Biomethane (biogas) is an alternative and renewable energy source produced through the anaerobic (oxygen free) digestion of organic matter whereby the organic matter is converted into a combustible biogas rich in methane (CH4) and a liquid effluent (Figure 1). In gen-eral, biogas consists of 55 percent to 80 percent meth-ane and 20 percent to 45 percent carbon dioxide (CO2). However, depending on the source of the organic matter and the management of the anaerobic digestion process, small amounts of other gases such as ammonia (NH3), hydrogen sulfide (H2 S), and water vapor (H2O) may be present. It is the methane component of the biogas that will burn or produce energy. The gas can be used to generate heat or electricity or both. It can be burned in a conventional gas boiler to produce heat for nearby buildings or to heat the digester, or used in a gas engine to produce electricity. As the organic material (feed-stock) is added to the system, the digested effluent is pumped from the digester. The effluent can be stored in a tank and later applied to the land at an appropriate time as a fertilizer without further treatment. Or, the ffluent can be separated into solids (fiber) and liquids. The solids can be composted prior to sale for use as a compost or animal bedding. The liquid still contains nutrient that can be sold or used on the farm as a liquid fertilizer as part of a crop nutrient management plan. Sources of organic matter that have been used to pro-duce biogas include animal manure, sewage sludge, municipal solid waste, food-processing wastes, and industrial wastes.
A typical biogas system consists of manure collection, anaerobic digestion, storage for digester effluent, and
gas handling and gas use equipment (Figure 2).
Anaerobic digesters have been used successfully in municipal and industrial wastewater treatment plants and on a number of livestock farms for many years. However, the use of anaerobic digestion technology on livestock farms in the Countries. for manure treatment and energy production has increased over the past few years (EPA AgStar 2006). According to the EPA AgStar pro-gram, some of the factors influencing the increased demand for anaerobic digesters are increasing technical reliability of anaerobic digesters through the deploy-ment of successful operating systems; growing concern of farm owners about environmental quality; increasing numbers of state and federal programs designed to share costs in the development of these systems; spiraling energy costs; the need for energy security; and emerg-ing new energy policies (federal and state) designed to expand growth of reliable renewable energy and green power markets.
Benefits and Challenges of Biogas Technology
Anaerobic digestion can convert organic wastes into profitable byproducts as well as reduce their environmental pollution potential. Anaerobic digestion offers the following benefits to an animal feeding operation and the surrounding communities:
• Electric and thermal energy.
• Stable liquid fertilizer and high-quality solids for soil amendment.
• Odor reduction.
• Reduced groundwater and surface water contamination potential.
• Potential revenue from sales of digested manure (liquid and solids) and excess electricity and/or process-
ing off-site organic waste.
• Reduction of greenhouse gas emissions; methane is captured and used as a fuel.
Revenue from possible reuse of digested solids as livestock bedding.
• Potential revenue from green energy and carbon credits.
The cost of installing an anaerobic digester depends on the type and size of system, type of livestock opera-tion, and site-specific conditions (EPA AgStar, 2006). In general, consider the following points when estimat-ing installation/operating costs:
• Estimate the cost of constructing the system.
• Estimate the labor and cost of operating the system.
• Estimate the quantity of gas produced.
• Estimate the value of the gas produced.
• Compare operation costs to benefits from operation
(include value as a waste-treatment system and the fertilizer value of the sludge and supernatant).The main financial obligations associated with building an anaerobic digester include capital (equipment and construction and associated site work), project development (technical, legal, and planning consultants; financing; utilities connection; and licensing), operation and maintenance, and training costs. In making a decision to install a digester, one must realize that the system will require continuous monitoring and routine maintenance and repair that should not be underestimated. Components should be maintained as recommended by the manufacturers because manure and biogas can be corrosive on metal parts. In fact, the
majority of digester failures over the past few decades were the result of management, not technological, problems.