Livestock production – whether it be cattle, swine, or poultry – results in manure as a byproduct. In the past, the only option to dispose of animal waste was as fertilizer on nearby fields. With the need to store manure, farmers may use anaerobic digestion as an option to produce renewable energy while reducing greenhouse gas emissions. Digesters have been used since the 1980s to allow hog operations to reduce odors and greenhouse gas emissions while providing on-farm, renewable energy.
According to the American Biogas Council, there is great potential for growth in the U.S. biogas industry. The Council estimates 8,600 dairy, poultry and swine farms are poised for development today. And the swine industry agrees. As an example, Smithfield Foods, Inc. announced a goal of reducing greenhouse gas emissions across all of their U.S. operations, including their swine farms, by 25% by 2025 – and included technology like anaerobic digestion in their announcement.
The dairy industry is the largest source of manure methane from livestock, and swine manure is noted as the second largest source. The dairy industry has been adopting anaerobic digestion systems, with roughly 280 systems registered on the Livestock Anaerobic Digester Database. Alternatively, 45 such systems involving swine are registered.
Why have so few swine operations implemented anaerobic digester systems? The vertically integrated nature of the swine industry is one factor whereas economic challenges also prevent swine farmers from adopting AD systems on farms. For example, many manure systems currently used on swine farms, such as hoop structures or deep litter, are not ideally suited for anaerobic digestion, making the investment cost much higher. Manure management systems on hog operations usually collect manure through slotted floors, gutters, or runoff, making it more challenging and expensive to install the infrastructure needed to transport the manure to an anerobic digester.
Anaerobic digesters found in hog operations tend to be larger than those found in other agricultural operations. This is because hog manure is handled more easily by adding extra water, and that additional water results in needing a larger, less efficient digester to process the manure and water mixture.
How an Anaerobic Digester Works
An anaerobic digester is an air-tight enclosure wherein enzymes and microorganisms convert the biodegradable organic matter contained in the manure into biogas and new microbial cells. Organic matter is the main source of odors coming from hog farms. All digesters must remain airtight because oxygen kills methanogens.
Microorganism communities found in digesters include acid formers and methanogens. Anaerobic digestion starts with the hydrolysis of organic matter by enzymes and microorganisms into smaller molecules that can be used by acid formers that produce volatile fatty acids. The produced volatile fatty acids are used by methanogens in methane and carbon dioxide which are the two main components of biogas. In addition to reducing the pollution from the uncontrolled disposal of organic wastes, anaerobic digestion can also help in inactivating insect eggs and larvae, weed seeds, and animal parasites.
Proper temperature and pH must be maintained for enzymes and microorganisms to efficiently digest organic matter. The biodegradation of organic matter found in hog manure produces just as much methane as other types of manure and wastes, yielding high amounts of almost 99% pure methane.
Types of Anaerobic Digesters
There are different designs of anaerobic digesters. The simplest is a covered anaerobic lagoon that is usually operated at ambient temperature. Without temperature control, biogas production varies throughout the year depending on the ambient temperature. The lagoon digesters are simple to construct through the covering conventional lagoons on farms, with and without linear to prevent ground water contamination, and installing biogas collection and processing systems. Therefore, they require the least financial investment and biogas can be used on the farm or processed into Renewable Natural Gas (RNG).
Complete mix systems are the second most common type of digester used on hog farms and are used to treat manure with total solids of 3-10 percent. These systems are usually operated at constant temperatures. Therefore, these digesters produce a steady output of gas. Plug flow digesters have a 70 percent conversion efficiency of volatile solids to biogas, which is more efficient than other types of energy conversions.
Hot water or other heating media can be used for maintaining constant temperatures of the digester. When Combined Heat Power (CHP) generators are used for producing electricity from the biogas, water can be heated through the CHP and used to maintain high amounts of bacterial activity.
Resulting Products & Their Uses
Anaerobic digestion produces two products: biogas and anaerobically digested materials (i.e., digestate). The digester converts around 50% of organic solids to biogas. The resulting biogas is a mixture of methane, carbon dioxide, hydrogen, hydrogen sulphide and water vapor. Biogas produced by an anaerobic digester can be used to produce electricity, provide heat or power cooling systems. This energy is most often used on the farm or sold to the local power company. Some hog operations compress the methane and inject it into the natural gas pipeline, eventually selling the methane through the Renewable Fuel Standard, the carbon market in California or even as fuel for vehicles.
Digestate contains the non- biodegradable fraction of manure and bacterial cells, stabilized organic solids, helpful bacteria, inorganic nitrogen (ammonia and ammonium), insoluble phosphorus, and micronutrients. It is discharged from the digester and can be used as fertilizer. The resulting digestate has smaller particle size and viscosity – i.e., it resists changing to a fluid, thus, making it better suited for being applied to the soil. Mineralization during digestion promotes the efficiency of any downstream nutrient recovery processes. In addition to using the digestate as a fertilizer, it can be processed into liquid and solid fractions that may be used as animal bedding or bio-based products like bioplastics.
Deciding to Build an Anaerobic Digester
Whether a hog operation chooses to utilize an anaerobic digester or not usually depends on if it makes sense financially and operationally. If the hog operation is located near homes or developed areas, investing in an anaerobic digester could help with odor concerns.
If using an anaerobic digester provides energy savings or revenue from selling the methane produced, economics will drive the decision-making process. The methane can also be traded for carbon credits, which can be certified and sold on the Chicago Climate Exchange.
There are also alternatives to taking on the cost alone. Given the narrow margins that producers typically face, there may be opportunities to band together with neighboring farms to share costs. One example is a partnership between Smithfield Foods and Dominion Energy and the resulting joint venture, Align RNG. The project focuses on 26 farms under contract with Smithfield. Located adjacently, the farmers were well-positioned to work together toward the end goal of reducing greenhouse gas emissions while producing renewable energy.
Learn more about Anaerobic Digesters.
Read on to learn more about three farms utilizing ADs.
