Principles of Air Filtration for Swine Facilities

Introduction

Air filtration is a commonly used biosecurity practice to minimize the spread of airborne diseases. Common airborne diseases include Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Mycoplasma hyopneumoniae, and influenza A virus in swine (IAV-S). These diseases present substantial productivity and economic losses for producers. For example, PRRSV is estimated to cost the US swine industry $664 million per year (Holtkamp et al., 2013). Air filtration systems require careful consideration to determine if they are needed and beneficial. This fact sheet will cover these key topics related to selecting a filtration system and managing the system.

Objectives

• Describe why air filtration is useful in swine production
• Document different types of air filters
• Characterize how air filters are installed on-farm

Why air filtration on swine farms?

There are multiple modes through which diseases can be transferred into swine farms. Common transmission routes include direct contact, oral, fomites, vectors, and zoonotic (Ramirez & Zaabel, 2012). While there are well-defined biosecurity management practices for these transmission routes, airborne disease transmission is typically only addressed by siting farms far apart. Airborne diseases pose significant health and economic threats to swine producers. For example, PRRSV was estimated to cost the US Swine industry $664 million in 2012 (Holtkamp et al., 2013). In swine dense states, such as Iowa and North Carolina, the ability to adequately space farms is not viable. This can create a significant challenge as research has shown that PRRSV can travel 2.92 miles (4.7 km) and is still viable to infect a farm (Dee et al., 2009). In addition to PRRSV, other diseases have been shown to be viable in airborne samples. These diseases include Porcine Epidemic Diarrhea Virus (PEDV) and influenza A virus in swine (IAV-S). For the respiratory viruses (PRRSV, IAV-S) virus was found to be viable of particles larger than 2.1µm (Alonso et al., 2015).

The use of air filters has been shown to reduce the possibility of airborne disease transmission (Alonso et al., 2013b; Dee et al., 2005). The use of air filtration as a biosecurity measure is an effective method and financially feasible when elevated disease pressure is present (Alonso et al., 2013a). The impact of filtered air on production and barn environment has rarely been researched, so a benefit in a low disease pressure area (not in key production state or areas experiencing expansion in the number of swine operations) is not likely to be enough to outweigh the cost of the system. The individual sites history or airborne disease outbreaks must be considered along with the existing biosecurity program in place on farm.

What kind of air filters are used on swine farms?

There are multiple types of filters based on the particle removal goal. The most universal system defines filters as primary, secondary, and final filters (Figure 1). In the US, filter removal rating is called the “Minimum Efficiency Reporting Value” or MERV (Figure 2). The MERV rating ranges from 1 to 20, as the number increases, the removal efficiency (i.e., traps smaller particles) increases as well (ASHRAE, 2012). Typically, as the MERV rating increases so does the financial cost of the filter and the resistance to airflow (i.e., pressure drop). Another key metric of filter performance is the dust loading capacity, which represents the mass of dust a filter can hold before airflow becomes restricted. Primary filters are used to stop very large particles, that is 3 to 10µm and have a high dust loading capacity. The MERV rating of primary filters range from MERV 1 to 8. An example of a common primary filter is a furnace filter in a residential unit. Primary filters are also often called “prefilters” in the swine industry. Typically, the primary or pre-filter in a filtered barn has a MERV-8 rating. Secondary filters are intended to catch finer particles, around 0.3 to 3.0 µm. The range of MERV ratings for secondary filters is MERV 9 to 16. In the swine industry, these are commonly called “V-bank filters” or “bag filters”. The common recommendation for “low density” swine regions for secondary filters is a MERV 14, while in “high density” areas a MERV 15 or 16 is appropriate. Depending on the specific brand of filter used alternative ratings can be used, consult a filter specialist or ag engineer when selecting and sizing an air filtration system for specific recommendations. Lastly, final filters are designed to stop nearly every particle in the air. These are commonly called “HEPA” filters with a MERV rating greater than MERV 17. In addition, there are final filter specific rating tests that target the finer particles. For example, a filtration system commonly found on sow farms utilize primary and secondary filters. Compared to boar studs, where primary, secondary, and final filters are utilized.

