Ionizer Assisted Air Filtration for Collection of Submicron and Ultrafine

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Ionizer Assisted Air Filtration for Collection of Submicron and Ultrafine ParticlesEvaluation of Long-Term Performance and Influencing Factors Bingbing Shi*,† and Lars Ekberg†,‡ †

Department of Energy and Environment, Chalmers University of Technology, Göteborg, Sweden CIT Energy Management AB, Göteborg 412 58, Sweden

‡

S Supporting Information *

ABSTRACT: Previous research has demonstrated that unipolar ionization can enhance the filter performance to collect airborne particles, aeroallergens, and airborne microorganisms, without affecting the filter pressure drop. However, there is a lack of research on the longterm system performance as well as the influence of environmental and operational parameters. In this paper, both field and laboratory tests were carried out to evaluate the long-term particle collection efficiency of a synthetic filter of class M6 with and without ionization. The effect of air velocity, temperature, relative humidity, and particle concentration were further investigated in laboratory tests. Results showed that ionization enhanced the filtration efficiency by 40%-units during most of the operation time. When the ionization system was managed by periodically switching the ionizer polarity, the filtration efficiency against PM0.3−0.5 was maintained above 50% during half a year. Furthermore, the pressure drop of the ionizer-assisted M6 filter was 25−30% lower than that of a filter of class F7. The evaluation of various influencing factors demonstrated that (1) air moisture reduced the increase of filtration efficiency; (2) higher upstream particle concentration and air velocity decreased the filtration efficiency; and (3) the air temperature had very limited effect on the filtration efficiency.

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from indoor air in various settings.8−19 Agranovski et al. (2006) demonstrated that continuous emission of unipolar ions could enhance the filtration efficiency of low-efficiency HVAC filters from 5 to 15% to 40−90%.8 Grinshpun et al. (2005) found that a powerful ionizer purifier had a high particle removal efficiency of 90% within 5−6 min and 100% within 10−12 min.9 When ionization is used alone, airborne particles and bioaerosols are conditionally removed depending on the ion concentration, particle size, and properties, as well as air flow velocity.9−11 When ionization is used together with an air filter, the efficiency of the filter is improved. Examples of factors influencing the magnitude of the efficiency increase are the properties of the filter medium and the distance between the ionizer and the filter.8,12−15 Results from Agranovski et al. (2006) and Park et al. (2011 and 2009) demonstrated that ionizers can increase the efficiency of filters made of both synthetic fibers and glass fibers. However, the increased efficiency of synthetic fiber filters was significantly higher than that of glass fiber filters.8,12,13 Similar findings were also presented in the research by Shi et al. (2012 and 2013) on

INTRODUCTION Exposure to fine and ultrafine particles is strongly related with respiratory and cardiovascular diseases/syndromes according to many reports.1−4 One common method to reduce personal exposure to particulate air pollution is the use of intermediate class filters (class M5-F9 according to EN779:2012, which roughly corresponds to MERV 9−15, according to ASHRAE 52.2). However, for small particles, either the efficiency of glass fiber filters is limited or the efficiency of charged synthetic filters (which may have a high initial efficiency) decays very quickly.5−7 Furthermore, many single-family houses and large buildings are equipped with low-packing-density filters for low filter cost and low filter pressure drop. However, these filters are also characterized by low filtration efficiency on airborne particles. For example, Jamriska et al. found that the removal efficiency on submicron particles in an office building was only 34% where the ventilation system was equipped with a “medium” filter.5 Thus, from both economical and health perspectives, there is a need to investigate techniques which can improve the removal efficiency of these low efficiency filters, without increasing their pressure drop. An ionizer operating upstream of a ventilation air filter is a promising technology to meet the above need. Many researchers have investigated ionization used for removal of airborne particles, aeroallergens, and airborne microorganisms © 2015 American Chemical Society

