Sapwood from a tree branch can be used to filter over 99 percent of E.coli from drinking water, according to the latest findings by MIT mechanical engineers, and this low-cost method of water filtration may prove to be a boon to developing countries.
How are the water filtration devices made?
Though likely many types of tree branches can be used to create water filtration devices, the MIT researchers used a small segment from a white pine tree branch that was an inch long. They peeled the bark off of it, and then inserted and fastened it into the end of a plastic tube with the sort of metal clamping band used with hoses in cars.
Then, the water polluted with the E.coli is poured into the tube, and the water which drips out through the xylem in the chunk of wood has been filtered of over 99 percent of the E.coli bacteria — 99.9 percent, to be exact. The chunk of sapwood can be used to filter 4-6 liters of water per day, enough for one person.
The MIT mechanical engineers who thought up this ingenious and inexpensive method of water filtration published their findings February 26 in PLOS One. According to the study’s senior co-author Rohit Karnik, after he listened to a talk about how water and sap flows through plants and tree branches, he realized that it “has to pass through these membranes in xylem.”
In living plants, xylem membranes help transport water and minerals to were they’re needed in a plant or tree, and it transports sap in trees. That talk gave Karnik the idea to try out a piece of sapwood, and find out if it would work as a water filtration method — and it did work, like a charm.
Though he has some reservations about how the water filtration system will handle more heavily polluted water, Rick Andrew, who is the global business development director of water systems at the National Sanitation Foundation International, has stated that the water filtration devices made with sapwood could be a low-tech method to purify water.
Getting easy and inexpensive access to pure water is a major problem in many developing countries. Dead and dry wood will not work however; sapwood must be used to make the water filtration devices,and plastic tubing and a metal clamp needs to be available to make the method work.
Also, it is important to note that while the sapwood is great for filtering particles of the E.coli bacteria which are larger than 70 nanometers wide, it wasn’t successful at filtering out smaller particles in the 20-nanometer range. Since smaller particles can’t be successfully filtered out by the white pine sapwood, viruses, which are smaller, can still get through and won’t be filtered out.
According to Rohit Karnik, it could well be possible that there are plants that would be even better suited as water filtration devices than the white pine branches that they used in the study.
Ideally, the wood that’s used should be thin enough it can be easily transported, but yet be one you can use for a few days before you’d need to throw it away. Karnik added that using sapwood chunks as a part of a water filtration device would be “orders of magnitude cheaper” than the types of “high-end membranes” that are currently available, and also, he reasoned, why should a membrane have to be fabricated, when they are cheaply available in nature?
The membrane-based water filtration systems are often relatively expensive, and some rely on a pump as a necessary part of the device.Others require chlorine, which can be expensive, and — while boiling water can be effective — you need to have enough fuel to be able to boil the water.
The method of using sapwood as a part of a water filtration device is not good for filtering a large amount of water for a lot of people at one time, but it might prove to be an inexpensive way to filter water for individuals.
When mechanical engineers or anyone else attempts to come up with a method to filter water, the ideal outcome is that the membrane or other water filtration method needs to be able to filter out microbes, but do it while maintaining a high flow rate through the filter. Sapwood is perfect to use as it satisfies both of these requirements.
Next, the researchers plan to test out the use of other varieties of sapwood from different species of trees to determine their relative effectiveness as part of a water filtration device. While further experimentation is needed to hit upon the very best types of sapwood — and the types that would be the most efficient and most widely available for using as water filtration devices in developing countries — the initial findings look like sapwood could potentially be a low-cost method of water filtration that eventually be used on a wide-spread basis in developing countries.
Written By Douglas Cobb