An All-in-One Concept of a Mobile System for On-Farm Swine Depopulation, Pathogen Inactivation, Off-Site Carcass Disposal, and Biosecure Cleanup

Abstract

Infectious animal diseases can cause severe mortality on infected farms. An outbreak challenges the system and forces difficult decisions to stop the disease progression. We propose an ‘all-in-one’ concept of a mobile system for on-farm swine depopulation and pathogen inactivation. The system uses vaporized CO₂ followed by heat treatment, broadening options for off-site carcass disposal and cleanup. A direct-fired heater supplies heat into the insulated trailer to reach and maintain the inactivation temperature for targeted pathogens. We developed a user-friendly model based on engineering principles for estimating site- and scenario-specific CO₂ amounts and times required to inactivate targeted pathogens. Multipoint CO₂ injection and improved distribution to animals follow the plug-flow reactor air replacement model. The model illustrates the depopulation and inactivation of two diseases, African swine fever (ASF) and the porcine reproductive and respiratory syndrome (PRRS) viruses. The model allows for dump trailer size, pig number, weights, and environmental conditions input. Model outputs provide users with practical information about the required CO₂ injection rate, temperature setpoints, and times to effectively depopulate and inactivate pathogens in carcasses. The concept could be adopted for a routine or a mass depopulation/treatment/disposal with a single or fleet of ‘all-in-one’ units.

1. Introduction

The endemic African swine fever (ASF) strains in Europe and Asia can result in severe mortality (90% or more) on infected farms [1]. A highly infectious disease outbreak challenges the production system and forces difficult decisions to stop the disease’s progression. Although recent attempts on DNA and live attenuated ASF virus vaccines have been reported with promising efficacy [2], there currently is no commercially available vaccine or treatment for ASF. Once ASF outbreaks occur, the current option is implementing a ‘stamping out’ policy where entire herds with infected animals are depopulated. Therefore, there is a critical need for developing and implementing rapid and humane depopulation procedures for a large number of pigs.

Carbon dioxide (CO₂) is one of the depopulation methods approved by the American Veterinary Medical Association (AVMA) for the humane euthanasia of large amounts of livestock [3]. CO₂ inhalation results in rapid depressant, analgesic, and anesthetic effects. Enclosed dump-bed trucks or trailers serving as CO₂ chambers were suggested for on-farm swine mass depopulation [4]. A North Carolina State University (NCSU) research team has validated this concept using relatively simple setups in the mid-2000s. Meyer and Morrow (2005) [4] presented a graphical concept of the CO₂ gas ‘wash-in’ and ‘wash-out’ exponential functions, using a hypothetical example of a closed chamber, originally filled with Gas A (e.g., air), into which Gas B (CO₂) is introduced. The air replacement model followed the continuously stirred tank reactor (CSTR) model, which assumes a single CO₂ entry and air/CO₂ mixture exit. The CSTR model makes it possible to predict the change in CO2 concentration when CO₂ is continuously introduced into the chamber, assuming the perfect stirring of the chamber. It was confirmed that 20% of the CO₂ flow rate (to meet the AVMA guidelines) was successful using a CO₂ gas cylinder attached to a dump trailer [5]. A growing body of knowledge confirms that supplying 20% CO₂ of the chamber volume suggested by AVMA can depopulate pigs. It is also generally accepted that achieving a 90% CO₂ composition gas would be sufficient for rapid depopulation. A mini-review of the current status of CO₂ vaporizer concepts [2,3,6,7,8,9,10,11,12,13,14,15,16,17,18] for swine depopulation is presented in Supplementary Materials.