Received: Revised: Accepted: Published: 6891

February April 27, April 29, April 29,

24, 2015 2015 2015 2015 DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

Article

Environmental Science & Technology Table 1. Summary of Experiments Carried Out purpose

tested object

measurement site

synthetic M6 filter with ionization synthetic M6 filter without ionization glass fiber M6 filter without ionization new glass fiber F7 filter without ionization old (7 months) glass fiber F7 filter without ionization old (7 months) synthetic M6 filter with ionization ionization system with carbon brushes

long-term test (7 months) to clarify filter performance regarding efficiency, pressure drop and ozone generation short-term test (hours) to provide reference performance data for a typical Swedish filter solution short-term test (hours) of supply air conditions (factors) that may influence the filtration efficiency short-term (hours) test chamber experiments to study ozone generation from the ionizing equipment

field laboratory laboratory laboratory laboratory

filter filter filter filter

test test test test

rig rig rig rig

laboratory filter test rig laboratory test chamber

Table 2. Specifications of the Filter Media Tested in the DCV System filter class tested filters #1 #2

EN 779 M6 F7

20

ASHRAE 52.221

fiber diameter (μm)

thickness (mm)

packing density

MERV 11−12 MERV 13

10−25 1−5

11 4.5

0.041 0.010

Figure 1. Sketch of (a) the air handling unit and (b) distribution of the ionization brushes over the cross section of the air handling unit in the field measurements.

class filters used in the field (nonlaboratory settings). The lack of long-term studies could turn out to be a major knowledge gap for the technology application in real buildings. This study is aimed to make up this knowledge gap by studying the longterm performance of an ionizer-assisted low class synthetic filter with respect to particle removal efficiency and filter pressure drop. In this study, the long-term performance in the field was measured during 216 days in an office building in Göteborg, Sweden. Particle removal efficiency of ionizer-assisted air filtration was evaluated for submicron and ultrafine particles as well as fine particles and PM10. The polarity of the ionization system was occasionally switched in order to maintain the system performance. In addition to the enhanced efficiency, the risk of byproducts emission (e.g., ozone) is another important issue considered in the experiments. To compare with the performance of the same type of filter without ionization, another series of measurements on the same type of filter without ionizer were conducted in a laboratory filter test rig during the same period. Outdoor air conditions during the long-term operation may have a big impact on the efficiency of ionizer-assisted systems; however, reports about the influence of such conditions are quite sparse. Thus, the influence of air relative humidity (RH) and temperature, outdoor particle concentration, and air

ionizer-assisted air filtration, with special focus on the influence of filter material on the enhanced efficiency for collecting ultrafine and submicron particles.14,15 Charged synthetic filters can show remarkably high initial efficiencies on ultrafine and submicron particles. However, the collection efficiency of such electret air filters might fall down quickly during the dust loading process.16,17 One reason is that the electrostatic effect, exhibited by the fiber surface, is screened by the deposited particles. Therefore, how to maintain a high efficiency during long-term operation in the field is of great interest. In this study, a specific charged synthetic filter was selected due to its substantially increased efficiency with ionization observed in previous research.15 The ionization technique was investigated as a solution to maintain long-term performance of the filtration system. Ozone is often generated as a byproduct of ionizer air purifiers, and can damage respiratory organs. However, some researchers have found that carbon fiber ionizers may produce high concentrations of stable unipolar ions while generating negligible amount of ozone owing to the fine diameters of the carbon fibers.18,19 Thus, an ionization system composed of 50 carbon fiber ionizers in two stages was applied in this study. Although previous results have demonstrated that an ionizerassisted low class filter can reach a particle removal performance of similar magnitude as a filter of high class, there are only a few reports on long-term performance of ionizer-assisted low 6892

DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

Article

Environmental Science & Technology velocity through the filter were also investigated in the laboratory filter test rig in this research.

