ASHRAE Journal - December 2010 - 139

Chamber C (ppb)

ft2] 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 Zhang10 for details). A full-scale environmental chamber (1,920 ft3 [55 m3]) 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).

600

Formaldehyde Filter Off On Off On

TVOC_as Toluene Off On Off

400

Environmental chamber testing

200

48 Elapsed Hours

Test Setup

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 ft2 (96.8 m2). There were 16 workstations in the room. The total amount of supply air for this room was 1,400 ft3/ min (2378 m3/h), while only 480 ft3/min (815 m3/h) was passing through the DBAF. The test room was maintained at 71.6°F (22°C) and 30% RH. Major compounds present in the air of the test space were: pentanal, toluene, hexanal, xylene, α-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 ft3/min (476 m3/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
December 2010

Figure 2: Full-scale environmental chamber testing of short-term performance of the DBAF: (left) concentration reduction due to DBAF operation; (right) test setup.
Room Air Sampling Exhaust Air AHU Return Air Botanical Filter
Air Sampling Before Filter Air Sampling After Filter

Outdoor Air

PTR-MS

Test Room

Supply Air

Figure 3: Field test setup with integration of DBAF system into HVAC and VOC monitoring by proton transfer reaction mass spectrometry (PTR-MS). 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.
ASHRAE Journal 139

ASHRAE Journal - December 2010

Table of Contents for the Digital Edition of ASHRAE Journal - December 2010