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UV LED 101: Fundamentals and Applications of UV LEDs in Water TreatmentRan Yin and Chii Shang, Department of Civil and Environmental Engineering, Hong Kong University of Science and TechnologyContact: +852-23587885 or cechii@ust.hk (Chii Shang)

IntroductionUltraviolet (UV) light irradiation has been recommended by the USEPA as an alternative to chemical disinfectants for water disinfection because of several advantages, including effective inactivation of chlorine-resistant pathogens, negli-gible formation of disinfection byproducts and ease of being retrofitted into existing treatment units (Pirnie et al. 2006). UV irradiation also has been used for direct photo-degrada-tion (direct photolysis) of emerging micropollutants (Kim and Tanaka 2009, Stefan and Bolton 2002). It also is used to activate oxidant precursors, including hydrogen peroxide (H2O2), persulfate (S2O8

2-)/peroxymonosulfate (HSO5-), and

chlorine (HOCl/OCl-), to produce reactive radical species for the degradation (indirect photolysis) of emerging micro-pollutants (Cater et al. 2000, Patton et al. 2016). The global market for UV disinfection installations/devices is esti-mated to reach 2.8 billion USD by 2020 (Allied Analytics LLP, 2014). Meanwhile, more and more UV-based advanced oxidation processes (e.g., UV/H2O2 and UV/chlorine) have been incorporated into the treatment trains in advanced drinking water production facilities and for advanced waste-water treatment targeting (in)direct potable water reuse.

Mercury-vapor UV lamps that currently dominate the market have some intrinsic drawbacks. They are fabricated with fragile quartz materials and contain mercury, which is toxic. Mercury-vapor UV lamps also require a lot of energy to operate and waste a lot of the input electric energy as heat (Würtele et al. 2011). These challenges always exist in real-world applications from large-scale water treatment facilities to small-scale portable devices. With rapid developments in semiconductor technology, UV light emitting diodes (UV LEDs) have emerged in recent years as new UV sources in the industry for curing, medical, security and other purposes. The special manner of light genera-tion gives UV LEDs some unique features, including compact-ness, the elimination of any risks of mercury-leaching and the ability to be switched on or off instantly. In addition, UV LEDs are robust and have relatively longer lifespans (achieved for UV-A/UV-B LEDs, predictable for UV-C LEDs), and have some other advantages over mercury-vapor UV lamps (Chen et al. 2017). The versatility of being able to provide UV at any specific wavelength widens the applicability of UV LEDs in water treatment. At the present stage, low wall-plug efficiency

and output power, and high costs are the major limitations of UV LEDs (especially UV-C LEDs). Nonetheless, the development of UV LEDs between 2007 and 2017 has followed Haitz’s Law that forecasts an increase in output per bulb by a factor of 20 and a decrease in cost by a factor of 10 per decade (Pimputkar et al. 2009). The UV-C LED market is expected to grow strongly from 7 million USD in 2015 to 610 million USD by 2021 (Yole Development, 2016).

The rapid development of UV LED technologies and cost reduction in the manufacture of UV LEDs call for engineers and researchers in water treatment areas to consider UV LEDs as UV sources in the foreseeable future. This article high-lights some fundamentals for any engineers and researchers interested in and preparing to use UV LED technologies. The article starts with a short review of the construction/fabrica-tion methods of UV LEDs, the light generation mechanisms of UV LEDs, UV LED array and reactor design, and some unique characteristics of UV LEDs that are related to water treatment. The applications of UV LEDs for water disinfection and in advanced oxidation processes are also briefly reviewed, followed by a discussion on the challenges and needs in a future where usages of UV LEDs are more prominent.

The basicsLight generation mechanismsUV LED is a solid-state semiconductor, which has a p-n junc-tion (also called the active region). The p-n junction is formed when a p-type semiconductor is brought into contact with an n-type semiconductor. Due to the presence of a concentration gradient, the holes on the p-side and the electrons on the n-side diffuse toward the p-n junction, resulting in negative charges on the p-side and positive charges on the n-side (Nakamura et al. 1991). The electrons in the current flow combine with holes on the p-side and excess energy is released in the form of light and heat (Chen et al. 2017). The difference in energy levels between the conduction band and the valence band, the so-called “bandgap,” decides the amount of energy released, as well as the wavelength of the light being emitted.

