Composting of animal Carcasses

Animal carcass composting for both routine and emergency management of food animal mortalities is a safe method of carcass disposal.{Berge, 2009 126 /id} It has been used in varying extent, depending on region and regulations, from routine composting of poultry carcasses, composting or road kill, to emergency composting of large animals and animal carcass composting in epidemic disease outbreak situations. There is a growing interest in using on-farm composting for the disposal of animal by-products and mortalities because the practice is relatively simple, effective, environmentally sound, and economical. The finished compost can be land applied and thereby provides an environmentally acceptable means of recycling nutrients and stabilized organic matter to the soil. Proper composting eliminates most pathogens and reduces spore-forming bacteria, prions and other selected pathogen levels.

The principles and elements of carcass composting.

Carcass composting differs from composting other materials such as manure and green waste and present some unique challenges. Carcasses are typically composted whole and do not present uniformly chopped substrate for microbial action, nor are these compost piles turned as frequently. Both of these factors contribute to a non-uniform compost composition at the end of the process.

Composting is a largely aerobic process in which bacteria, fungi and other micro-organisms convert organic material into stable humus. Composting animal carcasses requires precautions to minimize the potential spread of diseases, odors and liquids. The composting process depends on naturally present microorganisms to digest the organic components in the carcass. The carbon-based materials in the piles supply energy for the microbes while the carcass tissues and fluids supply nitrogenous materials for microbial protein synthesis. In most composting systems a carbon to nitrogen ratio (C/N ratio) of 25:1 to 30:1, moisture content of 50-60% and a temperature in the range 43-65°C are considered optimal for supporting the performance of the microorganisms that drive the composting process.{Glanville, 1997 37 /id} The target pH is neutral, although successful composting occurs at pH values between 5.5 to 9.0.{Nakasaki, 1993 108 /id} Regular monitoring of the compost is essential. Temperature monitoring is a key indicator of a properly functioning compost pile

Heat, water, carbon dioxide, ammonia, and volatile organic compounds are by-products produced in the process. Most of the digestion at the carcass level is anaerobic, but the liquid and gaseous by-products of the anaerobic process diffuse away from the carcass and into progressively more aerobic layers of the compost envelope where aerobic degradation to carbon dioxide and water takes place.

The microbial flora responsible for the decomposition of organic matter comprises a complex mix of organisms some of which are able to function and survive at temperatures high enough to kill mammalian and avian pathogens.{Nakasaki, 2005 105 /id} These complex microbial decomposer communities occur naturally in the environment and many of the mesophilic microbes are responsible for the continuous and normal decay of plant and animal tissues at ambient temperatures.

Composting systems are divided into 'open' and 'closed systems'. Open systems include windrows, static piles and bins. Closed, in-vessel systems are far less common and typically are used for small species such as poultry, nursery pigs, etc.

Most mortality composting operations employ naturally ventilated static pile processes. Animal carcass composting piles are typically constructed in layers, starting with a thick absorptive layer of carbonaceous plant material. Whole carcasses are laid on top of the base and covered with additional absorptive organic material. Succeeding layers of mortalities are added on a daily basis until the bin is filled, or until an appropriate freestanding pile height is reached. Bins containing poultry or similar small carcasses may contain many layers. Mature sheep and swine may include two or three layers of carcasses (Figure 1), while mature cattle are usually composted in a single layer with two animals placed back to back (Figure 2).

The success of naturally ventilated static pile composting processes depends on the characteristics and thickness of the materials used to envelope the carcasses. Water-holding capacity, biodegradability (for heat production), gas permeability (for O₂ penetration), and mechanical strength (to prevent compaction and loss of gas permeability) are the most important envelope material parameters. {Glanville, 2006 119 /id;Ahn, 2007 120 /id} Some common materials include: sawdust, woodchips, ground cornstalks, rice hulls, ground straw, corn silage, straw-manure mixtures, and poultry litter.

Composting times vary depending on the size of the carcasses, ambient temperature and other physiological factors. The estimated number of days for primary composting ranges from 10 days for fowl to 195 days for adult bovines.{Keener, 2007 47 /id} Primary composting is recommended for all carcasses to minimize the spread of infection, and allow for break-down of soft tissue. Following the primary composting period and cooling to 45-48°C the compost can be turned to stimulate the secondary compost heating phase in which bones will be degraded. Secondary composting is performed for an additional period of 10 to 65 days depending on carcass sizes.{Keener, 2000 49 /id}

The microbial risks associated with composting of animal carcasses

Animal carcasses are microbiologically active material that may contain viruses, bacteria, protozoa, parasites, prions, toxins, drug residues and other chemicals. All of these biologically active materials need to be reduced to a safe level or eliminated to minimize their potential hazard. Temperatures of 55°C (130°F) for three consecutive days as is achieved in proper composting kill most pathogenic bacteria and parasites and inactivate viruses.

