As public concerns about the various emissions and odors associated with pork production continue to mount and pork producers become increasingly committed to minimizing biosecurity breeches to protect herd health, composting has become a common method of mortality disposal.
In an effort to reinforce the basic principles and proper management of mortality composting, Karl VanDevender, Extension agricultural engineer and program leader of the livestock and poultry waste management program at the University of Arkansas, authored a recently updated Extension bulletin on the topic.
The purpose of composting is two-fold:
• To convert dead animal carcasses into a dark, humus-like material similar to potting soil, and
• To kill many disease-causing pathogens, which requires that temperatures above 131° F be maintained for at least three consecutive days.
The challenge with mortality composting, especially with larger animals, is the inability to maintain a uniform mixture of high-carbon bulking agent and carcass material. When placed in a bulking agent, the area around the carcass tends to be in high moisture, with low carbon-to-nitrogen (C:N) ratios and low-oxygen levels (Table 1).
“In essence, rather than a homogeneous mixture, mortality composting is the above-ground burial of mortality in a carbon material,” VanDevender explains. “As a result, adjustments need to be made for the mortality composting process to be successful.”
The most important adjustment is adding ample bulking material to absorb the water released as the carcass decomposes. Adding bulking agents accomplish three things:
1. Prevents seepage (leachate) from escaping the compost pile.
2. Serves as a biological air filter. The area near the carcass is generally anaerobic and generates objectionable odors. The bulking agent provides the biological activity necessary to “scrub” or convert the odorous gases, VanDevender says. A side benefit of effective odor control is it doesn’t attract flies and scavengers.
3. Provides insulation. Adequate bulking agent mass helps retain the heat generated by microorganisms. This is especially important in cold climates.
High-carbon alternatives for composting include: corn stover, sawdust, wood shavings/chips, hay, straw, chopped soybean stubble, rice hulls, peanut hulls, recycled paper/cardboard, yard waste/leaves, chicken and turkey litter, manure (horse, sheep, cattle, swine) and finished compost.
Mixing is Major
Another key component to successful mortality composting is “mixing” or “turning.” The goal is to mix the non-decomposed bulking materials at the edges of the pile with the lower C:N material near the carcasses. Normally, the small bones, soft tissue and objectionable odors are gone. Some larger bones, such as skulls and leg bones, may remain.
Turning adds oxygen, increases the C:N ratio and provides an opportunity to add water, as needed, for the second composting cycle to begin. At the end of the second cycle, there should be no soft tissue remaining, and bones should be brittle. If bones are not easily broken, they should be added to the new composting cycle when the next mortality is added.
There are two main components to mortality management — building the primary pile and managing the piles.
Figure 1 shows the six steps to building a primary compost pile. Begin by ensuring the base layer of bulking agent is deep enough (1 ft. minimum) to absorb excess water and filter odor. Material between mortality layers should be at least 6 in. thick. The top cover should be at least 1 ft. thick in bins or 2 ft. thick in static piles or windrows.
Depth of bulking agent will depend on the type of material and whether the pile is covered or not. Larger animals require more bulking agent due to higher volumes of liquid in the carcass. When in doubt, it is always better to add too much agent than to not have enough, VanDevender says.
Figure 2 (page 16) shows the five basic steps needed for mortalities to complete the compost cycles. The most significant factor is the time it takes for a carcass to compost, which is primarily dependent upon the size of the animal. Secondary factors include the type of bulking agent, initial moisture content, and the temperature achieved in the pile. Table 2 provides some guidelines to estimate days required for the primary cycle and when to turn the pile to begin the secondary cycle, using mortality weight as a guide.
When the second phase is complete, the compost can be used as a fertilizer source or blended with bulking agent to begin a new compost pile. In addition to reducing the amount of new bulking agent required, the recycled product helps “jump start” the next pile by inoculating it with actively growing microorganisms. No more than 50% recycled compost should be used. Higher levels may result in carbon deficiency and gradual deterioration of the composting process.
“The primary indicator of compost pile degradation and effectiveness is temperature,” VanDevender continues. “Active piles will maintain temperatures above 110° F., and more commonly operate in the range of 130° to 150° F.”
He suggests checking and logging the temperature of the pile at least twice a week using a thermometer with a shaft at least 36 in. long. The temperature log can help guide when to mix or turn the pile. If temperatures hover at 110° F. for a week or more, the pile should be turned to check the degradation progress.
Factors affecting degradation include porosity of the bulking agent, moisture and C:N ratio. “It is difficult to properly manage a mortality composting system without a thermometer,” he emphasizes.
Sizing and Siting
The most prevalent question producers struggle with is how big to build the composting facility. The farm’s or site’s daily mortality records provide the best guidance. Lacking adequate records, Table 3 provides estimates at three performance levels.
“The pounds of average daily loss are then multiplied by a volume factor to determine the total primary composting volume needed. The most commonly cited volume factor is 20 cu. ft. of capacity for each pound of average daily loss,” VanDevender explains. “The secondary composting volume is normally set to equal the total primary volume.
