Effect of Soilless Substrate Volumetric Water Content on Particulate Matter Reduction in a Plant-Based Biofilter System

Article information

J. People Plants Environ. 2024;27(6):575-584
Publication date (electronic) : 2024 December 31
doi : https://doi.org/10.11628/ksppe.2024.27.6.575
1Researcher, Urban Agriculture Research Division, National Institute of Horticultural and Herbal Science, Rural Development Administration, Wanju-gun 55365, Republic of Korea
2Professor, Department of Plant Biotechnology, Korea University, Seoul 02841, Republic of Korea
*Corresponding author: Jongyun Kim, jongkim@korea.ac.kr
First authorHyeong-Seok Lee, dlgudtjr1212@korea.kr
Received 2024 November 13; Revised 2024 November 21; Accepted 2024 December 12.

Abstract

Background and objective

This study was conducted to quantify the reduction of particulate matter (PM) through a plant-based biofilter system, with the moisture content of the soilless substrate set at at different levels to enable comparison.

Methods

For the experiment, a biofilter system (0.054 m3) consisting of an upper part with plants and soilless substrate acting as a filter and a lower part incorporating a wind blower for air circulation was constructed. Four Aglaonema commutatum ‘Snow white’ were planted in the biofilter system. Volumetric water content (VWC, %) of the substrate was set at different levels between 10% to 40%. The initial PM concentration in the sealed glass chamber (5.88 m3) was set to 500 μg·m−3, and the changes in PM concentration were measured 15 minutes after operation of the biofilter system.

Results

As the substrate VWC in the biofilter system increased, the amount of PM2.5 and PM10 reduction decreased, with a linear response (p < .001), mostly due to increased pressure drop (PD) and decreased airflow rate (AFR) of the biofilter system. As the substrate VWC increased (from 10% to 40%), the PD increased (from 0.040 to 0.118 kPa), and AFR decreased (from 112.7 to 76.8 m3·h−1).

Conclusion

Consequently, as the substrate VWC increased from 10% to 40%, the PD of the air passing through the substrate increased and the AFR decreased, resulting in a significant decrease in the PM reduction efficiency. Further research is required to determine the optimal substrate VWC that is effective in reducing PM without increasing the PD, while maintaining the proper growth of the plants.

Introduction

To cope with the depletion of energy resources caused by rapid urbanization and climate change issues caused by environmental degradation, the development of sustainable and environmentally friendly housing has become a crucial task for modern society (Alhinawy, 2024). Against this backdrop, passive houses, which minimize energy consumption and create an eco-friendly living environment through advanced insulation, airtightness, and energy recovery ventilators, have attracted attention (Schnieders and Hermelink, 2006; Taleb, 2014; Vlad et al., 2012). However, creating a comfortable living environment requires not only the proper management of lighting, temperature, and humidity, but also the maintenance of a clean indoor air quality. Notably, given that people today spend most of their time indoors (Babu and Suthar, 2020), and thus concerns about health problems caused by indoor air pollutants (e.g., particulate matter (PM) and harmful gases) have recently increased (Bălă et al., 2021; Syed et al., 2023), there has been growing interest in housing styles that can maintain and manage an improved level of indoor air quality (Elnaklah et al., 2021).

Improved indoor air quality can be partly achieved through natural ventilation, but this method can be challenging depending on external conditions such as rainfall, ultraviolet rays, and high levels of PM outdoors (Ekberg, 1994; Hong et al., 2017). Passive houses, a type of sustainable housing, minimize energy consumption through enhanced insulation and airtightness, but they have some limitations in terms of their capacity for indoor air purification due to the reduced ventilation they allow (Zeiler and Boxem, 2009). Moreover, while indoor air purification systems, such as air purifiers, have been developed to artificially clean indoor air, they have difficulty fulfilling the role of ventilation, as they only purify and circulate the air within the space where the system is installed (Choe et al., 2022). Even ventilation systems integrated with air conditioning units, which aim to address this issue, have limitations; in addition to energy concerns, they may also bring polluted outdoor air indoors (Noh and Yook, 2016).

