Development of an excimer UV light source system for water treatment

Transcription

1 Development of an excimer UV light source system for water treatment Zoran Falkenstein Ushio America, Inc. May 2001 Zoran Falkenstein

2 Development of an excimer UV light source system for water treatment Zoran Falkenstein Ushio America, Inc. May 2001 This document summarizes the development of a demonstration-stage excimer UV system for the treatment of conductive fluids. A brief introduction to current UV water treatment systems, the motivation for developing excimer UV water treatment systems, as well as their rationale in contrast to traditional UV systems will be given. After the design of the system components is described, the performance characterization of the combined excimer UV system is outlined. As an example, results for the application of a KrCl* excimer UV system are discussed for bromate removal in bottled spring water. 1. INTRODUCTION UV irradiation of water has been established as a mature alternative to chlorination for disinfection of drinking water, and is also readily applied for the degradation of (organic) compounds in water by advanced oxidation technologies (AOTs). a-c While disinfection of water is primarily achieved by standalone UV irradiation, advanced oxidation systems usually utilize a combination of UV light and oxidizing agent (namely ozone and/or hydrogen peroxide), and sometimes further include (dispersed) photo catalysts (Fe or Ti). b,c In all processes, the UV-treatment systems must be designed to effectively irradiate the fluid with (V)UV radiation, since the (V)UV photons will start the desired photo biological, photo physical or photochemical reaction(s). To achieve this, the UV source - while separated from the fluid by a quartz sleeve - is typically submerged into or imparted to the reaction chamber. a,c Such systems are currently widely used on large industrial scales for (a) degradation of bacteria in germicidal systems, or (b) degradation of organic compounds through radical formation (by synthesis of the organic compounds, ozone and/or hydrogen peroxide), and - in combination with (b) - also for (c) activation of a catalyst in UV/oxidation systems. Because the quantum efficiency of each particular process (a) (c) is maximized if the emitted radiation matches the light-induced process (which assumes that the photochemistry is well understood), intense radiation in a specific spectral wavelength range is desired. Unfortunately, the choice of UV light sources for each process is mainly determined by the availability of powerful, efficient UV light sources. With the exception of UV lasers and low-pressure mercury lamps, commercially available intense UV sources are mainly (doped) medium and high-pressure mercury lamps, Xe-arc and Xe-flash lamps, all of which emit into a broad spectral range. a Since the germicidal effect of UV radiation is highest at 260 nm, d most UV disinfections systems for drinking water (a) utilize the resonance radiation from low-pressure mercury lamps at 254 nm. Similarly, the most appropriate UV sources for the degradation of organic matter with UV oxidation systems are low and medium pressure mercury lamps (for the photodissociation of ozone), or medium and high-pressure mercury lamps (for photodissociation of hydrogen peroxide). a-c Alternatively, strong quasi-planck and gray radiators, such as Xe-arc and Xe-flash lamps, a can be applied because some of the continuum radiation will always fall into the wavelength range that is required for a particular photo sensitized process. The application of the latter two sources wrongly appears to be fundamentally weak (i.e., inefficient), but instead can be extremely effective and beneficial when the photochemistry is very complex and/or not very well explored. Zoran Falkenstein page - 1 -

