HOW POLLUTANTS ARE REMOVED FROM INDOOR AIR
There are many methods for removing pollutants from indoor air, but most buildings use replaceable filters built into their heating, ventilation, and air conditioning (HVAC) systems. The filters used by most buildings focus on removing PM from the air, however, some buildings use additional filters designed to remove VOCs and other chemicals (Seppanen & Fisk, 2006).
In addition to these more traditional methods for removing pollutants from indoor air, natural methods such as plants and algae can also be used to filter the air indoors (Pettit et al., 2018). Unlike HVAC filters that rely on capturing pollutants, plants are able to break down chemical pollutants into CO2 and other less harmful products (Mikkonen et al., 2018).
TRADITIONAL HVAC SYSTEMS
To improve IAQ in buildings with HVAC systems, outdoor air is first brought into the building. The air from outside typically only passes through a particle filter to remove PM from the air (Wang & Zhang, 2011). This filtered air is then heated or cooled according to what the building temperature is set to and is pumped into the building, and the air indoors is pushed out through the vents (Llewellyn et al., 2002).
LIMITATIONS OF TRADITIONAL HVAC SYSTEMS:
The simple particle filters used by many buildings generally remove very little PM, such as the minimum efficiency reporting value (MERV) filters that remove less than 20% of particles from the air (Stephens & Siegel, 2013). Using a more complex filter system to remove more pollutants from the air requires a more powerful fan system, increasing the energy consumed by the building (Pettit et al., 2018).
PLANTS AND GREEN WALLS FOR CLEANING INDOOR AIR
Plants are known to remove pollutants from the air around them, especially CO2, VOCs, and even PM through a number of natural processes (Irga et al., 2019; Torpy et al., 2014; Wang et al., 2014). These processes can be physical, such as particles sticking to the plant leaves (Irga et al., 2019; Paull et al., 2019), or chemicals, such as CO2 and some VOCs being absorbed and converted into sugars by the plant (Mikkonen et al., 2018; Su & Lin, 2015).
Green walls, also known as plant walls, living walls, or vertical gardens, are a collection of systems that allow plants to be grown along vertical surfaces (Bustami et al., 2018). Respira is classified as an active hydroponic green wall. ‘Active’ refers to the fans within the system that direct air through the root zone of the plants; ‘Hydroponic’ because the plants are grown using water rather than soil (Pettit et al., 2018).
HOW GREEN WALLS REMOVE POLLUTANTS
Although it may be assumed that the plants in the green wall are largely responsible for removing pollutants from the air around them, most pollutants are removed by the microbial ecosystem of bacteria and fungi living in and around the plant’s roots (Mikkonen et al., 2018). This microbial ecosystem is very diverse, with many different species being specialized to break down and metabolize a variety of chemicals, including many VOCs (Fleck et al., 2020). However, in order for the root microbes to remove VOCs, they must be able to access the polluted air.
Respira is designed with this fact in mind, as it brings polluted air directly to the plant roots using its fans and watering system. Since bacteria and fungi do not have lungs, they are not able to access pollutants in the air directly, instead, they must absorb pollutants through their cell membranes (Zhang et al., 2013). By using fans to draw air through the plant roots and a hydroponic growth medium, a pressure drop is created (Irga et al., 2018), allowing Respira to help direct VOCs out of the air and into the watering system. This allows the roots of the plants, and their microbial ecosystem, to be in direct contact with pollutants from the air.
IMPLEMENTATION
Thanks to grants and funding through MITACS and OCI, the VOC removal efficiency of Respira was tested at Toronto Metropolitan University (TMU), along with an investigation of the types of bacteria and fungi growing in the roots of the green wall plants
REMOVAL OF VOLATILE ORGANIC COMPOUNDS BY RESPIRA:
Preliminary tests of Respira show promising results for its ability to remove VOCs from the air. These tests were done in a sealed 30ft chamber where polluted air was allowed to circulate through Respira for 72 hours, with the exception of the dry control.
To determine how effective Respira is at removing VOCs, it was compared to other conditions, known as controls. For all tests, the fan and lighting systems were turned on. The first condition was the “dry” control, where Respira had no plants or water. The second condition was the “water” control, where water and fertilizer were added into Respira. The final condition was Respira when it was fully functional, with plants, water and fertilizer.
In the preliminary tests of Respira, three main VOCs were tested: benzene, toluene, and tetrachloroethylene (PERC). Benzene and toluene typically enter indoor air through adhesives, solvents, paints, and tobacco smoke (Health Canada, 2011, 2013). Tetrachloroethylene is usually found in homes near dry- cleaners who still use this particular chemical when washing garments, but can otherwise be introduced to indoor air through contaminated water and some fabric finishes (Health Canada, 2015; McKernan et al., 2008).
RESULTS
Preliminary tests of Respira showed that it could remove up to 2.68 mg of benzene and 3.77 mg of toluene in 72 hours, whereas water alone only removed up to 1.43 mg of toluene and no benzene. Interestingly, tetrachloroethylene concentrations slowly decreased at about the same rate no matter the testing conditions. This means any decrease in tetrachloroethylene over time is likely caused by a process known as dry deposition (Pettit et al., 2018), where chemicals in the air become stuck on solid surfaces, removing them from the air that was sampled.
FURTHER TESTING:
These preliminary tests were just the start of the research being done on Respira. In addition to more VOCs being tested, such as acetaldehyde and m- xylene, the types of bacteria and fungi in the plants housed by Respira are also being investigated.
CONCLUSION
Contrary to popular belief, plants do not effectively remove toxins from the air using their leaves. The primary source of their ability to remove toxins stems from the root zone where a build of microorganisms capture and permanently destroy the airborne pollutants. Through preliminary testing, Respira proves to be an effective way of removing indoor air pollution. Respira also offers a more sustainable method for improving IAQ in comparison to the conventional HVAC approach, which uses disposable filters that are then discarded into a landfill only to release PM back into the air as it breaks down.
Through further research, we aim to investigate whether the technology and naturally occurring ecosystems of bacteria living in the roots of plants could lead to a reduced need for ventilation in buildings, thereby reducing the energy consumption of the building.