Both pilot-scale and farm-scale experiments with pigs show that CO₂ can be effective and humane for euthanizing pigs [6,7,8,9,10,11,12,13,14,15,16,17]. The follow-up work by Meyer et al. (2014) [18] also demonstrated to be effective at euthanizing pigs (22–46 kg) in a waste dumpster using dry ice and liquid CO₂. When the CO₂ flow rate was maintained at 20% of the dumpster’s total volume per min, 60% of the chamber volume was filled within ~5 min, and all pigs were euthanized within 5 min. Humane depopulation using CO₂ required an air-tight chamber and an adjustable airflow control system from a liquid CO₂ source. Most recently, Pepin et al. (2022) showed the use of a full-size (2.4 × 12 × 1.02 m) dump trailer for swine depopulation with CO₂ in Minnesota [19]. Pepin et al. (2022) adopted large (4.5 m³) tanks (commonly used for storing liquid propane) for storage of CO₂ gas previously generated from liquid CO₂ at a separate location. The stored CO₂ gas can then be transported separately to the farm. Gas delivery lines are then connected to the enclosed dump trailer for depopulation. CO₂ was filling the trailer for 5 min and then was held for 10 min. This large-scale CO₂ depopulation system showed a 99.5% successful depopulation rate (201 of 202 pigs), proving that it is possible to build the system using a large mobile trailer. Previous studies [5,18,19] have suggested a concept for a farm depopulation system made from locally outsourced components. Still, a significant effort involving a multi-step process is required to achieve depopulation. The depopulation process involves numerous single points of failure (SPOFs). Thus, there is still a need to improve the CO₂-based depopulation concept and its implementation.

In addition to the limitations in CO₂-based depopulation, the carcasses can still remain infectious. Thus, another layer of complexity is immediately present; that is, proper biosecure disposal of infected carcasses is needed to manage the virus transmission risk. Proper treatment and disposal of swine carcasses can limit further disease spread [20]. Pathogens can remain in carcasses, spread into the environment, and severely inhibit recovery efforts from disease outbreaks. Rapid burial practiced during an emergency can be problematic. For example, the ASF virus was demonstrated to survive in the soil for a maximum of 3 weeks (in sterile sandy soil) to a minimum of 3 days (in swamp soil) [21]. Furthermore, transporting infectious carcasses from the affected farm increases the risk of spreading the disease beyond the containment area.

Common approaches for carcass disposal include burial, incineration, and composting [22]. However, off-site carcass disposal challenges biosecurity and threatens pathogen spread via air, water, soil, groundwater, vegetation, or fomites [23]. Therefore, a significant reduction in biosecurity risk is possible if pathogen inactivation can be achieved immediately after depopulation and before carcass removal from the site. Pathogen inactivation largely widens the options for carcass disposal when it can be considered not infectious. These can include the transport of carcasses to pre-approved, properly sized, equipped, and staffed management sites.

The actual implementation of depopulation and pathogen inactivation concepts is difficult in an emergency when there is limited time to communicate, organize labor and equipment, and read guidelines describing how to depopulate and dispose of carcasses. Furthermore, shopping for parts to assemble a relatively simple depopulation system may also be problematic in an emergency situation. While such relatively simple concepts can be beneficial, the actual adoption and commercial-scale use require consideration of farm practices and training (e.g., routine biosecurity vs. disease management) and socioeconomic considerations. Thus, while potentially useful, previous [5,18,19] concepts could be difficult for emergency use to respond to massive disease outbreaks without prior assembly, practice, and demonstration of readiness.

Thus, we propose an ‘all-in-one’ concept of a mobile system for on-farm swine depopulation, pathogen inactivation, off-site carcass disposal, and biosecure cleanup. The concept is based on commercially available mobile trailers that could be modified and sized for swine depopulation using liquid CO₂ and targeted pathogen inactivation with a direct-fired heater. In addition, we have improved the design of the CO₂ depopulation dumpster concept by using a plug-flow reactor (PFR) model that is inherently more efficient in the complete replacement of breathing air by CO₂ for the same depopulation chamber volume as compared with the current CSTR model used in the AVMA guidelines.

This paper presents the main features of a mobile system’s ‘all-in-one’ concept. Furthermore, the paper presents engineering principles used for the design we developed the model for estimating:

• The amount of CO₂ needed to depopulate for a wide range of swine numbers and weights, the size of a user-selected mobile unit, and standard sizes of liquid CO₂ cylinders (Section 2).
• The required time for targeted swine pathogen inactivation by heat (Section 3).

The developed model (‘Model for swine depopulation and pathogen inactivation needs.xlsx’ in Supplementary Materials) was then used to illustrate the depopulation of a swineherd in a mobile system followed by heat treatment and pathogen inactivation of two diseases of concern—that is, ASF and the porcine reproductive and respiratory syndrome (PRRS).