high voltage transformers (24 kV). The diameter of each carbon fiber is about 20 μm. Figure 1b indicates the location of the ionization brushes. Twenty-five brushes were distributed over the duct cross section about 2 m upstream of the filter bank. The remaining 25 brushes were distributed in the same manner but further 0.5 m upstream. Laboratory Filter Test Rig. Laboratory tests were carried out in a filter test rig designed in accordance with the standard SS-EN 779.20 In the direction of the airflow the test rig has an outdoor air intake, an air handling unit with a fan and coils for cooling, and a filter test section. The filter test section has more than 2 m straight ductwork before and after the tested filter. This ductwork has the same cross section dimensions as the tested filter, i.e. 0.6 × 0.6 m2. Tests were conducted at air flow rates ranging from 0.24 to 2.3 m3/s, which corresponds to the air velocities in the range of 0.67 to 6.4 m/s. The airflow rate, the pressure differential over the tested filter and the particle concentrations upstream and downstream of the filter were measured at locations prescribed by the standard SS-EN 779. In addition, air temperature and RH were measured upstream of the filter. The filters were challenged by the outdoor aerosol prevailing at the site of the laboratory, which is located in the city of Gothenburg, close to the site of the field tests. Laboratory Test Chamber. The ionizers were tested with respect to ozone generation in a full-scale test chamber. The test chamber has interior surfaces of polished stainless steel with a floor area of 7 m2 and an interior volume of 14.5 m3. The chamber was sealed (less than 0.1 air changes per hour) during the tests. A small table-fan was used to mix the air in the chamber. SI Figure S6 shows experimental setup of the chamber test. An unused ionizer brush was connected to ±24 kV, which were the same voltages as used in the field tests. Ozone measurements were repeated for different distances between the ionizer brush and the steel wall of the test chamber, which was grounded. The distance between the brush and the wall was varied between 2 and 30 cm. Measurements were also performed with an ionizer brush that had been used for 7 months in the field. In these measurements, the distance between the brush and the wall of the test chamber was 20 cm. Moreover, an additional experiment was conducted in the chamber using eight new ionizers connected to −24 kV. The distance between the brushes and the wall, and the intervals between the brushes were 20 cm. Measurement Equipment. In all experiments described above, the particle number concentrations were measured by a condensation particle counter (P-TRAK 8525. TSI) in the particle size range of 0.02−1.0 μm, and an optical particle counter (CI-500, CLiMET) in the following size fractions: 0.3− 0.5, 0.5−1.0, 1.0−5.0, 5.0−10.0, 10.0−25.0, and >25 μm. Additionally, an aerosol photometer (Dust-Trak 8532, TSI) was used to measure the total mass concentration of particles smaller than 10 μm (PM10). The ozone concentration was measured by an ozone instrument based on UV-photometry (Environics Series 300), with a detection limit of 1 ppb by volume. All the instruments were calibrated before the experiments. The air velocity and pressure drop were measured by a multifunction instrument of model Swema air 300 with two sensors of models SWA 31 and SWA 10. The pressure differential sensor, SWA 10, can measure from −300 to 1500 Pa with an accuracy of ±0.3 Pa plus ±1% of the reading. The air

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EXPERIMENTAL SECTION Two types of air filters were tested over a period of seven months, both in a laboratory setting and in the field. In this study, these tests are denoted “long-term tests”. An additional set of short-term tests were carried out in the laboratory. The tests comprised a synthetic fiber filter of class M6, a glass fiber filter of class M6, and a glass fiber filter of class F7. The synthetic M6 filter consists of polypropylene, modacrylic and polyester fibers. It was tested both with and without upstream ionization. The glass fiber F7 and M6 filters were only tested without ionization. All tests were carried out with the ambient outdoor aerosol of Gothenburg as upstream aerosol challenge. The tests comprised measurements of particle concentrations to enable determination of the filtration efficiency. Tests were also carried out in order to evaluate the influence of the supply air conditions on the particle filtration efficiency. These conditions included air RH, temperature, upstream particle concentration, and air velocity. Furthermore, measurements were performed in order to evaluate the risk of ozone generation from the ionization equipment. Table 1 summarizes the various experiments performed in this research. The properties of the tested filter media are shown in Table 2. In addition, scanning electron microscope (SEM) (Zeiss Supra 60 VP) images of the two filter media in Table 2 are presented in Figure S1 (a,b) in the Supporting Information (SI). Field Measurement Site. The field measurements were carried out at the air handling unit of an office building in Gothenburg. Figure 1a shows a sketch of the air handling unit. The nominal supply airflow rate is 4.8 m3/s. However, the system has a variable airflow rate with demand control with respect to room temperature and carbon dioxide concentration. Thus, during most of the time, the system was operating at an airflow rate substantially lower than the nominal value. The air handling unit has a comparatively large cross section area at the filter bank, which comprises nine filter units. This original design leads to an unusually low air velocity at the cross section of the filter bank. In order to increase the air velocity toward more typical values, 1/3 of the total cross section was blocked. Thus, six filter modules were used during the experiments. The air flow rate was logged by the control system of the air handling unit. The lowest airflow rate was found to be about 1.4 m3/s. This airflow rate prevailed at nighttime and during weekends. During daytime on weekdays, the airflow rate increased to a maximum of about 2.8 m3/s. Here, the air velocity in the cross section of the filters (face velocity) was about 0.8 m/s at 1.4 m3/s and was about 1.6 m/s at 2.8 m3/s. The outdoor air intake is located on the roof of the building. The air flow rate was determined by measurement of the pressure differential over a calibrated flow measurement device located at the inlet to the supply air fan. The air flow rate values were logged by the control system of the air handling unit. The outdoor air temperature and RH were monitored upstream of the filters by portable loggers. Upstream and downstream air samples for measurement of particles, ions and ozone were taken at the locations indicated in Figure 1a. The pressure differential over the filters was measured at points located immediately upstream and downstream of the filter bank. System for Ionization. The ionization system had a total number of 50 carbon fiber brushes (graphite) powered by two 6893

DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

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Environmental Science & Technology

measured upstream the filter bank, and about 300 ions/cm3 downstream. The second curve from the top shows the efficiency observed for the same filter when the ionizers were switched off temporarily. In addition, Figure 2 also shows the results obtained for the synthetic filter of class M6, and the glass fiber filter of class M6, when tested in the laboratory without ionization. As shown in Figure 2, the efficiency of the ionizer-assisted synthetic M6 filter is higher than the efficiency of F7 glass fiber filter during the first 190 days (∼6 months). Without ionization, the efficiency of the M6 synthetic filter rapidly decreased with operation time from above 92% to about 20% which is close to the filtration efficiency of M6 glass fiber filter. With ionization, the filtration efficiency of the M6 synthetic filter was increased by more than 20%-units and maintained about 40%-units increase during most of the operation time. The efficiency measured by the P-trak instrument (particles in the size range of 20−1000 nm) and by the Dust-trak instrument (PM10) are presented in SI Figures S3 and S4. The variation trends of these efficiencies are similar to the results shown in Figure 2. In Figure 2, there were three operation intervals separated by switching the ionizer polarity, i.e., 1st−26th days; 27th−110th day and 111th−216th day. After switching the ionizer polarity, the lowest filtration efficiency appeared after 7−10 days, afterward the filtration efficiency began to increase again. SEM images of the M6 synthetic filter operated with ionizer and without ionizer after long-term operation are presented in SI Figure S1 (c,d). More dust was loaded on the fibers of the M6 filter operated with ionizer (SI Figure S1(d)) than that on the fibers without ionizer operation (SI Figure S1(c)). In addition, it appears that the ion exposure does not destroy the filter fibers. Table 3 provides a summary of the filtration efficiencies observed for the synthetic filter of class M6 during the longterm measurement in the field. The efficiency increase compared to the no-ionizer case in the laboratory test is shown in brackets. From Table 3, it can be seen that the ionizer significantly enhanced the filtration efficiency, especially when the filter became dirty. Factors Influencing the Filtration Efficiency. Figure 3 shows the measured filtration efficiencies plotted against the RH, temperature, upstream (outdoor) particle concentration, and the air velocity in front of the filter (face velocity). Relative Humidity. Figure 3(a) shows the variation of air filtration efficiency with RH when the supply air temperature was 20 °C and the concentration number of the PM0.3−0.5 was 1 × 107 particles/m3. In the figure, the ionizer-assisted air filtration efficiency decreased with increasing RH, while the

velocity sensor, SWA 31, can measure from 0.1 to 10 m/s, with an accuracy of ±0.04 m/s at 0.1−1.33 m/s and ±3% at 1.33− 30 m/s. Additionally, outdoor air temperature and RH were monitored upstream of the filters by portable loggers.

