سیستم گوارش بی هوازی (ADS) برای مزارع لبنی متعدد : تجزیه و تحلیل GIS برای انتخاب محل بهینه

While anaerobic digester systems (ADS) have been increasingly adopted by large dairy farms to generate marketable energy products, like electricity, from animal manure, there is a growing need for assessing the feasibility of regional ADS for multiple farms that are not large enough to capitalize their own ADS. Using geographical information system (GIS) software, this study first identifies potential sites in a dairy region in Vermont, based on geographical conditions, current land use types, and energy distribution infrastructure criteria, and then selects the optimal sites for a given number of ADS, based on the number of dairy farms to be served, the primary energy input to output (PEIO) ratio of ADS, and the existing transportation network. This study suggests that GIS software is a valid technical tool for identifying the potential and optimal sites for ADS. The empirical findings provide useful information for assessing the returns of alternative numbers of ADS in this region, and the research procedures can be modified easily to incorporate any changes in the criteria for this region and can be applied in other regions with different conditions and criteria.

Anaerobic digester systems (ADS) have been adopted by more and more large dairy and livestock farms in the United States and many other nations in the past decade as a technology for generating marketable energy products, such as electricity and natural gas, from animal and food processing waste (Wang et al., 2011). Despite rising interest in ADS, adoption rates remain low compared to the number of dairy and swine farms in the United States that produce organic by-products suitable for ADS. According to the U.S. Environmental Protection Agency (EPA) (2012), there were 176 operating ADS on U.S. farms in 2011, producing about 541 million kilowatt-hours (kWh) of energy. It is estimated there are 81241 swine and dairy farms that are feasible candidates for ADS (United States Environmental Protection Agency, 2010). In addition to these farms, there are many smaller animal farms that could potentially use ADS to convert animal manure into marketable energy products. According to 2007 U.S. Agricultural Census data, there were a total of 1,083,022 animal farms in the United States with beef cattle, dairy cows, hogs and pigs, layer hens, and other animals (USDA, 2009). ADS are based on biological methanogenesis, a natural degradation process performed by microorganisms in numerous environments such as animal intestines and landfills (Chynoweth et al., 2001). ADS apply the methanogenic process to biological waste streams, including animal manure, municipal solid waste, grease sludge, spoiled animal feed, and other organic by-products, to generate biogas and other products such as compost and animal bedding. Biogas consists of 55–80% methane, and the remaining portion is primarily composed of CO2, hydrogen sulfide (H2S), and water (Lusk et al., 1996). 1.1. Inputs, outputs and revenue of ADS According to EPA (2012), most of the on-farm ADS in the United States have used methane to generate electricity to meet the farm needs, and farms then supply the excess electricity to the grid to serve household and business needs. It is also technically feasible to inject biogas into natural gas pipelines or use it in farm or transport vehicles (Hansen et al., 2007), but this requires that the biogas be scrubbed of impurities such as CO2, H2S, and water at additional system cost (Lusk et al., 1996). To illustrate the potential use of biogas in transport vehicles, a 10-farm cooperative in Indiana with six digesters has adapted their tractor trailer milk hauling fleet, carrying 300,000 gallons of milk a day, to be fueled by compressed biomethane from the digesters (Callahan, 2011). ADS also provide additional useful products to farms and businesses. The liquid by-product from the digesters contains all of the same basic nutrients as the manure and is spread or injected on nearby cropland. The digester process heats the manure to a temperature that kills many pathogens and after the digestion, the slurry is run through a screw-press separator, removing the solids from the liquid to provide materials that have been used as animal bedding and compost that is sold to farms, gardeners, landscapers, and homeowners. The heat from the generator can also be used to heat water for washing milking equipment and to warm the farmhouse or barns in the winter (Wang et al., 2011). In addition to the marketable energy products and additional economic benefits mentioned above, ADS provide many environmental benefits. First, they reduce the potency of greenhouse gases (GHG) released into the