The reason these feedstock materials have such an effect on the digestions is because they manipulate the pH and C:N ratio of the organic matter. The pH of an anaerobic digester needs to be maintained at a neutral 7.0 to produce the highest yield of methane. The systems also need a moderate C:N ratio in order to maintain yields. Carbon and nitrogen are both important minerals in the anaerobic production of methane. In a system with high carbon content, the nitrogen will be used before the carbon can be properly digested. However high nitrogen will result in ammonia production which becomes toxic to the bacteria at certain concentrations. Dairy manure tends to have a slightly low C:N ratio than is preferred, but this can be counteracted with the addition of sawdust to the organic matter (Homan et al., 2012).

Figure 1: A description of a typical manure biogas digestion process. Source: Envision Gundersen Health System (2018).

Explanation of Digester

There are several types of anaerobic digesters, including municipal wastewater digesters, but for agricultural purposes a manure biogas digester is used. A manure anaerobic digester serves many purposes on a farm. It reduces odor and pathogen exposure by enclosing manure in a closed off or isolated location. By capturing the methane in a closed system, it reduces greenhouse gas emissions.

There are several types of manure digesters. A covered, sealed lagoon operation uses a removable cover in which there is a piping system used to transport the biogas produced. Plug flow digesters are long, narrow, and often underground. The underground aspect improves the insulation of the system, so the tank doesn’t require as much additional heating for the digestion to take place. Dry digesters use more solid than liquid substrates and are operated in silo-like tanks. The final manure digester type, and the kind the Madison area digesters are, is a complete mix system. The digester is a heated tank with a mixing system and uses wastewater to help liquify the organic matter Corré and Conijn (2016).

The digester works through a relatively simple process. Manure is collected then transported from the farm to the digester. The manure is placed in the tank along with other organic matter the operation wishes to use. The liquid organic matter is constantly stirred and maintained at the optimum temperature. All the while the anaerobic bacteria digest it, creating methane which collects at the top of the tank. This methane is burned through a generator to create electricity or otherwise burned for direct heat of the digester system itself. The electricity produced can then be distributed on the local grid.

---

Materials and Methods

Sources and past case studies about anaerobic digesters were used for research purposes. Through research information about the processes behind anaerobic digestion and the operation of a biogas digester, understanding was reached. In addition, case studies of two biodigesters in Middleton and Sun Prairie were studied in order to understand their operations and impacts on the surrounding area. An Anaerobic Digester System Enterprise Budget Calculator was used for the analysis of cost and profit for a biogas digestion system. Tables and figures were collected from the data of these sources and studies and used in this paper to enhance the information presented.

---

Results and Discussion

Middleton Case Study

Process

Figure 2: Anaerobic digester in Middleton Source: Envision Gundersen Health System (2018a).

In this case, Gundersen Health System and Dane County in Wisconsin have partnered with three dairy farms, Blue Star Dairy, Hensen Brothers Farm and Ziegler Farm, on the GL Dairy Biogas Project aiming to turn cow waste from the farms into electricity. In total, more than 2,000 cows provide the adequate amount of manure and the manure is processed in three air-tight digester tanks. The produced methane is captured at the top of the digester and burned in a generator to create electricity (Gundersen Lutheran Medical Center, Inc. and Gundersen Clinic, Ltd., 2018a).

Impacts on area

Approximately 38% of Gundersen’s renewable energy production is produced by this project. The digester operation generates about 16 million kilowatt-hours of electricity annually. The produced electricity is added to the local grid in Dane County through Madison Gas and Electric and is enough to power approximately 2,500 homes and reduce fossil fuel carbon dioxide emissions by 9,979 tons per year. The process also creates a large amount of organic-fiber by-product that has many horticultural uses and is composted onsite. In addition to producing the energy, the project prevents more than 1.68 tons of phosphorus runoff to the water ways in Dane County every year. Phosphorus is the leading cause of green algae and other weed growth in Dane County’s lakes and comes from both urban and rural sources. It is estimated that 167.8 tons of algae will be reduced annually in the Yahara watershed as a result of this project. Besides improving the environment, this created six full time jobs on an ongoing basis to support the digester day to day operations (Gundersen Lutheran Medical Center, Inc. and Gundersen Clinic, Ltd., 2018a).

