Exploring nutrient recovery from hydroponics in urban agriculture: An environmental assessment

In light of global population growth and the increasing food demand in cities, new food production strategies have been developed to promote a more resource-efficient urban agriculture. Greenhouses with hydroponic systems have been proposed as sustainable systems for growing food in urban areas with a better control of plant growth. However, nutrient management in hydroponic agricultural systems is an environmental challenge and its efficiency could be improved from a circular economy standpoint. The goal of this study is to analyze the potential implementation of three nutrient recovery alternatives that promote re-use for urban hydroponics, i.e. direct leachate recirculation (DLR), chemical precipitation (CP) and membrane filtration (MF), and to study their environmental performance through life cycle assessment. The study focuses on the recovery of phosphorus (P), magnesium, potassium and calcium in a hydroponic tomato crop cycle carried out in an integrated rooftop greenhouse (i-RTG), located in the Metropolitan Area of Barcelona. The assessment shows that DLR was the most environmentally friendly option in terms of global warming (5.5 kg CO₂ eq. to recover 447 g of P) as opposed to CP and MF, which had 3 and 5 times more impact, respectively. Moreover, all three alternatives showed less eutrophication potential than the baseline scenario, which considered that 447 g of P were discharged into the environment. Meeting the crop's nutritional requirements through recovered nutrients helped save between 44–52% of global warming impacts with respect to new fertilizers when using DLR and MF. Oppositely, CP showed a 2% impact increase in global warming because this technology was only able to recover P and part of the magnesium. This study informs practitioners and decision-makers about the environmental benefits of applying circular thinking to nutrient management in urban agriculture to promote urban sustainability.