Ozone water disinfection is gaining ground
While chlorine and ultraviolet light are the standard means of disinfecting water, ozone is just as effective at killing germs. To date, ozone has only been used as an oxidizing agent for water treatment in large factories. Today, however, a consortium of projects from Schleswig-Holstein is developing a miniaturized ozone generator for use in smaller applications such as water dispensers or small household appliances. The Fraunhofer Institute for Silicon Technology ISIT supplied the sensor chip and electrode substrates for the electrolysis cell.
Compared to conventional disinfection means such as chlorine or ultraviolet, ozone dissolved in water has many advantages: it is environmentally friendly, remains active beyond its immediate place of origin, n has a short retention time in water and is therefore tasteless. Due to its high oxidation potential, ozone is very effective in fighting germs. It breaks the cell membrane of common pathogens. In Germany, ozone is mainly used to disinfect swimming pools and drinking water and to purify wastewater. Yet it is rarely used to disinfect water in household appliances such as ice makers and drink dispensers or in other devices such as shower toilets. MIKROOZON, a project funded by the state of Schleswig-Holstein and the EU, aims to change that. Researchers from Fraunhofer ISIT have teamed up with Itzehoe-based CONDIAS GmbH, founded in 2001 as a spin-off of the Fraunhofer Institute for Surface Engineering and Thin Films IST, and partner of CONDIAS, Go Systemelektronik GmbH, from Kiel. The three partners are developing a miniaturized ozone generator with integrated sensor technology and a microprocessor control system.
Direct production of ozone by electrolysis of water
“The ozone generator is very compact and can be integrated into systems and devices that require regular disinfection,” explains Norman Laske, researcher at Fraunhofer ISIT. “You just connect it to the water line and it will produce the right amount of ozonated water whenever needed. The ozone generator is only a few cubic centimeters in size and includes an electrolysis cell, a sensor chip, control electronics to regulate current and voltage, and electronics to read signals from the sensor. “The two electrodes are separated by an ion-conducting separator membrane,” says Laske. “When a voltage is applied to the electrodes, water is split by an electrolysis process. Due to the diamond layer covering the electrodes, this process first forms hydroxyl radicals, which then react to form mainly ozone (O3) as well as oxygen (O2). “
Diamond Coated Silicon Electrodes
The manufacture of electrodes with their boron-doped diamond layer is the know-how that gave CONDIAS GmbH its name. The company is already using a chemical vapor deposition process to coat large-scale electrodes required for the disinfection of ship ballast water. However, the electrodes required for the MIKROOZON generator are much smaller. They are made of silicon and have finely etched trenches that pass through the electrodes to form narrow slits on the reverse side. In order to be able to engrave these trenches with the required precision, the researchers at Fraunhofer ISIT had to have wafers manufactured to their own specifications.
To build an ozone generator, pairs of these electrodes are mounted back to back, with a separator membrane between them. The gases are released at the interface with the separator membrane and then escape through the trench structure on the other side of the electrode, where the turbulence of the water flow ensures that they are effectively dissolved and dispersed.
The Fraunhofer ISIT sensor chip is equipped with three sensors to measure conductivity, mass flow and temperature. These parameters must be monitored in order to control the electrolytic process. The sensor chip provides the data needed to control ozone production based on the quality and quantity of water used. “In order to ensure that there is sufficient ozone available for the required period, the temperature must be monitored,” says Laske. “This is because ozone breaks down faster at higher temperatures.” Conductivity correlates with the degree of hardness of the water: the harder the water, the higher the conductivity, which means more current must flow to achieve the desired effect. When equipped with a system to monitor these parameters, the ozone generator should be able to process up to 6 liters of water per minute – without the sensor chip, it is currently specified for 0.5 to 1.5 liters.
CONDIAS markets the mini-generator under the MIKROZON brand. “Each partner brought years of experience in their own area of specialization,” says Volker Hollinder, CEO of CONDIAS GmbH. “This created a product that can now be manufactured on an industrial scale. The spread of the coronavirus has highlighted the importance of disinfection. The use of chemical disinfectants is often problematic, as they leave harmful residues. Our system uses ozone generated by electrolysis to kill germs, so it does not produce any disinfectant residue.