UV and the Role it Plays in Advanced Oxidation Processes as a Barrier Against Emerging Contaminants
Introduction
Modern drinking water facilities face an array of complex and sometimes contradictory problems. On one hand the need to treat microorganisms that are becoming increasingly chlorine tolerant, while driving down the disinfection byproducts caused by high doses of chlorine, and at the same time treat the new contaminants that are emerging such as pesticides, caused by more intensive land use, or pharmaceutical products consumed in ever increasing quantities by an ageing population. Water scarcity will inevitably lead to more reuse of water, which will highlight the need to develop and add process barriers to remove these contaminants from the water supply. Few conventional drinking water processes can address this issue and almost no municipal wastewater process is capable of targeting these problem compounds.
Metabolized and un-metabolized pharmaceutical and personal care products (PPCPs) are not new, however their potential to cause effect on living tissue is now subject to much scrutiny. A study (1) by the U.S. Geological Survey published in 2002 brought attention to PPCPs in water. Following sampling of 139 susceptible streams in 30 states, detectable quantities of PPCPs were found in 80% of the streams.
PPCPs include:
- Sun screen products
- Prescription and over-the-counter therapeutic drugs
- Diagnostic agents
- Veterinary drugs
- Fragrances
- Cosmetics
- Nutraceuticals (vitamins)
Sources of PPCPs:
- Agribusiness
- Hospital residues
- Human activity
- Pharmaceutical manufacturing residues (well defined and controlled)
- Illicit drugs
- Veterinary drug use (antibiotics and steroids)
The USEPA maintains an active program called the Contaminant Candidate List (CCL) to identify contaminants in public drinking water that warrant detailed study and may require regulation under the Safe Drinking Water Act (SDWA). The most recent Contaminant Candidate List, CCL3, was finalized on September 22, 2009 and contained 104 chemicals or chemical groups, 12 microbiological contaminants, and for the first time includes 10 pharmaceutical compounds.
The list includes antibiotic pharmaceuticals such as erythromycin, and nine hormones: 17 alpha-estradiol, 17 beta-estradiol, equilenin, equilin, estriol, estrone, ethinyl estradiol, mestranol, and norethindrone
UV alone or in combination with selected chemical oxidants has the ability to produce large amounts of the hydroxyl radical (OH-). These species aggressively attack organic compounds, either by the abstraction of hydrogen atoms from water, (alkanes and alcohols), or it can add itself to the compound (olefins and aromatic compounds).
Relative oxidation power of main oxidizing species
Species Relative Oxidation Power
Chlorine 1.0
Hypochlorous Acid 1.10
Permanganate 1.24
Hydrogen Peroxide 1.31
Ozone 1.52
Atomic oxygen 1.78
Hydroxyl Radical 2.05
Positively charged hole on
Titanium Dioxide, TiO2+ 2.35
The table illustrates how powerful the hydroxyl radical is. It is non selective and initiates a complex cascade of oxidation reactions leading to mineralization of the organic compound.
History
Advanced oxidation processes (AOP) can be defined as "near ambient temperature and pressure water treatment processes which involve the generation of hydroxyl radicals in sufficient quantities to effect water purification" (2).
The earliest evidence of this phenomenon was recorded by Downes and Blunt (3), who observed the decomposition of H2O2 by sunlight in 1879, and the decomposition of H2O2 by UV was later observed by Thiele (4) in 1907. By 1922 Kornfeld (5) had developed reaction products from the photolysis of H2O2. The basic concepts of the modern AOP technologies are over 100 years old.
Today these processes are an essential tool in the removal of a number of microconstituent compounds such as N-nitrosodimethylamine (NDMA). NDMA is a known carcinogen and is effectively removed using UV light. California has recently established a public health goal for NDMA which will likely serve as an eventual regulation in the State. UV light at or close to 228 nm is used to photolyze this compound, effectively breaking the bonds within the molecule.
In the north of Holland, the PWN Water Supply Company successfully replaced breakpoint chlorination at their Andijk drinking water treatment plant by using UV/H2O2. The plant wanted to provide control against emerging organisms that are chlorine tolerant, while reducing byproduct formation and controlling organic contaminants. The effect of UV and H2O2 on 12 pesticides was studied. For an electric energy of 1 kWh/m3 conversion varied from 18% for trichloroacetic acid to 70% for atrazine. For a combination of 1 kWh/m3 and 15 g/m3 H2O2 all pesticides could be degraded by more than 80% (6).
In the UK, operators at the Mid Southern Water drinking water plant at Boxall's Lane, used UV light to effectively remove a wide variety of pesticide species from well water being abstracted from chalk aquifers. Atrazine, Simazine and Diuron in concentrations of 0.1 μg/l to 0.5 μg/l were successfully removed using UV light alone with a higher removal rate achieved when UV was combined with H2O2.
A 12 month study recently undertaken by Greater Cincinnati Water examined the ability of a low pressure and medium pressure UV system to reduce seven contaminants of interest. The study included: Atrazine, Metolachlor, MTBE, MIB, Ibuprofen, Gemfibrozi,l and 17-α-ethynylestradiol; some of these contaminants have been found in the Ohio River. The study examined the addition of up to 10mg/l of H2O2 in conjunction with the UV systems and recorded encouraging degradations under different process conditions (7).
How does UV Disinfection Work?
