Would local agriculture benefit if treated human wastewater from cities like Abidjan, Lagos and Kinshasa was applied to nearby fields? Yes, in some cases, say the authors of a new study published in Nature Sustainability.

The research team from the University of Illinois in the United States looked at 56 of the world’s largest cities on six continents to assess how safely treated human wastewater could be used to fertilize – and  sometimes irrigate – farms.

It’s a circular method to recover the nitrogen, phosphorus and potassium derived from crops humans eat, and send it back to the fields instead of applying costly fertilizers that often create environmental challenges of their own.

“We found, for example, that in Cairo, Egypt, if all of the nitrogen resources from wastewater were utilized, the city could cut Egypt’s nitrogen fertilizer imports by roughly half,” said graduate student John Trimmer, the lead author of the study. “This type of approach could also help smallholder farmers in places like sub-Saharan Africa gain better access to fertilizer than what is currently available.”

The idea takes some getting used to, and it wouldn’t work everywhere. The idea has limits if the distance or logistics for transporting heavy wastewater are too difficult to yield much ROI. In some cities, new technology would be needed to recover human waste safely, in a form similar to crystallized commercial fertilizers.

Casablanca, Cape Town and Karthoum also are among the dozens of cities the team assessed. For a look at the results, see the Nature article online or contact the University of Illinois.