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Botanical air filtration.

Botanical air filtration (BAF) using selected plants potentially can be integrated with architectural and interior design to provide a pleasant, attractive and natural indoor environment while at the same time reducing the concentration of indoor pollutants. How does botanical air filtration work? Is the technology effective for improving IAQ? What type of microbes are responsible for volatile organic compounds (VOCs) removal? Will it generate airborne microbes?

BAF uses either the plant leaves and/or the root bed to adsorb or absorb VOCs in room air, and relies on the metabolism process on the leaves or the microorganisms in the root bed for the VOC decomposition and degradation. Several studies have demonstrated the potential of BAF to remove indoor VOCs. (1,2,3)

The ability of plant leaves in adsorbing and metabolizing VOCs is very limited due to slow in-leaf diffusion and metabolization rate. Schmitz, et al., (4) concluded that assimilation and metabolism of formaldehyde by plant leaves appear unlikely to be of value for indoor air purification due to the low uptake rate.

Previous chamber studies might have given unwarranted claims by showing "remarkable" potential of plants in removing CO, C[O.sub.2], and VOCs such as formaldehyde, benzene, xylene, trichloroethylene, and ethylbenzene under chamber conditions. But when the results are scaled up to typical office or residential room scale and compared to typical outdoor ventilation/infiltration rates, the benefit from plants for pollutant removal becomes negligible. (5)

Dynamic botanical air filtration (DBAF) works by passing polluted air through the plant's root system. However, it offers good potential for practical application in treating indoor air. VOCs could become the potential carbon source for microbial communities in soil from the rhizosphere of plant roots (the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms) as indicated in the studies of Owen, et al., Holden and Firestone, and Fan and Scow. (6,7,8) One study using such a system has shown that three plants in a real office with an average floor area of 13 [m.sub.2] (140 [ft.sub.2]) were more than enough to reduce total VOC levels by more than 75%. (9)


Darlington, et al., (2) reported the field performance of a 10 [m.sub.2] (108 [ft.sub.2]) bioscrubber wall that used hundreds of species of indoor landscaping plants on a lava rocks root bed. The system had very low face velocity through the root bed (0.01 m/s [2.0 ft/min]), and required a relatively large area to deliver a significant amount of airflow rate to the space (360 [m.sup.3]/h [212 [ft.sub.3]/ min]). The bioscrubber resulted in lower formaldehyde and TVOC concentration levels in the room it served compared to an adjacent space without the bioscrubber.

Most recently, a modular DBAF has been developed (Figure 1). It delivered up to 600 [ft.sub.3]/min (1019 [m.sup.3]/h) air with a pressure drop of less than 0.4 in. w.c. (97 Pa) across the root bed. The root bed was 7.87 in. (0.2 m) thick and covered an area of 11.6 [ft.sub.2] (1.08 [m.sub.2], 1.8 m x 0.6 m). It consisted of a mixture of 50% activated carbon and 50% pebbles. Golden Pothos (Epipremnum aureum) was used with a density of eight plants per square meter. An auto-irrigation system was used to keep the volume metric moisture content of the root bed within the range of 0.1 to 0.3 [ft.sub.3] water/[ft.sub.3] of bed volume. The maximum face velocity through the root bed was 52 fpm (0.26 m/s).

Compared to the previous biofiltration system, (2) the root bed of the DBAF consisted of porous shale pebble and granular activated carbon instead of lava rocks. The activated carbon had a BET surface area of 900-1100 [m.sub.2]/g and 80% of the pore sizes were less than 10 nanometers, which was highly effective for adsorbing VOCs.

The plants used in the DBAF were more selective (e.g., Golden Pothos [Epipremnum aureum]) for ease of maintenance and more root-zone microbial community. The DBAF also has a higher face velocity (up to 0.26 m/s [52 fpm]) and delivers higher airflow rate per unit surface area (up to 1010 [m.sup.3]/h [556 [ft.sub.3]/min] through a 1.08 [m.sub.2] [11.6 [ft.sub.2]] root bed). It has the potential to be more easily adopted for indoor air cleaning either as part of an HVAC system or operated as a standalone unit to provide the required clean airflow rate. In the following sections, we summarize the performance test results for the DBAF (see Wang and Zhang (10) for details).

