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A Local Solution to the Challenge of Carbon Neutrality and Excess Phosphorus: Anaerobic Manure

Digesters

Riley Ebel Jessie Ralph

Liilia Namsing Aaron Yappert Beam Kitikhun

ENVS 401

Professor Klyza, Professor Baker-Medard, Diane Munroe

December 2015

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Table of Contents 1. Introduction ............................................................................................................................. 3

1.1 Carbon Neutrality ................................................................................................................ 3

1.2 Phosphorus in Lake Champlain ............................................................................................. 4

1.3 Project Overview ................................................................................................................. 4

1.4 Current Energy Profile ......................................................................................................... 5

2. Manure Digestion ..................................................................................................................... 6

2.1 Producing Biogas from Dairy Waste at the Goodrich Farm ....................................................... 6

2.2 Greenhouse Gas (GHG) Emissions Implications of Anaerobic Digestion .................................... 9

2.3 Benefits of Renewable Natural Gas .......................................................................................10

3. Methane Accounting ................................................................................................................10

3.1 Baseline Emissions .............................................................................................................11

3.2 Environmental Protection Agency (EPA) ...............................................................................11

3.3 California Air Resource Board (CARB) ................................................................................12

3.4 Regional Greenhouse Gas Initiative (RGGI) ..........................................................................12

3.5 Main Variables ...................................................................................................................12

3.6 Results and Discussion ........................................................................................................14

3.6.1 Baseline Emissions .......................................................................................................14

3.6.2 GHG Emissions Implication of the Switch from #6 Fuel Oil to Renewable Natural Gas ........16

3.7 Recommendations to Reach Carbon Neutrality .......................................................................18

4. Phosphorus .............................................................................................................................20

4.1 Threat to Bodies of Water ....................................................................................................21

4.1.1 The Implications of Phosphorus Runoff in the Lake Champlain Basin ................................22

4.2 Phosphorus and Agricultural Land Use ..................................................................................22

4.2.1 Fertilizer Spreading Practices .........................................................................................23

4.2.2 Additional Phosphorus Reduction Strategies ....................................................................27

4.3 Future of Phosphorus Reduction ...........................................................................................28

4.3.1 Phosphorus Separation Technologies ..............................................................................28

4.4 Middlebury College and Phosphorus Reduction .....................................................................30

4.5 Results and Discussion ........................................................................................................31

4.5.1 Models ........................................................................................................................31

4.5.2 Recommendations ........................................................................................................36

5. Conclusion ..............................................................................................................................36

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Acknowledgements .....................................................................................................................38

6. Bibliography ..........................................................................................................................39

Appendix A: EPA ........................................................................................................................43

Appendix B: CARB, COP ............................................................................................................46

Appendix C: RGGI ......................................................................................................................48

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1. Introduction

1.1 Carbon Neutrality In 2007, the Board of Trustees of Middlebury College adopted a policy objective of carbon neutrality by 2016 for campus operations. The college then became a signatory of the American College and University Presidents’ Climate Commitment in 2008. These two initiatives began the process of reducing greenhouse gas (GHG) emissions on campus and the conversion to clean fuel sources. The college’s first step in reducing GHG emissions came through the construction of a biomass gasification facility, which became operational in 2009. The plant draws on sustainably harvested biomass and waste wood and it reduced the use of on-campus #6 fuel oil by nearly one million gallons. Continued improvements in operational efficiency of the plant have reduced fuel oil usage even further. Coupled with this was an initiative to increase the energy efficiency of the campus. $1.7 million was spent on 62 efficiency projects, which led to a reduction in energy demand of 3.4 million kWh. With the addition of two solar projects, the college’s total reduction of GHG emissions was 55%, declining from 30,644 metric tonnes in 2008 to 13,848 metric tonnes in 2014 (Figure 1; Byrne, 2015b). The college is now attempting to reduce the remaining 13,848 metric tonnes of GHG emissions, specifically targeting the use of 640,000 gallons of #6 fuel oil, through the implementation of a renewable natural gas anaerobic manure digester. The proposed digester project will be built on the 850-cow Goodrich Farm in Salisbury, Vermont.

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Figure 1. Middlebury College Historical Fiscal Year GHG Emissions. (Data from Middlebury College Annual GHG

Inventory.)

1.2 Phosphorus in Lake Champlain One potential co-benefit of the manure digestion project at the Goodrich Farm is the production of a manure-derived liquid fertilizer that can be applied to fields through liquid injection. These application practices along with changes in manure management have the potential to reduce phosphorus runoff thereby improving the quality of nearby waterways. Due to the prevalence of agricultural land and dairy farms in close proximity to Lake Champlain and its tributaries, Lake Champlain receives an unhealthy dosage of nutrients from these facilities as a result of runoff events. One nutrient that has proven particularly problematic in the lake is phosphorus, which when released into local water bodies in excess amounts can lead to rapid algal growth and eutrophication (“Lake Champlain Basin Program: State of the Lake 2015,” 2015).

1.3 Project Overview The goals of this project are threefold. First, we wanted to assess if Middlebury College will achieve carbon neutrality through the implementation of the Lincoln Renewable Gas biomethane digester located at the Goodrich Farm. The second goal serves to assess how many offsets the college might need to purchase should carbon neutrality not be achieved in its entirety

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through the proposed digester project. Third, this project aims to assess, broadly, the impacts of phosphorus runoff into Vermont watersheds through traditional farm fertilization methods and discuss how the more concentrated liquid byproducts of manure digestion can be better utilized to reduce the ecological impacts of excess phosphorus in Vermont waterways. Achieving these goals will in part help the college serve as a leader in environmental sustainability as well as, and perhaps more importantly, illustrate the numerous advantages of small scale manure digester projects to not only the farm(s), but to the greater community through energy production and phosphorus reduction.

