Scenario

We are a three-person consult ing team for BioGas America and it is our goal to increase the adoption of anaerobic digesters on dairy farms by creating awareness through positive education.

Abstract

This study aims to explore that if Wisconsin dairy farmers implement the use of an anaerobic digester, then the benefits they should experience are a profitable return on investment and better environmental outcomes. The consulting researchers found through multiple case studies that the earnings of complete-mix anaerobic digesters eventually exceed their initial investment costs through the sale of solid and liquid by-products, electricity generated by combusting biogas, and savings on organic waste disposal. Depending on a farm’s size and contractor agreement, the profitability of energy sold may be highly lucrative. Environmental outcomes of anaerobic digesters include a significant decrease in greenhouse gas emissions and decreased observance of eutrophication; however, the long term effects on global warming are disputed when combusting biogas. Cattle manure has a low carbon-to-nitrogen ratio so the addition of a co-digestant input, such as dairy whey or vegetable produce, is deemed crucial for efficient methane production. The authors conclude that the benefits of operating an anaerobic digester outweigh any hazards as long as proper facility maintenance is observed.

Introduction

Currently, there are various negative environmental and social impacts resulting from poor manure management from livestock production, especially dairy farming. These include nitrogen runoff into water sources causing eutrophication of our nation’s great natural resources as well as the emission of methane, a very potent greenhouse gas. This same runoff can cause health issues if it contaminates drinking water; high nitrates concentration in water originating from manure has been linked to blue baby syndrome in dairy rich regions of Wisconsin. In addition, the dairy industry is direly hurting for alternate forms of income as prices of milk have been steadily below the cost of production in recent years. Through our proposal, we hope to enlighten farmers in Wisconsin and throughout the United States on a new revenue stream from their farm’s waste, as well as the environmental and social benefit of doing so.

Environmental Loss due to Manure Mismanagement: According to the EPA, agriculture in the United States accounts for approximately 9% of our total greenhouse gas emissions, which add to our existing atmosphere to trap more heat on Earth and perpetuate global warming. In more depth, manure management is responsible for about 14% of all greenhouse gases produced in crop and livestock production for food, which equates to approximately 1.25% of our nations greenhouse gas production EPA, 2019). The greenhouse gases that are typically emitted from manure are methane and nitrous oxide which are 25x and 298x more potent greenhouse gases than CO2, respectively (Land Trust Alliance, 2020). These gases are often emitted to the atmosphere when the manure is openly exposed in a manure lagoon or when it is spread onto fields. Manure lagoons are large dugout holes that are filled with liquid manure for storage until it is able to be returned to fields to replenish the soil with nutrients. A new way to mitigate this environmental danger is to create a storage system to harness these gases from escaping to the atmosphere and also be used as a renewable source. This is known as an anaerobic digester, and they are the topic of our research project.

Manure from dairy cattle, although considered waste to us, is highly packed with nutrients capable of feeding many organisms on this planet. This manure is often applied to fields to resupply nutrients to the soil, but sometimes it may run off into local water sources causing eutrophication. Coming from the Greek word Eutrophs, meaning over-nourished, eutrophication is the process by which the limiting nutrient to photoautotrophs (photosynthetic plants and microbes) is supplied and fuels their overgrowth. This often occurs when the nutrients from manure are washed off of fields during a rainfall and low into lakes and rivers. By supplying commonly limiting nutrients (phosphorus and nitrogen) to these organisms, they bloom; this bloom can turn a crystal-clear lake into a murky water lake with many weeds and poor water quality. Although a home to new types of organisms, the new ecosystem is not suited for certain species, including economically relevant species like trout and salmon.

