Phytoremediation and Greenwalls: The Next Revolution in Interior Plantscaping
It might sound like something from science fiction—but in the very near future most skyscrapers will probably incorporate greenwalls in their design to reduce heating and cooling costs and provide fresh, healthy air for the people inside.
The use of greenwalls and phytoremediation in interior landscape design is not a new or revolutionary concept—since the earliest civilizations, gardens and plantings have been used to visually enhance interior walls and provide places for growing value-added plants. In our own time the use of plantings on vertical spaces has been found to provide aesthetic and economic value and to promote the improved health and mental well-being of building occupants as well.
The historic challenge with trying to use greenwall systems has been the difficulty of growing on a vertical plane versus a horizontal one. In order to deal with this issue most early designs (dating from the 1970s) incorporated a step system to go vertical, basically using a series of offset planter boxes stepping back one on top of the other to allow light and service access.
The problem with this system is that although the plants are going up the wall, the visual appeal is lost once the line of sight is above the height of the viewer. Vertically-growing plants can't be seen, so plants with low-growing vine habits like ivy were typically used to grow over the edge of the boxes and cascade down the sides for visual effect.
Maintenance of these systems was pretty daunting—with limited access to the highest steps, the plants were difficult to access for pruning, fertilizing or watering. Few systems in the ‘70s or ‘80s included automated watering systems, either, so crews had to be sent in regularly to water and clean the plants.
A box two stories up a wall was just not feasible to maintain—typical atrium plantings of potted or raised bed-planted tropicals on a horizontal format became common and unless vertical plantings were in easy proximity for maintenance these systems were usually eventually replaced with artificial treatments.
Phytoremediation only began to gain credibility in the last 25 to 30 years. In the 1970s, NASA conducted studies on the use of plants to clean air in confined spaces—the idea was that as the plant transpired, it would take in carbon dioxide (CO2) and other airborne volatile compounds and respire oxygen (O2), thereby ‘remediating’ stale air.
The studies gave birth to a ‘plants for clean air’ campaign, promoting the use of indoor plants to help clean the air in old buildings with limited air-exchange capabilities—and the term ‘phytoremediation’ was coined to describe the process.
The development of phytoremediation systems as a process to reduce VOCs (volatile organic compounds) and CO2 in the air of enclosed buildings like skyscrapers has been an exciting new frontier for architects and horticulturalists. These systems combine the aesthetic appeal of a greenwall plant system with the useful function of plant transpiration to absorb and break down airborne contaminants while reducing energy use.
The process of using plants as a bio-filter—where old air passes through plant leaf and root systems and is cleansed, then is recirculated into the building space—provides a number of benefits. Regulations require the introduction of fresh outside air into a building’s HVAC (heating, ventilation and air conditioning) network and the venting of old air out.
The ‘fresh’ air—which is sometimes actually dirtier than interior air—then has to be either heated or cooled, depending on the season. If stale interior air could be remediated and recirculated, though, building operators could save significantly on the costs of heating and cooling fresh air.
In addition, studies have shown that ‘sick building syndrome,’ thought to be caused by dirty air, is deleterious to the health and mental well-being of building occupants—with the use of phytoremediation systems, however, worker sick days and medical claims decrease and worker productivity actually increases, both from the health benefits of breathing clean air and the comfort of having plants in the workplace.
So how do you build a plant bio-filter? The engineering challenges related to this task start with having a system that can integrate into the buildings HVAC system. Much like an air handler in a home’s central air system, a bio-filter system would require old air to be drawn into a common area, forced through the bio-filter and then recirculated through the building. Under this model, the best candidate for the location of the bio-filter would be the building lobby.
The next challenge is to design a bio-filter that will maximize the surface area of both leaf and roots so the air can pass over and easily flow back into the building. One solution is to build a wall, with plants growing on the vertical plane. Imagine a multistory honeycombed structure where the holes in the honeycomb have plant chambers installed and the surface of the growing medium is actually on a vertical plane. Old air could pass through the leaves, into and through the medium, over the roots and be channeled back into the HVAC system.
While this sounds like a simple solution, growing plants successfully in this manner presents a number of challenges. For instance, it requires a plant stabilization system—including a removable container that allows air movement—as well as a growing medium that allows high air-filtration without decomposition and irrigation that works effectively on a vertical plane.
Fertilization, maintenance, light, airflow and several other variables need to be considered too, along with choosing appropriate plant varieties to use in the wall. These challenges are currently being addressed by a number of companies attempting to enter the relatively new and burgeoning building phytoremediation market.
Although there are more than 20 varieties of plants that perform well at capturing VOCs and other air-based compounds, not all will work well in a vertical format and some are not aesthetically pleasing. The growing medium needs to be lightweight and very stable with no decomposition, good water-holding capacity, high porosity for air movement and the ability to harbor beneficial bacteria and microbes.
Plant stabilization is required to hold the plant in place vertically and the medium must be contained as well so it does not fall out of the container or the front. The plant containers themselves need to be structurally rigid enough to hold 30 to 40 pounds of plant irrigation system and growing medium, but have to be open enough to allow airflow. They also have to be easily removable (plug-and-play, if you will) so individual containers can be installed, replaced or maintained without too much difficulty.
Irrigation is a big issue as well—water will flow down, pool or puddle and it can harbor mold, algae and mildew as well as promote humidity in the recirculation system. A vapor system will deliver water in a warm vapor, allowing it to rise through the growing medium and condensate for plant availability, but its delivery must be frequent enough to prevent the media from drying out.
Lighting must also be addressed—not all systems will face the existing lighting, so supplemental lights will be required.
All in all, the benefits of integrated phytoremediation systems for buildings far outweigh the challenges scientists must face in developing them. There are a number of companies and researchers with systems in the development and pre-launch stages now, with clients ready and waiting to install them into their facilities when all the bugs are ironed out.
The health advantages to be gained from breathing better-quality air as well as the long-range economic returns from saving on heating and cooling costs make these systems very attractive to building owners and managers. From skyscrapers to mobile homes, phytoremediation systems can be scaled to any size of facility and should eventually be affordable for any budget.