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Research
Results for Aerosol Treatment
Technology Evaluation with the
Knorr 3-Stage Air Filtration and
Purification System
August 2, 2021 Report
Experimental Approach
A standard EPA, ASHRAE, or ASTM-approved method does not currently exist for evaluating
efficacy of aerosol treatment products or devices against bioaerosols. Utilizing EPA's
specialized Aerosol Test Facility in Research Triangle Park, NC and experience in
conducting research with aerosolized biological agents, a standardized approach that can
be applied across multiple technology types was developed to achieve the objectives of
this project. An objective of the test conditions was to maintain a high enough
concentration of viable aerosolized test virus in the air for 90 minutes in control conditions
(without technologies active) to be able demonstrate a 3-log (99.9%) reduction in airborne
virus concentration during test conditions (with technologies active) relative to the time-
matched control tests. For these tests, MS2, a nonpathogenic bacteriophage that infects
Escherichia coli, was used as the test virus.
A large test section of the aerosol wind tunnel in EPA's Aerosol Test Facility was sectioned
off from the recirculating wind tunnel to serve as the test chamber utilized for this
research. The 12 ft x 10 ft x 25 ft chamber provides a 3,000 ft3 conditioned space for
dissemination and sampling of bioaerosols and for evaluating different aerosol treatment
strategies (Figure 1). The chamber is temperature and relative humidity (RH) controlled,
and its air is high-efficiency particulate air (HEPA)-filtered prior to each test. A mock
heating, ventilation, and air conditioning (HVAC) system was designed and constructed in
the test chamber to simulate air circulation and exchange rates typical of a wide range of
general application settings. An Omni-Aire 1000V (Omnitec, Mukilteo, WA) negative air
machine (NAM), with the H EPA filter removed, serves to represent a cold air return,
recirculating air through the HVAC system and test chamber. Air then flows through the
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8-inch main duct before passing through six evenly spaced 6-inch branches that distribute
air back to the chamber through 10-inch round diffusers with butterfly dampers at the
distribution points. The system was constructed from galvanized steel materials, and the
NAM airflow rate is set to 350 cubic feet per minute (CFM), resulting in approximately 7 air
changes per hour (ACH) in the chamber. No outside air is introduced during testing. Two
metal fans (LASKO 2265 QM, Westchester, PA) placed in opposite corners of the chamber
facilitate mixing, operating at 1448 ft/min. In between tests, the chamber is opened, and
recirculated HEPA-filtered air is run through the chamber until negligible particle counts
are present in the air in preparation for the next test.
an
Figure 1. Test chamber schematics of A) side view and B) top-down view.
The bacteriophage MS2 (ATCC 15597-B1), a non-enveloped virus that infects the host cell
Escherichia coli (ATCC 15597), was used in this current study. Conducting research with
aerosolized viruses at these large scales necessitates the use of safer, e.g., Biosafety Level
(BSL)-l potential surrogate viruses, versus using more pathogenic agents, such as SARS-
CoV-2 (BSL-3). As a non-enveloped virus, MS2 is expected to be more resistant to chemical
inactivation than enveloped viruses (e.g., SARS-CoV-2), MS2 stock is prepared using a top
agar overlay technique, and samples are analyzed using a plaque assay with the E. coli
host.
In this study, four 6-jet Collison Nebulizers (CH Technologies, Westwood, N J) are used to
aerosolize MS2 into the test chamber. In each nebulizer, a 10 mL mixture of 1:4 parts MS2
stock (MS2 in SM Buffer) to 0.22 |im filter-sterilized deionized water with 6 drops of
Antifoam A (Sigma-Aldrich, St. Louis, MO) is nebulized over a period of 10 minutes prior to
the first bioaerosol sampling period. Aerosolized MS2 is sampled using SKC BioSamplers
(SKC Inc., Eighty-Four, PA) connected to air sampling pumps that draw air at a rate of 12.5
L/min. The sampling period for each aerosol sample is 10 minutes, resulting in a total
volume of 125 L of air sampled during each sampling period. Bioaerosol samples are taken
in duplicate from a 5 ft "breathing zone" height during each sampling period. The first
B)
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aerosol sample in each test is taken immediately following the 10-minute aerosolization
period (time = 0 sample). The count median diameter of aerosolized particles is 44 ± 2 nm
at the beginning of each test (time = 0 min), and it increases over the duration of the test to
85 ± 3 nm at the end of 90 minutes, as measured by a Scanning Mobility Particle Sizer
(Model 3080 Electrostatic Classifier/TSI 3022 Condensation Particle Counter, TSI Inc.,
Shoreview, MN).