Time: Years

Economic Loss due to Manure Mismanagement: More detrimentally, after the bloom causes great growth, it is followed by a massive die-off of photosynthetic microbes when the nitrogen or phosphorus is used up. These then float to the bottom of the water source and feed microbes that consume large amounts of oxygen from the water; this causes oxygen-depleted water, inhibiting growth of almost all aquatic animal species and creating dead zones (Scannone, 2016). A very prevalent dead zone with hypoxic water (less than 2ppm dissolved oxygen) that follows this pattern yearly is in the Gulf of Mexico near the headwaters of the Mississippi River. As the Mississippi River travels the length of the United States through some of the most heavily farmed regions in the country, it collects massive amounts of nitrogen and phosphorus from manure run-off that are then concentrated at this dead zone in the Gulf of Mexico (Bruckner, 2020). The Mississippi River acts as a funnel for fertilizer run-off from 41% of the United States’ tributaries, and delivers it to the Gulf of Mexico in a concentrated allotment (Nature Conservancy, 2020).

On a local scale throughout the rivers that drain into the Mississippi, this reduction in biodiversity has impacted trophy fish populations and the recreation industries that rely on them. In addition, eutrophication of rivers and the resulting decreased water quality is linked to diminished waterfront property value (Dodds & Bouska, 2008). With respect to dead zones in the Gulf of Mexico, this greatly impacts the commercial fishing industry and tour industry that relies on the vast fish biodiversity of the natural resource. Mitigating these environmental effects of manure is integral to the prolonged success and sustainability of many seemingly unrelated industries.

Social Loss due to Manure Mismanagement: Losing pristine water sources results in the loss of socially and culturally relevant activities. For one, recreational fishing of some of the most sought after species in the salmon family. Brown trout specifically require fast flowing water with a grave bottom to spawn or reproduce (Victorian Fishing Authorities, 2018). When organic pollutants cause plants to bloom and resulting sediment to build up, the water speed decreases, oxygen levels decrease, and there is no longer visible gravel for the salmon to utilize. There are various downstream effects on the rest of the ecosystem that arise from this such as decreased food source for piscivores, and the resulting shift of species populations. Entire habitats and populations are changed when too many pollutants flow into water sources from man-made sources including dairy farms. This not only decreases our ability to experience resources in their natural state, but also infringes on the rights of ‘the land’ as a stakeholder. Aldo Leopold, a Wisconsin conservationist, wrote the following in the Sand County Almanac:

“The land ethic simply enlarges the boundaries of the community to include soils, waters, plants, and animals, or collectively: the land... In short, a land ethic changes the role of Homo sapiens from conqueror of the land-community to plain member and citizen of it. It implies respect for his fellow-members, and also respect for the community as such (A Sand County Almanac, 1949).”

By recognizing that we are members of the land, we must respect the rights of the other stakeholders of the community. In doing so, we must check our practices so that they are not carelessly changing entire ecosystems.
Poor manure management can also cause health problems for some of the most vulnerable members of our society. Blue baby syndrome is a medical condition that occurs when water with too much nitrate concentration is given to babies to drink. Once in the blood, the nitrates are converted to nitrites and then react with hemoglobin to form methemoglobin. This alteration of our oxygen carrying molecule diminishes its ability to do its job, and results in babies who are deficient in oxygen (WHO, 2020). Natural nitrate concentrations in water is around 1 milligram per liter of water, while high levels are around 50 milligrams per liter of water. The high concentration of nitrates in water in developed countries is often due to fertilizer spread on farm fields that then leeches into the underlying water table. Although still uncommon, there have been a few cases in the Eastern Wisconsin due to the region’s high concentration of small dairy farms with primitive manure management practices.

Operation of an Anaerobic Digester: Within the manure of ruminating animals, there are microorganisms called methanogens that can convert present elements into methane gas and other by-products. Methanogens in manure originate in the intestines of ruminating animals; therefore, live cows also produce methane daily. This process is called methanogenesis and can be further described as a conversion of hydrogen and carbon dioxide into methane gas and water. The source of nutrients for methanogens is provided by a separate domain of microorganisms called archaea who breakdown fatty acids and other complex molecules during digestion. This breakdown results in the supply of hydrogen and carbon dioxide among other molecules (Reay et al, 2010).