3 2. RATIONALE UV systems for the disinfection of drinking water, as well as UV oxidation systems for contaminated ground water, drinking water, and industrial processing water have proven to be economically viable for a large array of (organic) contaminants and applications. They are cost effective enough to compete with traditional biological, physical and chemical water treatment technologies and furthermore do not produce secondary waste. Systems which employ UV sources up to 100 kw, and which can process volume flows of 100 m 3 per hour are commercially available. However, there are applications where current UV treatment systems (that is, the UV sources) are (a) not efficient enough (i.e., only little radiation is emitted in the desired spectral region) or (b) provide the wrong chemistry (i.e., emitted radiation spectrum leads to an undesirable chemical reaction pathway). Even though the overall radiant efficiency of mercury lamps is very high (roughly 60%, 50% and 40%), a (a) is understandable since each chemical compound (including ozone and hydrogen peroxide) call for specific, narrow-banded radiation spectrum. Unless the emission spectrum of a UV source is monochromatic, the effective radiant efficiency of a photochemical process is therefore always smaller than the overall UV source efficiency. In that respect, the UV efficiency of (doped) mercury lamps in the spectral range of 200nm to 400nm is rather 50%, <15%, and <10%. At the same time, since the choice of available UV sources is very much limited to produce broadband UV, undesirable reactions can be caused (b). This can easily be seen for example, when only a specific target species has to be decomposed out of a multitude of chemical compounds with broadband UV. To improve both (a) efficiency and (b) selectivity, spectrally selective, narrow-banded, efficient and intense UV radiation sources are needed. This is where we see excimer UV sources to be viable. We do not see a strong potential for excimer UV light sources for water treatment applications where low, medium and high-pressure mercury lamps are already successfully being applied. For these applications, the available UV emission spectra happen to be fairly well matched to key chemical bonds and species (such as peptide bonds in DNA, ozone and hydrogen peroxide), and the radiant efficiencies of these processes are sufficiently high to meet the cost requirements for bulk applications. This is particularly true for disinfections with low-pressure mercury lamps, and for most parts for UV/ozone and UV/hydrogen systems. Instead, we do see large potential for excimer UV light sources in cases where conventional UV systems fail because they provide either mismatched, or unselective (broad-band) spectral emission. In that sense, we are offering excimer UV light sources to provide selective, narrowbanded radiation at 172 nm, 222 nm, 282 nm and 308 nm that can because of their selectivity enable or improve UV treatment of specific water. Zoran Falkenstein page - 2 -

4 3. DEMONSTRATION SYSTEM To be able to do proof-of-principle experiments on a water treatment with (V)UV at 172 nm, 222 nm, 282 nm or 308 nm, we have developed two reaction chambers as schematically shown below. Metallic reactor housing Fluid inlet fluid High voltage electrode Fluid outlet As can be seen, the excimer UV source is sealing to the aluminum reactor housing with two silicon O- rings, which are secured with plastic end caps. The overall length of the reactor is 20 cm, the inner diameter 15 cm (system 1) or 7 cm (system 2). With an outer diameter of the (V)UV sources of 5 cm, the two reactors allow to irradiate fluids with a penetration depth of 5 cm or 1 cm, respectively. We have chosen these values to have practical, transportable UV excimer demonstration systems - although we do understand that each application will require its specifically optimized reactor dimension (i.e., penetration depth). The ideal penetration depth for a specific application can be obtained from the absorption spectrum of the fluid. It can also be seen that in our design the fluid is in immediate contact with the outer surface of the DBD lamp and acts as the low-potential (grounded) electrode. The inner high-voltage electrode remains in the center of the co-axial tube assembly and provides high voltage across the gas to generate excimer formation. Since water with the exception of de-ionized water, which does not need any UV processing is electrically conductive, it is possible to use the fluid itself as grounded electrode. Because water has a high dielectric constant (?=81) in comparison to the lamp components (for quartz?=3.7, for gas?=1), e the voltage drop across the fluid is fairly small (several 10 V for applied high voltages of several kv). Hence, the electrical losses are very small, while at the same time the design provides significant advantages. In comparison to using a metallic mesh-electrode (as grounded electrode of the excimer UV source), no absorption losses of the mesh electrode will occur. The radiant power and radiant efficiency can be increased accordingly to the reduction of electrode mesh surface (depending on the mesh size, this can be 20% to 50% of the lamp surface area). Also, in comparison to conventional UV treatment systems, no quartz sleeve has to be used. More importantly, the direct contact of the fluid with the UV light source will provide strong forced cooling of the UV source. This enables to drive the DBD UV light source at high electrical power (> 1 Wcm -2 ), which without forced cooling usually strongly reduces the efficiency of UV production. f,g Some photographs of the demonstration system are given overleaf. Zoran Falkenstein page - 3 -

