Inventions in solid state lighting, particularly the advancements in white light LEDs, have been driving the most significant transformation in lighting since the very first introduction of the incandescent light bulb. Headline societal impacts of LEDs-recently acknowledged with Nobel awards to several of the key inventors-include energy efficiency, long life and environmental benefits. Additionally, the versatility of design options opened up by the compact nature of LEDs has been touted across an array of applications in industries ranging from residential to commercial and industrial. 
Now, with recent inventions in ultraviolet (UV) emitting LEDs, we are on the cusp of a new revolution in disinfection. In particular, water disinfection options for a whole host of applications and industries are being reconsidered. Powerful, reliable UV LEDs are figuring prominently into more compact designs in everything from portable appliances to municipal systems.
As the most commonly used solvent in any laboratory environment, clean water is a staple and the quality of water is critical for experiments and core processes. Lab water purification systems typically use a combination of methods, including reverse osmosis, filtration and UV disinfection, and range from large centralized systems to small bench top units. Traditionally, these systems have used mercury lamps for UV disinfection.
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| Figure 1: Lab water purification system showing various points of UV disinfection throughout the system. |
In UV disinfection, light in the range of 250-280 nm disrupts the DNA of microorganisms rendering them unable to reproduce. The action spectrum for bacteria is commonly reported as 265-267 nm peak wavelengths, although wavelength susceptibility may vary among a large number of bacterial and viral strains. Low-pressure mercury arc lamps have become a common way of accessing the germicidal portion of irradiation spectrum. As a plasma, they emit a discrete wavelength at 253.7. Although this is not the optimum germicidal wavelength, there is sufficient emission to provide a means of DNA disruption and they have become the industry standard.
However, UVC emitting LEDs are emerging as a viable alternative. Here, the spectral response is continuous, as opposed to mercury’s line response, and the peak of the emission may be tailored to the most optimal germicidal wavelength by using breakthrough crystal growth technologies. (Figure 2)
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| Figure 2: Spectral comparison of low-pressure mercury lamps versus LED in relation to germicidal effectiveness curve. |
Enabling More Compact Designs
The UV disinfection that occurs in a typical lab water system makes use of a flow cell reactor-water flows into the cell, is disinfected by UV light and flows out of the cell free of target microbes. The efficiency of the flow cell system is largely dependent on the size and shape of the cell. Parameters important for optimal disinfection include UV power, the size of the cell, the amount of water to be disinfected and the flow rate of the cell.
Traditional disinfection systems using mercury lamps must adapt to the shape of the mercury bulb – a long, cylindrical tube. This shape dictates the footprint of the UV unit in the overall water system – limiting the system designer’s options in overall size and configuration of the disinfection reactor. UVC LEDs are a point light source in a durable, compact package, which allows for a multitude of arrangements and footprints. The decrease in the size required for the UV reactor allows for development of smaller bench top units and integration of UV disinfection at more stages of the process.
Decreasing Operating and Maintenance Costs
Mercury lamps have been known to require significant warm up time, anywhere from 50 seconds to ten minutes, to reach the required germicidal intensity. In addition, frequent on/off cycles can diminish lifetime by 50 percent or more. Consequently, mercury lamps in these systems are kept on all day-increasing power consumption and the frequency of lamp replacement. These fragile bulbs also contain mercury and thus must be carefully disposed of when replaced – typically about once a year.
UVC LEDs reach their full brightness in under a microsecond, which makes them ideal for on-demand disinfection applications. As on/off cycles do not impact their lifetime, systems using UVC LEDs for disinfection are able to take advantage of the full operating life of the LED for disinfection use. This efficiency also reduces the power consumption of the system and may significantly extend the lifetime of the end unit they are integrated into. In some cases, annual replacement of UV source is no longer required as a lamp may be designed which matches the lifetime of the product.
