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Composting Manure and Sludge E-479 06-08 Composting Manure and Sludge John M. Sweeten and Brent W. Auvermann* omposting is a controlled, isms typically do not tolerate such Cmicrobial process that converts high temperatures; as a result, the biodegradable, organic materials temperature of an anaerobic pile will into a stable, humus-like product. usually be 130 degrees F or less. The The microorganisms that do most of end products, or metabolites, of anaer- the work are thermophilic microbes obes include organic fatty acids, (bacteria, actinomycetes and fungi), aldehydes, alcohols, hydrogen which thrive in relatively high sulfide, and many other compounds temperatures (greater than 131 responsible for obnoxious odors. degrees F). This publication addresses only Composting is generally done under aerobic, thermophilic composting. aerobic conditions, in which atmos- pheric oxygen is present at 5 percent Advantages and Disadvantages or more by volume. (At sea level, Composting is an important treat- oxygen is normally present in air at ment process for many organic about 20 percent by volume.) wastes and residues, including Aerobic decomposition converts animal manure, municipal and biodegradable organic matter in industrial sludge, and solid or semi- manure or sludge to oxidized end solid crop residues. products, prima- I Anaerobic (very low rily carbon Major Advantages of Composting oxygen) conditions dioxide (CO2) occur in a compost and water. The • It produces a biochemically pile when excess mois- thermophilic stable product that has low odor and ture, fine particle size, good physical properties, and it or compaction reduce temperature range, from 131 attracts few flies. the flow of oxygen into • It significantly reduces the the pile. degrees F to 160 degrees F (55 volume of material that must be degrees C to 71 degrees C), inacti- stored, transported, disposed of, or vates most pathogens and weed used. seeds. Although there may be offen- • It is a forgiving, robust and sim- sive odors during the initial stages of ple process that can be done on-site composting, the final product gener- without a tremendous investment in ally has an inoffensive odor heavy infrastructure. characterized as musty or sweet. The improved physical properties of * Resident Director and Professor, By contrast, anaerobic decomposition compost include low moisture content (usually below 35 percent by Texas AgriLife Research and occurs when the oxygen concentra- Extension Center, Amarillo; tion in a compost pile drops below mass), more uniform particle size, Associate Professor and about 5 percent. Anaerobic organ- friable texture, reduced volume and Extension Agricultural Engineer, reduced weight. These properties Amarillo 2 lower the hauling costs per I High-temperature tions around the particles. Excess moisture often unit of active ingredient and composting is effective lowers the temperature in the pile and increases make it easier to spread the against many weed the amount of time required for compost to stabi- material uniformly. Aerobic, seeds and pathogens, lize and mature. In mechanical systems, a thermophilic composting but some are resistant. moisture content of more than 65 percent can also inactivates or kills most For example, many cause the composting material to agglomerate pathogens and weed seeds. spore-forming bacteria (“ball up”) and restrict airflow. (e.g., Bacillus an- thracis, or anthrax) or Phosphorus, potassium and hardened seeds (e.g., Before composting, wet organic materials should other mineral elements are Convolvulus spp., or be mixed with a dry material, or bulking agent, to retained in composted mate- bindweed) may survive reduce the initial moisture content. Suitable rial. While ammonia nitrogen in even the most active bulking agents include cotton burrs, sawdust, may be volatilized, or lost to compost piles. peanut hulls, corncobs, wood chips, rice hulls the atmosphere as a gas, total and brush trimmings. These materials also nitrogen usually remains stable as a proportion provide carbon, which helps microorganisms of total dry matter. Because of those advantages, break down organic matter. there is greater market potential for compost than for unstabilized organic wastes. Finished compost usually has a final moisture content of 20 to 30 percent. Some of this product Major Disadvantages of Composting can be recycled as a bulking agent, but it should • Equipment and labor costs may be prohibi- not be used as more than 50 percent of the tive. bulking agent for the next pile. • Market demand for compost may be tem- poral and difficult to sustain. Structure and Consistency • Obnoxious odors may be produced during To promote aeration, composting materials the initial stages of composting. should have enough pore space, or