atmosphere as a result of animal rearing by capturing and combusting methane, a GHG that has 21 times as much atmospheric warming potential as CO2 (EPA, 2010 and Pöschl et al., 2010). According to a recent estimate by the EPA (2012), the on-farm ADS in the United States prevented the direct emission of about 1.2 million metric tons of CO2 equivalent in 2011. Second, the combustion of biogas to generate electricity for the grid displaces the use of fossil fuels for energy generation and thus contributes to additional emission reductions of GHGs and other air pollutants (EPA, 2012) of the grid. Pöschl et al. (2010) estimated that, for every kWh of electricity produced from on-farm ADS, there is a net reduction of 414 g of CO2 emissions. Third, the ADS significantly reduce the odors associated with manure storage and spreading and may reduce nutrient load runoff into rivers and lakes by providing an improved option for waste management. Liquid manure that has had the fibers removed is more easily incorporated into the soil, thereby reducing potential runoff of pooled manure on the ground surface. It has been estimated that, if all animal manures on U.S. farms were treated through anaerobic digester systems (ADS), the resulting biogas could generate nearly 1 quad (1015 BTU or 1.055×1018 J) of energy each year, or 1.8% to 3% of annual U.S. electricity consumption (Cuéllar and Webber, 2008). However, it is a great challenge to expand ADS to more and more farms, especially small and medium farms. Besides animal manure from farms, there is great potential to convert organic by-products from food processing facilities into energy products via ADS, and some of the alternative wastes may in fact increase the biogas production from animal manure. Research has shown that adding whey, a by-product of cheese production, to on-farm ADS can increase biogas production from cow manure when steps are taken to maintain pH levels above 5.7 during the methanogenic stage of digestion (Ghaly, 1996 and Kavacik and Topaloglu, 2010). It is an accepted practice for cheese producers in Vermont is to pay a small fee to farms with ADS, known as a tipping fee, for dumping their whey into the ADS. The whey would otherwise need to be treated in an energetically consumptive manner at a wastewater treatment site at greater cost. Farms with ADS garner a small amount of revenue from the tipping fees and have also noted increases in biogas production. 1.2. Public policy and financial support for ADS The environmental and energy production benefits of ADS have spurred financial assistance from the U.S. Department of Agriculture (USDA), which has partially funded the installation of commercially proven livestock to waste ADS. Energy and conservation programs that have provided financial and technical support to on-farm ADS are presented in Table 1 (USDA – AgSTAR, 2012).Despite the benefits and planning and financial support of ADS in the United States, adoption rates have remained low compared to available sites for ADS and most interest has focused on large-scale systems. A study of ADS implementation in the United States found there to be no operating ADS on dairy farms with fewer than 400 cows (Lazarus and Rudstrom, 2007). Farm size has been found to affect the interest level of farm operators in ADS installation, with those on smaller farms having less interest in the technology, a result of the high capital expense of installation and the economies of scale typically regarded as necessary to support ADS (Swindal et al., 2010). The connection between ADS technology and large scale is argued to be largely a social construction promoted by its incorporation into the debates over agricultural industrialization (Welsh et al., 2010). The close association between ADS and large farms in the United States results primarily from government policy and programs that favor large-scale operations, as all the operating ADS are heavily subsidized by government agencies (Wang et al., 2011). ADS, it is argued, should instead be considered scale neutral, as they have been adopted by different sizes of operations, from small farms in China and the developing world, to large-scale industrial farms in the United States and Europe (Welsh et al., 2010). The focus of ADS investment at large farms in the U.S. can be viewed largely as a result of engineering and economic studies during the early phases of ADS farm adoption in the 1970s that found economies of large scale necessary to support adoption (Penn State Extension, 2013) and these perceptions have persisted into the research, development, and financial support systems of present ADS policy. Installing the ADS on large-scale farms alone will not realize the energetic potential from the majority of animal manure