Liquifying Urban Lungs: Assessing the Air Purification Potential of Photobioreactor "Liquid Trees" in Highly Polluted Cities

Abstract

Urban air pollution poses a major threat to public health globally, with over 4 million deaths each year attributed to outdoor ambient particulate matter exposure. Cities in developing countries tend to suffer disproportionately from dangerously high levels of air pollution due to reliance on legacy energy systems like coal-fired power plants. For example, Serbia has the highest rate of pollution-related deaths in Europe, with pollution concentrations up to five times WHO guidelines. The capital Belgrade hosts two highly polluting coal plants and vehicle emissions are increasing along with population density. While expanding urban greening through tree planting could help mitigate pollution, many cities lack adequate space. This paper examines an innovative solution called "liquid trees" - photobioreactors containing microalgae that absorb CO2 and release oxygen through photosynthesis. A single liquid tree bioreactor can purportedly match the air purification capacity of up to 200 sq meters of lawn. The LIQUID3 system developed in the Institute for Multidisciplinary Research at the University of Belgrade, Serbia, Belgrade uses native freshwater microalgae species housed in a 600 liter tank. Algal biomass can be harvested as fertilizer. The bench-like structure also provides lighting and device charging. By rapidly installing these solar-powered bioreactors in highly polluted urban pockets lacking space for trees, cities could substantially improve air quality. Results from lab testing demonstrate a single LIQUID3 unit can absorb as much CO2 as two mature trees according to designers. Field implementation in Belgrade has garnered interest for broader deployment in cities like New Delhi and Paris. Liquid trees present a feasible way to boost urban air purification, though limitations exist. Maximum carbon capture capacity requires abundant sunlight. Algal species must be carefully selected. And capital costs may be prohibitive for widespread use in lower income cities. While not a wholesale replacement for urban greening efforts, liquid trees exemplify the type of innovative nature-based solution needed to cleanse the air of harmful pollutants in space-constrained metropolitan areas. Their success highlights the importance of integrating natural biological processes into built environments to enable urban resilience