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Published online 12 October 2005
Published in J Environ Qual 34:2018-2023 (2005)
DOI: 10.2134/jeq2004.0435
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Preliminary Investigation of Air Bubbling and Dietary Sulfur Reduction to Mitigate Hydrogen Sulfide and Odor from Swine Waste

O. Grant Clark*, Brent Morin, Yongcheng Zhang, Willem C. Sauer and John J. R. Feddes

Dep. of Agricultural, Food and Nutritional Science, Univ. of Alberta, 4-10 Agriculture and Forestry Centre, Edmonton, AB, Canada, T6G 2P5

* Corresponding author (grant.clark{at}ualberta.net)

Received for publication November 15, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
When livestock manure slurry is agitated, the sudden release of hydrogen sulfide (H2S) can raise concentrations to dangerous levels. Low-level air bubbling and dietary S reduction were evaluated as methods for reducing peak H2S emissions from swine (Sus scrofa) manure slurry samples. In a first experiment, 15-L slurry samples were stored in bench-scale digesters and continuously bubbled with air at 0 (control), 5, or 10 mL min–1 for 28 d. The 5-L headspace of each digester was also continuously ventilated at 40 mL min–1 and the mean H2S concentration in the outlet air was <10 µL L–1. On Day 28, the slurry was agitated suddenly. The peak H2S concentration exceeded instrument range (>120 µL L–1) from the control treatment, and was 47 and 3.4 µL L–1 for the 5 and 10 mL min–1 treatments, respectively. In a second experiment, individually penned barrows were fed rations with dietary S concentrations of 0.34, 0.24, and 0.15% (w/w). Slurry derived from each diet was bubbled with air in bench-scale digesters, as before, at 10 mL min–1 for 12 d and the mean H2S concentration in the digester outlet air was 11 µL L–1. On Day 12, the slurry was agitated but the H2S emissions did not change significantly. Both low-level bubbling of air through slurry and dietary S reduction appear to be viable methods for reducing peak H2S emissions from swine manure slurry at a bench scale, but these approaches must be validated at larger scales.

Abbreviations: NRC, National Research Council


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MODERN SWINE OPERATIONS generate large volumes of slurry. The slurry is usually stored in earthen or concrete structures in an anaerobic condition that can produce toxic gases particularly during agitation and removal. When the slurry is disturbed during the agitation process, reduced sulfurs, volatile organic compounds, phenols, and others are suddenly released through off-gasing. Off-gasing during agitation presents a particularly hazardous occupational and animal health situation where slurry is stored within a barn or confined storage structure (Chénard et al., 2003; Norén, 1977). The most dangerous gas is H2S, which is colorless, odorless at high concentrations (>150 µL L–1), and potentially lethal (700–2000 µL L–1) (Atia et al., 2004). Regulatory agencies have therefore set workplace exposure limits for H2S. In Alberta, the Provincial Department of Workplace Health and Safety has set threshold limit values of 5 µL H2S L–1 for 8-h exposure and 10 µL H2S L–1 for exposure up to 15 min (Atia et al., 2004). Moreover, H2S (at low concentrations) and other S compounds comprise about one-half of the offensive odorants from swine waste (Shurson et al., 1998; Spoelstra, 1980).

If slurry is stored aerobically, as in aerated storage lagoons, very few sulfurous volatiles are produced (Banwart and Bremner, 1975), and aeration of stored manure is therefore one method of reducing the emission of toxic or malodorous volatiles (Zhang and Zhu, 2005). In anaerobic storage, however, H2S and other sulfurous volatiles are produced in the slurry by the anaerobic microbial decomposition of S compounds, especially sulfate (Arogo et al., 2000; Shurson et al., 1998; Banwart and Bremner, 1975). Arogo et al. (2000), for instance, showed a direct correlation between the initial sulfate concentration in swine manure slurry and the amount of sulfide produced, and Clanton and Schmidt (2000) described the state of knowledge of the metabolism and emission of S compounds in manure. The sulfate in slurry ultimately originates from the protein, minerals, or premix in the feed (either digested or spilled) and, to some extent, from S compounds in the drinking and wash water (van Kempen et al., 2002; Arogo et al., 2000).

