The organic solid waste that citizens deposit in brown recycling bins could currently produce the amount of compost needed to satisfy 8% of the nutrients demanded by urban and peri-urban agriculture, reducing environmental impacts such as soil eutrophication by 39% and global warming by 130%.

This is the main conclusion of a study carried out by the Institute of Environmental Science and Technology of the Universitat Autònoma de Barcelona (ICTA-UAB) that analyses the potential of compost produced from organic solid waste as a substitute for industrially produced mineral fertilizers. The study, published in the journal Waste Management, determines that the estimated production of compost by 2025 could supply 21% of the nutrient needs of urban and peri-urban crops in the Metropolitan Area of Barcelona (AMB), further reducing environmental impacts.

This would require “an increase in the selective collection of organic matter and compost production capacity according to the metropolitan waste management programme (PREMET25)’, says Juan David Arosemena, ICTA-UAB researcher and lead author of the study, who stresses that there is a great opportunity to further optimize the circularity of nutrients in cities and thus take advantage of all its benefits.

The 5,568 hectares of peri-urban and urban agriculture in the Metropolitan Area of Barcelona require a total of 1,474 tons of NPK nutrients (nitrogen, phosphorus and potassium) yearly, three essential elements for plant growth. Of these, nitrogen is the most demanded with 769 tons, followed by potassium, with 592 tons, and phosphorus, with 113 tons. Most of the nutrients needed are used by cereals (42%), vegetables (38%), sweet fruits (11%) and legumes (5%).
From the 5,106 tons of compost produced from the AMB's organic solid waste yearly, 113 tons of NPK nutrients (phosphorus 44 tons, potassium 37 tons, and nitrogen 32 tons) could be recovered, in addition to other elements.

Apart from nourishing crop fields, compost production has important environmental benefits for cities, such as reducing methane emissions from mixed waste treatment in landfills, using biogas as an energy source, and replacing industrial mineral fertilizers in agriculture. These industrial mineral fertilizers are not only made from finite resources (as in the case of phosphorus), but also require a lot of energy and fossil fuels to produce (as in the case of nitrogen). “Compost, on the other hand, is made from our waste and provides the same nutrients as the mineral fertilizer; we have to make the most of it, and thus close the cycle,” says Arosemena.

“However, to take full advantage of its benefits, certain barriers that currently hinder the circularity of nutrients in cities should be removed, such as the poor quality of agricultural soil in urban areas, the acceptance of compost as an alternative fertilizer and local regulations associated with organic fertilizer application”, says Arosemena. The study therefore presents policymakers with a full life cycle, from waste to crops, to quantify systematically the benefit of coupling nutrient flows within a city.