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SELECTING
A MANURE DIGESTER DESIGNER Expectations of success. Decisions to install a digester should be based on accurate information. Decision makers must be confident of a claims and predictions. Potential digester designer should be asked to address three topics: 1. The material balance (or accounting) of the system proposed? 2. The energy balance (or accounting) of the system proposed? 3. Experience with this type of material at a comparable scale? Material Balance or AccountingBiogas and fiber production should be accurately predicted. The sum of material entering a system should equal the sum of the material exiting the system. For example, if 100 pounds of total solids (dry matter) are placed in the digester, a total of 100 pounds should exit the digester in one form or another. Similarly, 100 pounds of nitrogen (or P, K etc) in should equal the sum of 100 pounds out. Constituents “in” are easy to comprehend as there is usually only one means by which to load the digester. Constituents “out” are less intuitive as material exiting the digester are in the form of liquid effluent, the biogas and the periodically cleaned out digester grit. Energy Balance or AccountingNet available electricity and hot water should be accurately predicted. The sum of the energy entering the digester should equal the sum exiting in a variety of forms. The energy units differ at various points of the digester. Energy content of material entering the digester is most often noted as volatile solids, an inaccuracy that has become a convention in spite its inadequacy. Incoming manure energy content may also be noted as COD (chemical oxygen demand), BOD (biological oxygen demand) or calories. Regardless of the energy unit employed, some fraction of this energy content is converted by methanogenic and other organisms into biogas (of which methane is a part). What is not converted to methane rich biogas remains. Methane rich biogas is used as a fuel to produce heat, torque or (with fuel cells) electricity. During the conversion process, a portion of the fuel energy is converted to heat (Btu’s) and a portion to electrical energy (kWh’s). Of heat energy, a portion is lost as radiant heat, a portion is lost in system exhaust, and a portion is recoverable in hot water, hot air or some other medium. Of recovered heat, some fraction is used to support the overall energy conversion and production process. The remainder is available for sale or other uses. Similarly, some portion of the electrical generation is used to support the overall energy conversion and production process. The remainder of the electricity is available for sale or other uses. The portion of the energy (heat or electric) needed to produce the power is referred to as “parasitic” or “station” power. ExperienceRelated experience is the best predictor of success. What works at one scale on one type of material does not necessarily work on another. Physical, chemical and biological systems perform differently as scale changes. Moreover, physical, chemical and biological characteristics are unique for different materials to be treated in a digester. As extreme examples, swirled whey in a quart laboratory reactor flows differently than a million gallons of oatmeal consistency dairy cow manure. Similarly, the effectiveness of bacterial action differs. Grit, abrasive fibers and extraneous objects in manure is considerably different than would be expected in human waste treatment or industrial wastes. Relatedly, manure constituents biological degradability differs from the constituents of human or many industrial waste streams. Whey and sawdust degrade differently. Costs and benefits should be reasonably predictable. Manure management is secondary to the main income generating purposes of the farm. Resources are simply too limited to expend much to build, maintain or tend manure treatment facilities. Specialized technicians are not affordable.
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SELECTION CHECKLIST:
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Selecting a Designer| Checklist | Available Designers Important Offer:
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