2. Model for Proposed CO₂ Vaporizer Concept to Meet AVMA Guidelines for Swine Depopulation

In this research, we have considered limitations to the CSTR model currently recommended by AWMA and aimed to improve the design by taking advantage of the inherent superiority of a PFR model. Our design concept resembles a PFR where multiple injection points deliver CO₂, which immediately sinks (greater density than air) to the animal breathing zone, completely removes breathing air in that zone, and continues to move down through the reactor until full with CO₂. Less dense air escapes around the circumference of the reactor top, which is hatch-covered. The PFR-based concept is more efficient than the CSTR currently assumed in the AVMA guidelines. The PFR-based concept completely replaces breathing air faster than the CSTR with the same CO₂ injection rate, thus proving a more sustainable solution. This is because less CO₂ is theoretically needed to meet AVMA’s ‘20% of the chamber volume per min’ injection rate guideline. In addition, the improved concept includes an ‘all-in-one’ platform for on-farm swine depopulation, carcass disposal, and biosecure cleanup (Figure 1). Figure 2 illustrates the liquid CO₂ injection, vaporization, and breathing air replacement.

Our model estimates the required liquid CO₂ flowrate needed to depopulate swine according to the AVMA guidelines. The net volume to be filled is estimated as the total trailer volume less the volume occupied by pigs. The pig volume is estimated based on body mass and dimensions [24].

$$\mathrm{Net}\mathrm{volume}\mathrm{of}\mathrm{the}\mathrm{trailer}\left({\mathrm{m}}^3\right)=\mathrm{Trailer}\mathrm{volume}({\mathrm{m}}^3)-\mathrm{Volume}\mathrm{occupied}\mathrm{by}\mathrm{pigs}({\mathrm{m}}^3)$$

(1)

The liquid CO₂ flowrate to fill the trailer’s net volume (Equation (1)) takes into account the total CO₂ rate exhaled from pigs in the trailer (Equation (3)). The latter is a function of total heat production and body mass (Equation (4)).

$${\mathrm{Flow}}_{{\mathrm{CO}}_2}=\raisebox{1ex}{$\mathrm{Net}\mathrm{volume}\mathrm{of}\mathrm{the}\mathrm{trailer}$}\!\left/ \!\raisebox{-1ex}{$300$}\right.-{\mathrm{CO}}_{2\mathrm{swine}}$$

(2)

where Flow_CO2 = the needed liquid CO₂ flowrate to fill the trailer’s headspace (m³/s), 300 = the equivalent of 5 min suggested by the AVMA guideline (s), CO_2swine = total CO₂ rate exhaled from pigs in the trailer (m³/s).

$${\mathrm{CO}}_{2\mathrm{swine}}={\mathrm{THP}}_{\mathrm{swine}}÷24.6÷1000$$

(3)

where THP_swine = total heat production per one trailer load of pigs (per batch) (kW) and 24.6 (kJ/L) is a conversion factor and 1000 converts L to m³.

$${\mathrm{THP}}_{\mathrm{swine}}=\mathrm{THP}\times \mathrm{Total}\mathrm{swine}\mathrm{body}\mathrm{mass}\mathrm{in}\mathrm{the}\mathrm{trailer}$$

(4)

where THP = total heat production per body mass, 14.95 × mass^−0.4 (W/kg) [25]. Total swine body mass = average pig body mass × the number of pigs in the trailer (kg).

This model (Equations (1)–(4)) is used in the ‘CO₂ estimate’ tab of the spreadsheet ‘Model for swine depopulation and pathogen inactivation needs.xlsx’ (Supplementary Materials). The ‘Read me’ tab guides the user through the model. The inputs are generally presented in the left spreadsheet column (highlighted in yellow) and require user input or selection (cells using red font). Model outputs are presented in the right column (highlighted in green). Users provide trailer dimensions, pig’s number per depopulation batch, and pig’s mean body mass. In addition, users must select a ‘commercial liquid CO₂’ cylinder from marketed standardized sizes. Model outputs are the recommended setpoint for both mass-based and volume-based setpoints for liquid and gas CO₂ delivery rates to the trailer to depopulate one batch of pigs under 5 min. The reason for several output formats is to provide users with options for selecting proper hardware (e.g., CO₂ valves for controlling liquid flowrate) and their setpoints. The model also estimates number of batches of similarly sized pigs that a single user-selected commercial CO₂ liquid cylinder could depopulate.