1.4 Current Energy Profile

Currently, the majority of energy used on the Middlebury College campus comes from the combustion of biomass, which consists of locally sourced woodchips. The second biggest energy source is #6 fuel oil followed by other fuel sources, such as #2 fuel oil, diesel, biofuel, and natural gas. This energy is mainly used for heating and cooling, generating electricity, and for college-owned vehicles. About 20% of electricity is generated on campus from the steam byproduct from the combustion of natural gas and biomass, while the rest is purchased from Green Mountain Power. Depending on daily energy needs, biomass derived energy is used first, followed by natural gas, and finally, during peak consumption days, fuel oil and biofuel. Energy usage tends to be the highest during the colder days in winter and the warmer days in summer, as heating and cooling needs peak respectively (Byrne, 2014).

The college voluntarily monitors all its greenhouse gas (GHG) emissions associated with the aforementioned energy sources. Carbon dioxide makes up about 98% of all the college associated GHG emissions, with the remaining 2% being methane and nitrous oxide. Middlebury College also includes emissions from college-funded travel and waste management in its annual GHG inventory (Byrne, 2014). During the fiscal year of 2013/2014 about 7,272 metric tonnes of carbon dioxide equivalent (MTCDE) were emitted by the combustion of #6 fuel oil, which is estimated to be about 50% of the college’s total carbon emissions. This was followed by emissions associated with college staff travel totaling to about 24%, and the combustion of #2 fuel oil and diesel, which made up about 13.5% of total emissions. Other emissions associated with electricity, vehicle fuel, and waste management made up the remaining 12.5% (Byrne, 2014). Switching from the use of fuel oils and natural gas to renewable natural gas (RNG) will reduce the current net carbon emissions further. The remaining emissions will be offset by investments into new energy efficiency projects on campus, and as a last resort through the purchasing of carbon offsets.

When estimating its net carbon emissions, the college includes emissions from the main campus as well as the Bread Loaf campus and the Snow Bowl. In 2006, the Snow Bowl became the first carbon neutral ski area in the country, after the college began purchasing carbon offsets from Vermont-based Native Energy. These offsets cover ski area emissions plus estimated carbon emissions for skier travel to and from the Snow Bowl.

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In the college’s carbon accounting scheme, only emissions released within the physical boundary of the college or from operations of which 50% or more is owned by the college are accounted for. Biomass is currently considered carbon neutral because the land where the woodchips are harvested from continues to grow trees that sequester carbon at a faster rate than the rate at which carbon dioxide is emitted by the college. When burned as a fuel source, there is no net increase in the amount of carbon dioxide in the atmosphere. The RNG is considered carbon neutral based on similar assumptions (Byrne, 2015b). This is different from burning fossil fuels, which causes a net increase of carbon dioxide in the atmosphere.

The majority of electricity purchased from Green Mountain Power is generated from nuclear power or from renewable energy sources like hydropower. Although nuclear energy is not considered a renewable source, the associated carbon dioxide emissions are very low and as such do not contribute much to the college’s carbon footprint.

2. Manure Digestion

2.1 Producing Biogas from Dairy Waste at the Goodrich Farm Biogas is a mixture produced from dairy wastes through anaerobic digestion, which

breaks down organic material in an oxygen-free environment. This mixture is mainly composed of methane and carbon dioxide and is produced naturally within manure piles or lagoons on dairy farms, where the gas and associated GHGs are released into the atmosphere (Krich et al., 2005). As of 2007, the uncontrolled anaerobic decomposition of manure in Vermont released about 140,000 metric tonnes of MTCDE into the atmosphere, of which 95% can be traced back to dairy farms (Dowds, 2009). Anaerobic digesters allow farms to control and capture biogas as a renewable source of energy to be used on site, or elsewhere.

Biogas can be stored before or after processing and can either be used directly in a combustion engine to create electricity and heat, or can be further refined and upgraded to biomethane or RNG. Anaerobic digestion occurs in two basic stages: organic waste products are first decomposed by acid-forming bacteria into fatty acids, hydrogen sulfide, hydrogen gas and carbon dioxide. Then, methane forming bacteria metabolize the fatty acids and hydrogen gas into methane and carbon dioxide gases. This raw biogas has a methane content between 55-70% and a carbon dioxide content between 30-45%, along with some other trace contaminants including water vapor, hydrogen, and hydrogen sulfide gases (Petitioner’s Exhibit LRNG-DKS-2, 2015). At this point, the raw biogas can be refined or “scrubbed” to remove carbon dioxide and trace contaminants. In order for biogas to be considered a RNG product and subsequently utilized as a direct substitute for geologic gas, the raw biogas must be refined to yield a methane content greater than 95%.

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Figure 2. Image of digester models to be used at the Goodrich Farm. (Image from Elmar Broering of WELTEC

Biopower in Petitioner’s Exhibit LRNG-DKS-2.)

Digester systems generally include a digester mechanism, equipment for handling and storing effluent gas, a flare and in some cases, equipment to refine biogas to renewable natural gas (Dowds, 2009). There are several different manure digester system types including complete mix (to be used at the Goodrich Farm), fixed film, and covered lagoon systems. The Goodrich Farm will be installing three complete mix digesters manufactured by WELTEC Biopower. The digesters are circular tanks made of stainless steel in which manure is heated and mixed with microorganisms (Figures 2 & 3; Hamilton, 2012). In these systems, biogas is displaced by incoming manure and biogas production is generally controlled by adjusting volume so liquids remain in the digester for about 20 to 30 days (Petitioner’s Exhibit LRNG-DKS-2, 2015). Biogas is stored in the tanks above the substrate and later pushed through a pipeline to the gas-upgrading unit. The digester system also includes a flare for biogas venting—if the gas cannot be transported directly to the upgrade equipment, the biogas emergency flare incinerates the gas. The flare is fully enclosed and has a capacity of 99% of gas production (Figure 4).

