Some very cool stuff buried in the article:

Gazing at the turbulent, rain-swollen river, Esculier highlights what he sees as a curious paradox: as a society, we are spending energy on treating our nitrogen-rich wastewater and destroying reactive nitrogen, while also, spending energy on making synthetic nitrogen fertiliser (whose production and use account for around 2% to 5% of greenhouse gas emissions). Treatment facilities capture around 10% of nitrogen from our sewage to be spread on crops, while 50% goes into the air, he says - and the remainder, into the river. Given a greater Paris population of 10 million people, this means “nitrogen from four million people goes into the Seine every day”.

If we used all the urine from greater Paris to fertilise wheat instead, it would be enough to produce more than 25 million baguettes a day," Esculier calculates.

Over the past decade, Esculier has tried to put some of those findings into practice, trialling ways to collect urine and use it as fertiliser. In its simplest form, he was familiar with this from his own family history: “One of my grandmothers used to tell her children to go and pee on the rhubarb,” he says, which gave the plant a boost of natural fertiliser.

Under a research programme called Ocapi, which Esculier leads, he and his team have organised various pilot projects aimed at collecting urine in cities which is then used by farmers to fertilise their crops. In one project, 20 volunteers collect their own urine and bring it to a drop-off point, where a farmer then collects it, stores it and uses it as fertiliser.

Esculier hands me a packet of biscuits produced as part of the Ocapi project. As the label proudly states, the Biscodor (or “Golden Biscuits”) are made with flour from “wheat cultivated with a fertiliser based on human urine”. I put them in my bag, curious to see what my colleagues in London would think of them.

The idea of separating urine at source is attracting interest on a larger scale.

Saint-Vincent-de-Paul, a planned new neighbourhood in Paris in the grounds of an old hospital, will feature urine-separating toilets as part of a recycling pilot programme by the City of Paris.

“It’s fairly rare in Paris to have a new neighbourhood, given that the city is essentially already built, so we don’t have many opportunities to test these kinds of things,” says Antoine Guillou, deputy mayor of Paris in charge of waste management, recycling and sanitation. He adds: “The idea is to test the separation of urine, and to see if it can be collected and used as fertiliser.”

The new neighbourhood in Paris’ 14th arrondissement will comprise around 600 households, “which is quite a considerable size for an experiment but is small compared to the whole of Paris”, Guillou points out.