Environmental Chamber Testing

A full-scale environmental chamber (1,920 [ft.sub.3] [55 [m.sup.3]]) was used. It operated under 100% recirculation mode, and contains the DBAF and a composite wood-based office workstation system as a simulated VOC emission source (Figure 2 right). Measurements show significant reduction of formaldehyde and TVOC when the DBAF was on (Figure 2 left).

Field Experiments and Performance Evaluation

To investigate the DBAF performance under realistic indoor pollutant concentration levels and realistic ventilation conditions, the DBAF system was integrated into the HVAC system of a newly constructed office room (Figure 3). The total area of the test room was 1,042 [ft.sub.2] (96.8 [m.sub.2]). There were 16 workstations in the room. The total amount of supply air for this room was 1,400 [ft.sub.3]/min (2378 [m.sup.3]/h), while only 480 [ft.sub.3]/min (815 [m.sup.3]/h) was passing through the DBAF. The test room was maintained at 71.6[degrees]F (22[degrees]C) and 30% RH. Major compounds present in the air of the test space were: pentanal, toluene, hexanal, xylene, [alpha]-pinene, formaldehyde and acetaldehyde. A proton transfer-mass spectrometer (PTR-MS) was used to monitor these target compounds in real-time.

DBAF system versus outdoor air ventilation. The DBAF system provided an equivalent clean air delivery rate (CADR) of 280 [ft.sub.3]/min (476 [m.sup.3]/h) for formaldehyde (Figure 4a). The 5% outdoor air plus DBAF system also resulted in a similar effect of 10~25% outdoor air ventilation for toluene removal (Figure 4b). Overall, if the quality of air in the space is maintained at the same level as the case of 25% outdoor ventilation, use of the DBAF system would result in about 15% saving in heating energy for the space.

Long-term performance. The DBAF system was operated continuously for 300+ days when the outdoor ventilation air was kept at 5% of total supply airflow rate to the space. The initial formaldehyde and toluene concentration were 17 ppb and 2 ppb, respectively. After the filter was running for 10 days, the room formaldehyde and toluene concentration decreased to 10 ppb and 1 ppb, respectively, and then kept at a relative constant level, meaning that the VOCs continuously emitted from the particleboards were removed by the 5% outdoor air ventilation plus DBAF.

Figure 5 shows that the single pass efficiency (SPE) of the botanical filter for formaldehyde kept around 60%. The SPE for toluene was negatively influenced by the water content in the bed, but was still kept at 20% 300 days later. Note that without the botanical filter, concentrations in the spaces would have been 30% higher than current results, due to the continuous generation of toluene and formaldehyde from the sources.

To better understand the DBAF system and optimize its performance, the microbes in the prototype system and their activities for formaldehyde removal were characterized. The microbes were isolated from the plant roots and pebbles. By sequencing the 16S rRNA genes, seven bacterial strains were identified from this system. Among them, a strain of Arthrobacter aurescens TC1 was found to remove 86% of formaldehyde with a starting concentration of 11.8 ppm. (11) Air samples also were taken at the exhaust air of the DBAF system, but no microbes were detected.



Myths and Facts

Myth: Certain plants are great indoor air purifiers. Fact: Plant leaves have very limited ability in adsorbing volatile organic compounds (VOCs). Potted plants have little practical benefit to indoor air purification. Dynamic botanical air filtration with air passing through the root bed that has adequate moisture content may remove water soluble and non-soluble VOCs over a long period of time under realistic indoor conditions (if properly designed and operated). The effectiveness of botanical air cleaning should also be evaluated in the context of other IAQ strategies including ventilation and emission source control.

