By Philip Jones and Mitchel Hansen

Water contamination events such as what occurred in Flint, MI naturally elevate concerns about the health effects of contaminated drinking water. It is fair to say that the current pandemic has also brought a whole new meaning to transmissible disease anxiety. It is therefore timely to review some aspects of potable water disinfection, specifically the application of ultraviolet technologies to the disinfection of water.

A brief history of UV disinfection
It is perhaps surprising that UV light has been recognized as an effective water treatment technology for more than 200 years and that ultraviolet germicidal irradiation (UVGI) was first used as a disinfection technique more than 100 years ago. Since then, UV has become firmly established in water treatment plants globally and in recent decades has become increasingly popular for residential (whole-house, POE) applications, where the use of chemical disinfection is both impractical and undesirable. Removing the need to transport, store and handle hazardous chemicals together with the elimination of DBPs has helped to drive the adoption of UV as a primary disinfection method although the lack of a disinfection residual still needs to be considered where a pipework distribution system is involved. The introduction of high-output, low-pressure mercury lamps with a specific spectral output of 253.7 nanometers (nm) wavelength allowed for smaller scale (e.g., residential) applications to become a highly practical proposition.

It could be said that the particular wavelength emitted from low pressure (UV) mercury lamps is a happy accident, given that it is very close to the peak germicidal wavelength for the inactivation of pathogens in the range of 262 – 265 nm. More recently, a very significant technology advance has been the development and introduction of UV-C LEDs, which offer several advantages including the absence of mercury and instant-on (no warm up period required) technology, allowing for intermittent rather than 24/7 power application. In 2012, UV-C LEDs made their first appearance into the commercial water disinfection system market. The initial systems were smaller than conventional mercury vapor systems; however, they typically had lower flowrates and cost several thousands of dollars. Since then, UV-C LEDs have primarily grown in POU applications with systems becoming more compact, making them easily integrated or retro-fitted into existing products. As the design of UV-C LED systems improved, they have finally become competitive in the POE UV disinfection market. The improvements of these systems were most noticeable in their robustness, reactor efficiency and cost of ownership when compared to conventional mercury systems.

Technology comparisons
Mercury vapor lamps in various iterations (medium- and low-pressure) have been at the core of UV disinfection for several decades. Although the use of LEDs in the mass visible light market have become commonplace during the last decade, their use in the disinfection arena was initially restricted to specialized niche applications due to relatively high cost and limited UV output efficiency. In recent years, great strides have been made in refining UV-C LED technology to the point where the cost-benefit analysis has become more favorable, firstly in POU applications (e.g., water dispensing) and, most recently, in POE applications. The early negative aspects of material toxicity, heat management, and cost have been addressed. All materials used are now reduction of hazardous substances (RoHS)-compliant, integrated heat sinks handle thermal management effectively and continuously improving cost versus output is well demonstrated by the graphical representation of Haitz’s Law (see Figure 1). Haitz’s law is the observed and forecasted improvement of LEDs over the years. This law has found that every decade, the cost falls by a factor of 10 and the amount of light generated increases by a factor of 20. This evolution is taking place at the same time that pressure to eliminate the use and subsequent disposal of mercury is increasing.

 

At their core, UV disinfection systems are based on two principles – the efficiency of the UV lamp and the efficiency of the reactor design. Typically, conventional UV lamps are more efficient, producing more UV light than heat when compared to UV-C LED light sources. The opposite is true with conventional mercury lamp reactor design, as the cylindrical design is typically less efficient than modern UV-C LED reactors, which can be custom-designed based on the flexibility of the LEDs. In addition to the cost versus output improvements noted above, UV-C LED reactor and UV output efficiency are both benefitting from continuous improvement at a rapid pace, accelerating the penetration of LED technology into higher-flow applications. A particular advantage offered by UV-C LED technology is the ability to tailor UV wavelengths for specific, targeted applications due to not being constrained by the 254nm output of low-pressure mercury vapor lamps.

While conventional mercury-lamp-based UV disinfection systems will continue to dominate some sectors of the market, more especially high-flow commercial and industrial applications, there are additional factors (such as cost of ownership and long-term maintenance requirements) that will drive the adoption of UV-C LED devices in a growing number of applications. Due to the unique properties of uv-c leds, fouling is much less of a concern as heat discharge is managed at the back of the system using heat sink technology rather than the quartz lamp/sleeve/ water interface, where fouling and consequent reduced UV transmission can occur in water containing hardness minerals. In conventional systems, instant -on LED technology allows for powering the LEDs only when water is flowing, thereby eliminating the heat build-up that results in fouling during stagnant (no flow) conditions often found in systems using permanently powered mercury lamps. Elimination of the familiar hot water shot associated with conventional systems not equipped with a temperature management dump valve is an added benefit. The instant on/off properties of LEDs and consequent ability to be powered only when needed drastically reduces lamp replacement intervals when compared with conventional mercury lamps. Based on average household water demand of 2-3 hours per day, the replacement interval for LEDs is typically five years, compared to annual replacement of mercury lamps. The on demand advantage, together with flow regulated power demand, means that the long-term electrical energy requirement is substantially lower for UV-C LED systems.

LEDS coming online
UV-C LED systems have been part of several case studies, including university pilot studies, National Science Foundation (NSF) grants and recently a Cooperative Research and Development Agreement (CRADA) between the US EPA, Washington University and a major UV manufacturer. These case studies, dating back to 2012, offer a reliability and longevity indicator that is often desired in something as important as potable water treatment. The primary goal of the US EPA CRADA agreement was to test and validate a water treatment system incorporating UV-C LED disinfection technology. The device targeted higher flowrates and more challenging water qualities than addressed with currently available UV-C LED commercial systems. The CRADA showed efficacy of the world’s first commercially available UV-C LED POE treatment system.

Growth of UV LED systems in the market
The increasing demands and need for sustainable solutions to health and quality of life related issues should guarantee an increasingly important place for UV-C LED technology in the realm of improved water treatment methodologies. Continuous improvement in performance and cost reduction is already allowing this to happen and will only increase market penetration in the future.

About the authors
Philip Jones is a qualified chemist and water treatment professional with 25 years of experience in the field. Mitchel Hansen is Marketing Manager for AquiSense Technologies, a world leader in UV-C LED disinfection systems. Jones and Hansen can be reached at [email protected] or (859) 869-4700.

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