For an overview of the subject, please refer to the summary document, Alternatives to Rendering for Meat Processors.
There is no fundamental difference between composting meat products and ordinary composting, but there are some special factors to keep in mind. The following summary includes a lot of information that is common to all composting, but stresses those aspects which you may want to pay particular attention to if you are considering composting as an alternative to rendering for meat processing wastes.
In a compost pile, you are enlisting a wide variety of organisms to accomplish the work of breaking down your waste materials. To help them do their work, you need to maintain the right conditions in the pile.
As the breakdown proceeds, the conditions will gradually change. To keep the processes working efficiently, either you will need to build a pile that will regulate itself, or you should expect to adjust the conditions yourself occasionally.
Here is a short summary of factors that you will need to consider:
A short discussion of these factors from the viewpoint of meat processing wastes appears below, followed by a list of additional composting information resources.
Compost piles can be built with moving parts (tumbling bins for efficient turning, fans for forced air, etc.), or with passive components (air pipes, pallets, or other ways of promoting air circulation), or can just be simple piles. The more elaborate methods are generally suitable for small quantities and rapid composting.
Unless you are prepared to make a major capital investment in your composting operation, or are severely limited in available space, you will probably want to start with the simplest possible methods. The most common method for composting large quantities of animal wastes is to place the materials in windrows. These are long, narrow piles that rely primarily on wind for aeration. They are relatively quick and inexpensive to build, since they don't require anything but space (no pipes or pallets to lay out). But they do require good maintenance practices and adequate turning.
Typical windrow dimensions are 5 feet high and 10 feet wide, and of arbitrary length.
The pile has three main components:
Bulking materials are added to provide space for air and water to circulate in the pile. They may also help retain and distribute moisture in the pile. A number of common farm wastes, such as straw, leaves, or corn stalks, make suitable bulking materials. Other materials, such as newspaper, bark, and sawdust, have also been used. You can expect to add at least two volumes or more of bulking agent for every volume of meat waste.
Among the organisms that you may find in your pile are:
Different organisms thrive under different conditions. You want to set the conditions to favor those organisms that will help you get the job done as quickly and as thoroughly as possible. The conditions that matter most to the organisms in the pile include:
Temperature: When breakdown is occurring normally, the pile will heat up. Measuring the temperature inside the pile is one of the best ways to monitor whether the pile is functioning as it should. The optimum temperature range is 130 to 140 degrees F. If the temperature falls below 110 and the composting process is not finished, there is probably a problem with air or moisture levels, and the pile will need to be turned or moistened. (Low nitrogen would generally not be a problem for a pile containing meat). If the temperature rises above 150, organisms in the hottest part of the pile will be killed off, and overall efficiency will decline.
Moisture: Enough moisture must be present throughout the pile for decomposition to take place. But too much water can flood the air spaces and give rise to anaerobic conditions.
Oxygen: Oxygen will be consumed during aerobic decomposition, so fresh air must continually circulate in the pile. Turning the pile is the quickest and most effective way to aerate it. Adding sufficient bulking material to the pile will also improve air circulation.
Nitrogen: Available nitrogen, in the form of organic compounds in the pile material or inorganic ions in water, is often a limiting factor in vegetable composting. (Nitrogen in the air is too inert for most organisms to use.) Meat has a relatively high nitrogen content, so this factor is not likely to be in short supply, except possibly toward the end of the composting cycle. The ample amount of nitrogen in a pile with a lot of meat has a down side -- incomplete breakdown can lead to the production of ammonia and related compounds, which cause odors and can leach into groundwater. With sufficient bulking material and air, the nitrogen can instead be converted primarily into the atmospheric gas form, which will not cause problems.
Other factors that may increase composting efficiency:
Meat is said to be slower to break down than typical vegetable matter, so the following estimates are based on conservative assumptions.
Under ideal conditions, it is possible to compost some materials is a little as 2 - 3 months. But if you are primarily in the meat processing, rather than the composting business, there is likely to be a limit to the amount of money and work you will want to invest in your composting operation. It seems reasonable to allow a full year for the complete cycle from freshly generated waste to finished compost ready for land application.
So a one year cycle seems like a good target.
A ton of meat waste by itself will occupy about 1.2 - 1.5 cubic yards. With bulking agent mixed in, a newly mixed pile will occupy about 5 cubic yards for each ton of meat waste. (It will gradually lose volume as the material breaks down.)
If you are using windrows that are two square yards in cross section, you should therefore allow at least 2.5 linear yards, or about 8 feet of windrow length per ton of meat to be composted.
The total length of windrow needed to keep up with your meat processing operation depends on the rate at which the waste is generated and on the length of time required to complete the composting cycle. Suppose for example that the material in your pile can be removed after one year. Then you can multiply the number of tons of meat waste generated per year times 8 feet per ton to find the total length of windrow in feet needed to contain the waste from your operation on a steady state basis (that is, assuming you can remove finished compost as the same rate as you add fresh waste).
For example, if your operation generates 250 tons of meat waste per year, and if you can complete the composting cycle in one year, you will need 2,000 linear feet of windrow. If the windrows are spaced twenty feet apart, the composting area would occupy 40,000 square feet, or about one acre.
You can use this as a rough guide to the area you will need for the windrows, scaling it up or down depending on the size of your operation. You will also need additional area for items such as bulking material storage, temporary storage of finished compost, and areas for material handling and equipment storage and maintenance.
Cornell University, Composting home page, http://www.cfe.cornell.edu/compost/Composting_homepage.html. Lists many resources on general composting topics, including odor management and numerous case studies.
USEPA, Office of Solid Waste, Composting index page, http://www.epa.gov/epaoswer/non-hw/compost/index.htm. Includes studies on finding uses for compost.
Appropriate Technology Transfer for Rural Areas (ATTRA) Farm-Scale Composting Resource List, http://www.attra.org/attra-pub/farmcompost.html. A detailed list of additional resources for large-scale composting operations.
University of Georgia, Food Waste Composting http://www.ces.uga.edu/pubcd/B1189.htm. Includes pictures of windrows and other pile methods, plus some data on land application, and on comparative costs.
Iowa State University, Leopold Center, Composting Dead Livestock: A new solution to an old problem, http://www.leopold.iastate.edu/pdfs/SA8.pdf . Reviews the Iowa rules, and provides valuable information on composting of animal carcasses.