3. Model for Proposed Heat Treatment and Pathogen Inactivation in Swine Carcass

Disposal of infected swine carcasses presents several challenges and considerations for disease outbreak management, routine mortalities disposal, biosecurity risk management, environmental pollution, loss of farming land (in case of burial), and farm profits. Inactivating pathogens in the carcasses with appropriate heat treatment on the farm can be advantageous because it allows for more off-site disposal options (Figure 3).

Therefore, we present a direct heat-type treatment model for inactivating two common pathogens (i.e., ASF and PRRS). The SM spreadsheet ‘Model for swine depopulation and pathogen inactivation needs.xlsx’ has two tabs (‘Time estimate for ASF inactivation’ and ‘Time estimate for PRRS inactivation’). The heat treatment model uses the same inputs as already provided for the CO₂-based depopulation and requires additional parameters as described below.

A direct-fired heater supplies heat into the insulated mobile trailer (Figure 3 and Figure 4). The user is prompted to input the technical specifications of the heater (i.e., heat output, combustion gas output), outside air temperature, temperature setpoint for the trailer inside (based on material safety considerations), initial carcass temperature, selected pathogen inactivated temperature, and holding time. Insulation (olive green in Figure 3) retains the heat and temperature and effectively shortens the required pathogen inactivation time. Thus, users need to input the insulation type, thickness, thermal conductivity, wall thickness, and thermal conductivity of the trailer steel.

The selected pathogen inactivation and hold times can be selected from available literature (

Table 1. Inactivation temperature and hold time needed for targeted swine pathogens.

Targeted Pathogen	Temperature (°C)	Hold Time (min)	Ref.
ASF	60	20	[28]
Foot and mouth disease (FMD)	70	30	[29]
Swine vesicular disease (SVD)	65	2	[30]
Porcine epidemic diarrhea virus (PEDv)	60	30	[31,32,33]
PRRS virus	56	20	[34,35]

). Table 1 is a general guideline for selecting required temperatures and hold times for heat treatment. Lower temperatures are also proven to be effective for selected pathogens, yet they would require longer hold times. Two primary mechanisms of inactivation have been identified for numerous viruses: (1) enzymatic degradation of unprotected virus nucleic acids at ~40 °C and (2) destruction of coat proteins in complete viruses at higher temperatures [26,27]. Figure 5 illustrates the key parameters considered in the heat treatment model for the swine carcass.

Our model estimates the overall time to reach the targeted temperature inside the entire carcass as the sum of times required for step-wise heat transfer from a direct-fired heater to the carcass core and heat hold time (Equation (5)):

$$\mathrm{Total}\mathrm{time}={\mathrm{t}}_{\mathrm{heat}}+{\mathrm{t}}_{\mathrm{inact}}+{\mathrm{t}}_{\mathrm{hold}}$$

(5)

where t_heat = time required to heat the net volume of the trailer up to the temperature setpoint (Ts, e.g., 100 °C, guided by material safety; s), t_inact = time to reach the targeted pathogen inactivation temperature (Tc; e.g., 60 °C for ASF, Table 1) in the entire carcass (s), t_hold = required hold time for targeted pathogen inactivation temperature in the carcass (s) (e.g., 20 min for ASF, Table 1).

The time required to fill the net volume of the trailer with combustion gas is based on user-selected heater specs (Equation (6)):

$${\mathrm{t}}_{\mathrm{fill}}=\mathrm{Volume}\mathrm{of}\mathrm{the}\mathrm{trailer}÷\mathrm{Heat}\mathrm{output}$$

(6)

where Volume of the trailer = net volume, i.e., trailer volume less the space occupied by carcasses (m³); same as the volume used for CO₂ depopulation, Heat output = combustion gas discharged from heater (m³/s).