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Figure 3. Diagram of different components of the WELTEC Biopower complete mix digester. (Image from Elmar

Broering in Petitioner’s Exhibit LRNG-DKS-2.)

Figure 4. Diagram of flare system included with digester manufactured by WELTEC Biopower.

Finally, the system within the control room has metering and measurement for three

major biogas components: Methane (0-100% vol), oxygen (0-25% vol) and hydrogen sulfide (0-3000 ppm). Digester tanks are heated using a forced hot water system utilizing coils that use non-upgraded biogas produced by the digesters for fuel. A natural gas boiler with 700 kW

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capacity burns the biogas produced by the digester. This heating process also works to reduce emissions by producing and using the gas to facilitate the digester onsite. Lincoln RNG is also exploring the use of a solar thermal system as a potential replacement for biogas fuel in the future (Petitioner’s Exhibit LRNG-DKS-2, 2015).

The pipeline-grade RNG would be transported from the digester facility to Middlebury College and other buyers via the underground pipeline constructed and operated by Vermont Gas Systems, Inc. (VGS). This pipeline would supply the “gas island” distribution facility in Middlebury, and it would later be incorporated into VGS’ expanding distribution network in Addison County once it has been completed (Vermont Public Service Board, 2015).

2.2 Greenhouse Gas (GHG) Emissions Implications of Anaerobic Digestion

There are two major beneficial greenhouse gas implications of anaerobic digestion designated through subsequent capture and combustion of methane on dairy farms. Firstly, because raw methane has a significantly higher global warming potential (GWP) than CO2, combusting processed RNG as CO2 rather than allowing methane to be released into the atmosphere inherently reduces direct carbon dioxide equivalent GHG emissions—the conversion of methane to CO2 and water serves as a net GHG emissions reduction (Dowds, 2009). Secondly, the combustion of biogas or RNG is used to generate heat or electricity, which displaces fuel consumption that would have happened in the absence of the digester. The latter potential benefit creates a more indirect emissions reduction.

It is important to note that the digester operation does release emissions through leakage and destruction of excess methane via flare and that the methane offset is equal to the difference between the emissions from baseline manure management practices and emissions from the digester system. Manure that is stored in slurries or anaerobic lagoon systems (like those previously used at the Goodrich Farm) release significantly more methane, creating high baseline emissions. Replacing these high GHG releasing practices with a digester process will result in significant emissions reductions, although recent protocols do require accounting for emissions from the digesters themselves, so we cannot assume a digester emissions factor of zero (EPA, 2008). Direct emissions of the digester systems—including the leakage from the digester and incomplete combustion of biogas noted above—may result in the escape of between 1 and 3% of total methane produced in the digester into the atmosphere (Dowds, 2009).

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2.3 Benefits of Renewable Natural Gas Renewable natural gas (RNG) or biomethane is a pipeline quality gas that is

interchangeable with natural gas. The natural process of anaerobic digestion produces biogas, a mixture of methane, carbon dioxide, and other impurities. Through a simple refinery process biogas can be converted to biomethane, which can then be used to generate electricity and heat. Biomethane has a methane content nearly identical to traditional natural gas and can therefore be mixed freely within existing natural gas distribution systems. The benefits of RNG lie in the lifecycle: biogas is generally thought to be a carbon neutral resource due to the fact that the carbon emitted during combustion was initially fixed by plants through the process of photosynthesis. By combusting RNG, carbon dioxide that was originally in the atmosphere returns to the carbon cycle, resulting in no change in the total carbon dioxide of the cycle. Additionally, if left in an anaerobic lagoon, methane and carbon dioxide in manure would naturally decompose and release CH4 into the atmosphere. By capturing and converting the dairy waste into a refined biogas that can be combusted, RNG replaces fossil fuel natural gas and also reduces the amount of potent greenhouse gases that would be released into the atmosphere as a byproduct of dairy farming (Eggleston et al., 2006). While we were unable to find peer reviewed literature attesting to the fact that biomethane is carbon neutral, in this report we analyze our data under the same assumption as Middlebury College and experts of other organizations that believe this renewable natural gas is carbon neutral.

3. Methane Accounting

Carbon accounting is the process by which organizations account for and report their greenhouse gas emissions (Schaltegger and Csutora, 2012). This involves the direct measurement of carbon and other GHG emissions as well as the measurement of carbon dioxide equivalents that will not be released into the atmosphere as a result of different mitigation programs like the proposed manure digestion project at the Goodrich Farm. In 2007 the college began an annual GHG emissions inventory, tracking its energy use and associated emissions across operations in which the college has at least a 50% stake.

Accounting for the carbon emissions reductions associated with Middlebury College’s use of RNG is not only important for estimating the benefits of a cleaner energy source, but also to account for the removal of methane from the atmosphere through the implementation of the digester project. Methane is a much stronger GHG with a global warming potential about 25-28 times that of carbon dioxide and it is the second most prevalent GHG emitted in the United States (Chianese et al., 2009). It is estimated that emissions associated with dairy cattle manure account for 35% of all the livestock manure methane emissions in the United States (McGinn and Beauchemin, 2012). Therefore, the implementation of this digester project not only allows

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the college to reduce its carbon footprint but it also benefits the environment by limiting the release of a highly potent GHG into the atmosphere.

