By Dr. Stephen R. Wilk, Optical Engineer, Xenon Corporation, Wilmington MA
Extensive use of Mercury Germicidal Light in the Food Industry
Ultraviolet light is used for a large variety of applications in the food industry . With high-performance UV light sources and equipment, water, air, and surfaces can be reliably disinfected, cleaned and treated. UV light has been shown to be effective on most bacteria, as well as viruses and molds.
On surfaces such as conveyor belts, UV light can be used to provide continuous decontamination in the form of a module designed to expose the belt to UV as it passes. The intense germicidal action of UV light renders micro-organisms inactive. Mercury-based lamps emit UV light at the 253.7 nm. UV breaks molecular bonds within microorganism DNA, producing thymine and pyrimidine dimers that can kill or disable the organisms. UV light is widely used in meat packing, dairy, baking, and fruit and vegetable industries to kill bacteria, viruses, and molds.
Why replace the current UV sources?
Because of the toxic and cumulative effects of mercury, especially methylmercury, in the environment, governments and companies have been moving away from its use as much as possible, passing legislation to enforce this. One important international agreement was the Minamata Conference Accords. Many nations adopted its language in formulating mercury guidelines. Mercury was still allowed for small-scale metal refining, for dental amalgams, and for compact fluorescent and similar lamps, but only at a very low rate of usage. Although germicidal lamps are not even mentioned, manufacturers have been moving away from the use of mercury germicidal lamps altogether.
What are the alternatives?
Mercury-free alternative ultraviolet sources should be easily available, not prohibitively expensive, long-lasting, and easily substituted, requiring no onerous conditions. Deuterium lamps have long been used as a reliable source of deep ultraviolet light, providing a continuous spectrum down to 160 nm. The drawbacks are that it requires several minutes to warm up to a stable operating temperature, and the lifetime of the lamp is relatively short. Older models last typically 2000 hours, but newer models have lifetimes two to three times as long. Xenon arc lamps can produce extremely bright, broad spectra, and are in wide use as solar simulators and as projector lamps.
Xenon lamps in many such applications have their ultraviolet output curtailed, and don’t provide much below 300 nm, but if the lamp exterior is made of fused silica (which transmits light down to 200 nm) xenon arc lamps can provide output to below 250 nm. Argon and Krypton lamps also provide outputs that peak in the visible, but which continue down to 200 nm. The main problem with these is that they are not very common.
Photo courtesy of Xenon Corporation, Wilmington MA
Xenon flashlamps and Krypton flashlamps produce broad-band spectra that cover from below 200 nm to the near infrared. These lamps require high voltage power supplies and produce very short but very intense illumination. There are several suppliers, and the lamps have a long lifetime.
Ultraviolet Light Emitting Diodes (LED)are now available down to about 255 nm. LEDs have the advantage of being small, relatively low voltage, and have long lifetimes. The relatively narrow bandwidths (typically about 15 nm) mean that, for a properly chosen center wavelength, virtually all of the photons go into the 260 nm DNA absorption peak. LEDs as a class are a relatively inexpensive option, although the ultraviolet LEDs tend to be more expensive than other LEDs.
Excimer lamps make use of the same Excited Dimers (properly Excited Complexes) as Excimer lasers. These are gas mixtures, typically involving noble gases, that emit when excited electrically by high voltage. The output can be continuous, and the spectrum is typically narrow, although broader than LED output. One drawback is that most currently commercially available excimer lamps do not have good overlap with the DNA absorption. Krypton Fluoride (249 nm) and Xenon Iodide (253 nm) have better overlap, but are not among the excimers currently offered by manufacturers.
What discussion there has been about finding mercury-free alternatives has centered almost exclusively around excimer lamps and ultraviolet LEDs. None appear to mention flashlamps, arc lamps, or other alternatives at all. (1, 2, 3, and 4) This apparent omission extends to agencies (such as universities, or the IEEE) who mention only UV LEDs and Excimer lamps to the exclusion of other alternatives (5, 6 , and 7) Clearly, the makers of flashlamps and other sources of ultraviolet light must present the case for their products and the advantages of using them are clearly and publicly set forth.
M. Masoud and D. E Murnic 2013. High efficiency fluorescent excimer lamps: An alternative to mercury based UVC lamps. AIP Review of Scientific Instruments.
Nazieh MASOUD, and Daniel MURNICK , 2013. High Efficiency, Fluorescent Excimer Lamps, an Alternative to CFLs and White Light LEDs. Light & Vis. Env. Vol.37, No.4
Ammoniums 2013. Why UV LED curing printers are the better investment. IT Enquirer
K. Holz, J. Lietard and M.M. Somoza 2017. High-Power 365 nm UV LED Mercury Arc Lamp Replacement for Photochemistry and Chemical Photolithography.ACS Sustain Chem Eng. 2017 Jan 3; 5(1): 828–834.
M. Broxterman, S. Korte, and T. Juslel 2017. Mercury Free UV Lamp for Disinfection and Purification of Drinking, Process, and Waste Water – An Approach to assessing its Innovation Potential and Possible Market Entry Strategies. Journal of Business Chemistry 2017, 14 (3)
Hasen 2016. How LEDs Will Change Water Purification. Enviro Protection April 28, 2016.
A. Voronov 2008. Mercury free UV-light sources based on excimer lamps. 17th International Conference on Gas Discharges and Their Applications.