Emergy analysis of biogas production and electricity generation from small-scale agricultural digesters

Ecological Engineering 37 (2011) 1681–1691

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Emergy analysis of biogas production and electricity generation from small-scale agricultural digesters Richard J. Ciotola a,∗ , Stephanie Lansing b , Jay F. Martin a a b

Department of Food, Agricultural and Biological Engineering, The Ohio State University, Agricultural Engineering Building, 590 Woody Hayes Dr., Columbus, OH 43210-1057, USA Department of Environmental Science and Technology, University of Maryland, 1445 Animal Sci./Ag. Engineering Building, College Park, MD 20742, USA

a r t i c l e

i n f o

Article history: Received 27 August 2010 Received in revised form 31 May 2011 Accepted 29 June 2011 Keywords: Anaerobic digestion Manure Costa Rica Renewable energy

a b s t r a c t An emergy analysis was performed to assess the relative sustainability and environmental impact of small-scale energy production using Taiwanese model plug-flow anaerobic digesters to treat livestock manure in Costa Rica. Emergy analysis quantifies all inputs to a system by converting them to solar energy equivalents, thus allowing for direct comparison of the diverse inputs of renewable energies, human labor and economic goods needed to construct and maintain anaerobic digestion systems. The digesters were located on the campus of EARTH University, Costa Rica, and the biogas was utilized to power a 40 kW generator that supplies electricity for farm operations. Separate emergy analyses were performed for the biogas production and the combination of biogas production and generation of electricity. Manure was the largest input in both analyses, accounting for 85.3% of the annual emergy input for biogas production and 66.9% for electricity generation from the biogas. The fraction of emergy inputs from renewable sources (˚R) was 66% for biogas production and 52% for electricity generation from the biogas. The transformities of biogas and electricity generation from the biogas were 5.23E+04 sej/J and 1.01E+06 sej/J respectively. The emergy yield ratios (EYR) were 2.93 for biogas production and 2.07 for electricity generation indicating that these digesters efficiently match purchased resources and renewable energies to produce energy from livestock manure. The generation of electricity from the biogas resulted in a decrease in the emergy sustainability index (ESI) from 5.67 to 2.22 and an increase in the environmental loading ratio (ELR) from 0.52 to 0.93. Using a generator to convert the biogas to electricity does decrease the sustainability of the system, largely due to the high emergy value associated with the electrical generation equipment and machinery, but these results demonstrate that the production of biogas and the generation of electricity from Taiwanese model digesters in Costa Rica are environmentally sustainable processes that result in the production of energy that is largely dependent on renewable and recycled energies. © 2011 Elsevier B.V. All rights reserved.

1. Introduction One of the ongoing challenges facing human society is to continue to provide energy in usable forms such as electricity and gasoline while fossil fuel reserves are declining, populations are increasing and global warming concerns are growing (Omer, 2008). One solution is to develop additional capacity to provide energy with more sustainable systems that have minimal negative impact on the environment. There is also a need to evaluate the many different alternative energy production systems. This will aid in the design and selection of the most sustainable systems. In this study, emergy analysis was used to determine the relative sustainability and environmental impact of small-scale

∗ Corresponding author. Tel.: +1 614 208 4739; fax: +1 614 292 9448. E-mail addresses: [email protected] (R.J. Ciotola), [email protected] (S. Lansing), [email protected] (J.F. Martin). 0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.06.031

transformation of energy from organic waste streams processed in anaerobic digesters. Because emergy analysis quantifies all inputs to a system by converting them to solar energy equivalents, it allows for direct comparison of the diverse inputs of renewable energies, human labor and economic goods needed to construct and maintain the energy production systems. The aspects of sustainability that were evaluated in this study are the relative efficiency of different biogas production systems to utilize purchased resources to exploit locally available resources with minimal negative impact of the environment. Emergy analysis quantifies these aspects with a series of ratios and indices that are described in Section 2.6. In an anaerobic digester, energy in the form of methaneenriched biogas is produced through microbial degradation of various types of organic matter inputs, most commonly livestock manure. During anaerobic digestion, complex organic molecules such as carbohydrates, proteins and fats are transformed through a multistep microbial-mediated biochemical pathway. The end

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products of this process include methane, carbon dioxide gas and inorganic forms of nitrogen and phosphorous (Gerardi, 2003). Benefits of utilizing anaerobic digestion include the production of renewable energy and a liquid fertilizer as well as a 50–90% reduction in organic pollutants (Archer and Kirsop, 1990; Lansing et al., 2008a). Utilization of the methane-enriched biogas also reduces the amount of this powerful greenhouse gas (21 times more global warming potential than carbon dioxide) that is released into the environment from traditional manure management systems. Additional benefits of anaerobic digestion include reductions in odor and pathogens associated with livestock manure (Powers et al., 1999; USEPA, 2004; Lansing et al., 2010). The few published emergy studies of biogas generation indicate that the process has a high level of sustainability depending on the type of system employed. Wei et al. (2009) analyzed a four-inone peach production system (FIOPPS) that utilized an 8 m3 buried ambient temperature digester treating swine manure. The authors concluded that the FIOPPS relied more on renewable resources and had less negative environmental impacts than conventional greenhouse peach production systems without biogas production. Zhou et al. (2010) completed an emergy analysis of a biogas project that featured a 200 m3 and 500 m3 heated up-flow anaerobic sludge bed reactors digesting poultry and swine manure. They determined that the biogas project relied more on renewable emergy inputs and had less environmental impact than conventional Chinese agricultural production systems. To date, there have not been any published studies evaluating the sustainability and environmental impact of the lower-cost Taiwanese model digesters. Because of the potential of these digesters to improve the livelihood of small farmers, and the vast numbers of small-scale digesters (>35 million) (Chen et al., 2010), it is important to have a better understanding of not only the economic costs and benefits, but also the potential costs and benefits to environmental support systems. Two Taiwanese model plug-flow anaerobic digesters, as detailed in Lansing et al. (2008b), were analyzed. In order to assess the relative sustainability of biogas production and electricity generation from the biogas, it was necessary to perform two separate emergy analyses. The first analysis only evaluated biogas production while the second evaluated the biogas production combined with electricity generation. This allowed for the evaluation of potential energy loss that occurred as manure was converted to biogas and from biogas to electricity through an “energy transformation hierarchy” (Odum, 1998). The objectives of this study were to evaluate the sustainability and environmental impact of Taiwanese-model small-scale digesters using emergy analysis and compare the resulting indices to other energy production systems to determine their relative sustainability and resource use.