Sun Prairie Case Study

Process

Figure 3: Anaerobic digester in Sun Prairie Source: Envision Gundersen Health System (2018b)

Gundersen Health System constructed a dairy digester on the Maunesha River Dairy for the Sunny Side Digester project. The biogas project started production in April 2014 and is the Gundersen’s second dairy digester project. Approximately 1,300 cows on the farm supply more than enough manure for the project. The process of this digestion operation is essentially the same as in the GL Dairy Biogas Project based in Middleton (Gundersen Lutheran Medical Center, Inc. and Gundersen Clinic, Ltd., 2018b).

Impacts on area

An amount of electricity produced by this project represents about 9% of Gundersen’s renewable energy production. The digester operation generates about 5 million kilowatt-hours of electricity annually, providing the power required to run about 530 homes. The electricity is added to the local grid in Dane County through Alliant Energy. The process also creates a clean, organic-fiber by-product that is used for cow bedding (Gundersen Lutheran Medical Center, Inc. and Gundersen Clinic, Ltd., 2018b).

Benefits

Manure management

Figure 4: Primary and secondary emission sources and sinks for a partial life cycle assessment of the carbon footprint of dairy production systems. Source: Rotz et al. (2010).

Manure is an inevitable consequence of livestock products generated from housed animals. These manures are traditionally recycled back to land for plants to source the nutrients. However, since they contain inorganic nitrogen and microbially available sources of carbon and water, they provide the essential substrates required for the microbial production of N2O and CH4, namely greenhouse gases (Chadwick et al., 2011). Indeed, manure management accounts for 38% of CH4 and N2O emissions from the global agricultural sector except for the land use changes (Crosson et al., 2011). In this context, conversion of manure to biogas by anaerobic digester systems reduce GHGs emissions by 39% per kilogram of milk based on the partial life cycle assessment modeled dairy farms in the United States (Fig. 4; Rotz et al., 2010).

Several Wisconsin-specific studies have reported certain environmental benefits from the adoption of the anaerobic digester for biogas production from manure. Aguirre-Villegas et al. (2017) conducted survey-based research in Wisconsin with a GHG emissions modeling to evaluate the effects of manure management on GHG emissions. The results show that permitted facilities, farms with concentrated animal operations that are regulated due to their size >1000 animal units, are able to reduce emissions significantly through anaerobic digesters when implementing manure processing. In fact, anaerobic digestion is the most effective strategy to mitigate GHG emissions both from energy and manure. Aguirre-Villegas et al. (2014) also reported that the installation of an anaerobic digestion system can reduce GHG emissions of manure management on a dairy farm by more than 40% when compared to a system without a digester, highlighting the benefits of anaerobic digestion systems in terms of net emissions reduction potential.

Renewable energy source

Biogas is a promising means of addressing global energy needs and providing multiple environmental benefits as summarized in Table 2 (Mao et al., 2015).

Table 2. Biogas environmental benefits analysis..Source: Maoet al. (2015)

More specifically, Cuéllar and Webber (2008) extensively analyzed the total potential to convert livestock manure into biogas in the United States by comparing the changes in GHG emissions between two different scenarios. Scenario A is that animal manure is collected either in a lagoon or left in the open and coal is burned to produce electricity. Scenario B includes the treatment of livestock manure in anaerobic digesters to convert the waste to biogas (Fig. 3). The results show that anaerobic digestion, a process that converts manure to methane-rich biogas, can significantly lower GHG emissions from manure which contributes to ten to 20% of greenhouse gas emissions from the agricultural sector in the United States. Using biogas as a substitute for other fossil fuels, such as coal, replaces two GHG sources with a less carbon-intensive source, namely biogas combustion. The calculated biogas energy potential was nearly 1 quad of renewable energy per year, amounting to approximately 1% of the United States total energy consumption. Converting the biogas into electricity using standard microturbines could produce 88 ± 20 billion kWh, or 2.4 ± 0.6% of annual electricity consumption in the United States. Replacing coal and manure greenhouse gas emissions with the emissions from biogas would produce a net potential GHG emissions reduction of 99 ± 59 million metric tons or 3.9 ± 2.3% of the annual emissions from electricity generation in the United Staes. It should be noted that this paper was published in 2008. Thus, a magnitude of the estimated impacts of the anaerobic digestion on emissions has potentially changed, although the positive effect of the biogas production from manure is likely to be consistent.

