System Approach to UV-C LEDs Water Disinfection

Jim Cosman, Mitch Hansen, and Molly McKain - May 15, 2017

Ultraviolet light has become a star player in the disinfection world. It is increasingly being used in key applications that require alternative disinfection options. UV disinfection has several advantages including:

1. Chemical free UV provides physical treatment without the use of harmful chemicals

2. No DBPs No risk of harmful disinfection byproducts being generated as with chemical treatment

3. Efficient pathogen inactivation UV is very effective against a wide range of waterborne pathogens, including chlorine-resistant organisms such as Cryptosporidium and Giardia

4. Low maintenance Robust technology that is easy to use and maintain

Historically, conventional gas-discharge (mercury) lamps were employed in water treatment systems to emit UV light. Recently, a new type of light source (UV LEDs) emitting in the germicidal wavelength range (250 to 285 nm), has been rapidly developing and implemented into small-flow water treatment systems. In addition to the core benefits of UV disinfection technology listed above, UV-C LEDs offer additional benefits, including:

1. Mercury-free Conventional UV lamps contain mercury, but UV LEDs are free of hazardous materials, which eliminates the risk of a mercury spill due to a lamp breakage

2. Compact footprint High-power-density UV-C LEDs and advanced controls allow for a much smaller footprint compared to traditional UV systems

3. Instant on/off Systems are intermittent-flow friendly and can instantly be switched on and off without any warm-up time requirements. This also enhances power savings and leads to a prolonged lamp life

4 Unlimited cycling Lamp life is not impacted by on/off cycles, allowing for unlimited lamp cycling

5. Temperature independent LEDs do not transfer heat into the water, thus limiting lamp fouling and ensuring a constant UV output, regardless of water temperature

On a higher level, LEDs offer the same security of physical disinfection as mercury lamps. It is important, however, to examine the fundamental differences between UV-C LEDs and mercury lamps. New thinking and design approaches are required to fully utilize the potential of this exciting new light-source. An overall systems approach is required to fully harness the enormous potential offered by UV-C LEDs. Below are the key engineering issues associated with UV-C LED system development.

Nature of light source, robustness, and reliability Principal design challenge is to understand a completely different light source, at slightly different germicidal wavelengths. Care must be taken to fully characterize its robustness and reliability. A key difference that sets the stage for this paradigm shift in design is the difference in emission profiles between these two lamps (Figure 1). From this diagram alone, it is easy to see the need for a change in UV reactor design geometry. Mercury lamps require a cylindrical design, whereas UV-C LEDs have a point-source emission profile and may use, but do not require, a cylindrical profile. In addition to having a fundamentally different emission profile, the spectral output of UV-C LEDs must also be examined. With the popularity of low-pressure mercury lamps, 254 nm has been thought of as the ideal wavelength, even though the peak germicidal effectiveness falls between 260 and 270 nm, depending on the specific pathogen. UV-C LEDs are quasi-monochromatic (majority of output falls within a 10-nm gap) but can be engineered in a variety of wavelengths within the germicidal range to target specific pathogens or the general peak pathogen UV sensitivity. (e.g., 255, 265, 275 nm, etc.). The designer’s choice of wavelength is usually dependent upon a combination of cost, lifetime, target pathogen and UV intensity output. In addition, given the lack of experience with this new innovative light source, manufacturers must extensively pretest UV-C LEDs to be fully confident in the robustness and emission of these devices over time. A UV-C LED measurement and test facility is critical to yield this important data.

Reactor design Principal design challenge is to increase the overall system efficiency by designing a highly efficient UV reactor. Because of the differences in lamp emission profiles, treatment chambers incorporating LEDs have the potential to be substantially different than those using conventional mercury-based lamps. The designer must find a way to efficiently distribute this new light source within the reaction chamber. Currently, commercially available UV-C LEDs have outputs below 100 mW (for a single device, around 3.5 mm2) compared to a 17-W, low-pressure mercury lamp (around 350 mm x 15 mm). Although single UV-C LEDs are not as powerful as single mercury lamps, their UV-C output and power density (output per unit area) has been increasing exponentially over the last few years. As a result, more effort must be placed on designing a highly efficient UV reactor to compensate and maintain the overall efficiency of the water treatment system. Some designs may be elegant, such as lining a pipe with LEDs (as shown in Figure 2) but overall efficiency must be the driving premise. Perhaps once UV-C LED technology has matured to the levels of visible LED technology and are exceptionally efficient, they can be placed in practically any configuration; but for now, UV-C LED systems can only be effective when built around exceptionally efficient reactor designs. There are only a few UV-C LED treatment systems available today, each with a different reactor design. Efficient reactor designs are not easily discovered. Development teams spend years in designing and validating novel reactor configurations that are more efficient than mercury-lamp-based systems. Advanced modeling tools, such as computational fluid dynamics and finite element analysis, are deployed alongside physical measurement techniques, such as bio-dosimetry and actinometry.

Thermal management Principal design challenge is to manage the heat emitted by UV-C LEDs to ensure long lamp life. One of the key benefits of LED-based systems is that they do not transfer heat into the water (Figure 3). LEDs emit all their light at the front surface of the device and heat at the back surface, whereas mercury lamps emit both light and heat through the same surface. UV-C LEDs emit more heat than mercury-based lamps, which must, in turn, be managed correctly. This can be done through a variety of ways and will aid in the longevity of the system. New skill sets or specialists are needed to effectively accomplish this task. If high temperatures are maintained at the LED junction, the UV output of the LED will diminish and over time the disinfection efficacy of the system will reduce. Implementing thermal heat-reduction techniques and heat monitoring is key to sustaining long-lasting UV LEDs. Depending on usage, the replacement interval of these lamps can be several years compared with annual replacement requirements of conventional mercury lamps.