Using trapezoidal rather than round ducts simplifies the mounting of the ducts to the structural wall and mounting the media to the ductwork. These trapezoidal ducts are mounted onto metal panels which are shingled onto the structural wall. The growth media goes on as two overlapping layers, each about 2 cm thick. Pumped water trickles down through the core of the two layers creating the vertical hydroponic system. Media have very little nutrient holding capacity, so nutrients for the plants and microbes must be delivered via the circulating water in the form of low concentrations of hydroponic fertilizers. Although the amount of fertilizer added is dependent on time of year (there is a seasonality of plant growth even indoors), rates of nutrient additions are roughly a tenth to a fifth what is used in most “agricultural” applications. Specific hydroponic fertilizer mixes are commercially available and should be used.

The wall is planted after the rest of the system is in place and tested. Although the infrastructure is typically installed at the final stages of the rough construction (i.e. the drywall being taped), the planting tends to happen close to the occupancy date. Rather than pre-growing the plants in the growth media, commercially produced mature plants are obtained from open market sources. These potted plants are carefully bare-rooted to remove the soil from the root mass and transplanted into the biofilter.

Installation of the plants into the biofilter system with the roots exposed and passed through slits in the growing media. Photo courtesy of Sean Corbett, Drexel University

To transplant, the roots are carefully slid through small slits cut in the outer layer of the media into a pocket between the two layers. The roots are then in the wet zone from the water circulating through the biofilter. After little more than a couple of weeks, these transplanted plants have re-established their root systems to the point that they could not be easily pulled from their slits. Over time, the roots can extend 6 to 10 feet or more (several meters) from the plant.

The plants used in the living wall biofilters fall under the general category of “foliage” plants. The major groups of plants include Ficus spp., Dracaena spp., Philodrenon spp., and Syngonium podophyllum. Each “type” of plant includes a number of species and/or varieties meaning there are more than 30 different types of plants that are typically used.

Plants are selected based on four criteria. First, plants are selected based upon their ability to form good relationships with the beneficial microbes that do the actual cleaning of the indoor air. Second, plants are selected that tolerate the unique conditions of the vertical hydroponic. Not all plants do well in the growth media. As noted above, these indoor air biofilters tend to focus less on the smaller herbaceous material than other plant wall systems. Third, the plants are selected also match the specific conditions of each installation in terms of light, temperature, and water conditions. The fourth factor is design. Leaf color, shape, and texture give the wall its distinctive look. Plant size is also taken into account in the design. Varying plant size gives the wall more visual depth. Large walls can easily handle plants over 3 feet (1 meter) in height which would be inappropriate on a smaller wall. Because of the differences in size, each plant covers between 1/2 to over 3 square feet (0.05 and 0.30 square meters) of wall area. This gives a final typical plant density of approximately one plant for every 1.3 square feet (eight plants per square meter), providing 70 percent coverage of the biofilter immediately after planting.

Although the installation of the actual biofilter is typically carried out by a single subcontractor, its integration of particularly larger systems into the building requires careful coordination with the entire design team. Providing an adequate environment for the biofilter requires supplying enough natural and artificial lighting for plant growth, which of course has both architectural and electrical considerations. Moving this volume of air through the biofilter means the system must be fully integrated into the mechanical design; control and monitoring of the biofilter has to be interfaced with any building automation or energy management system. Structural supports will of course also need to be provided for the substrate to mount the biofilter onto as well as the design of systems to service the biofilter after installation.

Conclusion

Living walls are an increasingly used architectural feature. Designers must decide between either soil-based or hydroponic systems. Hydroponic living wall systems have many advantages over traditional soil-based systems. The use of this more sophisticated approach addresses many issues inherent in soil-based systems. An indoor air biofilter is an aesthetic special case of hydroponic living walls that uses natural processes to clean and improve the overall indoor air quality in buildings. They contribute to green building and sustainable design by improving the indoor environment, the health and well-being of building occupants, and optimize the use of energy for ventilation. Architects who choose to properly design and specify hydroponic indoor air biofilters into buildings can provide their clients with all of these benefits while creating dramatic and appealing indoor spaces.

Peter J. Arsenault, FAIA, NCARB, LEED AP, practices, consults, and writes about sustainable design and practice solutions nationwide. www.linkedin.com/in/pjaarch

Alan Darlington, PhD, is a researcher in addition to being the founder and director of Nedlaw Living Walls. In 2001, after his first award of the Martin Walmsley Fellowship, Dr. Darlington commercialized the product of this research through the company, Air Quality Solution, which merged in 2008 with the Nedlaw Group to form Nedlaw Living Walls. www.linkedin.com/pub/alan-darlington/9/97a/a6

 

The science behind Nedlaw Living Walls indoor air biofilter had its start back in 1994 at the Controlled Environment Systems research facility at the University of Guelph, in Ontario, Canada. Early research was funded by the Ontario Center of Excellence (OCE) and by the European and Canadian Space agencies. The group gained worldwide recognition for their use of biological systems to improve indoor air quality. www.naturaire.com