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L. Chénard, S.P. Lemay and C. Laguë
Adapted from Focus on the Future 2003 – "Optimizing the Production System"
Summary
Four pig farms were studied to assess the barn workers’ exposure to hydrogen sulphide (H2S) while pulling pit plugs and power-washing production rooms. Results indicated that plug pulling generated high concentrations of H2S reaching 1,000 ppm in some cases. All the farms studied had plug pulling events that exceeded limits defined by the Occupational and Safety Regulations of Saskatchewan. The H2S released when a plug was pulled did not follow a predictable pattern over time and within the room space. Power washing generated lower H2S concentrations than plug pulling but workers were exposed for a longer time period. Based on this study, swine barn workers may be exposed to H2S concentrations that exceed acceptable limits when pulling pit plugs and power-washing rooms. Personal monitors should be provided to all barn workers and training and standard operating procedures are needed so workers can learn how to deal with routine operations and emergency situations when high H2S concentrations are generated.
Introduction
Hydrogen sulphide is a life threatening gas that is produced by the anaerobic degradation of liquid manure (Zhang et al., 1990). Bacterial activity in liquid manure can occur under aerobic conditions in the top layer of the manure and in anaerobic conditions lower in the manure pit or storage. If there is sulphate in the liquid manure, a group of sulphate reducing bacteria will produce H2S. The sulphate can come from the water supply, from the urine and from the feces (Zhang et al., 1990). Hydrogen sulphide after being produced in the manure moves by molecular diffusion, which is a very slow process which process is accelerated when the manure is agitated. As most swine barns are equipped with gutters accumulating manure, H2S can be released when manure flows or is being mixed.
Donham et al. (1985) completed a study to evaluate the chemical and physical parameters of liquid swine manure and their possible implications on the health of workers and pigs. They found a positive relationship between the total sulfide content of the manure and the H2S measured in the air. They also conclude that sulphate in the water can contribute significantly to total manure sulfides (Donham et al. 1985; Arogo et al. 2000) and that manure produced in building where the water source exceeds 150 mg/L of sulphate can present possible extra hazards (Donham et al., 1985). However, additional research is needed to better understand these relationships.
Different scientists reported that H2S concentration in pig barns during normal operations is low, generally under 5 ppm and often in the part per billion range (Strobel and Heber 1998; Ni et al. 1999; Jacobson et al. 1999). However, McAllister and McQuitty (1965) found that H2S could increase from 0 to 800 ppm in the transfer pit or lift station and could be sucked back into the pig rooms through open pits or piping when manure is agitated. Similar observations were made in Ontario when deep manure pit buildings were monitored. Background H2S concentration was close to zero but could peak to concentrations as high as 1,300 ppm as soon as the manure was agitated (Clarke and Patni, 1993).
Saskatchewan Labour (1996) regulates H2S exposure in the Occupational Health and Safety Regulation. The 8-h time weighted average exposure limit (TWA) is 14 mg/m3 (10 ppm) and the 15-min time weighted average contamination limit (STEL) is 21 mg/m3 (15 ppm) and at no time should those limits be exceeded. Saskatchewan Labour (1996) does not have a defined ceiling value for H2S however it is stipulated that the immediately dangerous to life or health (IDLH) level for H2S is 100 ppm.
Recent events in Saskatchewan led us to believe that barn workers may be exposed to high H2S concentrations while pulling pit plugs and power-washing rooms. Monitoring of those tasks was performed to evaluate this hypothesis.
Objectives
The goal of this study was to assess the H2S exposure of workers while performing specific manure management tasks in swine operations. Specific objectives, presented here, were:
Part 1: To measure H2S concentrations in the workers’ environment during the emptying of in-room shallow manure pits and the power washing of production rooms.
Part 2: To evaluate H2S distribution in rooms while pulling pit plugs in shallow pits.
Materials and Methods
Equipment
Dräger Pac III monitors equipped with a H2S sensor (Maximum capacity of 1000 ppm, Dräger, Lübeck, Germany) were used to measure and record H2S concentrations during all the events. The monitors were set to record data every 10 s and the data were downloaded from the monitors on a regular basis. The monitors had their first alarm set at 10 ppm and the second alarm set at 15 ppm to warn workers performing the events.
Stands were built to securely shelter the monitor at 1 m above the floor so pigs, if present, could not have access to the monitor and disturb the monitoring. The height was selected to simulate a worker leaning to pull a plug.
Safety measures
For the plug pulling monitoring, the person performing the task had to use a personal protective equipment (self contained breathing apparatus, SCBA) when the concentrations were such that the second alarm of the monitor would be activated. A second person was following the procedure and was equipped to safely intervene in case the SCBA, if used, would fail.
For the power-washing monitoring, the person performing the wash was instructed to leave the monitor in the room in a stand as soon as the second alarm (15 ppm) was reached and to come back and continue the wash when the concentration lowered below 15 ppm.
