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Waste to Worth: A Case Study of the Biogas Circular Economy in Pennsylvania
Stephanie M. Herbstritt1, Siobhan L. Fathel1,*, Brett Reinford2, Tom L. Richard1
Published in Journal of the ASABE 66(3): 771-787 (doi: 10.13031/ja.14889). Copyright 2023 American Society of Agricultural and Biological Engineers.
1Agricultural and Biological Engineering, The Pennsylvania State University, University Park, Pennsylvania, USA.
2Reinford Farms, Mifflintown, Pennsylvania, USA.
*Correspondence: suf474@psu.edu
Submitted for review on 30 September 2021 as manuscript number ES 14889; approved for publication as a Research Article and as part of the Circular Food and Agricultural Systems Collection by Associate Editor Dr. Ana Martin-Ryals and Community Editor Dr. Kati Migliaccio of the Energy Systems Community of ASABE on 8 November 2022.
Highlights
Abstract. Coupling agricultural production with sustainable bioenergy systems may help us improve the circular economy of the food system and work within planetary boundaries for climate stabilization. However, leading sustainable dairies often do not have data to support that claim. As a result, practical case studies of circular economies with measured data from commercially operating farms are lacking in the literature, which is instead dominated by hypothetical and theoretical analyses. To grow and scale commercial implementation of circular economy and sustainability principles, it is important to understand how commercial farms implement these principles within the constraints of market incentives and actual farm operations. We conducted a case study of a commercial dairy farm in Pennsylvania, where a well-managed anaerobic digester system serves as the basis for a circular farm economy and allows the next generation to grow the farm business and expand the portfolio of revenue streams. The farm recycles food and agricultural waste into heat, renewable electricity, and fertilizer to heat and power the farm, amend the soil, and reduce farm costs. We also highlight the potential to scale the case study farm's circular economy approach in Pennsylvania using the state's projected 2030 manure, corn stover, winter double crops, switchgrass, and food waste resources to produce energy via biogas or renewable natural gas (RNG). We estimate the state could generate 40 million MJ annually from such integrated anaerobic digestion systems, meeting 3% of its electricity consumption. Circular economies like this case study can be designed in food and agricultural systems to operate within the constraints of an operating farm and recycle waste, produce nitrogen- and phosphorus-rich soil amendments and reduce imports of synthetic fertilizers, reduce and offset fossil energy consumption and greenhouse gas emissions associated with crop and livestock production, regenerate natural ecosystems, help ensure agricultural resilience and sustainability, and provide economic benefits.
Re-configuring agriculture and food systems for a circular economy requires redirecting many of the linear flows that currently end up as outputs of the system, often as wastes and pollutants, into inputs that supply energy and materials for new production cycles. In the dairy sector, traditional integrated farms feed their crops to livestock and use their manure for crop nutrients. However, these systems were never perfectly balanced, and in any case, integrated dairies are now less common as farmers specialize in achieving economies of scale. As a result, most modern dairy systems are largely linear—reliant on external inputs like fossil fuel and synthetic fertilizer (Amin et al., 2018; Cruse et al., 2010)— and produce significant greenhouse gas (GHG) emissions (Peterson and Mitloehner, 2021; Rotz, 2018; Rotz et al., 2020) and nutrient losses that contribute to environmental degradation (Amin et al., 2018; Friedman, 2018).
Global livestock production contributes approximately 14.5% of the total anthropogenic GHG emissions across the supply chain (Gerber, 2013), with dairy production systems responsible for about 30% of these emissions (Opio et al., 2013). In the U.S., approximately 72% to 75% of the life cycle GHG emissions of dairy products occur before products leave the farm (Thoma et al., 2013). Moreover, global demand for dairy products in 2050 is projected to increase by 58% from 2010 (FAO, 2011). The specific greenhouse gases emitted from dairy farms include methane (CH4) from enteric fermentation in the animals; nitrous oxide (N2O) and CH4 from manure in housing and storage; anthropogenic CO2 from fossil fuel use; and N2O from the soils used to produce feed and forage.
Interest in reducing on-farm dairy GHG emissions coincides with an increased focus on sustainable dairy production systems. In 2020, the Innovation Center for U.S. Dairy developed an industry-wide effort coined "the Net Zero Initiative" to assist U.S. dairy in achieving net-zero carbon emissions, significant water use reductions, and improvements in water quality. Large corporate partners, such as Nestle and Starbucks, support these efforts, further motivating the dairy industry to reduce GHG emissions (U.S. Dairy, 2021). Given these motivations to increase dairy production sustainability, reduce GHG emissions, and address climate concerns, dairy production farms are tasked with creating effective systems to meet these needs.
Anaerobic digestion (AD) is considered a keystone technology for dairy system sustainability, reducing GHG emissions, adding circularity to carbon and energy flows, and enhancing farm nutrient management (Fagerström et al., 2018; Launay et al., 2022). Through AD, dairy farms can convert agricultural waste, typically manure, to on-farm energy, producing biogas and a digestate co-product. Biogas, which is a mixture of CH4, carbon dioxide (CO2), and other trace gases, can then either be used immediately to produce heat and electricity or can be upgraded into biomethane, also known as renewable natural gas (RNG), for injection into the natural gas grid. Once in the gas grid, RNG can be used for transportation in natural gas vehicles, for electricity production in gas turbines, for heat and industrial power, or as a feedstock for chemical synthesis (Thrän et al., 2014). The digestate is a nutrient-rich byproduct of the digestion process that can be used as a fertilizer and soil amendment. The digestate residue, typically a liquid slurry, retains almost all of the original nitrogen from the manure in the form of ammonium (NH4+), all of the phosphorus (P), and, depending on the feedstocks and conversion rate, about 30% of the carbon (Koszel and Lorencowicz, 2015; Launay et al., 2022; Möller and Müller, 2012; Al Seadi et al., 2013). This digestate is a more predictable and plant-available nutrient source than fresh manure, and digestion can help with odor control (Aguirre Villegas et al., 2014; Doyeni et al., 2021; Penn State Extension, 2012; Walsh et al., 2018). The conservation of N and P in the digestate reduces the need for synthetic fertilizers and thereby increases the system's circularity. AD on dairy farms can minimize waste while retaining the value of waste materials and providing a renewable energy source, which makes AD a potentially valuable strategy for a more sustainable, circular economy in the dairy industry (Stanchev et al., 2020).
However, leading sustainable dairies often do not have data to support those claims of circularity and sustainability. As a result, practical case studies of circular economies with measured data from commercial farms in the United States are lacking, and the literature is primarily dominated by theoretical analyses (Aui et al., 2019; Bacenetti et al., 2016; Fagerström et al., 2018; Loizia et al., 2019; Timonen et al., 2019; Yang et al., 2022). Yet to grow the circular economy, we need to understand how commercial farms implement circularity within the constraints of market incentives and farm operations. Feedstock availability, government policies, and environmental regulations influence AD market incentives (Vasco-Correa et al., 2018). Farmers and researchers must develop and document practical systems for recycling and reusing mass flows, not just theoretical systems, and identify data gaps.
In addition to anaerobically digesting livestock manure, dairies could integrate dedicated perennial energy crops, double-crops, annual crop residues, and food waste into their AD systems. Several on-farm AD systems in Pennsylvania already co-digest livestock manure with domestic food waste (U.S. Environmental Protection Agency (EPA), 2021). Co-digesting dedicated energy crops, double-crops, or annual crop residues is uncommon in the U.S. but has been demonstrated commercially in Europe and elsewhere (Braun et al., 2011; Dale et al., 2016a), including in ways that do not disrupt the food system and even increase dairy production (Dale et al., 2016b). More diverse feedstocks provide opportunities to improve the economic viability of AD by increasing biogas output (Zhang et al., 2013a; Crolla et al., 2013; Atandi and Rahman, 2012) and generating additional revenue through tipping fees from accepting domestic food waste (Bishop and Shumway, 2009). Co-digestion of sustainably sourced herbaceous feedstocks (perennial and winter crops and annual crop residues) with manure has the potential to increase biogas output (Zhangb et al., 2013; Braun and Wellinger, 2003) while also potentially producing substantially greater levels of ecosystem services than achievable through conventional farming systems (Asbjornsen et al., 2014; Finney et al., 2016). Planting hardy, deep-rooted perennial crops on marginal lands has the potential to decrease soil erosion, store carbon, and improve soil health (Blanco-Canqui, 2016). Similarly, double-cropping with winter crops protects the soil from erosion, may reduce drainage nitrogen loss (Malone et al., 2018), can boost annual forage production, and can be used as an additional feedstock for the AD system (Brown, 2006; Heggenstaller et al., 2008). Additionally, double crops coupled with AD may help sustainably intensify prime agricultural land (Launay et al., 2022).