5 The UV radiant efficiencies of DBD-driven excimer (V)UV light sources depend on the electron densities and the electron distribution function, and can be controlled mainly by the applied voltage frequency and shape, gas pressure, gas composition and gas gap distance. h For the most efficient excimers Xe 2 *, XeCl*, XeBr* and KrCl* typical efficiencies in absence of mesh electrodes, as in our design, can be estimated to be in the range of 10-25%. An exact measurement of the radiant power and radiant efficiency has to be performed by actinometry. a To summarize, when comparing the UV radiant efficiencies of DBD-driven excimer lamps to broadband or continuous UV light sources, one has to bear in mind that for DBD lamps practically all of the emitted radiation is radiated into a specific, narrow-band or quasi monochromatic spectrum. In contrast, broadband and continuous UV sources radiate only a small fraction of the overall emitted radiation into the same specific, narrow-band spectral region. To obtain a radiant power of 8-10 W in the same spectral range as given in the example above, a few 100 W would have to be applied to high-pressure discharge lamps. This alone would certainly not justify the use of excimers because of their high cost (in comparison to mercury lamps). But since many photo physical and photochemical processes are initiated by specific, narrow-band wavelengths (ideally the UV light source will emit all of its radiation into the desired spectrum), DBD-driven excimer UV light sources can offer to improve or solve specific photo physical or photochemical processes where broadband UV sources fail. To investigate specific applications in proof-of-principle experiments, we have developed the described system. This system is intended to merely determine whether selective (V)UV light at 172 nm, 222 nm, 282 nm or 308 nm can solve a specific problem in general (where conventional UV systems are very inefficient or even fail). In a second step, the reactor dimensions (i.e., penetration depth) have to be optimized (from the absorption spectrum), which will allow determining the performance of the UV process (i.e., how much power will be needed for a given volume flow). Finally, the system has to be upscaled to meet the requirements of volume flow and level of degradation. DBD-driven excimer UV sources can be powered to several kw to produce several 100 W of selective, narrow-banded (V)UV radiation. l 4. INITIAL RESULTS As outlined before, the major advantage of excimer UV light sources for water treatment is given by their spectral selectivity. This enables to selectively decompose specific (organic) compounds, and can prevent inadvertent side reactions. Even though almost the entire (V)UV spectral region of interest in UV water treatment can selectively be covered with possible excimers (Xe 2 *, F 2 *, Cl 2 *, Br 2 *, J 2 *, ArBr*, ArCl*, ArF*, KrJ*, KrBr*, KrCl*, KrF*, XeJ*, XeCl*, XeF* radiating from 172 nm to 351 nm), only four excimer systems are efficient enough to be used in powerful, economically viable UV light sources for water treatment. Still, by adding four spectrally selective, narrow-banded UV sources to the range of available Zoran Falkenstein page - 4 -

