How to effectively remove viruses from indoor air? This question becomes increasingly important as the fall approaches. Efficient indoor air purification is essential, especially in schools. Fraunhofer researchers are studying and optimizing various air filtering and purification techniques in the AVATOR project.
School is in full swing again in all of the German Länder – with full classes. Children and young people sit close to each other in classrooms, many of whom have not been vaccinated due to their young age. To minimize the risk of infection, state governments and ministries of education encourage the purchase of indoor air purifiers.
But what are the different indoor air purification techniques used for? Led by the Fraunhofer Institute for Building Physics IBP, researchers from 15 Fraunhofer institutes and institutions are studying this in the AVATOR project, short for “Anti-Virus Aerosol: Testing, Operation, Reduction”. They are also studying and optimizing new air purification techniques that are not yet available on the market.
Classic interior air filters
The most common method of purifying indoor air is through conventional indoor air filters. They suck air through a filter fleece, thus trapping viruses and releasing the purified air into the room. Fraunhofer EMI simulations show how such devices can be used effectively, taking a classroom example: with a properly adjusted air change rate and proper positioning, the aerosol concentration can be reduced by about half. after 10 to 15 minutes of operation.
The simulations also show that the concentration of aerosols depends on specific ambient conditions and that the concentration is not the same for all workstations in the class. To further purify indoor air from aerosols expelled during respiration, in particular from their viral load, researchers from Fraunhofer LBF and IAP have equipped the synthetics used for the manufacture of fleece with additives.
The filtering effect of fleeces is based on three different mechanisms. These surface effects are precisely what scientists modify by using additives to filter out smaller particles more efficiently. Large particles cannot pass through the fleece and are filtered out. Smaller particles are slowed down and trapped in the polar material due to inertia. “
Dr Gunnar Grün, Professor and Deputy Director of the Fraunhofer Institute for Interfacial Engineering and Biotechnology
Polar additives affect the performance of the filter with respect to the smallest particles that adhere to the filter material due to surface effects.
Grün is also the head of the AVATOR project
The overall performance of the filter is determined by the size of the particles that are separated the least. As these are usually very small particles (approx. 200-300 µm), the efficiency of the filter can be further increased by this coating. Although there are already approaches to improving the performance of filters with additives, filter mats optimized in this way are designed for typical oil-based test aerosols. However, the aerosols that people release into the air when they breathe are water-based and therefore behave differently.
“We were able to increase the efficiency in particular for these bio-aerosols”, explains Grün. Fraunhofer IMM researchers generate suitable water-based test aerosols using liposomes. They have also developed a device for the optical detection of viral particles in indoor air.
Air purification with plasma
Even though indoor air filters work well in classrooms and similar places, they are reaching their limits, especially when it comes to retrofitting, in environments such as cold and damp rooms or slaughterhouses, for example. In order to avoid additional air resistance in the system, low temperature plasma-based air purification systems, which remove viruses from the air, are a good solution.
In these air purification systems, viruses do not remain attached to the filter webs, but are deactivated in the plasma device and separated on electrodes. Here, too, the Fraunhofer researchers were able to make improvements to the AVATOR project. “Our colleagues at Fraun-hofer IPM have developed self-cleaning electrodes based on the technology of an industrial partner in the automotive sector. This eliminates the need for the usual cleaning process, ”explains Grün.
“Virus grill”: sterilization at overtemperature
Indoor air filters and plasma remove viruses from indoor air. With the “grill virus”, researchers have taken a completely different approach to preventing infections: by heating the air to over 90 degrees Celsius, viruses are rendered harmless. Although viruses stay in the air, they cannot multiply. They are inactivated and can no longer affect people. Fraunhofer IFAM in Dresden has already shown that this principle works. Very high heat recovery enables energy efficient operation of the air purification system and minimizes heat input to the room, which is especially important in classrooms, offices and other non-air conditioned rooms. Currently, scientists are further developing the device, with particular emphasis on miniaturization.
Together with a foam manufacturer, Fraunhofer ICT and Fraunhofer IBP have developed a particularly effective approach in open-plan offices. They use sound absorbing partitions to remove the viral load from the air and minimize the risk of infection. “The entire surface of the foam is coated with an antimicrobial silver compound. Thus, a high level of viral inactivation can be achieved during the airflow ”, summarizes Grün. A demonstrator already exists. The sound absorption function is also taken into account: especially in the field of human speech, that is to say around 1000 to 4000 hertz, these partitions provide high sound absorption.
Virucide for room disinfection
When unoccupied rooms need to be cleaned, virucides are used. However, these hazardous substances generally have to be transported to the site and stored until use. Fraunhofer IMM researchers have therefore developed a more practical alternative: a mobile reactor that produces the virucide peroxydicarbonate from a harmless solution of sodium carbonate. The virucide itself breaks down into non-hazardous components. The reactor is already functioning and toxicity tests are currently being carried out at Fraunhofer ITEM, studying the intensity of the effect of the virucide on microorganisms and whether there is a critical exposure for humans and the environment resulting from its use.
Validation of techniques with virus detection
Whether it is conventional ambient air filters, anti-virus screens or partitions: indoor air purification techniques must be rigorously validated in terms of efficiency. Three Fraunhofer institutes are involved in this validation: Fraunhofer ITEM, Fraunhofer IBP and Fraunhofer IGB.
Non-pathogenic viruses harmless to humans but similar to SARS-CoV-2 viruses in size, enveloping structure and RNA strand are nebulized and serve as test aerosols for the various purification techniques. These so-called substitution viruses are produced at scale, purified, then formulated in test aerosols and nebulized for the various purification techniques. In order to identify the effectiveness of new inactivation methods, researchers analyze infectivity and compare the total number of viruses before and after inactivation.