HYDROGEN SULFIDE EMISSIONS FROM A MECHANICALLY-VENTILATED SWINE BUILDING DURING WARM WEATHER

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1 Paper No An ASAE Meeting Presentation HYDROGEN SULFIDE EMISSIONS FROM A MECHANICALLY-VENTILATED SWINE BUILDING DURING WARM WEATHER BY Jiqin Ni (1), Albert J. Heber (1), Teng T. Lim (1), Ravikrishna Duggirala (2), Barry L. Haymore (3) Claude A. Diehl (2) and Alan L. Sutton (4) (1) Agri. and Bio. Eng. Dept Purdue University West Lafayette, IN 4797 (2) Monsanto EnviroChem Systems S. Outer Forty Road St. Louis, MO 6317 (3) ChemLink International Walfield Lane St. Louis, MO (4) Animal Sciences Dept Purdue University West Lafayette, IN 4797 Written for presentation at the 1998 ASAE Annual International Meeting Sponsored by ASAE Disney s Coronado Springs Resort Orlando, Florida July 12-16, 1998 Summary: Hydrogen sulfide (H 2 S) concentration in and emission from a new, grow-finish swine building with a deep pit was measured during three summer months. Hydrogen sulfide concentration, ventilation rate, temperature and pig number were continuously measured or recorded from June to September. Average daily mean (ADM) indoor H 2 S concentration was 173±21 ppb ranging from 38 to 536 ppb. The highest recorded 12-min H 2 S concentration was 1,624 ppb. The ADM H 2 S emission rate from the building, which had from 887, 21.9 kg pigs to 874, 98.6 kg pigs, was 582±57 g/d, or 7.±.9 g/d per 5 kg of pig weight. The mean H 2 S emission rate per 5 kg was much higher than values found in the literature. The H 2 S concentration was inversely proportional to ventilation rate. The H 2 S emission rate was directly proportional to ventilation rate, pig weight and indoor temperature. Keywords: Air quality, pig house, environment, pollution, ventilation The author(s) is solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of ASAE, and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Quotation from this work should state that it is from a presentation made by (name of author) at the (listed) ASAE meeting. EXAMPLE -- From Author's Last Name, Initials. "Title of Presentation." Presented at the Date and Title of meeting, Paper No. X. ASAE, 295 Niles Rd., St. Joseph, MI USA. For information about securing permission to reprint or reproduce a technical presentation, please address inquiries to ASAE. ASAE, 295 Niles Rd., St. Joseph, MI USA Voice: FAX: <hq@asae.org>

2 INTRODUCTION Hydrogen sulfide is considered the most dangerous gas among the by-products of manure decomposition. It has been responsible for many animal as well as human deaths (Field, 198; Anonymous, 1996). Hydrogen sulfide is colorless but has a very pungent odor like rotten eggs. Its density is 1.54 g/l and it is soluble to water. It originates from the anaerobic fermentation of manure. High concentrations of H 2 S are toxic to humans and animals (Hartung, 1988). A concentration of 5 ppm can cause dizziness, irritation of the respiratory tract, nausea, and headache. Death from respiratory paralysis can occur with little or no warning in concentrations exceeding 1, ppm (Field, 198). Usually, H 2 S is very low in animal houses compared with NH 3 and CO 2. It was measured at 9 ppb in a normally ventilated confinement building and 28 ppb after the ventilation was shut off for six hours (Muehling, 197). The H 2 S concentration was 166 ppb in two naturally-ventilated swine houses during a 63-day measurement reported by Heber et al. (1997). Much higher concentration of H 2 S was generated in air when the slurry was agitated. At that time, it could immediately rise to more than 1 ppm inside the barn and 15 ppm in the pit exhaust air (Patni and Clarke, 1991). Although several reports about acute H 2 S concentration in swine houses were found in the literature, information on typical H 2 S emission from animal facilities is lacking. Only two direct measurements of H 2 S production in swine houses have been found in available literature (Avery et al., 1975; Heber et al., 1997). Avery et al. (1975) presented the results of 8 samples in six swine units, where measured H 2 S concentration ranged from 12 to 2,174 ppb. The daily production of H 2 S was calculated as the average daily concentration of H 2 S multiplied by the total daily air flow and ranged from.35 to mg/d LU (LU: livestock unit, equals 5 kg of live weight). Heber et al. (1997) reported mean H 2 S emission rates of 843 and 93 mg/d LU from each of two naturally-ventilated, 1-head, finishing swine houses. Hartung (1988) attempted to estimate H 2 S emission on a national scale and calculated the total H 2 S emission from pig houses in Germany as.62 mg/h LU. The assumed H 2 S concentration was only 4 µg/m 3 and the assumed ventilation rate was 15 m 3 /h per LU. The objectives of this research related to hydrogen sulfide in a swine finishing building were to: 1. Measure concentrations on a continuous basis for a long period, 2. Measure emission rates at a high frequency for a long period, 3. Compare the measured concentrations and emission rates with the literature, and 4. Study influences of ventilation rate, pig weight and temperature on concentrations and emission rates. EXPERIMENTAL PROCEDURE Data presented in this paper were collected from one of four mechanically-ventilated, 1-pig finishing buildings involved in a large test of a new manure additive product. The building 2