Therefore, to overcome these limitations, research has been conducted on the development of a plant-based biofilter (PBB) system that combines the air purification effect of plants (Kim et al., 2010; Kim et al., 2008) with a biofilter system (Bang et al., 2013; Lee and Joe, 2008; Mannan and Al-Ghamdi, 2021). When installed in outdoor structures like balconies and connected to existing building air conditioning systems, biofilter systems can play an active role in ventilation by purifying polluted outdoor air and delivering clean air indoors. In this way, they can complement the limitations of current air purification systems (Wang and Zhang, 2011), offering many advantages when it comes to improving indoor air quality.

Previous studies have suggested several factors that influence the air purification effect of biofilter systems, including the type of substrate, temperature, pH, airflow rate (AFR) and empty bed residence time (EBRT). Among these, the volumetric water content (VWC) of the substrate and the pressure drop (PD) have been highlighted as key indicators affecting biofilter performance (Kennes and Thalasso, 1998; Ottengraf, 1986). It has also been suggested that an excessively high VWC can create stagnant zones within the substrate, increasing the PD and leading to an anaerobic environment, which inhibits microbial activity. Conversely, an excessively low VWC can deform the structure of the substrate, reducing its performance as a filter (Kennes and Thalasso, 1998). Moreover, the PD in biofilter systems varies depending on factors such as the type of substrate, particle size used, and VWC, which can significantly affect air purification efficiency. As such, it has been highlighted that it is crucial to develop technologies that maintain both the PD and the optimal substrate VWC when designing biofilter systems (Delhoménie and Heitz, 2005).

In Korea, technologies have been developed to maintain a consistent level of VWC within the substrate using humidified air (Lee et al., 2015a). Studies have been conducted that compare plant growth and PD according to the substrate VWC (Choi et al., 2014), as well as that examine changes in VWC according to the humidifying cycle and the effect on PM reduction (Lee et al., 2015b). However, the range of VWC suitable for plant growth has not been adequately considered. For this reason, further research is needed to determine the VWC conditions that can simultaneously maintain optimal plant growth and the effect of PM reduction. Moreover, for a plant-based biofilter system to work, plants must be grown in the substrate. It is generally known that plants exhibit poor growth in dry environments, and their air purification capacity diminishes under moisture stress (Kim et al., 2023; Park et al., 2019). However, it has been reported that, when using indoor plants, maintaining a lower VWC is more effective for preserving long-term ornamental value rather than promoting rapid growth (Nam et al., 2023), and that plants may dry out more slowly in low-light environments such as indoors (Lee and Kim, 2024). Based on this, it seemed that indoor plants would grow well in a peatmoss-based substrate with a VWC of 20%.

Given this context, we sought to investigate the effect of VWC on the PM reduction efficiency of biofilter systems by considering the range of substrate VWC levels that support the growth of indoor plants compared to plants grown in greenhouses or outdoors. To accomplish this, we measured the PD of biofilter systems and the AFR discharged from the wind blower at different soilless substrate VWC levels and compared these measurements with the PM reduction efficiency of the biofilter system. This study was conducted to obtain basic data for quantifying the optimal substrate VWC that can enhance the air purification effect of biofilter systems within the viable range for plant growth.

Research Methods

Plant-based biofilter (PBB) system configuration

The biofilter system used in this experiment to determine the effect of PM removal according to the soilless substrate VWC was designed to be separable into upper and lower sections for ease of substrate replacement and planting. The system was constructed using 5 mm thick transparent acrylic, 300 mm in length, 300 mm in width, and 600 mm in height. Its internal configuration is as follows (Fig. 1).

