A team of researchers at Massachusetts Institute of Technology has discovered natural and appealing way to use wood filters made from tree branches to purify drinking water. The filters can produce about four liters of drinking water a day – enough for the purposes of one person – more cheaply than chlorine-based purifiers or other high-tech filtration membranes. It is fashioned simply by removing the bark and cambium (i.e. the outer layers) of a tree branch, leaving the xylem – the water-conducting inner layer of the branch – called “sapwood.” The sapwood is then inserted into a plastic tube, and sealed with epoxy, or with a tube fastener. The water is then subjected to pressure and passed through the tube. The wood filter works because the pores in the xylem are the correct size to allow water to pass while prohibiting most microbes from passing through the tube. By using this filter, a person can remove more than 99.9 percent of E. coli bacteria from their drinking water. This compares favorably with ethanol-based hand sanitizers that boast of clearing 99.99 percent of germs. The wood filter works because the wood tissue has pores that are small enough to allow water to pass but not the microorganisms.
With regard to the filtering capacity of the wood, not all microbes are the same. Microbes come in roughly three sizes – large, medium, and small. In the large category are one-celled animals called protozoa: this category includes the amoeba and paramecium familiar to anyone who has taken a general biology laboratory class. Protozoa tend to be five to 20 micrometers (a micrometer is one thousandth of a millimeter) which is large enough to be seen with the unassisted eye, although they can be seen better with the aid of a lighted microscope. Bacteria fall into the medium category. E. coli can be 0.5 to two micrometers in size, which is too small to be seen without a microscope. Viruses fit into the smallest category; they tend to be one tenth the size of bacteria; too small to see even with a light microscope. To directly visualize a virus, one must make use of an electron microscope.
So what microorganisms does one have to worry about? Most of the disease-causing microbes that contaminate the water supply are either coliform bacteria, giardia, cryptosporidia, or hepatitis A. Coliform bacteria are a group of bacteria (including e. coli), that live in soil or vegetation and in the gastrointestinal tract of animals (“coliform” related to the “colon” – get it?). Most of them are innocuous, but some produce toxins that cause severe gastrointestinal discomfort.
Giardia are flagellated parasitic protozoa that live in the intestines of humans and animals. They are the causative agent of giardiasis, a disease characterized by symptoms such as nausea, diarrhea, abdominal cramps, weight loss, and gastrointestinal misery generally. Cryptosporidium parvum is another protozoan parasite that causes cryptosporidiosis. Cryptosporidium is spread by drinking water which has been contaminated with infected fecal material. Cryptosporidiosis is not usually fatal, although it can present a lethal threat to infants, the elderly, and the immune-compromised.
Hepatitis A is a small enteric virus, transferred through water contaminated by sewage that contains the virus. Symptoms include inflamed liver, weakness, nausea, lassitude, anorexia, fever and jaundice. The infection can be mild, requiring a couple of weeks of rest; or severe, which may result in damage to the liver or death.
There are already a number of ways currently available to purify drinking water. Physical processes use slow sand filters or biologically active carbon. Chemical processes include chlorination or flocculation (aggregating solid material in the water). Ultraviolet light can also be used – the ultraviolet light causes DNA damage that kills the microbes, but leaves the water unaffected. Microbes can also be eliminated by boiling. Unfortunately, these methods are either expensive, or beyond the technical capacity of the people who are most likely to be in need of clean drinking water. Boiling doesn’t require chemicals or high-tech equipment, but it is costly in terms of fuel.
There are, as yet, a few hurdles to be overcome in the use of wood filters. First among these is the problem of keeping the wood moist, as drying out completely eliminates its filtering capability. As is works out, coniferous (evergreen) trees are better suited for the purpose of filtration than deciduous trees, due to the structure of coniferous sapwood. Angiosperm wood, though, has smaller pores than coniferous wood; thus wood filters made from angiosperm branches can actually filter out virus particles, while coniferous wood cannot. The researchers have only tested one sort of pine tree as yet, however, and they hope to find a coniferous tree with smaller pore sizes. And finally, the wood filter is only useful for removing biological agents from the drinking water: some other technology must be employed for removal of chemical contamination, if it exists.
In conclusion, wood filters offer tremendous promise for providing safe drinking water to communities in need that cannot afford the more expensive methods for water purification. And perhaps the most appealing feature of the wood filter is its zen-like, back-to-nature simplicity. It makes one want to try it at home.
By Laura Prendergast