Compared to conventional means of disinfection such as chlorine or ultraviolet, ozone dissolved in water has a number of advantages: it is environmentally friendly, remains active beyond its immediate place of origin, has only a short retention time in water and is subsequently tasteless. Due to its high oxidation potential, ozone is very effective at combating germs. It breaks down the cell membrane of common pathogens. In Germany, ozone is chiefly used to disinfect swimming pools and drinking water and to purify wastewater. Yet it is rarely used to disinfect water in domestic appliances such as ice machines and beverage dispensers or in other fixtures such as shower-toilets. MIKROOZON, a project funded by the State of Schleswig-Holstein and the EU, aims to change this. Researchers from Fraunhofer ISIT have teamed up with the Itzehoe-based company CONDIAS GmbH, which was founded in 2001 as a spin-off from the Fraunhofer Institute for Surface Engineering and Thin Films IST, and CONDIAS partner Go Systemelektronik GmbH, from Kiel. The three partners are developing a miniaturized ozone generator with integrated sensor technology and microprocessor control system.
Direct production of ozone via water electrolysis
“The ozone generator is very compact and can be integrated in systems and appliances that require regular disinfection,” says Norman Laske, researcher at Fraunhofer ISIT. “You simply connect it up to the water line, and it will produce the right amount of ozonized water whenever required.” The ozone generator is only a couple of cubic centimeters in size and comprises an electrolysis cell, a sensor chip, control electronics to regulate current and voltage, and electronics to read the sensor signals. “The two electrodes are separated by an ion-conducting separator membrane,” Laske explains. “When a voltage is applied across the electrodes, the water is split by a process of electrolysis. Because of the diamond layer coating the electrodes, this process first forms hydroxyl radicals, which then react to form primarily ozone (O3) as well as oxygen (O2).”
Diamond coated silicon electrodes
In the facilities of CONDIAS GmbH, large-format electrodes for the disinfection of ship ballast water have already been coated by chemical vapor deposition. The electrodes for MIKROOZON are much smaller: they are made of silicon with finely etched trenches running through them. These open out at the back into narrow slit openings. In order to achieve the required precision in etching, the Fraunhofer ISIT researchers had to have wafer material specially produced according to their specifications. To turn the electrodes into an ozone generator, they are mounted in pairs back-to-back, with a separator membrane in between. The gases are released at the junction with the separator membrane and can be efficiently removed and dissolved by the trench-like structuring due to the turbulence of the water. The sensor chip developed at the institute is equipped with three sensors that measure conductivity, mass flux and temperature. These parameters are required to control the electrolysis process. The sensor unit thus provides the data to control the production of ozone depending on the water quality and quantity. "To ensure that sufficient ozone is available for the time of use, the temperature must be monitored. The higher the temperature, the faster ozone decays," Laske explains. Conductivity, in turn, correlates with water hardness: the higher the water hardness, the higher the conductivity and the more current must flow to achieve the desired effect. In the future, the ozone generator should be able to process up to 6 liters of water per minute thanks to the integrated measurement - it is currently specified for 0.5 to 1.5 liters without sensor technology. CONDIAS GmbH markets the miniature generator under the name MIKROZON®. "All partners combine many years of expertise from their respective specialties in this product, which can be manufactured in large quantities," says Volker Hollinder, CEO of CONDIAS GmbH. "The spread of coronavirus has shown how important disinfection is. Often, the use of chemical disinfectants is problematic because harmful residues remain. Germs are eliminated through electrochemical ozone production. There are no residues of disinfectants."