Keeping our supermarket shelves packed with food requires an industrial production process that is heavily dependent on a regular supply of nutrients such as phosphorous and potassium fertilisers, as well as energy-demanding nitrogen fixing.

At the same time the agriculture industry is spending money to ensure it is able to maintain sufficient fertility in soil, the world’s cities are disposing of vast quantities of these same nutrients every time they are flushed away as part of urban wastewater.

‘We grow our crops in the field, apply nutrient-rich fertilisers, eat the crops, excrete all of the nitrogen, phosphorus and potassium and then those nutrients end up at the wastewater treatment plant,’ says Jeremy Guest, an assistant professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign (UIUC). ‘It is a very linear, one-directional flow of resources. Engineering a more circular nutrient cycle would create opportunities that could benefit the environment, economies and agriculture.’

Rome is being seen as the city best placed to recycle nutrients from wastewater into food production (Image: Catarina Belova)

Guest and fellow researchers at UIUC analysed 56 major cities around the world to determine the feasibility of recycling these nutrients back into food production, based on the proximity of agricultural land to the urban centres where the food is then consumed. Of those analysed, Rome emerged as the best placed city to benefit from human-derived nutrients, while Los Angeles, New York and Boston were the three where this strategy was deemed least effective given the sprawling distances that would need to be overcome.

They also calculated that, overall, Asian cities would be best prepared to recover nutrients from city wastewater than those on any other continent, followed by Europe and Africa. The vast distances in Oceania mean this is the part of the world where the technique would be most difficult to implement.

‘Because this was developed as a global analysis, the method does not allow us to examine specifics for each city, such as the driving routes that would be required to haul nutrients or the locations of wastewater plants,’ continues Guest. ‘The results of this exercise should be taken as estimates of nutrient transport distances and are useful for identifying broad trends and locations that may warrant further investigation into reuse strategies.’

Jeremy Guest (left) and John Trimmer (right) evaluated the feasibility of using human-derived waste as a safe and valuable nutrient commodity (Image: L. Brian Stauffer)

Lead author of the study, UIUC’s John Trimmer, points to a few examples of the technology required for the safe extraction of nutrients from wastewater, such as Ostara, a company that recovers nitrogen and phosphorus as a compound called ‘struvite’, which is then sold as fertiliser.

‘Beyond the cities in which Ostara is currently operating, including Chicago and Madrid among others, and places where biosolids are already recovered, we are aware of several organisations working in countries such as Kenya, Uganda, Rwanda, and Ghana to recover resources from sanitation systems and develop successful business models,’ explains Trimmer. ‘These efforts can involve composting collected solids, growing black soldier fly larvae as animal feed, or other approaches.’

So, the next time you flush the toilet, think about where those nutrients might end up. They could be back with you before too long.

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