Hydrogen Sulfide Concentrations Around Swine Farms

By Carrie L. Tengman, Rodney N. Goodwin and Jose R. Bicudo

 

Manure production and storage in livestock operations is an environmental issue. Producers are interested in maintaining the highest quality manure for crop production, and they are concerned about impacts of the livestock operation on air, soil and water quality.

 

The livestock industries have been dealing with the air quality issue of odor for many years. Over the years researchers have completed projects designed to define, analyze and predict odor. These projects have identified many chemical components that influence odor and/or have implications for human health.

 

Hydrogen sulfide (H₂S) is a gas produced by anaerobic decomposition of organic material in livestock manure storage systems. Production and emission of H₂S to the ambient air is a continuous process that occurs in stored manure. Hydrogen sulfide has a low odor threshold and contributes to pork production odors. Agitation of stored manure and removal of manure for application to agricultural lands increases the emission rate of H₂S compared to normal operating conditions.

 

LITERATURE REVIEW

 

The concentration of H₂S downwind from swine production facilities has not been studied extensively. More research is being done to measure emissions from production facilities and the downwind effects because of air quality regulations being applied to the livestock industry.

 

Avery et al. (1975) found that H₂S production was correlated with several factors, including average outside temperature, pit area to building volume ratio, building air retention time, and daily sulfur intake.

 

A laboratory study by Arogo et al. (2000) reported that settled manure has distinct layers and each layer differs in sulfide production. Arogo found that the top of the three layers produce the highest total sulfide under the anaerobic conditions. Also, the low pH found in the lower layers creates a higher molecular H₂S concentration than the top layer.

 

Arogo et al. (2000) found that the potential for liquid to emit H₂S gas was driven by concentration in the liquid. Therefore, reduction of sulfate in the liquid may lead to reduced potential to emit H₂S gas.

 

Measurements of H₂S have shown increased concentrations inside animal buildings. Dissolved H₂S is released more rapidly when manure agitation starts, and decreases slowly as agitation progresses. The emission of H₂S rapidly decreases after agitation is completed as reported by Patni and Clarke (1991). Manure movement and splashing in the storage area appeared to be the dominant factors in rapid H₂S release, rather than diet and ventilation conditions, for example. Heber et al. (2000) documented higher H₂S emissions due to warmer weather and higher ventilation rates. The emission rates measured were directly proportional to room temperatures and building airflow rates.

 

Hobbs et al. (1999) measured gases from 112 d stored swine slurry that was stirred during sampling. H₂S was released at higher rates than other gases. They concluded that stirring resulted in the higher H₂S emission rate.

 

The Minnesota Pollution Control Agency (MPCA) regulates H₂S concentrations at the property boundary of livestock operations. Minnesota Rule 7009.0080 sets forth the H₂S standard as follows: H₂S exceeding 30 ppb (30-min average) more than twice in a 5 d period and 50 ppb (30-min average) more than twice in one year. However, recent changes to the regulation, Minnesota Statute 116.0713, provides producers an enforcement exemption during manure storage agitation and land application for a period of 21 days every year.

 

OBJECTIVES

 

The objectives of these projects were to measure:

• Project 1: downwind ambient air H₂S concentrations within the property boundary before, during and after manure storage agitation and manure removal from deep pit swine barns.
• Project 2: ambient H₂S concentrations continuously on swine farms for 30 days.

 

MATERIALS AND METHODS

 

Equipment
Single Point Monitors (SPM) (Zellweger Analytics, Inc., Lincolnshire, IL) utilize the MDA Chemcassette colormetric technique based on ASTM Standard D 4324-84 Standard Test Method for Hydrogen Sulfide in the Atmosphere by Rate of Change of Reflectance. A dry reagent medium – the Chemcassette tape – is used to collect and measure gas concentration. Upon exposure to the target gas, the Chemcassette changes color in direct proportion to the concentration. The photo-optics and electronics built into SPM monitors read the color intensity change and determine the gas concentration by comparison to a known gas response “calibration” pre-programmed in the instrument. The SPM detection limit is 1-90 ppb or 50-1500 ppb for the hydrogen sulfide calibration. H₂S concentrations from the 1-90 ppb calibration are 15-min average readings that the instrument itself averages after sampling for an entire 15-min period. The monitors set with the 50-1500 ppb calibration took 2-min average readings after a 2-min sampling period.