Myth: Microorganisms in the root bed of plants can consume the VOCs quickly to regenerate the ability of the root bed system in adsorbing/absorbing VOCs. Fact: While evidence of microorganisms' ability to decompose formaldehyde has been confirmed under controlled laboratory study, more data are needed for other VOCs. More importantly, the rates of biodegradation of VOCs by microbes need to be determined.


Myth: Botanical air filtration system may result in microbial contamination in indoor air. Fact: No data in the existing literatures reviewed indicated such a problem. For the DBAF system tested, microbes existed in the root bed of the plants and excess irrigation water, but no airborne microbes were detected in the air supplied by the DBAF system. More data are needed for a wider range of operating conditions.



The figures in this article are reprinted from Building and Environment by Zhiqiang Wang, Jensen Zhang, "Characterization and performance evaluation of a full-scale activated carbon-based dynamic botanical air filtration system for improving indoor air quality," Copyright 2010, with permission from Elsevier.


(1.) Wolverton, B.C. 1997. How to Grow Fresh Air: 50 Houseplants That Purify Your Home or Office. New York: Penguin Group USA.

(2.) Darlington, A., et al. 2000. "The biofiltration of indoor air: implications for air quality." Indoor Air 10:39-46.

(3.) Chen, W., J.S. Zhang, Z. Zhang. 2005. "Performance of air cleaners for removing multiple volatile organic compounds in indoor air." ASHRAE Transactions 111(1):101-14.

(4.) Schmitz, H., U. Hilgers, M. Weidner. 2000. "Assimilation and metabolism of formaldehyde by leaves appear unlikely to be of value for indoor air purification." New Phytologist. 147(2):307-315.

(5.) Girman, J., T. Philips, and H. Levin. 2009. "Critical Reviews: How well do house plants perform as indoor air cleaners?" Proceedings of Healthy Building; paper 667.

(6.) Owen, S.M., S. Clark, M. Pompe, K.T. Semple. 2007. "Biogenic volatile organic compounds as potential carbon source for microbial communities in soil from the rhizosphere of populus tremula." FEMS Microbiol Lett. 268:34-39.

(7.) Holden, P.A., M.K. Firestone. 1997. "Soil microorganisms in soil cleanup: how can we improve our understanding?" J. Environ. Qual. 26:32-40.

(8.) Fan, S. and K.M. Scow. 1993. "Biodegradation of trichloroethylene and toluene by indigenous microbial populations in soil." Applied and Environmental Microbiology 59(6):1911-1918.

(9.) Wood, R.A., et al. 2006. "The potted-plant microcosm substantially reduces indoor air VOC pollution: I. Office field-study." Water Air Soil Pollut. 175:163-80.

(10.) Wang, Z., J.S. Zhang. 2010. "Characterization and performance evaluation of a full-scale activated carbon-based dynamic botanical air filtration system for improving indoor air quality." Building and Environment doi: 10.1016/j.buildenv.2010.10.008.

(11.) Huang, W., et al. "Characterization of microbial species in a regenerative bio-filter system for VOC removal." Proceedings of IAQVEC 2010--the 7th International Conference on Indoor Air Quality, Ventilation and Energy Conservation in Buildings.

By Jensen Zhang, Ph.D., Member ASHRAE; Zhiqiang Wang; and Dacheng Ren, Ph.D.

Jensen Zhang, Ph.D., is professor and director and Zhiqiang Wang is a graduate research assistant at Building Energy and Environmental Systems Laboratory, Department of Mechanical and Aerospace Engineering, Syracuse University, Syracuse, N.Y. Dacheng Ren, Ph.D., is an assistant professor at the Department of Biomedical and Chemical Engineering, Syracuse University.*
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Title Annotation:Myths and Facts
Author:Zhang, Jensen; Wang, Zhiqiang; Ren, Dacheng
Publication:ASHRAE Journal
Article Type:Reprint
Geographic Code:1USA
Date:Dec 1, 2010
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