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Dust Spatial Distribution in a Typical Swine Building

ABSTRACT Dust has been implicated as a major contributor to the increased incidence of respiratory disorders among swine workers. Unlike gaseous contaminants, the dust is not uniformly distributed within swine buildings. Dust spatial distribution is an important variable to understand the nature of dust transportation and to implement appropriate control strategies. There is a lack of data on dust spatial distribution in livestock confinement buildings because of lack of adequate sampling techniques. In this project, a multi-point sampler has been developed using critical flow control venturis and was used to measure the dust spatial distribution in a typical swine building. Experimental results show that there is a high variation in the dust spatial distribution within the mechanically ventilated swine building. Ventilation rate, diurnal change of weather, oil sprinkling and air cleaning using an aerodynamic deduster all affect the dust spatial distribution.

Keywords: Dust spatial distribution, ventilation, multi-point sampler, swine building

INTRODUCTION

Confinement livestock housing at high animal density causes many problems such as poor indoor air quality (Carpenter, 1986), especially for cold climate buildings, in which the ventilation rate is low during winter. Many studies confirm the adverse effects of high contaminant concentration in livestock buildings on human health working in a contaminated atmosphere. (Dosman et al., 1988; Donham et al., 1989). Air quality has been an increasing concern for confinement livestock buildings.

Dust in enclosed swine buildings is primarily generated from feed grains, fecal materials, animal skin and hair, insects, and dead micro-organisms. They are comprised of viable organic compounds, fungi, endotoxins, absorbed toxic gases, and other hazardous agents. It has been proven that dust adversely affects animal health and productivity (Deboer et al., 1991). Dust also has direct damaging effects on the health of the operators (Senthilselvan et al., 1997). A considerable amount of data from the literature has shown that dust along with viable microorganisms, fungi, and absorbed toxic gases within airspaces of swine buildings, have been implicated as major contributors to the increased incidence of respiratory disorders among swine producers compared to nonfarm workers (Donham et al., 1989).

Unlike gaseous contaminants, the trajectories of the dust particles differ from the air streamlines within an airspace. Dust concentration depends largely on air distribution, relative locations to the dust sources, and activity level in the building. Consequently, dust can not be as uniformly distributed within a ventilated airspace as gaseous pollutants. It can be expected that there are spatial gradients of dust concentration within a ventilated airspace. Dust spatial concentrations in livestock buildings have been studied by Barber et al. (1991).

They found that there is a significant spatial variability of dust within the swine buildings. But more research on dust spatial distribution is needed to characterize the dust within livestock buildings.

The dust transportation and behavior in a ventilated airspace is very complicated because of combined effects of air flow turbulence, gravitational sedimentation, diffusion, coagulation, adhesion, and resuspension. One of the challenges in indoor air quality studies is to measure the dust spatial distribution so that the nature of dust transportation can be better understood and appropriate control strategies can be implemented. A clear understanding of the dust spatial distribution will provide useful information to control dust sources, improve the design of ventilation systems, and implement the control technologies. The objective of this project is to measure the dust mass spatial distribution within a mechanically ventilated swine building at different conditions to study the dust behavior and to evaluate the effectiveness of dust control strategies.

MATERIAL AND METHODS

Muti-point Dust Sampler

Most research to date has been based on monitoring dust concentration at only one representative location or on "grab" samples collected at two or three sites within the animal buildings. To study the dust spatial distribution and behavior, it is critical to measure dust concentrations across an airspace at multi-points during the same time period. Otherwise, the time required for each measurement point (typically on the order of hours or days for mass concentration) will introduce large errors in dust distribution patterns which are highly time dependent.

A muti-point dust sampler was developed by the authors using several critical venturis (Wang, et al., 1999). A conceptual design of the multi-point sampler is shown in Figure 1. It consists of a commercially available vacuum pump, a pressure monitor, a pressure regulator, and an array of critical venturis with air filters. When the air is drawn through a filter, the volumetric flow rate remains constant for all venturis as long as the pressure across the venturis is higher than the critical pressure drop. Since the critical pressure drop of the venturi was below 11 kPa, the pump was operated at a sufficiently high vacuum and a constant flow through the filters was maintained. This multi-point sampler was used in this study to measure the dust mass concentration in a cross section of a typical swine building.