Fig. 1

Diagram of the plant-based biofilter system used in this experiment : The passages of air with particulate matter is as follows : (A) polluted air is sucked into the upper part of the biofilter system while filtered by plants → (B) air is passed through the substrate and the air path filled with activated carbon → (C) air moves down to the lower part → (D) biofilter-passed airflows out through the PVC pipe connected to the wind blower. Yellow arrows : particulate matter flow; Blue arrows: airflow.

The upper section (300 mm L × 300 mm W × 300 mm H) included an irrigation unit for water supply and a planting unit for vegetation, while the lower section (300 mm L × 300 mm W × 300 mm H) housed a water pump, water tank, and wind blower. In the upper section, 16 air-path made by plastic mesh (Ø 23 mm, Daejin Nano, Anyang, Korea), each cut to 10 cm in length, to enhance airflow within the substrate. Moreover, 17 liters of horticultural substrate (Sunshine Mix4, Sun Gro Horticulture, Agawam, MA, USA) were used as the growth medium for the plants.

The PBB system is designed to draw outside air into the planting unit through the operation of a wind blower (TB-95-1, Innotech, Daegu, Korea; maximum AFR of 162 m3/h−1; static pressure of 22 mmAq) in the lower section. The air is purified by the filtering action of the plants and the medium, and clean air is then supplied indoors through PVC pipes (Ø 3.8 cm) connected to the blower.

Plant material

The plant material used in this experiment was the Aglaonema commutatum ‘Snow White’, a well-known air-purifying foliage plant commonly utilized in bio-wall systems. It was purchased in a 10 cm pot from a farm in Hanam-si, Gyeonggi-do. The plants were acclimatized for one month in an indoor growth chamber (temperature: 24.3 ± 0.6°C, relative humidity: 70.2 ± 7.8%, light intensity: 90.0 ± 24.4 μmol·m−2·s−1, mean ± SD), before being planted in the biofilter system at 20 cm intervals (four plants per biofilter system). The biofilter system with the plants was then placed in a specially designed glass chamber for PM experiments (1.4 m L × 2.0 m W × 2.1 m H; 5.88 m3), and the experiment was conducted after an additional two-week acclimatization period (temperature: 23.4 ± 0.1°C, relative humidity: 97 ± 1.1%, light intensity: 45.2 ± 0.9 μmol·m−2·s−1) to allow the roots to settle.

Investigation and analysis methods

In this experiment, the PM reduction effect of a biofilter system was measured according to the VWC of the substrate. The results were then compared with the PD and AFR of the biofilter system according to variations in the VWC. The aim was to provide basic data to quantify the optimal VWC that would enhance the air purification effect of PBB systems while supporting plant growth. To this end, the following points were investigated.

Measurement of substrate volumetric water content

The substrate VWC was set at four levels: 10%, 20%, 30%, and 40%. It was measured using FDR-type soil moisture sensors (SMEC-100, Spectrum Technologies, Aurora, IL, USA), which were connected to a datalogger (Watchdog 450, Spectrum Technologies), with overhead irrigation applied. The substrate VWC was measured by the substrate moisture sensors calibrated appropriately for the substrate (Sunshine Mix4) used in the experiment, using the calibration equation: VWC(%) = 1.1334 × sensor output + 10.193; r2 = 0.9198. A total of nine sensors were installed vertically in the substrate at a depth of 4 cm, spaced 8 cm apart. As most of the gravitational water drained into the water tank via the air-path installed in the biofilter system during irrigation, it was not possible to raise the VWC beyond a certain level. Therefore, for the 40% VWC treatment group, for example, the result corresponding to a VWC of 36.3% was used. During the experiment, the temperature (21.7 ± 1.5°C) and relative humidity (45.9 ± 10.4%) inside the glass chamber for the PM experiment were also measured using a data logger.