in the United States because there are so many small and medium farms across the country. For example, in the state of Vermont, there are a total of 1002 dairy farms and of these, 95.6% have fewer than 500 cows. Together these smaller farms account for 68.2% of the total number of cows in the state (United States Department of Agriculture, 2009). This pattern is similar across the United States, where a total of 66,606, or 95.3%, of dairy farms house fewer than 500 cows each, together accounting for 53.3% of the total number of dairy cows in the nation (USDA, 2007). The on-farm ADS have been primarily limited to large farms for at least two reasons. First, larger farms with a greater number of animals are more likely to be able to obtain capital from grants, loans, and their own funds to make the huge investment in ADS. For example, all the Vermont dairy farms that installed ADS between 2005 and 2008 had more than 700 cows, and their average initial investment on a system that generates electricity was $2.2 million (Wang et al., 2011). Second, the industrial research and development of ADS has focused on large farms because of the efficiencies of scale and lack of demand for small digesters. The revenue from ADS results mainly from the sales of electricity and the by-products of digestion, but output, and the resultant cash flow, are directly determined by the quantity of manure feeding into the system. The capital investment requirement of ADS, limited research and development of small-scale ADS, and the costs of grid interconnection have proven to be barriers to financial feasibility for smaller farms, but Swindal et al. (2010) suggest that one possible means to overcome these barriers is the construction and operation of community digesters, or ADS facilities accepting manure from multiple farms. Case studies and the experiences of nations in the European Union indicate that community ADS are technically and economically feasible. The European nations of Germany, Austria and Denmark produce the largest share of biogas on agricultural plants, with Germany leading production with over 4000 farm based biogas plants (Palela and Socaciu, 2012). Nearly half of the substrates used in German biogas plants are renewable raw materials, or field crops specifically grown for use in biogas plants (Palela and Socaciu, 2012). The Lemvig Biogas facility in Denmark is 100% privately owned by a cooperative of 69 local farmers and receives slurry from 75 farms and a variety of other producers of organic by-products suitable for co-digestion. The plant has been in operation since 1992, processes 500 t of manure and 120 t of organic by-products per day, produces 21 million kWh of electricity annually, and has stable economic performance that is comfortably better than break even (Task 37, 2013). The Ribe Biogas Plant in Denmark has been in operation since 1990, producing biogas for combined heat and power (CHP), and simultaneously solving agricultural, veterinary, and environmental problems in the area. Ribe is a farmer owned plant that provides 5.5 million cubic meters (1.9 million ccf) of biogas to the Ribe Fjernvarme CHP-plant, providing heat for 1000 homes and electricity for 1700 homes in the city of Ribe. The average transportation distance for organic materials entering the Ribe Biogas Plant is 15 km (9.3 miles) (Task 37, 2012). The transportation of animal manure and organic by-products from farms and other points of origin to centralized ADS will inevitably add to the amount of slow, heavy transportation vehicles on local roadways. Hadrich et al. (2010) examined the economics, including labor and transportation time, for dairy farms to transport liquid manure for land application. The study found that on average 175-cow dairy farms transported 1.53 million gallons of manure a year, primarily using 3000 gallon tank spreaders and a 120 horse power tractor. Farms as large as 1400 dairy cows were examined, and found to use significantly larger transportation vehicles, but it is expected that farms of this scale will likely capitalize ADS on-site, while smaller farms more similar to the 175 head will most frequently be transporting manure to community ADS. The average hauling rate of the 175-head farms for 1 mile (1.6 km) was 10,000 gallons per hour and 18 days were required to complete the task per year. Dairy farms of this size will generate about 4000 gallons of liquid manure per day, requiring about 1.3 trips using a transport vehicle with 3000 gallons of capacity. The additional transportation burden from these activities will be most concentrated near community based ADS and the impacts on roads, road users and rural areas should be examined during the planning phase of proposed projects. Due to data limitation, the quantitative impacts of manure transportation on local traffic and roadways are not included in this paper. 