VERTICAL GREEN WALLS vs BOTANICAL BIOFILTERS
What’s the difference?
While visually vertical green walls and botanical biofilters are almost identical, as the most prominent feature in them being the plants, there is a lot more going on under the hood of a biofilter than one usually realises. Botanical biofilters are distinctly made to have an increased air purifying capacity which entails a specialised substrate, ventilation components and other quirks of the system that combine and increase the sum of the powers of nature and technology.
Green wall benefits
Vertical green walls are used mainly for the purpose of aesthetics. People have a natural affinity for foliage, and the presence of nature, even if it’s purely visual, has a positive psychological effect on people. This can be explained by the phenomenon known as biophilic design. It has been proven that biophilic design reduces the level of acquired stress and improves cognitive abilities. This, in turn, increases employee and client satisfaction and reduces the number of days employees are absent from work.
But quite often green walls are marketed as much more than just indoor beauty gardens. They are presented as being capable of improving the indoor environment in a more physical sense – by removing pollution and increasing air humidity.
Green walls and pollution
While it’s true that plants are able to do so (just imagine how fresh the air of a forest feels), this effect is abysmally small. In one study it was determined that around 5 potted plants per square meter would be needed to remove all air pollution in a residence. This can be explained by the fact that most of the air pollution remediation is done by microorganisms living in the substrate, and that the air pollution needs to be brought to them, which in classical green walls is done only passively, or how much air is exchanged by chance. If a packed substrate like soil is used, only 5 cm of it is involved with the process.
Green walls as humidifiers
The same can be said about humidification. The amount of water released in the air is capped at how much is used in the system and usually this amount is no more than what potted houseplants would need, which is around a glass of water a week. Water evaporation is dependent on the vapour pressure present on the surface of water. If this vapour is only taken away passively, as is in vertical green walls, the rate of humidification is diminished.
Plants as CO2 removers
Also, plants usually have little to no impact on CO2 concentration indoors. It is so, because CO2 is removed via photosynthesis and indoor light levels are usually far too low for any significant impact on the rate of this chemical reaction. One would need to keep around 200 – 400 houseplants close-by to remove the CO2 they exhale, which basically means living in a small greenhouse.
Green wall mistakes
Furthermore, vertical green walls can even damage air quality. One is by releasing mould spores, which are created by fungal growth on humid, organic substrates like soil. Two – unfortunately, sometimes flowering plants that propagate via air are used in green walls, which make allergy inducing pollen. This should be a big no-no for gardens located indoors.
Botanical biofilter advantages
Botanical biofilters are designed to overcome most of the handicaps that are present in its grandaddy – the vertical green wall.
Most importantly, air is forced through the system actively, which means more pollution can be processed. Most dynamic botanical biofilters use a substrate that has high air permeability and surface area, like ceramic granules, thus increasing the amount of pollution that can be adsorbed. This substrate has to always be wet not only for the pollution to be absorbed, but also for microorganisms to be physiologically active and able to “eat” the pollution. Large surface area, air exchange rate and perpetually wet substrate means that a lot of water is used thus the rate of air humidification is greatly increased.
Microorganisms over plants
Plants in a botanical biofilter have more of a supporting role. They regulate the chemo-physical properties of the system and promote the growth of microorganisms by root exudates (acids and sugars) that the microorganisms feed on. While the microbes of classical biofilters are fed by inserting sugars or other feed in the system, plants are used in botanical biofilters because they have a stable rate of exudate excretion and because of the positive psychological impact described previously.
Unfortunately, while CO2 concentrations are usually the air pollution problem, people have most experience with, carbon dioxide remediation is a tall order. Pollution, like volatile organic compounds, can be abated because it makes up parts per million or even parts per billion in the air, while CO2 is a gas that is normally around 0,04% of the air volume or 4 parts per ten thousand particles in the air. It is indeed a problem of scale. While strong light can be used to increase the rate of CO2 absorption in plants, it is usually too costly, and the gain too small for it to be viable.
Lastly, inorganic substrates are usually used to increase the longevity of the system, which means that there is a lot less chance of mould propagation, as it needs organic matter to grow on. While it would be a lie to say, that there is no mould at all in dynamic botanical biofilters, because plants grow, roots and leaves fall and are decomposed just like in nature and also by fungi, there are no perpetual mould “carpets” that can sometimes be seen on soil.
Improving indoor air quality
While nature is capable of many beautiful things, including the restoration of good air quality, engineering a system is needed so these properties can be embiggened to a scale where they make a difference. Vertical green walls can make a space pleasing to the human eye, but botanical biofilters will be those that not only look the part, but improve the environment in a real physical sense. This is the strength of technology – when used correctly and precisely – synergy of the natural and clever to create a better world.
Irga P.J., Pettit T.J., Torpy F.R. 2018. The phytoremediation of indoor air pollution: a review on the technology development from the potted plant through to functional green wall biofilters, – Rev Environ Sci Biotechnol, 17: 395-415.
Pettit T., Irga P.J., Torpy F.R. 2018. Towards practical indoor air phytoremediation: A review, -Chemosphere, 208: 960-974.
Soreanu G., Dixon M., Darlington A. 2013. Botanical biofiltration of indoor gaseous pollutants – A mini-review, – Chemical Engineering Journal, 229: 585-594.
Torpy F.R., Irga P.J., Burchett M.D. 2014. Profiling indoor plants for the amelioration of high CO2 concentrations, – Urban Forestry & Urban Greening, 13(2): 227-233.