2. Methods 2.1. Study sites This study evaluated two Taiwanese model plug-flow anaerobic digesters that treated waste from the small-scale dairy and swine operations at the livestock farm on the campus of Escuela de Agricultura de la Región Tropical Húmeda (EARTH) University in Guácimo, Limón, Costa Rica (10◦ 4 N, 83◦ 38 W) (Fig. 1). The campus is located at an elevation of 50 m and has an average annual temperature range of 25–30 ◦ C. The digesters were constructed from polyethylene tubular bags that had a thickness of 0.2 mm. The dairy manure digester treated waste from approximately 60 Braham-Holstein cows while the swine digester treated manure from approximately 50 swine. Both the dairy and swine digesters were loaded by gravity from the milking parlor and swine corrals,

Fig. 1. Flushed dairy and swine manure are fed by gravity into two sets of Taiwanese model plug-flow anaerobic digesters. The combined biogas from the digesters powers a 40-kW generator. The effluent from both digesters flows into treatment wetland cells.

respectively, with manure flushed daily using water pumped from a nearby stream. The total combined length of the digesters was 63 m with a combined volume of 146 m3 and an average retention time of 26.5 days. Together they treated 6.7 m3 of wastewater per day and reduced the influent volatile solids content by an average of 81.7%. The digesters produced an average of 33.5 m3 /day of biogas, which contained approximately 21.7 m3 of methane. Biogas was stored in separate polyethylene storage bags and was pumped to a 1100 L triazine-complex absorption tower to remove hydrogen sulfide prior to being used in a 40 kW Cummins Power Generation biogas generator that was donated to EARTH University. This generator was specifically designed for the utilization of biogas and powered four milking machines, office equipment and small farm machinery for two hours daily during the farms morning peak electricity demand (Lansing et al., 2008b). 2.2. Emergy evaluation procedure The emergy evaluation procedure (Odum, 1996) used in this study included the following four steps: (1) constructing the system diagram, (2) building the emergy analysis table, (3) calculating the emergy indices and ratios and (4) making policy recommendations. Emergy analysis quantifies all renewable, non-renewable and purchased inputs to a system on the common basis of solar energy equivalents. It was developed as a means to compare different types of energy inputs to various systems. Emergy represents the total amount of energy both from renewable and non-renewable sources that are required to produce a product or service. A related concept, the transformity, is the amount of emergy that is required for each unit of system output. The transformity is calculated from the total emergy inputs divided by available energy produced. Product yields and emergy inputs can be used to calculate a variety of emergy indices and ratios, which can be used for analyzing and comparing different systems (Ulgiati and Brown, 1998). 2.3. Energy systems diagrams Two energy system diagrams were prepared for this analysis using energy systems symbols (Brown, 2004). Two of these diagrams were used to identify the line items for each entry to the emergy analysis tables. The first diagram was created for biogas

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Fig. 2. Energy systems diagram showing the labor, material and energy inputs requirements for the production of methane-enriched biogas and inorganic nutrients: (1) the loading of the digesters is accomplished through the interaction of human labor, pumped stream water and manure during the daily washing of the animal barns. (2) The construction, operation and maintenance of the digesters are the result of the interaction between human labor and purchased materials. All emergy flows are 1014 sej/year.

production and the second for biogas production combined with electricity generation (Figs. 2 and 3). 2.4. Emergy analysis tables Inputs and system products for both the dairy and swine digesters were evaluated as a single system. Two emergy analysis tables were created for this analysis: (1) biogas production and (2) biogas production combined with electricity generation (Tables 1 and 2). The line items in the emergy analysis tables include both the initial capital investment as well as the ongoing inputs required to operate and maintain the digesters. In order to convert initial capital inputs to an annual rate, initial inputs were divided by the twenty-year lifespan of the 40 kW generator (Lansing et al., 2008b). For each of these tables, the inputs

Fig. 4. The emergy signature diagrams identified manure as the largest input for the production of biogas (top) and manure and machinery as the largest inputs for the production of biogas combined with the electricity generation (bottom). The inputs are listed from left to right on the x-axis in order of increasing transformity.

for manure and labor were classified as a combination of renewable and purchased resources. The classification of a fraction of the manure as “purchased” does not imply that a portion of the manure was bought from the larger economy but rather indicates that some purchased resources are required to maintain the EARTH University farm that generates this waste stream. The percent allocation of manure and labor to the renewable and purchased categories is based on an emergy analysis of Costa Rica conducted at the University of Florida (Sahel project, 2000). They found that for the year 2000, approximately 68% of the annual emergy inputs to Costa Rica were from renewable sources while the remaining 32% was from non-renewable inputs. These percentages were used to allocate the manure and labor inputs into the renewable and purchased categories. In Table 1, the biogas yield in joules was calculated from the average daily production of biogas from both the dairy and swine digesters (33.5 m3 biogas/day) multiplied by the average percent methane (64.8%) in the combined biogas after H2 S removal. The daily production was then converted to annual production and multiplied by the energy content of methane (3.77E+07 J/m3 ) to obtain the biogas yield in joules/year.

2.5. Emergy signature diagram Fig. 3. Energy systems diagram showing the labor, materials and energy inputs requirements for the production of methane-enriched biogas, electricity and inorganic nutrients: (1) the loading of the digesters is accomplished through the interaction of human labor, pumped stream water and manure during the daily washing of the animal barns. (2) The construction, operation and maintenance of the digesters are the result of the interaction between human labor and purchased materials. (3) The operation, construction, maintenance of the 40 kW electric generator is accomplished through the interaction between human labor, building materials and the generator. All emergy flows are 1014 sej/year.

An emergy signature diagram (Rydberg and Jansén, 2002) was prepared for the production of biogas and generation of electricity from the biogas (Fig. 4). Signature diagrams provide a visual representation of the relative emergy inputs from different energy sources. With the exception of the concrete and machinery, the same inputs were used for both biogas production and combined biogas production and electricity generation from the biogas.

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Table 1 Emergy analysis table for the production of biogas at EARTH University, Costa Rica. Item

Unit

Units/year

Transformity (sej/unit)

Emergy (sej/year)

Renewable resources 1. Sunlight 2. Water 3. Manure (renewable) 4. Labor (renewable) Total

J m3 g h

5.77E+11 2.45E+03 9.50E+07 1.08E+02

1.00E+00 3.23E+11 9.70E+07 4.30E+12

5.77E+11 7.90E+14 9.21E+15 4.66E+14 1.05E+16

Purchased resources 5. Electricity 6. Plastic (digester and biogas storage bags) 7. Zinc (roofing) 8. Steel (roofing) 9. PVC (piping) 10. Wood (digester building frame) 11. Manure (purchased) 12. Labor (purchased) Total

J g g g g g g h

1.14E+09 1.01E+04 5.20E+03 8.72E+04 3.35E+03 1.46E+05 4.47E+07 5.10E+01

1.65E+05 5.87E+09 1.58E+10 4.15E+09 1.14E+10 8.80E+08 9.70E+07 4.30E+12

1.89E+14 5.95E+13 8.21E+13 3.62E+14 3.82E+13 1.28E+14 4.34E+15 2.19E+14 5.41E+15

J g g

2.99E+11 1.87E+05 2.20E+04

5.32E+04 8.48E+10 7.23E+11

1.59E+16 1.59E+16 1.59E+16

Products 13. Biogas 14. Ammonia nitrogen 15. Phosphate phosphorus

Transformity references for respective row number: (1) By definition, (2) Buenfil (2001), (3) Bastianoni and Marchettini (2000), (4) Ortega (2000), (5) Odum (1996), (6) Buranakarn (1998), (7) Tilley (2005), (8) Buranakarn (1998), (9) Cao and Feng (2007), (10) Buranakarn (1998), (11) Bastianoni and Marchettini (2000) and (12) Ortega (2000).