The time required to heat the net trailer volume accounts for heat loss via insulated trailer walls (Equations (7)–(10)):

$${\mathrm{t}}_{\mathrm{heat}}={\mathrm{Q}}_{\mathrm{total}}÷\left({\mathrm{Q}}_{\mathrm{heat}}-{\mathrm{Q}}_{\mathrm{loss}}\right)$$

(7)

where Q_total = the needed amount of theoretical heat transferred to headspace in the trailer up to the setpoint temperature T_S (kJ), Q_heat = heat output from heater (kW), Q_loss = total heat loss transferred by the process of conduction on wall surfaces with insulation (kW).

$${\mathrm{Q}}_{\mathrm{total}}=\mathrm{m}\times \mathrm{c}\times \left({\mathrm{T}}_{\mathrm{s}}-{\mathrm{T}}_{\mathrm{o}}\right)$$

(8)

where m = mass of headspace gas (kg), i.e., the volume of gas (CO₂) injected into the trailer headspace (m³) × CO₂ density (kg/m³) at setpoint temperature T_S, c = specific heat of CO₂ (kJ/(kg∙℃)) (adjusted for setpoint temperature, T_S), T_o = outside air temperature (℃).

$${\mathrm{Q}}_{\mathrm{loss}}=\frac{{\mathrm{k}}_{\mathrm{total}}\times \mathrm{A}\times \left({\mathrm{T}}_{\mathrm{s}}-{\mathrm{T}}_{\mathrm{o}}\right)}{\mathrm{d}}$$

(9)

where Q_loss = total heat loss transferred by the process of conduction on trailer wall surfaces with insulation (kW), k_total = thermal conductivity of steel with insulation (kW/(m·℃)), A = area of wall surfaces (m²), d = thickness of the wall (m).

$${\mathrm{k}}_{\mathrm{total}}=\frac{1}{\left(\raisebox{1ex}{$\mathrm{thicknees}\mathrm{of}\mathrm{steel}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{k}}_{\mathrm{steel}}$}\right.+\raisebox{1ex}{$\mathrm{thickness}\mathrm{of}\mathrm{insulation}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{k}}_{\mathrm{insulation}}$}\right.\right)}$$

(10)

where k_steel = thermal conductivity of steel (kW/(m·℃)), k_insulation = thermal conductivity of insulation (kW/(m·℃)).

The time to reach the targeted pathogen inactivation temperature in the entire carcass considers the heat transfer from the carcass surface (T_C; equal to the pathogen inactivation temperature, Table 1) to its core. The ‘core’ is assumed to be a homogenous solid rectangular prism of carcass with the total body mass and thermal conductivity of water (Equations (11)–(13)).

$${\mathrm{t}}_{\mathrm{inact}}={\mathrm{Q}}_{\mathrm{carcass}}÷\left({\mathrm{Q}}_{\mathrm{conv}}-{\mathrm{Q}}_{\mathrm{loss}}\right)$$

(11)

where Q_carcass = the needed amount of theoretical heat transferred to carcasses up to the targeted pathogen inactivated temperature, T_C (kJ), Q_conv = convective heat transfer to the carcasses inside the trailer (kW).

$${\mathrm{Q}}_{\mathrm{carcass}}=\mathrm{m}\times \mathrm{c}\times \left({\mathrm{T}}_{\mathrm{C}}-{\mathrm{T}}_{\mathrm{i}}\right)$$

(12)

where m = mass of carcasses in the trailer (kg), c = specific heat of carcasses (water, kJ/(kg∙℃)) (adjusted for setpoint temperature, T_S), T_i = carcass initial temperature (℃).

$${\mathrm{Q}}_{\mathrm{conv}}=\mathrm{h}\times \mathrm{A}\times \left({\mathrm{T}}_{\mathrm{S}}-{\mathrm{T}}_{\mathrm{i}}\right)$$

(13)

where h = convective heat transfer coefficient of combustion gas (kW/(m²·℃)), A = surface area of contact between carcass and gas in the trailer (m²).

4. Main Features of the All-in-One Depopulation and Heat Treatment Concept

This concept design addresses the dire gap in depopulation and carcass disposal capacity. The gap is exemplified by the recent shutdown of meatpacking plants caused by the COVID-19 pandemic [36]. Our solution concept provides farmers and the industry a practical option to make pig production more resilient to the challenges related to emergency responses to the human or animal pandemic. The concept design presented herein is a practical solution, with a fully functioning (depopulation and disposal) and scalable mobile unit for depopulation, heat treatment, biosecure disposal that can involve off-farm travel, and application to routine on-farm operations. The concept provides practical features (itemized in

Table 2. Main features of the proposed all-in-one system.

