Ventilation and Bedding Effects on Air Quality in Swine Confinement Systems

1. Introduction

The global landscape of meat production has undergone a dramatic transformation over the past few decades, marked by a substantial rise in intensive confined livestock operations to meet growing consumer demands [1,2]. This industrial approach to animal agriculture has introduced significant environmental challenges and raised pressing concerns about animal welfare. Concentrated Animal Feeding Operations (CAFOs) have fundamentally reshaped animal husbandry, often reducing animals to “cogs in a ma-chine” with limited regard for their natural behaviors or physiological needs [3]. Common practices include routine procedures such as piglet tail docking and tooth clipping, while animals are confined to restrictive spaces with metal flooring that inhibits natural behaviors like soil rooting. Furthermore, the environmental impact of these operations is considerable, particularly in terms of air pollution emanating from confined animal housing.

Ventilation systems in swine facilities often expel air containing high concentrations of odorous gases and particulate matter [2]. The emission of volatile organic compounds (VOCs) from these facilities has become a significant public health concern [4], contrib-uting to the formation of photochemical smog and respirable suspended particulates [5]. Particulate matter from swine houses also carries microorganisms and bio-aerosols, in-creasing the risk of respiratory infections in nearby communities [6]. Approximately 50% of odor emissions stem from livestock buildings (30%) and manure storage facilities (20%) [7]. The primary gaseous pollutants include ammonia, carbon dioxide, methane, sulfur compounds, and VOCs [8]. In response to these challenges, researchers and envi-ronmental engineers are actively developing and refining technologies to mitigate the environmental impacts of swine housing and manure management systems.

Various pollution control technologies have been explored to mitigate emissions from swine facilities, including activated carbon adsorption, wet scrubbing, and masking agents [9,10]. Among these, wet biofiltration has recently gained attention as a promising solution for managing low to moderate concentrations of odorous air and VOCs [11,12]. Biofilters have demonstrated the potential to reduce odor emissions by 40–83% in facilities near residential areas [13], with their effectiveness largely dependent on sufficient air contact time [14].

Innovative studies by Zong et al. [15,16] have focused on optimizing ventilation exhaust systems to identify cost-effective solutions for purifying air from swine housing. However, a critical gap persists in the literature: comprehensive investigations into the combined effects of bedding systems and ventilation strategies on odor mitigation and overall environmental impact. Addressing this gap is essential to developing integrated approaches for sustainable swine farming.

As global demand for meat continues to rise, the animal production industry faces an urgent need to develop engineering solutions that effectively manage gaseous odor pollutants while minimizing environmental and health risks. In this study, we adopted an integrated approach that incorporates bedding types, ventilation systems, and emission control technologies. This approach offers a promising pathway toward more sustainable and environmentally responsible livestock production.

2. Materials and Methods

2.1. Experimental Swine Houses Design and Operation

Three experimental swine housing models (M1, M2, and M3) were constructed to evaluate the effects of housing design on aerial environmental conditions (Table 1, Figure 1a–c, and Figure 2). The houses featured distinct biofilter configurations and flooring systems. M1 and M2 utilized fully slatted floors with underlying slurry storage pits, while M3 incorporated a sawdust bed. The biofilter systems varied across models: M1 employed vertically arranged cellulose materials, M2 utilized horizontally arranged cellulose materials, and M3 used a biofilter composed of biochar and flue gas desulfurization (FGD) gypsum. To maintain biofilter moisture, treated swine wastewater was recycled at a rate of 3.75 L/h, with the dripped water collected in a 2.8 m × 1.5 m × 1.0 m reservoir for reuse. Each housing unit accommodated nine fattening swine within a 12 m² area (3 m × 4 m), providing 1.3 m² per head—exceeding the typical field standard of 0.8–1.0 m² per head. The experiment was conducted over 16 weeks, from May 8 to August 26, 2018. The swine were provided with unrestricted access to food and water. Feed composition included 16% crude protein during the growing phase and 14.5% during the finishing phase. Each house was equipped with a distinct airflow pattern. In M1, air entered through an inlet duct, flowed downward, recirculated above the slatted floor, and exited through the vertical biofilter (Figure 1a). M2’s air entered through the inlet duct, flowed downward into the slurry pit, and exited through the horizontal biofilter (Figure 1b). M3 followed a similar airflow pattern to M2 but incorporated a sawdust bed to prevent infiltration (Figure 1c). Ventilation rates were automatically controlled (Vent System A/S, Roslev, Denmark) to maintain a constant temperature of 23°C, with a maximum flow capacity of 60 m³/head/h (2,160 m³/h or 0.67 m³/s). The system included recirculation of treated swine slurry into the pit to enhance microbial density and wetting of the biofilter to reduce micro-dust and odor emissions.

