According to the world health organization, air pollution is currently the greatest environmental risk to human health and is associated with around 7 million premature deaths each year. According to the German Federal Environment Agency, traffic is a primary contributor to poor air quality, especially in urban areas. Although legally binding limit values have to be met at official measurement stations, particularly in places where the exhaust air from ventilation systems of parking garages or metro stations is discharged without being filtered, higher levels of pollution can locally occur. There are no clear rules on how to deal with air pollutants in these areas, but there is a great need for efficient mitigation measures.

Biological air purification with plants has various advantages over technical systems. One of these are lower prices compared to the acquisition and operational costs of technical systems. In addition, certain pollutant groups can be partially degraded and metabolized by plants instead of just being stored. In an urban context, there are other positive effects of planted systems. These include for example the retention of rainwater, a climate-regulating effect and an increase in biodiversity. Finally, the biodegradability of the plants leads to a more sustainable circular economy...

Plant-based filters such as moss walls have been partially implemented in the past in urban areas. However, their efficacy was rather low especially due to the fact that the polluted air did not pass the filter structure, but was just in diffusive contact with the surface. Furthermore, the filtering materials could not be installed near the source of the pollutants, but typically on sidewalks, where concentrations were already strongly diluted. Thus, the aim of the publicly funded project BioLu is to develop a plant-based filter which can be used to clean the exhaust air from parking garages, which has been shown to contain a large variety of particulate and gaseous pollutants, mainly originating from the vehicle exhaust as well as tire and break wear. The filter shall be capable of being retrofitted to existing ventilation exhausts and directly penetrated by the exhaust air in order to maximize its efficacy.

An efficient plant-based filter medium needs to be constructed from at least four different layers: ...

In a first step, suitable individual layers for all stages of the filter were tested with respect to their filtration efficiency and air flow resistance. The results were fed to a self-developed calculation tool to predict the most promising candidates for a layered structure. Those combinations were then tested under realistic flow conditions to find the best compromise between filtration efficiency and pressure drop. The media were tested under dry conditions as well as fully saturated with water since both states can occur in the later application. First results show that the optimized layered structures can filter at least ...

In the next step, the project partner NIRA will establish large trial plots with the optimized combination of the materials and plants. The best procedures for laying the vegetation carrier on the field for pre-cultivation as well as for care, irrigation and harvesting will be investigated. Cuttings from the plant-based filter medium will then be exposed in a custom-built test rig to typical outdoor conditions (i. e., sunlight, rain, different temperatures, etc.) while a continuous air flow is passing through it like in the later application on the ventilation exhaust of a commercial parking garage. Changes in the filtration efficiency and pressure drop as well as the resistivity of the plants to these conditions will be regularly monitored.

The project BioLu is supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) on the basis of a decision by the German Bundestag.