	Features
1	One mobile unit is capable of assisting with all key tasks: loads pigs from the loading dock, completes depopulation, completes thermal treatment, delivers and off-loads mortalities, undergoes a biosecurity-driven clean up before returning to the farm to treat another batch of swine.
2	Rapid delivery of CO2 under the trailer cover meets the displacement of breathing air rate and time required by the AVMA guidelines. The displaced breathing air escapes at the top perimeter of the trailer.
3	The gas-delivery system can be operated by one person standing at a safe distance from the trailer.
4	CO2 liquid and propane cylinders can be safely installed and exchanged through the foldable ramp.
5	The feasibility of a refillable cylinders affixed to the mobile trailer can be explored.
6	A direct-fired heater supplies heat into the insulated mobile trailer.
7	Insulation minimizes the heat loss, retains supplied heat, and shortens the required time to inactivate pathogens.
8	Hydraulic lift: off-load the dump trailer using a hydraulic lift.
9	Swine carcasses can be unloaded on another location within the farm or at a centralized disposal site.
10	The mobile trailer can be effectively disinfected to minimize the risk of spreading diseases between farms.
11	The size of trailers can be selected from several manufacturers and many commercially available models to be cost-effective to manage on-farm disease-related risk.
12	The concept can be scaled up and developed into a small fleet of vehicles for startups and small businesses serving livestock production based on well-developed, popular, and road-approved hydraulic dump trailers.
13	The user-friendly model can be used to size the hardware, key operating, and techno-economic parameters.

and illustrated in Figure 6, Figure 7, Figure 8 and Figure 9) and a user-friendly spreadsheet model (in Supplementary Materials) that farmers can use to size fully functional mobile units to meet the AVMA criteria for depopulation.

5. Results

5.1. Estimation of the Required Liquid CO₂ for Swine Depopulation

An example of the calculations performed for estimating CO₂ quantity for depopulation is provided in the accompanying spreadsheet model (in SM), and key parameters are presented in

Table 3. Example of CO₂ quantity needed to depopulate a single batch of pigs.

Model Inputs		Model Outputs		Ref.
Mobile unit length	4.8 m (15.7 ft)	Mobile unit volume	12.1 m3 (427.3 ft3)
Mobile unit width	2.1 m (6.9 ft)
Mobile unit height	1.2 m (3.9 ft)
Pig body mass	100 kg (220 lbs)	Total pig volume	7.1 m3 (250.7 ft3)	[24]
Number of pigs	24	Heat production rate	2.5 W/kg	[25]
		Estimation of CO2 mass needed	5.0 m3 (176.6 ft3)
Conversion factor of heat to CO2	24.6 kJ/L	Exhaled CO2 production rate	0.239 L/s (0.49 CFM)	[25]
		Estimation of CO2 rate needed for filling the volume of the mobile unit	16.5 L/s (35.0 CFM)	[3,24]
		Estimation of CO2 volume for 5 min	4.94 m3 (174.5 ft3)
Mass of liquid CO2	0.45 kg (1 lb)	Volume of CO2 gas	0.23 m3 (8.1 ft3)	[37] (1 atm, 0 °C)
Commercial Liquid CO2 volume	68 kg (150 lb)	Number of batches for one liquid CO2 cylinder	7.0	CO2 LIQ 99.9% LC170 350 PSI [38]

. The example calculates the required CO₂ delivery rate (0.239 L/s) to fill 100% of the trailer headspace (12 m³) within 5 min (AVMA guidelines) to depopulate 24 pigs (100 kg each). Considering that 0.45 kg (1 lb) of liquid CO₂ converts to 0.23 m³ of CO₂ gas at 1 atm and 0 °C, a standard 68 kg (150 lb) CO₂ cylinder would be theoretically sufficient for depopulating seven batches of 24 pigs each.

5.2. Estimation of the Required Time for Heat Treatment of Carcasses and Targeted Pathogen Inactivation

An example of the calculations performed using the model for estimating the required time needed for pathogen inactivation in swine carcasses (‘Time estimate for ASF inactivation’ in Supplemental Materials) is presented in

Table 4. Example of required heat treatment time needed to inactivate ASF in a batch of swine carcass.