Filter performance on swine farms is impacted by the different types of media used to make the filters. The two commonly available types of filter media are synthetics and fiberglass. Synthetic media commonly has larger openings in the media and rely on a static charge and a higher airspeed to trap the particles. Fiberglass media has smaller openings and cannot be statically charged. Both filters offer the same removal efficiency when new, direct from the manufacturer. However, synthetic media can lose its the static charge from sitting in a warehouse for extended periods of time, becoming wet, or for unknown reasons. This loss of static charge reduces the removal efficiency by a typical interval of 3 MERV ratings.

How air filters are employed and managed?

An air filtration system can be integrated into a variety of ventilation systems that are currently used on swine farms. Typically, air filters are employed on sow farms with negative or positive pressure ventilation systems. On boar studs, filters are commonly used in negative or positive pressure ventilation systems or with commercial ventilation system featuring air conditioning. Regardless of the ventilation system, the most commonly used filter size is 24 in. by 24 in. There is a less common filter size of 24 in. by 20 in., this size is most frequently used in negative pressure ventilation systems with 4’ on-center truss spacing to allow for the filter boxes to fit between trusses(Figure 3).

In every type of filtration there are three key points to consider. First, air filters need to remain as dry as possible. Special consideration needs to be taken to protect the filters from water, rain, and the outside elements. The second key is that the filter grids that hold the filters in place needs to have an excellent seal to the structure and between the grids. The third key point is to keep and maintain an excellent seal between the primary and secondary filter to maximize the lifespan of the secondary filter. Each type of ventilation system presents different risks for the installation, maintenance, and operation of the filters.

Negative pressure ventilation systems are the most common type of mechanical ventilation system in the swine industry. The common location for installing air filters, primary and secondary, is in the attic over the ceiling inlets in the room (Figure 3, Alonso et al., 2013a). Depending on what kind of filter grid is used over the inlet, there could be two different filter sizes utilized. Adding air filtration to an existing system can be advantageous as it requires minimal changes to the facility structure. Common adaptations include filter grids installed and sealed above all inlets (Figure 4), addition of inlets for summer ventilation, and fan bump-out rooms for backdraft prevention. The downside is, when replacing the secondary filter, there is no way to prevent unfiltered air from entering the barn. Operationally, potential downsides include unfiltered air leakage (infiltration) through the barn shell, dust settling onto the filter media, and improper fan sizing, and the inability to inspect filters effectively once installed (Jadhav et al., 2015). Since filters require additional fan energy (due to increased resistance), common axial fans used in the swine industry are not designed to ventilate with air filters. Analysis of fan performance on existing facilities should be conducted prior to filter installation to see if adequate ventilation can be maintained and how to correct inadequate fan performance for an air filtration system. To maintain adequate ventilation rates, additional fans or high-pressure axial (direct-drive) fans may be required. There are additional costs associated with adding typical fans or swapping all fans to the high pressure version.

Positive pressure ventilated barns are a new trend in the swine industry (Ramirez et al., 2016; Smith et al., 2019). This type of ventilation system removes the infiltration risk by ensuring only filtered air enters the barn (leaks outward). Positive pressure barns typically require large vertical filter banks of both primary and secondary, eliminating the risk of dust settling into the filters, which is better for filter efficiency and longevity (Figure 5). The downside is that a positive pressure ventilated barn with filters will require a larger barn footprint. The other downside is that high pressure axial fans are required to operate the barn along with more advanced controllers.

Commercial ventilation systems will employ cooling units and specialized fans. Mechanical engineering firms can design the units for the farm and be involved in the maintenance and operation of the system. The downside is the larger electrical consumption and specialized components. The other consideration is that commercial units typically have side loaded filter grids. These grids present higher risk of air by-passing the filters when the filters are being installed, and during operation. This type of ventilation system has the highest costs and there is currently very little research to support benefits on sow farms.

Regardless of the type of ventilation system that the filters are utilized in, there are common operational tasks that must be completed to maximize filter effectiveness. When installing filters, there are a few items to check for. First, ensure that the filter is installed correctly. Most manufactures will mark the filter with a directional arrow for airflow. If the filter is installed in a vertical filter bank the pleats, or folds in the media, should run vertical as well. Once a filter is installed, an inspection of the filters seal to the grid and potential for bypass around the filter and grid should be conducted. Periodically throughout the lifespan, an inspection of the filter and checks for bypass should be conducted. Special attention should be given to checks at the start of winter and summer.