In addition to these environmental benefits, AD can provide significant energy resources and economic benefits to the agricultural community. A recent analysis estimated that Pennsylvania could produce 6,377 million kWh of electricity from biogas, or 1.3 billion cubic meters of RNG annually, by capturing and digesting wastewater, manure, food waste, and landfill waste (American Biogas Council, 2020). These digester systems would require >$1 billion in capital investments in rural communities but could recycle 1.23 million Mg (dry basis) of manure annually as liquid digestate (American Biogas Council, 2020), a valuable soil amendment. This potential would be greater with the addition of crop biomass. Increasing AD in Pennsylvania represents an opportunity to convert low-value materials to higher-value materials, produce electricity, heat, and RNG, and efficiently recycle energy, carbon, and nutrients.
Materials and Methods
To illustrate the use of AD to enhance the circular economy of the food system, we introduce a case study of a family-owned Pennsylvania dairy with an established AD system that has won a U.S. Dairy Sustainability Award. This case study is a practical example of a commercial dairy willing to share measured data from its farm operations to assess circularity. We modeled the mass flows of carbon, nitrogen, phosphorus, and energy entering and leaving the farm to quantify the system's circularity. Data gaps exist in the case study, and we highlight areas where researchers need additional measured data from commercial dairies to better understand, improve, and expand circular dairy systems. In this study, we filled these data gaps with literature values, prioritizing research reporting measurements on similar farms when possible. Then we follow with a resource assessment of Pennsylvania's potential crop, food waste, and manure biomass resources that could be converted into RNG or electricity using AD to estimate the potential of biogas and digestate to provide new sources of farm revenue and additional ecosystem benefits statewide. Finally, we discuss the potential advantages and uncertainties farm-scale digesters present to the environment, dairy farms, and the wider local community.
Case Study Description
In Pennsylvania and across the U.S., there is increasing interest in installing manure-fed digesters on swine and dairy farms to improve farm economics by selling renewable energy. The Commonwealth of Pennsylvania is home to approximately 30 manure-fed digesters (American Biogas Council, 2020). Historically, dairy farmers adopted digesters for nutrient and odor management, but in recent years, on-farm digesters have also seen increased revenue opportunities from renewable energy, including electricity (Goldstein, 2013) and RNG (U.S. Environmental Protection Agency (EPA), 2021). At least fourteen complete-mix anaerobic digesters in the state are co-digesting manure with food waste (U.S. Environmental Protection Agency (EPA), 2021), which provides additional economic revenue from food waste tipping fees.
The case study dairy farm, located in central Pennsylvania, has two on-site biogas digesters with capacities of 1,893 m3 and 5,678 m3. In addition to accepting manure from the farm's 1,025 dairy cows, the farm co-digests dairy manure with locally sourced food waste (locally sourced as defined by the 110th Congress, 2nd Session, 2008). The original, smaller digester was installed in 2008, followed by the larger digester in 2019 to meet the farm's growing size and increased demand to process off-farm food waste. An overview of the biogas production system is depicted in figure 1. This figure encompasses plant operations, biogas-to-energy conversion, and digestate handling.
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| Figure 1. Process diagram of current anaerobic digestion system at case study dairy farm. Light blue highlighted bubbles indicate end-use or -state of process, while red dotted lines returning to farm indicate that products are recycled. On left side, brown lines indicate manure coming into system; blue lines indicate food waste coming into system. Orange lines indicate in-system processes. Solid red lines indicate mass or energy flowing to end-use. |
Dairy manure is scraped daily from the free-stall barns and added to the digesters along with de-packaged food waste. The mesophilic digesters operate at 37° C and can produce 7,571 m3 of biogas per day, which is used for electricity generation and to meet the heating needs of the farm. Approximately 29.1 million liters of manure and 16,300 Mg (on a wet basis) of food waste are processed at the farm annually. The biogas produced in the digesters is scrubbed to reduce hydrogen sulfide (H2S) concentrations and then used for combined heat and power. Electricity is sold back to the grid as well as used on-farm through a net metering contract, and waste heat recovered from the engine generators is used for various on-farm activities, including heating the digester, heating water for the milking parlor, dairy barns and other buildings, grain drying, radiant heat, and pasteurizing milk for calves. The remaining digestate is separated into liquid and solid parts using a screw press. The liquid fraction is stored in a covered storage tank and is then used to fertilize surrounding fields, and the solid portion is recycled for livestock bedding.
The operation farms 453 hectares, all under no-till management, with winter double-crops used for small grain silage. The winter double-crop is cereal rye (Secale cereale) and is planted in rotation with corn (Zea mays) grown for grain and silage. The farm has planted triticale and winter wheat in the past but now focuses on rye. Crop residues on the farm are limited, primarily corn stover associated with grain production. Notably, the farm has recently planted 4 ha of switchgrass, a perennial warm-season grass crop, that, upon maturity, will be added as an additional feedstock to the AD system. Table 1 lists the case study farm's crop production by crop type and composition. On-farm crop production provides most of the dairy's feed needs, except for supplements used to enhance milk production. This on-farm feed production is unusual in Pennsylvania, where dairies usually grow most of their forage but purchase most of their grain (Malcolm et al., 2015). These grain purchases represent a major input of nutrients, so by growing its grain, the case study farm already has a higher potential for circular nutrient flows than most of its peers.
| Table 1. Case study farm crop production (area, average yield, and total Mg dry matter) by crop type (corn grain, corn silage, mixed hay, winter rye double-crop, corn stover, and switchgrass) and biomass composition—Mg carbon (C), Mg nitrogen (N), Mg phosphorus (P), crude protein (%), ash (%), volatile solids (%), nitrogen (%), carbon (%), and phosphorus (%). | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Crop | Area (ha) | Average Yield | Dry Matter (Mg) | Mg C | Mg N | Mg P | Crude Protein (%) | Ash (%) | Volatile Solids (%) | N (%) | C (%) | P (%) |
| Corn grain[a] | 231.3 | 381.1 bu ha-1 | 1,048 | 659 | 17 | 3.5 | 8.6 | 1.7 | 98.3 | 1.4 | 54.6 | 0.29 |
| Corn silage[b] | 151.5 | 51.9 ton ha-1 | 2,353 | 1251 | 33 | 6.1 | 8.8 | 4.3 | 95.7 | 1.4 | 53.2 | 0.26 |
| Mixed hay[c] | 65.9 | 9.9 ton ha-1 | 605 | 270 | 11 | 1.5 | 13.3 | 8.8 | 91 | 2.1 | 50.7 | 0.29 |
| Rye double-crop[d] | 402.6 | 19.8 ton ha-1 | 2,248 | 1171 | 60 | 9.8 | 16.1 | 9.6 | 90 | 2.6 | 50.2 | 0.42 |
| Corn stover[e] | 231.3 | 381.1 bu ha-1 | 1,048 | 641 | 17 | 3.1 | 8.0 | 7.3 | 92.7 | 1.4 | 53.2 | 0.26 |
| Switch-grass[f] | 4.1 | 18.5 tons ha-1 | 68 | 34 | 1 | 0.2 | 13.3 | 8.8 | 91.2 | 2.1 | 50.7 | 0.29 |
[a]Total dry tons were calculated using 15% moisture content and 56.6 lb bu-1. Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'corn grain and cob.' [b]Total dry tons were calculated using 67% moisture. Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'silage normal.' [c]Total dry tons were calculated using 10% moisture. Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'grasses, cool-season hay-mid maturity.' [d]Total dry tons were calculated using 67.7% moisture (moisture content of small grain silage reported in the Penn State College of Agricultural Sciences Feed Price List distributed via email by Virginia Ishler). Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'rye annual silage.' [e] Total dry tons are assumed to be equal for corn grain and corn stover. Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'corn silage normal (32% to 38% moisture) for corn grain. Crude protein, ash, and volatile solids (VS) were taken from Dairy One (2022) for the average of all reported corn stalks forage samples (2004-2022).%VS = 100 -%Ash. [f]Total dry tons were estimated, assuming 7.5 dry tons per acre. Crude protein, ash, and volatile solids were taken from The National Academies of Sciences, Engineering, and Medicine (2021) for 'grasses, cool-season hay-mid maturity.' | ||||||||||||
Mass Flows in the Case Study
To assess the circularity of the case study farm, we conducted a mass balance of the carbon (C), nitrogen (N), and phosphorus (P) mass flows across the farm boundaries and within the farm, as well as energy flows associated with the digester. While this mass flow analysis has implications for both air and water pollution off the farm, this study is focused on the circular versus linear nature of these flows within the farm system and does not attempt a cradle-to-cradle or cradle-to-grave life cycle analysis due to the limited data available at the case study farm.
