UV Takes Water Treatment Chemical-Free

A new UV process quickly breaks down pollutants in wastewater without help from chemical catalysts.

The system uses 172-nm light to trigger photolysis, splitting H₂O molecules into highly reactive hydroxyl radicals.

“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 the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB. “They are therefore able to decompose even very stable hydrocarbon compounds contained in harmful residues.”

A 172-nm UV radiation element. Courtesy of Fraunhofer IGB.

A challenge in developing the treatment system was that reaction takes place only within about 50 µm of the UV emitter and the hydroxyl radicals are extremely short-lived. Water flow must be controlled so that all of the reactor vessel contents are reliably and efficiently treated. The first industrial prototype, which the researchers will present at the IFAT trade fair in Munich next week, has a reliable throughput of 2.5 m³/h.

“A certain amount of variation is normal, since the processing speed depends of course on the degree of contamination as well,” Egner said.

A sensor system monitors the water output for any remaining harmful substances. The water is discharged only once impurities falls 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, the researchers said. The method offers advantages over chemical treatment processes that do not affect highly stable hydrocarbon compounds, including cleaning agent residuals, pesticides and pharmacological substances, they added.