Figure 5: Scenario A (left): business as usual. Livestock manure and coal-fired power emit greenhouse gases. Scenario B (right): biogas is produced and used for electricity generation, replacing two sources of GHGs (coal-fired power and untreated manure) with one source of GHGs (biogas combustion). Source: Cuéllar and Webber (2008).

Figure 6: Spatial distribution of recoverable dairy manure in high-priority states. Source: Milbrandt et al. (2018).

Milbrandt et al. (2018) also suggested that waste-to-energy (WTE) technologies including biogas production from manure present an opportunity to recycle organic waste material into renewable energy, while offsetting disposal and environmental costs. In this context, understanding the variability of individual WTE resource characteristics, including their location, amount, and quality, are considered as a key challenge to ensuring economic and environmental viability of WTE. The authors estimated the WTE resource potential in the United States and illustrated its geographic distribution. The results of analyses indicate that about 566 teragrams (Tg) of wet WTE resources are generated annually. This amount corresponds to about 1 exajoule (EJ), which is sufficient to displace about 18% of the highway diesel consumption on an energy basis in 2015. About half of this energy is generated by animal manure, in which dairy manure accounts for more than 50%. This indicates that there are abundant sources from farms for biogas production in Wisconsin and opportunities to manage such huge amount of manure for mitigating GHG emissions.

Profits

To assess economic benefits of installation of anaerobic digester at a farm, we employed “the Anaerobic Digester System Enterprise Budget Calculator” to estimate the capital costs, revenues and operating costs at representative dairy farms in Wisconsin, adopting the survey data reported by Aguirre-Villegas et al. (2017). The calculator was developed by Gregory M. Astill and Richard Shumway in Washington state university. The representative farms in Madison were classified into small, large, and permitted farms with 76, 465 and 2281 of wet cow equivalents, respectively Aguirre-Villegas et al. (2017). The anaerobic digesters installed on each farm were assumed to produce biogas from manure with co-digesting organic waste and used to generate electricity for selling. In addition, fiber separation which produces fibrous byproducts were factored in. These byproducts contribute to cow bedding as well as horticultural products (Gundersen Lutheran Medical Center, Inc. and Gundersen Clinic, Ltd., 2018a,b). Table 3 indicates Capital costs for installation of anaerobic digester at representative small, large, and permitted dairy farms in Wisconsin. All costs increased with higher number of the wet cow equivalents, directly correlating with the size of a farm. Among the costs for anaerobic digester installation, the costs for vessel ; biogas infrastructure for the digester appeared to be relatively constant as compared to the combined heat and power, which is used to generate electricity from the biogas (Table 3). The costs for fiber separation equipment also increased with number of wet cow equivalents, but these ratios in the total capital costs were considerably lower than that of the digesters.

Figure 7: Annual revenues and 10-years sum of net revenues from anaerobic digesters at representative dairy farms in Wisconsin.