UV LED chip fabrication and packagingThe active region of a UV LED is contained in a small semi-conductor chip. The chip is typically fabricated by growing

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semiconductors layer by layer (typically n-layer then p-layer) on top of a sapphire substrate (Figure 1). The layers are usually deposited through the metalorganic vapor phase epitaxy to make an epitaxial wafer, which is then cut into chips. Thermal management is very critical for LED chips, as heat is gener-ated during electron-hole recombination and the accumulated heat decreases the output power, shifts the emission spectra and shortens the UV LED’s lifespan. Using a flip chip design and chip surface texturing/shaping are common methods to improve heat dissipation in UV LED chips, and heat sinks are often necessary to be incorporated into the UV LED pack-ages/devices. A typical UV LED package includes electrical leads, an optical window/lens, a heat sink and some acces-sories (Morgan et al. 2006). UV LEDs are most commonly packaged in either a transistor outline design, a surface mount design or chip-on-board design (Chapman, 2013). Due to its compactness, effective heat dissipation and capability to have many chips placed on one board, COB is expected to become the most popular packaging method of UV LEDs when light emission from a single UV LED becomes insufficient.

Unique properties of UV LEDs Table 1 compares the major differences between UV-C LEDs and conventional UV-mercury lamps. Despite the several advantages of UV-C LEDs over conventional UV lamps listed, lamp life is always a major concern for UV LED users. Lamp life is affected by many factors including chip quality, packaging style and materials, heat dissipation ability, driving voltage, as well as application conditions. UV LED users should follow the data sheet provided by the manufacturers and operate the UV LEDs at a voltage or current lower than their maximum allowable values. This helps to protect the device and extend their lifespans. Other characteristics, such as moisture resis- u

Figure 1. The structural diagram of a typical UV LED chip. Figure courtesy of Bioraytron.

Table 1. Comparison of properties between UV-C LEDs and conventional UV

tance of the UV LEDs, also should be noted, and this applies especially to engineers in water treatment practices.

Attribute UV-C LED Conventional UV lamp

Mercury content None 20-200 mg

Lifetime (hours) 10,000 5,000-15,000

On/off cycles Unlimited Maximum 4 per day

Warm-up time Instantaneously Up to 15 minutes

Operating surface temperature

Same as process water 100-600°C

Durability Rugged semiconductor Fragile glass tube

Power supply 6-30 V DC 110-240 V AC

In addition, UV LEDs have some special electrical and optical properties, which should be noted by engineers and researchers for the better selection and appropriate operation of UV LEDs. As shown in Figure 2a, the UV LED spectra are usually reported by the manufacturer with emphasis on the emission wavelength at the highest intensity, along with the width of the emission peak (FWHM) (Pimputkar et al. 2009). Similar to other semi-conductors, UV LEDs have a special current-voltage relation-ship, as shown in Figure 2b, which stipulates that UV LEDs need a constant current but not a constant voltage to work, as a slight increase in voltage may result in a drastic increase in current over its allowable maximum.

The output power is usually linearly correlated with the forward current (Figure 2c). However, more heat will be generated with increasing forward current and the chip temperature will increase. The increase in temperature will result in a decrease in forward voltage (Figure 2d), as well as the radiant flux (Figure 2e). Excessive heat may also lead to a redshift in the emitted light (Figure 2f). UV LED chips and packing materials signifi-cantly affect their radiation pattern. Comprehensive assessment of the radiation patterns is requisite for the successful design of a UV LED system in a particular water treatment application. The aforementioned unique characteristics require a specific

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measurement method for the output of the UV LEDs, and proto-cols have been recently proposed by Kheyrandish et al. (2017) and Kheyrandish et al. (2018) to accurately measure the spectra and radiation profiles of UV LEDs. UV LED array and reactor designA single UV LED often does not provide enough light intensity

for water treatment on an industrial-scale. It is thus necessary to arrange multiple UV LEDs in one/several arrays to meet the intensity demand. Very few studies have focused on the UV LED array design for water treatment purposes. Further investigations are required to understand and optimize the complicated relationships among the radiation pattern from a single UV LED, the light distribution from a UV LED array