Filter lifespan is a challenging item to predict on a swine farm. The best approach is to routinely test a small sample of filters in a barn or filter bank. The most frequent test should be filter resistance (static pressure across the filter). For the swine industry, this test is commonly completed at a pressure drop of 0.15 in. of water column (static pressure) and the airflow is measured (Smith, et al., 2019b). To get the true airflow per filter on the farm the primary and secondary filter from the farm should be tested together. Other tests to consider at least annually is a filter efficiency (MERV rating). Note more frequent tests might be warranted if a synthetic secondary filter is utilized. Recent research on positive pressure ventilated barns has shown that a minimum sample size of 5.5% of all filters in a filter bank is representative (Smith, et al., 2019a). Other key findings include the worst case for filter lifespan is a filter bank with an intake within the immediate vicinity of a barns exhaust outlet. Regardless of the system routine filter checks and filter testing is the best approach to maximize the effectiveness and lifespan of the filters.

Summary

Including air filtration on a swine farm is an effective biosecurity practice for controlling airborne disease transmission. Financially, air filtration has been shown to be feasible for farms in swine dense regions, especially for farms that routinely have PRRSV outbreaks. There are three different types of air filters that are commonly used on swine farms. The type of filters utilized will depend on your farm’s specific ventilation system. Careful consideration should be given to the selection of the filters to be used on farm. Special care is needed during installation and operation of the filters to maintain an effective filtration system.

References and Citations

• Alonso, C., Davies, P. R., Polson, D. D., Dee, S. A., & Lazarus, W. F. (2013a). Financial implications of installing air filtration systems to prevent PRRSV infection in large sow herds. Preventive Veterinary Medicine, 111(3), 268–277.
• Alonso, C., Murtaugh, M. P., Dee, S. A., & Davies, P. R. (2013b). Epidemiological study of air filtration systems for preventing PRRSV infection in large sow herds. Preventive Veterinary Medicine, 112(1), 109–117.
• Alonso, C., Raynor, P. C., Davies, P. R., & Torremorell, M. (2015). Concentration, Size Distribution, and Infectivity of Airborne Particles Carrying Swine Viruses. PLOS ONE, 10(8), e0135675. https://doi.org/10.1371/journal.pone.0135675
• ASHRAE. (2012). ANSI/ASGHRAE Standard 52.2-2012: Method of testing general ventilation air-cleaning devices for removal efficiency by particle size.
• Dee, S., Batista, L., Deen, J., & Pijoan, C. (2005). Evaluation of an air-filtration system for preventing aerosol transmission of porcine reproductive and respiratory syndrome virus. Canadian Journal of Veterinary Research, 69(4), 293.
• Dee, S., Otake, S., Oliveira, S., & Deen, J. (2009). Evidence of long distance airborne transport of porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae. Veterinary Research, 40(4), 1–13.
• Holtkamp, D., Kliiebenstein, J., Neumann, E., Zimmerman, J. J., Rotto, H., Tiffany, Y., Wang, C., Yeske, P., Mowrer, C., & Haley, C. (2013). Assessment of the economic impact of porcine reproductive and respiratory syndrome virus on United States pork producers. Journal of Swine Health and Production, 21(2).
• Jadhav, H. T., Hoff, S. J., Harmon, J. D., Jacobson, L. D., & Hetchler, B. P. (2015). Infiltration Characteristics of Swine Finishing and Gestation Buildings: Review and Quantification. 435. http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1457&context=abe_eng_conf
• Ramirez, A., & Zaabel, P. (2012). Swine Biological Risk Management. Iowa State University. http://www.cfsph.iastate.edu/pdf/swine-biological-risk-management
• Ramirez, B. C., Hoff, S. J., Harmon, J. D., & Stinn, J. P. (2016). Thermal environment performance and uniformity assessment for a novel swine breeding and gestation facility. Presented at the ASABE Annual International Meeting, Orlando, Florida: ASABE. 1–9. https://doi.org/10.13031/aim.20162454577
• Smith, B., Hoff, S., Harmon, J., Andersen, D., Zimmerman, J., & Stinn, J. (2019). Quantification of Site Layout and Filter Characteristics on Primary Filter Airflow Reduction on Commercial Swine Sites in Iowa. AgriEngineering, 1(2), 291–302. https://doi.org/10.3390/agriengineering1020022
• Smith, B., Ramirez, B., Hoff, S., Harmon, J., & Stinn, J. (2019). Design and validation of a mobile air filter testing laboratory for animal agricultural applications. Agricultural Engineering International: CIGR Journal, 21(3), 39–50.