Knorr 3-Stage Air Filtration and Purification System
Results
The Knorr Brake Company has developed a patented 3-Stage Air Filtration and Purification
System for public transit vehicles, including tramway/light rail, metro, regional, and
intercity passenger vehicles. This system can be installed either in newly built vehicles or
retrofit into existing HVAC units. This technology was selected for evaluation based on
prioritization from research stakeholders, including those in the transit industry. The
system comprises the Merak Intense Field Dielectric (MIFD) filter, which uses an
electrostatic discharge to charge a physical arrest filter, an Ultraviolet-C (UV-C) radiation
component (one 30" 86W bulb with a peak wavelength at 254 nm), the Merak Dielectric
Barrier Discharge (MDBD) bipolar ionization component, and two blower fans operated by
one motor (to circulate air through the unit). The test unit evaluated in these laboratory
studies was wired with a control box that allows individual actuation of each of the MIFD,
UV-C, and MDBD modules (and the blower). The unit was placed in the middle of the test
chamber (Figure 2), and tests were conducted with the NAM recirculating air through the
HVAC system to facilitate mixing in the chamber (~7 ACH). Air flow through the Knorr
blower was approximately 1255 CFM with the MIFD arrest filters in place and approximately
1350 CFM with the arrest filters removed, resulting in 25-27 ACH in the chamber, treated by
the Knorr unit. All tests were conducted at 22 ± 2 °C and a RH of 30-35%[1]. This RH was
selected because virus viability is reduced at intermediate RH[2], and an objective of the
test design is to maintain a sufficiently high concentration of viable aerosolized MS2 after
90 minutes to allow for demonstration of a 3-log reduction in tests with technologies active
relative to control conditions throughout the duration of each test. During each
experiment, the Knorr blower was powered prior to MS2 aerosolization in order to circulate
airthrough the unit. Following MS2 aerosolization, the time = 0 min sample was taken, and
subsequently the component(s) to be evaluated was powered. The initial concentration of
MS2 recovered at time = 0 min for each test and control experiment conducted was > lxlO7
plaque-forming units (PFU) percubic meter (m3).
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Figure 2. Location of the Knorr 3-Stage Air Filtration and Purification System in the middle of
chamber during testing.
Figure 3 shows the concentration of MS2 at each sampling timepoint (normalized to the
initial time = 0 min MS2 concentration) during each test. Three control tests were
conducted on 6/16/21,6/21/21, and 7/19/21 with only the Knorr unit's blower on, and with
the three components intact, but not powered. Three additional "No Filter" control tests
were conducted on 6/22/21,6/23/21, and 6/24/21, similar to the other control tests with
only the blower powered, but with the arrest filters of the MIFD component removed.
Three tests to evaluate the MIFD component powered alone were conducted on 6/28/21,
6/29/21, and 7/7/21. Both the MIFD and MDBD components were powered in tests
conducted on 6/30/21,7/1/21, and 7/8/21. All three components (MIFD, MDBD, and UV-C)
were powered for tests conducted on 7/13/21,7/14/21, and 7/15/21. Two tests were
conducted with just the MDBD and UV-C components powered on 6/16/21 and 6/17/21.
One test was conducted with the MDBD powered alone on 7/20/21. The component
combinations run in triplicate were prioritized based on stakeholder interest in evaluating
different configurations for installation in their vehicles.