Part 1 - Worker’s exposure
Plug pulling
Four swine production sites or farms were selected to complete this study. Two of these farms are owned by the Prairie Swine Centre (PSCI) and the other two are owned by independent corporations. On each farm, monitoring was to be performed four times in each production section: gestation, farrowing, nursery and grow-finish and each set of replicates was done once in the summer-fall period of 2001 and in the winter of 2002.
For monitoring the plug pulling events, the monitor was placed at 1 m from the floor in the monitor stand within a 1 m radius from the plug hole. General information on the room (inside and outside temperatures, ventilation, production status, number of pigs, manure depth in the pit) was gathered before the event. The time was also recorded through out the monitoring event to indicate when the monitor was started, when the plug was pulled, when the plug was put back in the sewer hole and when the procedure ended. If noticed, any event of plugs popping out in other rooms was also recorded.
Power-washing
No specific instruction for power washing was given to the worker with the exception of wearing the H2S monitor at the waist or breast level (monitor attached to a pocket). The monitor was worn with the sensor facing the floor and it was covered with an open face plastic bag to avoid water infiltration into the sensor. The person would provide the same general information as for the plug pulling. The worker was instructed to leave the room, placing the monitor in the monitor stand in the room, if the second monitor alarm was turned on. They would only return when the alarm would shut off.
Part 2 - Concentration patterns
For the two PSCI sites, supplemental monitors were used to evaluate the H2S concentration distribution within the room. For the summer-fall period, four other monitors were placed in monitor stands in the room with: one monitor placed within a 1 m radius of one of the operating fans, one placed in a 1 m radius of the door and the others placed evenly in the alleyway to simulate the exposure of bystanders. In the winter period, along with the monitor placed at the plug, three monitors were placed in stands along the manure pit that was drained.
Results and Discussion
Part 1 - Worker’s exposure
Plug pulling monitoring
Results from four barns monitored in this study indicate that plug pulling generated high concentrations of H2S. A total of 119 monitoring events have been performed over both the summer-fall and winter periods. Table 1 presents the absolute maximum values reached for each section of each farm and for both seasons combined. The number of events that exceeded the IDLH level (100 ppm) on the total number of events performed is also indicated.
The results show that maximum H2S concentrations measured during plug pulling events greatly exceed Occupational Health and Safety levels. In some cases, the maximum recorded reached 1,000 ppm. Farms 1 and 2 have reached peak H2S concentrations that are in the same order of magnitude while lower maximum concentrations were recorded on farms 3 and 4. All farms had different buildings designs (room sizes, number of animals, building management and some had different buildings for different production stages). All of the farms had negative pressure ventilation and shallow manure pits.
Overall, 29% of the monitoring events performed during plug pulling (35 events over 119) resulted in concentration equal or higher than the IDLH. All farms had at least one event generating a concentration higher than IDLH.
According to the Occupational Health and Safety Regulations section 90 (Saskatchewan Labour, 1996), no employee can stay within an environment where the H2S concentration reaches IDLH. This means that the worker has to be away of this contaminated environment until the concentration lowers or special procedures must be used to ensure his safety. The use of protective respiratory and safety equipment (such as a self-contained breathing apparatus) and the use of specific procedures (e.g. communication at all time with another worker, both workers trained on the use of the protective equipment and on rescue procedures, etc.) are necessary if a task has to be finished or performed within such an environment.
The short-term exposure limit (STEL, set at 15 ppm, Saskatchewan Labour, 1996) becomes an important parameter to evaluate workers exposure when manure management tasks result in a rapid increase of the H2S concentration without reaching the IDLH level. The STEL value was calculated for each monitoring events and Table 2 gives the maximum STEL values obtained during both summer and winter periods combined and the number of events when the STEL value was exceeded.Overall, 48% of all monitored events generated time weighted averages exceeding the value given by the regulation. The plug pulling events monitored on farms 1 and 2 resulted in higher maximums and STEL values compared to those monitored on farms 3 and 4. In addition, fewer plug pulling events on those last two farms yielded H2S concentration in the workers’ environment which were above the maximum allowable STEL value. However, plug pulling events that generated STEL higher than the limit defined in the regulation did occur in each and every farm.
The H2S released when a plug was pulled did not follow a predictable pattern. Figure 1 shows typical variations of H2S concentrations as the plugs were being pulled (at time 0:00) and put back in place in a grow-finish room and a gestation room.
Figure 1. Plug pulling events performed in a grow-finish room and a gestation room during the summer and winter period, respectively.
In the grow-finish room, the maximum value was reached within less than 4 min after the plug had been pulled while in the gestation room, the concentration increased and went through a number of intermediate peaks before reaching its maximum. In most cases, the concentration decreased rapidly after the plug was put back in place.