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RESULTS As already mentioned, all filter tests were carried out with the ambient outdoor aerosol as upstream challenge aerosol. The office building used in the field measurements and the laboratory are located in the same part of Gothenburg city. Neither of the sites is influenced by particularly heavy traffic, although there are streets trafficked by cars and buses, etc., within 100−200 m. Long-term measurements have shown that the concentration of particles within the size range 0.3−0.5 μm typically varies from 0.1 × 106 particles/m3 to 40 × 106 particles/m3. SI Figure S2 shows a summary of upstream particle concentrations measured in the field and laboratory experiments. Filtration Efficiency over 7 Months. Figure 2 shows the filtration efficiency values for particles in the size range 0.3−0.5

Figure 2. Filtration efficiency values measured for the synthetic M6 filter tested in the field with ionizer “on” and “off”, together with values measured for an identical synthetic M6 filter and a glass fiber filter of class M6 tested in the laboratory without ionization. The data for a glass fiber filter of class F7 tested in the laboratory without ionization is also included.

μm observed for the synthetic M6 filter when tested in the field. Both the laboratory tests and the field measurements were conducted at an air velocity of 1.6 m/s (corresponding to 9700 ± 400 m3/h in the field measurements). The uppermost solid curve represents the data obtained when the ionization was on. Then, an ion concentration of about 5.0 × 105 ions/cm3 was

Table 3. Filtration Efficiency and Efficiency Increase (within brackets) When Ionizer Was Active during the Long-Term Operation Testa particles

a

efficiency item

d(0.02−1μm) P-trak

d(0.3−0.5μm) CLiMET

d(0.5−1μm) CLiMET

d(1−5μm) CLiMET

PM10 Dust-trak

total average max min 1−26th days 27th−116th days 117th−216th day

78% (17%) 97%(0%) 64%(30%) 92%(10%) 79%(21%) 72%(37%)

67% (22%) 100%(8%) 36%(20%) 90%(23%) 73%(42%) 51%(34%)

66%(13%) 98%(6%) 36%(14%) 91%(16%) 74%(35%) 50%(27%)

78%(5%) 99%(0%) 59%(13%) 95%(3%) 85%(21%) 67%(17%)

87%(31%) 100%(4%) 68%(37%) 95%(17%) 90%(53%) 81%(47%)

The data were collected when the air handling unit was operating at 9700 ± 400 m3/h. 6894

DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

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Figure 3. Filtration efficiency of the synthetic M6 filter, both with and without ionization, plotted against upstream (a) relative humidity; (b) air temperature; (c) particle concentration; and (d) supply air velocity. The particle concentration is for PM0.3−0.5.

Pressure Drop. Figure 4 shows the pressure drop over the filter plotted against the filter face velocity. The dashed curves

efficiency values measured without ionizer increased slightly with increasing RH. The figure also shows that the filtration efficiency with ionization is more sensitive than the filtration efficiency without ionization to the variation of outdoor RH. Temperature. Figure 3(b) presents the variation of the air filtration efficiency with air temperature when the RH of the supply air was 45% and the number concentration of PM0.3−0.5 was 1.2 × 107 particles/m3. The filtration efficiencies observed both with and without ionization, varied slightly with air temperature. Furthermore, temperature has no reported effect on particle filtration in the previous research. Thus, the influence of temperature on the filtration efficiency should be tiny. Outdoor PM. Figure 3(c) shows the variation of the air filtration efficiency with outdoor particle concentration when the supply air temperature was 20 °C and the RH was 65%. The figure shows that the air filtration efficiencies, both with and without ionization, decreased with increasing outdoor particle concentration. However, a relatively stable efficiency was observed when the outdoor particle concentration was higher than 1 × 107 particles/m3. This is because when the particle concentration increases and the ion emission is constant, the charge per particle due to ionization decreases. Thus, the Coulombic capture per particle decreases which results in a reduction of the filtration efficiency. Air Velocity. Figure 3(d) shows the variation of air filtration efficiency with the supply air velocity when the supply air temperature was 20 °C, the RH was 45% and the number concentration of PM0.3−0.5 was 1.2 × 107 particles/m3. The figure shows that the air filtration efficiencies decreased with increasing air velocity through the filters, both with and without ionization. Moreover, the ionizer-assisted filtration efficiency is more sensitive to the variation of air velocity than the efficiency without ionization.