3.1 Baseline Emissions In order to evaluate the methane reduction associated with the digester project, the first

task was trying to quantify the baseline methane emissions associated with the current manure management practices at the Goodrich Farm. Baseline emissions refer to the emissions that would have occurred from a manure management system prior to the implementation of an emissions mitigation project. In this case, the digester will replace the existing manure management system with the intent of capturing and destroying the otherwise released methane (Eastern Research Group, Inc., 2011). At the Goodrich Farm, the majority of the manure is stored as a liquid in earthen ponds outside the animal housing facilities, and smaller amounts are spread on fields as fertilizer throughout the year (Smith, 2015). As such, the baseline methane emissions for this project are those associated with the aforementioned manure management practices.

There is currently no standardized method for accounting for the methane emissions associated with different manure management systems on dairy farms. Therefore for this project, we evaluated three different protocols in an effort to understand how methane accounting results vary given different methods and to recommend which is the most appropriate for the digester project at the Goodrich Farm.

We used models from the U.S. Environmental Protection Agency (EPA), the California Air Resources Board (CARB) and the Regional Greenhouse Gas Initiative (RGGI). The EPA model represents methane accounting at the federal level, while the CARB model is very similar to models widely used in the private sector and is a component in one of the nation's most robust state-level carbon cap-and-trade programs. The RGGI model has been adopted by the Vermont Legislature and, along with the CARB model, represents site-specific projects with reasonable accuracy.

3.2 Environmental Protection Agency (EPA)

The U.S. EPA protocol for accounting agricultural methane emissions was developed based on the Intergovernmental Panel on Climate Change (IPCC) Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories Tier II protocol. This is one of the models used by the U.S. EPA to generate their annual report on national GHG emissions and sinks (EPA, 2014). In order to calculate the baseline methane emissions based on this model, we used both site-specific data and literature data to estimate the baseline emissions. See Appendix A for details regarding calculations.

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3.3 California Air Resource Board (CARB)

CARB uses the California Compliance Offset Protocol for Livestock Projects (COP), which provides a means to quantify and report GHG emissions reductions associated with installation of a biogas control system (BCS) or digester for manure management on dairy and swine farms. The protocol is based on the Climate Action Reserve’s Livestock Project Protocol Version 2.2 and includes updates from version 3.0, published in 2011. This protocol can appropriately quantify GHG reductions at an offset project anywhere in the U.S. and was developed through Climate Action Reserve, which is a California Offset Project Registry. Both site-specific data and literature data were used to estimate baseline emissions. See Appendix B for details and calculations.

3.4 Regional Greenhouse Gas Initiative (RGGI)

The Regional Greenhouse Gas Initiative (RGGI) is a mandatory carbon dioxide cap-and-trade state-level consortium for power plants located in the Northeast United States. Member states can engage in carbon credit trading with the intent of capping GHG emissions in the region. Credits can be earned through the permanent destruction of GHG emissions that would otherwise have occurred under normal conditions. In this regard, RGGI outlines multiple accounting schemes to assess the reduction in GHG emissions on a project-by-project basis. Under RGGI, anaerobic manure digesters that meet operational requirements constitute permanent and additional reductions in methane emissions and therefore are eligible to earn carbon credits. Member states, including Vermont, are free to adopt RGGI regulations that detail, among other things, how baseline emissions should be calculated for manure digester projects (RGGI, 2013). Similarly to other emission calculation protocols, the RGGI protocol quantifies methane emissions that occur within the project boundary given the existing manure management system. The RGGI protocol, through the calculation of eliminated methane emissions, will allow regulators to assess how many offset credits a project may receive. See Appendix C for details regarding calculations.

3.5 Main Variables Baseline modeled methane emissions from anaerobic storage or treatment systems can be quantified based on a number of important variables. Although the three protocols (EPA, CARB and RGGI) differ slightly in treatment of some of these variables, there are a few essential and consistent contributors to methane emissions on dairy farms.

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Livestock Category and Population Designation of livestock category, e.g. lactating dairy cows, heifers, etc., accounts for differences in methane generation among livestock type due to differences in the organic solids within their manure. Generally, manure from milking cows produces more methane during anaerobic decomposition than heifers; as such, designating type of animal is important to accurately determine methane emissions. Similarly, animal population is essential to calculate methane emissions: population is monitored monthly and dictates the amount of methane produced on the farm. Organic Solid Content in Manure

Organic solids, specifically volatile solids (VS), represent the portion of manure, that when anaerobically digested, leads to a methane by-product (Eggleston et al., 2006). VS excretion rates are best calculated with the use of published sources that estimate average daily excretion based on feed, livestock type and storage system. Variations in VS concentrations are important to note: for example, dairy cows contain more VS than heifers and storage in a slurry or lagoon allows for more efficient breakdown and methane production than dry stacking or other methods. The amount of volatile solids available for degradation within a system also depends on the previous month’s available and degraded volatile solids within the system. The maximum methane producing power in manure (B0), which ultimately dictates how much methane is released by organic solids in manure varies by species and diet, and is also based on biodegradable volatile solid. Temperature at the Site

Liquid-based systems are very sensitive to temperature fluctuations, so accurate and specific monthly or annual temperature data are important to consider. Temperature and climate at the site directly affect natural digestion rates and in a location with high seasonal temperature variation like Vermont, methane production from a lagoon is not constant. Generally, warmer temperatures facilitate more VS breakdown and subsequent methane emissions. Manure Management System Characteristics

This variable includes the type of system used to manage the manure and usually a system specific methane conversion factor that reflects the portion of B0 that is achieved. Retention time and temperature are important factors in the management system: manure that is managed as a liquid in warm conditions promotes methane formation and has high system methane conversion factors. Manure that is managed as dry material in cold temperatures produces significantly less methane. At the Goodrich Farm, the manure is held in a lagoon as a

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liquid and produces varying amounts of methane depending on changing temperature throughout the year.