We separated carbon flows into two mass balances: (1) biogas methane converted to its carbon mass equivalent, and (2) other carbon. Annual, perennial, and double crops in the case study system take carbon dioxide from the atmosphere and convert it to carbon in plant biomass through photosynthesis. For simplicity, we assumed the carbon in measured crop yields, soil storage, and crop and microbial respiration equaled the carbon coming into the system via photosynthesis. We assumed that carbon in plant biomass that does not stay in the system in soil storage or leave the system in respiration moves through the system as animal feed for dairy cattle. For the farm's annual crops, we assumed a 0.5 harvest index (Pennington, 2013), which means that 50% of the annual corn grain crop’s aboveground biomass is left on the soil and available for soil storage or respiration. For the farm's double crops, we assumed 150 kg C ha-1 is stored in the soil annually (Ramcharan and Richard, 2017). For the farm's perennial grasses, we assumed 700 kg C ha-1 (70 g C m-2) would be stored in soil and belowground biomass, which is an average of 40 to 100 g C m-2 from Anderson-Teixeira et al. (2009). We assumed that (1) 85% of the carbon in the system would be lost through crop and microbial respiration (Nicholson et al., 2017), and (2) carbon in animal feed either contributes to animal growth and respiration, milk production, and digester outputs or is lost through manure and enteric methane emissions. Enteric methane emissions were estimated from data reported by Min et al. (2022). The carbon in milk produced by dairy cattle flows out of the system for human consumption, some of which ends up in food waste that is not recycled in the system. We assumed one-third of the milk produced would be wasted downstream (FAO, 2017). Carbon in manure is captured in the case study system and put into the two anaerobic digesters. The carbon flows through the digesters as biogas or carbon in liquid and solid digestate. We estimated carbon in biogas on a mass basis, assuming biogas is 40% carbon dioxide and 60% methane on a volumetric basis, and closed the carbon balance on the digester by estimating an average annual capacity factor of 55% of the peak measured biogas flows. We assumed a 2% leakage rate of biogas from the digester (Liebetrau et al., 2013). We estimated carbon and nutrients in the liquid digestate based on the case study farm's liquid digestate production volume and composition. For the solid digestate, we applied an average solids concentration of 26.6% (Wright et al., 2004) to the measured solids volume and carbon concentration of the solids, estimating the bulk density of the digestate solids based on its total solids content (Wang et al., 2019). Because the digestate solids are recycled to the barn as bedding that again returns to the digester, we closed the digester mass balance on N and P with those recirculating solids.
The mass of carbon in biogas was converted to and categorized as energy. The energy in the biogas was converted to heat and power, with all the heat and about 20% of the electricity used on the farm and the balance of the electricity sold to the electric grid. Each kg of methane produced is equivalent to 53.2 MJ of energy when it is combusted with oxygen (O2) on a 1:1 molar basis to form carbon dioxide. We also assumed the engine generator sets had a 25.5% electricity conversion efficiency (Herringshaw, 2009) and a 47% thermal conversion efficiency (Bacenetti et al., 2016), with 70% of the captured heat returned to the digester and 30% used for hot water, pasteurization, and other heating requirements of the farm.
Nitrogen was input or recycled in the farming system from atmospheric deposition (estimated from data reported by Burns et al., 2021), the application of recycled liquid digestate and recycled solid digestate as livestock bedding, and annual nitrogen contributions from cropping systems. Nitrogen was also introduced into the system via food waste biomass imported from outside the system boundary. Nitrogen not exported as protein in the milk or recycled in the system as fertilizer was assumed to be lost in runoff and leaching, NH3 volatilization, and denitrification. Nitrous oxide, ammonia volatilization, and dinitrogen gas losses, as well as losses due to runoff and leaching, were estimated from literature values. We subtracted the mass of nitrogen in liquid digestate from the farm's crop nitrogen needs, assuming a 37% nitrogen use efficiency (NUE), to estimate synthetic nitrogen fertilizer needs. We also estimated the amount of nitrogen that would leave the system via culled cows, assuming 0.0240 kg N per kg dry mass body weight of a culled cow (Lahart et al., 2021) and the farm's culled cow rate. Table 2 includes specific assumptions for the nitrogen mass balance assessment.
Like nitrogen, phosphorus was input or recycled in the system via recycled liquid and solid digestate, and the food waste was added to the system. Phosphorus leaves the system in milk for human consumption or through runoff and leaching. We also estimated the amount of phosphorus that would leave the system when cows are culled, assuming 0.008 kg P per kg of a calf's dry mass body weight and 0.007 kg P per kg of an adult cow's dry mass body weight is phosphorus (Peterson et al., 2017). This analysis did not include livestock feed supplements to enhance milk production or reduce enteric methane emissions.
The system boundaries within the circular economy's context depend on upstream and downstream processes. We used a Sankey diagram to visualize the mass flows in the system. In a Sankey diagram, the links' width is proportional to the magnitude of flow rates between nodes, allowing clear identification of system productivity, efficiency, reuse, and loss. In this process, we have external flows entering the system via food waste added to the digester and from the atmosphere, with flows leaving the system in the form of food for human consumption, nitrogen and phosphorus runoff and leaching, emissions and respiration, and electricity after it leaves the farm.
Potential Statewide Biomass Resource and Biogas Production
We quantified Pennsylvania's biogas and electricity potential for crop biomass, food waste, and manure using (1) annual crop residues, perennial grass biomass, and food waste estimates for 2030 reported in the U.S. Department of Energy Billion-Ton study (U.S. Department of Energy, 2016); (2) double-crop winter rye estimates from Feyereisen et al. (2013); (3) current milk cow production numbers reported by the USDA (2020); and (4) manure production estimates per cow following the ASAE D384.2 MAR2005 standard for manure production (ASABE, 2005). Though an 11% decline in milk cows (Center for Dairy Excellence, 2021) is expected by 2030, this is also likely to be offset by an increase in milk production per cow, which correlates with manure production, so we assumed manure volumes would remain steady. While there may also be a corresponding improvement in feed efficiency, this effect will likely be small in the timescale of interest, so we ignored its impact on milk and manure production. We assumed milk cows produce manure at a rate of 68 kg cow-1 day-1 at 87% moisture content (ASABE, 2005).
We limited our biomass analysis to estimates of corn stover, winter rye, and switchgrass to represent crop residue, double-crop, and perennial grass feedstocks similar to the crop portfolio of our case study farm. We used county-level 2030 estimates for corn stover and switchgrass for Pennsylvania from the U.S. Department of Energy (2016) under the base-case scenario that assumes a 1% annual yield increase in energy crops from 2015 to 2040 at $70 dry ton-1. Likewise, we used the statewide 2030 food waste estimates from the U.S. Department of Energy (2016) under the base-case scenario for 2030.
