Xylem Filtration

Project Overview

Objective: Characterize xylem as a filter, determine minimum length to reject bacteria, and develop method for re-wetting to retain filtration properties

Role: Undergraduate senior thesis, primary researcher for topics covered in this paper, designed & performed experiments, wrote final technical paper

Skills: Sample preparation, hypothesis testing, statistical analysis, UV-Vis turbidity testing, Dynamic Light Scattering, general lab work, Technical writing

Result: Determined length based on cutoff size of feed solution and identified two polymer coatings that allow for re-wetting while retaining filter properties

Project Description

Safe drinking water remains inaccessible for roughly 1.1 billion people in the world. As a result, 400 children under the age of 5 die every hour from biological contamination of drinking water. Studies have been done to show that plant xylem from the sapwood of coniferous trees is capable of rejecting 99.99% of bacteria from feed solutions. Additionally, 4 L/d of water can be filtered with a ~ 1 cm2 filter area using a transmembrane pressure of 5 psi, an amount sufficient to meet the drinking needs of one person. However, the main drawback of xylem is that its permeability drops by a factor of 100 or more after being left out to dry for only a few hours. This project sought to characterize the performance of the xylem as a filter, determine the minimum length at which the xylem is effective for filtering bacteria, and increase the xylem’s ability to rewet (retaining its permeability and rejective capabilities) after drying through the use of polymer coatings. Potential techniques for decreasing the minimum particulate size the xylem can filter were also investigated, with the aim of allowing the membrane to filter viruses.

The xylem exhibited different permeabilities and rejection rates depending on the direction the feed solution was flown through it. To maximize both of these properties, the side of the xylem that normally faces the pine needles should be loaded into the hosing first. These differences were small but still statistically significant. For bacteria and other particulates in the 1 µm range, a sample length of 0.25in was determined to be sufficient for complete rejection. Since the transmembrane pressure required to drive flow through the xylem scales with length, this would also be the optimal size for removing bacteria from the feed solution. However, for particles that are below the cutoff size of 100 nm, the rejection rate increased with the length of the sample. So, the optimal length of the sample is highly dependent upon the composition of your feed solution. As a real world parallel, water from the Charles River in Cambridge, MA was filtered using the xylem. When observed under a microscope using a hemocytometer, the xylem appeared to remove many of the biological agents that were in the feed solution. As with all dead-end filtration setups, the xylem filter is susceptible to fouling, and so the permeability of the xylem decreases with continued use. It has been shown that the mechanism by which the xylem fouls depends greatly on the feed solution, specifically the size of the dissolved particles that are being retained. Both molecular weight 10K and 40K PVP showed promising results as potential coatings to solve the drying problem xylem face. Overall, plant xylem from the Eastern White Pine, Pinus Strobus, shows remarkable potential as a low-cost, biodegradable water filter for developing countries to remove bacteria sized particles. The entire system can be powered entirely off gravity fed flow and scaled in size to accommodate both individual needs and supply potable water for a community.

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