3.6 Results and Discussion

3.6.1 Baseline Emissions Baseline emissions represent methane released from standard manure storage in open-air slurry pits. Because the digester will be drawing manure from two farms in addition to the Goodrich Farm, baseline emissions were evaluated for all three farms. The neighboring farms store manure in open-air pits similar to those used on the Goodrich Farm, and because they are located within Addison County, they experience the same average monthly temperature. The three protocols showed considerable variability in the baseline emissions (Figure 5). The EPA protocol estimated emissions 63.1% lower than the CARB protocol and the RGGI protocol estimated emissions 32.1% lower than the CARB protocol. Though differences among the estimated baselines are unsurprising, the magnitude of the differences suggests that methane accounting protocols have yet to achieve sufficient accuracy for informed decision-making. This being said, some of the difference can be explained by the scope of the protocols. The EPA model is designed to function as a national GHG inventory and therefore uses a state-specific conversion factor for volatile solids into methane. Though this may be sufficient on the national scale when site-specific data cannot be attained for every digester project in the United States, it fails to best estimate a single project.

The CARB and RGGI protocols are much more sensitive to site-specific factors but still show substantial differences. The RGGI protocol applies a factor of 1/2 to all manure added to a slurry pit over the course of the reporting period (typically one month). Alternatively, the CARB protocol applies a factor of 4/5 to all manure added to the slurry pit over the course of the reporting period. These factors are intended to model the proportion of volatile solids within the manure available to be broken down throughout the reporting period. This factor is necessary because manure added later in the reporting period will have less time to break down while manure added at the start of the period will spend more time in the digester. This factor attempts to rectify these differences. We believe that the difference between these factors in the CARB and RGGI models may contribute to the different modeled baseline emissions (See Appendices B and C for equations).

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Figure 5. Total calculated baseline methane emissions of all project-associated farms. Values above bars represent

metric tonnes of CO2 equivalents released per year. Standard manure management practices include 100% of manure contained in open-air slurry pits and are the same across all three farms. Total manure inputs from 1800

milking dairy cows and 400 heifers.

In order to facilitate an understanding of how these baseline emissions would factor into the college’s net emissions, we averaged the CARB and RGGI baseline emissions. We did not include the EPA estimate due to its imprecision for single-digester sites. This average value, 5,907 MTCDE, is representative of typical farm emissions. Native Energy, a Vermont-based energy and offset consulting firm, estimated, prior to project permitting, that Goodrich Farm would produce 5,500 MTCDE per year prior (Byrne, 2015a). However, calculations were not provided and after speaking with regional experts, this value seems to reasonably reflect the emissions at a typical farm the size of the Goodrich Farm. Given these two corroborating sources, we think the averaged CARB/RGGI value is sufficiently accurate. However, it must be acknowledged that the significant differences between the models suggest there is need for a comprehensive review of methane accounting in the United States to better represent baseline emissions. Furthermore, though we can use standardized variables, no generalization can replace site-specific measurements of the Goodrich Farm conducted by trained professionals.

Though the digester will lead to a net energy output, there are several project components that will require significant energy inputs. The digester tanks are heated by a forced hot water system that will require 497 MTCDE per year of energy via carbon neutral biogas produced by the digester itself. Furthermore, the biogas upgrade equipment will require an estimated energy input that will lead to emissions of 895 MTCDE per year; however, this energy will be provided

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through the existing power grid and will constitute net release of carbon (Steckler, 2015). Additional manure will be trucked to the Goodrich Farm to supplement manure produced by the host farm. This trucking will emit 97 MTCDE. Collectively, these energy inputs will amount to 992 MTCDE per year to operate the digester system. These emissions must be subtracted from the overall modeled baseline emissions to determine the offset credits provided by the project. After subtraction, the final credits earned through the project total 4,915 MTCDE per year (Figure 6).

Figure 6. Offset credits accrued through the destruction of methane via anaerobic manure digestion. *Baseline emissions represent an average of the baseline emissions of the CARB and RGGI protocols. Values above bars

represent MTCDE.

3.6.2 GHG Emissions Implication of the Switch from #6 Fuel Oil to Renewable Natural Gas

Most of Middlebury College’s energy budget is used to facilitate heating and cooling on

campus and #6 fuel oil remains the main fuel source. #6 fuel oil is a liquid fuel derived from petroleum that is considered a residual fuel. It is an extremely heavy, viscous liquid that gives off some of the highest concentrations of particulate matter and GHG emissions of any fuel. Natural gas, which is primarily composed of methane, is a much lighter gas that produces significantly lower GHG emissions when burned in a boiler than residual or distillate fuels. Currently, the use of #6 fuel oil at the college emits 7,272 MTCDE per year. By switching to carbon neutral renewable natural gas, the college will be able to eliminate all #6 fuel oil use, and the associated 7,272 MTCDE per year.

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Figure 7. Projected college emissions after full implementation of the Goodrich Farm anaerobic manure digester.

Values above bars represent MTCDE.

The anaerobic digester located at the Goodrich Farm will lead to a 84.5% reduction in the college’s remaining GHG emissions, from 13,848 MTCDE to 1,661 MTCDE (Figure 7). Since the initial commitment to carbon neutrality in 2007, the college will have made a 94.5% reduction in overall GHG emissions. These results are based on the averaged baseline emissions from the CARB and RGGI protocols, the college gaining 100% of project credits, while taking into account project emissions. As such, this project is not only reducing methane emissions at the Goodrich Farm, but also reducing carbon outputs on campus. However, due to contractual negotiations, it is possible that the college may not receive 100% of the credits associated with the digester project. If the college were to receive a portion of the credits relative to its gas purchase, 75% of total project output, then remaining emissions would be 2,890 MTCDE (Figure 8).

