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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 roots. It is interesting to note that different plants have different tolerances to the anaerobic conditions associated with the flooding of the inter-particle spaces in the soil. We typically think of this as the tolerance of the plant to overwatering but it is actually the lack of oxygen in the soil, not the excess water, that damages the plants. When properly aerated, there is no such thing as too much water to the roots of almost all plants. Another issue that arises from soil being a collection of particles is that these particles will eventually succumb to gravity and therefore cannot be applied to the vertical surface without some sort of containment system. Soil will only work if it is contained in some sort of “pot.” Containment systems that are arranged so that their openings are not horizontal must also contend with the soil slouching out of the container. The soil frequently succumbs to gravity and sloths off the wall. This is also true for the entire plant; with the torque forces that the plant applies to the soil, the entire soil ball can be leveraged out of the container. Both of these issues are frequently addressed by covering the soil with a mesh, leaving only a small amount of soil around the base of the plant uncovered.

Containers used in living wall systems range in size from less than a quarter of a pint (100 ml) to many quarts (liters). But irrespective of their size, the containments offer a physical barrier to the expansion of the plants’ root systems which in the long run will reduce the viability of the plants — i.e., they become pot bound. This is much more of an issue with small containers than large ones and will influence plant selection and longevity of the system. The containment of the soil also creates barriers to the flow of water through the system. Water cannot disperse freely horizontally because of the sides of the container.

Correcting the water flow is sometimes addressed by watering each container separately to avoid the requirement that water flow between vertical containers. This has some advantages but it is difficult to balance water delivery particularly to the entire wall. For example, the upper sections of the wall receive their water from the water supply system while the lower sections of the wall receive water from the water delivery system and drained from the container above. Frequently under these conditions, it is the bottom of the wall that appears to be “overwatered” and exhibits flood injury while the top appears to be under drought stress. The last issue related to the use of soil-based plant walls is the longevity of the soil media. One way for a synthetic or engineered soil to withstand the intensive maintenance of the plant wall is the inclusion of organic material such as peat, fibers, or wood chips. This will help with the soil structure but being organic the material will decompose and the soil media will need to be replaced typically within a few years. This can be a very costly process.

Hydroponic-Based Living Plant Wall Systems

As mentioned previously, an alternative approach to soil-based planting is the use of hydroponic-based living plant walls. Hydroponics is increasingly used in agriculture for high value crops where the reliability of production, and the quality and consistency of the end product are paramount. Hydroponics can be defined as a cultural method where the plants are grown in an inert rooting material. The hydroponic rooting material typically offers little water holding capacity and is simply a means to support the plant (keeping it upright). The hydroponic root substrate is also inert in terms of the plant’s nutrition. All of the plant’s essential nutrients must be supplied from the irrigation water.

Hydroponics has made great headway in the traditional greenhouse production systems for a number of reasons. Agricultural hydroponics are much more sophisticated production systems than traditional soil systems which means that they can be easily integrated in an automated control system, giving better control and monitoring of the plant’s growth environment. While soil mixes tend to be inconsistent because of variability of the various components, the simpler (in terms of its composition) fabricated hydroponic media also streamlines the production of plants. The simple synthetic inert media used in hydroponics, such as rockwool or horticultural foam, gives the provider direct and immediate control over what is happening in the plant’s root environment (the “rhizosphere”). Lastly, hydroponic designs facilitate the efficient use of resources. Although not impossible with soil culture, hydroponic irrigation systems lend themselves to be designed as a closed loop such that the water and its contained nutrients can be collected after use and circulated back through the system. This substantially reduces the amount of water and nutrients used by the system, fitting it better into sustainable design. All these benefits are directly applicable to the living wall venue. Based on the above, it is clear that two critical components of a hydroponic system are the rooting or growing media and the irrigation system, both of which we will look at in more depth.

Hydroponic Rooting Media

The critical component of hydroponic living plant walls is the rooting media. This material must be inert and free from contaminants that could interfere with plant growth. The material must allow water to flow freely while maintaining good air spaces. The main advantage of the hydroponic media is that it retains its characteristics under the very intensive management seen on plant walls. Despite the very rapid and extensive flush of water down the wall, the internal structure of the hydroponic media will need to hold its structure for many years. A number of hydroponic living wall rooting media are now available. Most providers use either rockwool, felt, or a mat material as a rooting substrate. Because of its use in traditional agriculture venues, more is known about the cultivating of plants in rock wool than felt and matting. Rockwool has an advantage in that it has a substantial water holding capacity per unit of surface area. However, when saturated with water, this means that rockwool is quite heavy with weights of the rooting media of 20 pounds per square foot (100 kg/square meter); this is four or five times the operating weight of wet matting or felt. Therefore rockwool requires a substantial support system. Matting and felt are produced as sheets which allows for less restrictions on root growth and more uniform water flow down its face. Also, because of the lower weight of the felt and mat-based systems, less structure is required to support the material. Basically, the media needs a good bulk density to pore space ratio meaning that the media needs to have a great deal of internal space available for both the water and air which are needed for plant growth. This available space can be further refined into the ratio of water holding capacity to available air spaces — i.e. how much of the internal structure is occupied by water and air under normal operating conditions. The media must be capable of holding a good reservoir of water without sacrificing available air channels and allow oxygen to get to the roots. The porosity not only allows good air movement but also good drainage when water is present in excess as happens during watering cycles. The other important consideration with using material with a low bulk density is the ease of shipping and installation. During installation, hydroponic rooting material frequently has weights of less than an ounce per quart (several grams per liter) of volume compared to the soil mixes that are typically on the order of a pound or more per quart (hundreds of grams per liter). This is important if the installation is taking place several stories up in the air. The rooting media must also be benign. This is required first from the basis of toxicity to the plants and other building occupants. The use of the material in the built environment adds a higher degree of concern about the toxicity of the material than typically encountered under “normal” hydroponic situations. This relates to the entire life cycle of the material, under the complete range of conditions and situations found in the built environment. The media should also be inert in terms of the nutrient balance of the plants. The media should never absorb or release nutrients circulating in the irrigation water. This is managed by selecting media with low cation (positively charged ion) exchange capacity (CEC). With low CEC, the manager knows that all the fertilizer that goes into the system is being available to the plants and not binding to the rooting medium. Remember, the best hydroponic rooting media is simply a support system for the plant’s roots. The last factor of concern for the media is longevity. Many organic media such as coir (coconut fiber) are available and work well but simply do not have the life expectancy needed. The material has good water holding capacity and porosity but is broken down within a few months of installation. Mat-based rooting media has been installed in some biofilters for close to 10 years and still meets the performance specifications.

