Understanding the difference between a green vertical wall and a living wall biofilter.

Updated: Aug 2, 2021

It is commonly noted that people currently spend on the order of 80 to 90 percent of their time indoors. This has several impacts. First it means we are predominantly breathing indoor air and, as a result, indoor air quality (IAQ) has been the focus of numerous studies, standards, and programs that seek to create healthy indoor environments. Common approaches to achieving better IAQ results, particularly in green building design, include careful selection of materials used and increasing ventilation rates. Second, time spent indoors typically means that we are living life deprived of interaction with nature. To overcome both of these indoor environmental concerns, it has been common to incorporate plants into indoor environments. However, potted plants alone can have only limited impacts. An emerging option that is more effective and more appealing for many designs is to use a vertical wall of hydroponic plants. Designed properly, these plant walls not only provide a connection to nature, they can provide real and significant improvements to indoor air quality as well.

Living Plant Walls Overview

There is increasing interest in the integration of natural systems such as living plant walls and green roofs into the built environment. Of the two, living walls is the less mature industry. There has been considerable consolidation in the green roof industry over the past few years with the design community settling on a few well-tested methods of design, typical of sector maturation.

Corus Quay, Toronto, ON

There are two very distinctive categories of plant walls. The first being a plant façade where the plants are rooted at the base of the wall and with the aid of a mechanical system, the plant “climbs” the vertical surface. The second group of walls is where the plants are planted into rooting material that is attached to the vertical surface or “wall” and not at the base as seen in plant façades. While green roofs frequently use prairie grasslands or savannas as a natural analogy for their design, the non-façade type green walls are frequently described as cliff type ecosystems. Two relatively

distinctive approaches are used for the culture of planted vertical surfaces (nonfaçade) and are commonly available. Both are based upon traditional agricultural systems adapted to the vertical plane. The first is simply a modification of conventional potted plant culture. In these systems, plants are rooted into separate pots that may be arranged anywhere from having the pots parallel to the ground and opening outward from the wall to the pots aligned parallel to the wall and opening towards the pot above it.

The second alternative planting approach is to modify traditional hydroponics to the vertical condition. Hydroponics is a method of growing plants that does not use conventional soil. It is commonly thought that plants need soil to survive but this is entirely not true. Plants need water, air, light, nutrients, and support. Soil facilitates many of these requirements but is not in itself required. Hydroponics is a cultural method where the role of the “soil” or planting media has been reduced to little more than a support substrate such that this media does little more than keep the plants from falling over.

Living Plant Walls and Biofiltration

The increasing interest in living plant walls is well founded for a number of reasons; these plant walls can greatly improve the built environment through connecting the occupants to the natural world while occupying only a minimum footprint in the building. The aesthetic appeal of the plant walls is exceptional which has also been demonstrated to improve occupants’ emotional wellbeing. Further, indoor air biofilters (a special subgroup of living plant walls) have been clearly able to demonstrate improvements in the physical qualities of indoor environmental quality (IEQ). In fact recent studies conducted by the University of Guelph in Canada have demonstrated that indoor living wall biofilters reduce common indoor air pollutants by 30 percent. This completely biological (i.e. natural) method of maintaining the quality of indoor air has become recognized as an exceptionally functional and very aesthetic system that can truly enhance indoor environments in many ways. A particularly effective means of creating this indoor biofiltration is to use hydroponically grown plants in the system. By utilizing many of the benefits of hydroponic growing techniques, they are able to integrate engineering technologies to create an interior plant scape that effectively removes common indoor contaminants and improves the living environment. At its heart, the hydroponic plant wall is an indoor vertical wall of green plants. However, the plant wall is most effective when it is actually an integrated part of the air handling system for the building to form a biofilter. Ambient air is actively forced through the wall of plants and as the dirty air from the space comes in contact with the growing (rooting) media, contaminants are moved into the water phase where they are broken down by beneficial microbes in the root zone. Highly specialized biological components on the hydroponic media and roots of the plants actively degrade pollutants such as formaldehyde and benzene in the air into their benign constituents of water and carbon dioxide. The clean air is then dispersed throughout the space by a fan system that may be built into the system or may be remote. In essence, the indoor air biofilter is a part of the air handling system for the building with plants integrated right into it as a living air filter. From an internal processing standpoint, the biofilter is an adaptation of two separate processes. First is biofiltration, which is described as the passing of a contaminated air stream through a biologically active substrate where beneficial microbes use the pollutants (such as VOCs) as a food source. The second process is phytoremediation, which uses green plants to help the growth of these beneficial microbes.

