Opportunities for green infrastructure to optimize the urban metabolism of resources

Green infrastructures such as urban and peri-urban agriculture, vertical farming, urban forests, green rooftops, parks, gardens, have the great potential to improve the lives of city dwellers by improving air quality, reducing heat waves, providing natural spaces for recreation, and promoting food production. However, these infrastructures also require energy and water, and could further intensify local and global impacts, such as water depletion, greenhouse gas emissions, and eutrophication. It is essential that the implementation of green infrastructure strategies is conceptualized from a systemic approach which includes other urban processes so that we move towards a more circular use of resources. In this article, we illustrate the potential of such synergies between urban agriculture (UA) and waste water management at two levels 1)recovery of nutrients in a hydroponic tomato crop production to reduce mineral fertilizer demand, and 2) recovery of nutrients from wastewater treatment plants to meet the demand of fertilizers for an urban area. For both cases, the Metropolitan Area of Barcelona is used as the case study, and the Life Cycle Assessment methodology is used to quantify the environmental impacts. For (1), we find that the direct leachate recirculation technology reduces global warming potential, eutrophication, and fossil resource scarcity by more than half when compared to the business as usual linear fertilization scheme. Study (2) estimates that nutrient recovery from the wastewater treatment plants is up to ten times greater the fertilizer demand of urban and peri-urban agriculture, providing ample opportunity for its expansion in terms of resources.