Destruction of Organic Matter in Water using Ultraviolet Light

Transcription

1 Destruction of Organic Matter in Water using Ultraviolet Light J. R. Cooper, Ph.D. Chief Technology Officer NeoTech Aqua Solutions (formerly Ultraviolet Sciences)

2 Presentation Overview General discussion of the use of ultraviolet light for water purification Approaches for removing organics with existing ultraviolet light based technologies Technologies under development to improve organic removal from water Summary

3 Ultraviolet Light for Water Purification Ultraviolet light (UV) is not a new purification technology First used for disinfection in 1910 Marseilles, France Ultraviolet use to remove organics had been primarily in niche applications until recently Ultrapure water for semiconductor processing Last years have seen use of UV in more and more varied applications

4 Spectrum of Ultraviolet Applications Emerging Applications Disinfection Remediation Wavelength : 254nm TOC Reduction DHP Direct Hydroxyl Production Wavelength : 185 nm AOP Advanced Oxidation Photolysis Direct Disassociation Wavelength : up to 350 nm Existing Applications Wavelength : 200 to 300 nm H2O2 or O3 Added

5 Organic Removal by UV General Considerations Ultraviolet light breaks up molecules rather than removing them Process leaves only H 2 O and CO 2 in general No residual organic wastes if process is run to completion Incomplete process may lead to more or different organic molecules at outlet than at inlet Organic contaminants are rarely a single compound there are usually a number of compounds

6 Organic Removal by UV General Considerations This multiple contaminant problem lends itself well to broad spectrum solutions or multiple barrier solutions Combinations of UV, filtration/ro, chemical, and adsorptive processes may be required to achieve the desired performance Incomplete UV processes may convert contaminants into compounds more readily removed by complementary technology

7 Organic Removal by UV General Considerations Parameters of importance for ultraviolet remediation Ultraviolet transmittance (UVT) of water Includes effects due to Turbidity Chlorination Organic content Soluble iron and manganese concentration Dissolved and suspended solids Scale formation Fouling Dose provided by the ultraviolet system Dose specified by manufacturers is actually irradiance energy per unit area Affected by all of the above plus flow rate, system characteristics, and age

8 Presentation Overview General discussion of the use of ultraviolet light for water purification Approaches for removing organics with existing ultraviolet light based technologies Technologies under development to improve organic removal from water Summary

9 How Does Ultraviolet Light Remove Organics?

10 Direct Hydroxyl Production Direct production of hydroxyl ions from water (aka TOC removal ) UV Photons break water into H + and OH ions Hydroxyl ions oxidize organics Used to produce ultrapure water for processing semiconductor wafers since the 1970 s Nothing added to water highly desirable quality for end users at wafer fabrication facilities

11 Direct Hydroxyl Production Direct production of hydroxyl ions from water (cont d) Requires highly energetic photons Wavelengths less than 200 nm vacuum ultraviolet Almost exclusively 185 nm low pressure mercury lamps Low intensity leads to large systems Low efficiency leads to high power consumption Other options under development to address these shortcomings Not effective against all organic compounds NDMA, carbon tetrachloride are examples

12 How Does Ultraviolet Light Remove Organics?

13 Advanced Oxidation Processes (AOP) Enhanced production of hydroxyl ions from additives to water Hydrogen peroxide or ozone added to water UV photons interact with hydrogen peroxide or ozone to create hydroxyl ions Hydroxyl ions oxidize organics Usage becoming more common in both wastewater and drinking water applications.