One potential method to reduce peak H2S emissions from manure slurry is to agitate the slurry by very low-level bubbling of air, the intent not being to oxidize the volatile compounds in the slurry, but to promote their gradual release at innocuous rates throughout the storage period. As a result, a much smaller volume of H2S would remain in the slurry on removal. There have been many studies of continuous aeration of slurry to reduce the formation and emission of volatile organic compounds related to odor (e.g., Burton and Sneath, 1995), and some study of short-term aeration to reduce odor potential of manure slurry (Zhang and Zhu, 2005). Zhang and Zhu (2005) showed that even short-term aeration has the effect of reducing total solids, total volatile solids, biochemical oxygen demand, N concentration, and volatile fatty acids in the slurry. Even in the latter study, however, the airflow rate was maintained at 1.2 L s–1 m–3 for up to 4 d. Bubbling of air or N2 gas through manure slurry at comparable rates (e.g., 4.2 L s–1 m–3; Yasuhara and Fuwa, 1978) has long been used as a method of stripping volatile compounds for identification. By contrast, in the study reported here the airflow rates were very much lower, the highest being 0.04 L s–1 m–3. To the knowledge of the authors, this practice has not been investigated from a safety standpoint.

Since feed is ultimately the major source of manure S, another method of mitigating manure H2S emissions is to lower dietary S levels. This can be done by reducing excess nutrients, selecting low-S ingredients, or including additives that improve digestive efficiency or alter the microflora in the large intestine (Powers and Van Horn, 2000; Van Kempen et al., 2002). Although there has been much research of the use of diet manipulation to reduce odor and ammonia emissions, few studies have focused on diet manipulation specifically to lower H2S emissions from slurry. In one exception, Whitney et al. (1999) found that a mean reduction of 23% in the S concentration of the feed of nursery pigs during a 5-wk period tended to reduce H2S emissions from the stored manure, although this tendency was not reported as being significant. A few studies have looked at the effect of diet on S or sulfide concentrations in slurry of manure from cattle (Bos taurus) or swine. For instance, Stevens et al. (1993) showed that a reduction in dietary protein in dairy cows reduced sulfide concentration in the manure slurry. Shurson et al. (1998) were able to reduce S excretion from pigs by 30%, without affecting their growth, by selecting low-S feed ingredients. Hydrogen sulfide production from manure has also been measured in some studies focused on pig diet manipulation to lower ammonia (NH3) or odor production. Kendall et al. (1998), for example, reduced both NH3 and H2S emissions from pig manure slurry by 40% by reducing dietary crude protein and supplementing the feed with synthetic amino acids. Generally, this kind of diet manipulation can be done without compromising the performance of the animals or the final quality of the carcass (Van Kempen et al., 2002; Powers and Van Horn, 2000; Shurson et al., 1998). Kendall et al. (1998), however, did observe a tendency (P < 0.61) in pigs fed a low-protein diet to deposit about 1.3 mm more back fat during the 6-week study as compared with the control group, perhaps due to higher net energy in the low-protein diet.

The primary objective of this study was to evaluate, at the bench scale, the two aforementioned potential methods for reducing H2S emissions from swine manure slurry. Bench-scale methods offer much greater control over experimental and environmental variables than do production-scale studies, in which potential treatment effects of diet on manure emissions are often masked or confounded (Clark et al., 2005). The objective was addressed through two experiments: in the first, air was bubbled at low rates through stored manure slurry and H2S emission rates were measured during a 4-wk period; in the second, dietary S was adjusted and H2S and odor emission rates were measured from the stored slurry.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experiment 1
A bench-scale experiment was performed as a preliminary evaluation of low-level air bubbling to release H2S from stored swine manure slurry. Slurry was obtained from the main holding tank of the Swine Research and Technology Centre (Univ. of Alberta, Edmonton, AB), comprising mixed, liquid swine manure slurry from an experimental farrow-to-finish herd of about 1000 pigs. Nine 15-L slurry samples were placed in 20-L plastic vessels (Fig. 1) .