2.6. Emergy ratios and indices System inputs were first aggregated into three categories, which were then used with the system yield to calculate the emergy ratios and indices. Renewable inputs (R) are those that are available from the local environment such as sunlight or surface water. Purchased resources (F) are imported from outside the system at a cost and include items such as plastic and concrete. Non-renewable emergy flows (N) are provided from within the system but are limited in the amount available, such as soil or groundwater that are utilized faster than their rate of replacement (Ulgiati and Brown, 1998). There were no locally available non-renewable resources utilized in the EARTH production system. Finally, the system yield (Y) is the total emergy in the output of the system measured in solar

emjoules (sej); this value is equal to the sum of all the emergy inputs. The emery inputs and product yields were used to calculate the following emergy ratios and indices; the fraction renewable (R) = R/(R + N + F), the emergy yield ratio (EYR) = Y/F, the environmental loading ratio (ELR) = (F + N)/R and the emergy sustainability index (ESI) = EYR/ELR (Ulgiati and Brown, 1998). The fraction renewable (R) is the percentage of emergy flows into the system that are provided by renewable resources. The emergy yield ratio (EYR) is used to determine how efficiently a production process captures locally available resources. Larger EYR values indicate a more efficient use of emergy invested from outside the system (F) to harness local resources. The environmental loading ratio (ELR) is a measure of the impact a process has on the environment with

Table 2 Emergy analysis table for combined biogas production and electricity generation at EARTH University, Costa Rica. Item

Unit

Units/year

Transformity (sej/unit)

Emergy (sej/year)

Renewable resources 1. Sunlight 2. Water 3. Manure (renewable) 4. Labor (renewable) Total

J m3 g h

5.77E+11 2.45E+03 9.50E+07 1.09E+02

1.00E+00 3.23E+11 9.70E+07 4.30E+12

5.77E+11 7.90E+14 9.21E+15 4.68E+14 1.05E+16

Purchased resources 5. Electricity 6. Plastic (digester, biogas storage bags and scrubbing tank) 7. Zinc (roofing) 8. Steel (roofing) 9. PVC (piping) 10. Concrete (generator building and scrubbing tower) 11. Machinery (generator and electrical equipment) 12. Wood (digester building frame) 13. Manure (purchased) 14. Labor (purchased) Total

J g g g g g US$ g g h

2.54E+09 1.70E+04 5.70E+03 9.56E+04 3.46E+03 2.67E+06 2.00E+03 1.46E+05 4.47E+07 5.12E+01

1.65E+05 5.87E+09 1.58E+10 4.15E+09 1.14E+10 7.00E+07 1.93E+12 8.80E+08 9.70E+07 4.30E+12

4.20E+14 9.97E+13 9.01E+13 3.97E+14 3.94E+13 1.87E+14 3.86E+15 1.28E+14 4.34E+15 2.20E+14 9.78E+15

J g g

2.00E+10 1.87E+05 2.20E+04

1.01E+06 8.49E+10 7.23E+11

2.02E+16 1.59E+16 1.59E+16

Products 15. Electricity 16. Ammonia Nitrogen 17. Phosphate Phosphorus

Transformity references for respective row number: (1) By definition, (2) Buenfil (2001), (3) Bastianoni and Marchettini (2000), (4) Ortega (2000), (5) Odum (1996), (6) Buranakarn (1998), (7) Tilley (2005), (8) Buranakarn (1998), (9) Cao and Feng (2007), (10) Brown and Mclanahan (1996), (11) Sahel Project (2000), (12) Buranakarn (1998), (13) Bastianoni and Marchettini (2000), and (14) Ortega (2000).

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larger values indicating greater environmental stress. The emergy sustainability index (ESI) is measure of the overall sustainability of a production process (Ulgiati and Brown, 1998). 2.7. Sensitivity analysis A sensitivity analysis was performed in order to assess the relative importance of each of the emergy inputs to the biogas production and utilization systems. This procedure involves doubling and halving each of the emergy inputs and recalculating each of the emergy ratios and indices (Martin et al., 2006). 3. Results 3.1. Renewable resources The total annual emergy input from sunlight for both biogas production and electricity generation from the biogas was 5.77E+11 sej/year. Additional renewable resources included water, and a fraction of the overall manure and labor contributions. Water pumped from a nearby stream was used to clean the animal stalls and transport the diluted manure to the digesters. 7.90E+14 sej/year of water were used annually for the stall washing, and thus, loading of the digester feedstock. The annual renewable inputs supporting manure and labor were 9.21E+15 sej/year and 4.66E+14 sej/year respectively. An additional 2.19E+12 sej/year of renewable inputs in the form of labor were required for the utilization of the biogas for electricity generation. 3.2. Purchased resources Except for the fraction of labor and manure that was supported by purchased resources, all the inputs in this category were materials required for the construction and maintenance of the biodigesters and supporting infrastructure including the buildings, scrubbing tower and the generator. Non-renewable labor inputs were 2.19E+14 and 2.20E+14 sej/year, respectively for biogas production and biogas plus electricity generation. Non-renewable inputs used to produce the manure were 4.34E+15 sej/year for both biogas production and biogas plus electricity generation. The highest non-labor transformity among the purchased resources was for the machinery and electrical equipment associated with the generator, 3.86E+15 sej/year. The remaining purchased resources, which included electricity, plastic, PVC, concrete, zinc, steel and wood were 8.58E+14 sej/year for biogas production and 1.36E+15 sej/year for electricity generation from the biogas. 3.3. Emergy signature diagrams The largest total emergy input for both biogas production and electricity generation from the biogas was manure (Fig. 4). Manure is 85.3% of the total annual emergy inputs in the biogas production process and 66.9% for the combination of biogas production and electricity generation. The machinery was the largest single purchased input, accounting for 19.1% of the annual emergy input for combined biogas production and electricity generation, with the remaining purchased inputs accounting for 29.2% of the total emergy input. 3.4. Emergy ratios and indices Four emergy ratios and indices were calculated for biogas production and electricity generation from the biogas (Table 3). The fraction renewable (˚R) was 0.66 for biogas production and 0.52

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Table 3 Emergy ratios and indices for the production of biogas and the generation of electricity at EARTH University, Costa Rica.