We used Valli et al.'s (2017) farm-scale AD results to estimate biogas production from corn stover, winter rye, manure, and food waste. For corn stover, we used Valli et al.'s (2017) findings for corn silage; they observed a yield of 0.679 m3 biogas kg-1 volatile solids for corn silage containing 96% volatile solids, with 89% of the volatile solids degraded during digestion. Similarly, for winter rye, we used Valli et al.'s (2017) findings for triticale silage at 94% volatile solids, which were 0.594 m3 biogas kg-1 volatile solids, with 78% of the volatile solids degraded during digestion. For food waste, we use their findings for potato scraps containing 96% volatile solids at 0.696 m3 biogas kg-1 volatile solids, with 87% of the volatile solids degraded during digestion. For manure feedstock, we assumed the farm's cattle slurry solids fraction contained 83% volatile solids, with 55% of the volatile solids degraded during digestion for a
| Table 2. Nitrogen, phosphorus, carbon (other), carbon (methane), and energy mass flows as represented in the Sankey diagram, listing all assumptions. All non-methane carbon is represented as carbon (other). Mass flows are categorized as an 'input' to the system, a process or flow occurring in the system, 'in-system,' or an 'output' from the system. System boundaries are outlined in figure 2. | ||||||||
| From Node | To Node | Category | ¦ Nitrogen | ¦ Phosphorus | ¦ Carbon (other) | ¦ Carbon (methane) | ¦ Energy | Notes |
| kg N yr-1 | kg P yr-1 | kg C yr-1 | kg C yr-1 | 100 MJ yr-1 | ||||
| Atmosphere | Annual crops | Input | -- | -- | 2,550,500 | -- | -- | Mass flow of net primary production of carbon via photosynthesis into harvested crop biomass and soil. |
| Atmosphere | Perennial crops | Input | -- | -- | 353,081 | -- | -- | Mass flow of net primary production of carbon via photosynthesis into harvested crop biomass and soil. |
| Atmosphere | Double- crops | Input | -- | -- | 1,573,974 | -- | -- | Mass flow of net primary production of carbon via photosynthesis into harvested crop biomass and soil. |
| Atmosphere | Soil | Input | 7,697 | -- | -- | -- | -- | Nitrogen deposition; assumed 9 kg N ha-1 from Burns et al. (2021) for the Chesapeake Bay watershed where the case study farm is located. |
| Annual crops | Soil | In- System | 16,978 | 3,135 | 641,097 | -- | -- | Mass flow of crop residues to the soil from annual crops assuming a corn grain harvest index of 0.5 (Pennington, 2013) to estimate corn stover and composition of corn stalks (silage normal) from National Academies (National Academies of Sciences, Engineering, and Medicine, 2021). Corn root biomass is assumed to be in steady-state with soil C. See table 1 for assumptions. |
| Annual crops | Livestock | In- System | 50,105 | 9,614 | 1,909,403 | -- | -- | The farm’s measured crop production (dry basis) was calculated using assumptions for corn grain and corn silage, respectively (grain and cob nutrient requirements for dairy cattle, and silage normal, respectively, from the National Academies of Sciences, Engineering, and Medicine, 2021). See table 1 for assumptions. |
| Double- crops | Soil | In- System | 21,192 | 9,796 | 402,639 | -- | -- | Assumed 150 kg C ha-1 (550 kg CO2-e ha-1) will be stored in the soil annually based on Ramcharan and Richard's (2017) results for fertilized rye compared to winter fallow. At 15% conversion of C input to soil organic carbon (see below), this requires 1000 kg C ha-1 input. Also assumed a C:N ratio of 19:1 based on farm's crop production data and estimated composition from Dairy One (2022). |
| Double- crops | Livestock | In- System | 60,080 | 9,796 | 1,171,335 | -- | -- | The farm's measured crop production (dry basis); assumed 50.2% C, 2.6% N, and 0.42% P for winter rye (National Academies of Sciences, Engineering, and Medicine, 2021). See table 1 for assumptions. |
| Perennial crops | Soil | In- System | 6,999 | 1,543 | 48,991 | -- | -- | Assumed belowground biomass potential is 70 g C m-2 (Anderson-Teixeira et al., 2009) and 0.1 Mg ha-1 N (Stewart et al., 2016) for the 97 hectares in perennial grasses, which includes 4 ha recently planted in switchgrass. Assumed an equal amount of P in aboveground and belowground biomass. |
| Perennial crops | Livestock | In- System | 11,324 | 1,543 | 269,618 | -- | -- | The farm's measured crop production (dry basis); assumed 50.7% C, 2.1% N, and 0.29% P (National Academies of Sciences, Engineering, and Medicine, 2021). See table 1 for assumptions. |
| Perennial crops | Digester | In- System | 1,448 | 197 | 34,473 | -- | -- | Farm's perennial energy crop area (4 ha) and estimated yield (17 Mg ha-1 from Field et al. (2020) assumed 50.7% C, 2.1% N, 0.29% P (grasses, cool-season hay-mid maturity from National Academies of Sciences, Engineering, and Medicine, 2021). See table 1 for assumptions. |
| Livestock | Milk | Output | 58,145 | 9,636 | 706,702 | -- | -- | Milk production from farm's 1,025 Holsteins based on average milk production per cow reported by USDA (2020). Assume milk is 87% water, 4% nitrogen and 52% carbon (dry basis), and 93 mg P per 100 g of milk from (Chandan and Kilara, 2011), who report whole milk solids (dry basis) are 5.56% ash, 22.22% protein casein, and 4.76% whey protein, and assuming the average nitrogen content of proteins is 16%. |
| Milk | Human consumption | Output | 58,145 | 9,636 | 706,702 | -- | -- | Mass is carried through the system. |
| Human consumption | Food waste | Input | 17,444 | 2,891 | 235,332 | -- | -- | Assumed one-third of milk produced on-farm is wasted downstream (FAO, 2017). |
| Livestock | Manure | In- System | 80,940 | 28,438 | 1,052,221 | -- | -- | Farm's actual manure production: measured production is less than estimated by ASABE (2005); assumed 10.2% solids (Wright et al., 2004), 48.1% carbon from 3.7% N dry weight, 0.13% P dry weight, C:N ratio of 13:1 ASABE (2005). |
| Livestock | Atmosphere | Output | -- | -- | 143,788 | 77,724 | -- | Enteric methane emissions estimated from Min et al., 2022. Livestock respiration was calculated based on 1,025 Holsteins and rates in Prairie and Duarte (2007). |
| Manure | Atmosphere | Output | 81 | -- | -- | 2,385 | -- | CH4 and N2O emissions from digestate storage were estimated from Min et al. (2022). |
| Manure | Digester | In-System | 80,859 | 28,438 | 1,052,221 | -- | -- | Mass is carried through the system. |
| Table 2 (continued). Nitrogen, phosphorus, carbon (other), carbon (methane), and energy mass flows as represented in the Sankey diagram, listing all assumptions. All non-methane carbon is represented as carbon (other). Mass flows are categorized as an 'input' to the system, a process or flow occurring in the system, 'in-system,' or an 'output' from the system. System boundaries are outlined in figure 2. | ||||||||
| From Node | To Node | Category | ¦ Nitrogen | ¦ Phosphorus | ¦ Carbon (other) | ¦ Carbon (methane) | ¦ Energy | Notes |
| kg N yr-1 | kg P yr-1 | kg C yr-1 | kg C yr-1 | 100 MJ yr-1 | ||||
| Food waste | Digester | Input | 91,638 | 15,186 | 1,113,769 | -- | -- | Assume milk is 13% solids, 4% nitrogen, and 52% carbon (dry basis), and 93 mg P per 100 g of milk from (Chandan & Kilara, 2011), who report whole milk solids (dry basis) are 5.56% ash, 22.22% protein casein, and 4.76% whey protein, and assuming the average nitrogen content of proteins is 16%. |
| Digester | Liquid digestate | In- System | 163,760 | 11,674 | 842,709 | -- | -- | Digestate is stored in a covered storage tank. The values reflect the farm's measured liquid digestate production and its measured composition. |
| Digester | Solid residue | In- System | 8,737 | 31,950 | 207,751 | -- | -- | The solid residue nutrient composition was calculated by subtracting the measured nutrient composition of the liquid digestate from the nutrients entering the digester in food waste and manure. The values reflect the farm's measured solid digestate production and its measured composition, and they assume a density of 993 kg m-3 (Wang et al., 2019) |
| Liquid digestate | Fertilizer | In-System | 163,760 | 11,674 | 842,709 | -- | -- | Mass is carried through the system. |