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  		Fig. 1. Manure slurry storage vessel.

 
Air was continuously introduced into the slurry in the vessels through a 6-mm diameter plastic tube inserted through the vessel side and attached to an aeration stone (Fig. 1). An air compressor (Mo. CP6502516, Coleman Powermate, Kearney, NE), equipped with a precision pressure regulator (Mo. R352, Arrow Pneumatics, Broadview, IL), supplied air to a distribution manifold. The air flow from the distribution manifold to each vessel was controlled with an independent flow meter (Cole-Parmer, Vernon Hills, IL) preceded by a disposable dust filter (part no. 9900-05-BK, Parker Hannifin Corp., Haverhill, MA).

Air was bubbled through the slurry at three different rates: 0 (control), 5, and 10 mL min–1. These rates were estimated to be low enough to maintain anaerobic conditions in the slurry and still strip suspended H2S. For instance, dilute pig manure slurry, as used in Experiment 1, can be expected to have a typical 5-d biochemical oxygen demand (BOD5) in the order of 5 g L–1 (Zhang and Zhu, 2005). Airflow of 10 mL min–1 through 15 L of slurry sample during the 10-d storage period would have supplied about 1400 mg O2 L–1 manure slurry. The estimated oxygen transfer efficiency for fine-pore diffusers such as the aeration stones used in this study is about 10% (Solomon et al., 1998), so the total oxygen supplied to the slurry would have been equivalent to about 2.8% of the BOD5. This estimate was not, however, confirmed by direct measurement.

A multi-gas detector (Toxi Ultra; Biosystems, Middletown, CT) with an H2S sensor head was used to measure the H2S concentration in the exhaust air from the headspace of each vessel. The sensor head (4HS/LM CiTiceL; City Technology Ltd., Portsmouth, UK) was based on a 3-electrode electrochemical sensor with a nominal range of 0 to 100 µL L–1, a resolution of 0.1 µL L–1, and a T90 response time of ≤30 s (City Technology, 2003). This sensor is cross-sensitive to SO2 by less than +20% at 100 µL L–1 (City Technology, 2003). The H2S concentration in the outlet air was measured on Days 1, 7, 14, 21, and 28. The detector was calibrated every sampling day using a 9 µL L–1 H2S span gas mixture, and then inserted inline with quick-connect fittings between each vessel and outlet flow meter, in turn. The detector was left inline for at least 1 min on each outlet line to allow the reading to stabilize.

Outlet air was drawn continuously from each vessel through a 6-mm diameter plastic tube extending through the lid of the vessel, about halfway from the center (Fig. 1). The air was drawn from all storage vessels through a common outlet manifold by a vacuum pump (Mo. MOA-V111-AE; Gast Mfg. Corp., Benton Harbor, MI) and exhausted to a fume hood. The flow rate from each vessel was regulated with a separate flow meter preceded by a dust filter to protect the meter. Based on preliminary trials, a ventilation rate of 40 mL min–1 was chosen to keep the headspace H2S concentration in the optimal range of the detector (0–100 µL L–1). At this flow rate, 2 h were required for a complete air change in the 5-L headspace of each vessel. Since air was bubbled through the slurry at only 0, 5, or 10 mL min–1, the remaining volume of make-up air (40, 35, or 30 mL min–1, respectively) was drawn through the imperfect seal around the edge of the vessel lid. It can be assumed, therefore, given the long air-change time and distributed nature of the influx, that the headspace was well-mixed.

