Using secondary currents induced by air curtains to improve trapping of plastic particles

Plastic waste is ubiquitous in aquatic environments. It has become an environmental and societal problem due to its fragmentation into microplastic particles. We analyze the potential of secondary currents induced by air curtains to i) drive plastic particles into river lateral retention basins and ii) increase the probability of permanent retention in the sediment bed. Plastic particles simulate macroplastics, with h/d = 100, where d is the diameter of the plastic particle and h is the flow depth.    We mapped the efficiency of retention (percentage of mass of plastic particles permanently retained) to flow, air curtain and basin parameters. The latter are the Froude number, dimensionless air mass flux, basin aspect ratio and normalized plastic load. Measurements included Laser Doppler anemometry to map the flow field, LSPTV to track plastic particles. We have found that the secondary currents induced by air curtains can increase the plastic retention time in zones with separated flow. However, the absolute increase in permanent retention is low. Permanent retention seems only possible if vegetation or human-made structures are present in the retention zone. This combination should constitute a scalable retention solution employing cost-effective techniques, with low impacts on the biota and a low carbon footprint. Acknowledgements: This work was partially supported by Foundation for Science and Technology's through funding UIDB/04625/2020 (CERIS research unit).