| Solid residue | Bedding | In-System | 8,737 | 31,950 | 207,751 | -- | -- | All solid digestate is recycled as livestock bedding on the farm. |
| Bedding | Digester | In-System | 8,737 | 31,950 | 207,751 | Bedding is recirculated through the system and into the digester. | ||
| Fertilizer | Soil | In- System | 163,760 | 11,674 | 842,709 | -- | -- | Mass is carried through the system; all fertilizer is applied to the soil storage. |
| Digester | Biogas | In- System | -- | -- | 446,103 | 669,154 | -- | The farm's measured biogas production: assumes 40% of biogas is carbon dioxide on a volumetric basis and estimates an average annual capacity factor of 55% of the peak measured biogas flows. |
| Biogas | Atmosphere | Output | -- | -- | 1,128,640 | -- | -- | CO2-C, including biogas CO2 plus CO2 from combusted CH4 plus 2% leakage. Leakage was assumed by rounding the average reported by Liebetrau et al. (2013) (1.87%) up to 2%. This value is more conservative than the 1% assumed by Veltman et al. (2018) in their modeling study. Liebetrau et al.'s (2013) analysis included measured data from 10 biogas plants. |
| Soil | Atmosphere | Output | 20,879 | -- | 1,645,120 | -- | -- | Carbon leaves via crop and microbial respiration or remains in soil; it is assumed that 15% stays in the soil and 85% is released to the atmosphere (Nicholson et al., 2017). N2O losses estimated for land application of digestate from Holly et al. (2017) (1% via N2O). NH3 and denitrification/nitrification losses are assumed to be 28.5 kg N ha-1 and 14 kg N ha-1 respectively (double cropped rye, broadcast application, Pennsylvania dairy farm scenario from Castaño-Sánchez et al., 2022). |
| Soil | Annual crops | In- System | 23,186 | 4,135 | -- | -- | -- | Calculated based on the farm’s manure management plan, assumed 37% Nitrogen Use Efficiency (NUE) (Cassman, 2002). |
| Soil | Double- crops | In- System | 32,217 | 6,216 | -- | -- | -- | Calculated based on the farm’s manure management plan, assumed 37% NUE (Cassman, 2002). |
| Soil | Perennial crops | In- System | 5,187 | 979 | -- | -- | -- | Calculated based on the farm’s manure management plan, assumed 37% NUE (Cassman, 2002). |
| Soil | Runoff / leaching | Output | 20,192 | 344 | -- | -- | -- | Nitrogen leaching and phosphorus loss are assumed to be 44.6 kg N ha-1 and 0.760 kg P ha-1 (double cropped rye Pennsylvania dairy farm scenario from Castaño-Sánchez et al., 2022). |
| Soil | Soil organic matter | In- System | 20,737 | -- | 290,315 | -- | -- | Carbon leaves via crop and microbial respiration or remains in soil; assumed 15% stays in the soil and 85% is released to the atmosphere; Assumed a C:N ratio of 14% (from Herbstritt, 2022) to estimate N in soil organic matter. |
| Biogas | Heat and power | In- System | -- | -- | -- | -- | 47,465,341 | Mass carried through the system after converting carbon flows to methane; assumes 53.2 MJ kg-1 for methane. |
| Heat and power | Captured heat | In- System | -- | -- | -- | -- | 22,308,710 | Assumed thermal efficiency of 47% (average of digesters in Bacenetti et al., 2016). |
| Captured heat | Milk | In- System | -- | -- | -- | -- | 6,692,613 | Assumes 30% of captured heat is used for farm operations, mostly in the milking parlor. |
| Captured heat | Digester | In-System | -- | -- | -- | -- | 15,616,097 | Assumes 70% of captured heat is used to heat the digester. |
| Heat and power | Electricity | In- System | -- | -- | -- | -- | 12,103,662 | Assumes 6.7 MJ m-3 to electricity. Assumed electric efficiency of 25.5% (Herringshaw, 2009). |
| Electricity | Livestock | In- System | -- | -- | -- | -- | 2,420,732 | Approximately 20% of electricity generated remains in the system and is used for farm operations. |
| Electricity | The grid | Output | -- | -- | -- | -- | 9,682,930 | Approximately 80% of the electricity leaves the system. |
yield of 0.429 m3 biogas kg-1 volatile solids, as in ASABE (2005) for lactating dairy cows. For perennial grasses, we used Niu et al.'s (2015) conversion of switchgrass to biogas (0.2688 m3 biogas kg-1 volatile solids and 95.8% volatile solids). To further conceptualize biogas potential for the state, we estimated electricity production, assuming 1 m3 of biogas is equivalent to 38.2 MJ and an electricity conversion efficiency of 25.5% (Herringshaw, 2009), and discussed the potential economic impact for the state.
Results and Discussion
Mass Flows in the Case Study
Nitrogen, phosphorus, methane (displayed as carbon), and non-methane carbon mass flows are visualized in figure 2. The mass flow values and any assumptions used in estimating flows are in table 2.
The case study farm feeds its 1,205-head dairy with feed produced from annual crops (corn grain and corn silage), double-crops (winter rye), and perennial grasses (cool-season hay), all of which capture carbon dioxide from the atmosphere to produce biomass. The carbon, nitrogen, and phosphorus masses fed to livestock from these individual categories of crops are found in table 3. The dairy produces 1,347 Mg (dry basis) of milk on average annually, which is transported off-farm and sold to consumers. This analysis did not include carbon costs associated with packaging and transporting milk offsite.
 |
| Figure 2. Nitrogen, phosphorus, carbon (other), carbon (methane), and energy mass flows are in kg yr-1 (N, P, C) and 100 MJ yr-1 (energy), visualized in a Sankey diagram. Mass flows are colored by nutrient category. Widths are proportional to mass. Node lengths do not represent any scale. System boundary is edge of farm property and is represented by black dashed outline. Atmosphere, electric grid, runoff and leaching to surface and subsurface water, human consumption, and food waste fall outside of system boundary. Boundary highlights mass flows that remain on case study farm and those that flow out of or into farm. |
| Table 3. Total case study farm carbon (Mg C), nitrogen (Mg N), and phosphorus (Mg P) mass flows from crops to livestock by crop category (annual crops, double crops, perennial crops). | |||
| Crop category | Mg C | Mg N | Mg P |
| Annual crops | 2,550 | 67 | 13 |
| Double crops | 1,171 | 60 | 10 |
| Perennial crops | 304 | 13 | 2 |
Energy and Carbon
The farm is self-sufficient in meeting its energy needs, except for fuel for farm equipment and vehicles. The farm's biogas is combusted in two combined heat and power engine generator sets, rated at 130 kW and 499 kW. The heat from the generators, which is all used on the farm, primarily heats the digesters with about 30% for farm operations. The farm also consumes about 20% of the electricity produced through a net-metering contract and thus sells about four times more electricity to the grid than is used, resulting in monthly energy income. By creating an excess of energy that is returned to the electric grid, the energy analysis highlights the broader off-farm benefits to the circular economy.
Our analysis indicates that the carbon entering the case study farm is approximately equal to the carbon leaving the case study farm (our calculations estimate the net fluxes are within 0.1%). However, a carbon balance is different from a greenhouse gas balance, for which we did not have measurements to estimate. On the positive side, our analysis does not consider the greenhouse gas offset credits associated with the renewable electricity sold off the farm. However, our analysis also does not account for several on-farm greenhouse gas emissions, such as methane produced by the cows, fugitive methane leakage at the digesters, methane slip from incomplete combustion at the engine generator set, nitrous oxide emissions associated with crop production, or CO2 emissions from burning fossil fuels in farm tractors and equipment. On-farm fossil fuel emissions could be estimated from vehicle fuel records but are likely to be small relative to the CH4 and N2O emissions (Chianese et al., 2009; Malcolm et al., 2015) that are both much larger and very difficult to measure on an operating farm without expensive instrumentation. We also did not consider the carbon capture and storage of the CO2 emitted from the engine generator set, which totals over 2 million kg of carbon (>7.4 million kg of CO2) annually.