14 Advanced Oxidation Processes (AOP) Converting hydrogen peroxide or ozone to hydroxyls requires longer wavelength ultraviolet light than direct hydroxyl production from water Uses wavelengths that lamps produce more efficiently UV accelerates natural decomposition of both of these oxidizers Faster removal/shorter holding time

15 Advanced Oxidation Processes (AOP) nm wavelengths most efficient Coincides with wavelengths for UV disinfection Ozone absorbs UV strongly limits maximum effective concentration Peroxide does not absorb UV strongly need more UV to dissociate, but allowable concentration is unlimited

16 Advanced Oxidation Processes (AOP) Peroxide concentration required is 1X 2X the concentration of contaminant to be removed Peroxide generated offsite requires hazardous material trucking, storage, and personnel training Ozone concentration range similar Ozone concentration limited Generated onsite no storage requirement, but ozone generation uses a lot of electricity Both low and medium pressure mercury lamps used Low pressure: Higher efficiency, larger size, more lamps, used when fouling is a concern Medium pressure: Lower efficiency, smaller size, fewer lamps, shorter lamp life, 800 C operation

17 How Does Ultraviolet Light Remove Organics?

18 Direct Photolysis UV photons absorbed directly by organic molecule Molecule broken apart by photon UV only process UV wavelength must match compound s dissociative absorption curve Can be very efficient if absorption curve matches peak output wavelengths or if absorption curve is broad

19 Direct Photolysis The main present day application of direct photolysis is NDMA removal Both low pressure and medium pressure mercury lamps used in practical applications Neither is very efficient, but today these are the only available light sources for implementing this process on a reasonable scale Cost and size limit use of mercury lamp based systems to high need applications

20 Presentation Overview General discussion of the use of ultraviolet light for water purification Approaches for removing organics with existing ultraviolet light based technologies Technologies under development to improve organic removal from water Summary

21 Ultraviolet based Technologies Under Development Alternate technologies are being developed to address the shortcoming of present day ultraviolet light based organic remediation Once developed, these may stand alone or may be used in combination with other technologies, including other UV technologies A significant amount of performance validation will be required once these technologies become commercial

22 Technologies Under Development Titanium dioxide catalyzed AOP Hydroxyls produced at TiO 2 water interface when ultraviolet light is present UV wavelengths longer than Direct Hydroxyl Production essentially chemical free AOP Higher efficiencies with no chemicals or consumables Small low flow (<< 1 GPM) systems available Prone to fouling Requires contaminant to be in close proximity to catalyst Higher flow systems will have complicated geometries

23 Technologies Under Development Ultraviolet light emitting diodes (UVLED s) Has long term promise as high efficiency, low cost light source Capability to operate at wavelengths below ~240 nm requires new, as yet undiscovered semiconductor material Limited to disinfection and AOP applications Best performance today is orders of magnitude more costly ($/UV watt) than mercury lamps Efficiency and output power are both significantly lower than low pressure mercury lamps

24 Technologies Under Development Ultraviolet light excimer (excited dimer) lamps Two types Excimer phosphor combination Direct excimer UV production Both use a plasma discharge, like mercury lamps Both are mercury free Both can generate multiple wavelengths of interest Direct hydroxyl production and direct photolysis both possible

25 Technologies Under Development Excimer phosphor lamps Same physics as plasma TV pixels Electrical discharge in xenon produces 172 nm UV, which excites phosphor coated onto walls Special phosphors needed to emit UV wavelengths Much lower efficiency than visible phosphors, because excitation photon energy much closer to emitted photon energy. Efficiency lower than excimer UV conversion efficiency Multiple wavelengths from a single lamp possible Must use UV compatible materials Cost and availability are concerns

26 Technologies Under Development Direct Excimer UV lamps Ultraviolet produced directly from electrical discharge in excimer gas mixture Rare earth halogen mixtures (ArF, KrF, KrCl, XeCl ) used instead of phosphors to generate desired wavelength Broad linewidths cover most of UV C spectrum ( nm) Theoretical efficiency very high (>20% for most) Corrosive gases require lamps to be fabricated from special materials

27 Direct Excimer UV Spectrum Xe 2 KrI ArF KrF KrBr KrCl XeI XeBr XeCl

28 Summary After decades of use in niche applications, ultraviolet light based technologies are beginning to see expanded use for removing organic matter from drinking water and wastewater at high flow rates The performance characteristics of each of these technologies must be understood to use them properly, particularly in a multiple barrier organic removal system New ultraviolet light based technologies are being developed that are intended to overcome the weaknesses of the current generation of these technologies

29 Questions?