Model Inputs		Model Outputs		Ref.
Heat output from direct-fired heater (2 × heaters, Qheat)	191 kW (650,000 BTU/h)	Specific heat of CO2 in the trailer (c)	0.82 kJ/(kg∙°C)	GUARDIAN® Forced Air Heaters, Model name: Guardian 325
Setpoint temperature inside the trailer (TS)	100 °C			Specific Heat Formula Q = mc∆T Q = heat energy (kilojoules, kJ), m = mass of a substance (kg), c = specific heat (kJ/(kg∙K)), ∆ is a symbol for “the change in” temperature, T(K) mass = volume (m3) × density (kg/m3)
Outside air temperature (To)	0 °C	Density of CO2 in the trailer	1.98 kg/m3
Net volume of headspace	5.0 m3	The needed amount of theoretical heat energy transferred to the trailer up to the targeted temperature (Qtotal)	811 kJ
Thermal conductivity of carbon steel with insulation (ktotal)	1.36 W/(m2·°C)	Stored heat in the trailer, excluding heat loss	13.5 kW
Total heat loss through the walls in the trailer with insulation (Qloss)	177.1 kW	Time required to fill the needed amount of theoretical heat energy with heat loss (theat)	1.0 min
Carcass initial temperature (use 38 ℃ if heating follows euthanasia; use relevant T for processing stored carcass, Ti)	38 °C	Specific heat of carcass in the trailer (c)	4.17 kJ/(kg∙°C)
ASF pathogen inactivated temperature in the carcass (Tc)	60 °C	The needed amount of theoretical heat energy transferred to the carcass up to the targeted temperature (Qcarcass)	220,350 kJ	Specific Heat Formula
Heat transfer coefficient of forced convection gases (h, range: 25–300)	25 W/(m2·°C)	Time to reach the pathogen inactivated temperature in the entire carcass (tinact)	248 min
Convective heat transfer to the carcasses (heat source, Qconv)	15.6 kW	Required hold time for pathogen inactivated temperature in the carcass (thold)	20 min	[28]
Heat is transferred by the process of conduction on the floor surface with insulation (Qloss)	882.5 W

. As a result of calculating the required time needed for ASF pathogen inactivation in swine carcass (100 kg, 24 pigs), the total time is 4.5 h using 190 kW (650,000 BTU/h) of heat in the winter (outside temperature: 0 °C). It takes 1 min to raise the inside temperature of the trailer to the targeted temperature when the heat loss was reflected from the temperature difference between the outside temperature and the inside temperature in the trailer (t_heat). When the internal setpoint temperature was maintained, it took ~4.1 h to reach the ASF inactivation temperature to the center of the carcass through internal convection (t_inact). After that, the holding time (20 min) that must be maintained for ASF pathogen inactivation was reflected (t_hold).

Another example of the estimated time needed for PRRS pathogen inactivation in swine carcass (‘Time estimate for PRRS inactivation’ in SM) is presented in

Table 5. Example of required heat treatment time needed to inactivate PRRSV in a batch of pig carcass.

Model Inputs		Model Outputs		Ref.
Heat output from direct-fired heater (2 × heaters, Qheat)	191 kW (650,000 Btu/h)	Specific heat of CO2 in the trailer (c)	0.85 kJ/(kg∙°C)	GUARDIAN® Forced Air Heaters, Model name: Guardian 325
Setpoint temperature inside the trailer (TS)	100 °C			Specific Heat Formula
Outside air temperature (To)	30 °C	Density of air in the trailer	1.78 kg/m3
Net headspace volume	5.0 m3	The needed amount of theoretical heat energy transferred to the trailer up to the targeted temperature (Qtotal)	530 kJ
Thermal conductivity of carbon steel with insulation (ktotal)	1.36 W/(m2·°C)	Thermal energy accumulated in the trailer, excluding heat loss	66.6 kW
Total heat loss through the walls in the trailer with insulation (Qloss)	123.9 kW	Time required to fill the needed amount of theoretical heat energy with heat loss (theat)	0.13 min
Carcass initial temperature (use 38 ℃ if heating follows euthanasia; use relevant T for processing stored carcass, Ti)	40 °C	Specific heat of carcass in the trailer (c)	4.17 kJ/(kg∙°C)
ASF pathogen inactivated temperature in the carcass (Tc)	56 °C	The needed amount of theoretical heat energy transferred to the carcass up to the targeted temperature (Qcarcass)	160,200 kJ	Specific Heat Formula
Heat transfer coefficient of forced convection gases (h, range: 25–300)	25 W/(m2·°C)	Time to reach the pathogen inactivated temperature in the entire carcass (tinact)	181 min
Convective heat transfer to the carcasses (heat source, Qconv)	15.1 kW	Required hold time for pathogen inactivated temperature in the carcass (thold)	20 min	[34,35]