The design of anaerobic digesters vary with a facility’s anticipated use and volume capacity; although, the basic layout of the system remains somewhat uniform. Because of its high liquid content, animal manure and other organic waste products will combine into a slurry as they flow into the base of a large tank. This tank is the main body of the anaerobic digester and the site of methanogenesis. Fresh inputs will sit in the tank for about 15 days as the digestion process occurs in a complete mix system which we will be observing below (EPA, 2011). The material is occasionally agitated via a rotating mechanism to allow digested manure, or digestate, to float to the bottom as its density increases . The agitation also allows methane gas to travel above the liquid-solid mixture and reside at the top of the tank. An outflow pipe is built into the top of the tank to allow the gas to be pumped out, stored elsewhere, or moved to a site for immediate use. Another common feature of industrial anaerobic digesters is a methane balloon which expands as more gas accumulates at the top of the tank since more gas molecules provide more pressure to lift the balloon. This feature would be implemented if the biogas is not to be stored elsewhere.

Biogas, which includes methane and other gaseous compounds, is used as an alternative fuel source whose refinement determines when and how it is used. A farm’s anaerobic digester may produce biogas for on-site use or to be sold to local energy utility companies. At either destination, the biogas may be combusted to generate heat and power as well as electricity via turbine rotation. In 2018, the U.S. Energy Information Administration estimates that twenty-nine livestock facilities across the United States produced about 266 million kWh worth of electricity in the form of biogas (EIA, 2019). Utility companies have the additional choice of compressing biogas into a usable vehicle fuel similar to refined natural gas.

Thesis

Methods

 All information was obtained through a public search of the facilities in question. Most statistics were provided by Crave Brothers Farm LLC official website and records kept by the Environmental Protection Agency’s AgSTAR program. In 2007, Crave Brothers Farm started the construction of an anaerobic digester on their property in partnership with Clear Horizons LLC (EPA, 2014). Their goal was to make their dairy operations more sustainable through the use of manure recycling and biogas production. As of 2009, the Crave Brothers have two seven hundred fifty thousand gallon tanks within their digester system so that they are capable of handling the fifty-five thousand gallons of manure produced by their fifteen hundred cows (Damask, 2018). An additional twenty-five hundred gallons of whey and dairy production waste is added to the manure slurry flowing into the digester.
  Similarly in Mifflintown, Pennsylvania, Reinford Farms experiences the same environmental and financial benefits of owning an anaerobic digester. Information on their anaerobic digester was collected from third party articles promoted on their website, and data collected by the EPA’s AgStar program. The setup between Reinford and Crave Brothers are essentially the same, albeit on a different processing scale, so any difference in effects observed are because of farm specific requirements.

Figure 1: (Left) Reinford Farm, Pennsylvania (Right) Crave Brothers Cheese Farm, Wisconsin

Results

Environmental and Social Impacts

By preventing the methane from emitting into the environment, anaerobic digesters have been found to reduce on-farm emissions due to manure by 60%. As stated earlier, current greenhouse gas emissions from manure compose of a little over 1.25% of our nation’s emissions. However, if we were to introduce anaerobic digesters onto every livestock operation so all manure is processed in a digester, we could reduce our nations greenhouse gas emission by 75 million metric tons (using the estimation that the USA produces 6000 million metric tons of CO2 per year and manure emissions count for 1% of those emissions) (Statista, 2018). Removing that many tons of greenhouse gases is the equivalent to taking close to 10 million cars off the road. For calculations, see Figure 4 in the appendix. This is not accounting for the release of CO2 when the methane is burnt for energy.

