Shining a Light on Water Disinfection: LEDs aren’t just a Television Upgrade
Have you ever wondered how your drinking water gets from its source to your home? It varies a bit depending on where you live, but public drinking water generally goes through a series of separation and filtration steps to remove particulates (solids), followed by a disinfection step to kill/deactivate remaining pathogens (disease-causing organisms), such as bacteria and viruses. The water is then ready to be distributed to homes and businesses. A diagram of this process is shown in Figure 1.
The Current State of Water Disinfection
Since it is primarily responsible for removing the pathogens that cause waterborne diseases like cholera, typhoid, and giardia, the disinfection step is pretty important when it comes to making water safe to drink. Two of the most common methods for disinfection in US water treatment facilities are chemical and UV disinfection, which are both very effective at removing most waterborne pathogens. However, chemicals commonly used for disinfection (such as chlorine) can form dangerous byproducts when they interact with certain compounds—and these disinfection byproducts can stay in drinking water! Chemical disinfection also requires storage of the original chemicals onsite, which comes with other potential hazards. UV disinfection is accomplished by exposing water to light between 250 and 280 nm in wavelength, which is lower wavelengths than humans can see. This wavelength range is specifically designated “UV-C” and deactivates pathogen DNA, making them harmless. UV irradiation of water only requires electricity and doesn’t form byproducts. However, most UV disinfection systems use mercury lamps, which are not terribly energy efficient and are problematic and expensive if they break.
Despite the relative success of these technologies, there are still estimated to be 4-32 million cases of gastrointestinal illness due to public drinking water issues each year in the US.1 This is often caused by inconsistent disinfection of water (due to incorrect chemical concentrations, improper mixing, and incomplete irradiation, among others), so there is definitely room for improvement. One of the emerging alternative methods is to use of LEDs as sources of UV radiation, and researchers at the Arizona State University are seeking to make this method more effective.
LEDs: Advantages and Research Gaps
LEDs, or light emitting diodes, are basically tiny lightbulbs that are used in a wide variety of technologies, ranging from laptops, to household lightbulbs, traffic lights, and TVs. Usually LEDs are designed to emit visible light that humans can see, but they can also be made to emit UV light. They generate light from the movement of electrons, which makes them more efficient, cheaper, safer, and longer-lasting than the mercury-based lamps that currently dominate the UV-treatment market. What’s the catch? Because LEDs are so small, they have only a small surface area that emits radiation. If water doesn’t get exposed to enough radiation, it doesn’t get disinfected, so this is a big issue that needs to be addressed before LEDs can be used more widely. In many cases this is overcome by combining large groups of individual LEDs together, but these arrangements have faults in terms of functionality and efficiency.
Giving LEDs a Boost using Optical Fibers
Recently, a team of researchers at Arizona State University led by Mariana Lanzarini-Lopes, sought to enhance the radiation from LEDs using side-emitting optical fibers (SEOFs). These are a special type of optical fiber that lets light out the sides, as well as the end of the fiber… somewhat like a long, flexible glow stick. Some examples of traditional end-emitting optical fibers are shown in Figure 2A and B. They carry light from a source to the end of the fiber, like a hose does for water. SEOFs, in this analogy, would be if you poked holes in the hose so that water was let out all along the hose’s path. Using SEOFs to deliver light from the LED would not only enhance and redistribute the light given off, but their flexibility would enable UV radiation to get to the corners and hard-to-reach places inherent to unique water treatment tank geometries.
One of the challenges with these SEOFs is maximizing the light emitted (given off) over the length of the fiber. This means that more light will leak from the fiber, into its surroundings… enabling more water to be disinfected! Lanzarini-Lopes and collaborators approached this challenge from two angles: (1) enhancing scattering of light, and (2) minimizing light absorption by the protective coating. Their second objective is especially tricky when delivering UV light, because many of the materials traditionally used to protect the fibers, or cladding, absorb UV light. The researchers optimized the SEOF design as shown in Figure 2D, by adding silica nanoparticles to scatter light, treating the fiber with a special liquid, and covering it with a protective cladding that does not block UV-C light. They tested their system by measuring its ability to kill E. coli (one of the major culprits of gastrointestinal illness), and found it to be successful! There is still a lot of research that needs to be done to scale up this technology to be used in water treatment plants, but these researchers are off to a good start!
LEDs are useful at home too!
The way an LED bulb turns electricity directly into light makes it much more energy efficient (up to 90%) than a traditional incandescent lightbulb, which needs to heat its filament to an extremely high temperature before it can produce light. Its lack of a filament also means that an LED bulb doesn’t give off as much heat (wasted energy) and doesn’t burn out in the same way. On average, LED bulbs last 25,000 hours—that’s about 20 times longer than your average incandescent bulb, and 3 times longer than compact fluorescent (CFL bulbs)! If you haven’t started replacing your household lightbulbs with LEDs, consider making the investment.
Source Article: M. Lanzarini-Lopes, B. Cruz, S. Garcia-Segura, A. Alum, M. Abbaszadegan, and P. Westerhoff, Nanoparticle and Transparent Polymer Coatings Enable UV-C Side-Emission Optical Fibers for Inactivation of Escherichia coli in Water, Env. Sci. Technol. DOI: 10.1021/acs.est.9b01958, 2019. https://pubs.acs.org/doi/abs/10.1021/acs.est.9b01958
Cover Photo Source: https://www.freepik.com/free-photo/glass-water-macro-shot_3533843.htm