 

PROJECT DESCRIPTION

 

Site Selection, Project 1
Monitoring was conducted in November 1998 and October-November 1999. Data resulted from six farms described in Table 1. Each farm used deep pits for manure storage. Farms were located in Minnesota, Iowa and Illinois.

 

The duration of monitoring varied as well as agitation duration. Table 3 establishes the time of day that monitoring and agitation took place and gives the total hours for each activity. The numbers in bold italics are times agitation was in progress, and monitoring was not conducted prior to or after agitation. At Farm C and Farm E monitoring began at 1845 h and 1300 h, respectively. Agitation was already in progress. Monitoring was not done prior to agitation. At Farm F1 monitoring stopped at 700 h while agitation was still in progress. Monitoring did not take place prior to or after agitation at Farm F2, signified by the bold italicized times in Table 3.

 

A weather station was set up on each site to collect temperature, relative humidity, wind speed, wind direction, and barometric pressure data.

 

Each monitoring station consisted of two Single Point Monitors for the measurement of H₂S concentrations, with the exception of monitoring stations used on Farm A where a single SPM was used per station. One of the two SPM monitors was calibrated for 1-90 ppb concentrations and the second SPM was calibrated for 50-1500 ppb. The data presented is based upon overall 15-min averages utilizing the data from the appropriate calibration. Concentrations below 30 ppb are considered to be low. High concentrations will be considered to be greater than 50 ppb. These concentrations are used for reference based on the Minnesota H₂S statute.

 

Site Selection, Project 2
A total of five farms were selected based on type of housing, production system, and storage of manure. Detailed information on each farm is given in Table 2. A number of related factors were considered during the site selection process. These included size of the facility (animal units), property line setback distance, and accessibility. All facilities were included in this study after consent of the producer.

 

H₂S air emissions were measured continuously for a period of approximately 30 days at each site using the Single Point Monitor (SPM). The SPMs were located at varying distances and directions from the emission sources (buildings or earthen basins), so that continuous downwind detection of H₂S was possible through at least one of monitor. The SPMs were calibrated for measurement of 1-90 ppb concentrations.

 

When available, five SPMs were used to measure H₂S emissions. The primary goal of this exercise was to collect at downwind locations. Prevailing wind direction and emission source configuration played a major role in finding suitable locations for the monitors on field. Weather stations that automatically recorded wind speed, wind direction, temperature, relative humidity and barometric pressure were used in most sites. When sites were close enough, only one weather station was used for a pair of sites. A typical set-up is shown in Figure 1.

 

RESULTS AND DISCUSSION

 

Project 1
At Farm A1 five monitoring stations were set up (15, 30, 60, 90 and 120 m) downwind from the barn that was agitated. 75-min elapsed from the time agitation began until the H₂S concentrations were greater than 30 ppb, and peaked greater than or equal to 90 ppb after 150-min shown in Figure 2. The decrease of H₂S below 30 ppb occurred within 80-min after the end of agitation at all distances except 15 m. As distance increased, H₂S concentrations decreased up to the distance of 90 m in this study.

 

Four monitoring stations at Farm A2 were set up (15, 30, 60 and 90 m) downwind from the barn that was being agitated. High concentrations of H₂S were already present prior to agitation, possibly due to agitation activities the previous day. Very low wind speeds may also have contributed to the high concentrations. No single peak was evident this day because readings were greater than the SPM upper detection limit (90 ppb). After agitation the stations at distances of 30-90 m dropped to below 30 ppb 285-min prior to the decrease at 15 m. It took 5h for concentrations at all distances to fall below 30 ppb after agitation.

 

At Farm B four monitoring stations were set up (15, 30, 60 and 90 m) downwind from the barn that was being agitated. 255-min after the start of agitation H₂S levels were above 30 ppb and peaked after 360-min of agitation shown in Figure 3. The peak level of H₂S was 671 ppb at 15 m. After 420 minutes of agitation, the response of H₂S concentrations downwind was immediate. The concentrations at all distance were zero upon the end of agitation even with very low wind speeds.

 

Agitation was in progress at Farm C prior to SPM setup (15 and 30 m) north of the north barn. The south barn was being agitated. Distance measurements are those taken from the north barn. An additional 60 m should be added for distances from the south barn. Within 15-min the H₂S concentration was greater than 30 ppb and peaked after 60-min at a level of 191 ppb. Low levels of H₂S were reached 75-min prior to the end of agitation. This drop in H₂S may be due to the manure storage being nearly empty and the fact that agitation took place the previous day, depleting the dissolved H₂S.

