M. C. Wiles§*,
J. C. Amburgey§,
D. C. Borger*,
L. B. Willett*,
H. M. Keener,
and D. L. Elwell1
§College of Wooster Department of Chemistry
*The Ohio State University Department of Animal Sciences
The Ohio State University Department of Food, Agricultural, and Biological Engineering
1For more information, contact at: Department of Food, Agricultural, and Biological Engineering,
The Ohio State University, Ohio Agricultural Research and Development Center,
121B Agricultural Engineering Building, 1680 Madison Ave.,
Wooster, OH 44691; 330-263-3862; e-mail: email@example.com.
Offensive odors evolved during the decomposition of swine waste have resulted in complaints from neighbors in agricultural areas. This study showed that the chronological monitoring of the formation of volatile fatty acids (VFAs) could be used in conjunction with the composting process to optimize conditions that minimize the release of malodorous compounds. Mixtures of swine waste and sawdust (3.5:1) were placed in 91 kg reactor vessels and constantly aerated over a 21-day period to chronologically monitor fermentation and formation of acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids. Composting conditions were shown to be adequate based on carbon to nitrogen ratios and moisture contents. The 21-day aerobic treatment decreased all VFAs retained in the biomass by 50 to 100% with an average of 87%. Chronological monitoring of volatile emissions from the vessels showed that prior to peak gas volatilization, when the vessels attained peak composting temperatures, acetic acid was emitted in the greatest amounts, followed by butyric, propionic, valeric, isovaleric, and isobutyric acids. Following peak gas volatilization, butyric acid accumulated in the greatest amounts followed by acetic and propionic acids, valeric, isovaleric, and isobutyric acids. Peak VFA emissions occurred simultaneously with the greatest headspace temperatures, peak rates of O2 uptake, and peak production of condensate, ammonia, and CO2. Therefore, stringent control of these factors may decrease VFA emissions. Fewer VFAs were emitted from those vessels that quickly heated and were active for short periods of time, whereas much greater amounts of VFAs were emitted over longer periods of time from those vessels that possessed adequate conditions for composting for longer periods of time.
The major malodorous compounds in swine manure have been identified as volatile fatty acids (VFAs) including acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids, as well as the aromatic compounds phenol, p-cresol, indole, and skatole (Chen et al ., 1994; Schaefer, 1977; Williams, 1984). On average, slurries of swine waste have been shown to contain more VFAs than those of cattle waste (Cooper and Cornforth, 1978). However, laboratory experiments have indicated that VFAs persist only when oxygen is absent from waste. Rapid decomposition of VFAs occurred when air was passed through a waste sample, but decomposition occurred more slowly if air was passed only above the sample. When anaerobic conditions were re-established, VFAs did not re-form unless protein hydrolysate or glucose was added, which suggested that aeration was effective to eliminate VFA precursors (Cooper and Cornforth, 1978). In addition, aeration has been shown to eliminate the production of methane and hydrogen sulfide and to reduce the production of ammonia (Stevens and Cornforth, 1974). The purpose of the present study was to determine whether the chronological monitoring of the formation of VFAs could be used in conjunction with the composting process to optimize conditions that minimize the release of malodorous compounds.
To chronologically monitor the formation of VFAs in conjunction with the composting process, identical reactor vessels were used. Approximately 200 lbs (91 kg) of composting mixtures of swine waste and sawdust (3.5:1) were placed in insulated 54 gal (200 L) stainless steel drums and continuously aerated for 21 days. Water vapor and volatilized gases in the headspace areas were condensed, and the liquid condensate was collected in separate polyethylene bottles over 12-hour intervals. In addition, VFAs within the vessel contents before and after the trial were extracted using slightly acidic nanopure water, centrifugation, and filtration. Quantitative analysis of liquid condensate samples and filtered extracts was performed using an HP5890 Gas Chromatograph (Hewlett Packard, Palo Alto, Calif.) equipped with a flame ionization detector.
In addition to VFA emissions, ammonia emissions, carbon dioxide generation, and oxygen taken up by microbial activity were monitored via the volatilized gases in the headspace areas of the vessels. Portions of the vessel contents were used to determine percent total carbon, percent total nitrogen, retained VFAs, and moisture contents.
