By Brooke K. Mayer, PhD, PE
Ultraviolet (UV) light refers to the portion of the electromagnetic spectrum between 100 and 400 nm in wavelength (Figure 1). The UV-C portion is also called the deep UV or germicidal range as these wavelengths damage genetic material (DNA and RNA), thereby inactivating microorganisms. The peak germicidal wavelength, where DNA absorbs the most light is approximately 260 nm. Greater light absorption corresponds to more damage, as the UV photons change the nucleic acid structure, rendering the microorganisms incapable of replication (i.e., non-infectious).
UV irradiation was first shown to inactivate microorganisms in the latter part of the 19th century, ushering in a period of exploration of the impact of wavelength, lamp and ballast development, etc. for water/wastewater treatment applications. The first UV installation was in the early 1900s (Figure 2). However, advances in the production and use of chlorine gas in 1910 effectively halted the use of UV in North America (although not in Europe). Accordingly, chlorine paved the path toward one of the great achievements of the 20th century, water/wastewater disinfection, which dramatically improved human health protection.
In the United States, interest in UV as a water/wastewater treatment technology renewed in the 1970s in response to increased awareness of the deleterious environmental and health impacts of residual chlorine, as well as concern over the transport of hazardous chemicals. Of particular concern for wastewater systems were whole effluent toxicity, aquatic food chains, and overall ecological health of receiving waterbodies, which required chlorine-based wastewater systems to add additional chemical dechlorination processes. On the drinking water side, recognition of the formation of potentially carcinogenic disinfection byproducts when oxidizing disinfectants such as chlorine react with naturally present organic matter dramatically impacted disinfection perspectives. These concerns led to increased investigation and implementation of alternative disinfection methods including chloramines, ozone, chlorine dioxide, and UV.
Thus, the boost in UV development came from the regulatory realm. In particular, interest in UV was heighted by its unique advantages, including its physical mechanism of disinfection (as opposed to chemical oxidation), the absence of chemical additions, freedom from handling and storing toxic chemicals, and avoidance of chemical residual.Additional benefits include effective pathogen inactivation and simplicity of retrofits due to small footprints and low headloss. In spite of these benefits, widespread use and acceptance of wastewater UV technology took several decades,2 with an even greater lag in drinking water applications.
In the early stages of development, UV lamp technology was predominantly low pressure (LP), which translated into the need for a large number of lamps to treat large flows, often making UV an impractical choice from an operation and maintenance perspective. Moreover, operational issues such as poor electrical components and leaky connections deterred usage in the early years. Improvements in design addressed many of these issues. For example, improved lamp design enabled the installation of fewer lamps, which increased cost effectiveness for higher design flows, reduced operation and maintenance concerns, and allowed for automatic wiper systems to address fouling issues. Malley (2010) noted that in almost all cases of wastewater facility energy audits, UV energy use accounted for a very small, almost insignificant, portion of the facilities’ overall power usage.
Additionally, alternatives to traditional LP UV configurations continue to evolve, with some at technological maturity, and others subject to ongoing research seeking improvements to increase their market viability. Alternatives include low pressure high-output (LPHO) lamps, medium pressure (MP) lamps, light emitting diodes (LEDs), and excimer lamps, among others. Here, pressure refers to the vapor pressure in the mercury arc lamps, which dictates the wavelength of light emissions. Low pressure UV emits nearly monochromatic light at 254.7 nm, near the peak germicidal wavelength. Medium pressure lamps have polychromatic emission spectrums from approximately 200 to 320 nm. In general, fewer MP lamps are needed to disinfect higher flows at the same UV dose, but higher power input is required. Project engineer Richard Protasowicki suggests a basic rule of thumb of using LP or LPHO for systems for flows up to 2 to 3 mgd, whereas MP may be more suitable for higher flowrates. However, selection should be on a case-by-case basis accounting for site-specific details.
Non-mercury-based LEDs emit light over a relatively narrow distribution with peaks at variable wavelengths depending on the LED composition. They are a rapidly developing emerging technology with most testing at the laboratory level to-date. LEDs have a range of potential benefits, including high-intensity near-monochromatic outputs at user-defined wavelengths, longer life spans, and lower energy inputs. However, UV LED efficiency is currently low, with less efficient conversion of energy to germicidal output compared to LP and MP technologies.
