Water Resources Engineering (WRE) connects engineering hydrology and hydraulics with global, economic, environmental, and societal issues. Our student Becky Meissner makes this connection here…

The article “Water filter from the sapwood in pine tree branches” was published on ScienceDaily online on February 26, 2014 in tandem with the publication of the journal article from which its conclusions were drawn, titled “Water Filtration from Plant Xylem” in the open source journal PLOS One. The research discussed draws upon the hydraulics of sap transport from the roots to shoots of a plant to examine water filtration using the same mechanism. This use of natural hydraulics is framed in the context of filtering water in situations where the water’s hydrologic pathway has tarnished its quality, specifically in terms of the presence of bacteria in the water. The researchers examined the ability of white pine sapwood (xylem, the inner vascular tissue of trees) to remove red dye particles and E. coli bacteria from water. The red dye particles removed were all above 70 nm in size, resulting in an easily perceptible change in color (see Figure 1). In addition, more than 99% of bacteria were removed from the sample, agreeing with the conclusion that this filtering technique would work well for bacteria, most of which are larger than 200 nm in size (Figure 2). In terms of water resources engineering, cavitation is known to damage any transport system where it occurs, from pipelines to spillways1, so it is logical that there would be a mechanism in plants preventing cavitation2. The small size of the pit membrane pores that are useful for water filtration are also sensible for plants to have to avoid cavitation by maintaining a high flow velocity (compared with velocity within the transport vessel), according to mass continuity of flow1. Despite the theoretical usefulness of this naturally existing technology, nature itself is more random than constructed filtration systems. There remains a need to provide a larger sample size of experiments reporting success, involving different plant structures & aberrations (eg. bud scale scars) as well as possible comparisons between woody plants and flowering plants, which have longer vessels but better filtration capacity.

This research is incredibly important in the field of global health, in terms of inexpensive water treatment and containment of waterborne diseases. The authors of the original paper introduce their research within the context of the current high monetary, fuel and health costs of other water treatment technologies such as chlorination, synthetic membrane filtration, boiling water and using carbon or sand filtration systems3. Access to these filtration technologies is often limited by economic status and access to electricity or sunlight. The development of sapwood as a filtration technology would bring water filtration to many areas of the world limited by sunlight and electricity availability. An article published in 2008 in Environmental Science and Technology4 discusses available technology with calculated efficiencies in terms of production and bacterial removal, listing all examined technologies as effective, but short-term solutions. In order to make sapwood treatment an option that exceeds the efficiency of currently available technology, a mechanism to develop its long-term sustainability for each user must be developed. When a method of cleaning this natural filter is developed, it can last longer and provide cheap and easy filtration for people all over the world in remote locations using little of their economic and natural resources.

Figure 13. The water with dye before filtration (left) and after white pine sapwood filtration (right).

Figure 1. The water with dye before filtration (left) and after white pine sapwood filtration (right).

Figure 23. SEM image of E. coli trapped in the pit membrane of the xylem after filtration. Scale bar in bottom right is 2 microns.

Figure 2. SEM image of E. coli trapped in the pit membrane of the xylem after filtration. Scale bar in bottom right is 2 microns.

 

References

(1) Wurbs, RA and James, WP. Water Resources Engineering. New Jersey: Prentice Hall Inc. 2002.

(2) Choat B, Cobb AR, Jansen S (2008) Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function. New Phytologist. 177: 608–625.

(3) Boutilier MSH, Lee J, Chambers V, Venkatesh V, Karnik R (2014) Water Filtration Using Plant Xylem. PLoS ONE 9(2): e89934. doi:10.1371/journal.pone.0089934

(4) Sobsey MD, Stauber CE, Casanova LM, Brown JM, Elliott MA (2008) Point of use household drinking water filtration: A practical, effective solution for providing sustained access to safe drinking water in the developing world. Environmental Science and Technology 42: 4261–4267.

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