3 studied in this paper was untreated. Results presented here consisted of three months (June 26 to September 25, 1997) of measurements, a total of 89 days that included about 17 reliable data subsets. Each data subset consisted of a 6 or 9 minutes of continuous measurement of all variables except for gas concentrations that were sampled for 1 or 15 minutes at each location. The x m building had a 2.4 m deep pit under a fully slatted floor with a pit surface area of 89 m 2 (Figure 1). There were twenty, 3.3 x 5.9 m, pens on either side of a.76 m wide center aisle. The floor consisted of concrete slats. The building had a 2.26 m high flat ceiling with.8 m thick loose-fill cellulose insulation. The side walls consisted of a.81 m high,.15 m thick concrete skirt and 1.52 m high curtains. Manure depths in the pit increased from 148 cm on June 26 to 181 cm on September 25. No manure was removed from the pit during the test..46 Pit fan Fiberglass insulation Attic Pit chimney Curtain.8 Concrete skirt Slatted floor Room baffled ceiling inlets Pen partition Curtain * * Underfloor manure pit Manure depth * Cleanout port Unit: meter North 1.57 * ' * Pit fan gas sampling points AD 592 temperature sensors 9 6, 13 36' 18' ' * * Attic Room * * * Manure pit Pit fan chimneys Pit headspace gas sampling points Pressure transmitter Wall fans Wall fan gas sampling points Figure 1. Cross section (top), floor plan (middle) and side view (bottom) of the building with measurement and sampling locations. * 3

4 There were four pit ventilation fans and five wall ventilation fans in the building. The four,.46 m diameter, variable-speed pit fans (Aerotech AT18F), each installed at the top of a vertical chimney, operated continuously. The building had curtains on both side walls and on the west end wall. The building was tunnel-ventilated in the summer and cross-ventilated in the winter. One, 9 cm diameter and four, 12 cm diameter exhaust fans were located on the east wall of the building to create a tunnel ventilation effect during hot weather. Hydrogen sulfide was sampled at three location groups: 1) pit headspace (six sampling points), 2) pit fans (four sampling points), and 3) wall fans (five sampling points). Multiple sampling points were connected in parallel to the gas sampling system. Four sampling points at the inlets to the four 12 cm diameter wall fans were controlled individually with computer-operated solenoids. They were open only when the corresponding fans were operating. The gas sampling tube for the 9 cm wall fan was always open. Gas at each location was sampled and measured continuously for 15 minutes before switching to another location group. Blocks of data were averaged and stored every 2 seconds. Forty-five minutes were allocated during each 9-min cycle to measure gas concentrations in this building at these locations. Thus, 16 sampling periods were obtained daily for gas concentrations at each location. The sampling time for each location was reduced from 15 to 1 minutes on August 14, after which 3 minutes every hour were allocated for this building and the number of sampling periods increased from 16 to 24 each day. The first 3-min H 2 S concentration data in the 15 and 1-min measurements were ignored to allow the instruments to equilibrate. Thus, 12 or 7-min gas concentration data were obtained in each period. Hydrogen sulfide was first converted to sulfur dioxide (SO 2 ) with a H 2 S converter (Model 34, TEI, Inc., Mansfield, MA). The converted SO 2 was measured with a pulsed fluorescence SO 2 analyzer (Model 43C, TEI, Inc.) with a sample flow rate of 1. Lpm. The analyzer was based on the principle that SO 2 molecules absorb ultraviolet (UV) light and become excited at one wavelength, then decay to a lower energy state emitting UV light at a different wavelength, as described by the following chemical reaction: SO 2 + hv 1 SO 2 * SO 2 + hv 2 (1) As the excited SO 2 molecules decay to lower energy states, they emit UV light that is proportional to the SO 2 concentration. A photomultiplier tube (PMT) detected the UV light emission from the decaying SO 2 molecules. The SO 2 analyzer had a measurement range of.5 to 1. ppm and response times of 8 sec (1 sec average time) to 32 sec (3 sec average time). Their precision was 1% of reading or 1 ppb (whichever is greater) according to the manufacturer. The H 2 S measurement instrument was calibrated with certified gas eight times (6/26, 7/3, 7/11, 7/21, 8/5, 8/27, 9/5 and 9/17), once every one or two weeks. Building airflow was the sum of the airflow from the wall fans and the airflow from the pit fans. Airflow rates from the wall fans were calculated based on fan curves (Eqns 2 and 3) supplied by the manufacturer and the static differential pressure between indoor and outdoor air, which was measured with a ±25 Pa (accuracy.25%) differential static pressure transmitter (Model C1, Dresser Industries, Stratford, CT). 4

