With more than 70% of the Earth covered in seawater, turning seawater into drinking water — that is, desalination — would seem like the answer to our current global water crisis. But even though more than 18,000 desalination plants are operating globally, according to 2015 estimates by the International Desalination Association, we’re still far from the day when most of us are picking up a bottle of desalinated water, or getting desalinated water from the tap. Technology for large-scale desalination has seen a lot of progress in recent years, but still suffers from two long-running problems: it’s expensive, and it’s bad for the environment.

The cost of desalination

Converting seawater into drinking water at a large scale takes an enormous amount of energy, which makes it a costly effort. “Desalinated water is cheaper than bottled water, but 275x more expensive than currently available farm water in the central valley of California,” Richard Muller, a professor of physics at the University of California, Berkeley, writes for Forbes. And water for agriculture is where the need is greatest. While desalination is cost-effective at a small scale — like drinking water, which taps into 10% of the Earth’s freshwater supply — it doesn’t work for agriculture, which uses 70% of Earth’s freshwater.

Of course, Muller was pointing to an old-fashioned plant for his estimates. Newer technology does not require as much energy, or money, to turn seawater into drinking water. But in many parts of the world — for example, the Middle East, which has long invested heavily in desalination — the older tech, known as thermal, is dominant. Thermal plants take in seawater, heat it until it turns into freshwater vapor, keep it, and then pump out the brine wastewater. The heat requires a lot of energy and thus drives up cost.

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By contrast, reverse osmosis (RO) is the newer, more energy-efficient (and therefore less expensive) tech. RO pushes seawater through a series of special membranes that filter out salt and other impurities. While not cheap compared to the cost of water from a river or aquifer — and certainly not a reliable substitute for agriculture’s water needs — the expense of RO desalinated water is lowering as the filter technology is refined further to require less pressure (and thus less energy, thus less cost) to push the water through the membranes.

The fallout

Turning seawater into drinking water carries unintended environmental consequences, however. First, the energy required to run desalination plants, especially thermal plants, can add more fossil fuel pollution to the air. While solar energy is slowly being adopted to power desalination plants, the size and number of solar panels needed have prevented the large-scale adoption of this alternative energy source.

Second, the process of desalination creates a toxic byproduct damaging to the environment: brine. Currently, brine — that is, highly concentrated saltwater, often contaminated with chemicals that keep the plants’ plumbing running smoothly — is often dumped back into the ocean from whence the seawater came, where it “sinks to the seafloor and wreaks havoc on ecosystems, cratering oxygen levels and spiking salt content” affecting sea life, reports Matt Simon for Wired. With thermal desalination technology, only 25% of seawater taken in converts to drinkable water; the remaining 75% leaves as brine. With RO, it’s roughly 50-50, Simon reports.

Here, progress is being made, too. Diluting brine before pumping out the wastewater can minimize its effect on oceanic environments. Or, it can be pumped into pools, evaporated and have the salt harvested. Regardless, the chemicals that keep seawater from gunking up the pipes of desalination plants would still make it into the environment, and this is where the next innovation is truly needed. Because in a world ravaged by climate change, even with energy- and cost-effective RO desalination technology, the pollution may not be worth it.