Animal agriculture is faced today with discovering innovative ways to dispose of livestock mortality. This need has been brought on by the disappearance of rendering plants, concerns over burial and ground water pollution, and the economic cost and other issues related to incineration. Composting of dead animals is one option that is now available. This fact sheet is an overview of the principles of composting, and the management practices for composting swine mortalities.
Composting is a natural process where bacteria and fungi decompose organic material in a predominantly aerobic environment. During the composting process, microorganisms break down organic materials into a stable mixture called compost. The compost resembles humus, and is an ideal soil amendment.
Under controlled conditions, composting is usually done in two stages, primary and secondary composting. In the primary stage, a high rate of biological activity results in rapid composting and high temperatures in the pile. This is where most of the organic breakdown occurs. The secondary stage has lower biological activity resulting in slower composting and lower pile temperatures. The secondary stage allows the compost to complete the biological activity and stabilize, also called curing.
While composting occurs naturally, the process requires proper conditions to occur rapidly, minimize odor generation, and prevent nuisance problems. Conditions that must be controlled in the composting process are the material mix, moisture levels, porosity, and temperature.
The proper compost mix requires both carbon and nitrogen at the proper C/N ratio. The proper C/N ratio will result in a composting process that generates little odor, yet offers an environment where microorganisms can flourish. Generally, a C/N ratio that is higher than 25:1 is satisfactory. Most waste materials have a C/N ratio that is too low to compost. In order to compost these materials, amendments that contain a high C/N ratio must be added. Plant materials such as wood chips, sawdust, or straw are ideal amendments for on-farm composting.
Proper moisture levels and a stable porous structure for the composting mass are two other conditions required in the mixture of materials for proper composting. Like all living things, bacteria need water. To encourage bacterial growth and rapid composting, the mixture should be 50 to 60% moisture. If the mixture feels moist, yet when a handful is squeezed no water drips from it, the mixture probably has adequate moisture.
Second, the bacteria that are encouraged to grow in a compost pile are aerobic (require oxygen). Open spaces must be maintained to provide oxygen and allow air to penetrate and move through the pile. Ideally 35 to 50% of the pile volume would be small open spaces to allow air through the pile.
The aerobic bacteria that are of interest for the composting process grow at two temperature ranges: mesophilic bacteria (middle temperature bacteria, up to 100oF), and thermophilic bacteria (high temperature bacteria, up to 150oF). As the bacteria begin to break down the materials in the pile, heat is generated and the pile heats up. As the pile warms up, different bacteria will flourish with higher temperatures. As temperature increases, the mass of composting material will be more active and be broken down faster. Above 150oF, the rate of composting will decrease as bacteria are inactivated, or even destroyed by the excessive temperatures.
As the pile heats up, warm air within the mixture will rise and move out of the pile, while fresh air will be drawn in to replace it. This process exhausts carbon dioxide (CO2) created in the pile, and maintains an aerobic environment for the bacteria. In addition, temperatures that remain above 130oF for three days will destroy disease causing bacteria within the pile. Internal pile temperature is an indication of the current biological activity within the mixture and how well the pile is composting.
The composting process will generate and regulate its own temperature. However, to maintain high temperatures, the pile must have some insulation. A layer of inactive material (sawdust or finished compost) placed over the entire pile will insulate the pile. The insulation layer should be a foot or more in depth.
Discussions and articles on composting dead animals almost always gravitate towards principles of satisfying the following:
Unfortunately, strict application of those standards should only be done when dealing with a consistent, thoroughly mixed pile. The reality is that a pile in which a dead animal is composted is an inconsistent mixture. Therefore, composting a dead animal must be approached in a slightly different way.
Composting dead animals can be visualized as an above-ground burial in a biofilter (primary stage). The compost pile in this case is an inconsistent mixture with a large mass of material (the pig carcass) having a low C/N ratio, a high moisture content and nearly zero porosity. This mass is surrounded by a material (the carbon amendment) with a high C/N ratio, moderate moisture levels, and good porosity.
The decomposition process is anaerobic (lacking oxygen) in and around the animal carcass. But as gasses and liquids are produced and diffuse away from the carcass, they enter an aerobic zone. Here the gasses are trapped in the surrounding material, ingested by the microorganisms, and degraded to CO2 and H2O. Thus the surrounding material supports bacteria to form a biological filter, or a biofilter.
With this scenario, turning the pile is to be avoided until the carcass has been decomposed. For swine composting this period is generally three months after the last pig has been placed into the pile. After this time, the compost is moved to a secondary area where it is allowed to cure for an additional 3 months. Moving the pile introduces air back into the pile and mixes the contents of pile.
With larger animals, some bone fragments will remain after completion of the composting process. However, these bone fragments will be quite brittle and pose no health risks or danger to tractor tires or other equipment when land applied.
In order to monitor the composting process, it is necessary to measure and record temperatures of the compost pile. Pathogen kill can be confirmed by monitoring the internal pile temperature. Progress of the pile can also be surveyed from temperature records. Temperatures should be taken at several points near the pigs placed in the pile. Temperature recording can be done easily with a three-foot probe thermometer (1/4 inch probe diameter is recommended). Data recorded should include date, size, number of animals added, and the internal temperature of the pile.
Because of its ability to shed rain water from the pile, sawdust is recommended as the carbon amendment to be used in swine composting. Moisture level is a concern when composting outdoors. Exposure to rain and snow may result in high-moisture levels, which may adversely affect the composting process.
|Starting a compost Pile||Adding Pigs to a compost Pile|
Sawdust base 1 to 2 feet deep.
Skim off top sawdust.
Place layer of pigs on base.
Place new layer of pigs on pile.
Cover with two feet of sawdust.
Re-cover layer of pigs with 2 ft. sawdust.
|Figure 1. Construction of a composting pile for swine mortality.|
The practices of composting swine mortality are very simple. The methods recommended by the Ohio Swine Composting Development Team are listed below. Figure 1 shows the process.
When additional pigs are placed in the pile, follow these steps:
Pile management (Figure 2) is a simple cycle, based on a four pile rotation. Each pile is constructed for 90 days, composts for 90 days, then is turned and cured for 90 days. After this 270 day cycle, the compost is complete and can be land applied, or recycled into the pile.
In order to reduce sawdust requirements, finished compost can be used in place of sawdust. Up to 50% of the sawdust requirements can be replaced with finished compost. Covering the carcasses with compost also inoculates the pile with bacteria to help start the process. Recycle rates above 50% may limit carbon availability, thus interfering with the composting process.
Step 1.) Days 0 to 90
Build first primary composting pile (P1)
Step 2.) Days 91 to 180
Build second primary compost pile (P2)
Allow P1 to compost
Step 3.) Day 180
Move P1 into secondary composting area
Step 4.) Days 181 to 270
Build a third primary composting pile (P3)
Allow P2 to compost and P1 to cure
Step 5.) Day 270
Move P2 to secondary composting area
P1 is finished--recycle, store, or haul
Step 6.) Days 270 to 360
Build a fourth primary composting pile (P4)
P1 can be recycled into P4
Allow P3 to Compost and P2 to cure
Step 7.) Day 360
Move P3 to secondary compost area
P2 is complete recycle--store or haul
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