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PRELIMINARY RESULTS
Temperature
Examples of average daily interior temperatures during warm and cold weather are shown below. Preliminary evaluation of these and similar data from other trials suggest several trends that have important implications for emergency composting of infected animal carcasses.
Why Temperature is Important
Heat has long been recognized as an effective agent for killing pathogens in food, water, and other media. For this reason, the range of temperatures within mortality composting operations, and the variability of these temperatures throughout the pile and over time, are important in assessing which cover materials may be best suited for use in disposing of diseased carcasses.
NOTE: Although high temperatures are sometimes indicative of high microbial activity and rapid carcass decay it is worth noting that high internal temperatures, by themselves, do not necessarily guarantee rapid carcass decomposition. For further discussion and examples of this phenomenon, see "Decomposition" in the Preliminary Results section of this website.
Comparing Cover Material Types
Figure 1. Average temperatures near carcass surface for three cover materials tested during trial # 1 (constructed in August 2002).
As shown in figures 1 and 2 (summer and winter trials), temperatures during the early months are consistently highest in test units constructed with silage, and are generally lowest in piles constructed with ground cornstalks. Temperatures at comparable locations in piles constructed with alternative cover materials (yard waste compost, dry leaves, and straw over manure) are usually somewhere between those in the silage and cornstalk test piles.
Figure 2. Mean temperatures near carcass surface for three cover materials tested during trial # 2 (constructed in November 2002).
Favorable moisture content, and the presence of readily degradable organic compounds produced by the ensiling process, are believed to be key factors supporting rapid development of high temperatures in silage test units.
Temperature vs Location
During the early phases of carcass decay, average temperatures in the "core" of the composting windrows (between the carcasses) and at locations surrounding the outer surface of the carcasses, are quite similar. Locations nearer to the surface of the piles (external envelope), are cooler, reflecting the influence of much colder external air temperatures.
The extent of the difference between temperatures in the external envelope and in the central region (core and carcass surface locations) of the pile is affected by the ability of the cover materials themselves to produce heat (without benefit of moisture or organics released from the carcasses), and physical characteristics such as permeability and thermal conductivity, that affect the ability of cover materials to retain heat.
Figure 3. Average daily temperatures in central core (between carcasses), at outer surface of carcasses, and in outer envelope of silage trial # 1.
As shown in figure 3, temperatures within the external envelope and at the core of silage test units often differ by 10 °C or less, reflecting the relatively good ability of silage to produce and retain heat.
Figure 4. Mean daily temperatures in central core (between carcasses), at outer surface of carcasses, and in outer envelope of cornstalk trial # 1.
Temperatures in the external envelope of test units constructed with ground cornstalks are typically more than 10 °C lower than in the center of the pile (fig. 4) and during very cold weather they are nearly the same as external air temperatures. Very low moisture content (<15%), and a carbon-to-nitrogen ratio that is 2-3 times greater than the typically recommended range of 20-30, make it relatively difficult for bacteria to proliferate and produce heat in the outer envelope where carcass moisture and nutrients are less available. Compounding the inherently low heat-producing capacity of cornstalks, this material also has porosity (measured as free air space) that is nearly twice that of silage, and air permeability more than 5 times greater than silage...two factors that greatly increase the potential for heat loss.
Temperature vs Time
During the first 4-8 weeks following construction, temperatures at the outer surfaces of carcasses covered with corn silage often exceed 50 °C during cold weather (fig 2) and 60 °C during warmer seasons (fig 1). Initial temperatures of 40 °C are not unusual in the cornstalk trails during warm weather, and are often closer to 30 °C during cold weather.
As time progresses, pile compaction and depletion of degradable carcass tissues lead to reduced heat production and retention. When this occurs, temperatures gradually approach external air temperatures (figs 1 & 2).
While mechanical mixing would help to sustain temperatures by increasing oxygen levels and redistributing remaining nutrients, turning is not used during this research because the project is designed to simulate disposal of carcasses resulting from a contagious disease which might be transmitted via air if piles were turned.
Hot Enough to Kill Pathogens?
One way to put the composting temperature data into context, is to compare them with U.S. Environmental Protection Agency time/temperature requirements for reduction of human pathogens in municipal sewage sludge that is composted and marketed for public use. EPA rules stipulate that composted sewage sludge must be subjected to time/temperature conditions equivalent to 53 ° C for 5 days, 55 degrees for 2.6 days, or 70 ° C for at least 30 minutes.
Temperatures within mortality compost piles constructed using silage easily meet these requirements, while those constructed using cornstalk often fall short of these specific conditions.
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