Faculty Research: Professor Rohit Karnik

Need a Water Filter? Peel a Tree Branch

By Jennifer Chu, MIT News Office (edited for MechE Connects)

 

If you’ve run out of drinking water during a lakeside camping trip, there’s a simple solution: Break off a branch from the nearest pine tree, peel away the bark, and slowly pour lake water through the stick. The improvised filter should trap any bacteria, producing fresh, uncontaminated water.

In a paper published in the journal PLoS ONE, co-author Associate Professor Rohit Karnik and a team of researchers demonstrate that a small piece of sapwood can filter out more than 99 percent of the bacteria E. coli from water to produce up to four liters of drinking water a day – enough to quench the thirst of a typical person. They say the size of the pores in sapwood – which contains xylem tissue evolved to transport sap up the length of a tree – allows water through while blocking most types of bacteria.

Close Up of Bark Filter

Karnik says sapwood is a promising, low-cost, and efficient material for water filtration, particularly for rural communities where more advanced filtration systems are not readily accessible.

“Today’s filtration membranes have nanoscale pores that are not something you can manufacture in a garage very easily,” Karnik says. “The idea here is that we don’t need to fabricate a membrane, because it’s easily available.”

 

Tapping the Flow of Sap

There are a number of water-purification technologies on the market today, although each has its own benefits and drawbacks. For people at the bottom of the economic pyramid, high upfront cost is a major deterrent to using water purifiers.

Sapwood may offer a low-cost, small-scale alternative. The wood is comprised of xylem, porous tissue that conducts sap from a tree’s roots to its crown through a system of vessels and pores. Each vessel wall is pockmarked with tiny pores called pit membranes, through which sap can essentially hopscotch, flowing from one vessel to another as it feeds structures along a tree’s length. The pores also limit cavitation, a process by which air bubbles can grow and spread in xylem, eventually killing a tree. The xylem’s tiny pores can trap bubbles, preventing them from spreading in the wood.

“Plants have had to figure out how to filter out bubbles but allow easy flow of sap,” Karnik observes. “It’s the same problem with water filtration where we want to filter out microbes but maintain a high flow rate. So it’s a nice coincidence that the problems are similar.”

 

The Size is Right

To study sapwood’s water-filtering potential, the researchers collected branches of white pine and stripped off the outer bark. They cut small sections of sapwood measuring about an inch long and half an inch wide, and mounted each in plastic tubing, sealed with epoxy and secured with clamps.

The team flowed inactivated, E. coli-contaminated water through the wood filter. When they examined the xylem under a fluorescent microscope, they saw that bacteria had accumulated around pit membranes in the first few millimeters of the wood. Counting the bacterial cells in the filtered water, the researchers found that the sapwood was able to filter out more than 99 percent of E. coli from water.

Karnik says sapwood likely can filter most types of bacteria, the smallest of which measure about 200 nanometers. However, the filter probably cannot trap most viruses, which are much smaller in size, or salt.

 

Making Filtration Devices

Building upon these results, Krithika Ramchander, an MIT Tata Center fellow and mechanical engineering student, is working with Karnik to figure out how to make filtration devices from xylem. “The properties of xylem as a filter are not well known,” says Karnik. “We have conducted several studies to advance our understanding of this material, with the goal of being able to design filtration devices.”

Ramchander has already developed a simple process to dry xylem filters without blocking them – a critical problem identified early on in the study – and has demonstrated gravity-driven filtration.

This research was supported by the MIT Tata Center and the James H. Ferry Jr. Fund for Innovation in Research Education.