| What is Anaerobic Digestion?
Anaerobic digestion is the process of generating biogas from organic material, such as manure produced by livestock operations. Biogas consists of 60 to 80% methane, 20 to 40% carbon dioxide, and several trace compounds such as hydrogen sulfide and ammonia. Biogas produced from manure digestion can be burned for heat or electricity or both.
Why Anaerobic Digestion?
Technical feasibility
Biogas can be produced on site using a digester. Manure is fed into the digester where it is heated and retained for several days under anaerobic conditions (no oxygen). Under these conditions, anaerobic bacteria thrive and break down the manure to produce biogas. The biogas is collected and can be used for a number of purposes:
- Burn for heating the digester
- Burn for water and space heating
- Fuel for compressor engines
- Fuel for electric generators
The most successful digesters utilize all energy produced. The most efficient use of biogas is direct heating. If biogas is burned in an electric generator, the heat produced by the engine must be harnessed and used in order to achieve optimum efficiency. The electricity produced can be used on site or sold into the power grid.
Once the digestion process is complete, the digested manure, or effluent, is released from the digester and stored until it is land applied. The digestion process breaks down many of the solids contained in the manure. As a result, the effluent causes less stress on pumps and agitation equipment than manure does. Also, effluent is homogeneous and may not require agitation prior to land application.
Renewability
Most livestock operations produce a continuous year round supply of feedstock that is required for anaerobic digestion. For such operations, on farm anaerobic digestion could provide a continual source of heat and electricity.
Environmental benefit
Several anaerobic digesters found in North America are used primarily for manure treatment rather than energy production. When compared to manure, effluent has little or no odour and contains fewer weed seeds and pathogens. Also, digestion converts the nutrients found in the manure to a form that is more readily available to crops.
Cost effectiveness
Provided the livestock operation is a suitable candidate for anaerobic digestion and the digester is operated at maximum efficiency year round, digestion can be a cost effective method of energy production and manure treatment.
What are Some Problems with Anaerobic Digestion?
In order for anaerobic digestion to be cost effective, the biogas must be used as it is produced. Heat and electricity demand for livestock operations can vary drastically from season to season making it difficult to match energy demands with energy production. Combination operations, such as livestock and greenhouse operations, can make better use of the energy produced.
Anaerobic digestion works best with liquid manure systems. Livestock operations that use solid or semisolid manure management may not be good candidates for biogas production.
The equipment used for biogas production is specialized and requires a high capital investment. The operation and maintenance of a digester requires specialized training and skills as well as several hours each week devoted to the system upkeep. Also, biogas is an explosive gas that must be handled with proper safety procedures and equipment.
Due to its low energy value per unit volume, the use of biogas as a vehicle fuel is not feasible. Compressing the biogas to increase the energy per unit volume is too costly as is separating the methane found in biogas.
Is My Operation Compatible?
The feasibility of anaerobic digestion depends on a number of factors including the type of livestock, manure management system, and your heat and electricity requirements. A careful evaluation of your operation must be completed before anaerobic digestion is considered.
The last few decades have seen significant changes in anaerobic digestion technology and the systems we have today are more feasible than past systems. As technology develops and the energy costs increase, anaerobic digestion may become suitable for more livestock operations.
For additional information, check the following web pages:
Anaerobic digester, Agdex 768-1
Anaerobic Digesters: Frequently Asked Questions, Agdex 768-2
Biogas Energy Potential in Alberta, Agdex 768-3
Integrating Biogas, Confined Feedlot Operations and Ethanol Production, Agdex768-4
Biogas Distribution – Rural Utilities Division of Alberta Agriculture and Food.
Incentives for biogas production – Alberta Bioenergy Producer Credit Program
For more information
Navaratnasamy, Mahendran
Engineer
Agricultural Technologies Section
Agriculture and Rural Development
3rd fl JG O'Donoghue Building
7000 - 113 Street
Edmonton, AB
T6H 5T6
Phone: 780-415-2358
Fax: 780-422-9745 |