The biofilter improves the indoor environment in a number of ways. First in terms of its impact on contaminant levels in the air, a single pass through a hydroponic biofilter can remove 90 percent of harmful chemicals. Second, a hydroponic biofilter improves the aesthetics of the indoor space. There are increasingly strong links between greening the indoor space and the wellbeing of the occupants. Greening the space has been shown to reduce stress levels, increase work productivity, and reduce absenteeism. Because of the combination of these documented results, plant wall biofilters are one of the few indoor uses of green plants to receive recognition from the U.S. Green Building Council LEED® program as an innovative means of improving the indoor air quality. They have recognized these systems as a unique use of green plants which leads to a substantial positive impact on the indoor environment.

A hydroponic biofilter uses a combination of natural plant systems and manufactured air movement systems to move and filter air for superior indoor air quality.

Much of the effectiveness of the indoor air biofilter is because of the hydroponic nature of the system. To have a real impact on indoor air quality, the biofilter must be able to deliver very large volumes of air in a very efficient manner directly to the beneficial (i.e., contaminant eating) microbes, which are typically found on the plant roots. The beneficial microbes responsible for the degradation of the contaminants in the indoor air biofilter are also present in “normal” soils. But in typical soils whether in vertical walls or with potted plants, the microbes are not adequately exposed to the contaminants to have a substantial impact on air quality. To be biologically degraded, the contaminants must first be exposed to the microbes. This is very difficult in normal potted plants because the pot itself forms a barrier to the movement of gases into the soil. In other words, the container acts as a barrier between the microbes and the contaminants. The soil itself is also an extremely highly resistant pathway for air to interact with the microbes — it is simply too difficult for the air to penetrate into the soil to have any real impact on air quality. Further much of the surface of the soil is covered with plant material which acts as an additional barrier to the exposure of the soil microbes to the airborne contaminants. Instead of soil-based growth medium then, the use of open matted hydroponic rooting material means that air can be easily drawn through the mat, placing the air in close contact with the root zone of the plants and their associated microbes. This type of system can support air fluxes up to 20 cfm per square feet with pressure drops of less than a quarter of an inch of water. In order to understand more about the differences between soil-based and hydroponic-based systems, let’s look at each more in depth.

Issues With Soil in Plant Walls

The inert nature of hydroponic rooting media is very different from the typical cultivation of plants in soil where the rooting substrate facilitates many aspects of the life of the plant. The simplest is that the physical structure and weight of the soil offers the method of anchoring the plant in place. But more than this, natural soil is composed of particles of a range of sizes and origins, with organic and inorganic constituents. These particles can range in size from tiny clay particles to large grains of sand. The small particles clump together to form larger aggregates which give the soil its structure. Tiny spaces between the particles in the clumps can become filled with water during times of plenty and will act as a reservoir for the plant when water is needed. The large spaces between the aggregates act as channels for air to deliver the oxygen required for normal metabolism of the roots and allow for water drainage. Soil can act as a reservoir of nutrients for the plants in a manner similar to its water-holding ability. Electrically charged surfaces of the particles bind with the charged nutrient ions during times of plenty and slowly release the materials to be taken up by the plant. The management of soil-based systems can take advantage of the capacity of the soil to “hold” water and nutrients which works as a buffer to the actions of the manager or gardener. The manager knows they can rely on the soil to add water and nutrients to the plants without their constant input. Soil culture is typically less complicated, but with all of the buffering action from the soil, one never really knows what the plant is getting.

Plants and rooting media need to be benign so as not to introduce anything detrimental into the space they serve, such as this restaurant where a healthy environment is particularly advantageous.

However, soil systems may not necessarily be the cultural method of choice for use in living wall venues. Compared to plants grown in native soils, plants on a wall are much more intensively managed. Being of a substantial shallower profile than field soils, soil on a wall has to be watered more frequently than in the field. Even with careful engineering, repeated watering can break down the aggregates that give the soil its structure, and with the loss of structure, the channels that allow air to be delivered to roots are also lost. Anaerobic conditions are created and literally cause the roots to suffocate from the lack of oxygen. Anaerobic conditions also encourage a number of root pathogens which can also stress the