Table 4 summarizes revenues and operating costs of anaerobic digester at representative small, large, and permitted dairy farms in Wisconsin. The revenues from anaerobic digesters are consisted of tipping fees for the co-digested organic waste, electricity generated by the produced biogas, and cost saving nutrition application (the revenue from saving cost for the application of fertilizers by using the separated solid during the digestion). For fiber separation, digested fiber for cow bedding and cost saving nutrition application contribute to the revenues. On the other hand, operating costs of anaerobic digester consist of combined heat & power and digester maintenances, while fiber separation was from liquid/solid separator maintenance. The sum of the revenues and operating costs are the annual revenue for each farm size. The total revenues for small, large, and permitted farms were 29,031, 190,386, and 943,522 US$/year respectively. This indicates that all size of farms can make profit from the first year of operataion. Based on the annual revenue, estimated payback periods were 93.7, 15.7, and 4.5 years for small, large, and permitted farms respectively. Taking this into account it is appears that, the installation of an anaerobic digester on a small farm (76 of wet cow equivalents) in Wisconsin is not feasible under the modeled condition in this study. In the case of the permitted farms, it is possible to get 943,522 US$/year after a 4.5 years payback period from the operation of an anaerobic digester. It should be noted that the number of cows in a farm, namely, the wet cow equivalent was fixed to estimate the payback period.

Figure 7 shows annual revenues and 10-years sum of net revenues from anaerobic digesters at representative small, large, and permitted dairy farms in Wisconsin. As shown in Table 4, the annual revenues were increase with the size of farms. The 10-years net revenue – capital cost was calculated by {(annual revenue – annual operating cost) × 10} – total capital cost to evaluate a total balance of payments if a farm installs and operates an anaerobic digester for 10 years. Among small, large, and permitted farms, only the permitted farms pay off the capital cost and make a profit from the anaerobic digester. Therefore, the results from the calculator suggest that, under our models, the installation of anaerobic digester is feasible in a large-scale farm which operates a concentrated animal operation with >1000 animal units. Our results also imply that relatively small-scale farms can get benefit through the anaerobic digester if they collect the manure for an anaerobic digester to produce biogas and electricity, as in the case of Middleton and Sun Prairie.

---

Limitation

With every innovation and system, there are downfalls and inefficiencies. In anaerobic digester systems the limitations come in the form of cost, risk, and uncertainties surrounding the systems. As previously stated, while there are thousands of sites available for operation of digesters, not many farmers are using or even considering biogas digesters as a solution. Biogas digester operations, especially in the United States, are not proposed as a plausible technology unless the dairy cow population providing for the digester are near a thousand or more. This means farm size is a limitation to the biodigester operation. Farm size, or rather manure input, which is directly affected by the number of cows contributing to the system is a limiting factor. Since a digester is not considered high yielding enough unless it harnesses the input of manure sourced from approximately a thousand, the startup can be costly. The reasons surrounding the cost have to do with the electrical and political climate of the state the operation is in. The electrical industry is run as monopoly meaning wholesale electricity is the norm. Retail electricity is less available and harder to source. Biogas digesters are retail sources of electricity and is the reason they are less frequently used. Because revenue cannot be sustained solely on electricity it can be trickier to generate profit. This leaves two options for biogas digester systems. The anaerobic digester system can create revenue from other byproducts, which could include biogas, phosphorus rich fertilizer or compost, and other fiber products. Otherwise the problem of revenue can be counteracted by the partnerships of farms. If small farms partnered together to invest in a digester operation, then all the farms involved can contribute to the manure to run it. In addition, while a biogas digestion system can work efficiently, it is easily affected by environmental changes. Temperature and feedstock material must be monitored diligently in order to maintain an optimum pH and C:N ratio. Since it is sensitive, personnel need to be onsite or within reach of the digester operation, in order to keep it running properly and optimize the yield of energy. Without proper funding to employ and manage a biogas digester system it cannot be under operation. Lastly these biogas digesters can have social limitations. For there to be anaerobic energy sources there needs to be a push for a more sustainable future. Those who support composting and carbon neutral projects would find biogas digesters a solution.

