In some cities, trees have become a true luxury item: either because there is no space left (or there is no interest in allocating it for this purpose), because the ground is sealed by asphalt or concrete or because pollution prevents their development. This happens in large cities all over the planet, from India to southern Europe. India has released a solution that does not need rain and does not grow: it is a green water tank that does the work of ten trees. A liquid tree.
Context. In cities there are two overwhelming realities: They concentrate around 70% of carbon dioxide emissions and almost half of the population lives in them. Some Spanish cities such as Madrid, Barcelona, Seville or Murcia deserve special mention, among those with the lowest proportional tree cover on the continent and those with the most deaths due to the heat island effect, according to a study by specialists from the Barcelona Institute for Global Health. published in The Lancet.
It is not so much a question of having many trees (Madrid, for example, has them), but of having proportional tree cover and here the Spanish state needs to improve, he says. this study of 744 European cities and the recommendations of the European Commission.
Al fresco liquid tree. “Liquid trees” are, in a nutshell, urban photobioreactors. Inside there is a closed system with microalgae in aqueous solution to absorb carbon dioxide and release oxygen as if it were a real plant.
Up to this point, everything is more or less as if it were a tree, but with the advantage of not needing soil, land to plant it in, or taking root. And that the cleaning function of the liquid tree is equivalent to two 10-year-old trees or 200 square meters of grass, according to the Multidisciplinary Research Institute of the University of Belgrade, to whom they came up with the concept in 2021 following the assignment of the United Nations Development Program (UNDP) to combat air pollution in the Serbian capital. The first prototype was called LIQUID 3 and was planted in Stari Grad.
Why is it important. Because cities are the epicenter of the global emissions problem and if we have already seen that today a good part of the world’s population lives in cities, in 2050 it will be even worse: the UN estimates that the figure will rise to 68%.
As explains Dr. Ivan Spasojevicone of the inventors of LIQUID 3, the goal is not to replace forests, but to use this system for urban areas where there is no space to plant trees. Under certain conditions of high pollution, trees suffer to survive, but according to the scientist, algae are not affected.
How it works. As you can see in the image on the cover or in the video below, LIQUID 3 is a kind of aquarium with 600 liters of fresh water where there are single-celled microalgae (which we can find in any pond) continuously doing photosynthesis. The contaminated air is introduced in the form of bubbles thanks to the pumping system and a photovoltaic panel provides electricity for both the pump and the nighttime LED lighting.
Furthermore, maintenance is minimal: every month and a half you have to remove the biomass generated, which serves as fertilizer (not for the liquid tree, obviously) and replace the water and minerals.
They clean more than a lifelong tree. The main reason for this liquid tree compared to a traditional tree is efficiency: while parts such as the trunk, branches or roots do not photosynthesize, everything in the algae is productive. According to the UNDP Serbiathat makes them between 10 and 50 times more efficient than conventional trees. The startup Liquid Trees has quantified the CO₂ removal capacity of its liquid tree at 1.83 kg of CO₂ per kg of biomass produced.
From prototype to first street trees. Liquid trees are not something new: as we have already seen, the concept dates back to 2021. However, it has not remained a mere prototype and that’s it. The technology is escalating. In 2024, the Kerala University of Fisheries and Ocean Studies and the company Lo Carbon Solutions they installed India’s first outdoor liquid tree in Kerala: a 1,000 liter tank equivalent to 10 mature trees. Almost at the same time, the DS business group and the startup Liquid Trees they planted a 1,600 liter unit equivalent to six mature trees.
Yes, but. Leaving aside something obvious such as that if the electricity contribution does not come from a renewable source, the real carbon balance is worse than the figures suggest or that it is data provided by interested parties and not externally audited, a scientific review by researchers at the Kerala University of Fisheries and Ocean Studies published in the International Journal of Plant and Environment lists some limitations of the concept, among them a fairly obvious one: investment in infrastructure and maintenance is not comparable to planting lifelong trees.
And that’s without talking about the environmental cost: an architectural design study from the University of Alcalá calculation that a photobioreactor façade takes more than 11 years to compensate for the CO₂ emitted during its own manufacturing. Finally, no city has yet implemented the technology at scale. What exists are prototypes and specific pilots, not deployed urban solutions.
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