The objective of the study was to recover and quantify the malodorous compounds from composting waste as well as from volatile emissions. Adequate composting conditions were confirmed using the carbon-tonitrogen ratio, moisture content, oxygen availability, and aeration to displace excess heat generated by microbial activity. Microbial activity was monitored with O2 utilization and CO2 generation.
The greatest headspace temperatures and peak rates of O2 uptake, as well as peak production of condensate, VFAs, ammonia, and CO2, occurred simultaneously. As heat produced from microbial action increased the temperature, gas volatility increased, and VFAs and ammonia were released with water vapor. Throughout the trial, the moisture content of the composting mixtures decreased by 24 to 45% as a total of 4.54 to 6.95 gal (17 to 26 L) of condensed gases were produced per vessel. The vessels that produced the fewest VFAs also attained the highest internal temperatures and produced the greatest amounts of condensate (Figure 1). Peak ammonia production and peak internal temperatures were reached quickly, and the moisture released was sufficient to produce inadequate composting conditions that ceased microbial production of VFAs. However, in the vessels that heated and produced ammonia more slowly and over a longer period of time, the peak vessel temperatures were lower, and more than twice as much of each VFA was emitted (Figure 2). Thus, composting mixtures of apparently identical materials did not react to a given set of conditions equally. The conditions of the specific mixture influenced the amount of VFAs and, therefore, the amounts of odor emitted during composting.
Figure 1. Average ammonia and VFAs emitted from those vessels that heated quickly and whose contents remained at elevated temperatures for a short period of time.
In the compost vessels, acetic acid was emitted in the greatest amounts before peak production of condensate and peak composting temperatures, followed by butyric, propionic, valeric, isovaleric, and isobutyric acids, in order of decreasing emissions. After peak production of condensate, butyric acid was emitted in the greatest amounts followed by acetic and propionic acids which showed approximately equal emissions. Valeric, isovaleric, and isobutyric acids were produced in the least amounts throughout the entire trial. Extraction of solid samples obtained as the vessels were emptied confirmed that the 21-day aerobic treatment decreased all VFAs in the biomass between 50 to 100%, with an average of 87%. The reactor vessel apparatus was an effective method to measure malodorous compounds and ammonia emitted during the decomposition of livestock waste while simultaneously monitoring the environmental conditions of the composting process. In addition, aeration was confirmed to be an effective method for the control of VFAs and ammonia and, thus, odors produced by their emission.
Figure 2. Average ammonia and VFAs emitted from those vessels that heated slowly and gradually and whose contents remained at elevated temperatures for a longer period of time.
The results obtained in this study indicate that monitoring the chronological formation and fermentation of VFAs can be used in conjunction with the composting process to optimize conditions that minimize the release of malodorous compounds. Because peak emissions of VFAs occurred simultaneously with the greatest headspace temperatures, peak rates of O2 uptake, and peak production of condensate, ammonia, and CO2, stringent control of these factors may decrease VFA emissions. Fewer VFAs were emitted from those vessels that quickly heated and were active for short periods of time, whereas much greater amounts of VFAs were emitted over longer periods of time from those vessels that possessed adequate conditions for composting for longer periods of time.
Chen, A., P. H. Liao, K. V. Lo. 1994. Headspace analysis of malodorous compounds from swine wastewater under aerobic treatment. Bioresour. Technol. 49: 83-87.
Cooper, P., I. S. Cornforth. 1978. Volatile fatty acids in stored animal slurry. J. Sci. Fd. Agric. 29: 19-27.
Galliher, T. Testing Lab for the School of Natural Resources. The Ohio State University/Ohio Agricultural Research and Development Center, Wooster, Ohio. Personal Communication. March 1999.
Schaefer, J. 1977. Sampling, characterization and analysis of malodours. Agric. Environ. 3: 121-127.
Stevens, R. J., I. S. Cornforth. 1974. The effect of aeration on the gases produced by slurry during storage. J. Sci. Fd. Agric. 25: 1249-1261.
Williams, A. G. 1984. Indicators of piggery slurry odour offensiveness. Agric. Wastes 10: 15-36.