Not only do we kill life under the footprints of our buildings. But the processes that produce our building materials, and in other ways support our existence, also produce a toll on the environment. This waste takes the form of air, water, and land pollution. There are other effects as well.
For example, due to the acceleration of Himalayan glacier reduction three major water courses, the Ganges, Mekong, and Yangzi rivers, are providing less and less potable and irrigation water for one of the most, if not the most, densely populated areas on earth. Mark Twain once said, “Whiskey is for drinking. Water is for fighting over.”
The last time I heard many of the countries in this region are having armed conflicts and also several of them have nuclear arsenals. In Europe a similar problem is developing with Alpine glacier reduction. In the U.S. the Ogallala Reservoir system is being increasingly stressed. This is the major source of subsurface water between Montana and Texas. Think Dust Bowl.
Our ability to reuse the water we pull up from wells, melt from glaciers, and collect from precipitation will be critical in avoiding untold misery heading our way. By reducing the extent to which we ship our gray water to salinated water bodies and by increasing our ability to reuse this water, we can start stemming this tide of waste.
I first became curious about this subject when I was trying to figure out a way to put green roofs on structures that were burdened by a large cooling load. Most hot areas are short on water. So, irrigation water for green roofs had to come from some source other than potable sources. Ahah! Gray water! I had heard of plants cleaning grey water from wetland and bio-swale research. In the course of my readings I ran across some phytoremediation literature. This was followed up by an encounter with a Phytoremediation conference I attended almost by accident.
I found out that there are annual conferences, peer reviewed journals, and a growing body of research literature and field studies on the subject. This seemed like a natural fit for the world of green roofs and living walls. The first two things I learned was that it was a new and interdisciplinary field and that this was a complex topic.
The term phytoremediation was first coined in 1995. This shows how new a field of study this is. It is the study of the remediation effects of plants and their immediate environs. Over the past 40 or so years there have been a number of attempts to use plants to clean up our messes. One of the earliest of studies was NASA in trying to find a way to clean up air pollution on space vehicles¹. NASA came up with a list of plants that targeted certain pollutants such as volatile organic compounds, formaldehyde, benzene, etc. NASA also carried out experiments in plants cleaning up NASA sites on earth. When compared to the expense and energy embodiment of manufactured filtration equipment, plants were shown to be more efficient.
The Army Corps of Engineers have been using plants to clean up munitions ranges by planting plants that target pollutants like toluene and heavy metals.
A simple Google search will point out a host of more sources of information on this topic.
As we learn more about phytoremediation mechanisms, we have spotted previous errors in our assumptions and conclusions of these processes. For one thing, there are several forms of phytoremediation. For example, if the process takes place in the rhizosphere, it is called “rhizo,” if in the plant tissue it is called phyto…
Another distinction is the phytoremediation process itself. There is sequestration, hyper-accumulation, volatilization, extraction, filtration, and stabilization that have been identified so far.
Degradation, comprised of rhizodegradation, phytodegradation, and phytovolatilization, molecularly transforms the contaminant. In the rhizosphere (in the area bordered by 1 inch distant from the root hairs) fungi, bacteria, microbes, plant enzymes, and soil chemistry combine to break down contaminants such as petroleum hydrocarbons, PCP, perchlorate, pesticides, PCB’s, and other organic compounds. In the plant tissue, through phytodegradation, organic compounds such as chlorinated solvents, methyl bromide, DDT, PCB’s, phenols, nitriles, and nutrients are chemically transformed. In the leaf area through phytovolatilization plants aerate and dilute pollutants such as arsenic, tritium, mercury, and chlorinated solvents. This process also finishes the job of phytoremediation started by other such processes that partially work on the pollutants before reaching this stage.
Extraction, comprised of phytoextraction/phytomining, rhizofiltration, and phytovolatilization, is a process whereby plants extract contaminants from soils.