Figure 1. A multi-point dust sampler

Experimental Setup

The building used for the experiment is shown in Figure 2. This building is comprised of two identical rooms each consisting of 11 pens. Each pen was equipped with one two-hole feeder and a nipple drinker. Partially slotted floor was used in both rooms. Two pens in the middle of the room were keep empty to set up the multi-point sampler. Each room had 72 pigs weighing approximately 160-240 lbs fed with mashed dry corn meal. Fresh air entered each room through slotted air inlets. Each room has two exhaust fans with a total capacity of 4.3 m³/s. During winter operation, the room air temperature was approximately 18-22°C. A feedback control system was used to activate the fans or heater to control air temperature in the rooms. The fan duty cycles were recorded using timers connected to the fans. Each room temperature and relative humidity was measured using a calibrated hygrothermograph.

Figure 2. Swine building (top view)(all dimensions in meters)

Dust concentrations at 27 points within each room were measured (Figure 2). Measurement points were uniformly distributed in the central cross section (sampling plane) in the room, as shown in Figure 3.

Figure 3. Sampling point set up (side view) (all dimensions in meters)

The dust collector located upstream of each critical venturi was a 37 mm diameter (0.8 μm porosity) filter housed in a holding cassette. As the room air velocity at most of the sampling points was less than 0.5 m/s, the dust sampling inlet was oriented perpendicular to air flow to keep all sampling close to isokinetic conditions. Filters were dried in a desiccant drier for 24 hours and weighed on a precision electronic balance prior to the dust collection. Before sampling, the sampling rate of each filter was calibrated with a calibrated rotameter. The sampling rate of each filter was 19.2 ± 0.2 L/min. The start time and the stop time of sampling were recorded. Each measurement was over an approximate twenty-four hour period except for daytime and nighttime sampling (diurnal effect study). The samplers were dried in a desiccant drier for 24 hours after sampling and immediately weighed again on the precision electronic balance. The dust mass in each filter was calculated and recorded. The dust mass concentration in each point was calculated using the following equation.

(1)

where:

C_m = mass concentration (mg/m³)
m = net mass increase of the filter after sampling (mg)
Q = sampling rate of each filter (L/min)
t = sampling period (min)

RESULTS AND DISCUSSION

The dust mass concentrations in 27 points were measured using the multi-point sampler at different conditions. The experimental cases and results are summarized in the Table 1.

Table 1 Test cases and experimental results

Effect of Ventilation Rate on Dust Spatial Distribution

Ventilation is effective in the control and dilution of gaseous contaminants. It is also widely believed that ventilation systems have a direct effect on the dust spatial distribution. Ventilation will remove the dust from the airspace, but at the same time ventilation may increase air movement and stir up dust and keep it in the air. All measured results show that there is a high variation in the dust spatial distribution within the mechanically ventilated swine building (Figures 4 – 6).

a

b

Figure 4. Effect of ventilation rate on dust spatial distribution (mg/m³): (a) Low ventilation rate at 26% fan duty cycle, Cave =4.56 mg/m³; and (b) High ventilation rate at 68% fan duty cycle, Cave =4.05 mg/m³.

Cases 1 and 2 were measured at similar indoor and outdoor conditions (Figure 4). The only difference is the average ventilation rate. The fans in case 1 were running only 26% of the time, whilst the fans in case 2 were running 68% of the time. With low ventilation rate, there was a zone of higher dust concentration next to the feeder and dust was more symmetrically distributed across the building section. With high ventilation rate, there is a zone of high dust concentration near the air inlet side. This could be a dead ventilation zone. The dust spatial distribution is similar to the flow pattern. It appears that ventilation rate has a direct effect on the dust spatial distribution. However, the measured overall average dust mass concentration had little difference between these two cases although the ventilation rate in case 2 was 2.6 times higher than the case 1. The possible reason is that dust production rate increases with the increase of ventilation rate. This verifies that ventilation rate has less effect on the overall dust removal.

Effect of Diurnal Change on Dust Spatial Distribution

Figure 5 shows the dust spatial distribution changes with the diurnal change. The measured spatial dust concentrations show that the overall dust level during the daytime was much higher than that during the nighttime even though the daytime had a higher ventilation rate. One explanation for this phenomenon is the animal activity. Compared with nighttime, pigs are more active during the daytime. They are eating, walking and playing, and disturbing more dust. The activities of farm workers might be another factor affecting dust production during the daytime. Comparing the dust spatial distribution patterns, the dust was more symmetrically distributed across the section during nighttime because of low ventilation rate.

a

b

Figure 5 Effect of diurnal change on dust spatial distribution (mg/m³): (a) Nighttime in control room, Cave =4.23 mg/m³; and (b) Daytime in control room, Cave =7.14 mg/m³.

