In 2011, World Health Organization (WHO) reported that 768 million people were drinking water exposed to contamination by pathogens and chemicals. These people had no effective means of filtering the water before consumption.
“Low cost, easy-to-use and effective methods of water purification are needed to provide clean drinking water to a significant fraction of the global population,” says Associate Professor Rohit Karnik, mechanical engineer at the Massachusetts Institute of Technology, U.S.A.
Karnik and his research team might just provide a solution. In their paper published last month in PLoS One, they demonstrated for the first time that the xylem of gymnosperm plants can effectively filter bacteria from water.
Xylem tissues in vascular plants work like pipes in which water and minerals move from the roots to the leaves. The xylem tissue of gymnosperm plants, like conifers, is made up of tracheids—short, close-ended segments that are bundled together in parallel. Many pits line the tracheid wall, and the pits are covered with semi-permeable membrane. As water is transported in the xylem, it moves out of one tracheid into another through the membrane of these pits.
Karnik tested the ability of the pit membranes to filter bacteria from water. His team fitted white pine (Pinus strobes) branch cuttings—stripped of its bark—into one end of a plastic tube. They then used this simple setup to filter a solution containing high concentration of the bacteria Escherichia coli.
They found that the xylem filter removed “at least 99%” of E. coli; the filtrate contained E. coli concentrations of 200-750/ml.
Assistant Professor Daniele Lantagne of Tufts University, U.S.A., who has worked with water treatment programs in more than 40 countries, acknowledges the potentials of xylem filters to reduce water-borne pathogens. She emphasizes however, that xylem filters prototypes must first be “tested according to the World Health Organization guidelines for evaluation of household water treatment products”.
According to the current WHO standards, safe drinking water should contain “no detectable E. coli in every 100 ml sample.”
Other water-borne pathogens like viruses may also slip through the xylem. Karnik’s team showed that the xylem filter could not remove particles smaller than 80nanometer. Prof. Jonathan Ball, virologist at University of Nottingham, U.K., noted that “a cut-off of 80nm would remove some (e.g. poxviruses) but not all (e.g., noroviruses) viruses harmful to humans.”
Nevertheless, xylem filters are cheap, light and easy to set up. A possible design, Karnik suggests, could be “a water container placed a few feet above” a faucet that has a “replaceable xylem filter inside”—no pumps required—that could filter 4 liters of water daily. These factors make xylem filters a promising alternative to established water treatment methods like sand filters, solar-disinfection, chlorination and other expensive membrane filters.
“What we have is just proof-of-concept that xylem can be used for filtration,” says Karnik. He hopes to produce a prototype in the next 2-3 years, and is interacting with water treatment experts in developing countries. “Our eventual goal is to enable practical implementation of xylem-based filters.”