The Biomimicry Manual: How Does Nature Make Saltwater Drinkable?
It’s frightening to think, but one of every six people in the world today doesn’t have enough safe water to drink. Within 30 years, thirst will spread to three-quarters of the world’s population. But surely this clever ape can figure out how to tap our watery planet’s vast oceans? We can’t drink saltwater, of course, but doesn’t desalination offer tantalizing potential? We already do it in some of the world’s driest spots, but in general, it’s still an expensive proposition, fraught with environmental disaster. But even as we humans struggle to meet the freshwater challenge in a sustainable way, nature is busy doing it. Every day, tide in and tide out, fueled by sunshine and emitting nothing more than sea salt. How do they do it? Find out in todays entry of The Biomimicry Manual.
Humans use lots of technologies to make fresh water from salty. Generally though, it boils down to either evaporating water and condensing it somewhere else, leaving the salt behind, or actively pushing saltwater through a semi-permeable membrane, leaving salt on one side and fresh water on the other.
Either way, you have to draw huge amounts of seawater through an intake screen, with the unfortunate side-effect of killing a lot of fish, invertebrates, birds, and even mammals. Smaller creatures that make it through (like plankton, eggs, larvae, and little fish) will die during processing. Various chemicals have to be added to the water along the way, to prevent clogging, scaling, and corrosion of filters and pipes, and then removed for drinking. The salty leftovers are laced with these chemicals, along with concentrated lead, iodine, and nitrate-heavy agricultural runoff. The whole process generally requires a lot of energy for heating or pumping or transporting, and many places with big water problems (like Mexico City or New Delhi) are far from the coast.
Yes, human desalination is expensive, energy-intensive, and a bit of an environmental nightmare. And yet nature pulls it off every day with none of these side-effects. Marine animals keep their blood only a third as salty as seawater (a serious feat, especially for critters like penguins, whales, sea snakes, and seals, whose ancestors were land-dwellers). How do they do it?
Mammals simply make really salty pee, two and a half times saltier than seawater. Birds and reptiles can’t do that: their kidneys just aren’t built for it. Penguins and gulls take care of it by sneezing out salt, while sea turtles and crocodiles cry salty tears, and sea snakes ooze it out under their tongues. All of them use salt glands, with a sodium-potassium ion pump and a counter-current exchange mechanism to move salt out of their blood. The design has evolved more than once. It must be a great idea.
Plants can’t tolerate salt either, but a select few have figured out how to beat it. In the last tiny pockets of Southern California coastal salt marsh, Pickleweed sequesters salt in its succulent stems. Once a segment is salt-saturated, it turns red and falls off. Bye salt. And in tidal saltmarshes all over the world, silver-leaved Saltbush fills tiny balloon-like bladder cells, each mushrooming off the tip of a special hairy ‘trichome’ cell. When the bladder is full, it pops open to dash salt out like a microscopic salt-shaker. Why re-invent the wheel? Can we harness these low-footprint strategies, with artificial wetlands acting like ‘Living Machines’ to make fresh drinking water?
The undisputed champion of saltwater living is the mangrove. These trees perch on stilts, their roots directly in the salty coastal ebb and flow. Twice a day, the tides wet their feet. It’s not an easy lifestyle, but mangroves have figured it out more than once (‘mangrove-ness’ has evolved at least a couple of times in unrelated plants), with a few different strategies. Many are so good at getting rid of salt you can actually drink their root-water. Good to know, next time you’re competing on ‘Survivor.’
Red mangroves use this neat energy-free trick to make their drinking water: evaporation wicks moisture from their leaves, creating a vacuum that sucks saltwater through their root membranes up through the tree, leaving salt behind. Pretty cool. The Aquaporin company is doing exactly this, mimicking the fatty membrane channels commonly seen in nature. Their bio-inspired filters pass water through, excluding all other particles and ions.
Black mangroves do it differently. Despite their name, their leaves are chalky white with excreted salt crystals. Other mangrove species sequester salt in their oldest leaves, shedding them, lifeless, into the water below. Still others collect salt in their most delicious root-parts, to be trimmed away by hungry crabs. I’m not sure how we can use that, but who doesn’t love feeding crabs?
With a planet of nine billion thirsty people, why not look to nature for answers? It may be that sipping our way through our oceans is not sustainable either, but until we figure out how to lasso an icy comet, we have to find a way. Nature’s salty mentors are out there showing us how, with sunshine and sea salt.
An evolutionary biologist, writer, sustainability expert, and passionate biomimicry professional in the Biomimicry 3.8 BPro certification program, Dr. Tamsin Woolley-Barker blogs at BioInspired Ink and serves as Content Developer for the California Association of Museums‘ Green Museums Initiative. She is working on a book about organizational transformation inspired by nature.
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