Figure 2. Typical optical and electrical properties of a UV LED. (a) UV LED emission spectrum, (b) current-voltage curve, (c) radiant flux-current curve, (d) forward voltage-temperature relationship, (e) radiant flux-temperature relationship, (f) redshift in peak wavelength with increasing driving current. (Figures 2(a), (b), (c) courtesy of Bioraytron, and Figures 2(d), (e), (f) courtesy of AquiSense Technologies)

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and the UV dose delivered to the target microorganisms, oxidant precursors or pollutants. Nonetheless, the special characteristics and shapes of UV LEDs offer engineers new ways to think about reactor design. In addition, little informa-tion can be found on the difference in fouling and in poten-tial new cleaning methods between conventional UV lamps and UV LED-based reactors in the literature, which need to be considered in the design and operation of any UV LED module for water treatment in the water engineering field.

Application for water disinfectionAs emerging UV light sources, UV LEDs are being studied for their application in microbial inactivation in water. The efficiency of UV LEDs in microbial inactivation has been found to be highly dependent on wavelength and the micro-organism species. UV-A LEDs (315-400 nm) are generally ineffective (Song et al., 2016). The required UV dose to achieve one log inactivation of Escherichia coli (E. coli) at 365 nm has been reported to be 13,846 mJ/cm2 or higher, which is four orders of magnitude higher than the typical UV dose for 4-log E. coli inactivation required by LP UV lamps at 254 nm (8 mJ/cm2).

UV-B LEDs (280-315 nm) are more efficient than UV-A LEDs, but are still not efficient enough for microbial inacti-vation (Chevremont et al. 2013). The required UV dose has been reported to be 94.8 mJ/cm2 for one log inactivation of E. coli at 310 nm, which is significantly lower than that required by UV-A LEDs but still far higher than the values required by UV-C LEDs (1.0-30.5 mJ/cm2) (Oguma et al. 2013). The inactivation efficiency is also significantly affected by the microorganism species. Moreover, the wavelength-depen-dency on the inactivation efficiency was found to vary with microorganism species. For MS2, the inactivation efficiency was found to decrease with increasing wavelength (e.g., from 255 nm to 275 nm) (Beck et al. 2015). However, UV LEDs at 275 nm was found to be more effective than 255 nm against E. coli and T7.

There is still a debate on whether there is a synergism in microbial inactivation when UV LEDs at two/multiple wavelengths are used simultaneously for water disinfection. Chevremont et al. (2012) have reported that the combined use of 280 and 365 nm UV LEDs provided two times higher log-inactivation than the sum of each individually. Nakahashi et al. (2014) have also reported such synergism when 254 and 365 nm UV LEDs are simultaneously used for Vibrio para-haemolyticus inactivation. However, Oguma et al. (2013) did not observe any synergistic effect in E. coli inactivation under the simultaneous irradiation from 265, 285 and 310 nm UV LEDs. Beck et al. (2017) also reported no dual wave- u

length synergism in inactivation of bacteria and virus or for the damage of DNA/RNA by using UV LEDs. These incon-sistent findings suggest the need for more research/trials with better control of the experimental conditions to demonstrate the benefit of the combination of UV LEDs at different wave-lengths to improve the overall disinfection efficiency.

Application in advanced oxidation processesUsing UV LEDs to drive advanced oxidation processes (AOPs) to activate photocatalysts (e.g., TiO2) and oxidant precursors (e.g., H2O2, S2O8

2-/HSO5-, and HOCl/OCl-) and

generate reactive radical species to degrade pollutants have also been investigated. It is recommend for readers to refer to a recent review paper, which summarizes the detailed advances in the application of UV LEDs in AOPs available in the related literatures (Matafonova and Batoev 2018). UV LEDs are mostly studied as an alternative to conventional UV lamps in heterogeneous AOP systems, such as the TiO2-based photocatalytic AOPs. However, most of the studies have been conducted in lab-scale batch slurry reactors and there is still a long way to go for the implementation of the UV LED/TiO2 photocatalytic AOPs in real-world water treat-ment applications.