At the start of the trial, the shaft of a paint mixer was inserted through a hole in the center of each lid and sealed in place with vinyl tape (Fig. 1). At the end of the storage period (Day 28) the slurry was agitated for 30 s by spinning the paint mixer with an electric drill. Hydrogen sulfide concentrations were measured from the headspace of each vessel before agitation and then after agitation at 5-min intervals for 20 min.

The data were analyzed using the SAS software package (SAS Institute, 2001) with an {alpha} value of 5% (P ≤ 0.05) throughout. The SAS Mixed Procedure was used to analyze the headspace H2S concentrations (Wang and Goonewardene, 2004; SAS Institute, 1996). It was expected that the concentrations would increase when the slurry was agitated, and so the measurements taken during storage and after agitation were analyzed separately. The model used to analyze the storage data included aeration rate (experimental treatment) and storage day as fixed variables. Denominator degrees of freedom were calculated using the Kenward-Roger algorithm (SAS Institute, 1996). Repeated measures were specified, with the storage day as the associated time variable and using an unstructured covariance matrix. The model used to analyze the concentration data collected immediately after agitation included the elapsed time since agitation, the aeration rate, and the interaction of the two primary factors. Elapsed time was used as the temporal variable associated with the repeated measures and computational options were specified similar to those used for the analysis of the storage data. Table 3 indicates the numerator and denominator degrees of freedom and the significance levels of the fixed effects included in the models.


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  		Table 3. Factors included in statistical models and significance of their effects.{dagger}

 
Experiment 2
Six barrows, average initial body weight of 42 kg, were housed individually in stainless steel crates. After an initial 10-d adaptation period, pairs of pigs were fed diets with three levels of S according to a repeated 3 x 3 Latin square design.

The formulations of the diets are presented in Table 1. The diets were formulated to contain different levels of S by adjusting the concentration of the S-containing amino acids (methionine and cysteine). One diet (A) was marginally deficient in the S-containing amino acids, a second diet (B) fulfilled the National Research Council (NRC, 1998) standards for those amino acids and can be considered as the experimental control, and the third diet (C) contained an excess. All diets met the NRC (1998) standards for digestible energy and apparent ileal supply of all amino acids except methionine and cysteine. Vitamin and mineral content met or exceeded NRC (1998) standards. Samples of the diets were ground through a 0.5-mm mesh screen before analyses. Analyses for dry matter, gross energy, crude protein, crude fat, crude fiber, ash, S, and the S-containing amino acids were performed according to the standards of the Association of Analytical Communities International (2000). Partial chemical compositions of the experimental diets are shown in Table 2. The crude protein and S contents of Diets A, B, and C were proportional to the content of the S-containing amino acids. The pigs were fed equal amounts twice daily (0800 and 1600 h) that together supplied three times the daily maintenance requirement for digestible energy (NRC, 1998). Water was mixed with the feed at a mass ratio of 2.5:1.


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  		Table 1. Diet formulation for Experiment 2.

 

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  		Table 2. Partial characterization of the diets in Experiment 2.

 
Each of the three feeding periods in the experimental design comprised 14 d, including 7 d for adaptation and 7 d during which manure slurry (feces and urine) from each pair of pigs was collected in 200-L barrels stored at room temperature (18°C). At the end of each period, the slurry in each barrel was homogenized, analyzed for moisture and S content, and divided between two 20-L vessels (as per Experiment 1). The 5-L headspace of each vessel was ventilated at a rate of 40 mL min–1 and air was bubbled through the 15-L slurry sample at 10 mL min–1.

