If only humans could drink seawater without dying, we wouldn’t find ourselves floundering in a water crisis. To not die, first you have to boil saltwater and collect the pure vapor, or get yourself a fancy membrane that filters out all the salt and, conveniently, sea life.

This is the controversial idea behind large-scale desalination—great big expensive facilities that turn saltwater into a liquid that won’t kill you. The classic criticism of desal is that it takes a tremendous amount of energy to process seawater, and we really shouldn’t be burning any more fossil fuels than we need to be. But a less chattered-about problem is the effect on the local environment: The primary byproduct of desal is brine, which facilities pump back out to sea. The stuff sinks to the seafloor and wreaks havoc on ecosystems, cratering oxygen levels and spiking salt content.

Unfortunately, scientists haven’t had a good idea of just how much brine the 16,000 operating desal facilities worldwide have been producing. Until now. Researchers report today that global desal brine production is 50 percent higher than previous estimates, totaling 141.5 million cubic meters a day, compared to 95 million cubic meters of actual freshwater output from the facilities. Bad news for the environment, to be sure, but things aren’t altogether dire: Desal tech is rapidly evolving, so plants are getting far more efficient, both in the brine they produce and the energy they use.

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Desalination facilities typically fall into one of two categories: thermal and membrane. With thermal, you suck in seawater, heat it up to get the pure vapor, and pump the remaining brine back out to sea. With membranes, you push seawater at great pressures through a series of filters, which pull out all the salt and other contaminants.

Thermal is the more old-school method—prior to the 1980s, 84 percent of desalinated water went through this process. Since the beginning of the new millennium, though, a particular kind of membrane technology, reverse osmosis (we’ll call it RO for short), has proliferated exponentially. RO facilities now produce 69 percent of desalinated water worldwide.

Why? Because RO is cheaper and more efficient. Advances in membrane technology mean facilities require less and less pressure, and therefore energy, to filter seawater. As an added benefit, RO produces less brine. With thermal, 75 percent of the water you bring in might leave as brine. With RO, it’s more 50-50 freshwater to wastewater.

“It also depends on the feedwater,” or input water, says Edward Jones, coauthor on the new study and an environmental scientist at Wageningen University in the Netherlands. “Reverse osmosis is least efficient when you're desalinating highly saline water, such as seawater. And it gets more and more efficient as feedwater salinity drops.”