Factors that affect composting
Although decay of organic materials under normal environmental conditions is sometimes slow, most organics ultimately break down to humus that helps to build topsoil. In much the same way that livestock producers improve animal growth rates by creating a favorable environment (adequate food, water, and shelter), organic decompostion rates also can be accelerated by providing favorable conditions for bacterial growth and reproduction. The process of accelerating biological decomposition by controling moisture content, temperature, oxygen, and carbon to nitrogen ratios (C:N) is called composting.
Water (moisture content)
Many factors affect the composting process, but moisture content often is the most crucial. Wastes that are too dry (less than 40 percent moisture content) decay slowly because they lack sufficient water for survival of bacteria. At moisture levels above 60 percent, small pore spaces that allow oxygen to move into the compost become filled with water.
Lacking sufficient oxygen for rapid growth, aerobic microorganisms that produce relatively little odor are soon replaced by anaerobic organisms that produce highly odorous organic acids and hydrogen sulfide. For optimum performance, maintain moisture content between 40 and 60 percent. The compost should be moist but not soggy. If moisture can be squeezed from a handful of compost material, it probably needs to be mixed with drier material. To prevent excess moisture and the problems it can cause, on-farm animal mortality composting facilities in Iowa often have a roof to prevent absorbtion of excess moisture during seasons when frequent rainfall occurs.
Heat is an important byproduct of bacterial activity. Internal temperatures within properly sized composting operations often reach 120150 degrees F. This temperature range stimulates rapid growth of thermophilic (heat-loving) bacteria that promote decay.
As an added benefit, exposure to high temperatures helps to kill disease-causing microorganisms (pathogens), thereby helping to reduce the risks of disease transmission from infected materials. Recognizing this, the USEPA lists composting as an accepted method for treating municipal sewage sludge to reduce pathogens. Similarly, researchers in the poultry industry report that, if done properly, two-stage composting of poultry carcasses inactivates pathogenic viruses that cause avian influenza, avian adenovirus, Newcastle disease, and infectious bursal disease. Similar poultry studies indicate that composting also can destroy several types of Salmonella, Listeria monocytogenes, Pasteurella multocida, and Aspergillus fumigatus. Fewer studies have focused specifically on swine pathogens, but the limited results have been encouraging. Peak composting temperatures of 140 - 158 degrees F achieved during swine carcass composting trials in North Carolina were sufficient to destroy several types of swine pathogens. Eleven of 15 samples retrieved from composting swine after 177 days had negative test results for Salmonella. In the same study, pseudorabies virus placed in scintillation vials was inactivated after 29 days inside the compost bin, and Erysipelas rhusiopathieae in culture tubes were not viable after 245 days. Other studies indicate that viable S. cholerasuis were not detected in the carcases of experimentally infected pigs after seven days of composting, and Actinobacillus pleuropnumoniae did not survive 35 days after infected swine were composted.
Since mortality composting usually is done inside unheated structures, it is important to construct bins that are large enough so that sufficient internal heat will be generated and retained during cold weather. As shown in the adjacent chart, temperatures of compost located near to external sidewalls (sidewalls exposed to wind, as constrasted with walls between bins) of composting bins are noticeably cooler during cold weather than in the "core" area of the bin. Potential effects of the cool zone can be minimized by keeping the carcasses 912 inches away from the bin edge and constructing bins large enough (individual bin floor areas of 100- 200 square feet) to have substantial "core" volume where heat is retained.
Ventilation (oxygen availability)
Although organic decay readily occurs under anaerobic (no oxygen present) conditions, the term "composting" typically refers to aerobic (oxygen-using) decay processes. In most cases, aerobic microbial activity is considered more desirable than anaerobic decay because the by-products of aerobic processes (water, carbon dioxide, and heat) are not offensive and produce a heat-treated product that is low in viable pathogens or weed seeds. Anaerobic decomposition, by contrast, produces little heat and generates highly odorous products such as hydrogen sulfide and organic acids.
To keep a composting process sufficiently aerobic, oxygen concentrations of at least 5 percent within the compost pile are typically recommended. Since highly active aerobic microorganisms use oxygen rapidly, however, maintaining aerobic conditions throughout a compost pile, particularly at the core, requires constant monitoring, and fan-powered aeration or frequent mechanical turning. While some industrial composting operations attempt to maintain continuous aerobic conditions, farm and small community composting operations rarely take such expensive measures. Instead, small system operators focus on keeping the outer layers of the compost pile aerobic through passive ventilation. This is done by adding coarse-textured materials, like the wood chip mixture shown here, to their compost feedstocks to maintain a porous compost structure that allows oxygen to diffuse into the outer layers of the pile.
Nutrition (carbon:nitrogen ratio)
Like all living things, the bacteria and fungi that decompose organic materials need carbon and nitrogen to grow and reproduce. Bacteria grow quickly, and break down organics most rapidly when their food source contains about 25 times as much carbon as nitrogen. Decomposition will occur, although more slowly, when carbon-to-nitrogen (C:N) ratios are as low as 10:1, or as high as 50:1.
Because carbon and nitrogen analyses are expensive, farmers and operators of small municipal or industrial composting operations learn to look for the symtoms of inadequate C:N ratios rather than relying on frequent testing of compost feedstocks. If C:N ratios are low (less than 15:1), excess nitrogen in the mixture will be released as ammonia. If ammonia odors become a problem, compost operators counteract this by adding a high-carbon material, such as sawdust, to raise the C:N ratio of the mixture and reduce ammonia production.
If C:N ratios are high (greater than 50:1), this too, causes slow decay. But since reduced decay rates also can be caused by high or low moisture content, low oxygen concentrations, or by low C:N ratios, diagnosis of high C:N conditions must be made in the context of these other possible causes. If moisture content appears adequate, the frequency and strength of ammonia emissions are not unusual, and more frequent turning of the compost does not improve the rate of decay, then high C:N may be the cause of slow decay. If incremental additional of nitrogen in the form of manure or additional animal carcasses appears to improve the rate of decomposition, this further confirms a high C:N condition and points to a need to adjust the future proportion of compost feedstocks to avoid this problem.