MIT's wooden water filter gets put to real-world use
It's a sad fact that many of the people who most need water filters are those who are least able to afford them. A newly demonstrated MIT technology could help, in that it uses locally sourced wood to inexpensively purify contaminated water.
Running lengthwise within the "sapwood" of a tree's trunk and branches are straw-like conduits known as xylem.
These draw water up from the roots and distribute it throughout the tree, in the form of sap. The xylem walls consist of thin membranes made of lignin and cellulose. Sap can pass through tiny pores in these membranes, but potentially harmful bubbles within that liquid cannot.
Back in 2014, MIT scientists demonstrated that this same principle allowed filters made of pine sapwood to thoroughly remove E. coli bacteria from tainted water – in the lab, at least. Now, the researchers have created a practical, low-cost, wood-based filtration system that could actually be put to use in impoverished regions, where it would be built by local people following instructions.
The process begins with the slicing of cross-sectional discs out of tree branches – pine trees that are native to parts of India were used in the study, although other types should also work.
Those discs are soaked in hot water for an hour, after which they're dipped in room-temperature water, dipped in ethanol, then left to dry. The ethanol treatment keeps the xylem membranes from sucking in and sticking together when drying – thus impeding water flow – which was a problem with the 2014 filters.
Once one of the wooden discs has dried, it's placed in a plastic holder at the bottom of a 1 meter-long (3.3-ft) cylindrical filtration device. Contaminated water is then poured into a receptacle on top of that device, subsequently being pulled by gravity down through the wood and out of a spigot.
When tested over a two-year period by local users in India, the system was found to remove over 99 percent of E. coli and rotovirus from tainted water, at a filtration rate of 1 liter (33.8 oz) per hour. The wooden filters remain effective after being stored in dry form for at least two years, and can each be used to treat 10 to 15 liters (2.6 to 4 US gal) of water before needing to be replaced.
"Because the raw materials are widely available and the fabrication processes are simple, one could imagine involving communities in procuring, fabricating, and distributing xylem filters," says MIT's Prof. Rohit Karnik. "For places where the only option has been to drink unfiltered water, we expect xylem filters would improve health, and make water drinkable."
Open-source instructions for building the filtration system are available online. It can be seen in use, in the following video.