5 Q f,9 = P (2) Q f,12 = P (3) where: Q f,9 = airflow rate for the 9 cm fan, m 3 /h, Q f,12 = airflow rate for the 12 cm fan, m 3 /h, and P = static pressure, Pa. Airflow rates from the pit fans were estimated based on fan control voltage. The four chimney fans were controlled in parallel by a single speed controller. An airflow/voltage relationship was determined from three other identical buildings that were equipped with full-size airflow rate sensors (FanCom Model FMS 5, Techmark, Lansing, MI). Pit fans operated at full speed most of the time during this test. Air temperatures were measured with semiconductor sensors with stainless steel probes (Model AD592, Computer Boards, Inc., Mansfield, MA). Indoor temperatures measured at seven locations equally spaced along the center of the building length 2 m above the floor, and outdoor temperature were used in this paper. A detailed description of the test installation was given by Heber et al. (1998). The building was filled with 616 pigs on June 19, four days after the building was emptied. A second group of young pigs was moved into the building on June 26, making the total number of pigs 887 with an average weight of 21.9 kg. The number of pigs was maintained except for 13 deaths during the three-month test. By September 25, there were 874 pigs weighing about 89.6 kg each. Pigs were weighed before entering the building. The average weight of the pigs on a certain day (nth day) of the test was calculated based on the beginning weight and with Eqn 4, in which the pig growth rate, Q Wp, was assumed to be.745 kg/d. W p n ( n) = W () + Q (4) p i= 1 wp where: n = number of days in the building, d, Q Wp = rate of pig growth, kg/d, W p () = weight of pig on the first day, kg, and W p (n) = weight of pig on nth day, kg. Manure samples were obtained from the manure pit (on 7/1, 7/24, 8/6, 9/12 and 9/26), and analyzed in the Purdue Animal Sciences Waste Management Laboratory. The building had three cleanout ports. The north side of the building had two cleanout ports located toward the end of the building. One cleanout port was located on the south side at the center. The top sample was obtained by lowering a cup attached to a rod to obtain a sample from the surface of the pit in the cleanout ports. Profile samples were obtained by lowering a probe sampler (Coliwasa, Nutrient Resource Management, Inc. Columbus, OH) to obtain a core sample from the top to the bottom of 5

6 the pit. Probe samples were poured into a bucket and thoroughly mixed. A subsample of the mix was taken and stored in a sealed 8 oz. plastic bottle, placed in a styrofoam container with ice, and transported to the laboratory. Total nitrogen (TKN) was determined by the micro-kjeldahl nitrogen method of Nelson and Sommers (1972). Ammonium nitrogen (NH 4 + -N) was determined using the steam distillation method of Bremner and Keeney (1965). For dry matter, the samples were analyzed gravimetrically at 9 C. For P and K, manure samples were wet ashed by refluxing with concentrated HNO 3 for three hours prior to analysis of the digest. Analysis of K was determined by atomic absorption spectrophotometry. Analysis of P was evaluated according to Murphy and Riley (1962). Mean manure characteristics are listed in Table 1. Table 1. Mean manure characteristics based on five samples. ph Dry Matter Total N Ammonia P K % ppm ppm ppm ppm Top of the pit Pit profile Mean Hydrogen sulfide emission rate at each period (6 or 9-min) was the product of the mean H 2 S concentration (sampled 7 or 12-min) and the mean airflow rate (measured 6 or 9-min). Hydrogen sulfide emission from the wall fans was calculated by multiplying the H 2 S concentrations measured at locations.5 m upstream of the fans and the sum of the fan airflow rates. Hydrogen sulfide emission from the pit fans was calculated by multiplying the H 2 S concentration measured at the pit fan chimney entrances and the sum of the pit fan airflow rates (Figure 1). Total emission from the building was the sum of the emission from the wall fans and the emission from the pit fans. The outside H 2 S concentration was neglected and assumed zero. Hydrogen Sulfide Concentration RESULTS AND DISCUSSIONS There were H 2 S concentration differences between the pit headspace, pit fans and wall fans (Figure 2). The average daily mean (ADM) concentrations for these three locations were 158±21, 197±23 and 169±2 ppb (mean ± 2 times the standard error or 95% confidence interval), respectively. The pit fan H 2 S concentration was the highest among the three locations. The ADM building H 2 S concentration (averaged over the three locations) was 173±21 ppb. The minimum and maximum daily means were 38 and 536 ppb, respectively (Figure 2 and Table 2). The ADM building H 2 S concentration was a little lower than the mean H 2 S concentrations of 221 and 18 ppb reported by Heber et al. (1997), who obtained these values during 63 days of continuous measurement in two naturally-ventilated swine buildings of similar size during cold weather. Avery et al. (1975) reported concentrations between 12 and 2174 ppb in six swine 6