Figure 4. Pressure drop of the glass fiber F7 filter and the synthetic M6 filter together with the calculated pressure drop reduction percentage.

show the values obtained when testing unused filters in the laboratory test rig. The solid curves represent the corresponding values for the same filters after they had been in operation for 7 months. The pressure drops observed during the field measurements are also shown in the figure as discrete data points. The dotted curve at the top of the diagram in Figure 4 shows the percentage reduction of the pressure drop (PRP) observed for the M6 filter compared to the F7 filter. The curve indicates 6895

DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

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indicates that the ionizer-assisted M6 filter could provide the same level of air cleaning for small particles as a traditional F7 filter. In Table 3, the values in the brackets are the efficiency increases (percentage points) for different sized particles. From low to high, they follow the order of ΔEFd(1−5 μm) < ΔEFd(0.5−1 μm) < ΔEF d(0.3−0.5 μm). It means that the efficiency increase is larger for small particles than for big particles. This result agrees well with the finding by Romay et al. (1998)23 and previous research on the efficiency increase for different particle sizes.8,12−15 The Coulombic capture could be the major cause of a higher efficiency increase on small particles. This is because the Coulombic capture increases with decreasing particle size,23,24 and moreover, it becomes significant when an ambient aerosol is substantially charged by ions generated from an ionizer. It is important to evaluate the factors that influence the longterm field measurement results. The long-term filtration efficiency values presented in Figure 2 were influenced by the outdoor RH and particle concentration. Because the outdoor RH was 70% ± 10% which falls in the stable efficiency range of RH in Figure 3(a), the outdoor air RH has a small influence on the filtration efficiency in Figure 2. During the long-term measurements, the outdoor particle concentration in the size range 0.3−0.5 μm was (1.43 ± 1.1) × 107 particles/m3, which mainly falls within the observed concentration range with nearly stable filtration efficiencies in Figure 3(c). Thus, a further concentration increase is not expected to cause any substantial drop of the efficiency. Elevated efficiencies observed occasionally during the long-term test could be caused by low outdoor particle concentrations. In Figure 3(a), for the case without ionizer, the moist air makes the filter wet which will reduce the porosity of the filter medium, which, in turn, results in the increase of the filtration efficiency. For the case with ionizer, the moist air could yield a lower ionization output than dry air, which results in a smaller efficiency increase than that of the case with dry air. These findings are very important for the field application of the system, while there are few reports on it. The results in Figure 3(d) demonstrated that air velocity affects the filtration efficiencies in different ways. For the case without ionization, previous study shows that the electrostatically induced forces and the mechanical mechanisms dominate the total filtration efficiency.22,26−30 The induced force decreases substantially with increasing air velocity, while for the mechanical mechanisms, the increasing air velocity substantially decreases the effect of air diffusion but does not influence the effect of interception. The effect of impaction increases with increasing air velocity, but this mechanism mainly influences particles larger than the observed PM0.3−0.5. Therefore, the effect of increased air velocity was negative to the filtration efficiency. For the case with ionization, the Coulombic force, together with the induced force and the air diffusion, decreases with increasing air velocity. It could be the reason why the result shows that the ionizer-assisted filtration efficiency is more sensitive than the filtration efficiency without ionization to the variation of air velocity. Considering the effects of the studied factors, this ionizerassisted air filtration system is expected to show very high efficiency when the system is operating at low RH, low outdoor particle concentration and a low air velocity. Therefore, it will be beneficial to install a prefilter and a heating/cooling coil in front of this ionizer-assisted air filtration system to provide low

that the average pressure drop of the M6 filter is about 30% lower than that of the F7 filter without ionization at air velocities toward the lower end of the scale, while at higher velocities, it is about 25% lower. Note that the high air velocities shown in Figure 4 are not typical, in office buildings the air velocity is normally below 2.5 m/s. Ozone Generation. The ozone measurements performed in the field showed about equal concentration upstream and downstream of the ionizer-assisted filtration. As can be seen in SI Figure S5, the upstream and downstream concentrations deviated ±3 ppb, regardless of the airflow rate. The result may be explained by measurement uncertainties. However, there may be a small amount of ozone generated that falls below the detection limit of the method used. The consequence of any such ozone generation is that the concentration increases by a few ppb only, regardless of the airflow rate. The measurement in the test chamber showed the ozone concentration did not change with time when the distance between the ionizer brush and the test chamber wall was 20 cm or longer (see SI Figure S7). However, when the distance between the brush and the wall was decreased to 4 cm, there was a tendency of ozone concentration increasing toward 7−8 ppb. At 2 cm distance, the concentration increased dramatically toward 24 ppb. Since the shortest distance between an ionizer and AHU duct surface is about 20 cm in the field experiment, the ozone generation from an ionizer should be very low. Additionally, the increase of the chamber ozone concentration from the negative ionizer was larger than that from the positive ionizer, which is in agreement with the finding from Chen and Davidson (2003).25 The ozone concentrations measured with an ionizer that had been in operation for 7 months were slightly higher (2 ppb) than the concentrations observed for a new one (unused brush). During this measurement, the distance between the brush and the wall was 20 cm. The total ozone generation from eight negative ionizers was presented in SI Figure S8. Results show that the total ozone generation rate is 1.9 mg/h, which means 0.24 mg/h for each ionizer.

