Lifetime Versatility of UVC LED Technology for Water Purification Systems

By Sébastien Blumenstein, PhD

In response to the COVID-19 pandemic caused by SARS-CoV-2, UVC LEDs have emerged as a pivotal technology for disinfecting surfaces, as well as creating safer environments in public areas, working spaces and households.¹ The germicidal effect of UVC has been exploited for over a century in various application fields. Today, UV-enabled water disinfection is a standard method for appliances ranging from large-scale water treatment plants to POU water purification systems. With the rapid technological progress of UVC LEDs in the past decades, various types and sizes of UVC LED-based water purification systems² have been proven a viable disinfection response to water-relevant pathogens, such as Pseudomonas, E. coli and Legionella.³

UVC LED technology for water purification today
Mostly found today in POU appliances (see Image 1), water purification systems that utilize UVC LEDs offer a range of advantages compared to traditional UV lamp systems by being more ecological, safer, reliable and cost effective. One outstanding feature of UVC LEDs is the ability to be instantly switched on and off without any drawback on lifetime. Thus, UVC LEDs can be activated on demand via flow sensors or simply by dispensing signals (e.g., from solenoid valves), which means that lifetime is only used when a purification system is activated by the consumer, i.e., when water is dispensed. Compared to traditional systems, in which UV lamps are often continuously activated, there is no need for extra warm-up time or for limiting cycling management to prevent accelerated degradation. This allows for UVC LED technology to offer a clear flexibility enhancement.

The instant on-off switching property translates into a specific water purification product lifetime, which equally depends on the appliance specifics as well as on the user behavior. Different use-cases illustrate the wide range of possible lifetime outcomes by representative system properties (flowrates) and consumption behavior (daily dispensed water volume). The analysis of the following use-case comprises the quantitative evaluation of end-of-service life (EOSL) in years, as well as the total water volume capacity to demonstrate the lifetime versatility of UVC LED-based water purification systems.

Lifetime versatility of UVC LEDs: A use-case
The lifetime definition of integrated UVC LEDs is crucial to the performance and reliability that a water purification system provides.³ Water disinfection system designs typically require a minimum total optical power to meet the UV fluence rate that ensures a specific disinfection performance claim, often specified in terms of a relative reduction rate (percentage or log-reduction value) for a water-relevant pathogen (e.g., Pseudomonas aeruginosa). As UVC LEDs degrade slowly and continuously over the time in which they are on (on-time), system designs should consider the output power of UVC LEDs at end of life as well as the related reliability value that statistically quantifies the level of confidence in ensuring the required optical power over the full product lifetime. Because of this, customers should pay attention to the confidence level that manufacturers attach to the disinfection performance claim of their products over lifetime.

For example, this use-case assumes that for a given UVC LED-based water disinfection system, a certain disinfection performance level is guaranteed for a total UVC LED on-lifetime of 500 hours. For a meaningful product lifetime discussion, two parameters are considered:

• EOSL in units of years (y)
• The treated total volume capacity in liters (L)
  The EOSL describes the effective lifetime in terms of years during which the water disinfection product (e.g., a UVC LED water reactor) is applied to an operative system after which the performance claim expires and when the manufacturer would typically recommend a replacement. The total capacity specifies the consequentially dispensed water volume used during that time. Both measures are evaluated as a function of:
• Flowrate in liters per minute ( L/min)
• Average daily water consumption in liters (L)4
  A simplified model considers that UVC on-time is used whenever water flows at a specific flowrate through the disinfection system during days of use. Additionally, the activation of UVC LEDs in regular intervals is recommended in order to prevent potential microbial growth, specifically prohibiting biofilm formation during periods of water stagnation, i.e., when the appliance is not in use (water maintenance). Mathematically, this model can be expressed by Equation 1.

Applying 500 hours total UVC LED on time, as well as an automated water maintenance cycle of two minutes UVC LED activation every 12 hours, sets the basis for Equation 2 in the quantitative analysis shown in Equation 2.

Figure 1 illustrates EOSL values calculated by Equation 2 for a range of flowrates between 0.5 and three liters per minute, and average daily water consumption between one and 50 liters. These parameter ranges have been chosen to represent various application scenarios such as domestic under-the-sink filtration systems (UF [typically up to three L/min], RO [typically less than one L/min] and others), commercial bottled water coolers (ranges down to smaller one L/m), POU water coolers in office environments (typically 1.5 to 2.5 L/min) as well as addressing professional POU water appliances in public and hospitality spaces (typically greater than two L/min). Within these parameter ranges, the shortest EOSL amounts to 0.8 years for 50 liters consumed per day at a flowrate of 0.5 L/min (Figure 1, bottom right corner) and the longest reaches 19 years for one liter consumed per day at three L/min (Figure 1, top left corner). While these values are calculated extremes, the majority of EOSL values (75 percent) in this evaluation lie between two and 11 years, with a mean of 5.6 years. This points out the impressive breadth of possible effective lifetimes of a UVC LED-based water purification product in the field, especially when compared to typical UV lamp replacement schedules between six and 12 months.