1.3. Policy implications As indicated in Table 1, there are substantial Federal programs in place to provide financial support for planning, technical project management and capitalization of ADS. Results from Wang et al. (2011) suggest that state and local support of ADS are also critically important. States that have seen the greatest adoption of ADS are those that also have some of the highest renewable energy goals. A major conclusion from the administrator of the Vermont Cow Power program is that the 50% and greater failure rates of ADS in the 1970s stemmed from support programs that helped to finance facility construction, but did not provide ongoing support to aid with operation and maintenance. This resulted in ADS cessation by all but those who perceived significant benefits from the waste and odor management properties of ADS, and thus kept going in the face of maintenance and upgrade expenses (Raker, 2011). State level legislation of feed-in-tariff (FIT) rates for the renewable energy generation of ADS has been shown to be crucial in sustaining ADS operations, providing stable energy revenue sufficient to realize positive cash flow. The state of Vermont experienced a resurgence of ADS installation beginning in 2004. The energy rate received by farms with ADS was established at the standard wholesale rate for energy, which fluctuates regularly. In 2008, the energy rate received to farms fell over 50% to below $0.04 per kWh, jeopardizing the economic feasibility of all operating ADS. State legislation was quickly established to provide an energy rate structure at an interim price of $0.08 per kWh, and ultimately a FIT rate of $0.141 (Wang et al., 2011). Local supports from community members and energy customers have provided significant support to dairy farms operating ADS. In the state of Vermont, more than 3000 electricity customers have voluntarily signed up for the Green Mountain Power (GMP) Cow Power program, in which participating customers pay a $0.04 per kWh premium for electricity generated from cow manure on dairy farms for 25%, 50%, or 75% of their electricity consumption (Wang et al., 2011). The first Cow Power biogas facility came online in 2005 and as of February 2013, there are 12 dairy farms participating in the program (Green Mountain Power, 2013). The increased adoption of ADS is due to increased government investment in renewable energy, increased consumer demand for locally sourced renewable energy products, and technical and financial support from regional electrical utility providers. While there are favorable signs for increasing the utilization of biogas from animal manure on U.S. farms and other organic by-products, the overall rate of adoption is very low. There are three major pathways for more animal farms to use and benefit from ADS. The first is the continued expansion of ADS on large farms through significant efforts and collaboration among manufacturers, research institutions, government agencies, large farms and farm organizations, and utility companies. The second pathway is to develop small-scale ADS for small and medium farms through research and technical transfer. For example, small-scale and low-cost ADS have been installed on extremely small dairy farms in China, and such ADS have the potential to be adopted by small animal farms in the United States. The third pathway is to develop centralized or regional ADS to serve multiple farms. This study is motivated by the potential of the third pathway, the lack of information and studies on its feasibility, and related questions such as how to select optimal sites for centralized ADS in a specific region. Although there are potential economies of scale for the centralized digester, manure transportation and handling costs can offset the economic savings if there are not sufficient suitable dairies willing to participate in close proximity to the proposed facility (ESA, 2011). Therefore this exploratory study will specifically present a logical framework for guiding the process of identifying potential ADS sites, selecting the optimal sites for a given number of ADS using GIS software, report the results of a case study for Addison County in Vermont, and finally summarize the major conclusions. The research procedures based on GIS software and the empirical results of the optimal sites for a wide range of ADS are expected to shed some light on the potential for and feasibility of ADS for multiple farms and to provide useful information to community planners, government agencies, utility companies, dairy farms, extension specialists, and others who are interested in centralized ADS for multiple farms.