2.2. Analysis of PM and Odorous Compounds

Particulate matter was measured using an aerosol mass monitor (GT-331, SIBATA) oper-ating at a precise flow rate of 2.83 L/min. This equipment allowed for the simultaneous quantification of PM10, PM2.5, and total suspended particles (TSPs). For the analysis of volatile organic compounds (VOCs), gas chromatography coupled with mass spectrometry (GC/MS, Agilent GC6890N/5975C) was employed. Air samples were collected over five-minute intervals using 1-L Tedlar bags (No. 22053, Restek, Bellefonte, PA, USA) to ensure accuracy and consistency in sample capture. The analysis followed the protocol detailed by Kumari et al. [17], enabling precise identification and quantification of indi-vidual VOCs. The VOC analysis included specific compound groups. Sulfur compounds, such as dimethyl sulfide and dimethyl disulfide, were measured for their known contri-bution to odor intensity. Volatile fatty acids, including acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, and isovaleric acid, were quantified due to their signif-icant role in swine facility emissions. Indoles (indole and skatole) and phenols (p-cresol) were also included, as these compounds are critical indicators of odor and air quality in livestock environments. Ammonia (NH₃) and hydrogen sulfide (H₂S) concentrations were determined using a Gastec probe (Gastec Co., Ltd., Kanagawa, Japan), providing real-time data on these key gaseous pollutants. The removal efficiency of the biofilters was calculated using a standard equation, as outlined by previous studies [18].

RE = ((C₁ − C₂)/C₁) × 100

(1)

where C₁ represents inlet NH₃ concentrations and C₂ represents exhaust NH₃ concentrations.

2.3. Airborne Bacteria Analysis

Airborne bacteria were monitored by measuring the total bacterial count (TBC), total Escherichia coli (TE), and total fecal coliform (TC) using the plate settling method. Standard Petri dishes (90 mm × 15 mm, SPL Life Sciences, Pochun-si, Korea) were employed for this purpose. Each bacterial group was cultured on selective media: Tryptic soy agar for TBC, Chromocult Coliformen agar for TE, and m-FC agar for TC. The plates were incubated under controlled conditions at 35°C for 24 hours to facilitate the growth of TBC, while TE and TC plates were incubated for 48 hours to allow for adequate bacterial growth. After the incubation period, bacterial colonies were counted and the results were expressed as colony-forming units per minute (cfu/min) of settling, providing a standardized measurement of airborne bacterial contamination.

2.4. Statistical Analysis

Statistical analysis employed ANOVA at 5% significance level, with Tukey’s test for comparing quantitative factor averages using SPSS software (SPSS Inc., Chicago, IL, USA)..

3. Results and Discussion

3.1. NH₃ and H₂S Reduction

The management of gaseous emissions, particularly ammonia (NH₃) and hydrogen sulfide (H₂S), remains a critical challenge in swine production facilities. These emissions affect both animal welfare and environmental quality [19]. Three pilot swine houses (M1, M2, and M3) were evaluated for NH₃ concentrations (Figure 3). M1 exhibited significantly higher NH₃ levels (9.07 ppm, p < 0.05) compared to M2 (6.10 ppm) and M3 (5.94 ppm). These findings align with research by Arogo et al. [20], who reported similar NH₃ concentration patterns in conventional versus alternative housing systems. The sawdust bedding system (M3) initially demonstrated superior NH₃ control, consistent with findings from Huang et al. [21], who attributed this to high organic carbon content binding with NH₃ molecules. However, after day 14, NH₃ levels in M3 increased significantly, potentially due to reduced absorption capacity of bedding material latter stages. This pattern aligns with observations by Gallmann et al. [22], who documented similar temporal changes in bedding-based systems. The increased NH₃ emissions post-day 14 correlate with findings from Cao et al. [23] regarding the impact of urine and feces accumulation on bedding degradation processes. H₂S concentrations varied significantly among systems, with M1 averaging 0.63 ppm, M2 at 0.57 ppm (p < 0.05), and M3 showing undetectable levels (Figure 4). These results support findings by Ni et al. [24], who demonstrated the effectiveness of alternative housing systems in H₂S reduction. The peak H₂S concentration in M1 (3.2 ppm) exceeded the recommended maximum of 3.0 ppm [25], consistent with observations by Grant and Boehm [26], regarding anaerobic conditions in conventional systems. M3 with sawdust bedding shows lower H₂S emission than that of the pit slurry system. This study suggested integrating sawdust bedding into a pit slurry system to keep H₂S in the permissible range.

