cabinet. The power and control cabinet consists of a typical electrical
cabinet that houses the lamp power supplies, the control electronics and user
interface. They are available in polycarbonate, epoxy-coated steel and 304
stainless steel. Standard cabinets carry a NEMA 12 environmental protection
rating. NEMA 4 (waterproof) and NEMA 4X (waterproof and corrosion resistant)
also are available. LP systems are powered by ballasts while MP systems require
transformers. Cabinet cooling is a concern and typically accommodated
by fans or cabinet coolers. User interfaces and controls
vary by product line and range from membrane keypads with digital displays to
simple switches and LED’s. All power and instrument wiring from the
cabinet to the chamber is included with the system.
UV Treatment Systems (Options)
UV monitor. The
monitor detects the UV output of the lamp(s) at the interior wall of the treatment chamber. Monitoring is valuable in tracking lamp and system performance. A monitor is recommended in all disinfection applications in fresh, reverse osmosis/deionization (RO/DI) and other clean water.
Quartz sleeve cleaning.
The cleaning mechanism removes deposits that may accumulate on the quartz
sleeve, which would block the emission of UV light. It is recommended for use
in applications where the water is high in organics or dissolved iron or
manganese. The wiper can be actuated automatically or manually with automatic
being preferred to ensure regular cleaning and maintain system performance.
High purity finish.
Chambers may be electropolished and passivated to better than Ra25 or Ra15
interior surface finishes. These highly polished finishes eliminate any
microsurface texture where organisms be retained. This option is preferred in
pharmaceutical, electronics and often food and beverage applications.
Specialty quartz. In
addition to standard quartz for lamps and sleeves, ozone-free quartz options
are available to eliminate the production of ozone in the system. This is
critical in deozonation, recirculated indoor, high iron content and air
applications. A special high purity quartz, which accentuates high-energy
vacuum UV, is used in TOC destruction and other photolytic applications.
Sample, drain and bleed systems. Taps and controls are available to sample fluid,
drain and bleed the UV systems. Bleeding may be required in intermittent flow
MP systems to maintain lamp cooling.
Packages are available that include a wide range of test and documentation such
as component validation and testing, material certifications and certification
to standards. This is valued in pharmaceutical applications.
Options include upgrading material to 304 stainless steel and upgrading
environmental protection rating to NEMA 4 or NEMA 4X. Frequently used in
outdoor installations and food and beverage applications.
Power and control upgrades. Options include variable power control to increase power to maintain
consistent dose over the life of the lamps, data logging to allow data
downloads of all system parameters and dose monitoring to measure and control
the actual, absolute UV dose.
Applications of Ultraviolet Technology
The most common application of UV systems is in the disinfection of water. Most
industrial uses of water require removal of microorganisms to protect the
product being made, industrial process, environment or worker health and
safety. For consumables such as foods and beverages, water used in the process
must not contain any biological contamination that would result in human health
hazards and product liability. In many processes water of exceptional purity is
needed as a process fluid.
RO, activated carbon filtration and DI commonly are employed
with UV being an integral part of these processes. Industrial wastes must be
treated to protect the environment, and many process waters including cleaning
and rinse fluids must be free of contamination as well.
Often, chemicals such as chlorine are used to disinfect
water. UV eliminates or greatly reduces the use of disinfection chemicals along
with their cost, storage, environmental and worker hazards and potential to
damage other process equipment.
Ultraviolet systems are highly effective at deozonation. Ozone commonly is used
as a disinfection chemical and as an oxidant for organic compounds. Often,
residual ozone can cause harm to process equipment or cause health hazards.
Typically, UV doses of three to four times that of disinfection are required.
Chlorine and chloramines commonly are used to provide residual disinfection of
water. Chlorine will damage RO membranes and other process equipment or
otherwise affect a product or process. UV provides a reliable, cost-effective
alternative to activated carbon or bisulphite injection or other chlorine
removal systems that can cause microbiological problems and high maintenance
costs. These applications require extremely high doses of UV.
Destruction of organics.
Many large or nonpolar organic molecules contaminating water can be broken down
by UV light and become harmless or easily removed by activated carbon or DI.
High purity, RO/DI water requires very low organics. UV also can break down
many organic contaminants that pose environmental or health hazards.
Disinfecting air. Ultraviolet
light provides a very effective means of disinfecting air. Hospitals, food
processors and clean rooms require air that is free from living organisms that
may cause harm. Tanks for storing sweeteners, syrups, ingredients and other
fluids must allow air to vent when draining or filling. UV can protect these
fluids from contaminated air.
Selecting the Right UV System
UV transmission. The
first factor to consider is transmission of the water or fluid to germicidal
wavelengths. To determine transmission, a 10 mm (T10) or 40 mm (T40) quartz
cell is filled with the fluid, and UV light is passed through it measuring the
amount not absorbed. Typical fresh water has a transmission of 80 percent in a
40 mm cell (T40 = 80 percent), RO/DI water usually is T40 = 95 percent, and
typical prefiltered wastewater is T10 = 60 percent. Many fluids less than T10 =
50 percent cannot be effectively treated with UV unless there is a high rate of
recirculation. Estimates of transmission are common, and fluids should be
tested for their UV transmission when selecting a system.
Flow rate. Every UV
system will have a maximum flow capacity at each transmission value. Typically,
the higher the transmission, the greater the capacity. Each system will be
limited as to how low a T-value it can treat based on its design. Some
high-power systems hydraulically are limited in that the lamps can treat more
water than chamber velocities allow.
UV dose. Most rated
flow capacities are for a 30 mJ/cm2 (or one times the disinfection) dose at the
end-of-lamp-life (8,000 hours continuous operation). All UV lamps gradually
become more opaque due to use and are oversized initially to compensate. Other
applications require much higher doses. A deozonation dose of three times
disinfection would reduce a system’s capacity by a factor of three.
LP. It is important
to understand when LP or MP lamps should be used. LP lamps relatively are
inexpensive, but cannot treat as much fluid per lamp. For typical fresh water
disinfection with no unusual conditions, LP is much more cost effective at flow
less than 250 to 350 gpm. As transmission falls, dose requirements increase or
other factors come into play such as the equal-price flow rate becomes lower.
Also, since LP systems output only 185nm and 254nm wavelengths, many fluids
besides water, which may be harmed by other wavelengths, can be treated.
MP. At higher flow
rates, fluid temperatures above 70° C or below 40° C, high dose
requirements or where added features are critical, MP systems are more
economical. MP systems are available with a broader range of options. Above 250
to 350 gpm (or less for low transmission fluids or higher dose requirements), the
use of a single high power lamp is more cost effective due to fewer lamps,
fewer chambers and controls, decreased installation and piping costs, and
greatly reduced maintenance and service costs. Also, temperature extremes have
no effect on MP lamps. For MP systems, the use of a single lamp with a single
monitor allows for greatly increased monitoring, control and process security.
There is no uncertainty with one lamp.