porosity, to • Some states require a permit for the con- allow air to flow freely through the matrix. struction of composting facilities. Bulking agents are usually necessary to increase the porosity of fine-textured materials (such as Factors Affecting Composting Rates sewage sludge or poultry manure) and absorb The primary factors that affect biological activity excess moisture. The amount of bulking agent and composting rates are moisture content, phys- may range from less than 1:1 (volume/volume) ical structure and consistency, aeration, nutrient to more than 5:1, depending on particle size and balance, pH, and temperature. initial moisture content. Moisture Content Aeration The microbes responsible for aerobic, ther- A compost pile must be aerated to support mophilic composting require moisture. The aerobic microbial activity. Aeration also removes moisture content should be 40 to 60 percent (by released moisture and excess heat that would mass) initially. If the moisture content is below otherwise reduce microbial activity. Generally, about 35 percent, the decomposition rate will be more air is required for moisture and tempera- much slower. Water should be added to dry ture control than for microbial activity. Air within manure to initiate composting. or exhausted from the composting materials should contain 5 to 15 percent oxygen. If the moisture content of compost material is too high, water in the pore spaces displaces air and There are two methods of aeration. With the first, restricts gas movement, creating anaerobic condi- the compost pile is turned mechanically using 3 front-end loaders or specially designed compost decrease in carbon, but usually a smaller turners. The pile should be turned several times decrease in nitrogen. per week initially, and less often later in the process. The compost should then be stockpiled If the nitrogen content is low and the C/N ratio for curing, during which final degradation is high (above 30), microorganisms must recycle occurs. During the curing process fungi and the nitrogen through many generations of actinomycetes may appear; they primarily digest bacteria as the carbonaceous matter is decom- hemicellulose. posed. During composting, most nitrogen is immobilized and stored in the bodies of the The second aeration microorganisms, although some nitrogen is lost I Compost organisms method is a forced-air as ammonia or nitrogen gas. As a result, the C/N consume a variety of system that either blows or ratio normally decreases during the composting carbohydrates, fats and proteins, some of draws air through the process. which are more easily compost. The aeration rate digested than others. should be sufficient to An optimum C/N ratio, below which nitrogen Lignin is among the create measurable free ceases to be a rate-limiting factor, depends least digestible of the oxygen in the exhaust air. partially on the potential rate of decomposition carbohydrates. Cellu- The aeration rate can be of the carbon source. For example, a readily lose and hemicellulose reduced as composting available carbon source such as sugar creates an are among the more digestible. progresses. Once immediate nitrogen demand and requires a low composting is complete, the C/N ratio. On the other hand, cellulose and material should be aged in a stockpile for about 2 lignin are more slowly degradable and have a months to complete its stabilization. low nitrogen demand, requiring a higher C/N ratio. Carbon, Nitrogen and Other Nutrients Microorganisms that decompose organic The ideal initial C/N ratio for composting most residues require nitrogen and other nutrients for manure and sludges is between 20:1 and 30:1. metabolism and reproduction. The amount of Because the C/N ratio for livestock manures is nitrogen required per unit of organic matter usually about 10:1, it is often desirable to add varies with the type of microorganisms involved, carbon-rich material (e.g., crop residues or but it is generally accepted that the sawdust) to raise the C/N ratio. These materials carbon/nitrogen (C/N) ratio of the wastes will may also reduce the moisture content and influence the increase the porosity of the compost. decomposition rate. I Mechanical turning mixes Adequate phosphorus (P) and potassium (K) Carbon is used to compost materials more thor- build microbial cells oughly, making it less likely that must also be present to compost organic wastes. and to supply anaerobic (very low oxygen) Phosphorus is a constituent of microbial proto- energy for microor- zones will develop within a pile. plasm, and potassium is necessary for regulating ganisms. Much of It also ensures that all of the osmotic pressure relationships within bacterial the carbon is material in a pile, including the cells. Ideally, the phosphorus content
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