UV LEDs for water disinfection

The treatment of water by irradiation with UV light from a mercury lamp is already a widely used technology. The UVC radiation triggers biochemical reactions in microorganisms that kills them or is preventing their growth. The primary mechanism is the formation of bonds between adjacent nucleobases (cytosine and thymine), on the same strand of DNA or RNA, which blocks the reproduction of the microorganisms. A water treatment with UVC radiation eliminates the undesirable use of physical disinfectant.

Apart from the UV-Pearl plant US-based Aquionics to disinfect small amounts of water (<1 m3 / h) no water treatment plant on the basis of UVC LEDs commercially available is available. The reason for this is that UVC LEDs in the range 250-280 nm are not technically matured, therefore are available only as a utility model and their price is currently much higher than that of mercury vapor lamps.

The aim is to develop UVC LEDs, with conversion efficiencies and lifetimes as close as possible to that of mercury lamps or even exceed this. But long before this goal is achieved, technical solutions with UVC LEDs can already be successful, there can be low conversion efficiencies or high production costs offset by other advantages of UVC LEDs, such as their long lifetimes, optimally adjusted peak wavelengths, or dimming compact design.

Through the introduction of UVC LEDs as radiation sources, the manufacturers of water treatment plants can expand their product range or even change profoundly. In addition to the aspects of mercury freedom, break resistance and service life extension of the radiation source the extremely fast turn-on and the very large dynamic range of UVC LEDs result new, more effective control concepts for water disinfection systems. For example, the system does not start for several minutes before the actual flow through the reactor with water in order to bring them to the corresponding UV intensity level. In addition, the wavelength of UVC LEDs can be optimally adapted to the composition of the to be treated water. Only the displacement of the wavelength of 254 nm (mercury lamp) of 265 nm, ie in the maximum of the DNA absorption band biodosimetric efficiency will increase by ~ 10%. By combining UV-LEDs of different wavelengths there can be used so called AOP's (Advanced Oxidation Processes), where UV radiation in combination with hydrogen peroxide or titanium dioxide generated OH radicals, which in turn degrade organic contaminants in water oxidation.

Specific Effects

Besides the above main advantages of UVC LEDs to mercury lamps are, depending on the target market of water disinfection, following specific effects of particular relevance:

Decentralized treatment systems have typical water flow rates from 1 m3 / h or less and targeting markets such as Fountains of self-supplying, developing and emerging countries with a lack of high-quality, centralized water supply system, the countries of the first world in areas without water network infrastructure, mobile applications of any kind (eg ships, aircraft, rescue and emergency cases) or hygienic particularly challenging water intakes (eg, hospitals, food production ). In these applications, it is advantageous that UVC LEDs can be operated at low DC voltages from batteries or solar systems and that they are low maintenance, mechanically robust and free of toxic substances. UVC LEDs offer a great opportunity, the fundamental problem of UVC water sterilization, namely their lack of sustainability to work around, by the principle of "Endstellenentkeimung" is passed, in the sterilization directly at the tap.

In particular for medium and large flow rates during water treatment, energy efficiency is essential. UV-radiation sources should be able to operate with the small mean flows that are at wastewater treatment plants up to a factor of 6 below the maximum design flow rates in a very low power range. While conventional mercury lamps can only be regulated on 50-30% of the maximum power down, there is no such lower limit for UVC LEDs. Even if the UVC LEDs were only half as efficient as mercury lamps, this advantage of UVC LEDs would clearly reduce the average power consumption. The energy saving effect, combined with an extended light source life and new reactor designs would significantly reduce in a large-scale solution with a water flow rate in the range ~ 100 m3 / h in particular the operating costs.