Hydroponic living plant walls are based on vertical applications of traditional hydroponic systems but with particular attention to the growing media and irrigation methods used.

Hydroponic rooting media used in living plant walls must meet all of these criteria and those that are unique to the vertical plant systems. Unlike soil which requires containment systems, mat and felt hydroponic rooting media is self-supporting. The media does not require a container to be used on a vertical surface. The mat and felt material can be installed as continuous sheets which covers a substantial vertical height of the wall. The sheets can be overlapped to ensure adequate hydraulic conductivity between sheets. The height of the single sheet (close to 4 feet or 1.2 meters) minimizes the transitions and the overlap of the sheets minimizes the possibility of perched water tables within the wall. Horizontal spread of water is not truncated by the presence of vertical walls of the containers. Alternatively, the lack of horizontal “baffles” in the hydroponic could lead to the cohesive forces of the water pulling the flowing water into streams. These channels of water would only flow through narrow sections of the media while the rest of the wall would go dry. Selection of media with an adequate grain can further influence the formation of water channels. The grain is determined by the alignment of the media’s fibers. Most of the fibers in rockwool run parallel to its long axis, so installing the pieces so the grain is perpendicular to the flow of water and will discourage channeling and give more uniform water horizontal distribution but may encourage formation of perched water tables. Mat rooting material is a spun polyester mat held together with epoxy resins that has a random alignment of fibers which works to disseminate any channels. The channeling of the water is most pronounced when the water passes through dry media because as with soil, it is difficult to initially wet the media. With experience, it has been found that once wet, the roots of the plants acts as baffles and largely stop the formation of water channels giving good horizontal spread of water.

Irrigation of Living Plant Walls

With the growing media established, the focus now turns to irrigation of the plants. The simplest type of irrigation system is an open loop type design. With this design, water is released to the top of the wall (typically from a domestic cold water supply) and flows down the wall to a drain at the bottom. The water comes in contact with the wall once and is shuttled to the drain. Nutrients are typically injected into the irrigation water or more rarely, integrated to the rooting substrate. Either way, the nutrients that have dissolved into the water and have not been taken up by the plants are washed down the drain along with the water. This approach is extremely consumptive in resources. It places very high demands on the water usage and the waste water system both in terms of volume of effluent generated and nutrient loading of the system. For this reason all but the smallest plant walls should not have open loop irrigation designs.

A simple closed loop hydroponic living plant wall system uses a basin to collect and store water than can be re-circulated to the plants.

A more sustainable design is to install closed loop irrigation system. Here, the water that passes down the plant wall is collected in a reservoir to be reused. Being a closed system, it also means that the nutrients in the water that were not taken up on their pass through the root zone will be available to the plants on their next circuit rather than being ejected into the environment via the drain. Rather than relying on the pressure from the domestic water system to deliver the water to the top of the wall, a closed loop system uses a pumping system to lift the water to the top of the wall. The reservoir may be integrated in the base of the wall or may be remote. There were fears that the cycling of water repeatedly through the growth media would encourage the proliferation of root diseases. As in agricultural hydroponics, it is now believed that this fear is overrated. Root diseases are problematic when the plants are already under stress such as when the plants are being flooded (overwatered). Because of transplant shock, newly established plant walls are susceptible to root diseases and care should be taken at that time. With good plant care, root diseases offer little threat. A certain volume of water is lost to the vapor state as it circulates through the system largely via transpiration of the plants. Therefore the reservoir needs to be topped off either manually or automatically with either a mechanical or electronic system to replace the evaporated water. Alarms can be installed to warn when the levels in the reservoir are either excessively high or excessively low. Systems can also give warning when there is a pump failure to ensure the wall does not dry. To minimize the risk associated with mechanical failure of the circulating pump, it is frequently advantageous to install a second pump and, similar to many other mechanical installations, have them arranged in a lead lag configuration. Integration of the irrigation system into the building automation system ensures good protection of the owner’s investment into the plant wall.

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 wellbeing 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. For more information, please visit: Originally published by: Peter J. Arsenault, FAIA, NCARB, LEED AP, practices, consults, and writes about sustainable design and practice solutions nationwide. 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.


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