Biogas Technology

Components of a typical biogas digester

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).

anerobic digester system

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.