destruction of organic matter in water using ultraviolet … · destruction of organic matter in...

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


• 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


• 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





• Summary

How Does Ultraviolet Light Remove Organics?

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.


• 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


• 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





• 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


• 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


• 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


• 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


• 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

Questions?

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