Since the byproduct of digestion is mostly solid effluent and flush water, there is a great reduction in the amount of fertilizer runoff into water sources. NOAA estimates that each year, dead zones cost the Gulf of Mexico’s seafood and tourist industry approximately $82 million (Nature Conservancy, 2020). If we were able to reduce even just half of the nitrogen and phosphorus run-off by using anaerobic digesters (currently not all fertilizer run-off is from manure), we could save that industry $41 million dollars. Taking into account the millions of recreation dollars lost each year in other water sources due to eutrophication is almost incalculable, and is outside the scope of this research.

Profit to Farmers

However, since each digester is paid for by each individual farmer (at least for now) and it is unreasonable to think that every livestock operation in the United States will get an anaerobic digester on their farm in the near future, the benefits that farmer will see themselves should also be brought up. On the Reinford’s 1000 dairy cattle farm in Pennsylvania, they process close to 12,000 gallons of manure a day and produce 140 kilowatt hours of electricity per hour around the clock. This equates out to approximately 1.2 million kWh per year. Since this is more than enough to cover their electricity costs, they sell it back to the grid and receive around $20,000 per year instead of paying $5,000 per year on electricity. According to Brett, the farm manager, they expect a return on investment time of 8 years (Huso, 2018). This farm, and many others benefit from tax credits and federal grants to offset the high upfront cost of creating a multi-million dollar digester.

Through a rough energy balance of the system, one may see the truth in the claim that a digester is a carbon negative facility. Carbon negative means that more electricity is produced than the amount consumed in running the biodigester and making dairy products. Manure and other matter enter the tanks through a gravity fed pipe and methane gas rises through the collection valve on its own, so no energy is needed here. Energy input is required to maintain the tanks’ internal temperature of 99 ℉ and for the screw press separator to divide the finished digestate. Captured methane is combusted on-site to provide the heat for warming the tank. The Crave Bros upgraded their facility with an engine generator capable of producing 633 kW. Clear Horizons, their development partner, maintains the system and can remotely control the digester’s settings from their office in Milwaukee, WI. In their agreement with the Crave Bros, Clear Horizons have the rights to anything produced by the digester, such as solids, liquid sludge, biogas, and electricity generated. However, the farm generally buys back the solid and liquid digestate to be used as cow bedding and liquid fertilizer, respectively. Any excess electricity not reinvested in the digester is sold to WE Energies, Inc by Clear Horizon. Since both Reinford Farm and the Crave Bros have excess electricity to sell, their anaerobic digesters earn them profit as well as save them fees on energy bills.

Increasing Efficiency of Anaerobic Digestion

Anaerobic digestion has illustrated enumerated advantages, but the potential of manure for biogas production is not fully utilized due to high nitrogen content in animal manure leading to an imbalance of carbon to nitrogen (C/N) ratio. In order to provide all the requirements needed for anaerobic digestion and compensate for the carbon deficiency of manure, another carbon-rich substrate needs to be co-digested with the manure to improve the efficiency of anaerobic digestion. Lignocellulosic biomass residues are a promising substrate to balance the C/N ratio feedback needed for proper anaerobic digestion. The process allows conversion of organic waste materials to bio-energy (i.e. biogas) while leaving behind nutrient rich residues that can be used as fertilizer. In a study done by Wang et al. addition of 4.6 kg of wheat straw (an abundant agricultural waste with high lignocellulose content) per ton of cattle manure improved biogas yield by 10% due to the synergic effects of the materials (2012). 