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Figure 8. Projected remaining emissions for the college per offset credit scenario. Contractual negotiations may

dictate that the college does not receive 100% of project credits, as they are only purchasing 75% of the project’s gas. Values above bars represent MTCDE under the two credit allotment scenarios. Data from Middlebury College

Annual GHG Inventory

3.7 Recommendations to Reach Carbon Neutrality

1) Utilize the maximum capacity of digester Our results are based on a 2,200-cow population contributing manure to the digester. The

digester being installed at the Goodrich Farm, however, has the capacity to process manure from about 4,000 cows. So, there is potential for more methane emission offsets if more farms contribute manure to the digester or the population of cows at the Goodrich Farm increases in the future. Most possible manure additions in the future would likely be from other farms in the area. A calculation of baseline emissions at maximum digester capacity with a population of 2,700 milking cows and 800 heifers, totaling 3,500 cows, shows an additional 3,000 MTCDE of possible offsets from this additional manure being added to the digester. With additional manure comes additional trucking and associated emissions adding up to an extra 100 MTCDE; however, the increased digester offset benefits significantly outweigh the costs. Therefore, using more of the digester capacity could be a good way for the college to increase offsets in the future.

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2) More efficiency projects on campus The college has invested $1.7 million in 62 different efficiency projects since 2008 that

have contributed to the carbon neutrality goal; these include a number of solar projects, efficient design in new buildings and sustainable dining among many others. Despite this robust list of projects, there are many other options available to the college to continue reducing the carbon footprint on campus. Weatherizing the older buildings on campus would be a great way shrink our carbon footprint through increased energy efficiency. As a majority of fuel used on campus goes towards heating and cooling, a tighter envelope in some of the older buildings would help minimize heated or cooled air from escaping through windows, doors and other cracks. Additionally, the school could look into the installation of more solar projects on campus to meet energy demand. Whether these installations are on roofs or elsewhere, solar panels directly on campus would provide a self-contained renewable energy source.

3) Buy offsets As a last resort, Middlebury can purchase offsets from various sources that would

counteract the unavoidable GHG emission by paying for reductions at projects that avoid or capture GHG somewhere else. Ideally, as Middlebury continues to move towards the carbon neutrality goal, offset purchases will decrease and eventually not be required at all once emissions are considered at net zero (Figure 9).

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Figure 9. Middlebury College’s carbon emissions reductions through the digester project and projected remaining emissions that would need to be offset.

4. Phosphorus Phosphorus is an element that is crucial for all forms of life. It is an important mineral nutrient in many agricultural systems because it fosters the growth of crops; sufficient quantities of phosphorus are critical for the development of plant seeds and roots. While phosphorus is a naturally occurring substance in many ecosystems, it is also generally a limiting nutrient for further plant growth (Hart et al., 2004). A limiting nutrient is a chemical that is necessary in an ecosystem to support plant growth but may only be available in small quantities (Bachman, n.d.). Once the supply of this nutrient has been depleted, plant growth stops unless additional amounts of the nutrient are applied.

Over the course of this project, our group not only wanted to explore the benefits of anaerobic manure digestion in terms of achieving carbon neutrality, but we also wanted to determine if manure digestion could have any co-benefits for the ecosystems surrounding digester sites. One of the byproducts of manure digestion and separation is a manure derived

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liquid fertilizer with a high nutrient content. Vermont, like many other places, has been struggling with phosphorus accumulation in its bodies of water and this product could provide an option to mitigate excessive agricultural runoff into Lake Champlain’s watersheds through its application practices. In this section we explore the potential co-benefits of manure digestion on phosphorus runoff as well as additional ways farmers can reduce their field runoff.

4.1 Threat to Bodies of Water Although phosphorus is necessary for farmers to achieve maximum yields, the loading of phosphorus into waterways through runoff can be extremely detrimental to aquatic environments. Phosphorus is also a limiting nutrient in bodies of water for aquatic life (Bachman, n.d.). While the terrestrial levels of phosphorus that are lost due to agricultural runoff may be insignificant to the farmer who is applying fertilizers, small amounts of additional phosphorus can still wreak havoc on aquatic ecosystems (Hart et al., 2004).

Phosphorus loading into waterways during runoff events is an environmental problem because it can lead to the eutrophication of a body of water (Hart et al., 2004). Eutrophication is “a biological process in which there is excessive algal growth in water due to an excess of nutrients and in particular phosphates and nitrates. The eventual decomposition of the algae depletes the water of available oxygen, resulting in a sterile body of water” (Schaschke, 2014). Although eutrophication can be a natural process, it has been accelerated by human activities in many watersheds. Blooms in the growth of algae can occur with the inputs of phosphorus that occur during runoff events. According to some research, one pound of phosphorus can trigger the growth of 300 to 500 pounds of algae (“What’s the Fuss about Phosphorus?”, n.d.). Unfortunately, this rapid and excessive growth can be detrimental to other parts of aquatic ecosystems. Algal growth can prohibit light penetration into the water body, which impacts aquatic plant growth and affects the visibility of some fish species. Additionally, other aquatic life forms depend on dissolved oxygen in the water and if the oxygen levels drop too much, areas of the water body can become dead zones where there isn’t enough oxygen to support life (Chrislock et al., 2013). It is important to note however that agriculture isn’t the only source of phosphorus pollution in waterways. Stream bank erosion also contributes to the amount of phosphorus that ends up in bodies of water. Soils containing phosphorus are washed into waterways as stream banks gradually erode under the flow of water, releasing the nutrients into the water as well. Additionally, wastewater treatment plants are point sources of phosphorus pollution into watersheds (“Lake Champlain Basin Program: State of the Lake 2015,” 2015). In some regions, urban activities contribute the most phosphorus runoff into nearby watersheds. Paved roads prevent the absorption of these nutrients into the soil. These impermeable surfaces allow rain to wash nutrients accumulated on these solid surfaces directly into storm drains and water bodies. Fertilizing personal lawns can also contribute to phosphorus runoff from urban areas (“What’s the Fuss about Phosphorus?”, n.d.).