According to our estimates, soil carbon is accumulating at a rate of about 290,000 kg yr-1. Inputs include annual crop and double-crop residues, perennial crop belowground biomass and leaf litter, and carbon in digestate. As these inputs are returned to the soil, they contribute to the farm's terrestrial carbon storage, helping to regenerate the natural ecosystem's functions and drawdown carbon from the atmosphere. Although carbon is accumulating in the system, on a carbon-dioxide equivalent mass basis, enteric methane emissions leaving the system account for 29% of the carbon being fed to livestock, assuming a global warming potential of 34 as in Camargo et al. (2013) and IPCC (2013). Therefore, the reduction in enteric methane emissions from livestock represents a significant opportunity to improve the carbon benefit of the system.
Although the farm has invested in the liquid-solid separation of the digestate, only about 2% of the carbon in the digestate is recycled to the livestock barns as bedding, with 98% of the digestate carbon returning to the soil. This is similar to the results of Aui et al. (2019), who report that approximately 94% of the total digestate solids flow to the liquid digestate stream after liquid-solid separation. Though the carbon in digestate has likely been converted to a more stable carbon than raw manure (Launay et al., 2022), there is considerable uncertainty about how digestate carbon contributes to long-term soil carbon storage. Long-term measurements on-farm of the soil carbon pool, soil, and microbial respiration are needed to understand the system.
It is important to note that the crop residues and energy crops added to this farm digester were not previously producing methane emissions (as the manure was), but rather were degrading aerobically directly to CO2. Converting such feedstocks to methane in a digester creates new risks of fugitive CH4 emissions. Even a small percentage of methane leakage from a digester may negate the carbon benefits of feedstocks that otherwise degrade aerobically (Grubert, 2020; Herbstritt et al., 2022). In addition to sealing leaks, any downstream engines or gas turbines must achieve nearly complete combustion of the methane, as even ~2% unburned methane (methane slip) dramatically reduces the GHG benefits (Moscato et al., 2020). It is also worth noting that upgrading biogas to RNG would require a significant upfront cost and access to a pipeline to be integrated into the natural gas grid (Walker et al., 2018). On-farm and downstream methane leakage is highly variable and, thus, must be more widely measured on commercially operated farm digester systems to assess the overall greenhouse gas impacts, including this case study farm where measured leakage data is not currently available.
Nitrogen and Phosphorus
Within the case study farm, some nitrogen is deposited from the atmosphere onto crop fields, but most of the nitrogen in crop biomass comes from liquid digestate recycled in the system and applied as a soil amendment. Of the total nitrogen and phosphorus in food waste and livestock feed that comes into the farm, 77% of the nitrogen and 32% of the phosphorus ends up in liquid digestate. Approximately one-third of the nitrogen in digestate comes from off-farm food waste. Less than 3% of the nitrogen and phosphorus leaves the farm in culled cows.
The digester produces 32 million L of liquid digestate and 1,800 Mg of solid digestate annually. The solids content of the separated liquid digestate was 4.6% solids, similar to the average total solids content of 5.25% reported by Wright et al. (2004) from case studies of five digesters in New York State. Our case study farm returns liquid digestate to annual, perennial, and double-crop fields as a soil amendment and meets 100% of their crop phosphorus needs.
Like phosphorus, the farm is almost self-sufficient in nitrogen fertilizer. Our mass flow analysis estimates the farm can meet 78% of its crop N needs from waste products recycled in digestion (assuming 37% crop NUE), while 22% of its nitrogen comes from synthetic N fertilizers. The farm occasionally side-dresses N fertilizer, but more data is needed to quantify the necessity of this practice. Anaerobic digestion converts much of the protein N from fresh manure to ammonium nitrogen. This higher ammonium concentration results in a liquid fertilizer that is more readily available to plants and has, on average, a higher nitrogen utilization percentage than manure (Alburquerque et al., 2012; Lukehurst et al., 2010).
However, the farm currently applies its liquid digestate by broadcast spray application, which is associated with significant ammonia N losses to the atmosphere. Injecting digestate into the soil could reduce ammonia volatilization associated with broadcast spray application, which the case study farm is evaluating by participating in a manure injection trial. More measured data from commercial dairies is needed to quantify the N losses from liquid digestate in broadcast applications and the potential benefits of digestate injection. Still, by taking a waste product (manure) and producing a more predictable fertilizer source (digestate), anaerobic digestion may reduce the overapplication of organic fertilizer and the offsite export of nutrients in nonpoint source pollution.
Solid digestate is returned as livestock bedding for the farm’s dairy cattle. By reducing the need for synthetic nitrogen fertilizer and livestock bedding, this aspect of the farm’s circular economy also reduces overall farm costs and increases profitability.
One of the negatives this analysis revealed is that the farm in the case study is accumulating phosphorus and nitrogen in the system, with more phosphorus accumulation than nitrogen. This nutrient accumulation leaves the possibility of additional N losses to the atmosphere and excess N and P running off or leaching into downstream waters, especially as soils become P-saturated (Dodd and Sharpley, 2015). However, the farm follows a nutrient management plan to reduce potential losses. Nevertheless, the consequence of excess nutrients building up in the liquid digestate could present an opportunity to capture those nutrients as a commercial fertilizer (digestate) to increase on-farm revenue.
The Importance of Food Waste in Circularity
The case study farm produces 66% of its biogas from off-farm food waste sources, highlighting the potential to increase the circularity of food and agricultural systems when farms capture and recycle external waste sources. The farm accepts 16,329 Mg (wet basis) of food waste annually, estimated at 2,123 Mg on a dry basis. This food waste is primarily past-date or off-spec cow milk, fruit and vegetable produce, and dog food. Estimates are that about one-third of fresh milk is wasted downstream (FAO, 2017), which would be 404 Mg (dry basis) per year for this farm. Thus, the one-third of the farm’s milk that is likely wasted downstream is less than 20% of the food waste they accept into their system, meaning they take in four times more food waste than is generated off-farm from their own milk. This considerable input of external food waste beyond that associated with farm products represents a nutrient input that substitutes for purchased fertilizer nutrients for the case study farm. If other farms in the state were to improve the circularity of the food system by recovering downstream waste, less synthetic chemical fertilizer would be required if nutrients were captured and distributed based on crop needs. Of course, only a subset of farms can recover such a disproportionate share of that downstream food waste, but many more could recover food waste than are currently doing so. Still, off-farm N and P inputs must be managed cautiously to avoid the buildup of excess nutrients in the system, which can increase the potential for nutrient losses and pollution.
The food waste is converted in the digester into mostly biogas and liquid digestate, with the methane in the biogas burned to produce carbon dioxide and electricity. Conventional food waste disposal in landfills and other waste treatment systems, often in poorly contained anaerobic environments, produces methane emissions that have a 100-year global warming potential of 28 to 36 times that of carbon dioxide (IPCC, 2014). Eliminating these fugitive emissions at landfills reduces greenhouse gas emissions and improves the system's overall carbon and nutrient balance. And as previously mentioned, when the liquid digestate is land-applied as a soil amendment, some of the carbon will remain in the soil.
The Importance of Crop Feedstocks
Food waste and manure are the only feedstocks currently fed into the farm's digester. However, we also included the future flow of switchgrass into the digester in the analysis presented in table 2 and the Sankey diagram in figure 2. This future feedstock is included in the perennial crops category, which also includes hay that is fed to livestock. The farm plans to co-digest switchgrass when the recently planted four hectares reach maturity in one to three years, when it is likely to yield at least 13 Mg ha-1 based on county-level modeled yields for Pennsylvania (Field et al., 2020). The farm planted switchgrass on marginal cropland to enhance environmental quality and ecosystem services. The farm does not need the switchgrass for livestock bedding or feed, so it intends to feed it to the digester to generate more energy and income. Planting switchgrass on land that is unsuitable for food or feed production offers an opportunity to increase and restore the profitability of the land and provides several ecosystem services as well as an additional bioenergy feedstock (Asbjornsen et al., 2014).
It is important to note that harvesting the switchgrass does not reduce the water quality benefits, and the income may encourage the adoption of this perennial crop by landowners (Jiang et al., 2019). The same is true for double crops like winter rye (Malone et al., 2018; Malone et al., 2022; Herbstritt et al., 2022). This case study farm is currently feeding its double-cropped rye biomass to its cows for milk production, so we did not include that as a feedstock to the digester, although some spoiled feed and low-quality forage biomass are likely to be added. For farms with digesters that are not currently double-cropping, adding a double-crop to fields in annual food crop production would increase energy feedstock supplies without disrupting the food supply (Dale et al., 2016b; Launay et al., 2022; Herbstritt et al., 2022).
