UV applications in water and environmental technology
In water and environmental engineering the deciding quantity is not the irradiance at the reactor wall but the radiation that remains available inside the medium. Dissolved substances, turbidity and the absorption of the matrix itself attenuate it, and they do so differently at each wavelength. Assessing or scaling a process therefore requires the transmittance of the medium, the optical path length and the residence time alongside the measured irradiance.
Author: Dr. Mark Paravia
Dr.-Ing. Mark Paravia is the managing director of Opsytec Dr. Gröbel GmbH in Ettlingen and heads the accredited calibration laboratory. Following his research on pulsed xenon excimer discharges at the Institute of Lighting Technology at KIT, his current focus is on optical radiation measurement technology. He is a recognized UV expert, vice-chair of the DIN Standards Committee FNL 7 “Optical Radiation,” and a member of the DVGW Project Group on UV Disinfection.
What tasks does UV radiation perform in water treatment?
Optical processes serve four distinct purposes in water treatment, and they must not be measured the same way. Disinfection targets the inactivation of micro-organisms. In direct photolysis the dissolved compound absorbs the radiation itself and is altered by it. Advanced oxidation combines radiation with hydrogen peroxide, chlorine, persulfate or ozone to generate short-lived radicals. Photocatalytic processes rely on irradiated semiconductor surfaces or particles.
From that distinction follows the first decision in any project: which wavelengths trigger the intended mechanism? A plant with low-pressure lamps and near-monochromatic emission calls for a different spectral assessment than a polychromatic medium-pressure lamp or a UVA-driven photocatalytic reactor. Public drinking water supply carries further requirements specific to equipment and procedure; these are collected under guidelines, norms and standards in UV.
Why does the water matrix change the result?
The water matrix is not a side condition; it is frequently the governing one. Dissolved organic matter competes for photons and scavenges radicals at the same time. Carbonate and bicarbonate act as radical scavengers. Nitrate absorbs in the short-wave range and can itself react photochemically. In chlorine-based processes, halides shift the radical chemistry and may favour halogenated by-products.
Results obtained in ultrapure water therefore transfer only partly to real process or waste water. A degradation rate achieved in laboratory water can drop substantially in a real matrix without anything changing on the optical side. Matrix data – dissolved organic carbon, alkalinity, nitrate, chloride – belong in the description of the experiment just as much as the radiation data.
How far does the radiation reach into the medium?
Irradiance measured at a surface does not describe what arrives deeper in the medium. Attenuation follows the optical path length and depends on wavelength: short-wave components are usually weakened more strongly than long-wave ones, so the spectrum shifts with depth. Inside an absorbing medium, a broadband source effectively becomes a different source.
For laboratory work this means a reading at a defined position becomes transferable only when layer thickness and transmittance are stated with it. In disinfection experiments on liquid samples the mean fluence within the sample is the governing quantity, not the surface value. Vessel shape contributes as well: a thin film, a Petri dish, a stirred beaker and a flow-through reactor each present a different radiation field, even under the same lamp.
What does energy demand reveal about a process?
A process that degrades quickly in the laboratory is not automatically the more economical one. The established comparison is the electrical energy required per order of magnitude of removal – the energy needed to reduce the concentration of a target compound in a defined volume by a factor of ten. That figure makes visible how often a higher degradation rate is bought with a disproportionate rise in energy input.
There is a direct consequence for measurement. The denominator of that figure comes from chemical analysis and the numerator from an electricity meter – but two plants become comparable only once it is also known what share of the electrical input actually arrives as usable radiation inside the medium. Without that intermediate quantity, a chemically better process cannot be told apart from one that is merely coupled more efficiently.
Why transformation products belong in the assessment
A fall in the parent concentration is not the same as harmlessness. Photolysis and oxidation generate intermediates and end products that may be more persistent or more toxic than the starting compound. Several studies from the user community therefore examine the formation of transformation products alongside the degradation kinetics, together with their biodegradability and toxicity.
