Destruction of Organic Matter in Water using Ultraviolet Light
J. R. Cooper, Ph.D.Chief Technology OfficerNeoTech
Aqua Solutions
(formerly Ultraviolet Sciences)
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
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 10‐15 years have seen use of UV in more and more varied applications
“Spectrum”
of Ultraviolet Applications
AOPAdvanced Oxidation
PhotolysisDirect
Disassociation
Remediation
Wavelength : 254nm
Wavelength : up to 350 nm
DHPDirect Hydroxyl
Production
Wavelength : 200 to 300 nmH2O2 or O3 Added
Wavelength : 185 nm
Disinfection
Existing Applications
Emerging Applications
“TOC Reduction”
Organic Removal by UV – General Considerations
• Ultraviolet light breaks up molecules rather than removing them
– Process leaves only H2
O and CO2
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
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
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
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
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
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
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.
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
Advanced Oxidation Processes (AOP)
• 200 –
300 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
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
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
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
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
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
Technologies Under Development
• Titanium dioxide‐catalyzed AOP– Hydroxyls produced at TiO2
‐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
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
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
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
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 (200‐320 nm)
– Theoretical efficiency very high (>20% for most)– Corrosive gases require lamps to be fabricated
from special materials
Direct Excimer
UV Spectrum
160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
Xe2KrI
ArF KrClKrF
XeClKrBr XeI XeBr
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