Limitations

Alongside the several benefits indicated by use of anaerobic digestion, there are a few disadvantages that exist. In Europe, financial incentives have led to a surge in anaerobic digestion installations to produce heat and/or electricity from biogas in a combined heat and power (CHP) plant. However, whilst anaerobic digestion has potential to reduce greenhouse gas emissions, its environmental impacts on a life cycle basis are uncertain and depend largely on the feedstocks used. There are arguments illustrating how the majority of impacts towards global warming potential (GWP) are higher in biogas than in natural gases. For example, when comparing AD CHP, natural gas CHP, natural gas boiler, and oil boiler in the UK, biogas combustion composed of 57% of the combined total ozone depletion potential. However, the discrepancy in environmental impacts between biogas and fossil fuel alternatives can be mediated by changes in how we produce and store biogas. The impacts can be reduced by using cover storage for digestate and recovering methane for use in CHP. We can also use improved techniques for digestate application on farmland. Further savings on GWP can be achieved by using high methane-yielding feedstocks, such as maize silage. By correctly applying certain techniques when producing biogas, we can lower its environmental impacts.

Figure 2: A scheme of anaerobic co-digestion of animal manure and lignocellulosic residues for biogas production along with its potential applications.

Conclusions

After this analysis, it can be seen that there are various benefits of implementing an anaerobic digester on a dairy farm in the Midwestern region. First, the savings on energy input to the facility and sale of excess electricity generated via methane combustion eventually pay for the initial investment cost and then turn a profit. Second, as biogas is captured in the digester the amount of methane dispersed to the atmosphere is reduced, thereby diminishing a large contributor to greenhouse gas emissions. Finally, when used as a fertilizer, digested manure has a reduction in nutrients that are liable to cause eutrophication in downstream bodies of water. This saves the local ecological system from disruption due to algal blooms and the seafood industry, such as in the Gulf of Mexico, will not lose its source of product.  

For future direction, anaerobic digesters can play roles in different types of manure, expanding upon dairy farms. In a study done by (Kafle and Chen, 2015), both chicken and swine manure actually had higher biogas yield compared to dairy manure (Figure 3). This illustrates potential to incorporate AD onto other types of animal farms thereby potentially increasing our biogas production and use.

Figure 3. Daily biogas productions from different manures.

Figure 4. Calculations of anaerobic impact on carbon emissions

Citations

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

Erik Ohman

I am majoring in Microbiology with plans to become a researcher and discover new scientific technologies to improve food production efficiency that also protect our environmental resources. I am motivated to pursue this field of study because I am a stakeholder of both food production and environmental preservation. I was raised on a small beef cattle and swine farm in Eastern Wisconsin which is where I gained my affinity to food production. I am also an avid outdoorsman and conservationist; I enjoy hunting, fishing, hiking, and kayaking in our nation’s great outdoors. The project of implementing anaerobic digesters is a perfect blend of my passions as anaerobic digesters have the ability to improve farmers’ economic livelihood and protect our natural resources through manure mitigation, all while using in-depth microbial sciences that I have learned through my major’s core classes. In the future, I can foresee myself working for a real company that develops anaerobic digestion systems.

Valerie Nguyen

It wasn’t until college did I realize a passion for environmental sustainability. Currently majoring in Genetics and Genomics, I decided to obtain a certificate in Food Systems and combine the two areas of interest in order to help create more sustainable agricultural systems through the adaptations of crops and microbes. By studying anaerobic digesters, I can apply knowledge from my core classes into a specific subject matter that directly affects farmers in the United States. Hopefully, I can continue studying digesters that incorporate my knowledge in microbiology/genetics to improve efficiency and environmental impact as its use becomes more popular.

Jaime Pham

Hi folks! I am a junior pursuing a degree in Biological Systems Engineering with a concentration in bioprocesses. One day I found myself declared in this major with quite literally no sense of how that happened. However, once I got to know the possibilities of problem-solving in the natural environment, I never wanted to transfer! BSE originally only focused on agricultural problems but has expanded to include topics related to food processing, natural resources, and renewable energy from a biological standpoint (which is what I love to study). Researching anaerobic digesters fits pretty well into what I have already been studying (mechanical design, agricultural effects on the environment, etc) but also explores topics I don’t normally consider such as social issues, economic gains, and long term consequences. I’m excited to explore other complex systems, such as digesters, that have a lot to offer in environmental solutions and have their own hazards to mitigate.