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4.1.1 The Implications of Phosphorus Runoff in the Lake Champlain Basin In the state of Vermont, phosphorus runoff has created a serious problem in Lake Champlain. Lake Champlain sits on the border of Vermont and New York with the northern end extending into Canada. At 120 miles in length and reaching nearly 400 feet deep, Lake Champlain is the sixth largest lake in the United States (“Lake and Basin Facts,” n.d.). A large amount of phosphorus ends up in Lake Champlain after flowing through the extensive Lake Champlain Basin watershed. According to the Lake Champlain Basin Program’s most recent “State of the Lake Report,” for every square mile of lake surface area there are eighteen square miles of watershed draining into the lake (“Lake Champlain Basin Program: State of the Lake 2015,” 2015). Lake Champlain’s tributaries deposit nearly 921 million tonnes of phosphorus into the lake each year. However, certain watersheds and tributaries contribute far more to this total phosphorus loading than others. The three primary sources of the excess quantities of this nutrient that end up in the lake are runoff from developed land use, agricultural land use, and sediments from stream banks that are gradually eroded and deposited into waterways. These various sources of phosphorus loading in the Lake Champlain watershed contribute different quantities in each tributary (“Lake Champlain Basin Program: State of the Lake 2015,” 2015).

The excess phosphorus that flows into Lake Champlain through these tributaries can also lead to increased algal growth and eutrophication. Some algal blooms of bacteria such as toxic cyanobacteria, also known as blue-green algae, can be dangerous to humans and other living organisms. This type of bacteria can be particularly harmful if ingested and poses a threat to the use of lake water as untreated drinking water (“Lake Champlain Basin Program: State of the Lake 2015,” 2015). Currently, Vermont has a number of programs in place in an attempt to reduce runoff to waterways. These programs include protecting stream banks from erosion, fighting for higher water quality through new water quality laws, and establishing a Total Maximum Daily Load for phosphorus runoff (which is currently under review for implementation) (“Lake Champlain Basin Program: State of the Lake 2015,” 2015; "Lake Champlain Phosphorus TMDL: A Commitment to Clean Water," n.d.).

4.2 Phosphorus and Agricultural Land Use

The state of Vermont is situated in a temperate zone where soils are prone to periods of weathering. Vermont’s climate has therefore required farmers to use additional fertilizers on their fields in order to achieve higher crop yields because agricultural soils have a low naturally present nutrient content (Aguiar, 2015). In order to achieve maximum possible yields in agricultural settings, farmers must add additional phosphorus to the soils in the form of fertilizers (Hart et al., 2004). According to Murray Hart et al., global phosphate use greatly increased

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throughout the 20th century likely because farmers were encouraged to continue applying additional phosphorus to their fields (2004). We now know that the continued application of phosphorus to fields doesn’t necessarily lead to greater benefits for crops because phosphorus can be easily over applied, especially when fields are fertilized with manure. The ratio of nitrogen, another limiting nutrient important for plant growth, to phosphorus in manure is around 1:1; however, the quantity of phosphorus needed by most crops is about eight times less than the amount of nitrogen (Zhang et al., n.d.). This means that farmers apply fertilizer to meet the nitrogen needs of their crops and as a result, excess phosphorus builds up in the soil because there is more available than can be absorbed by the soil and used by crops. As phosphorus continues to build up in the soil due to these fertilizer application techniques, excess nutrients become susceptible to runoff from precipitation or flooding and end up flowing into nearby watersheds.

4.2.1 Fertilizer Spreading Practices

For farming operations that work with large quantities of animals to supply a product such as meat or milk, one of the byproducts is the large quantity of manure produced by the livestock. This manure contains significant amounts of key nutrients for crops such as nitrogen and phosphorus. This allows for a cyclical farming operation as farmers fertilize fields growing feed, like corn, for their livestock with the manure produced by these animals. In Vermont, dairy operations are common and follow this cycle of using animal manure to fertilize feed products.

Access to large amounts of manure provides farmers with a couple of options as to how they can apply the manure to their fields. Some farmers spread manure in a more solid state that contains at least 15% solids over the surface of their fields. The consistency of manure in solid form can vary and can make it difficult for farmers to evenly spread this type of fertilizer. Manure can also be used on fields in a liquid form through various distribution techniques (Kaasik, 2012). Through the course of our research, we have explored the use of one of the byproducts of anaerobic manure digestion as a liquid fertilizer for fields. After the digester process has captured methane from the manure, the manure is put into a separator, which physically separates the solid and liquid components of the manure. At the Goodrich Farm, this is done using a screw press (Figure 10). While the dry solids go on to make bedding for the cows, the separated liquid provides an alternative to traditional manure spreading techniques (Smith, 2015).

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Figure 10. Screw press separator. (Image from AGRO).

Phosphorus loading into watersheds as a result of fertilizer inputs on agricultural land

depends on how these inputs are applied and as a result, various theories exist as to the best manure management practices. According to a study by Kleinman et al., the authors suspected that the addition of phosphorus to the soil surface through application of manure or other fertilizers causes a temporary spike in the amount of phosphorus that can potentially be washed off of the field and into a waterway. For this reason the timing of fertilizer application can play a key role in phosphorus loading. However, it may not always be easy to time fertilizer application perfectly, especially if an unexpected rainstorm hits soon after manure is applied to fields. The study also finds that the concentrations of phosphorus already present in the soil can impact the amount of runoff (Kleinman et al., 2002). This finding has important implications for the current and common practice of overloading agricultural soils to achieve necessary nitrogen concentrations. Over application can lead to more phosphorus in runoff as the soils gradually become highly concentrated with this nutrient.