Yield (sej/year) Fraction renewable Emergy yield ratio Environmental loading ratio Emergy sustainability index

Calculation

Biogas

Electricity

R+N+F R/(R + N + F) Y/F (F + N)/R EYR/ELR

1.59E+16 0.66 2.93 0.52 5.67

2.02E+16 0.52 2.07 0.93 2.22

for combined biogas production and electricity generation. The difference between these two values is primarily due to the increase in non-renewable inputs needed to produce the generator and electrical equipment. The emergy yield ratio (EYR) was lowered from 2.93 to 2.07 by the generation of electricity. Utilization of the biogas to power the generator increased the environmental loading ratio (ELR) from 0.52 to 0.93. Electricity generation from the biogas lowered the emergy sustainability index (ESI) from 5.67 to 2.22. 3.5. Yields and transformities The yield (Y) is the sum of all the renewable, non-renewable and purchased emergy inputs required to produce a particular product or service (Ulgiati and Brown, 1998). 1.59E+16 sej/year were required for the production of biogas while the generation of electricity from the biogas required an annual input of 2.02E+16 sej/year. The digesters produced an average of 2.99E+11 joules of biogas each year with a resulting transformity of 5.32E+04 sej/J. The 40 kW generator produced an average of 2.00E+10 joules/year of electricity with a transformity of 1.01E+06 sej/J. 3.6. Sensitivity analysis The sensitivity analysis illustrated the relative importance of each input on the emergy ratios and indices. When individual inputs were doubled and halved, manure changed all four ratios and indices by more than 10% for both biogas production and electricity generation from the biogas (Fig. 5). When electricity was generated from the biogas, doubling and halving of the machinery input also changed all four ratios by more than 10%. Doubling the inputs of zinc and steel decreased the ESI by 12.2% and 10.1% for biogas production only. For both biogas production and electricity generation from the biogas, doubling of the water input increased the ESI by 12.9% and 11.7% respectively. None of the other inputs caused changes greater than 10% in the ratios and indices when doubled or halved, revealing that these parameters do not have as large of an impact on the final values of the emergy ratios and indices. 4. Discussion 4.1. System inputs This study met its objectives by quantifying the annual inputs and products on the common basis of solar emergy. The lack of non-renewable (N) inputs indicates that this production system does not deplete locally available natural resources such as soil or groundwater. The analysis also showed that biogas production and electricity generation from the biogas from this anaerobic digestion system relied on renewable energy for more than half of the inputs (˚R > 0.5). The fraction renewable (˚R) is an appropriate way to compare different production systems that have been classified as “renewable” because most renewable energy systems have both renewable and nonrenewable system inputs.

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Fig. 5. Results from the sensitivity analysis showing the effects of doubling and halving the emergy inputs for manure and machinery. (FR: fraction renewable; EYR: emergy yield ratio; ELR: environmental loading ratio; ESI: emergy sustainability index.)

The ˚R for biogas production at EARTH University (0.66) was less than the 0.78 determined by Wei et al. (2009) for biogas production in a buried fixed dome digester. It was also lower than the 0.87 calculated by Zhou et al. (2010) for biogas production from two large, heated reactors. While these comparisons may indicate a less sustainable system at EARTH University, it is necessary to review the assumptions of these particular studies in order to adequately compare these systems. In this study, the manure inputs were assigned to both renewable and purchased categories. In contrast, both Wei et al. (2009) and Zhou et al. (2010) did not consider any portion of the manure to be classified as a purchased input. Had all the manure inputs in this study been classified as renewable, the ˚R for biogas production would have been 0.93. The classification of manure as an entirely renewable resource has also been used in other types of production systems such as hot peppers (Kalaf, 2007) and the combined production of rice and ducks (Xi and Qin, 2009). Despite the trend in the literature of classifying manure as a renewable resource, the separation of this input into renewable and purchased fractions is a more accurate reflection of the types of emergy responsible for manure production. Items without nonrenewable or purchased component should be limited to energies that are provided entirely by the environmental support system. Although manure is a waste stream, there are still non-renewable and purchased inputs necessary for its production. These occur in the form of products and services that are supplied by the greater economy of Costa Rica and used to maintain the EARTH University farm and the animals that produce the manure. Because of this connection to the national economy, the percent non-renewable emergy flow for Costa Rica (Sahel Project, 2000) was used as the most appropriate estimate of the purchased fraction of the manure. In this study, the classification of labor inputs was also divided into both renewable and purchased portions, as it was assumed that a worker would be sustained by a combination of renewable and

purchased resources. Other authors have also made the assumption that both renewable and purchased energies support human labor (Martin et al., 2006; Rydberg and Jansén, 2002; Panzieri et al., 2002; Guillen-Trujillo, 1998). Unlike the classification of manure as entirely renewable, the division of labor into renewable and purchased fractions is a more common practice in emergy analysis. The only climatic input included in this study, sunlight, was a very small percentage of the overall input (0.003%). Additional renewable energy from wind and rain was not a significant input to the EARTH digesters, and thus, not included in the analysis, as only the largest climatic input is included in emergy analyses (Odum, 1996). Solar energy is responsible for the tropical climate in Costa Rica and the ability to produce relatively consistent amounts of biogas year round without additional heating. Anaerobic digestion is a temperature dependent process, with increasing biogas yield at higher temperatures. In ambient temperature digesters such as the ones at EARTH University, heat energy is transferred into the digester slurry from the surrounding environment. The main source of heat transfer from the environment is through sunlight, even though some additional heat transfer would occur from the air and soil that is in contact with the digester. This study calculated that the total quantity of heat transferred into the system by estimating the amount of solar emergy received in Guácimo, Costa Rica within an area equal to that of the digesters (120.3 m2 ). Because sunlight is such a small percentage of the emergy signature the assumption about the amount of solar input is not likely to substantially alter the results. The results of the sensitivity analysis prove that doubling and halving the solar input resulted in less than 1% difference in the emergy ratios and indices. It is interesting to note that a large amount of purchased energy would be needed to heat a digester located in a temperate climate to maintain the high year-round ambient temperature provided, with a relatively small emergic value, by the supporting environmental services (sunlight) to a digester located in the tropics. 4.2. Transformities of the system inputs In the literature, there is often more than one transformity value reported for an input, and the value most appropriate for the particular study must be chosen. For example, there are two different values for the transformities of water reported in the literature. Buenfil (2001) reported a value of 3.23E+11 sej/m3 for “rivers and streams” while Odum (1996) reported a value of 4.85E+04 sej/J for the “chemical stream emergy”. Despite the different methods of calculating water transformity, both calculations yielded similar results for annual emergy input: 7.90E+14 sej/year and 5.86E+14 sej/year. Because water is a minor contributor to the emergy signature the choice of either transformity would have yielded similar results. The transformity from Buenfil (2001) was chosen due to the consistency of the units with the water usage data in this study. In addition, there are various transformities in the literature for the electricity required to pump the wash water from the streams and run the biogas and scrubbing tower pumps. The transformity for electricity, hydroelectric power in Brazil (Odum, 1996), was chosen because more than 80% of the electricity generated in Costa Rica is from hydroelectric power (Anderson et al., 2006). In the absence of any published transformities for galvanized steel roofing, the best approximation was obtained by using separate inputs for steel and zinc in the percentages typically used for this type of roofing. The transformities for these inputs were obtained from Buranakarn (1998) and Tilley (2005), respectively. The transformity for manure, 1.13E+08 sej/g, calculated by Bastianoni and Marchettini (2000) was modified for use in this study in order to avoid the double counting of dairy and swine stall

