Ultraviolet germicidal irradiation, or UV-disinfection of drinking water is dated back to 1910, but has been used on a bigger scale since the mid 1950’s.
That means that for more than 50 years, UV-lamps have been in the service of providing us with safer drinking water, with no disruptive innovations until the UVC-LED (light emitting diode) was first introduced in 2002.
UVC-LED, like other LED light sources has the benefit of instantly reaching full output power when switched on, it also can manage an unlimited number of On/Off cycles. Due to this UVC-LED products are optimal for applications where you need intermittent functionality to match with the flow of water. For example, point of use (PoU) applications that could be ranging between water dispensers, drinking fountains, showers and drinking water taps in your kitchen, mobile home, or boat.
UV-lamps are mostly referring to low or medium pressure mercury vapor lamps. Which as you can tell by the name, contain mercury. Due to this, mercury vapor lamps have for example been banned in the EU since 2015 for lighting applications. But since there has been no comparable replacement product there is an exception from the ban for disinfection use.
So, to get to the details on where UVC-LED differ from UV-lamps, and why there still is an exception from the ban, we start with the so called “wall-plug efficiency”, meaning, how much output of UV- light do you get compared to the power you feed your UV light source with?
- Low pressure mercury lamps are usually in the range of 30-40% efficiency
- Medium pressure mercury lamps are considered <10% efficiency
- UVC-LED is dependent due to different suppliers being in different stages of innovation and development, some focusing more on durability, while others are instead focusing on higher efficiency, but to date (December 2021) it is between 3-6% efficiency. Although some manufacturers are expecting to reach 8-10% efficiency already during 2022.
Now that seems like a big difference in efficiency you might say. Yes, it is, but that leads us to the next difference between the different light sources, wavelength!
Wavelength is referring to where in the UV-spectrum the most light is emitted. Ultraviolet light is ranging from 100 nm (nanometers) to 400 nm, and the UVC or short-wave UV light is between 100 nm to 280 nm. The wavelength considered to be most effective on average in damaging RNA and DNA and thus inactivating bacteria, viruses, and protozoa by destroying their ability to reproduce, rendering them harmless, even if they are not killed, is the 260-265 nm range. However, since this is an average, you might correctly have drawn the conclusion that some organisms are more sensitive to a shorter wavelength, while others dread a longer wavelength.
- Low pressure mercury lamps generate 85-90% of its light at 253,7 nm. But still has a wider range in lower intensity, down to 185 nm, and can produce ozone if these shorter wavelengths are not filtered by the casing of the lamp. Meaning that it is almost monochromatic, but not in the “optimal” range, however due to high intensity it still has very high efficacy.
- Medium pressure mercury lamps have a wavelength range from 200-600nm. This is a very wide range, and therefore they can manage to have higher efficacy for organisms that is outliers in the wavelength sensitivity.
- LEDs are monochromatic and target a specific wavelength with a tolerance of ± 5 nm. The most common commercially available UVC-LED today has a wavelength of 275 nm, but LED in the wavelength of 265 nm and 255 nm are becoming available on the market. The 275 nm LEDs reach a higher light power per $, making it more effective than the 265 nm at the same cost.
As mentioned before, the LED are instant On/Off, reaching full capacity the moment they are switched on, much unlike the UV-lamp, that needs a warm-up period before it starts to emit UV-light and reaches its full efficiency, therefore UV-lamps are usually left on for the main period of the day when they are being used, this will inevitably lead to heating of the water in the periods when there is now flow of water through the system.
With the difference in targeted wavelengths, and wall-plug efficiency there are some benefits and disadvantages for both technologies. With a wider range of wavelength, you might target a wider range of organisms, but with a monochromatic solution like the LED, it is possible to target specific types of organisms, or combine different LEDs to gain a wider range of wavelength. The higher efficiency of the low-pressure mercury lamps enables it to be used in applications with very high flow rates, like municipal water treatment plants.
Another difference worth mentioning is the lifespan, UV-lamps can have a longer life span in terms of active hours, although UVC-LED has reached a similar level in the past years. But since the UVC-LED can be run intermittently, the installation lifespan for the LED is much longer than the UV-lamp. An additional benefit from this possibility to have intermittent operation is that it is possible to reach a significantly lower power consumption.
In summary, UVC-LED still has some ground to cover before it can compete with UV-lamps in high flow rate applications in terms of efficiency. But the flexibility, instant reaction, small size, no mercury, and monochromatic characteristics makes it better suited for small, low to medium flow applications, and it is just a matter of time before UVC-LED outperform all UV-lamps, as visible light LED has changed the lighting industry.
