In the U.S. the use of composting to manage on-farm swine mortality has increased from 10.5 to 35.9% from 1994 to 2006 (USDA:APHIS, VS, CEAH 2001 and 2007). Very little is known about the emissions coming from the composting of on-farm mortality. Traditionally, the most popular method of composting has been the open static pile (OSP) in bins, piles, or windrows, with management of primary, secondary and curing stages. In recent years, other systems of composting have been introduced to farmers, including the use of in-vessel (IV) systems; of which the most popular are rotating drums. The claimed advantages of IV composting of mortality are accelerated decomposition, odor control, little or no leachate leaving the composting site as a potential discharge, and greater control of the composting process. It is not known if the acceleration of the IV system results in less gaseous losses occurring during composting. The impacts of using rotating drum IV or OSP composting systems and of composting whole or ground carcasses on air emissions were assessed in this research.

The objectives were to: 1) Compare the quantity of emissions from IV and OSP mortality composting systems; 2) Measure the impact of grinding carcasses on emissions when composted in IV and OSP composting systems; and 3) Estimate energy consumption and economic costs of the IV and OSP animal tissue composting systems.

Dairy manure compost, horse stall bedding, wood shavings and swine mortality compost were blended to achieve initial moisture content of 40 to 60%, and a carbon-to-nitrogen ratio of 25:1 to 30:1, based on chemical analyses of materials used. Carcasses were placed in IV units or OSP’s, either whole or ground.. In-vessel and OSP systems were housed in individual chambers at the Michigan State University Animal Air Quality Research Facility when emissions, including ammonia, hydrogen sulfide, carbon dioxide, and oxygen, were measured continuous for days 1 through 20 (primary phase) and 65 through 80 (secondary phase) of composting.

In-vessel and OSP composting systems were used in the primary phase only and all composts were in OSP’s during the second phase or collection period. No emissions measures were taken during the 45-d interim between phases because of limited resources available to conduct this project. The two phases or periods for emissions sampling were chosen when planning the experiment to represent two portions of active composting during which different amounts of emissions would be anticipated.

Temperature within each batch was measured daily, while moisture content of the compost batches was measured twice weekly. Moisture contents of 40 to 60% were maintained during composting. Compost stability or maturity was determined by measuring oxygen consumption or carbon dioxide evolution rates in the last week of the secondary phase. Temperatures achieved in both phases indicated excellent composting activity.

Oxygen consumption was not affected by compost system, carcass form and phase of composting. Carbon dioxide emission was greater (P < 0.05) in the primary phase than in the secondary phase. Mass of carbon dioxide per day tended to be greater with use of the IV system of composting (P = 0.07). These findings suggest that the decomposition was occurring at a greater rate early in the process and in the IV system.

When considering the emissions from both phases together, the IV system emitted more (P < 0.05) non-methane total hydrocarbons, ammonia, and sulfur dioxide, and less (P < 0.05) methane, nitric oxide, and nitrous oxide than the OSP system. When considering the primary phase alone, the IV system generated about 95% less methane than did the OSP system (0.31 vs. 6.7 g/d), less nitrous oxide (-1.00 vs. 1.94 g/d), more non-methane total hydrocarbons (4.13 vs. 0.19 g/d), and more ammonia (86.96 vs. 5.04 g/d); all differences P < 0.05 and amounts shown being for the IV and OSP systems in the primary phase, respectively. The emissions in the two phases were compared and were greater (P < 0.05) for methane, non-methane total hydrocarbons, ammonia, nitric oxide, and sulfur dioxide in the primary phase as compared to the secondary phase, but not for nitrous oxide which was greater (P < 0.05) in the secondary phase than it was in the primary phase. Carcass form did not affect amounts of emissions. Emission patterns or emission rates over time were examined to determine at what time in the composting process differences in emission amounts occurred.

For a 2000 head finishing swine farm with a 2% mortality rate, we estimate that mortality composting on the farm for a 20-d primary phase would emit 1.40 and 0.94 tons of carbon dioxide equivalents (CO2e) annually depending on which method of composting was used (IV or OSP, respectively). Carbon dioxide emissions accounted for 99.9 and 80.3% of the CO2e from IV and OSP systems in the 20-d primary phase. A 10 to 20-fold greater amount of CO2e would be emitted from primary composting as compared to secondary composting. Our measurements indicate that from days 65 to 80 of composting, only 0.07 and 0.08 tons of CO2e would be emitted for IV and OSP, respectively. The dramatic decrease is believed to be a reflection of the greater anaerobic and aerobic microbial activity in the primary phase. Carbon dioxide emissions accounted for 99.8 and 35.9% of the CO2e from IV and OSP systems in the 15-d secondary phase. If we assumed that emissions we observed in the secondary phase of composting would be emitted for all the other days of a complete composting process that would last a total of 6 months (i.e. 20 d in primary emission amounts and 160 d of secondary emission amounts as measured in the present study) then the total CO2e annually from mortality composting would be 2.10 and 1.76 tons for IV and OSP systems, respectively. As a portion of the total CO2e emitted annually from a 2000-head finishing farm, that from composting (either method) is much less than the emissions from animal production, manure storage, and manure application to fields (32.9, 116.7, and 29 ton, respectively; Maycher, 2003). But this is a very conservatively estimate and likely inaccurate as the time from our primary phase to our secondary phase, the time during turning of OSP compost material, and the time from our secondary phase to completion were not measured and the emission amounts may differ from those we measured in our 15-d secondary phase. After conducting our study, we still do not know how much the mathematical modeling done here underestimates or overestimates the GHG emission from the entire composting process. In conclusion, whether carcasses were ground or left whole did not change greenhouse gases (GHGs) emission and so air quality improvements is not a justification for the added expense and energy used to grind carcasses pre-composting. Total emissions of GHGs emitted during the first weeks of very active composting are greater with the IV composting system, than those emitted from an OSP system.

The results of this research were presented at the annual meeting of the US Composting Council in Santa Clara, California on January 26, 2011 and included as preliminary data in a grant proposal submitted to USDA/NIFA/AFRI – Research Climate Change in 2010 (not funded). A CIG grant focused on GHG emission from composting swine mortality was written for submission in February 2011 (not funded).

Dale W. Rozeboom
Professor/Extension Specialist
Department of Animal Science
Michigan State University
2209 Anthony Hall
East Lansing, Michigan 48824
Ph: (517) 355-8398""
Fax: (517) 353-1699""
Email: [email protected]