UV-source for germ-free water

In many sewage treatment plants, it is customary to sterilize microbes with ultraviolet rays. However, the cost of the process depends largely on the efficiency and service life of the lamps used. Now, an improved UV lamp could hold the key to increased efficiency.

Operators of municipal sewage treatment plants are increasingly tending to use UV radiation as a final stage to disinfect their effluent water. This is particularly done if there are bathing lakes or rivers downstream. The sterilization process has also proved to be an elegant solution in processing drinking water and water for swimming pools, as no chemicals are needed.

Ultraviolet radiation with a wavelength of around 254 nanometers causes such severe genetic damage to bacteria, fungi and viruses that if the dose is high enough, almost all germs lose their ability to proliferate. But the mercury-vapour lamps generally used for this purpose have a drawback: their performance gradually deteriorates and they only last for less than one year when in constant operation. With an efficiency ranging from 5 to 35%,  most of the electrical energy charged does no more than heating up the water.

A better lamp is being developed in an EU project known as "Plas Light". It is likely to have a long service life, especially as it does not require any electrodes that can be worn down. Companies and research institutes from four EU countries are partners in the project – including scientists from the Fraunhofer Institute for Chemical Technology ICT and the Fraunhofer Technology Development Group TEG.

The new UV source operates in a completely different way from conventional lamps: Magnetrons generate microwave radiation which is guided into a closed chamber containing a gas mixture. The microwave energy turns the gas into a plasma phase that emits UV. In a prototype that's been developed, the radiation passes through two quartz glass plates measuring 40 x 40 centimetres into the water that is flowing across.

"The composition of the gas is a very important factor," reveals TEG project manager Anja Flügge, "because it allows us to adjust the wavelength of the emitted radiation within certain limits – in contrast to conventional lamps. In this way, we can adapt the UV source to different types of microbes."

The UV-source is incorporated in a mobile test rig, consisting mainly of an open-topped channel as a model of a flume or sluice. This enables the scientists to treat and analyse the water on site at a customer’s facility under realistic conditions.

The German researchers presented their system at IFAT in Munich in April.