Just days before the 2016 Summer Olympics began in Rio de Janeiro, the Brazilian city faced an unsavory problem: how to handle its overwhelming sewage. Nearly half of Rio’s municipal wastewater flowed untreated into Guanabara Bay, where the waters were so polluted by sludge that direct contact was deemed a health hazard to Olympic athletes.

Chinese cities are being encircled by untreated sludge—a pungent, viscous mixture of human excreta and stormwater runoff. Four years ago in Beijing, trucks loaded with untreated sludge from the city’s largest wastewater treatment plant were illegally dumping mountains of sludge in the outskirts of the capital as “free fertilizer” for peri-urban farms. Similarly, in the southern China city of Guangzhou, disposing toxic sludge into the nearby river required nothing more than a hired boat. From the far western desert city of Urumqi to the eastern metropolis of Shanghai, more and more of China’s cities are struggling to manage mountains of sludge and municipal waste, as well as floods of wastewater and stormwater runoff.