PM concentration (μg·m−3) measurement

The biofilter system, with a target volumetric water content of the substrate, was placed in a glass chamber for PM experiments. PM samples (JIS test powders 1, Class 11, APPIE, Kyoto, Japan) were injected to adjust the PM concentration inside the chamber to 500 μg·m−3, after which the biofilter system was operated. The PM concentration inside the glass chamber was measured using a aerosol monitor (DustTrak 8533, TSI, Shoreview, MN, USA) over a period of 2 hours, with measurements taken at 5-minute intervals (2 minutes of measurement followed by 3 minutes of waiting), in accordance with the operating time of the biofilter system.

The PM reduction effect of the biofilter system according to the substrate VWC was estimated using the following equations [Eq. (1), (2)]. These equations were applied to calculate and compare the PM reduction (PR; μg) and the PM reduction efficiency (PRE; %) of the PBB system based on the initial PM concentration (500 μg m−3) in the glass chamber, and the PM concentration (μg m−3) measured 15 minutes after the biofilter system was operated for each treatment.

(1) PMreductionoftheBiofiltersystem(PR,μg)={(Ti-T)-(Ci-C)}×CV
(2) PMreductionefficiencyoftheBiofiltersystem(PRE,%)={(Ti-T)-(Ci-C)}/Ti×100
  • PR: Particulate matter reduction (μg)

  • PRE: Particulate matter reduction efficiency (%)

  • CV: Chamber volume (m3)

  • Ti: Initial PM concentration in the glass chamber where a biofilter system to be operated is placed (μg·m−3)

  • T: PM concentration in the glass chamber 15 minutes after operating the biofilter system (μg·m−3)

  • Ci: Initial PM concentration in the glass chamber where a biofilter system not to be operated is placed (μg·m−3)

  • C: PM concentration inside the glass chamber 15 minutes after installing the biofilter system not to be operated (μg·m−3)

Airflow rate (AFR; m3·h−1) and pressure drop (PD; kPa) measurement

The AFR (m3·h−1) discharged from the wind blower of the biofilter system was estimated as follows. A vane anemometer’s (Testo 416, Testo, Lenzkirch, Germany) measuring probe was inserted and fixed at the outlet of the PVC pipe (∅︀ 3.8 cm) connected to the blower. The real-time air velocity (m·s−1) was then measured and multiplied by the cross-sectional area of the pipe (45.36 cm2) to determine the AFR (Fig. 2). The PD (kPa) across the biofilter system was estimated by connecting a manometer (TPI 621, TPI, Beaverton, Oregon, USA) to the hand valves (Figs. 2B and 2C) installed at the top and bottom of the biofilter system. A silicone tube was used to connect the manometer to the valves, and the air pressure difference was measured before and after the air passed through the substrate. To this end, as shown in Fig. 2, an acrylic plate (300 mm L × 300 mm W × 10 mm H) with an inserted pipe was attached to the top of the biofilter system, and a hand valve was inserted into the pipe (Fig. 2B). Additionally, a pipe was connected between the biofilter system and the wind blower at the bottom, with a hand valve inserted into the pipe (Fig. 2C).

Fig. 2

Diagram of measuring the airflow rate (m3·h−1) and pressure drop (kPa) of the biofilter system. Airflow rate (m3·h−1) is calculated by surface area (45.36 cm2) of the pipe and air velocity (m·s−1) of the biofilter system measured by the vane anemometer attached to the outlet of the PVC pipe. Pressure drop (kPa) is measured by the manometer connected to the hand valves installed in the position before and after the air in the biofilter system passes through the substrate. The direction of air movement is as follows : A → B → C → D. Yellow arrow : particulate matter flow; Blue arrows: purified airflow.