A wide variety of potential microbial pathogens may be found in animal carcasses and the microbial hazards of carcass composting was recently reviewed.{Berge, 2009 6 /id} Most studies indicate that composting will efficiently eliminate viral agents. Bacterial pathogens, unlike viruses and parasites, can survive outside the host organism if composting temperatures are inadequate to destroy these organisms. An additional concern is the potential for regrowth of micro-organisms that were not completely eliminated if conditions subsequently become favorable. Ova of the parasite Ascaris lumbricoide, are especially resistant to destruction and have therefore been accepted as a benchmark or proxy for microbial destruction achieved by various treatment systems. Bacterial pathogens potentially found in meat, food scraps, manure, sludge and other organic residuals are destroyed by exposure to the time-temperature regimes obtained in a well managed composting environment. However, the static compost pile coupled with the non-uniform composition of carcass compost presents special conditions that warrant additional research on the potential risks of spore-forming bacteria, materials handling, and the final disposition of the compost product. There have been concerns about prion agents remaining in compost. A recent study of the degradation of prions during composting indicates that there may be degradation in composting, providing further safety to composting.{Huang, 2007 44 /id}. Berge et al. concluded that carcass composting achieves adequate levels of microbial pathogen reduction. Further studies were encouraged to determine the fate of spore-forming bacteria and prion agents in carcass composting.

Carcass composting environmental concerns

In a situation where one method of carcass disposal is evaluated for approval, it is necessary to estimate and compare the risks associated with alternate methods of carcass disposal that are currently approved, such as rendering and incineration. For example, when evaluating risks of on-farm composting, it has to be evaluated to not only rendering, but also with the transport and handling of carcasses to rendering.

The transportation of fallen stock from the premise of origin to a site of further processing or disposal may entail risks for spread of contagious diseases.

Emission of green house gases (GHG) CO2, CH4, and N2O is a consequence of this microbial-driven process of composting and it is dependent on several factors, including moisture content, C/N ratio, aeration method and the type of amendment used {Hao, 2004 128 /id} the A study of adding calf mortalities concluded that even though the emissions of GHG increased when calf mortalities were added to manure during windrow composting, the quantities were relatively small, and the improved the final compost product. An evaluation of GHG gases in composting carcasses in relation to other rendering or carcass disposal practices should be done.

Conclusions

Carcass composting for routine and emergency food animal mortalities is a safe and economical method of carcass disposal. The microbial hazards have been evaluated, and indicates that there viral, bacteriological and parasitical agents are effectively reduced or destroyed. Composting may be a good alternative to other forms of carcass disposal involving transport and rendering while avoiding the hazards and odors associated with. Th resulting product can be used as a fertilizer with high nitrogen content and thereby nutrients are recirculated in the ecosystem.

Figure 1. Placement for small carcasses (swine, sheep, calves, poultry) in static pile composting.

Composting of animal carcasses. A safe and environmentally sound approach to take care of animal mortalities - Image 1

Figure 2. Placement for large carcasses (cattle and horses) in static pile composting.

Composting of animal carcasses. A safe and environmentally sound approach to take care of animal mortalities - Image 2

References

Ahn, H.K., Richard, T.L., Glanville, T.D., Harmon, J.D., Reynolds, D.L., 2007. Laboratory determination of compst physical parameters for modeling airflow characteristics.

Berge, A.C., Glanville, T.D., Millner, P.D., Klingborg, D.J., 2009. Methods and microbial risks associated with composting of animal carcasses in the United States. J. Am. Vet. Med. Assoc. 234, 47-56.

Glanville, T.D., Richard, T.L., Harmon, J.D., Reynolds, D.L., Ahn, H.K., Akinc, S., 2006. Environmental impact and biosecurity of composting for emergency disposal of livestock mortalities. Iowa State University.

Glanville, T.D., Trampel, D.W., 1997. Composting alternative for animal carcass disposal. J Am Vet Med. Assoc 210, 1116-1120.

Hao, X., Chang, C., Larney, F.J., 2004. Carbon, nitrogen balances and greenhouse gas emission during cattle feedlot manure composting. J. Environ. Qual. 33, 37-44.

Huang, H., Spencer, J.L., Soutyrine, A., Guan, J., Rendulich, J., Balachandran, A., 2007. Evidence for degradation of abnormal prion protein in tissues from sheep with scrapie during composting. Can. J Vet Res 71, 34-40.

Keener, H.M., Elwell, D.L., Monnin, M.J., 2000. Procedures and equations for sizing of structures and windrows for composting animal mortalities. Applied Engineering in Agriculture 16, 681-692.

Keener, H.M., Foster, S.S., Moeller, S.J., 2007. Ohio's farmstead composting program- a decade of success.

Nakasaki, K., Nag, K., Karita, S., 2005. Microbial succession associated with organic matter decomposition during thermophilic composting of organic waste. Waste Manag. Res. 23, 48-56.

Nakasaki, K., Yagushi, H., Sasaki, Y., Kubota, H., 1993. Effects of pH control on composting of garbage. Waste Manag. Res. 11, 117-125.

This article was originally published in One Health newsletter in 2010. Engormix.com thanks the newsletter and the author for this huge contribution.