. In this example, the outside temperature was set as summer (30 °C); also, the carcass temperature was set to 40 ℃ to reflect summer. The required heat treatment time was about 3.3 h.

For illustrative purposes, the needed amount of CO₂ for depopulation and the estimated time for inactivating target pathogens for batches of pig of different sizes (30, 60, 135 kg) using one size of ‘all-in-one’ unit was investigated. The amount of CO₂ required was inversely proportional to the pig mass, and the treatment time required for the pathogen inactivation was proportional to the pig mass. In other words, a lower amount of CO₂ is required to depopulate large pigs, but a longer time is then required for heat treatment (

Table 6. Estimated ASF and PRRS inactivated time. Scaleup example for the all-in-one concept using the same trailer to depopulate and thermally treat 3 different pig sizes (30, 60, 135 kg). Outside set temperature: 0 °C and carcass temperature: 38 °C.

Different Pig Sizes	Maximum Number of Pigs (Based on Available Floor Area) That Can Be Loaded into the Trailer (12 m3)	Recommended CO2 Gas Delivery Rate (L/s) to the Trailer for Depopulation Suggested by AVMA	Estimated ASF Inactivated Time (h)	Estimated PRRS Inactivated Time (h)
30 kg	50	25	2.9	2.5
60 kg	30	22	3.5	2.9
135 kg	19	15	4.8	4.0

).

The performance of our prototype needs to be evaluated in future research. Research results should be used for further refinement of the prototype. Full-scale trials (without animals and then, if warranted and approved, with animals) should be conducted to test and prove that the trailer meets the AVMA guidelines regardless of weather conditions. In full-scale trials, the post-heat treatment cooling down time of the trailer chamber for on-road transport must also be considered due to safety reasons. According to the calculations, heat loss from trailer continues after the heater is turned off (summer, 30 °C: 124 kJ/s, winter, 0 °C: 177 kJ/s), it may take about 20~30 min (winter ~ summer, respectively) to discharge the 221,000 kJ accumulated in the trailer. A real-time monitoring system for measuring the CO₂ displacement rate in the covered dump trailer space could be installed (e.g., the chemical and laser type of CO₂ sensor [39,40]). Plastic barrels can be used to represent pigs and effectively reduce the volume of the chamber space that pigs occupy. The gas leakage from the chamber can be controlled by the coupling of the roof hatch cover. The risk of frostbites that is of concern when using liquid CO₂ [4] can be managed by properly sizing the trailer height. For example, a 100 kg pig is ~60 to 75 cm in height [24] should have sufficient headspace clearance from the CO₂ nozzles. General information on pig handling and loading to the trailer is available in the Swine Care Handbook [41].

6. Conclusions

Outbreaks of animal diseases cause severe losses and force difficult decisions aimed at stopping the disease progression. We proposed an ‘all-in-one’ concept of a mobile system for on-farm swine depopulation and pathogen inactivation. It uses vaporized CO₂ followed by heat treatment, broadening options for the off-site transfer of mortalities, disposal, and cleanup. We developed a user-friendly model based on engineering principles used for estimating site- and scenario-specific CO₂ amounts and the total time required to inactivate targeted swine pathogens. The model illustrates the depopulation for two diseases of concern, i.e., African swine fever (ASF) and the porcine reproductive and respiratory syndrome (PRRS) viruses. The suggested model confirmed that the CO₂ rate required for depopulation and the heat treatment time of targeted pathogens provides users a set of practical answers based on inputs representing realistic conditions (pig size, temperature, pathogen type, and so on). The concept could be adopted for a routine or a mass depopulation/treatment/disposal scenario with a single unit or fleet of units.