As mentioned previously, Donham et al. (1985) suggested a possible link between the high sulphate content of the water and the high H2S content in the manure. Water test results were obtained from the different farms. The sulphate content for the different water sources supplying farm 1 ranged from 832 to 1,800 mg/L; for farm 2, the sulphate content of the unique source of water was 1,190 mg/L. Farms 3 and 4 have the same water source and its sulphate content was 101 mg/L. Although more in-depth analysis would be required to draw strong conclusions, maximum H2S concentrations from farms 1 and 2 for all barn sections are generally higher than the ones obtained in farms 3 and 4 as shown in Table 1. This means that a farm provided with a water source rich in sulphate would likely produce more H2S during manure handling than a farm with a lower water sulphate content. However, even if farms 3 and 4 were provided with a low sulphate water, H2S concentrations still exceeded occupational health and safety regulations. A low sulphate content of the water will contribute at reducing worker exposure to H2S but it will not eliminate the problem.
Power-washing monitoring
Power washing generated lower H2S concentrations than plug pulling. In total, 107 power-washing events have been monitored during this project with 53 and 54 events performed respectively during the summer and winter periods. Out of these events, 19 (seven in the summer and 12 in the winter) resulted into H2S concentrations that exceeded the STEL within the workers’ environment (Table 3).
Overall, 25% of the power-washing events performed in the nursery sections of the barns resulted in high H2S concentrations. Those conditions exceeded the limits provided in the regulations as well as for 22% of the events in the farrowing and 7% in the grow-finish sections. Only two farms did some power-washing in the gestation room; only a small part of the rooms was cleaned and these events did not generate conditions that exceeded the STEL. Farms with higher water sulphate content had more power-washing events generating high H2S concentrations. Table 4 gives the average conditions in which those events occurred. As power washing generally takes time, in some cases, the STEL was reached a while after the task started and was exceeded for a long period of time, which in some events was more than 30 min.
Part 2 - Concentration patterns
From all the monitoring events performed during the summer time in farms 3 and 4, at least one event per barn section resulted in higher H2S concentrations being reached elsewhere in the room than at the plug level (Fig. 2). The plug area is the location where most of the maximum concentrations were measured but no real trend can be observed for the other locations where the peak concentrations were reached.
Of all the monitoring events performed to evaluate the concentration patterns during the winter period (eight in each barn section), at least three events per barn section resulted in higher concentrations elsewhere above the pit than at the plug (Fig. 3). Once again, the evolution of the concentrations at the different locations above the pit follows a somehow similar pattern of increase and decrease with different magnitudes and no real trend can be observed to characterize the location where the peak occurs. This indicates that the manure movement in the pit is an important contributor to the H2S released in the air during those events representing from 33 to 50% of the events.
No predictable pattern of distribution could be observed for a preferred location where the peak would be reached. This means that a worker pulling the plug and walking away from it may not get to a safer area if he stays in the room; the same comment applies to a bystander staying in the room.
Conclusions
Plug pulling can generate high concentrations of H2S as in some cases the maximum recorded in some events reached 1,000 ppm (which was the maximum detectable concentration for the sensors used in this study). All four farms had plug pulling events that could present health and safety risks to workers and exceeded limits defined by the Occupational and Safety Regulations of Saskatchewan (Saskatchewan Labour, 1996). Farms that have a water source with a high sulphate content are more at risks of high H2S concentrations being released in their swine buildings. The H2S released as a plug is being pulled does not follow a predictable pattern when considering the level that will be reached, the concentration variations during the event and the time at which the peak concentration will be observed.
Figure 2 Hydrogen sulphide concentration pattern during plug pulling event in a nursery room during the summer period.
The concentration distribution in a room while the plug is being pulled does not follow a predictable pattern. While most of the highest concentrations were generally recorded at the plug or sewer hole, in some cases it was recorded somewhere else in the room or over the pit. A worker pulling the plug and walking away from it may not be in a safer position if staying in the room, and the same comment applies to a bystander.
Power-washing generates lower concentrations than plug pulling. However, as the task to be performed generally takes more time, the STEL can be reached a while after the task started (close or more than an hour) and be exceeded for a long period of time, which in some of the monitored events was more than 30 min.
Further research to improve building design, manure management systems and the development of engineering controls for existing buildings is needed to lower the risk of worker H2S exposure. Before such improvements are in place, personal monitors should be provided to all swine barn workers as H2S may be present in other areas than where the plug is pulled (ex: transfer pit room, plug popping situations). Training and standard operating procedures are needed so workers can learn how to deal with routine operation and emergency situations that can generate high H2S concentrations.
Figure 3 Hydrogen sulphide concentration pattern during plug pulling event in a nursery room during the winter period.
Acknowledgements
The authors would like to acknowledge the financial contributions provided by Sask Pork and the Agricultural Development Fund (ADF) program of Saskatchewan Agriculture, Food and Rural Revitalization to support this research project. Thanks must also be given to Sask Pork, Alberta Pork and Manitoba Pork for strategic funding to the research program at PSCI. In kind contribution was also provided by the following organizations as they completed the monitoring in their barns to complement the work performed at PSCI: Big Sky Farms Inc., Heartland Pork Management, PIC Canada Ltd (Aurora) and Quadra Group (Chesterfield Farm). Technical support at PSCI was provided by Robert Fengler and Caroline Ferh. Guidance was also provided by Shelley Kirychuk and Niels Koehncke of the Institute for Agricultural Rural and Environmental Health of the University of Saskatchewan.
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