As described for Experiment 1 (above), the oxygen supply would have been well below the BOD5 for the slurry. The BOD5 of the slurry in this experiment would likely have been greater than that in Experiment 1, since the feces and urine were collected directly and were undiluted. Pigs produce about 3.1 kg BOD5 d–1 per 1000 kg live animal mass (ASAE, 2003). In this experiment, a total of about 30 L of slurry was collected over 7 d from each pair of 40-kg pigs. The BOD5 of the slurry would therefore have been about 58 g L–1. As in Experiment 1, the 10 mL min–1 airflow would have supplied about 1400 mg O2 L–1 manure slurry during the 10-d storage period. At an estimated 10% oxygen transfer efficiency (Solomon et al., 1998), the total oxygen supplied would have been equivalent to about 0.2% of the BOD5, although no direct measurements of BOD5 were made to confirm this estimate.

The H2S concentration in the headspace of each vessel was measured once each day (as per Experiment 1) for 12 d. On Day 12, the slurry in each vessel was agitated for 30 s (as per Experiment 1). Hydrogen sulfide concentrations were measured before agitation and 20 s after agitation. During the final storage period the slurry was left in the vessels until Day 14, and odor concentrations were measured by olfactometry on Days 9 and 14 (Feddes et al., 2001; European Committee for Standardisation, 1999).

The data were analyzed using the SAS software package (SAS Institute, 2001) using an {alpha} value of 5% (P ≤ 0.05) throughout. The headspace H2S concentration data were normalized with a logarithmic (base 10) transformation, as verified with the SAS Univariate Procedure. The SAS Mixed Procedure was used to the test the concentration data for the significance of the effects of fixed variables, according to a full 3 x 3 Latin Square design (Wang and Goonewardene, 2004; SAS Institute, 1996). The fixed variables included in the model were the diet fed during each feeding period, the diet fed during the preceding feeding period (i.e., carryover effect), storage time, and animal pair (i.e., animal effect). The storage time was specified as the temporal variable associated with repeated measures of the headspace H2S concentration, and a compound-symmetric covariance matrix was used (SAS Institute, 1996). The statistical significance of the effects of the fixed variables and interactions included in the model are shown in Table 3. The least-squares means of the log-transformed H2S concentration data were compared to distinguish significant differences between the diet effects (Table 4). One-tailed Student's t tests were used to determine the significance of any difference between paired headspace concentration measurements taken immediately before and after agitation of the stored slurry, averaged across feeding periods and storage vessels for each dietary treatment (Table 4).


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  		Table 4. Concentrations of dietary S, manure slurry moisture, S, and headspace H2S in Experiment 2.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Experiment 1
The headspace H2S concentrations during storage for the three different bubbling rates are shown in Fig. 2 . The concentration from the 5 mL min–1 bubbling treatment was significantly higher than both the control and the 10 mL min–1 treatments (P < 0.05), whereas the concentrations at the latter rates two did not differ significantly from one another. The day on which the measurements were made had no significant effect on the values, nor was there a significant interaction between the aeration rate and the storage time (P < 0.05).



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  		Fig. 2. Headspace H2S concentration during slurry storage for different air bubbling rates in Experiment 1. Error bars show standard deviation of the mean within treatments (n = 3).

 
The H2S concentrations in the outlet air after agitation of the slurry are shown in Fig. 3 . The time elapsed after agitation and the bubbling rate both had significant effects on concentration (P < 0.05). Upon agitation, the H2S concentrations in the headspaces of the bubbled vessels were dramatically lower compared with the unbubbled vessels. Five minutes after agitation, the mean concentrations for the 10 mL min–1, 5 mL min–1, and control treatments were 3, 47, and >120 µL L–1 (out of range of the detector), respectively. At that time, therefore, the headspace concentration corresponding to the 10 mL min–1 bubbling rate was below the 8-h exposure limit (5 µL L–1) as regulated in Alberta (Atia et al., 2004), whereas the concentration for the 5 mL min–1 and control treatments treatment took about 10 min and more than 20 min, respectively, to decline to about the same level. At the highest bubbling rate (10 mL min–1), there was no discernable change in the concentration of H2S 5 min after agitation as compared with pre-agitation levels.