 

At Farm D three monitoring stations (15, 30, and 60 m) were set up downwind from the barn to be agitated. The concentration of H₂S did not rise above 30 ppb until 210-min after the start of agitation. The concentration at 15 m peaked after 270-min at 222 ppb. Low levels of H₂S were reached within 15-min after agitation ended. The barn was agitated for a total of 300-min, just 23% of the total time that monitoring took place. The other 77% of monitoring time showed low levels of H₂S at all distances.

 

Two SPM stations (30 and 60 m) were set up at Farm E downwind from manure agitation. Agitation was in progress and had been for several hours prior to monitoring. H₂S concentrations peaked at 291 ppb and dropped off to low levels within 90-min following the peak, about 30-min prior to the end of agitation. There were very strong winds during the time of monitoring, but the actual wind speeds were not measured.

 

On Oct. 27 Farm F1 monitors were setup (15, 30, and 60 m) on the north end of the barns while pit agitation was occurring in the south barn. Therefore, 45 m must be added to the reported measurements for the actual distance from the south barn. High concentrations of H₂S were measured 480-min after the start of agitation and reached the peak 10-min later at 202 ppb. Concentrations fell below 30 ppb 550-min prior the end of agitation for a short period of time before the Farm F2 data began collection.

 

At Farm F2 SPMs were set up at distances of 15, 30, and 60 m. Agitation in a different pit started Oct. 28 at 845h. High levels were present and H₂S peaked after 15-min at 15 m. After 135-min of agitation H₂S concentrations were low. This is due to the wind direction changing. Monitoring was discontinued because of the property boundary restriction.

 

Project 2
Results from continuous monitoring of H₂S measured at different distances from the sources are given in Table 4.

 

Readings were not taken at property lines or in locations to which the general public had access. This was done in order to avoid compliance monitoring. The Minn. R. 7009.0200 prohibits violation of the state hydrogen sulfide ambient standards beyond the property line or in locations to which the general public has access.

 

The number of readings that exceeded 30 ppb was highest at the finishing site. The 15-minute average concentrations were higher than 30 ppb for about 40% of the time (i.e. 12 days out of the 30-day monitoring period). A maximum of 450 ppb of H₂S was recorded at 5 m downwind from the barns during the whole monitoring period at this site. However, it should be noted that the H₂S monitors were located only 5 m from the barns.

 

H₂S concentrations downwind of the farrow-to-finish site were also high. The 15-minute average concentrations were higher than 30 ppb for about 18% of the time (about 5.5 days out of the 30-day monitoring period) at a distance of 18.3 m from the north basin. Emissions from the manure storage basins appeared to have a significant effect on the downwind concentration. H₂S concentrations decreased significantly after the manure was pumped from the basins (between 8/17/99 and 8/21/99), as shown in Figure 4. The maximum H₂S concentration registered after manure was pumped was about 40 ppb.

 

Although the H₂S monitors were located at similar distances from the sources at the nursery, farrow-to-finish and wean-to-finish sites, the number of times the 30-ppb value was exceeded in the nursery and wean-to-finish sites was about 7 and 20 times less than in the farrow-to-finish site, respectively. A straw cover considerably reduced H₂S emissions (by 97%) from the basin at the nursery site. With the exception of the manure agitation and pumping events, H₂S concentrations at both the nursery and wean-to-finish sites never went over 60 ppb. Mean H₂S concentrations at these sites varied from 4 to 10 ppb at about 15 to 35 m from the sources.

 

H₂S concentrations near the hoop barn were very low as compared to the other sites. The 30-ppb value was exceeded only once during the monitoring period. H₂S concentrations were generally below 10 ppb, with a mean of 1.7 ppb at 9 and 15 m from the barns.

 

Continuous monitoring of H₂S at the various sites indicated that there seems to be a diurnal pattern in the H₂S concentration measured at different distances downwind from the sources. This pattern is graphically shown in Figure 5 for the wean-to-finish site by plotting all data in a single 1,440-minutes cycle. The H₂S concentrations increase during early morning hours (from midnight to 5:00 am), decrease during the day (from 5:00 am to 4:00 pm), and start increasing again in late afternoon till midnight. Similar patterns were observed for all sites.