7 buildings from 8 air samples analyzed with the methylene blue method. The 8 samples were taken on ten days with eight samples per day over a 2-3 week period (Table 3). Variations of H 2 S concentration among 7 to 12 minute averages within a day were more visible. Figure 3 shows an example, in which there were not only diurnal variations of H 2 S concentrations, but also concentration differences between the three measurement locations. The highest H 2 S concentration during any 7 or 12 min measurement was 1,624 ppb. 7 H 2 S concentration, ppb Pit head space Pit fans Wall fans 6/26 7/3 7/1 7/17 7/24 7/31 8/7 8/14 8/21 8/28 9/4 9/11 9/18 9/25 Date Figure 2. Daily mean hydrogen sulfide concentrations at different locations. Table 2. Some statistics of daily means of measured variables (n=89). Mean ±2xSE Min Max Std. Dev. Daily mean H 2 S conc., ppb Total H 2 S emission, g/d H 2 S emission, g/d LU Ventilation, m 3 /h 158,75 9,66 46, ,213 45,38 Pig number, head Pig weight, kg Total pig weight, kg 48,783 3,641 19,418 78,351 17,77 Indoor temperature, C Outdoor temperature, C Indoor H 2 S concentration should be influenced by the outdoor ambient background concentration since there was always an indoor-outdoor air exchange. In this study, the outside or background concentrations of H 2 S were obtained in five independent measurements (Table 4). Two of the 7

8 measurements were conducted during summer at a location 1 m east of the building with a strong easterly wind. The mean of the two summer measurements was 23.7 ppb. The three winter measurements, in which the measurement locations were between two buildings that were spaced 18.3 m apart, did not give detectable H 2 S concentrations. The high ambient H 2 S concentration in summer as compared with winter has not been discussed in the literature. Table 3. Comparison of mean hydrogen sulfide concentration in and emission from swine buildings selected from three previous reports. Avery et al., 1975 Hartung, Heber et al This Unit 2 Unit 4 Unit Treated Control research Season NA NA NA NA January - March Summer Building type Farrowing Finishin g Finishin g NA Finishin g Finishin g Finishin g Manure pit type (1) Partial Full Partial NA Full Full Full Pig weight, kg 7,618 29,376 3,6 NA NA NA 48,783 Manure depth, cm NA NA NA NA NA NA Ventilation type Mech. Mech. Mech. NA Natural Natural Mech. Airflow, m 3 /h NA 8,296 4,681 NA NA NA 158,75 Airflow m 3 /h LU NA ,627 Sample number 8 (2) 8 (2) 8 (2) NA 1,5 1,5 1,7 Concentration, ppb 2, Emission, mg/d LU 14.8 (3) 4.3 (3).4 (3) ,984 Note: (1) Full pit was under the entire floor while partial pit was not; (2) Eighty samples for each unit, 1 days with eight samples per day over a 2-3 week period; (3) As reported in the cited reference and after unit conversion; NA = not available. 8