Conclusion

Biogas digesters have had a positive impact on Madison and the surrounding area. The Middleton and Sun Prairie digester add electricity to both the Madison Gas and Electric and Alliant energy gird, which in turn powers thousands of homes in the area. Both operations reduce greenhouse gas emissions of the farms that are collected from by reducing exposed manure content. The biogas digesters help to reduce runoff pollution in the area’s waterways. Not only do the biogas digesters work to aid the public and the environment, but it also benefits the farmer. In addition to reduced emissions and better manure management practices, the byproducts of biogas digestion serve practical purposes on farms. The byproducts can act as fertilizer, compost for farms or local horticultural purpose, and cow bedding. With greenhouse gas emissions being one of the critical issues causing climate change, biogas digesters are an innovation agriculture can look to as a solution. Biogas digesters, if more widely considered and implemented, could have the positive effect on the Madison area already exhibited, but to a greater degree.

---

Citations

“A Short History of Anaerobic Digestion.” Penn State Extension. The Pennsylvania State University; September 4th, 2012.

Aguirre-Villegas, Horacio A., and Rebecca A. Larson. "Evaluating greenhouse gas emissions from dairy manure management practices using survey data and lifecycle tools." Journal of cleaner production 143 (2017): 169-179.

Aguirre-Villegas, Horacio Andres, Rebecca Larson, and Douglas J. Reinemann. "From waste-to-worth: energy, emissions, and nutrient implications of manure processing pathways." Biofuels, bioproducts and biorefining 8.6 (2014): 770-793.

Chadwick, D., Sommer, S., Thorman, R., Fangueiro, D., Cardenas, L., Amon, B., & Misselbrook, T. (2011). Manure management: Implications for greenhouse gas emissions. Animal Feed Science and Technology, 166, 514-531.

Corre, W.J. and J.G. Conijn. “Biogas from agricultural residues as energy source in Hybrid Concentrated Solar Power.” Science Direct. Procedia Computer Science 83 (2016) 1126 – 1133.

Crosson, P., Shalloo, L., O’brien, D., Lanigan, G. J., Foley, P. A., Boland, T. M., & Kenny, D. A. (2011). A review of whole farm systems models of greenhouse gas emissions from beef and dairy cattle production systems. Animal Feed Science and Technology, 166, 29-45.

Cuellar, Amanda D., and Michael E. Webber. "Cow power: the energy and emissions benefits of converting manure to biogas." Environmental Research Letters 3.3 (2008): 034002.

Homan, Evan. “Biogas from Manure.” Penn State Extension. The Pennsylvania State University; March 5th, 2012.

Mao, C., Feng, Y., Wang, X., & Ren, G. "Review on research achievements of biogas from anaerobic digestion." Renewable and Sustainable Energy Reviews 45 (2015): 540-555.

Milbrandt, A., Seiple, T., Heimiller, D., Skaggs, R., & Coleman, A. (2018). Wet waste-to-energy resources in the United States. Resources, Conservation and Recycling, 137, 32-47.

Rotz, C. A., Felipe Montes, and D. S. Chianese. "The carbon footprint of dairy production systems through partial life cycle assessment." Journal of dairy science 93.3 (2010): 1266-1282.

Tilman, David and Michael Clark. "Global diets link environmental sustainability and human health” Macmillan Publishers Limited. 2014.

Thoma, Greg et.al. "Greenhouse gas emissions from milk production and consumption in the United States: A cradle-to-grave life cycle assessment circa 2008.” International Dairy Journal, 31 (2013) S3eS14.

“Turning Cow Waste Into Energy.” Envision Gundersen Health System. Gundersen Lutheran Medical Center, Inc.Gundersen Clinic, Ltd. 8420-4_0718. 2018a.

“Turning Cow Waste Into Energy-Sun Prarie.”Envision Gundersen Health System. Gundersen Lutheran Medical Center, Inc. Gundersen Clinic, Ltd. 10874-2_0117. 2018b.

---

Acknowledgements

This project would not have been successful without the contributions of the outstanding students in our Food Systems, Sustainability, and Climate Change class.  We would particularly like to acknowledge the wonderful and challenging questions, and the specific knowledge that students with different areas of expertise provided.

About the Authors