For homogeneous AOP systems, some studies used UV-A LEDs to enhance the efficiency of the photo-Fenton and photo-Fenton-like AOPs. Most of these processes have performed best under acidic conditions (e.g., pH ~ 3), which is why their applications in real water treatment are limited. A few studies have attempted to combine UV LEDs (e.g., 260, 285 and 300 nm) with S2O8

2-/HSO5- to generate sulfate radi-

cals and hydroxyl radicals for emerging pollutant degrada-tion (Oh et al. 2016) but in pure water only. UV-B and UV-C LEDs also have been tested in UV/H2O2 AOPs to degrade pollutants. The pollutants are degraded faster when using UV LEDs at lower wavelengths, which is due to the higher light absorbance of H2O2 at lower wavelengths. This also makes the UV-C LED/H2O2 AOPs less attractive in real-world appli-cations, due to the high cost and low wall-plug efficiency of the UV-C LEDs at this stage. Wang et al. (2017) investigated the activation of chlorine by UV LEDs at 280 and 310 nm to degrade carbamazepine. The UV/chlorine process was found to be more efficient than the UV/H2O2 process at the same wavelength and oxidant dosage. Little discussions have been made on the relationships between the UV LED emission spectra and oxidant absorp-tion spectra (Figure 3), and the relationships between UV LED wavelengths and system efficiency (Yin et al., 2018).

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Comparisons between UV LEDs of different wavelengths are not straightforward because the UV doses of the UV LEDs are different and need to be normalized. We recently exam-ined the wavelength-dependency on chlorine photolysis and subsequent radical formation using UV LEDs at 255, 265, 280 and 300 nm (Yin et al., 2018). The chlorine photolysis and subsequent formation of hydroxyl radicals (HO•) and reactive chlorine species (RCS) decreased with increasing wavelength at pH 6 (i.e., 255 nm was the best), and none-theless increased with increasing wavelength at pH 7 and 8 (i.e., 300 nm was the best). The wavelength dependency on chlorine photolysis was larger at alkaline pH than at acidic pH, and the pH dependency was larger at longer wavelength.

Figure 3. Absorption spectra of several oxidant precursors and emission spectra of UV LEDs

Conclusions and recommendations UV LEDs are promising UV light sources with several advantages over conventional UV lamps, even though a full takeover has not happened yet. Due to their compact nature, UV LEDs are very suitable for small-scale disinfection devices for point-of-use applications. Several small-scale UV LED (especially UV-C LED) based disinfection devices are available on the market now (e.g., PearlAqua platforms from AquiSense Technologies, portable water bottle UV purifier from Camelbak, Ellie portable baby bottle sterilizer and multi-purpose sanitization device from Rayvio, etc.) and more are anticipated to come out in the coming years.

In addition to the applications in drinking water and reused water treatment, UV LEDs also have been applied in the

fields of food and beverage (e.g., RayVio XR Series), curing (e.g., FirePower UV LED curing lamp from Phoseon) and medical (e.g., wearable UV LED medical devices, such as stethoscope). The value of the global market for point-of-use UV-C LEDs will increase from 28 million USD in 2016 to 257 million USD in 2021, according to the 2016-2021 UV LED Application Market Report (LEDinside, 2016). Devel-oping UV LED-based disinfection and/or advanced oxida-tion technologies for water treatment requires the selection of UV LEDs with appropriate emission wavelengths, with due consideration for the lifespan and cost of the UV LED units and their energy consumptions as well. More efforts should be put on system design, such as the design of UV LED arrays and

reactors, selecting wavelengths and combina-tions of wavelengths, and hybrid disinfection/oxidation technologies, etc., so that multiple tasks can be accomplished more efficiently with fewer UV LED bulbs and/or devices.

AcknowledgementsThis study was supported by the Hong Kong Research Grants Council (grant number 16202217). n

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