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4.4. Joint transformity of the co-products

1.00E+07 Transformity (sej/J biogas)

1687

1.00E+06

1.00E+05

1.00E+04 This study

Zhou et al., 2010

Wei et al., 2009

Fig. 6. Comparison of biogas transformities of three biogas production systems.

wash water. The emergy analysis of the Puerto Rican dairy farm (Bastianoni and Marchettini, 2000) includes 2.61E+12 J of “agricultural water” as one of the renewable inputs into the co-production of milk, methane and manure. This value was subtracted from the total emergy of the yield and the difference was used to re-calculate the modified transformity used in this study, 9.70E+07 sej/g.

4.3. Transformity of biogas One method to evaluate the relative sustainability of different systems is to compare the transformities of the products. Those with greater relative transformities require the input of more emergy to produce the same amount of product. Comparison of transformities evaluates the total emergy involved in product formation and does not differentiate between renewable, non-renewable or purchased resources (Fig. 6). The EARTH University digesters required an input of 1.59E+16 sej/year to produce 2.99E+11 J biogas/year, which resulted in a transformity of 5.23E+04 sej/J biogas. In comparison, the two large-scale mechanized digesters in the farm biogas project (Zhou et al., 2010) used 1.48E+18 sej/year to produce 5.50E+12 J biogas/year. The resulting transformity for the farm biogas system was 2.69E+05 sej/J biogas, which is about five times greater than the methane transformity from the EARTH University digesters. The digesters studied by Zhou et al. (2010) did produce a greater yield per cubic meter of digester than the EARTH University digesters; 7.86E+09 J biogas/m3 and 2.05E+09 J biogas/m3 of digester respectively. However, the digesters analyzed in Zhou et al. (2010) also required over nine times the annual emergy inputs to construct and maintain the system compared to the EARTH University system. The 8 m3 buried fixed-dome digester used in the FIOPPS system (Wei et al., 2009) required an annual input of 1.28E+16 sej to produce 6.28E+09 J biogas/year resulting in a biogas transformity of 2.04E+06 sej/J. This value is approximately 38 times greater than the transformity for the biogas produced by the EARTH University digesters. The annual emergy input into the EARTH digesters, 1.59E+16 sej/year is relatively similar to the annul inputs to the FIOPPS. The large difference in the transformities is a result of the lower biogas yield in the FIOPPS as compared to the EARTH digesters. This difference can be explained by both the greater total volume (146 m3 ) of the EARTH University digesters as well as their location in a tropical climate. The rate of biogas formation is reduced at lower temperatures (Gerardi, 2003) and the digester studied by Wei et al. (2009) is located in a temperate region of China with winter temperature as low as −13.2 ◦ C. The EARTH digesters are also more efficient, producing 2.05E+09 J biogas/m3 of digester while the FIOPPS produced 7.85E+08 J biogas/m3 of digester.

In order to better evaluate the relative sustainability of different biogas production systems, it is necessary to also consider the various co-products that are formed and how they are utilized. The EARTH University digesters produced inorganic nitrogen and phosphorus as co-products. These products occur in the liquid effluent from the digesters and can be used as fertilizer resulting in additional benefits from anaerobic digestion. Other emergy analyses of biogas production have also quantified co-products. Zhou et al. (2010) included “biogas slurry” and “biogas residue” as co-products that are utilized as inputs to livestock production and aquaculture. Wei et al. (2009) identified peaches, peach branches and swine as co-products with biogas in the FIOPPS. One method of evaluating the relative sustainability of systems producing more than one product is to use the joint transformity of the co-products (Bastianoni and Marchettini, 2000). This value is the ratio of the annual emergy input divided by the total energy of the co-products. The joint transformity of the farm biogas project (Zhou et al., 2010) co-products was 2.28E+05 sej/J, a reduction of 15.1% from the biogas transformity. Likewise, the joint transformity of the coproducts for the FIOPPS, 2.06E+05 sej/J was 89.9% lower than the biogas transformity. The joint transformities from these prior studies were higher than the individual transformity for the methane produced from the EARTH University digesters. The co-products from the EARTH digesters, ammonia nitrogen and phosphate phosphorus were quantified in grams rather than joules, so a conversion to joules is necessary in order to calculate the joint transformity. Zhou et al. (2010) used the Gibbs free energy of the biogas slurry to convert this co-product from kg to joules. Assuming that this Gibbs free energy value is similar to the one for the EARTH digester effluent, the energy of the co-products was calculated 1.34E+10 sej/year resulting in a joint transformity of the 5.09E+04 sej/J. When the co-products are included in the analysis, the EARTH University digesters still have a lower joint transformity than the systems studied by Zhou et al. (2010) and Wei et al. (2009). This indicates that the EARTH University anaerobic digesters are producing more energy for relatively less emergy inputs. 4.5. Transformity comparisons with other energy production systems Other forms of energy production have transformities similar to the biogas transformity at EARTH University (5.23E+04 sej/J) (Table 4). Transformities 4.87E+04 sej/J and 3.15E+05 sej/J have been reported for ethanol from sugarcane (Pereira and Ortega, 2010; Giampietro and Ulgiati, 2005). The transformity of biodiesel was 3.90E+05 sej/J from soybeans and 2.31E+05 sej/J from sunflower seeds (Giampietro and Ulgiati, 2005). These comparative transformity values illustrate that a number of different renewable energy systems can be utilized and yield similar ratios of required emergy for each energy output. Due to the diversity of renewable energy production systems and their unique feedstock inputs, combined with a shortage of published emergy analyses, further studies are needed to improve the evaluation of the sustainability of the various renewable energy production systems. Since biogas has the same potential uses as non-renewable natural gas, natural gas values were compared with methane biogas derived from systems that rely on renewable inputs. Odum (1996) calculated a transformity for natural gas use in boilers of 4.80E+04 sej/J. This is the value that is most commonly used for natural gas in emergy studies (Bastianoni et al., 2005) Bastianoni et al. (2005) determined the transformity of “petroleum natural gas” to be 4.35E+04 sej/J which is less than the biogas transformities calculated from studied by Wei et al. (2009) and Zhou et al.