 

Further data analysis and research is clearly needed in order to identify which factors affect the animal/hydrogen sulfide ambient air concentration relationship.

 

This project was designed to collect very specific information on the downwind hydrogen sulfide concentrations during agitation. Measurements of weather were made on a limited basis and may have provided more insight to the reasons for increases and decreases in hydrogen sulfide downwind. For most of the farms it took a long period of time to measure increased downwind hydrogen sulfide emissions from the manure storage due to agitation. Measurements of sulfide levels in manure throughout manure agitation could provide information on hydrogen sulfide emissions.

 

CONCLUSIONS

 

Monitoring the downwind ambient air surrounding livestock facilities is not a simple task. Several factors influence the success of the monitoring and the results. The parameters that effected the results are not completely apparent in this data. Research results show that primarily low levels of H₂S were present prior to agitation of the manure storage and that upon the end of agitation H₂S again fell to low levels. Figure 6 shows the average values for the farms where monitoring took place. This graph shows the time lag from the start of agitation to the peak concentrations of H₂S, and also documents the fall of H₂S concentrations once agitation has stopped. Overall, H₂S concentrations dropped to zero within 105-min after agitation. The distance from the H₂S source also had an effect on the concentrations that were detected downwind. The drop of H₂S concentrations below 30 ppb occurred prior to the end of agitation and up to 5h post agitation. Manure and manure storage characteristics were not measured, but may also play a role in H₂S concentrations.

 

Manure agitation is an added cause of H₂S emissions. The 30-day monitoring of H₂S in different directions from the farms showed a natural variation in concentration over a 24hr period that was repeated. Higher concentrations were measured in the early morning and late evening. This appears to tell us that weather is a major component of H₂S emissions.

 

REFERENCES

 
ASTM. Standard Test Method for Hydrogen Sulfide in the Atmosphere by Rate of Change of Reflectance. D 4323-84 (Reapproved 1997)e1. In: Annual Book of ASTM Standards. Philadelphia, American Society for Testing and Materials. Vol 11.03.
 
Arogo, J., R.H. Zhang, G.L. Riskowski, D.L. Day. 2000. Hydrogen sulfide production from stored liquid swine manure: a laboratory study. Trans. ASAE 43(5):1241-45.
 
Avery, G.L., G.E. Merva, and J.B. Gerrish. 1975. Hydrogen sulfide production in swine confinement units. Trans. ASAE 18(1):149-51.
 
Hobbs, P.J., T.H. Misselbrook, T.R. Cumby. 1999. Production and emission of odours and gases from aging pig waste. J.Agric. Engng. Res. (1999) 72, 291-298.
 
Ni, J.Q., A.J. Heber, T.T. Lim, C.A. Diehl. 1999. Continuous measurement of hydrogen sulfide emission from two large swine finishing buildings. In ASAE/CSAE-SCGR Annual International Meeting. Toronto, Ontario Canada, July 18-22. Paper No. 994132. ASAE, 2950 Niles Rd., St. Joseph, MI 49085-9659 USA.
 
Patni, N.K. and S.P. Clarke. 1991. Transient hazardous conditions in animal buildings due to manure gas released during slurry mixing. Applied Engineering in Agriculture 7(4):478-484.
 
Minnesota Pollution Control Agency. 2000. Minnesota Rules, Chapter 7020, Animal Feedlots. Saint Paul, MN.

 

Table 1. Swine farm identification is listed, and the dates on which the manure storage pits were agitated and monitored. Monitoring took place for the measurement of downwind hydrogen sulfide concentrations during manure storage pit agitation and pumping.

 

Table 2. Details of sites selected

 

Table 3. Average downwind hydrogen sulfide concentrations at 15, 30, 60, 90, and 120 meters from the point of agitation at swine farms using deep pit storage. Averages were calculated from the 15-minute average readings given by the SPM.

^aeach farms description and date of agitation/monitoring can be found in table 1.
^bagitation was in progress prior to monitoring or that agitation continued after monitoring stopped.

 

Table 4. Mean H₂S concentrations and number of times the 30 or the 90 ppb levels were exceeded in all sites

^awith 95% confidence interval.
^bmean was not calculated because there were too many values recorded as "greater than 92 ppb" (the maximum limit of detection for the monitor used).
^cthe monitor was programmed to detect H₂S concentrations between 50 and 1,500 ppb only.