Filtering seawater salt may require a lot of energy or specialized engineering, but a thin membrane made of porous wood can fix that.
In membrane distillation, salt water is pumped through a film, usually made of some type of polymer with very narrow pores that filter the salt and let only water molecules pass through. Jason Ren and his colleagues at Princeton University, in New Jersey, have developed a new type of membrane made of natural wood instead of plastic.
The linden trees are a genus of trees of the Malvaceae family (previously classified in their own family, the Tiliaceae), native to the temperate regions of the northern hemisphere. It comprises about thirty species, which are distributed throughout Asia, Europe and eastern North America. The membrane developed to filter seawater It is made of a thin piece of American linden, which undergoes a chemical treatment to remove additional fibers in the wood and make its surface slippery for water molecules.
One side of the membrane is heated so that when water flows on that side it vaporizes. Next, the water vapor travels through the pores of the membrane to its colder side and leaves the salt behind, condensing like fresh water. This requires much less energy than simply boiling all salt water because there is no need to maintain a high temperature for more than one thin layer of water at a time.
This method filters approximately 20 kilograms of water per square meter of membrane per hour, which is not as fast as polymer membranes. The researchers think it may be because they did not have the equipment to make their membrane so thin: it is 500 micrometers thick, while polymer membranes are generally closer to 130 micrometers thick.
Making wood membranes thinner should not be too difficult with the right equipment, according to Ren: "The functional part of the membrane is a micrometer thick. The rest is just a support structure to make it more difficult to to break".