Experimental design and statistical processing

To compare the PM reduction effect according to the substrate VWC in a plant-based biofilter system, each VWC treatment group (10%, 20%, 30%, and 40%) was randomly assigned with three replicates. The experimental results were analyzed through regression analysis and analysis of variance (ANOVA) using SAS 9.4 (SAS Institute, Cary, NC, USA) and SigmaPlot 14.5 (Grafiti, Palo Alto, CA, USA). As a post-hoc test for the ANOVA, Duncan’s multiple range test was used to compare the means, and the significance between the means of each treatment was tested at the α = 0.05 level. Furthermore, the correlation between the substrate VWC and the PM reduction efficiency (PM2.5, PM10), AFR, and PD of the biofilter system was analyzed using Pearson’s correlation coefficient (PCC).

Results and Discussion

Comparison of PM reduction effects by substrate volume water content

The experimental results showed that when the PBB system was operated, the PM concentration in the experimental glass chamber decreased, in all substrate VWC treatment groups. However, a comparison of the PM reduction (PR; μg) of the biofilter system according to substrate VWC levels showed that the reduction of both PM2.5 and PM10 decreased as the VWC increased (p < . 001; Fig. 3). Moreover, the PM reduction efficiency (PRE; %), which is estimated by dividing the PR of the biofilter system by the initial PM concentration in the experimental glass chamber, also showed a tendency to decrease proportionally as VWC increased (Table 1). As the substrate VWC increased from 10% to 20%, 30%, and 40%, the PRE for PM2.5 of the biofilter system decreased from 40.7% to 34.6%, 20.2%, and 15.7%, while the PRE for PM10 decreased from 41.2% to 35.1%, 20.0%, and 16.4%, respectively. In other words, it was found that the PRE for PM2.5 and PM10 decreased by 25% and 24.8%, respectively, as the substrate VWC of the biofilter system increased from 10% to 40%. This is consistent with the findings of previous research (Delhoménie and Heitz, 2005), which showed that the gas exchange area between the outside air and the biofilter layer is inhibited when the substrate VWC in the biofilter system is excessive. It appears that as the substrate VWC increases, the amount of air passing through the filter layer of the biofilter system decreases, which in turn reduces the PR.

Fig. 3

Relationship between volumetric water content of the substrate and particulate matter (PM2.5, PM10) reduction.

Particulate matter (PM2.5, PM10) reduction efficiency of plant-based biofilter system according to the volumetric water content of the substrate

The results of this experiment were consistent with the findings of previous studies, which emphasized that the substrate VWC is the main factor affecting the air purification efficiency of biofilter systems (Delhoménie and Heitz, 2005; Kennes and Thalasso, 1998). In particular, by numerically quantifying the linear decrease in the PM reduction effect as the VWC increases within the range of the VWC where plants can grow, it was found that the optimal substrate VWC should be considered to maximize the PM reduction performance when operating biofilter systems. In this experiment, the amount of PM reduction of the 40% VWC treatment, which had the highest VWC, was reduced by 1.5 times or more compared to the PM reduction of the 10% VWC treament. This suggests that maintaining a high substrate VWC for good plant growth may act as a factor inhibiting the air purification effectiveness of the biofilter system. Previous studies have focused on developing technologies to stabilize the substrate VWC for proper plant growth (Kim et al., 2016; Lee et al., 2015a) and evaluating plant growth (Choi et al., 2014; Kim et al., 2016), but the results of this experiment suggest that a high VWC, which is intended to support plant growth, may have the undesired effect of reducing the PM reduction effect of biofilter systems. However, due to the environmental characteristics of indoor plants, it has been reported that the drought response in plants is reduced under low light intensity (Lee and Kim, 2024); moreover, for indoor plant maintenance, a low VWC is effective in preserving the ornamental value of the plants (Nam et al., 2023). Based on these, it appears that maintaining a low VWC in biofilter systems using indoor plants will maximize the PM reduction effect while also ensuring the efficient growth and maintenance of the plants.

