Cities face a real challenge with increasing water stress, as some are already running out of water. In 2018, Cape Town was poised to become the first major global city to hit 'Day Zero', the day when the taps would be turned off due to drought, high demand and inadequate supply. The metropolis came close. It isn’t alone – Istanbul, Mexico City and Chennai have all faced water crises recently.

This is the first in a series of papers that highlight new ways of thinking about global water resources. This paper explores circular cities, where the use and reuse of water and wastewater resources are optimized, thereby unlocking the true value and potential of urban areas.