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"Liquid Tree" - oxygen production with microalgae
(discuss.tchncs.de)
The University of Belgrade’s Institute for Multidisciplinary Research invented an urban photo-bioreactor. Liquid 3 cleans the air and serves as a bench in the Belgrade city centre.
It's a tank filled with microalgae, a solar panel on the top generates electricity for USB charging ports for citizens and a pump that pumps air into the tank where the algae process the CO² and produce Oxygen.
https://balkangreenenergynews.com/liquid-tree-to-combat-air-pollution-in-belgrade/
Also how well it would hold up in a cold snap. Would be fairly limited to warm climates I imagine but even warm climates get a few days of freezing temperatures occasionally.