On the optical side this means the absorption spectra of the intermediates matter as well. If a product forms that absorbs more strongly in the applied waveband than the parent compound, the photon distribution inside the reactor changes while the experiment runs. A transmittance measured once at the start then describes only the opening phase.
Which measurement equipment supports water and environmental work?
For irradiance and dose in laboratory and process applications, a radiometer such as the RMD Pro with a sensor matched to the process is the usual choice. Which waveband and which sensor design make sense depends on the source and on the installation; the selection is described under selection of UV sensors.
A spectroradiometer such as the SR900 belongs wherever polychromatic sources are characterised, spectral change is tracked over lamp life, or UV and visible contributions need to be captured together. It covers 200 to 1100 nm.
Reproducible laboratory exposure comes from irradiation chambers with a fixed geometry; a dose controller such as the UV-MAT ends the exposure when the target dose is reached rather than after a set time. In plant operation, permanently installed sensors take over continuous monitoring; digital versions such as PLC.D pass sensor type and calibration status to the control system along with the reading.
What does the published research show?
The following work by users points at what governs practice: the influence of the matrix, the way the reactor is run, and the assessment of what remains in the water afterwards.
Shows on steroid hormones how strongly a background of organic matter slows photocatalytic degradation at a coated membrane.
Influence of organic matter on the photocatalytic degradation of steroid hormones by TiO2-coated polyethersulfone microfiltration membrane
Liu, Siqi, Pattabhiramayya C. Edara, and Andrea I. Schäfer. "Influence of organic matter on the photocatalytic degradation of steroid hormones by TiO2-coated polyethersulfone microfiltration membrane." Water Research 245 (2023): 120438.
Sets adsorption, photolysis and advanced oxidation side by side on the same compound, with kinetics and reaction pathways.
The fate of aqueous betrixaban during adsorption, photolysis, and advanced oxidation: Removal, kinetics, and reaction mechanisms
Jasemizad, Tahereh, Lev Bromberg, and Lokesh P. Padhye. "The fate of aqueous betrixaban during adsorption, photolysis, and advanced oxidation: Removal, kinetics, and reaction mechanisms." Journal of Water Process Engineering 44 (2021): 102430.
Examines the photo-transformation products of a pharmaceutical and the influence of pH, concentration and temperature on their formation.
Initial fate assessment of teratogenic drug trimipramine and its photo-transformation products – Role of pH, concentration and temperature
Khaleel, Nareman DH, et al. "Initial fate assessment of teratogenic drug trimipramine and its photo-transformation products – Role of pH, concentration and temperature." Water Research 108 (2017): 197-211.
Judges the products of a UV degradation explicitly by biodegradability and toxicity, not by removal alone.
UV-photodegradation of desipramine: impact of concentration, pH and temperature on formation of products including their biodegradability and toxicity
Khaleel, Nareman DH, et al. "UV-photodegradation of desipramine: impact of concentration, pH and temperature on formation of products including their biodegradability and toxicity." Science of The Total Environment 566 (2016): 826-840.
Describes a continuously operated structured reactor for the degradation of dissolved contaminants.
A new continuous flow-through structured reactor for the photodegradation of aqueous contaminants
Fernandez-Perez, Amparo, et al. "A new continuous flow-through structured reactor for the photodegradation of aqueous contaminants." Journal of Environmental Chemical Engineering 6.4 (2018): 4070-4077.
Links advanced oxidation of a cytostatic drug to subsequent aerobic biodegradation and toxicity testing.
Removal of the anti-cancer drug methotrexate from water by advanced oxidation processes: Aerobic biodegradation and toxicity studies after treatment
Lutterbeck, Carlos Alexandre, et al. "Removal of the anti-cancer drug methotrexate from water by advanced oxidation processes: Aerobic biodegradation and toxicity studies after treatment." Chemosphere 141 (2015): 290-296.
The environmental technology topic field currently holds 21 papers; the full listing is available under publications by customers by topic.
Adjacent application fields: UV disinfection, photocatalysis, photocatalytic hydrogen production.
