Serbia scientists have developed urban bioreactors, dubbed ‘liquid trees’, to help fight the problem of air pollution in the capital Belgrade. 

The liquid trees, named LIQUID3, are solar-powered tanks that contain 600 litres of water and micro-algae. They work by binding carbon dioxide and producing pure oxygen through photosynthesis. 

The tanks, now installed on some streets in Belgrade, emit a green glow due to the micro-algae.

According to the developers’ website, LIQUID3 replaces one adult tree or 200 square metres of lawn, and the technology also works during the winter. 

“LIQUID3 represents an alternative sustainable concept of greening of urban environments that show limited space and heavy pollution,” the developers said. 

In 2022, LIQUID3 was picked for the Best Innovation Award by the UNDP Serbia, the Ministry of Environmental Protection and GEF.

Belgrade and other Southeast European capitals suffer from high levels of pollution, worsened by the proximity of major coal-fired power plants.

For a few days in November, data from IQAir showed that capitals of Bosnia & Herzegovina, Croatia and Serbia were among the most polluted cities in the world. 

Air pollution is a growing concern for Serbians, sparking occasional protests. As pollution levels spiked in November, several thousand people joined a protest in Belgrade.

Coal remains an important part of Serbia’s energy mix, and, as in other Western Balkan countries, is heavily polluting. 

According to CEE Bankwatch Network, the coal plants of the Western Balkans continued to massively exceed emissions limits in 2021. Overall, in 2021 coal power stations in the region emitted five times as much sulphur dioxide as allowed and 1.8 times as much dust, the NGO said.

This contributes to the premature deaths of thousands of people every year. 

Artificial trees are being explored as an option to fight climate change. Among the pioneers of the technology is Arizona State University engineering professor Klaus Lackner, who designed a mechanical tree that could absorb up to 32 tonnes of carbon dioxide a year — far more than a real tree. However, issues such as the cost of the trees and what to do with the carbon dioxide they take from the air still need to be resolved.