Comparison of airflow rate and pressure drop by substrate VWC

To determine the cause of the change in PM reduction effectiveness according to the VWC of the substrate in the biofilter system, the AFR (m3·h−1) discharged from the wind blower at the lower section and the PD (kPa), which represents the pressure difference between the air before and after passing through the substrate, were compared. The results showed that as the substrate VWC increased from 10% to 40%, the AFR gradually decreased, from 112.7 to 90.9, 81.1, and 76.8 m3·h−1, while the PD increased, from 0.040 to 0.075, 0.094, and 0.118 kPa, respectively (Fig. 4). Furthermore, a correlation analysis between the substrate VWC in the biofilter system and the PRE for PM2.5 and PM10, AFR, and PD revealed that the substrate VWC had a strong negative correlation with the PRE for PM2.5 and PM10, as well as with the AFR, while it exhibited a strong positive correlation with the PD (Table 2). This finding aligns with those of a previous study (Choi et al., 2014), which showed that the PD in planted biofilter systems tripled under high substrate VWC conditions (28.7%) compared to under relatively dry substrate VWC conditions (18.5%). As the substrate VWC increases, the pores within the substrate become saturated with moisture, resulting in a decrease in porosity. Consequently, the air passing through the substrate encounters more resistance, decreasing air permeability and increasing the PD (Choi et al., 2014). This increase in PD requires a higher blower speed (Detchanamurthy and Gostomski, 2012). Even in this experiment, as the substrate VWC increased from 10% to 40%, the PD rose by 195%, while the AFR discharged from the blower decreased by 32%, which appears to have reduced the PM reduction effectiveness of the biofilter system.

Fig. 4

Relationship between the volumetric water content of the substrate and airflow rate (m3·h−1), pressure drop (kPa) of the biofilter system: (A) airflow rate, (B) pressure drop.

Pearson’s correlation coefficient between the biofilter system substrate’s volumetric water content (VWC), airflow rate, pressure drop, and particulate matter reduction efficiency (PM2.5, PM10)

In general, as the moisture content of the air increases and the relative humidity rises, the moisture absorption of PM suspended in the atmosphere also increases; this leads to an increase in the mass of PM absorbing moisture, causing it to coagulate and settle, thereby reducing the overall PM concentration (Kim et al., 2023). Based on these characteristics, there has been a growing interest in developing PM reduction technologies that utilize moisture. These include fine water spray systems that reduce PM by adsorbing PM onto water droplets and promoting its sedimentation (Tsai et al., 2003). More recently, drone systems that spray water particles have been developed to reduce PM concentrations in work environments (Choi et al., 2021). However, in the biofilter system used in this experiment, the high substrate VWC increased the PD and thereby decreased the AFR, which also decreased the PM reduction effect; this decrease in the PM reduction effect was more significant than the PM reduction effect caused by the increased relative humidity from the substrate VWC and the adsorption of PM onto the water droplets on the substrate surface. Therefore, it appears that the PM reduction effect of the biofilter system decreased as the VWC increased. In the future, if the PD and air velocity of the biofilter system can be adjusted to maintain an optimal AFR regardless of the substrate VWC—by using a high-power blower or developing technology to enhance airflow within the substrate—further research will be required to compare the effect of the substrate VWC on the PM reduction effect of the biofilter systems.

This study differs from previous studies (Delhoménie and Heitz, 2005; Kennes and Thalasso, 1998) that suggested potential differences in air purification effects based on VWC, in that it quantitatively assessed the PM reduction effect of the biofilter system according to substrate VWC. However, it has the limitation of not comparing the effects of reducing gaseous pollutants, such as volatile organic compounds (VOCs) including benzene and toluene, as well as the assessment of plant growth based on VWC. The air purification effect of biofilter systems is important not only for the reduction of particulate matter but also for the removal of gaseous pollutants. Moreover, since plants have varying VOC removal capabilities depending on the VWC (Kim et al., 2023), it is crucial to measure the VOC removal effect when developing biofilter systems. Future research should focus on quantifying the optimal VWC range that can maximize both the optimal growth of plants and the air purification effect of the biofilter system. This should involve a comprehensive evaluation of plant growth and VOC removal, along with a comparison of PM reduction effects based on the substrate VWC of biofilter systems.