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• Control - 6/15/21
¦ Control - 6/21/21
Control - 7/19/21
Control (No Filter) - 6/22/21
Control (No Filter) - 6/23/21
Control (No Filter) - 6/24/21
-MIFD-6/28/21
•MIFD-6/29/21
MIFD - 7/7/21
• MIFD + MDBD- 6/30/21
¦ MIFD + MDBD-7/1/21
MIFD +MDBD- 7/8/21
¦ MIFD + MDBD + UV - 7/13/21
¦ MIFD + MDBD + UV - 7/14/21
• MIFD + MDBD + UV - 7/15/21
• MDBD + UV - 6/16/21
MDBD + UV - 6/17/21
¦ MD8D - 7/20/21
• Control
• Control - No Filter
¦MIFD
• MIFD + MDBD
- MIFD + MDBD + UV
MDBD + UV
¦MDBD
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Figure 3. A) Concentration of MS2 recovered from aerosol samples throughout the control
and Knorr components tests, normalized to the initial time = 0 min concentration for each
test. Each timepoint represents recovery from duplicate bioaerosol samplers as determined
by plaque assay, and the error bars represent pooled standard deviation at each sampling
timepoint. B) Normalized log10 MS2 concentration at each sampling timepoint, averaged
over each set of components tests (three replicates for each control and component test type,
except for MDBD + UV [n=2] and MDBD [n=l]). Error bars represent standard deviation in
averaged MS2 recoveries for each test condition. MIFD = Merak Intense Field Dielectric (MIFD)
electrostatic filter; MDBD = Merak Dielectric Barrier Discharge (MDBD) bipolar ionization
component; UV= UV-C radiation component.
Total particle (number) concentration, measured by a Scanning Mobility Particle Sizer
(Model 3080 Electrostatic Classifier/TSI 3022 Condensation Particle Counter, TSI Inc.,
Shoreview, MN), was also recorded during each experiment. Figure 4 shows the particle
concentration at each sampling timepoint during each test. Powering the MIFD lead to the
greatest reduction in particle concentration, but particle concentrations were also reduced
with the MDBD component operating. The arrest filter in the MIFD also reduced particle
concentrations even when the filter was not charged.
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Control - 6/15/21
Control - 6/21/21
Control - 7/19/21
Control (No Filter) - 6/22/21
Control (No Filter) - 6/23/21
Control (No Filter) - 6/24/21
MIFD- 6/28/21
MIFD - 6/29/21
MIFD - 7/7/21
MIFD + MDBD-6/30/21
MIFD + MDBD- 7/1/21
MIFD + MDBD-7/8/21
MIFD + MDBD + UV - 7/13/21
MIFD + MDBD + UV - 7/14/21
MIFD + MDBD + UV - 7/15/21
MDBD + UV - 6/16/21
MDBD + UV - 6/17/21
MDBD - 7/20/21
Figure 4. Particle concentration measured at each timepoint during testing. Error bars
represent standard deviation in measured particle concentration during each sampling
timepoint.
Figure 5 shows the calculated log10 reduction in MS2 recoveries at each sampling timepoint
between the average of the control tests with the arrest filter in place and the tests with
components active (Figure 5A), as well as the calculated log10 reductions comparing the
average of the control tests without the filter in place to the component tests (Figure 5B).
All control and test conditions are represented by an average of three replicate tests,
except for the MDBD + UV tests (two replicates) and the MDBD alone test (one replicate).
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A)
Control With Arrest Filter
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MIFD ¦ MIFD + MDBD ¦ MIFD + MDBD + UV BMDBD + UV ¦ MDBD
Figure 5. The calculated log10 reduction at each testing timepoint between the average
recoveries ofMS2 from the A) the control tests with the MIFD arrest filter in place and B) the
control tests without the arrest filter in place. Error bars represent pooled standard error
from the test and control experiments.
[1] EPA recommends that indoor relative humidity be maintained between 30% and 50%
(https://www.epa.gov/iaq-schools/moisture-control-part-indoor~air-quality-design-tools-
schools ).
[2] Lin, Kaisen, and Linsey C. Marr. 2020. "Humidity-Dependent Decay of Viruses, but Not
Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics." Environmental Science &
Technology 54 (2):1024-32. doi; 10.1021/acs.est.9b04959.
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