ADS on animal farms can reduce manure odors and GHG emissions, generate renewable and low-carbon electricity, produce valuable by-products, and provide an additional source of farm revenue, but the construction of ADS requires significant capital investment and has been concentrated on large farms in the United States (Wang et al., 2011). This study is motivated by the growing need to assess the feasibility of regional ADS for multiple dairy farms that are not large enough to capitalize their own ADS. Such regional ADS located on appropriate sites can utilize cow manure from multiple dairy farms to produce electricity and other products such as animal bedding materials, compost, liquid fertilizer, and heat. This paper has presented a methodology and case study for identifying suitable areas and potential sites for regional ADS in Addison County, a dairy region in Vermont, and for selecting the optimal sites for 1 to 15 ADS in this region. The analysis provides useful information for assessing the feasibility and returns of alternative numbers of ADS in the region, and the framework can be easily modified and applied to other regions with different criteria for suitable areas, potential sites, and optimal sites. Findings from this study suggest four major conclusions. First, it is technically feasible to adapt commercially available GIS software to the task of identifying optimal sites for regional ADS that will use animal manure from multiple farms to generate electricity and other energy products. GIS software provides a valuable technical tool for identifying sites with conditions suited to ADS that would be accessed by multiple dairy farms and for evaluating these sites for optimization to maximize the energy return of the systems. Second, in the case study region of Addison County, Vermont, the analysis indicates that increasing the number of ADS sites, within the range of 1 to 6 regional ADS, increases the total number of farms that can be serviced and that, beyond 6 ADS, the number of total dairy farms to be served will not change. Major benefits of more than 6 ADS are the reduction in the average manure transport distance and the number of farms to be served by each site. However, both of these benefits decrease as the number of regional ADS increases, suggesting that it is possible to determine the optimal number of regional ADS sites by calculating the point at which the marginal benefit of one additional ADS site is greater or equal to the marginal cost of constructing that additional ADS site. Further analysis of the trade-offs between construction, operation, and transportation expenses will be vital in the ultimate determination of the optimal number of community ADS for a given region. Third, it is likely that the success or failure of regional ADS will hinge on the ability of the system to take advantage of synergistic relationships between ADS operations and the farms that produce manure. For example, systems that rely on petroleum to fuel manure transport vehicles will ultimately have system profitability strongly tied to the price of diesel fuel, but systems that convert a portion of the biogas to compressed natural gas (CNG) and use this in CNG-fueled transport vehicles may achieve a closed loop transport system in which a portion of the energy output is used to operate the system and the remainder is the net gain available for sale or other uses. The scrubbing of biogas impurities to allow it to be used in vehicles and storage at fueling depots are additional costs that need to be evaluated for this type of system, but market precedents for this model exist for study. Also, dairy farms have found significant value in the by-products of the digestion process of generating biogas. The dry matter from the ADS has been used as animal bedding, which offsets the need to purchase and transport other materials such as woodchips. The liquid by-product is used as a fertilizer and is applied to crop fields. Farms accessing community ADS will have an economic incentive to recover these materials and transportation systems will need to support not only moving manure from farms to the ADS but also moving the dry-matter bedding and liquid effluent back to the farm. Fourth, although regional ADS serving multiple farms have not been implemented in Vermont because of the real and perceived challenges and expenses associated with transporting manure, this may change if a market price is applied to manure that reflects its use as a resource in ADS that generate energy and other marketable products rather than a waste requiring disposal. For example, the operational revenue reported by Wang et al. (2011) for on-farm ADS in Vermont is $1263 per day for a 1212-cow operation, or $1.04 per cow per day. To put this in context, a 200-cow dairy farm will produce about 4588 t of dairy cow manure per year (The Ohio State University, 1993), and will generate approximately $76,000 in potential annual energy revenue when the cow manure is used as an input of ADS. At the reported transportation rate of animal manure of $0.08 per ton per km, and an average one-way distance from farm to ADS of 9.2 km (the distance with 3 regional ADS, as per Table 2), the cost to transport the manure is approximately $6750 annually. After this transportation cost is accounted for there is $69,250 remaining in annual revenue potential, which translates to $15.10 per ton of manure. Policy incentives toward large and regional ADS for multiple farms will encourage farms, communities, and utility companies to evaluate the technical challenges inherent in these systems and weight them against the economic feasibility of converting organic by-products into marketable energy products. This exploratory study has found that GIS software can be used to identify the optimal sites for any given number of ADS based on region-specific criteria but there are certain technical and economic hurdles that are not addressed in this study and require more research. For example, while individual dairy farms typically return the nutrients in manure to the land by spreading manure on fields, regional ADS would need to assess alternative methods of efficiently returning to the fields the liquid effluents that contain the nutrients from the manure. Additionally there is great need for information about the returns to scale of ADS in terms of energy production in relation to manure input and the trade-offs between fuel and labor expense for transportation. The impact of manure transportation on local traffic and roadways should be examined and included in the comparison of capital investment savings from avoiding the construction of multiple ADS on many small and medium farms. The findings from this study and other relevant case studies indicate that there are viable pathways for small and medium dairy farms to access ADS technology using community based ADS and that agricultural, energy and transportation policy makers should consider this pathway when considering GHG reduction, renewable energy generation, and farm economic development strategies.