A new publication (EB-420) is available from University of Maryland Extension. Manure as a Natural Resource: Alternative Management Opportunities is written as an overview of some existing technologies. Many new ideas are proposed for the region, and understanding the science behind them is imperative to deciding which option you may want to follow.
Manure, as a source of organic matter and plant nutrients, is an excellent conditioner for soils. It is a component of agronomic production, cycling nutrients between soils, plants and livestock. However, in areas where limited land is available for application, excess soil nutrients can lead to water quality issues. Local restrictions on manure application necessitate finding alternative uses. The simplest method is to transport manure to nutrient-deficient land. Manure can be composted into a higher-quality fertilizer or have the nutrients extracted and sold separately. Manure also has an energy value, and where feasible, anaerobic digestion, pyrolysis, or gasification could be options.
Shipment to Nutrient-deficient Regions is Simplest Solution for Dealing with Excess Manure The major restriction to transporting manure is the cost of hauling and handling. As fuel costs rise, the distance you can ship manure will decrease [1, 2]. Transporting manure is only feasible if the price of commercial fertilizer rises enough to make hauling manure a cheaper option [1]. The future scarcity of rock phosphate provides us with motivation to recycle phosphorus locally [1]. Therefore, any improvement in manure transport may offset future global shortages in fertilizer for the Mid-Atlantic region.
Based on 2009 commercial fertilizer prices, the University of Georgia determined that poultry litter had an estimated value of $80/ton [3]. However, poultry litter was sold at $10-55/ton, reflecting actual demand. The contrast between the estimated value and the actual price could be due to limited understanding of the full value of chicken litter, which includes both nutrients and organic matter. To make transporting litter profitable, public policy would need to subsidize and promote it, similar to the Maryland Department of Agriculture Manure Transport Program [2, 4]. Common methods of transporting manure include truck, rail, and barge [2, 5, 6]. In Europe, pipelines also have been proposed to move liquid manure slurries [7]. One way to increase transport distance is to reduce the volume of manure and concentrate the nutrients. While maximum profitable distances of raw manure transport can be 30-40 miles [8], concentrating nutrients could expand the distance to 185–260 miles [5, 9]. There are several methods of concentrating nutrients, including mechanical (pelletizing, baling) and biological (composting). However, any additional processing of manure will incur its own costs and should be considered accordingly.
Pelletizing Compacts Manure Using High Temperatures and Pressure to Create a More Uniform, Dense Product Due to their density, a larger amount of pellets can be shipped compared to raw manure, easing storage and transportation [12, 13]. However, the need for a large drier means on-farm pelletizing is usually not cost effective [10, 11]. Nutrients can also be added to the pellets, enhancing their agronomic value. Costs associated with the pelletizing process can be up to $40-50/ton [2, 12] and the pellets may have an estimated value of $100/ton [2]. Any additional handling of manure will have energy and other associated expenses, which could annul the savings in transport. Baling is Another Option for Pre-packaging Manure Baling and wrapping poultry litter with plastic has been tested in Arkansas where baling litter was cheaper than other options, costing only $5-10/ton [2, 6]. Barge transportation on the nearby Mississippi River made shipping baled litter economically feasible over long distances [6]. Bales can also can be shipped on flat-bed trucks, removing the need for specialized trailers. The use of flat beds lowers the cost of shipping by enabling backhauling where a trucker returns to the originating point with a full load [1]. Since it costs almost as much time and fuel to drive empty as fully loaded, backhauling makes economic sense. However, a successful litter baling company was not operating when this report was published. Composting is a Well-known Solution to Manure Handling, Particularly for Reducing Odors and Pathogens Compared to pelletizing or baling, composting can be a much simpler process, requiring only a bucket loader and storage space. When performed correctly, composting encourages microorganisms to break down the manure (figure 1), causing a reduction in total mass [14, 15]. Benefits include improved handling, odor and transportation. However, these benefits may be offset by additional costs, including bulking agents, storage space, and the loss of nitrogen [15]. Bulking agents (materials high in carbon, such as wood chips, straw) are used to increase the quality of composts. When feeding on manure, microbes prefer good moisture, a pH 5.5-8.0 and temperatures greater than 130°F. In addition, the microbes require 30 carbon (C) atoms for every one nitrogen (N) atom, which is known as the C:N ratio [15]. Not all livestock manures have the same C:N ratio, especially if they are mixed with different types of beddings. If a manure (for example, chicken litter and pine shavings) has a C:N greater than 30, the compost may be ineffectively broken down. On the other hand, excessive nitrogen (C:N less than 20) may limit composting, since high levels of ammonia are toxic to microbes. Therefore, bulking agents (cereal straw, pine shavings, dry switchgrass) can be used to adjust C:N ratios as well as moisture content. Phosphorus and potassium are concentrated by composting, but nitrogen can be lost as a gas. The reduced odor, pathogens, and ease of handling can make up for nitrogen loss. For more information on composting, see University of Maryland Extension publication, Backyard Composting (HG-35).
Benefits Moves nutrients to needed areas. Cycles nutrients in the region.
Costs Fuel limits transport distance. May require additional equipment. May require additional inputs
Manure, as an Organic Material, Can be Used to Produce Energy
Methods of energy production can be biological (anaerobic digestion) or use heating (incineration, pyrolysis, or gasification). Energy production is more labor- and capital-intensive than composting and, unless it is done of the farm, will include transportation and maintenance costs.
Anaerobic Digestion is a Tested Practice
Anaerobic digestion refers to the microbial breakdown of organic materials under anoxic (lack of oxygen) conditions. Many microbes require oxygen, but there are others who have adapted to anoxic environments. The smell of sulfur and methane from marshes are a sign of anaerobic microbes living in saturated, low-oxygen soils.
A valuable resource produced from anaerobic digestion can be a biogas like methane (CH4), which can be burned to produce heat or energy [16]. Fertilizer is another byproduct of anaerobic digestion, produced from the remaining “digested” manure or liquid effluent. Similar to composting, nutrients like phosphorus and potassium are more concentrated in the fertilizer byproduct than in raw manure, but very little nitrogen is lost [17-19]. Read More with Pictures and Graphs on PDF.