3.2. VOCs and PM

Volatile organic compounds (VOCs) and particulate matter (PM) emissions from swine housing facilities are significant environmental and health concerns in livestock produc-tion [27]. This study evaluated three different swine housing systems to assess their impact on VOC (Table 2, Figure 5) and PM emissions (Figure 6). The results revealed significant differences in VOC concentrations across the three housing systems. M1 exhibited the highest VOC concentration (884.29 ppbv), followed by M2 (605.31 ppbv), and M3 (84.86 ppbv) (p < 0.05). These findings are consistent with those of Hutchings et al. [28], who reported that slurry pit systems generally produce higher VOC emissions compared to alternative bedding systems. The sawdust bedding system (M3) demonstrated an ap-proximately 85% reduction in VOC emissions compared to the pit systems, aligning with results from similar studies [29]. The recirculation of treated wastewater in M2 led to a 31% reduction in VOC emissions compared to the conventional pit system (M1). While phenolic compound concentrations were similar between M1 (0.86 ppbv) and M2 (0.94 ppbv), M3 showed significantly lower levels (0.56 ppbv, p < 0.05). This trend corresponds with the findings of Gallmann et al. [22], who observed lower phenolic emissions in bedding-based systems. Sulfuric compound emissions followed a similar pattern to VOCs, with M3 (2.44 ppbv) exhibiting significantly lower concentrations than M1 (14.42 ppbv) and M2 (5.06 ppbv). The sawdust bedding system demonstrated a reduction efficiency of 51.77–83.07% for sulfuric emissions. However, in contrast to VOC results, the sawdust bedding system (M3) generated higher PM levels. Specifically, PM10 concentrations in M3 were 312.0 μg/m³, and PM2.5 concentrations were 164.29 μg/m³, significantly exceeding those in M1 and M2 (p < 0.05). This observation aligns with the work of De Rooij et al.. [30], who reported elevated PM levels in bedding-based systems due to animal activity and bedding material properties. The pit bedding systems (M1 and M2) exhibited lower PM concentrations, reaching only 48–72% of M3’s PM2.5 levels and 37–79% of M3’s PM10 levels. These results suggest a trade-off between VOC and PM emissions when selecting housing systems. This study demonstrates that while sawdust bedding systems are effective in reducing VOC emissions, they may contribute to higher PM concentrations. Additionally, the use of recirculated treated wastewater shows promise in reducing VOC emissions in pit systems. These findings provide valuable insights into emission control strategies for swine production facilities.

3.3. Aerial Microbes’ Concentration

The concentration of airborne microbes in swine housing facilities is a significant concern for both animal and worker health. Understanding how different housing designs influence microbial dispersion is essential for optimizing facility management practices [30]. This study compared three swine housing systems (M1, M2, and M3), focusing on total bacterial count (TBC), total coliforms (TC), and total E. coli (TE) concentrations (Figure 7). The sawdust bedding system (M3) exhibited significantly higher concentrations of all measured microbial parameters compared to the slurry pit systems (M1 and M2). These findings are consistent with the work of Lühken et al. [31], who reported elevated airborne microbial loads in bedding-based systems. The slurry pit systems (M1 and M2), which featured slot-plastic porous floors separating the airspace from the pit space, demonstrated lower airborne microbial concentrations. This effect is explained by Bilić et al. [32], who documented the effectiveness of spatial separation in reducing microbial contamination in the air. In contrast, the sawdust bedding system (M3), lacking spatial separation, resulted in higher microbial dispersion, particularly when the bedding material dried. These results align with findings from Zhao et al. [33], who noted a relationship between bedding moisture content and microbial aerosolization. The increased microbial concentrations in M3 were also linked to enhanced pig activity, supporting the observations of Kim et al. [34], who noted that enriched environments with natural behaviors promote greater microbial dispersion. The sawdust bedding, while beneficial for animal welfare, encouraged natural behaviors such as rooting, digging, and increased movement patterns, which in turn led to greater microbial dispersal. As noted by Luiken [35], such behavioral patterns in bedding-based systems are key contributors to higher microbial loads in the air.

4. Conclusions

This comprehensive study of three swine housing systems (M1, M2, and M3) highlighted signifi-cant trade-offs between various emission types and environmental parameters. The sawdust bedding system (M3) demonstrated superior performance in reducing certain gaseous emissions, achieving an 85% reduction in volatile organic compounds (VOCs) compared to conventional systems and maintaining hydrogen sulfide (H₂S) at undetectable levels. However, this system exhibited notable limitations, particularly in particulate matter (PM) emissions and airborne microbial concentrations. In contrast, the recirculated treated wastewater system (M2) emerged as a promising intermediate solution. It achieved a 31% reduction in VOC emissions compared to the conventional pit system (M1), while avoiding the elevated PM levels observed with the sawdust bedding system. These findings suggest that hybrid approaches may offer the most balanced solution for emission control. Future research should focus on developing integrated systems that combine the advantages of each approach while minimizing their respective drawbacks, thereby optimizing both environmental and operational outcomes in swine production.