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Table 4 Comparison of the transformities of different forms of energy. System

Product

Transformity (sej/J)

Reference

Sugarcane, Brazil Solar Coal Petroleum natural gas Natural gas Natural gas EARTH University Crude oil Oil Solar Energy crops Coal power plant Sunflower seed Dairy farm, Puerto Rico Farm biogas project Soybean Dairy farm, Puerto Rico EARTH University FIOPS

Ethanol Heat Coal Methane Methane Methane Biogas Oil Oil Electricity Methane Electricity Biodiesel Methane Biogas Biodiesel Electricity Electricity Biogas

4.87E+04 1.58E+04 4.00E+04 4.35E+04 4.80E+04 5.22E+04 5.32E+04 5.40E+04 5.42E+04 8.92E+04 8.94E+04 1.60E+05 2.31E+05 2.48E+05 2.69E+05 3.90E+05 1.19E+06 1.59E+06 2.04E+06

Pereira and Ortega (2010) Paoli et al. (2008) Odum (1996) Bastianoni et al. (2005) Odum (1996) Bargigli et al. (2004) This study Odum (1996) Bastianoni et al. (2005) Paoli et al. (2008) Jury et al. Odum (1996) Giampietro and Ulgiati (2005) Bastianoni and Marchettini (2000) Zhou et al. (2010) Cavalett and Ortega (2010) Bastianoni and Marchettini (2000) This study Wei et al. (2009)

(2010). However, this transformity was calculated for natural gas formation rather than usage and it does not take into account the additional emergy required for extraction and distribution. The increase in natural gas transformity due to factors such as extraction, refining and distribution have not been widely evaluated in the emergy literature. Odum’s (1996) transformity for natural gas use in boilers was not an analysis of a particular system. The transformity was calculated from the coal transformity based on a 20% greater efficiency of natural gas compared to coal use in boilers. One study (Bargigli et al., 2004) did investigate “natural gas extraction form North Sea off-shore platforms and its refining and transport to the final user.” These authors reported a solar transformity (w/o economic inputs) of natural gas at 5.22E+04 sej/J. This value is somewhat greater than the 4.35E+04 sej/J for natural gas formation as reported by Bastianoni et al. (2005). Both values are less than the EARTH University digesters, which means that natural gas currently has a slightly higher “energy return on investment” (Hall et al., 2008) than biogas. Because natural gas is a non-renewable resource, there is a need to develop more sustainable methods of providing this form of energy. Bentley (2002) estimated that society is approximately half-way to peak worldwide production of natural gas, after which production would decline rapidly. As non-renewable resources such as oil and natural gas become scarcer, the remaining amounts become increasingly more expensive and more energy must be used for discovery and extraction (Hall et al., 2008). From an emergy standpoint, this would mean that increasing amounts of purchased inputs would be needed to locate and extract the remaining reserves resulting in an increasing transformity for natural gas with time. At some point it is likely that biogas production, through more sustainable processes such as anaerobic digestion, will produce a product with a lower transformity than natural gas. Also, unlike natural gas extraction, the production of biogas from anaerobic digestion provides the added benefit of treating wastes and producing additional products such as inorganic nitrogen and phosphorous. In addition, aspects such as global warming potential were not taken into account in this study. Unlike the combustion of natural gas, carbon dioxide (CO2 ) released from the combustion of biogas does not directly lead to increased atmospheric CO2 levels. This is due to the fact that the carbon in biogas was fixed from atmospheric CO2 by the plants that were used to feed the livestock. Instead, anaerobic digesters are known for reducing global greenhouse gas emissions. Standard agricultural manure management

practices include storing manure in an anaerobic lagoon, which releases methane to the environment. By capturing and combusting methane-enriched biogas in an anaerobic digestion system, the methane, a greenhouse gas 21 times more powerful than CO2 , is converted to CO2 , the same CO2 fixed by the plants fed to the animals producing the manure. However, the construction operation and maintenance of an anaerobic digester does require some fossil energy inputs (Ishikawa et al., 2006). 4.6. Emergy ratios and indices The emergy ratios and indices for biogas production and electricity generation from the biogas showed that conversion of biogas to electricity resulted in a reduction in the relative sustainability of the system. This was due to the greater amount of purchased inputs required to construct and maintain the hydrogen sulfide scrubbing tower and 40 kW generator and the energy lost when biogas is converted to electricity. This in part is due to heat losses in the generation of electricity, which is a process that is about 30% efficient (Jeong et al., 2009). The benefit of transforming the biogas to electricity is the production of higher transformity and higher quality energy source, as electricity has more utility than biogas. Whether or not this investment is worthwhile depends on the context of the biogas generation. A facility such as EARTH University has the resources to invest in the necessary inputs for electricity generation and has on-farm demands for electricity. In contrast to EARTH University, the majority of the 1000 small farmers in Costa Rica that have Taiwanese model digesters installed on their farms use the biogas directly for cooking (Lansing et al., 2008a). A more sustainable system with less environmental stress would be obtainable at EARTH with a more appropriately sized generator. Lansing et al. (2008b) reported that a 15 kW generator would have cost $25,000 less than the 40 kW generator that is currently in use and would perform more efficiently than the larger generator for the relatively small amount of daily biogas produced. Another way to increase the sustainability of electricity generation from biogas would be to use a combined heat and power generator that captures some of the waste heat which can be used to heat the digester, thereby increasing biogas production efficiency. Through an emergy analysis, comparison between the relative sustainability of the EARTH University digesters and the current

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published emergy studies on anaerobic digesters: the FIOPPS (Wei et al., 2009) and the farm biogas project (Zhou et al., 2010) can be conducted. The ESI for the EARTH University system was 5.67 as compared to 5.74 for the FIOPPS, indicating a similar level of sustainability. However, the ESI calculated in this study was far below the ESI value of 50.13 calculated by Zhou et al. (2010). This comparison highlights the need to look beyond the ratios and indices when comparing the sustainability of different production systems with emergy analysis. The assumptions used in the emergy analysis can have a large effect on the values of the ratios and indices. For example, Zhou et al. (2010) considered all inputs of manure, urine, flushing sewage and human labor as entirely renewable inputs. Had these authors subdivided these inputs between renewable and purchased as was done in this study; their calculated ESI would have been lower. For this reason, it would be advantageous to develop more standard methods for conducting emergy analysis, in particular a method for subdividing inputs between different categories.