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DISCUSSION The results of long-term measurements presented in Figure 2 show that the ionizers significantly enhanced the filtration efficiency, especially when the filter became dirty. This means that ionizers applied in front of filters could be used to extend the lifetime of charged synthetic filters and substantially increase the efficiency of low-efficiency filters. The temporal decay of the filtration efficiency of the M6 synthetic filter is possibly because the electrostatic force decreases as the filter medium becomes more and more covered by collected dust. Finally, the particle collection efficiency almost completely relies on the mechanical filtration mechanisms22 and the filter medium type (glass fiber or synthetic fiber) becomes less important. The filter of class F7 showed only a small change of its filtration efficiency over the 7-month period of operation. According to SP Swedish Technical Research Institute, an F7 filter, operated under real-life conditions, should show at least 50% efficiency for 0.4 μm sized particles. This criterion was met during the tests, both of the F7 filter and the ionizer-assisted M6 filter. The ionizer-assisted M6 filter actually showed even higher efficiency values than the F7 filter, for 0.4 μm sized particles, during the first 6 months of operation. The result 6896

DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

Article

Environmental Science & Technology

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RH and low particle concentration in the supply air. Moreover, a large cross section area at the filter bank in the air handling unit is recommended in order to achieve a low filter pressure drop and high particle collection efficiency when the air velocity at the filter face is low. Additionally, particle composition could influence the efficiency of the ionizer-assisted air filtration. Thus, further research is needed on the uncertainties induced by the particle composition. Because ozone generation rate from one ionizer in the chamber test was 0.24 mg/h, the detectable ozone concentration increase from the ionizer was 1.2 ppb for the lowest field air flow (5000 m3/h) and 0.6 ppb for the highest field air flow (10 000 m3/h). There are few reports on the long-term performance of air filter assisted with ionizer, therefore this study contributes valuable knowledge on the long-term performance. The tested ionizer supported air filtration system equipped with a synthetic filter of class M6 exhibited similar efficiency as a nonionization glass fiber filter of class F7. The efficiency of the ionizer-assisted synthetic fiber filtration was maintained equal to or higher than the efficiency of a traditional F7 filter without ionization during more than 6 months. The pressure drop of the ionizer-assisted synthetic M6 class filter was 25−33% lower than that of a traditional F7 class filter made of glass fiber. Additionally, the ozone generation from the ionization equipment is very limited. To our knowledge, this is the first study showing that switching ionizer polarity several times during the long-term running can improve the filtration efficiency of the system. The influence of supply air conditions on the ionizer-assisted air filtration were rarely investigated in previous research, therefore the research presented in this paper fills in a knowledge gap on this topic.

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ASSOCIATED CONTENT

* Supporting Information S

SEM images of the filter media, the long-term filtration efficiencies measured by P-trak and Dust-trak, and ozone concentration measurements. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b00974.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 46-31-772 1150; fax: 46-31-772 1152; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by FORMAS (The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning) Grant (242-2007-1583) and BELOK (the BELOK Network of Swedish building owners) Grant (2011:6). We appreciate the contributions from Transjoinc AB, Sweden, and Vokes air AB, Sweden, who provided the tested ionization system and tested filters. We gratefully also acknowledge the contributions from Akademiska Hus AB, Sweden, who provided access to the field ventilation system.

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REFERENCES

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DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898

Article

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DOI: 10.1021/acs.est.5b00974 Environ. Sci. Technol. 2015, 49, 6891−6898