One of these nutrients is phosphorus, a key ingredient in fertiliser. We need fertilisers to meet the demands of the planet’s growing population, but there is a limited supply of phosphorus. Once it finds its way into the sea it becomes impossible to recover.

And yet we all excrete about half a kilogram of the stuff a year, making cities a potentially rich source of the element. In the Netherlands human sludge is already being processed to recover phosphorus and recycle it into a high-tech fertiliser which will not leach into the environment.

Reporter: William Kremer

Photo: Getty images