Excimer (exciplex) lamps are another emerging mercury-free UV alternative. They rely on the excitation of rare gas or rare gas-halogen dimers, with a wide range of emitted light wavelengths depending on the lamp’s dimer composition (e.g., NeF* emits at 108 nm, whereas XeF* emits at 351 nm). Recent disinfection and advanced oxidation studies have evaluated the use of KrCl* excimers (222 nm emission) to treat water, air, and surfaces. One advantage of this system is that far-UV-C emissions at 222 nm are not very harmful to human tissue, which reduces the hazards typically associated with UV operations. Additionally, excimer lamps are reportedly nearly two times more efficient than LP lamps.
The range of UV system configurations can be adapted to meet different end users’ needs, which vary across wastewater, drinking water, and water reuse sectors. Global projections for the past decade suggested that UV installations would increase across all three market sectors. However, wastewater applications were the initial focus and remain the most promising larger-scale long-term market for UV, with up to 15,000 new facilities projected between 2010 and 2020.
In the drinking water realm, UV acceptance was historically slower (at least in North America), but the dual risks of chemical (DBPs) and microbial (chlorine-resistant Cryptosporidium, in particular) contaminants repositioned UV to ‘stun the drinking water industry’ nearly 25 years ago. Notably, improved microbial assays in the late 1990s reversed the previous perception that UV was not effective against Cryptosporidium and Giardia, leading to a rapid uptick in UV research and adoption in response to the US EPA’s Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). Up to 7,000 new facilities were projected from 2010 – 2020. The latest AWWA Water Utility Disinfection Survey (2017 data)[10 showed that of the 375 systems, 70% of respondents used chlorine, making it by far the most commonly used disinfectant at US drinking water treatment systems. In decreasing order of use prevalence, chlorine was followed by chloramines, chlorine dioxide, ozone, and UV (Figure 3). For the first time in the history of the Survey, UV usage was on the same magnitude as ozone and chlorine dioxide, largely due to the LT2ESWTR coupled with advances in UV disinfection technologies.
Safety, risk management, DBPs, maintenance of storage and feed systems, and maintaining sufficient residual are issues leading some utilities to consider chlorine alternatives. Among chloramines, chlorine dioxide, ozone, and UV, UV is by far the most popular chlorine alternative for systems considering switching disinfectants, illustrated by the dramatic increasing trend in UV technology adoption in the past 10 years (Figure 4). Moreover, future growth of UV adoption appears to have greater potential than the other alternatives.
Beyond the LT2ESWTR, California’s Title 22 Water Reuse Standards boosted UV applications for the explicit purposed of water reuse as Snider et al. (1991) demonstrated that UV could meet Title 22 disinfection requirements. Beyond conventional disinfection, water reuse often features advanced oxidation processes (AOPs), including UV-based AOPs such as UV/H2O2. Increases in water reuse practices and guidance led to projections of up to 100 new UV-based AOP facilities (primarily in the water reuse sector) between 2010 and 2020.
Accordingly, with continued technological development, regulations and guidance, and growing market sectors, the future of UV looks bright.
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- Protasowicki R. Ultraviolet Disinfection of Drinking Water: A Growing Trend. Water Wastes Dig. Published online 2021. https://www.wwdmag.com/disinfection/article/10917786/ultraviolet-disinfection-of-drinking-water-a-growing-trend
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- Payne EM, Liu B, Mullen L, Linden KG. UV 222 nm Emission from KrCl* Excimer Greatly Improves UV Advanced Oxidation Performance in Water Treatment. Environ Sci Technol Lett. Published online 2022. doi:10.1021/acs.estlett.2c00472
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- Malley JP. Where We’re Headed 20 Years After UV Technology Stunned the Drinking Water Industry. J Am Water Works Assoc. 2018;110(12):58-60. doi:10.1002/awwa.1199
- AWWA Disinfection Committee. Emerging Trends in Disinfection: Lessons From AWWA’s Disinfection Survey. J AWWA. 2021;113(1):20-28. doi:10.1002/awwa.1648
- Snider KE, Darby JL, Tchobanoglous G. Evaluation of ultraviolet disinfection for wastewater reuse applications in California. Published online 1991.
About the author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and Doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.