As clean freshwater has become scarcer around the world—especially in arid regions such as the Middle East and North Africa—those countries that can afford it have increasingly turned to desalination. That energy-intensive process extracts salt from sea (or other saline) water, transforming it into water that’s fit for human consumption. There are now nearly 16,000 desalination plants either active or under construction across the globe.

“[But] they don’t just produce desalinated water,” explains Manzoor Qadir, a researcher at the United Nations University in Canada. “They also produce brine.”

Brine is the concentrated salt water that’s left after desalination. But Qadir says, “there is no comprehensive assessment” of how much is being produced. So, Qadir and his colleagues decided