6 broad-band or continuous UV light sources, a large amount of UV water treatment processes become possible, cost efficient or feasible. To illustrate this concept, we will discuss the removal of bromate in (bottled) spring and drinking water as an example where excimer UV sources can be applied for water treatment. The common technology in drinking (spring) water processing involves pumping, ozonation, shipping, storing, ozonation and bottling. Next to many other chemical compounds (both organic and inorganic), also ppb levels of bromides can be found in the raw water. During the first ozonation in the water processing, the bromides are converted into bromate (BrO 3 - ), so that in ozonated spring water typical bromate levels of 10 to 20 ppb are found. The generated bromates remain in the water, and will be bottled after the second ozonation. The second ozonation (about 100 ppb of ozone) before bottling is performed to guarantee the sterilization of both the water bottle and bottle cap. Because bromates pose strong effects on health, new legislative regulations call for technologies that either mitigate the formation, or lead to the reduction of bromates in (bottled) drinking water. Decomposition of bromates with UV radiation from low-pressure, medium-pressure and high-pressure mercury lamps has been performed. The fundamental result is that bromates can effectively be decomposed. However, in presence of ozone (from the second ozonation before bottling), the UV light also very strongly decomposes ozone. When looking at the spectral absorption of bromate and ozone, it becomes clear that UV is extremely strongly absorbed in the spectral range of nm by ozone, c while bromate moderately absorbs in the range of nm. Hence, the emission of a broadband UV source will decompose bromate, and extremely efficiently decompose ozone (which is the reason why mercury-based UV sources are so effective in UV/ozone systems). Essentially no ozone will remain in the fluid. According to water bottling companies, this is not acceptable since a remote ozone level is absolutely critical to meet hygiene requirements in the bottling/capping process. Considering the absorption spectra of bromate and ozone, a well-suited wavelength range to selectively decompose bromate (and not ozone) is nm. Since the absorption of ozone in the Hartley band decreases with decreasing wavelength in this spectral range, m lower wavelengths are more preferable. One efficient excimer UV source that emits in this spectral range is KrCl* (222 nm), so that some ozone decomposition has to be expected. For proof-of-principle tests of bromate removal with UV at 222nm in absence of ozone, a water bottling company provided raw spring water with 14 ppb of bromides, which was enriched with 20 ppb of bromates to simulate ozonated spring water. The raw spring water was treated with the 222 nm UV system as described above. A volume flow of 1 gallon per minute resulted in a reduction of bromate of 30%. This result shows that bromate (in absence of ozone) can be cleaved with UV at 222 nm, although it does not tell anything about byproduct formation. To see whether the cleaved bromate quickly reacts back to form bromate, an additional test at 1 liter per minute was performed, resulting in a reduction of bromate of 60%. Based on the fact that the removal increased with higher UV energy density supplied to the water (regardless of the absolute value of 60% or 32%, respectively), this result shows that the fragments of bromate do not back-react quickly to reproduce bromate. For proof-of-principle tests of bromate removal with UV at 222nm in presence of ozone, the same water company ozonated the raw spring water (containing 14ppb of bromides, as well as 20 ppb of doped bromate). Due to the oxidation of bromides to bromate, this resulted in a final bromate concentration of 47 ppb (which confirms full chemical conversion). The ozone concentration was measured at 700 ppb. With 222 nm UV irradiation of the fluid at a volume flow of 1 gallon per minute, the bromate concentration was reduced to 32 ppb (a 32% reduction), while the ozone concentration reduced to >50 ppb. This result shows that bromate (in presence of ozone) can be cleaved with UV at 222 nm, while complete ozone decomposition can be avoided. Again, the results do not say anything about byproduct formation. Zoran Falkenstein page - 5 -

7 When analyzing these results, it is important to understand that the absolute values of the removal efficiencies (30% reduction at 40 W electrical power for a volume flow of 1 gallon per minute) are not very meaningful, as they have been obtained with a non-optimized reactor. To be able to give information on the effectiveness, the same degradation experiments have to be performed with an optimized system (i.e., penetration depth), the results are merely aimed to fundamentally proof the concept. In contrast, it is virtually impossible to selectively photodissociate bromate (and not ozone) with mercury-based UV systems because they provide the wrong or broadband emission. 5. LITERATURE a. A. Braun, M. T. Maurette, E. Oliveros, Photochemical Technology, (New York, John Wiley & Sons, 1991) b. O. Legrini, E. Oliveros, A. Braun, Chem. Rev. 93 (1993), c. J. R. Bolton and S. R. Carter, Aquatic and Surf. Photochem. 23A (1994), d. B. Gellert, Contr. Plasma Phys. 31 (1991), e. R. Weast, CRC Handbook of Chemistry and Physics, (Boca Raton, Florida, CRC Press, 1988) f. K. Stockwald, Dissertation, University of Karlsruhe-TH, 1991 g. Z. Falkenstein and J. Coogan, J. Phys. D: Appl. Phys. 30 (1997), h. B. Gellert and U. Kogelschatz, Appl. Phys. B 52 (1991), i. V. Schorpp, Dissertation, University of Karlsruhe-TH, 1991 j. Z. Falkenstein and J. Coogan, J. Phys. D: Appl. Phys 30, (1997). k. L. Rosocha, Treatment of Hazardous Organic Wastes Using Silent Discharge Plasmas, (Springer Verlag: Nato ASI Series, Vol. G34, Part B, 1993) m. H. Okabe, Photochemistry of small molecules, (New York, John Wiley & Sons, 1978) Zoran Falkenstein page - 6 -