5. Conclusions Emergy analysis is an appropriate method to evaluate the sustainability of alternative energy production, because it accounts for all inputs to the system. Most energy production systems rely on both renewable and non-renewable inputs and an emergy analysis compares them all on a common basis. This study highlights the reduction in sustainability of an anaerobic digestion system when biogas is used to generate electricity, reflected in the decreasing ESI as well as the increasing transformity value of electricity produced from biogas. However, the generation of electricity does produce a product with higher energy quality and increased utility. Whether or not the conversion to electricity and the subsequent loss of sustainability is justified is a decision that needs to be made by the beneficiaries of the production system. In the case of EARTH University, the resources to generate electricity and the need to power the milking operations makes the conversion to electricity a logical use of the biogas. Also, the absence of a large-scale use for direct burning of the biogas at the EARTH University farm makes the conversion to electricity a more appropriate choice for the energy produced. The sustainability of electricity generation from biogas can be increased by maximizing engine conversion efficiencies, while minimizing the associated costs. In the case of EARTH University, this could be accomplished by using a more appropriately sized generator for the farm. Additional studies of biogas production systems in other countries should be conducted to determine the range of values for the emergy indices and ratios across a broad range of feedstocks, digester designs and climatic regimes, which would allow for a greater understanding of the variation between different settings and assist in designing of the most efficient and sustainable biodigestion systems. While this study focused on the operation and performance of a biogas system, it is also recommended that future analyses be conducted at the farm and regional scale to assess the impact of these digesters at larger scales. In addition, more emergy studies are needed that focus on the raw materials used in other renewable energy production systems, which would provide a broader selection of appropriate transformities for use in subsequent emergy studies. In the face of increasing human energy demands, depleting reserves of fossil fuels, and the apparent sustainability of anaerobic digestion systems, it is recommended that the small-scale production of biogas in Costa Rica should be expanded beyond the 1000 systems that were in operation in 2008.

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Acknowledgments This study was made possible by the contributions from the following individuals at EARTH University: Dr. Raúl Botero, Joaquín Víquez and Helen Martínez. Support was also given by The Ohio State University and the Department of Food, Agriculture and Biological Engineering and the University of Maryland’s Department of Environmental Science and Technology as well as the contributions to this research from the Ohio State University’s Targeted Investments in Excellence-Carbon, Water and Climate grant. Appendix A. 1. Sunlight Area: (21 m × 2.55 m) + (21 m × 1.59 m × 2) = 120.3 m2 Insolation: 5.61 kWh/m2 /day (NASA, 2010a) Albedo: 0.35 (NASA, 2010b) Albedo correction: 5.61 kWh/m2 /day × (1 − 0.35) = 3.65 kWh/m2 /day = 1.31E+07 J/m2 /day Energy input = 1.31E+07 J/m2 /day × 120.3 m2 × 365 days/year = 5.77E+11 J/year 2. Water Volume input = 6.7 m3 /day × 365 days/year = 2.45E+03 m3 /year 3. Manure Mass of dairy manure: 0.083 × (60 cows × 43,100 g/cow/day × 365 days) = 7.83E+07 g/year Mass of swine manure: 0.800 × (50 swine × 4200 g/swine/day × 365 days) = 6.13E+07 g/year Total mass of manure: 7.83E+07 g/year + 6.13E+07 g/year = 1.40E+08 g/year Total renewable fraction: 0.68 × 1.40E+08 g/year = 9.50E+07 g/year Total purchased fraction: 0.32 × 1.40E+08 = 4.47E+07 g/year 4. Labor Digester construction: 4 farm workers: 15 h each × 4 = 60 h 1 professor: 8 h 1 electrical consultant: 15 h Biogas production: (60 + 8)/20 = 3.4 h/year Electricity generation: (60 + 8 + 15)/20 = 4.2 h/year Digester operation and maintenance: 13 h per month × 12 months/year = 156 h/year Total labor input (biogas production): 156 h/year + 3.4 h/year = 159.4 h/year Total labor input (electricity generation): 156 h/year + 4.2 h/year = 160.2 h/year Total renewable fraction (biogas production): 159.4 h/year × 0.68 = 108.4 h/year Total purchased fraction (biogas production): 159.4 h/year × 0.32 = 51.01 h/year Total renewable fraction (electricity generation): 160.2 h/year × 0.68 = 108.9 h/year Total purchased fraction (electricity generation): 160.2 h/year × 0.32 = 51.25 h/year 5. Electricity Water pump (1 hp): 1.53E+06 s/year × 746 J/s = 1.14E+09 J/year Biogas and scrubbing tower pumps (1 hp): 1.88E+06 s/year × 746 J/s = 1.40E+09 J/year Energy input (biogas production): 1.14E+09 J/year Energy input (electricity generation): 1.14E+09 J/year + 1.40E+09 J/year = 2.54E+09 J/year 6. Plastic Digester and storage bags (biogas production): 1.27E+05 g Digester and storage bags (electricity generation): 1.57E+05 g Replacement rate (1 bag every five years): 3.92E+03 g/year Biogas scrubbing tower (1100 L): 1.05E+05 g Mass of plastic (biogas production): (1.27E+05 g/20 years) + (3.92E+03 g/year) = 1.01E+04 g/year Mass of plastic (electricity generation): (1.57E+05 g/20 years) + (3.92E+03 g/year) + 1.05E+05 g/20 years) = 1.70E+04 g/year 7. Zinc Roofing material galvanized steel. Assume 16 oz/ft2 (MRS, 2010) Assume G90 coating: 0.9 oz Zn/ft2 = 25.5 g Zn/ft2 = 2.73E+02 g Zn/m2 (ASTM, 2010) Area of dairy digester building roof: 198.4 m2 Area of swine digester building roof: 182.4 m2