Conclusion

This study was conducted to provide foundational data for improving indoor air quality by introducing a plant-based biofilter system. It focused on quantifying the optimal range of substrate VWC that promotes both desirable plant growth and the air purification effect of the biofilter system. As the substrate VWC increased, the PD of the air passing through the substrate increased and the AFR emitted by the wind blower decreased, which in turn reduced the PM reduction effect of the biofilter system. This appears to be the result of a decrease in porosity and quantity of airflow as the pores within the substrate filled with moisture, increasing the resistance of the air passing through the substrate.

Therefore, to improve the PM reduction efficiency in future biofilter systems, it is crucial to maintain an optimal substrate VWC. Building on this, further research should focus on identifying plant species capable of effectively purifying VOCs and thriving in low VWC environments, as well as examining the effects of such conditions on plant growth and maintenance within these systems. Furthermore, technologies should be developed to enhance the physical structure of the substrate, modify its type and combination, optimize the air passages within the substrate and more. These technologies should be used to maintain a seamless airflow within the substrate, while keeping the PD low, even as the VWC increases, all while supporting proper plant growth. To this end, continued follow-up research is needed to maximize the air purification effect of biofilter systems.

Notes

This paper was funded by the research project of Rural Development Administration (PJ016057)

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Fig. 1

Diagram of the plant-based biofilter system used in this experiment : The passages of air with particulate matter is as follows : (A) polluted air is sucked into the upper part of the biofilter system while filtered by plants → (B) air is passed through the substrate and the air path filled with activated carbon → (C) air moves down to the lower part → (D) biofilter-passed airflows out through the PVC pipe connected to the wind blower. Yellow arrows : particulate matter flow; Blue arrows: airflow.

Fig. 2

Diagram of measuring the airflow rate (m3·h−1) and pressure drop (kPa) of the biofilter system. Airflow rate (m3·h−1) is calculated by surface area (45.36 cm2) of the pipe and air velocity (m·s−1) of the biofilter system measured by the vane anemometer attached to the outlet of the PVC pipe. Pressure drop (kPa) is measured by the manometer connected to the hand valves installed in the position before and after the air in the biofilter system passes through the substrate. The direction of air movement is as follows : A → B → C → D. Yellow arrow : particulate matter flow; Blue arrows: purified airflow.

Fig. 3

Relationship between volumetric water content of the substrate and particulate matter (PM2.5, PM10) reduction.

Fig. 4

Relationship between the volumetric water content of the substrate and airflow rate (m3·h−1), pressure drop (kPa) of the biofilter system: (A) airflow rate, (B) pressure drop.

Table 1

Particulate matter (PM2.5, PM10) reduction efficiency of plant-based biofilter system according to the volumetric water content of the substrate

Particulate matter size (μm) Particulate matter reduction efficiency (%) p-value

Substrate volumetric water content (%)

10 20 30 40
2.5 40.7 a z 34.6 b 20.2 c 15.7 d p < 0.001
10 41.2 a 35.1 b 20.0 c 16.4 c p < 0.001
z

Mean separation within rows using Duncan’s multiple range test at α = 0.05

Table 2

Pearson’s correlation coefficient between the biofilter system substrate’s volumetric water content (VWC), airflow rate, pressure drop, and particulate matter reduction efficiency (PM2.5, PM10)

Variables Pearson’s correlation coefficient (r)

VWC (%) Airflow rate (m3·h−1) Pressure drop (kPa) PM2.5 (%)
Airflow rate (m3·h−1) −0.923***
Pressure drop (kPa) 0.985*** −0.957***
PM2.5 (%) −0.966*** 0.905*** −0.936***
PM10 (%) −0.950*** 0.895*** −0.918*** 0.996***
***

significant at p < .001.