9 H 2 S concentration, ppb Pit headspace Pit fans Wall fans Ventilation Time, hour of day Total ventilation rate, m 3 /h Figure 3. Hydrogen sulfide concentrations in the pit headspace, at the pit fans, and at the wall fans and total ventilation rate on September 12, Table 4. Ambient background hydrogen sulfide concentration. Date Start time Measurement duration, min Mean H 2 S concentration, ppb June 9 9: June 18 13: December 18 15:45 95 December 29 18:2 1 December 3 8: June mean December mean 17 Hydrogen Sulfide Emission Hydrogen sulfide emission is expressed as emission from the building and emission per LU. The latter definition related the H 2 S emission to animals in the building for comparison with other buildings. The total pig weight increased from 19,418 kg on June 26 to 78,351 kg on Sept. 25. The mean H 2 S emission rate from the building was 582±57 g/d. The maximum daily mean emission rate was 1,882 g/d as compared to the minimum of 135 g/d. The average daily mean (ADM) H 2 S emission was 7.±.9 g/d LU and ranged from 1.9 to 2.1 g/d LU (Table 2 and Figure 4). The daily mean H 2 S emissions were between 2 and 6 g/d LU over half the time (Figure 5). 9

10 H 2 S emission, g/d Total per LU /26 7/3 7/1 H 2 S emission per LU, g/d. 7/17 7/24 7/31 8/7 8/14 8/21 8/28 9/4 9/11 9/18 9/25 Date Figure 4. Daily mean hydrogen sulfide emission rate Frequency, % H 2 S emission per LU, g/d More Figure 5. Frequency of daily mean H 2 S emission rates. The average daily mean H 2 S emission rate of 6,984 mg/d LU obtained in this study was much higher compared to the literature. The H 2 S production rates reported by Avery et al. (1975) ranged from.35 to mg/d LU, which were only a tiny fraction of the rates measured in this study. Table 3 lists three of the six swine units from Avery et al. (1975). Unit 2 was a farrowing building with a partial pit and had the highest H 2 S production rate. Units 4 and 5 were finishing 1

11 buildings with full and partial pits, respectively. Unit 5 had the lowest production among the reported six units. The daily H 2 S production in Avery et al. (1975) was calculated as the average daily concentration of hydrogen sulfide multiplied by the total daily airflow. This technique may have resulted in lower calculated emission rates. The daily mean H 2 S emission rate in this study was about 5 times higher than the 14.9 mg/d LU estimated by Hartung (1988). The difference probably arose from the assumed values of H 2 S concentrations and airflow rates. Hartung (1988) used an average H 2 S concentration of 4 µg/m 3 (about 3 ppb) and an average ventilation rate of 15 m 3 /h per LU (Table 3). The H 2 S concentration assumed by Hartung (1988) was much lower than most of the reported measurement values and the ventilation rate was less than 1% of the rates measured in this study, which averaged 1627 m 3 /h per LU. Higher ventilation rates were measured in this study because of the hot summer weather. The daily mean H 2 S emission rates reported by Heber et al. (1997) were 93 and 843 mg/d LU in a building treated with a manure additive and an untreated building, respectively. These emission rates were about 12-13% of the values obtained in this study. Lower ventilation rates in cold weather were probably the reason for the lower H 2 S emission rates. Influences of Ventilation Rate, Temperature and Pig Weight The daily mean H 2 S concentration was inversely proportional to ventilation rate (Table 5). This can be clearly seen in Figure 3 with hourly mean data. The negative relationship can be explained by the dilution effect of ventilation on the gas concentration in the building. The ventilation rate in the test building was automatically adjusted to control indoor temperature. Its daily fluctuation pattern was quite closely related to that of outdoor temperature (Figure 6). Therefore, the H 2 S concentration was also inversely proportional to indoor and outdoor temperatures. Table 5. Correlation coefficients of hydrogen sulfide concentration and emission rate with other variables. H 2 S concentration Building H 2 S emission H 2 S emission per LU H 2 S concentration 1 Total H 2 S emission.36 1 H 2 S emission per LU Ventilation rate Total pig weight Indoor temperature Outdoor temperature