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Area of generator building roof: 36.75 m2 Biogas production: Roof area: 198.4 m2 + 182.4 m2 = 380.8 m2 Mass of Zn: 380.8 m2 × 2.73E+02 g Zn/m2 = 1.04E+05 g Zn Annual mass input: 1.40E+05 g Zn/20 years = 5.20E+03 g Zn/year Electricity generation: Roof area: 198.4 m2 + 182.4 m2 + 36.75 m2 = 417.6 m2 Mass of Zn: 417.6 m2 × 2.73E+02 g Zn/m2 = 1.14E+05 g Zn Annual mass input: 1.14E+05 g Zn/20 years = 5.70E+03 g Zn/year 8. Steel Roofing material galvanized steel Assume 16 oz/ft2 (MRS, 2010) Assume G90 coating: 15.1 oz steel/ft2 = 428 g steel/ft2 = 4.58E+03 g steel/m2 (ASTM, 2010) Area of dairy digester building roof: 198.4 m2 Area of swine digester building roof: 182.4 m2 Area of generator building roof: 36.75 m2 Biogas production: Roof area: 198.4 m2 + 182.4 m2 = 380.8 m2 Mass of steel: 380.8 m2 × 4.58E+03 g steel/m2 = 1.74E+06 g steel Annual mass input: 1.74E+06 g steel/20 years = 8.72E+04 g steel/year Electricity generation: Roof area: 198.4 m2 + 182.4 m2 + 36.75 m2 = 417.6 m2 Mass of steel: 417.6 m2 × 4.58E+03 g steel/m2 = 1.91E+06 g steel Annual mass input: 1.91E+06 g steel/20 years = 9.56E+04 g steel/year 9. PVC 2 schedule 40 PVC pipe: 0.68 lb/ft = 1.01E+03 g/m (TET, 2010a) Biogas production: 63 m 2 PVC pipe × 1.01E+03 g/m = 6.38E+04 g PVC pipe 6.38E+04 g PVC pipe/20 years = 3.19E+03 g/PVC year Replacement rate: 25% of pipe every five years (assume 4 replacements in 20 years) (4 × (0.25 × 3.19E+03 g/year))/20 = 1.59E+02 g PVC Total annual mass input: 3.19E+03 g/PVC + 1.59E+02 g/PVC = 3.35E+03 g PVC/year Electricity generation: 65 m 2 PVC pipe × 1.01E+03 g/m = 6.58E+04 g PVC pipe 6.58E+04 g PVC pipe/20 years = 3.29E+03 g/PVC year Replacement rate: 25% of pipe every five years (assume 4 replacements in 20 years) (4 × (0.25 × 3.29E+03 g/year))/20 = 1.65E+02 g PVC Total annual mass input: 3.29E+03 g/PVC + 1.65E+02 g/PVC = 3.46E+03 g PVC/year 10. Concrete Assumed concrete density: 2400 kg/m3 (Portland, 2010) Surface area of generator building: 36.75 m2 Assumed foundation thickness (1 ft): 0.3048 m Concrete volume (foundation): 36.75 m2 × 0.3048 m = 11.2 m3 Concrete mass (foundation): 11.2 m3 × 2400 kg/m3 = 2.69E+04 kg = 2.69E+07 g Assumed wall dimensions (0.5 ft thick and 8.0 ft tall): 0.1524 m thick and 2.44 m tall Concrete volume (walls): 4 × (6.1 m × 0.1524 m × 2.44 m) = 9.07 m3 Concrete mass (walls): 9.07 m3 × 2400 kg/m3 = 2.18E+04 kg = 2.18E+07 g Concrete mass in generator building: 2.69E+07 g + 2.18E+07 g = 4.87E+07 g Surface area of cylindrical scrubbing tower platform: 2 m2 Height of cylindrical scrubbing tower platform: 1 m Concrete volume (tower platform): ␲ × (0.8)2 × 1 = 2.0 m3 Concrete mass (tower platform): 2.0 m3 × 2400 kg/m3 = 4.80E+03 kg = 4.80E+06 g 11. Machinery Cost of 40 kW generator: 40,000 USD Annual USD input: 40,000 USD/20 years = 2000 USD/year 12. Wood Area of dairy digester building: 198.4 m2 Area of swine digester building: 182.4 m2 Length and width of each building (assume square structures): Dairy digester building: 14.09 m Swine digester building: 13.51 m Support beams: Volume of wood in support beams (4 per building - 4 × 4 × 8 ): 8 × (0.1016 m × 0.1016 m × 2.44 m) = 0.2015 m3 Density of oak wood: 750 kg/m3 (TET, 2010b) Mass of wood in support beams: 0.2012 m3 × 750 kg/m3 = 151 kg = 1.51E+05 g Roof frame: Assume constructed with 2 × 4 boards (0.0508 m × 0.1016 m) Framing boards assumed every one meter with length same as building sides:

Dairy digester building roof: 14 boards = 197.26 m Swine digester building roof: 13 boards = 175.63 m Total length of boards in both roofs: 197.26 m + 175.63 m = 372.89 m Total volume of wood in roof frames: (0.0508 m × 0.1016 m × 372.89 m) = 1.92 m3 Total mass of wood in roof frames: 1.92 m3 × 750 kg/m3 = 1443 kg = 1.44E+06 g Bamboo side rails: Assumed density of bamboo poles: 300 kg/m3 (TET, 2010b) Assumed dimensions of bamboo poles: 1 m length × 0.0508 m diameter Volume of each bamboo pole: ␲ × 0.0026 × 1 = 2.03E−03 m3 Number of bamboo poles: 2165 Total mass of bamboo poles: 2156 poles × 2.03E−03 m3 × 300 kg/m3 = 1317 kg = 1.32E+06 g Total mass of wood and bamboo: 1.15E+05 g + 1.44E+06 g + 1.32E+06 g = 2.91E+06 g Annual mass input of wood and bamboo: 2.91E+06 g/20 years = 1.45E+05 g/year 13. Biogas Average daily biogas production: 33.5 m3 /day Average methane percent in the biogas: 64.8% Average daily methane yield: 33.5 m3 /day * 0.648 = 21.7 m3 /day Energy content of methane: 1012 BTU/ft3 × 1055 J/BTU × 35.31 ft/m3 = 3.77E+07 J/m3 (Hearth, 2010) Combined biogas yield of dairy and swine digesters: 21.7 m3 /day × 365 day/year × 3.77E+07 J/m3 = 2.99E+11 J/year 14. Electricity Generator conversion efficiency: 1 kWh/2.2 m3 biogas = 0.4545 kWh/m3 biogas Annual electricity generation: 33.5 m3 biogas/day × 365 day/year × 0.4545 kWh/m3 biogas = 5.56E+03 kWh/year 15. Ammonia nitrogen NH4 -N produced from the dairy digester: 73 g/day × 365 day/year = 2.66E+04 g/year NH4 -N produced from the swine digester: 440 g/day × 365 day/year = 1.61E+05 g/year Total mass of NH4 -N produced per year: 2.66E+04 g/year + 1.61E+05 g/year = 1.87E+05 g/year 16. Phosphate phosphorous Dairy wash water usage per day: 2.2 m3 /day × 1000 L/m3 × 365 day/year = 8.03E+05 L/year Average dairy digester effluent concentration: 6.5 mg PO4 -P/L Annual PO4 -P produced from dairy digester: 6.5 mg PO4 -P/L × 8.03E+05 L/year = 5.22E+06 mg PO4 -P/year = 5.22E+03 g PO4 -L/year Swine wash water usage per day: 4.5 m3 /day × 1000 L/m3 × 365 day/year = 1.64E+06 L/year Average swine digester effluent concentration: 10.2 mg/L Annual PO4 -P produced from dairy digester: 10.2 mg PO4 -P/L × 1.64E+06 L/year = 1.68E+07 mg PO4-P/year = 1.68E+04 g PO4 -P/year Total mass of PO4 -P produced per year: 5.22E+03 g PO4 -P/year + 1.68E+04 g PO4 -P/year = 2.20E+04 g PO4 -P/year

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