Preble Soil and Water Conservation District

Harold Keener, Davis Elwell, and Terry Mescher

Animal agriculture is faced today with discovering innovative ways to dispose of their mortalities. This need has been brought on by the disappearance of rendering plants, concerns over burial and ground water pollution, and the economic costs and related issues of anaerobic decomposition in tanks. Composting of dead animals is one option that is now available.

This brief article presents background information on composting dead animals in a rational, scientifically sound fashion.

Composting is a natural, biological process of decomposition of solid material in a predominately aerobic environment. In it, bacteria, fungi and other small organisms break down organic materials (raw) to stable, usable organic substances called compost. The microorganisms use the most readily available parts of the waste as their food source, and the compost that remains afterwards resembles humus and makes a good soil amendment.

While composting occurs naturally, efficient composting requires properly prepared and maintained conditions and needs to be done so as to keep odor and other nuisance problems at a minimum. Under controlled conditions, composting is generally accomplished using two stages, primary (high rate) and secondary (curing). Following is a discussion of items, which need to be accomplished in order for a composting operation to run properly.

Efficient composting begins with a proper mix of materials. Most raw waste materials such as manures, garbage, or animal remains will not compost properly by themselves. Amendment materials are needed to make a mixture that will provide a good diet for the bacteria. Most often, amendments suitable for farm operations should have a high carbon (C) content. These are materials such as wood chips, sawdust, or straw. They provide the energy source needed for bacterial growth. With good bacterial growth, the nitrogen (N) and other chemicals in the raw waste that produce the strongest odors will be taken up into the bacterial mass (biomass), and both odor production and attraction of flies and vermin will be minimized. Thus, a recipe for a mixture of waste and amendments that produce a good C to N ratio (C:N) is fundamental to proper composting. Generally a mix with a C:N greater than 25:1 is satisfactory. Such a ratio is normally achieved if three units, by weight of high C materials, by weight are mixed with one unit of high N material.

There are two other things required for a proper mix of materials. The first is the right moisture level, and the second is a stable, porous structure for the composting mass. Like all other living things, bacteria need water. For good bacterial growth and the resulting rapid composting of waste, mixture should be 50-60% moisture. As a rule of thumb, this means that a handful of the material should feel moist but that squeezing the handful should not make water drip from it. Second, bacteria, or at least the bacteria that should be encouraged to grow in the composting material, also need oxygen (are "aerobic"). So open spaces throughout the mixture are important so that air can move through the compost. Generally a porosity of 35-50% is satisfactory. Both of these requirements will be met when a proper recipe is used for a mixture of composting materials. It is very important to avoid excess water because of the potential for odors and leachate conditions.

Piling enough material together so that it will retain heat and then letting the bacteria go to work do actual composting of a mixture of waste and amendments. In practice on the farm, this can be carried out in an open pile of material or in a structure. Bacteria are naturally present on all organic materials, and nothing needs to be done to obtain them. As the bacteria break down the organic compounds, heat is generated and the pile warms up if it is a few feet high by several feet across. Maximum practical depth is limited to 7 feet.

The aerobic bacteria that are of interest for composting come in two broad classes: mesophilic (liking middle temperature; up to 100 degrees F) and thermophilic (liking high temperatures; 100-150 degrees F). As the pile warms up, different species of bacteria will flourish as higher temperatures are achieved. In general, as temperature increases the mass of composting material will be more active and will be broken down faster. This activity, however, will decrease above 150 degrees F as bacteria are inactivated or even killed by excessive temperature. On the other hand, elevated temperature causes the air in the pile to rise, to exhaust the carbon dioxide that has been produced, and to draw fresh air and oxygen into the pile. Additionally, high temperatures above 130 degrees F for three days will kill off pathogenic bacteria that might otherwise be harmful. Thus, temperature of the material in the pile is an indication of how well it is composting, and monitoring temperature is an essential part of properly running a composting system.

Finally, a properly constructed pile should be covered with a layer around a foot thick of inactive material. This may be mature compost or fresh amendment material. This will serve as a biofilter to absorb odorous chemicals released by the active compost. It will also keep flies and vermin out of the pile.

When composting dead animals, it is necessary to measure and record temperatures of the pile to insure that a minimum of 130 degrees F has been reached for three days to indicate that the composting process is taking place properly. This is easily done with a 3-to-5 foot long (recommend ¼ inch diameter stainless steel) thermometer readily available from numerous equipment suppliers. Data recorded should include date, size and number of animals added, and temperatures at several points near previously placed dead animals.

Articles and discussions on composting dead animals almost gravitate toward principles of satisfying 1-carbon/ nitrogen balance, 2-temperature, 3-moisture, and 4-porosity, i.e., oxygen level, which have been found to work throughout the composting industry. Unfortunately, strict application of hose standards should only be done when dealing with a homogeneous mixture. The reality is that composting a dead animal is a heterogeneous process, unless one goes through the trouble of grinding the animal.

Composting a dead animal can be visualized as burial above the ground in a biofilter system (stage 1). The compost system in this case is heterogeneous with a large mass of material (the carcass) with a low C:N ratio of 5 to 12, a high moisture level of 70-80%, and nearly zero porosity enveloped in a zone of material (the carbon source) of medium to high C:N (25 to 100), moderate moisture level (45-55%) and good porosity (35-50%). The C:N varies with the material placed next to the carcass and in many cases involves two types of material with a high C:N ratio material further out from the carcass and interfacing with the atmosphere. The decomposition process is anaerobic in and near the surface of the carcass. But as the off-gases of the process diffuse outward, they enter an aerobic zone in which they are trapped in the bio-film layers and degraded to carbon dioxide and water and incorporated into biomass, much as would happen in a bio-filter system. With this composting scenario, turning the compost is to be avoided until the carcass has been decomposed. For small carcasses, three months are recommended while for animals over 100 pounds, six months or longer may be needed before turning the compost pile. Following this first turning, the material is generally allowed to compost for an additional period (stage 2) of 3 to 6 months before disposal on the land as finished compost. Sometimes with very large animals bone fragments will remain, but these are quite friable (easy to break up) and pose no health risk or danger to tractor tires or other equipment. It should be noted, to minimize the need for sawdust in stage 1, stage 2 materials are used to replace part of the sawdust, with it being placed next to the decayed animal.

Composting dead animals is being done successfully throughout animal agriculture. However, the application of rational principles based on treating the system as a heterogeneous matrix has been lacking in the literature. Recognizing the process as burial of a low C:N, high moisture, low porosity material in a bio-filter system leads to a better understanding of what the composting system for dead animals needs to achieve in order to maintain air quality and meet temperature criteria for pathogen destruction.

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Photo courtesy of USDA-NRCS