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 well-being 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. Illustration courtesy of Nedlaw Living Walls, Inc.

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. Photo courtesy of Nedlaw Living Walls, Inc.