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Anaerobic Digestion on Swine Operations

Photo: Dr. Robert Burns, University of Tennessee

For the latest statistics on operating swine digesters, visit AgSTAR's projects Web page.

For more information on the potential of AD, see AgSTAR's Market Opportunities report.

Anaerobic digestion (AD) of animal manures shows great promise in the United States and has environmental and economic advantages over land application of untreated manure. Although dairy farms have experienced steady growth in digester use, swine operations have lagged behind. Understanding current manure management and storage practices and barriers and solutions to AD can help to determine ways to increase its use.

Current Manure Management and Storage Practices

Depending on the region of the country, swine operators handle manure differently. Swine farms in some Midwest states, for example, typically involve deep pit storage, which allows excretions to drop into storage pits underneath slatted floors of the barn. Manure is pumped out once or twice a year and transported via drag line for land application and injection up to four miles away. Some digestion of manure does occur in the pits, resulting in odor issues and the release of methane into the atmosphere. A number of operations send vented air from barns through a bio-filter (i.e., a bed of activated media such as wood chips) for odor control. However, due to the infrequent removal of manure, operations using deep pit storage must modify manure management processes and structures to incorporate AD systems.

Flush collection, in which flowing water is used to move manure, is another common manure management practice, particularly in North Carolina. The collected dilute manure has traditionally been stored in outdoor uncovered lagoons. To make AD a more economical option, swine farmers have been taking steps to reduce the dilution of manure by switching to scrape or pull plug manure collection, thus reducing the size of the AD system needed and increasing the biogas production per gallon input potential.

Barriers and Solutions to Increasing AD

Current barriers that limit AD adoption on swine farms fall into the following categories:

Knowledge: In general, the swine industry does not fully understand AD systems and their associated benefits. Some misconceptions include that available nutrients are reduced when manure is digested, that biogas production cycles do not match on-farm energy needs, or that extremely large herds are required for a system to work. Without well-documented examples of successful AD systems on swine farms and their associated benefits, farmers usually do not seriously consider these systems as an option.

Two swine operations with successful digesters include:

Educating swine producers and others in the industry regarding the processes, benefits, and drawbacks of treating their manure with AD could increase system use. Improving knowledge could be accomplished by:

Technology: Relatively few examples exist of successfully installed AD systems on swine farms compared to dairy farms. Slow adoption and past failures have led to negative perceptions and assumptions about the technologies offered. This leads prospective owners to conclude that the technology is not well established and may be excessively risky. Farmers are concerned about the availability of qualified, experienced designers, installers, and servicing technicians, and about being able to determine whether technology providers are legitimate.

Some technological changes could help increase AD adoption. These include:

Economics: The primary challenge for the swine industry is staying profitable in a low profit-margin business. Producers must also find ways to cost-effectively comply with regulatory requirements, including manure management and nutrient management, which requires producers to have access to a sufficient number of crop acres in order to apply nutrients at recommended agronomic rates.

AD systems are typically seen as complex projects with large up-front capital costs. Additionally, typical swine manure management is designed to minimize the need for structures, and most swine operations keep personnel to a minimum. The majority of swine producers are located in areas with low electricity prices, limiting the potential for AD as an income stream. Dairy based AD systems have the added economic benefit of providing digested fibers for bedding. Swine AD systems typically do not have the economic benefit of providing a fiber source since swine manure has a lower fiber content than dairy manure and swine production systems traditionally do not need fibrous bedding. While there are numerous other benefits (e.g., odor reduction), they are difficult to quantify and do not typically result in cash for the farm.

Improving the economics of AD systems is key to increasing their use. Economics could be enhanced through the following measures:

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