Effect of Dust Source Control and Air Cleaning on Dust Spatial Distribution

Effects of two dust control technologies on dust spatial distribution were evaluated: air cleaning using aerodynamic dedusters and dust source suppression using oil sprinkling. In the deduster treatment, the ratio of air flow rate through the dedusters to the room ventilation is 32%. The dedusters have a dust removal efficiency of 85%. Apparently, large flow rate for the deduster is required to improve the room air cleaning efficiency. The measured spatial dust concentrations with dedusters show that the overall dust level is approximately 20% lower than the control room (Figure 6b). The high dust concentration zone near the air inlet side disappeared. This indicates that some dust was removed from the dusty air. As the equipment operation affected the airflow pattern, the dust spatial distribution was different from the control room. Dust spatial distribution and dust level are very closely related to the dust source and dust production rate. It has been proven that oil sprinkling can control the dust source and reduce the dust production rate (Zhang et al., 1996). The measured dust spatial concentrations with oil sprinkling treatment show that the overall dust level is much lower than the control room. This indicates that oil sprinkling at regular frequency can significantly reduce the dust level. As oil sprinkling reduces most of the big size particles, therefore, the dust spatial distribution after treatment was more close to the air flow pattern because the small size particles are more likely to follow the air streamline (Figure 6c).

a

b

c

Figure 6 Comparison of dust spatial distribution for dust source control and air cleaning with control (mg/m³): (a) Control room, Cave =5.02 mg/m³; (b) Air cleaning (deduster), Cave =3.82 mg/m³; and (c) Dust source control(oil sprinkling), Cave =0.82 mg/m³.

CONCLUSIONS

Dust concentration depends largely on air distribution, relative locations to the dust sources, animal and human activity level in the building, and air cleaning technologies. Based on the experimental results, the following conclusions are summarized.

1. Unlike gaseous contaminants, the trajectories of the dust particles may differ from the air streamlines within an airspace. There is a high variation in the dust spatial distribution within the mechanically ventilated swine building.
2. Ventilation system has a direct effect on the dust spatial distribution. But increasing the ventilation rate does not effectively reduce the overall dust level because the dust production rate increased with an increase of ventilation rate.
3. There is a large variation in overall dust level with the diurnal change. The overall dust level during the daytime is much higher than the nighttime, likely due to the animal activities which changed the airborne dust production.
4. Measured dust spatial distribution shows that air cleaning can reduce the dust level. To improve the overall dust removal efficiency, large flow rate of the dedusters is required.
5. Oil sprinkling at regular frequency is an effective measure to control the dust level.

NOMENCLATURE

ACKNOWLEDGEMENT

The authors thank Steve Ford and Peter Stroot for their technical assistance in the setup of dust sampling system. We also extend our thanks to Brian Anderson, Jerry Edwards, Al Gutival and J.R. Hubele for their help on the measurement.

REFERENCES

Barber, E.M., J.R. Dawson, A. Battams and R.A.C. Nicol. 1991. Spatial variability of airborne and settled dust in a piggery. J. Agr. Eng. Res. 50:107-127

Carpenter, G.A.. 1986. Dust in livestock buildings--review of some aspects. Journal of Agricultural Engineering Research 33: 227-241.

DeBoer, S., W.D. Morrison and L.A. Braithwaite. 1991. Effects of environmental quality in livestock buildings on swine health and productivity: A literature review. Transactions of the ASHRAE 97(2): 511-518.

Donham, K.J., P. Haglind, Y. Peterson, R. Rylander and L. Belin. 1989. Environmental and health studies of workers in Swedish swine confinement buildings. British Journal of Industrial Medicine 40: 31-37.

Dosman, J.A., B.L. Graham, D. Hall, P. Pahwa, H.H. McDuffie, M. Lucewicz and T. To. 1988. Respiratory symptoms and alterations in pulmonary function tests in swine producers in Saskatchewan: Results of a survey of farmers. Journal of Occupational Medicine 30: 715-720.

Senthilselvan, A., J.A. Dosman, S.P. Kirychuk, E.M. Barber, C. S. Rhodes, Y. Zhag and T. Hurst. 1997. Accelerated lung function decline in confinement swine building workers. CHEST. 111(6): 1733-1741.

Wang, X., Y. Zhang, L. Zhao and G.L. Riskowski. 1999. Development of a multi-point aerosol sampler using critical flow control devices. Transactions of ASHRAE (accepted).

Zhang, Y., A. Tanaka, E. M. Barber and J. J. R. Feddes. 1996. Effect of frequency and quantity of sprinkling canola oil on dust reduction in swine buildings. Transactions of the Ame. Soc. Agr. Engr. 39(3): 1077-1081.