UV light is created by a low pressure, high output amalgam lamp, or by a medium pressure lamp. The low pressure, high output lamps use an amalgam of mercury to produce a single (monochromatic) line output at 254 nm. These lamps operate independently of fluid temperature. Their main advantage is the high electrical efficiency which can be up to 40%. Amalgam lamps up to 800 watts are now being used by ETS and atg.
Medium pressure lamps are used where footprint is more important than electrical efficiency or where a broader spectral output is required; these types of lamp are 15%-18% efficient. Medium pressure lamps are polychromatic and produce a continuous spectral output from 190 nm to the long wave visible light and infrared parts of the spectrum. Electronic ballasts or constant wattage transformers are used to continuously vary the lamp output to compensate for lamp aging and to ensure that a uniform UV dose is delivered to the fluid.
Wipers are used to keep the optical path free from fouling. Iron, calcium, and a wide variety of organic contaminants from municipal water will foul the quartz sleeves negatively impacting system performance.
UV light is absorbed by the DNA of all living organisms and the damage that results inhibits normal cell function. This process of dimerization is caused when cross bonds in the DNA structure absorb sufficient energy that they vibrate and eventually break effectively preventing replication and rendering the cell non viable.
As can be seen from the diagram above, DNA broadly absorbs UV in the germicidal region, with the wavelengths between 250 nm to 270 nm capable of being most strongly absorbed and thus doing the most damage. The UV line at 254 nm that is produced by an amalgam lamp and the continuous lines produced by a medium pressure lamp are all absorbed, thus damaging the DNA. At normal disinfection doses organisms are not able to repair themselves and the damage is permanent. No organisms have demonstrated any tolerance to UV light.
The Science of Photolysis
Conventional ozonation or hydrogen peroxide oxidation of organic compounds does not completely oxidize many species to CO2 and H2O. In a number of reactions, the intermediate oxidation products can be more toxic than the initial compound. Completion of the oxidation reactions is often achieved using UV light.
Ozone readily absorbs UV light to form OH- from a H2O2 intermediate, as shown below:
O3 + hv → O2 + O(1D)
O(1D) + H2O → H2O2 → 2OH-
The absorptivity of H2O2 for UV light at 254 nm (the wavelength produced by low pressure or monochromatic lamps) is very low. It is greatly increased when polychromatic lamps (medium pressure lamps with broader spectral output) are used and further increased when high quality synthetic quartz is selected with enhanced UV transmittance below 240 nm.
The direct photolysis of hydrogen peroxide leads to the formation of hydroxyl radicals
H2O2 → HO2- + H+ → H2O2
HO2- + hv → OH- + O-
These reaction mechanisms are complex and varied. The illustration below highlights some of the potential breakdown pathways.
From the table of relative oxidation power, the Titanium Dioxide, TiO2+ process would seem to offer the most potential. This photocatalytic oxidation process is now being examined by ETS in the USA and atg in the UK.
UV Reactor Design and Modeling Tools
The older S type design (inlet at bottom, outlet at top) or the U type design (inlet and outlet on top or bottom) have both largely been replaced by the in line type, shown in the pictures throughout the article. This design is (a) simpler to install and (b) optimizes the flow pattern across the UV lamps. The use of computational fluid dynamic (CFD) models easily highlights the hydraulic inefficiencies caused by inlet bends.
System sizing is affected by thee variables: flowrate, lamp power, and transmittance of the fluid being treated.
The flow profile is produced from the chamber geometry, flowrate, and the particular turbulence model selected.
The radiation profile is developed from inputs such as water quality, lamp type, and the transmittance and dimensions of the quartz sleeve.
Proprietary CFD software simulates both the flow and radiation profiles. Once the 3D model of the chamber is built, it is populated with a grid or mesh that comprises of thousands of small cubes. Points of interest, such as at a bend, near a sleeve surface, or close to the wiper mechanism use a higher resolution mesh, while other areas within the reactor use a coarse mesh. Once the mesh is produced, hundreds of thousands of virtual particles are fired through the chamber. Each particle has variables of interest associated with it and are harvested after they exit the reactor. Discrete phase modeling produces delivered dose, headloss, and other chamber specific parameters.
When the modeling phase is complete, selected systems are validated using a third party to provide oversight that determines how close the model is able to predict system performance. Validation uses non pathogenic surrogates such as MS2 or T1 to determine the reduction equivalent dose (RED) ability of the reactors. Most reactors are validated to deliver 0.5 log to 6 log reductions of Cryptosporidium within an envelope of flow and transmittance.
Installation contractors find the in line type both simpler to install and they occupy less space, which in a pipe gallery can often be very beneficial.
Conclusion
UV will continue to play an active role as a disinfection barrier against the chlorine tolerant organisms. As available water supplies dwindle and we are forced to use and eventually reuse water, the removal of microcontaminants and PPCPs will become more pressing. Conventional wastewater plants were not built as a barrier to these nuisance compounds so cannot be expected to effectively remove them. Oxidation using UV light and a number of oxidants would seem to be the next logical step.
References
(1) http://www.groundwater.org/
(2) Glaze et al Ozone: Sci. Eng., 1987 , 9, 335-352
(3) Gmelin (1966) 8th Edition Verlag Chemie , 3, 2275-2285
(4) Gmelin (as previous)
(5) Gmelin (as previous)
(6) Kruithof IOA 1998 p 331-348
(7) Maria Meyer Greater Cincinnati Water WQTC 2009
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