Through phytoextraction, metals, perchlorate, and organic chemicals are taken up and concentrated in plant tissues. Through rhizofiltration, fungi, algae, and bacteria bind metals, nitrate, ammonium, phosphate, and pathogens. Through phytovolatilization contaminants such as Se, tritium, Hg, chlorobenzene, and other chlorinated solvents are diluted and dispersed in the air and soils in lower concentrations for further exposure to all the above processes.
Contaminant Immobilization, comprised of phytostabilization and rhizofiltration, prevents contaminant movement, or leaching. Through phytostabilization and rhizofiltration contaminants such as metals, phenols, phosphates are trapped and concentrated.
Other processes such as sequestration, hyperaccumulation have also been used to describe various processes. Fundamentally, plants and their immediate environs of the rhizosphere and the air around the foliage and stems serve to molecularly alter, concentrate and render inactive, or dilute for further processing many contaminants. Researchers at Penn State University have worked with the potential of plants cleaning grey water. Phyto-active living walls are in use cleaning indoor air pollution.
Different plants have different processes and targets. Their effectiveness can be greatly enhanced by our controlling what goes into the gray water in the first place. It can also be enhanced by some simple filtration and aeration mechanisms to prepare the pollutants for processing by the plants and their allies.
Each of these species has specific targets, carries out its phytoremediation activities with one or more specific phytoremediational process, which may or may not include a number of regions of activity, from the rhizosphere to the air sheath adjacent to the leaf and stem structure.
In providing a proper habitat and ecology for these plants designers have to consider the impact of plant metabolic rates on the effectiveness of the phytoremediation process chosen. A plant in dormancy has a very low metabolic rate. This effects transpiration rated as well as the speed of biochemical activities. This has direct bearing on how much cleaning takes place. Another factor is what I call the “second hand of green roof design.” The first hand is finding plants that work well on a particular roof. The second hand is designing one’s roof so certain plants are happy.
Good green roof design does both at once. This is especially true once we depart from the “as light as we possibly can and as drought tolerant as possible” and approach the realm of the multifunctional green roof, the truly working roof.
This is a very knew area of scientific inquiry and we are just scratching the surface. However, enough successful work has been done that compels us to start incorporating these tactics and strategies in our built environment as partners in progress. Our green standards are abysmally too low for any serious self-congratulations. By using grey water as green roof and living wall irrigation now, we can start evaluating the phytoremediation effects as we go. After mechanical filtration and phytoremediation, along with control of the contaminants we introduce into our environment, I see no reason why we can’t start using green water for a wide array of uses from laundry to washing food to washing ourselves, to irrigating food crops, to irrigating green roofs and living walls in areas short on potable water.
The Active Phytoremediation Wall System; Graphic by: CASE and SOM via Inhabitat
In addition to the free literature mentioned above, there will be two discussion panels on this subject in the Greenroofs & Walls of the World™ Virtual Summit 2013, starting yesterday, February 12 through March 13, 2013 held by Greenroofs.com, in association with the World Green Infrastructure Network (WGIN).
The beauty of a virtual conference is that you never have to miss events that are scheduled live. You can always return to a virtual venue later and see everything on demand at your convenience.
 B.C Wolverton, Anne Johnson and Keith Bounds, “Interior Landscape Plants for Indoor Air Pollution Abatement, Final Report –September 15, 1989.” Stennis Space Center, Mississippi, MS 39529-6000.
Publisher's Notes: We are extremely fortunate to have Patrick participating in a live Q & A Session regarding this article on February 13 at 4:00 pm EST plus leading our two Panel Sessions on Phytoremediation. Don't miss:
Patrick Carey, GRP, has a degree in architecture and lives in Seattle, WA, and also has backgrounds in Philosophy and Professional Theatre. Director of the Northwest EcoBuilding Guild's Green Roof Project since 2000, Patrick is also principal of hadj design, a green roof design-build company. hadj has designed and installed over 75 green roofs that range in size from chicken coops to complete houses to commercial installations. hadj design has pioneered the cross-training of its crews in all aspects of green roof installation and has taken on the challenge of getting green roofs of all scales up and running.
Patrick is also a trainer for the Green Roofs for Healthy Cities' Green Roof 101, 201, 301, & 401 Courses.
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