Rising at an altitude of 5 200 m in the Peruvian Andes, the River Rímac carves a path of 200 km down to the coastal desert, through the city of Lima and into the Pacific Ocean. The Rímac is the lifeline of Lima’s water supply, providing most of the drinking water of its 9.6 million inhabitants, and used to irrigate much of its 12 500 ha of peri-urban farm land. It is also the Lima region’s main means of waste disposal – the discharge of untreated effluent from mines, factories and human settlements has led to levels of contamination that have been described as “catastrophic”. As its population grows, at the rate of almost 200 000 a year, Lima has become increasingly vulnerable to water scarcity, which will be aggravated by climate change. Meanwhile, rising incomes are creating demand for a greater variety and higher quality of food, even as urban expansion pushes agriculture onto less productive land. A recent urban water management study proposed a “grey‑to‑green” solution: to reduce stress on Lima’s water resources – and boost food production – by treating and re-using 300 million tonnes of wastewater a year to irrigate the city’s green belts and farmland.