“All mortality composting facility designs need to have two areas designated for primary composting and one or more areas for curing (storage) phases. The two most common factors overlooked when designing (composting) facilities is the storage of new bulking material and having a place to store the finished compost before it is used,” he says.
When choosing a site for a composting facility, it is important to consider the proximity to the production facilities and human habitats. The facility must be close enough for easy access, but far enough away to avoid animal and human health risks.
All-weather access is important. The site should be well drained and meet all local and state regulations. Access to water is important to maintain moisture levels for effective composting. Electricity for lighting for after-dark work should be considered.
“Ideally, the facility should not be visible to the public,” VanDevender reinforces. “If this is not possible, it should blend in with the rest of the facilities on the farm. The main concept is to not call unnecessary public attention to the disposal of swine mortality. The composting facility, just like the rest of the farm, needs to be kept neat and orderly.”
Local regulations may affect the type of composting facility a producer can build and where it can be built, so be sure to check with the local Extension, conservation district or Department of Natural Resources office, he reminds.
Additional information on sizing and types of composting facilities can be found at http://www.extension.org/pages/Composting_Swine_Mortality/print/.
Comparing Mortality Disposal Options
To provide a cost comparison of the two mortality disposal options, University of Nebraska Extension Engineer Chris Henry and retired Extension Farm Management Specialist Larry Bitney budgeted the annual costs for disposing of mortalities in a 300-sow, farrow-to-finish operation (Table 1). Annual mortalities were estimated at 40,000 lb./year (110 lb./day).
These assumptions were made:
• The labor or equipment used to remove dead animals would be the same for all alternatives, so those costs were not included.
• Labor costs of $15/hour were used to move dead animals to the disposal method of choice.
• Fixed costs include depreciation, interest on the non-depreciated balance of the disposal options, repairs, property taxes and insurance.
Three options were compared: Incineration with afterburner and fuel tank, low- and high-investment composting bin systems.
The initial investment of a 500-lb. capacity, lined, thermostatically controlled incinerator, including afterburner, fuel tank and fuel lines, was estimated at $7,626. The incineration rate was assumed to be 78 lb./hour with a fuel consumption of 2.2 gal. of diesel fuel/hour. Annual fuel consumption was estimated at 1,128 gal. Fuel cost was $3/gal.
The estimated life of the incinerator was 5,000 hours (roughly 10 years). The interest rate was 7%; an annual repair cost of 3% of the investment was used. Electrical costs were negligible.
It was assumed that the incinerator would be positioned near the facility, so no additional labor costs to move dead animals were included.
Composting Bin Options
High-investment and low-investment composting bin options were compared. Both options had 10 x 14-ft. bins with 6-ft. high walls.
The high-investment composting option included seven bins with a roof, sidewalls above the bins, and a concrete apron in front. One bin was used to store carbon material. Concrete work was hired, but wood framing and roof construction was done by farm staff. Estimated cost: $18,500.
The low-investment option included six bins, with no roof, sidewalls or front apron. Storage of the carbon source was either outside in a pile or in a nearby building. Estimated cost: $7,465.
The useful life of both options was estimated at 15 years; interest rate was 7%, and the annual repair cost was 2%.
The annual sawdust requirement for both composting options was estimated at 80 cu. yards/year at a cost of $7.50/cu. yard ($600/year).
A skid-steer loader with a ½-cu.-yard bucket was used to transport dead animals, move the carbon source (sawdust) from the storage bin, move and mix primary and secondary material, and load final material into a manure spreader. The loader cost was estimated at $14.60/hour.
Composters were sized to fill the primary bin in 90 days, where material would remain another 90 days before being moved to a secondary bin. After 90 days in the secondary bin, one-third of the material was recycled to a new primary bin and the remainder spread as crop nutrients.
Labor and machine requirements for both options were:
• Daily labor for loading with sawdust and dead animals was 1.83 hours/week, plus 0.67 hours of loader time/week.
• Moving material from the primary to secondary bin was 1.25 hours of labor and loader time, four times/year.
• Moving material to a recycling bin and spreading remaining material was 3.67 hours of labor, 2.0 hours of loader time, 1.67 hours of tractor and spreader time, four times/year.
• Labor and machine costs were estimated to be slightly higher for the low-investment composting facility because the carbon source was not stored in the composter.
The Bottom Line
With diesel fuel at $3.00/gal., the costs to operate the incinerator and the high-investment composting alternative are nearly equal — $0.14/lb. of mortality. The low-investment composting option was slightly less expensive.
Henry emphasized that mortality records for a specific operation will provide more accurate cost estimates.
The complete Extension bulletin (EC727) is available at http://elkhorn.unl.edu/epublic/live/ec727/build/ec727.pdf. It includes a rendering option and discusses the various safety and groundwater issues associated with burying livestock mortalities.