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  		Fig. 3. Headspace H2S concentration after start of agitation of slurry in Experiment 1, corresponding to different air bubbling rates during storage. Error bars show standard deviation of the mean within treatments (n = 3). Unfilled markers and a dashed line indicate concentrations that exceeded the range of the instrument (>120 µL L–1).

 
The highest bubbling rate (10 mL min–1 in 15 L of slurry) corresponds to only about 650 mL min–1 m–3 slurry. If this approach were to scale proportionately to an industrial setting, then a 50000-L holding tank, for example, could be stripped of dangerous concentrations of dissolved H2S by bubbling air at about 35 L min–1. This conjecture must be investigated with trials at production-scale facilities.

Experiment 2
The pigs remained healthy throughout the experiment and readily consumed their daily feed allowances. Increased S content of the diet (Table 2) resulted in a corresponding increase in the slurry S concentration (Table 4). The concentration of S in the drinking water was found to be about the same throughout the experiment (17.7, 18.0, and 17.4 mg L–1 during Periods 1, 2, and 3, respectively).

The concentration of H2S in the headspaces of all vessels was greater than zero (P < 0.05) during storage. The dietary treatments did affect the H2S concentrations (Table 4), whereby Diet C resulted in concentrations that were higher (P < 0.05) than those resulting from Diets A and B. There was also a significant carryover effect, whereby the H2S concentration was influenced (P < 0.05) by the diet fed to the same pair of pigs during the feeding period before the one in which the manure slurry was collected. A significant animal effect (P < 0.05) was also accounted for in the model.

The day on which the headspace H2S concentrations were measured (i.e., storage time) also had a significant effect on the measurements (P < 0.05) and there was a significant interaction between this factor and the diet (P < 0.05). The effect of the storage time was most pronounced for the high-S diet (Diet C), because the emissions associated with that diet varied the most during a given storage period. This is illustrated in Fig. 4 for the first storage period of Experiment 2. Similar differences in variability were also evident for the two subsequent storage periods, but with the peak concentrations associated with Diet C occurring at other times during storage. Since the focus of this study was mainly on the emission spike immediately after disturbance of the manure slurry, this variability during storage was not investigated further.



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  		Fig. 4. Headspace H2S concentrations during the first storage period of Experiment 2, corresponding to three experimental diets. Error bars show standard deviation of the mean within treatments (n = 2).

 
Immediately after agitation, there was a slight but significant increase (P < 0.05) in the H2S concentration in the headspace air corresponding to Diet B only (Table 4). This confirms that the 10 mL min–1 bubbling rate most effectively stripped H2S suspended in the slurry, since there was no rise in H2S due to the sudden release of dissolved H2S when the slurry was agitated. This contrasts with the pronounced rise in concentration when, for example, unbubbled manure was agitated in Experiment 1 (Fig. 3).

The odor concentration of the air exhausted from the headspace of each slurry storage vessel was measured on Days 9 and 14 of the third storage period (Fig. 5) . The dietary treatments had no effect on odor concentration (P > 0.05) in this investigation.



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  		Fig. 5. Odor concentration of inlet and outlet air from manure slurry storage vessels in Experiment 2. Diets (A, B, and C) had no effect on odor concentration (P > 0.05).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of this study show that low-level bubbling of air through stored swine manure slurry might be a viable technique to reduce the levels of H2S released when the slurry is agitated during removal. Also, diets that include an excess of S increase the emission of H2S from slurry when it is disturbed. There was no effect, however, on odor concentration at the S levels used in this study. Since these conclusions were drawn from the results of a bench-scale study, the effectiveness of the methods at an industrial scale must be verified through further research.


   ACKNOWLEDGMENTS
 
This work was supported financially by Alberta Agriculture, Food, and Rural Development. The authors thank K. Sauer and the staff of the Swine Research and Technology Centre and the AFNS Olfactometry Laboratory (University of Alberta).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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