Figure 2 depicts three selected horizontal cross-sections of Figure 1 together with total capacity values. One can clearly recognize the non-linear dependency for EOSL and total capacity on the average daily consumption, as well as their reversed curve progression (Equations 2 and 3). When daily consumption increases, more water gets treated and less UVC LED on-time is used for water maintenance (in relative terms), while the effective product lifetime decreases due to longer UVC LED usage periods per day.

For the parameter range of this use-case, treated water volumes at EOSL are found between 5,000 liters (14 years) and 73,000 liters (2.2 years) with a mean of 36,000 liters. Again, this demonstrates the wide range of possible water purification applications served depending on user behavior and system flowrate. Figure 2 also emphasizes the consequences at different flowrates. As shown, tripling the flowrate results in up to five years longer EOSL, while total capacity can increase by a factor > 2.5. Because a certain water volume is treated faster at higher flowrate, UVC LED activation periods are shorter. Therefore, less UVC LED on-time is consumed per day for a specific daily water consumption.⁵

Additionally, product lifetime is impacted by periods of non-use or days of no water consumption. For example, a UVC disinfection system used for domestic under-the-sink filtration might be used daily, whereas a professional POU system in a hospitality environment could experience one day off during the week, while a water cooler installed in an office space might even go through a full weekend of water stagnation. Noteworthy, this emphasizes the need of an automated water maintenance cycle to prevent microbial growth during such periods of water stagnation.

Covering all three scenarios, Figure 3 illustrates EOSL and total capacity values over daily water consumption at a fixed flowrate of two L/min. When considering a UVC disinfection module attached to a water dispensing unit in an office space, EOSL decreases with fewer closed-office days, which is mainly due to the intuitive fact that average daily consumption takes place during work days.(4)
In contrast, the total water volume treated increases, because less UVC LED on-time is effectively consumed for the automated water maintenance routine over product lifetime. Overall, the effective lifetime of the same UVC LED-based water purification system can vary up to two years depending on the application environment. Knowing the end user’s application environment and identifying daily average consumption helps manufacturers and servicers to optimize replacement schedules and thus, reduce costs significantly.

Lifetime flexibility powers more versatile disinfection solutions
These application examples give quantitative evidence for the versatility of UVC LED-based water purification products provided by a large-scale lifetime flexibility. The intrinsic ability of UVC LEDs to be activated on demand only extends end-of-service-life (EOSL) expectations substantially compared to systems based on traditional UV lamps. In particular, moderate water volume consumption scenarios that apply to domestic POU, for instance, benefit from replacement cycles easily exceeding 10 years.

This analysis gives confidence to manufacturers and servicers of POU water dispensing appliances that switching to a UVC LED solution provides a multiple-year, maintenance-free disinfection feature, adding value to their business through predictable planning for replacement, as well as to their customers, with reduced maintenance expenses. UVC LED technology continues to progress at a rapid pace, which will continuously widen its market penetration into water, food and beverage, hospitality and building spaces.
Today, the lifetime versatility of UVC LED-based disinfection systems already serves a large application range by easily adapting to diverse end-customer usage behaviors. IoT-compatible units with functional indications are available as customizable components for OEMs, as well as autonomous plug-and-play modules for retrofit and optional accessory to serve a broad range of water purification appliances.²

References

1. F. Javier García de Abajo, Rufino Javier Hernández, Ido Kaminer, Andreas Meyerhans, Joan Rosell-Llompart and Tilman Sanchez-Elsner, “Back to Normal: An Old Physics Route to Reduce SARS-CoV-2 Transmission in Indoor Spaces,” ACS Nano ASAP, 2020. DOI: 10.1021/acsnano.0c04596
2. GWI market map. “Reduced scalability obstacles spell bright future for UV-LED systems,” Chief Technology Officer, Vol 21, Issue 3 (March 2020) https://www.globalwaterintel.com/global-water-intelligence-magazine/21/3/market-map/reduced-scalability-obstacles-spell-bright-future-for-uv-led-systems
3. A.W. Miller, R.M. Mariita, Disinfection Performance of UVC LEDs against Pseudomonas aeruginosa, Escherichia coli and RNA Bacteriophage Qβ at Different Conditions, Conference Proceeding, UV Technology Advances 1, IUVA Americas Conference, Orlando, FL (2020) https://www.iuva.org/2020-AC-Conference-Tuesday
4. Here, the definition of average consumption per day refers to days of usage only. Days of non-usage are respected by the evaluation with an extra term in Equation 1.
5. Please note, that the disinfection performance of a water purification system will typically differ depending on flowrate, which is not discussed in this article.

About the author
As Field Application Engineer, EMEA, Dr. Sébastien Blumenstein has technical responsibility for Crystal IS UVC LED projects, including water disinfection applications. He supports in integrating off-the-shelf Crystal IS water disinfection products as well as customized solutions to meet specific customer needs. Prior to Crystal IS, Blumenstein worked at Excelitas, a photonics technology leader delivering OEM optical solutions. He holds a PhD in physics from the Technical University of Darmstadt (Germany). Blumenstein can be reached at [email protected].

About the company
Crystal IS, an Asahi Kasei company, is a manufacturer of high-performance UVC LEDs. Crystal IS products are suitable for monitoring, disinfection and sterilization in a variety of applications, including commercial and consumer POU water purification, and infection control in air and on surfaces in healthcare industries. For more information, visit cisuvc.com.