Potential Biomass Resource and Biogas Production in Pennsylvania
There is potential for other farms in the state to adopt similar circular systems. The Billion-Ton study (U.S. Department of Energy, 2016) estimates Pennsylvania could produce 735,480 Mg of corn stover and 23,961 Mg of switchgrass (all on a dry basis) by 2030. Based on modeled results in Feyereisen et al. (2013), another 2.1 million Mg (dry basis) of biomass could be available from double-cropping winter rye on annual cropland in Pennsylvania, totaling almost 3 million Mg of biomass from crop residues, double-crops, and perennial grasses for bioenergy. An additional 293,092 Mg of food waste was also projected for Pennsylvania’s 2030 base case scenario.
Pennsylvania milk cows total 525,000 today (USDA, 2020). At 68 kg manure cow-1 day-1, these dairies produce over 4,641 Mg day-1 (dry basis) of manure that could be digested, with the leftover digestate applied as a soil amendment. Currently, most farms in Pennsylvania apply undigested manure. Digesting this manure before the land application could help improve soil health by converting the nutrients in manure to a more accessible form, such as ammonium, for plants to use. As previously discussed, nitrogen in the form of ammonium is easily volatilized, and in a surface application of the digestate in a no-till system, a significant portion of the nitrogen will be volatilized and lost to the atmosphere, reducing the circularity of nutrient flows. Therefore, it is important to minimize the surface area of the digestate that is exposed to the air after application, which can be achieved by subsurface injection of the digestate (Lukehurst et al., 2010).
In total, we estimate Pennsylvania could produce 3,347 GWh of electricity annually from these estimates of corn stover, switchgrass, winter rye, food waste, and manure without changing the current cropland area. This estimate would increase if unproductive or unprofitable annual cropland were converted to dedicated energy crops or if more efficient power generation systems, such as fuel cells or gas turbines, were used to produce electricity from the biogas. With the average annual per capita electrical power consumption estimated at 11,331 kWh (Lippert, 2014), biogas from annual crop residues, winter rye, switchgrass, food waste, and manure could offset 3% of the total electrical power consumption in Pennsylvania without reducing the current area for annual food crops.
Discussion
Anaerobic digestion has the potential to be a keystone technology for the circular economies of livestock farms like the case study dairy analyzed here. Recycled on-farm manures and biomass, as well as off-farm food wastes, can regenerate agroecosystems with perennial and double crops, provide economic benefits to help ensure agricultural resilience and sustainability, and offset fossil energy consumption. However, widespread commercial implementation of circular economy principles in the U.S. dairy sector requires more data about how farms successfully implement circularity within the constraints of market incentives and farm operations. Where possible, we measured a commercial dairy farm's mass flows of C, N, and P, as well as energy flows to understand its circularity. When necessary, we used literature values, selecting measured data from the literature over reported modeled results. Our analysis provided insights into several aspects of circular economy systems as well as data limitations of commercial dairy farms that should be addressed for future analyses.
On-Farm Energy Self-Sufficiency
A core tenet of a circular economy is the conversion of waste to resources and, thus, to design waste out of systems (Stahel and MacArthur, 2019). In Pennsylvania, there has been a decrease in the number of dairy farms in favor of larger, consolidated operations (Winsten et al., 2010) that require imported feed. This trend has decoupled nutrient flows between crops and livestock, resulting in grain imports to dairy regions that are often produced with synthetic fertilizers (Hilimire, 2011). These decoupled production systems create an imbalance wherein farms must bring in animal feed, fertilizer, and energy for livestock and crop production. This results in significant inputs of N and P to the region, and the embedded GHG emissions associated with these inputs further increase the GHG emissions associated with dairy production. Integrated crop-livestock systems can reduce this nutrient imbalance and help reduce GHG emissions. However, better accounting of GHG emissions is needed to understand the impact of practices implemented on commercial farms.
Perhaps the most valuable resource highlighted in this case study is the energy generation potential from organic wastes. Producing on-farm energy can further reduce the GHG emissions associated with dairy production systems (Malcolm et al., 2015). In this case study, we present a farm that can provide all its electricity needs for livestock and crop production (e.g., heating of barns and wash water, milk pasteurization, electricity) with a large surplus of energy that is returned and sold to the electric grid. Furthermore, by importing food waste to feed the digester, this case study farm can satisfy the vast majority of the farm's feed and fertilizer needs. Tipping fee income from the food waste, net-metering to discount on-farm electricity costs, and off-farm sales of excess electricity all make the digester a profitable technology to increase circularity.
In both the case study farm analysis and our statewide calculations of AD potential for Pennsylvania, food waste is included as a feedstock for anaerobic digestion. This flow brings additional feedstock, carbon, and nitrogen back into agricultural production, which is then converted into energy, nutrients, and soil carbon. Due to the volume of food waste being brought into the farm and its high digestibility, the food waste feedstock is responsible for most of the biogas production at the case study farm. We estimate the food waste feedstock produces 66% of the biogas, manure produces 33% of the biogas, and once fully established, the 4 hectares of switchgrass will produce 1% of the biogas. Biogas production estimated from data from Valli et al. (2017) and Niu et al. (2015) is within 7% of the measured biogas produced at the case study farm. In this case, food waste provides a significant feedstock for biogas production and represents an additional 1,113,769 kg of carbon and 91,638 kg of nitrogen that is processed and upgraded into energy and higher-value products rather than being wasted in a landfill. While an increasing number of landfills now have liners and recover landfill gas as RNG, the nutrients are not recovered, and the undigested carbon does not contribute to agricultural soil health. Production of synthetic N fertilizer is energy-intensive, and the resulting embedded GHG emissions represent the largest part of the GHG footprint of most field crops (Camargo et al., 2013). These embedded emissions can be reduced by recovering nutrients from waste and using them as a replacement for synthetic fertilizer.
Nutrient Cycling
In addition to designing out waste from circular economies, another important goal of circular economies is to reduce toxicity (Ellen MacArthur Foundation, 2013). Nutrients associated with crop fertilizers are of particular importance to the health of the Chesapeake Bay watershed, where this case study farm is located (Beegle, 2013). The case study shows the potential to reduce and potentially eliminate synthetic fertilizer use for many farms and, combined with fertilizer injection to further capture a significant portion of the nitrogen as well as phosphorus added to the system. The case study farm uses digestate for approximately 78% of their crop nitrogen needs and 100% of their crop phosphorus needs, only occasionally using synthetic fertilizer to top-dress or side-dress fields for N. Synthetic nitrogen fertilizers are highly mobile and difficult to conserve in agriculture (Jarvis, 1993), yet with the digester and complementary soil, manure, and crop management, the farm can meet most of the needs of their crops with recycled nutrients from the digestate.
The farm also mitigates nitrogen pollution by creating fertilizer application setbacks from streams and drainage features and by using no-till farming practices, both of which help reduce nitrate loading to surface water. However, there are opportunities to further decrease nitrogen losses to the environment, including the previously discussed injection of digestate rather than broadcast application; applying digestate and fertilizer only in the late spring or summer rather than in the fall; applying nitrification inhibitors to limit nitrate loss as well as nitrous oxide emissions during denitrification; and incorporating edge-of-field practices such as riparian buffers to capture runoff, nutrients, and sediment (Veltman et al., 2018; Jiang et al., 2020; Akiyama et al., 2010); furthermore, additional soil testing data could help the farm understand whether they need to side-dress nitrogen in any particular year or field.
The farm is currently participating in a digestate injection trial to evaluate this potential option to mitigate nitrogen losses to the environment. If the farm were to invest in fully applying its digestate via injection, this would reduce the ammonia emissions and thus add more nitrogen to the soil. By injecting digestate without adjusting the nutrient management plan, the farm risks having a surplus of nitrogen in the soil, which could increase nutrient leaching and runoff in the ground and surface waters. More research is needed to synchronize the farm's nutrient needs with careful timing and appropriate quantities of digestate application, such that digestate is not applied during a period of low plant nutrient uptake or high rainfall and N leachate is minimized. These and other paths offer a range of opportunities for the farm to further decrease nitrogen losses and thereby increase the circularity of this nutrient in the farm and food systems.
