New system uses UV light to treat water without chemicals

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In the near future, there may be a way of more effectively treating waste water without the use of harmful chemicals. Scientists from the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) in Germany have developed an industrial photochemical reaction system in which water can be treated at high flow rates by UV light without having to add chemical catalysts. An initial prototype of the system will be displayed at this year’s IFAT in Munich, Germany between 5-9 May.

Certain substances found in waste water are harmful to the environment – yet waste water treatment plants only remove a portion of these contaminants. For instance, bacteria commonly employed in the biological treatment stage have no effect on persistent substances, which include highly stable hydrocarbon compounds. The result is that cleaning agent residuals, pesticides and pharmacological substances are released into environmental waters. The loading from these kinds of harmful substances in the North Sea, for instance, is already clearly measurable today.

In the photochemical reaction system developed by the team at the Fraunhofer IGB, in collaboration with international industrial partners, breaks down resilient and harmful molecules thoroughly, without having to add chemicals such as hydrogen peroxide.

In the system, photolysis or photochemical dissociation is employed. The principle of photolysis is based on splitting water molecules using photons the shorter the wavelength of light, the higher the photons’ energy. Researchers therefore use light sources in this system that emit UV light exclusively in the region of 172 nanometres – which are extremely energetic photons. As soon as these photons enter water, they split the water molecules, forming highly reactive hydroxyl radials as a result. ‘These hydroxyl compounds have an even higher reaction potential than atomic oxygen, for example,’ explained 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.’

However, there are difficulties with using this approach. The process takes place only 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 around 50 micrometers deep surrounding the external surface of the glass. So, in order to be sure no harmful particles escape untreated, the water must be controllably and verifiably directed through this boundary layer.

In addition, because the active hydroxyl compounds are extremely short-lived, it is important to ensure every single hydroxyl radical formed is used on the chemical reaction and not wasted. If no ‘fresh’ molecules are found for the hydroxyl to react with during this time interval, the energy of the hydroxyl radicals goes unused.

But, after testing the prototype system, the experts in Stuttgart have been successful in controlling the movement of the water so precisely that all of the reactor vessel contents are reliably and efficiently treated.

The first industrial prototype has a through-put of 2.5m2 per hour. And, to be sure the water is discharged only if its quality is impeccable, the unit is equipped with a sensor system located at the discharge port that monitors the water for harmful substances. The water is only discharged once the quantity of impurities falls below a maximum permitted value.