12 25 35 Ventilation rate, m 3 /h Total ventilation Indoor T Outdoor T Temperature, C 6/26 7/3 7/1 7/17 7/24 7/31 8/7 8/14 8/21 8/28 9/4 9/11 9/18 9/25 Date Figure 6. Daily mean outside and inside temperatures and building ventilation rates. The daily mean H 2 S emission rate was directly proportional to H 2 S concentration, ventilation rate, pig weight and temperatures. Total H 2 S emission rate was the product of H 2 S concentration and ventilation rate. It is obvious that with higher H 2 S concentration and ventilation rate, there should be higher H 2 S emission rate. Fresh manure production is directly proportional to pig weight. Since H 2 S was produced by microbial activities in the manure, it could be expected that higher fresh manure production provided more H 2 S. Higher temperature increases microbial activity but also results in higher ventilation rates. Therefore, its positive correlation to the H 2 S emission rate could also be expected. However, H 2 S release from manure did not seem to be only influenced by these variables. Ni et al. (1998) reported that the concentration and release rate of H 2 S were less stable than those of NH 3 and CO 2 in emptied and cleaned swine houses with constant temperature and ventilation rate. Factors influencing the release of H 2 S from manure and emission from swine buildings need more investigation. CONCLUSIONS The following conclusions were drawn from 89 days of continuous measurement of H 2 S concentration in and emission from the large swine facility during warm weather: 1. The average daily mean indoor H 2 S concentration was 173±21 ppb. The minimum and maximum daily mean H 2 S concentrations were 38 and 536 ppb, respectively. 2. There were wide diurnal fluctuations of H 2 S concentrations in the swine house. There were also moderate differences in H 2 S concentrations between the pit headspace and the pit fans and wall fans. These variations demonstrated the necessity of measuring the H 2 S concentration for a sufficiently long time and selecting appropriate measurement locations to obtain reliable data. 12

13 3. The mean of two summer measurements of outside H 2 S concentration was 23.7 ppb. More measurements are needed to confirm this relatively high value as compared to winter. 4. The average daily mean emission of H 2 S was 582±57 g/d from the building or 7.±.9 g/d per 5 kg of pig weight. The daily mean H 2 S emissions were between 2 and 6 g/d per 5 kg of pig weight over half of the days. Emissions reported in the literature were from one to three orders of magnitude lower. 5. The daily mean indoor H 2 S concentration was inversely proportional to ventilation rate. The daily mean H 2 S emission from the building was directly proportional to ventilation rate, pig weight and indoor temperature. ACKNOWLEDGEMENTS This research was supported, in part, by the Multi-State Consortium on Animal Waste, the Purdue University Agricultural Research Program, and Monsanto Enviro-Chem in St. Louis, MO. The authors also acknowledge the collaboration and assistance of Heartland Pork, Inc. REFERENCES Anonymous (1996). "Nightmare on the hog farm: Hydrogen sulfide claims two lives in Minnesota." The Biobulletin 3(2):1-3. Avery, G.L., G.E. Merva and J.B. Gerrish (1975). "Hydrogen sulfide production in swine confinement units." Transactions of the ASAE 18: Bremner, J.M. and D.R. Keeney (1965). "Steam distillation methods for determination of ammonium, nitrate and nitrite." Anal. Chim. Acta. 32: Field, B. (198). "Rural health and safety guild: Beware of on-farm manure storage hazards." Cooperative Extension Service, Purdue University, West Lafayette, Indiana. S-82. Oct p. Hartung, J. (1988). "Tentative calculations of gaseous emissions from pig houses by way of the exhaust air." In: Volatile emissions from livestock farming and sewage operations. Nielsen, V.C. et al., (Eds). London, Elsevier Applied Science. pp Heber, A.J., R.K. Duggirala, J. Ni, M.L. Spence, B.L. Haymore, V.I. Adamchuk, D.S. Bundy, A.L. Sutton, D.T. Kelly and K.M. Keener (1997). "Manure treatment to reduce gas emissions from large swine houses." In: International Symposium on Ammonia and Odour Control from Animal Production Facilities. Vinkeloord, The Netherlands, Oct Voermans, J.A.M. and G.J.Monteny (Eds). pp Heber, A.J., B.L. Haymore, J. Ni and R.K. Duggirala (1998). "Measurements of gas emissions from commercial swine buildings." In: ASAE Annual International Meeting. Orlando, Florida, July Paper No ASAE, 295 Niles Rd., St. Joseph, MI

14 Muehling, A.J. (197). "Gases and odors from stored swine waste." Journal of Animal Science 3: Murphy, J. and J.P. Riley (1962). "A modified single solution method for the determination of phosphate in natural waters." Anal. Chim. Acta 27: Nelson, D.W. and L.E. Sommers (1972). "A simple digestion procedure for estimation of total nitrogen in soils and sediments." Journal of Environmental Quality 1: Ni, J., T. Lim, A.J. Heber, C. Diehl and R. Duggirala (1998). "Gas release from deep manure pits in cleaned and emptied swine finishing buildings." In: Animal Production Systems and the Environment. An International Conference on Odor, Water Quality, Nutrient management and Socioeconomic Issues. Des Moines, Iowa, July 2-22, pp. 6. 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):