Anaerobic Digestion as a Carbon Emission Mitigation Strategy
An engineered AD system can mitigate more GHG emissions than can be sequestered in the soil via terrestrial carbon sequestration (Cole et al., 1997; Veltman et al., 2018). Our mass flow analysis shows that the energy created from waste feedstocks offsets more GHG emissions than agricultural conservation practices and that adding biomass crops could further increase GHG mitigation. However, a digester system that leaks even a small percentage of methane may negate the additional GHG benefit of biomass crop digestion. Typical methane losses are reported in the range of 1% to 3% (Liebetrau et al., 2013), and >10% loss has been reported when upgrading biogas to RNG (Börjesson and Berglund, 2007), so these fugitive methane losses can be substantial.
The large CO2 flow back to the atmosphere after biomass combustion represents a potential opportunity to increase the climate benefits of digesters. The roughly 7.4 million kg yr-1 of CO2 released annually from biogas combustion represents a large opportunity for downstream carbon benefits such as bioenergy carbon capture for permanent storage in underground geologic formations or other CO2 utilization strategies. In this case study farm, only about 5% of the livestock feed and food waste carbon entering the system is ultimately stored in the soil and represents only about 10% of these annual biogas combustion emissions. Sixty percent of those CO2 emissions are from biogenic methane combustion and thus represent a fossil fuel offset many times greater than soil carbon sequestration, indicating the importance of on-farm biogas production (an engineered solution) as a greenhouse gas mitigation strategy. This scale differential also highlights the importance of (1) conducting comparative analyses that do not conflate soil health practices with greenhouse gas mitigation practices and (2) validating the scale of different options with commercial-scale practices and measured results. However, data quantifying the fugitive methane being emitted from the electricity generated from the farm’s biogas is rarely collected from commercial farms, and the GHG offsets are rarely quantified unless the biogas is upgraded to RNG for the California Low Carbon Fuel Standard marketplace. Offsets include emissions that would have otherwise been emitted by the farm by using an alternative regional energy source in the absence of the energy from the digester or from the food waste feedstock that was diverted to the digester that would have otherwise degraded in a landfill. Data about both fugitive methane (leakage and methane slip) and offset benefits are needed locally to understand the greenhouse gas balance. Farms aiming for net-zero emissions need to be measuring emissions and quantifying offsets to understand if they are meeting their goals.
Potential Advantages of Farm Scale Digesters to the Farm and Wider Community
- Farm-scale digesters provide an opportunity for dairy farmers in Pennsylvania and elsewhere to improve their environmental quality, farm economy, and relationships with the larger community. The digesters provide the case study farm with a source of income that increases the economic viability of the operation while also providing an opportunity for increased environmental stewardship, which is important not only to the owners but also to their customers. This case study farm initially explored AD as a solution to control farm odors for their local community, and the system was successful in that regard. But the digesters now serve a more central role that has allowed the farm to: (1) diversify and grow its portfolio and revenue streams, (2) gain energy independence, (3) reduce the need for synthetic nitrogen fertilizer and instead produce and use a fertilizer with more plant-available nutrients, (4) repurpose food waste and offset their own farm's estimated downstream food waste, and (5) connect to their local community through education and demonstration partnerships.
- A key aspect of this study is to consider the broader importance of incorporating AD into dairy. The case study farm provides a lens through which to consider similar farms and opportunities to incorporate AD for biogas production throughout the state. It is important to note that while Pennsylvania has a large dairy industry, most PA dairy farms milk less than 300 cows, while economic analysis suggests Pennsylvania dairies need 1,000 to 2,000 cows to profitably implement anaerobic digestion with existing technologies and incentives (Leuer et al., 2008). Lower cost technologies for smaller dairies and aggressive policy incentives, such as for biogas to electricity, renewable natural gas, and carbon capture and storage, are needed for widespread implementation.
Our resource availability estimates show unused potential for digesting corn stover, switchgrass, winter rye, food waste, and manure, which could be maximized by considering crop profitability and productivity across the landscape. There has been an increased focus on identifying unprofitable agricultural areas and evaluating the benefits of incorporating perennial grass crops in these areas (Kreig et al., 2021; Brandes et al., 2018; Soldavini and Tyner, 2017). If these less productive fields can be converted into bioenergy crops through farm landscape management strategies, there may be economic benefits such as increased farm revenue and increased biomass feedstock availability for local energy production, along with various ecosystem services, such as increased wildlife habitat, decreased soil erosion, and nutrient loss. There are opportunities to further understand the potential impacts and benefits of incorporating perennial grasses throughout farm landscapes.
The benefits of increasing on-farm AD have implications that may positively impact communities near where the digester is operating. In addition to reduced odor, as previously mentioned, community benefits could include reliable, renewable electricity, decreased greenhouse gas emissions, and reduced water pollution from crop fertilization. These benefits to the local community could be further enhanced by converting unprofitable fields into perennial grass crops that could provide additional ecosystem services. However, all potential benefits vary based on the AD system and specific site, feedstock, and environmental conditions.
Conclusions
This case study presents a carbon, nutrient, and energy circularity analysis of a real commercial dairy farm using measured data where possible. The case study farm has AD at its core, which operates to increase circularity in several ways. The case study shows that livestock systems, which are a source of many linear agriculture problems, can serve as a powerful solution to the circularity of the food system when coupled with engineered solutions like AD. Biogas to electricity and RNG are examples of circular economies for livestock systems that can recycle wastes in ways that reduce nitrogen pollution, reduce carbon emissions, increase self-sufficiency, and provide economic benefits that can help ensure agricultural resilience and sustainability while offsetting fossil energy consumption.
This mass flow analysis revealed that only about 5% of the carbon flowed into the farm is stored on-farm in the soil, while on-farm biogas production can offer greater carbon benefits as a greenhouse gas mitigation strategy. However, this analysis made assumptions regarding mass flows important to GHG analysis, including enteric and barn emissions of methane and field emissions of nitrous oxide. These GHGs have such large carbon dioxide equivalent global warming potentials that mass emissions within the whole-farm margin of error for carbon (for methane) or nitrogen (nitrous oxide) could exceed predicted climate mitigation benefits. Off-farm GHG benefits are also uncertain, including the offset value of biogas electricity and of diverting food waste from the local landfill. Thus, better quantification is needed to document overall farm GHG mitigation on this and other commercial farms.
The case study farm we discussed is almost completely self-sufficient in meeting its energy needs, except for fuel for equipment and vehicles. The farm is almost entirely able to meet its crop nutrient requirements by applying liquid digestate from food waste and manure as a soil amendment. The farm connects with the broader food system both through the sale of milk off the farm and the inflow of food waste and the associated carbon and nutrients from off-farm sources, demonstrating the potential of a circular economy of food and agriculture. However, the accumulation of excess nutrients in the system may be an unintended consequence of bringing food waste into the system and needs to be explored in future analyses, especially the potential for excess P to accumulate and be available for nutrient pollution of surface water. Still, the digester system, run by the farm's next generation, reduces overall farm costs by reducing fertilizer and bedding costs, and income from food waste tipping fees and electricity sales allows the farm to be profitable and financially sustain itself.
Additional examples of practically operating dairy farms are needed to understand the circular economies of the milk and dairy sector, and more data from commercial dairies need to be collected over longer timescales to reduce uncertainties about these systems' benefits and unintended consequences. But the analysis thus far does indicate anaerobic digestion can help farmers meet the increased demand for dairy products while also providing renewable energy. Anaerobic digestion is well positioned to provide economic and environmental benefits to dairy farms and to the greater food and agricultural systems they serve.
Acknowledgments
This research is supported in part by Sustainable Agricultural Systems grant number 2020-68012-31824 from the United States Department of Agriculture’s National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the U.S. Department of Agriculture. https://nifa.usda.gov/
Brett Reinford is the owner and operator of the case study farm, which is in partnership with Penn State University and Iowa State University on "Grass to Gas" education and demonstration under the Sustainable Agricultural Systems grant number 2020-68012-31824.
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