Water treatment using UV light

Researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB (Fraunhofer IGB) have developed a photochemical reaction system in which water can be treated at high flow rates by UV light without having to add chemical catalysts.

There are numerous harmful substances in wastewater that should not find their way into the environment, yet many resist being broken down by biological wastewater treatment plants. Bacteria commonly employed in the biological treatment stage have no effect on persistent substances, which include highly stable hydrocarbon compounds. This results in cleaning agent residuals, pesticides and pharmacological substances reaching environmental waters.

Fraunhofer IGB researchers, together with international industrial partners, have developed a chemical reaction system that breaks down resilient and harmful molecules thoroughly and efficiently, without having to add chemicals like hydrogen peroxide. The researchers are essentially utilising the ‘self-healing’ power of water aided by photolysis (ie, photochemical dissociation).

The principle of photolysis is based on splitting water molecules using photons. The shorter the wavelength of light, the higher the photons’ energy. The researchers used light sources that emit UV light exclusively in the region of 172 nm - ie, extremely energetic photons. As soon as these photons enter water, they split the H₂O molecules, forming highly reactive hydroxyl radials as a result.

172 nm UV radiation element. © Fraunhofer IGB.

“These hydroxyl compounds have an even higher reaction potential than atomic oxygen, for example,” said Siegfried Egner, head of the Physical Process Technology department at IGB. “They are therefore able to decompose even very stable hydrocarbon compounds contained in harmful residues.”

The process takes place in the immediate vicinity of the UV emitter - a rectangular, flat glass element that is positioned in the reactor vessel. When power is applied to the element, the hydroxyl radicals form a thin reactive boundary layer only about 50 µm deep surrounding the external surface of the glass. In order to be sure no harmful particles escape untreated, the water must be controllably and verifiably directed through this boundary layer.

Not only must the researchers ensure entire contents of the reactor vessel is treated, they also require certainty that every single hydroxyl radical formed is also used for a chemical reaction. This is because the extremely reactive hydroxyl radicals are extremely short-lived; if no ‘fresh’ molecules are found to react with during this time interval, the energy of the hydroxyl radicals goes unused.

The experts have been successful in controlling the movement of the water so that all of the reactor vessel contents are reliably and highly efficiently treated. A sensor system located at the discharge port monitors the water for harmful substances and the water is discharged only once impurities fall below a maximum permitted value. The entire unit is fully automatic and programmable - for instance, it can be switched on and off depending on the electrical power rates on offer.

The researchers presented a prototype of the technology, with a throughput of 2.5 m³/h, at this year’s IFAT trade show in Munich.

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