One of the more common methods of applying manure fertilizers to fields is through surface spreading (Figure 11). This technique is commonly seen in Vermont where the manure is held in lagoons. The unseparated mixture is pumped into tanks that are transported through the fields by truck and the manure is sprayed on the soil surface from the back of the truck.

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Figure 11. Example of farmer using surface spreading manure application on field. (Image from Utah State

University Manure Solutions). According to Dan Smith, manure at the Goodrich Farm is spread onto the fields twice a

year, once after first cutting and once after harvest. The Goodrich Farm applies nearly a quarter of their manure fertilizer with this technique (Smith, 2015). The problems highlighted earlier by Kleinman et al. often occur when manure is spread using this technique. Furthermore, Kleinman et al. express concerns that surface applications, such as the spreading of manure or fertilizers on field surfaces, can increase the vulnerability of these field inputs to runoff (2002).

Alternatives to this type of surface fertilizer application are injection, knifing, or immediate incorporation (Kleinman et al., 2002). Although integrating phosphorus into the soil can increase soil erosion (also a contributor to phosphorus loading), the contribution of phosphorus runoff from these mixed soils amounts to less than runoff from soils that have only received surface fertilizer application. Mixing the soil and added fertilizer together increases the absorption of the phosphorus into the soil and decreases P surface runoff (Kleinman et al., 2002). These incorporation techniques are possible for liquid forms of fertilizer including the liquid byproduct of manure digestion and separation.

The fertilizer byproduct of the manure digestion and separation project changes the way that the Goodrich Farm and the other farms that provide manure for this project will fertilize their fields. In the case of the Goodrich Farm, this change in the consistency of the manure fertilizer will lead to a change in fertilizer application practices. Rather than spreading partially solid manure from the back of a truck driving through fields, farmers can now use the drag-lining method to inject the high nutrient content liquid product into the soil (Smith, 2015). In this method of liquid injection, a tractor is connected to a hose, which delivers pumped liquid

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fertilizer or manure to the tractor. The tractor also drags a piece of tillage equipment. The tillage equipment allows for the liquid, which is pumped through the hose, to be incorporated into the soil as the tractor drives up and down the fields (Wright and Bossard, n.d.).

The drag-lining method of fertilizer injection, also known as the drag hose method (Figure 12), is not the only existing method that can be used to inject the liquid byproduct of manure digestion and separation into the soil. Some other existing means of manure injection were developed to allow the farmer to inject the fertilizer into the soil without tilling the soil. These other methods of fertilizer injection include trailing shoe spreaders––a practice that allows the liquid manure to be inserted into the soil surface, open slot injectors––a system that inserts liquid fertilizer a small distance into the soil by making small openings in the soil with discs or knives, and closed slot injectors––in which the liquid fertilizer is injected to a shallow depth in the soil and the slots are closed afterwards (Kaasik, 2012).

Figure 12. Example of drag lining or drag hose method of manure incorporation (Image from Houzz).

As mentioned above, these methods of liquid injection allow for more efficient mixing of

the fertilizers and the surrounding soils. According to Dan Smith, liquid injection can also reduce some of the negative odors associated with spreading manure on fields (2015). However, when considering methods to reduce potential phosphorus runoff from agricultural land, there are several other practices and standards that should be taken into account.

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4.2.2 Additional Phosphorus Reduction Strategies

i. Conservation Management Practices There are many factors that can affect the quantity of phosphorus washed into

surrounding waterways from agricultural land. As a result, various recommended conservation agricultural practices exist, but the ‘best management approach’ to these agricultural practices depends on specific factors that vary on a farm-by-farm basis. For this reason, in order to make a widespread difference in the amount of phosphorus runoff we see from agricultural operations, farms will likely use various ‘best management practices’ to protect their soil from runoff and erosion.

These conservation practices include a variety of tillage forms, crop rotation, and conservation drainage. In addition to these farming practices, riparian buffer zones near waterways can reduce the potential for the loading of phosphorus into watersheds (Weisman, n.d.). In Vermont, according to Act 64, these vegetative buffer zones are required to help protect local aquatic ecosystems that neighbor agricultural lands. This regulation also requires that manure application on agricultural lands not occur within twenty-five feet of a waterway and that farmers maintain a buffer with perennial vegetation (Vermont Agency of Agriculture, 2006). The decision on which of these conservation practices is best to use in a given farming situation depends on the characteristics of the agricultural land.

ii. Total Maximum Daily Load

In Vermont, one of the ways that government agencies are working to control the amount

of phosphorus that ends up in Lake Champlain is through a phosphorus Total Maximum Daily Load (TMDL). A TMDL sets a limit for the amount of phosphorus that can enter the lake in order to meet water quality standards. This standard essentially places a cap on the amount of phosphorus entering Lake Champlain’s watersheds ("Lake Champlain Phosphorus TMDL: A Commitment to Clean Water," n.d.). While the EPA rejected Vermont’s 2002 Lake Champlain TMDL in 2011, the development of a new TMDL has been underway since 2013. These updated water quality standards for the state have been finalized and the implementation process will commence over the course of the next two years while the EPA monitors state efforts (Watershed Management Division, 2015).

The implementation of a new TMDL for Lake Champlain would require polluters to find ways to reduce their impact. Although TMDLs are an important part of pollution regulation, in Vermont they also put pressure on farmers to come up with solutions to reduce their phosphorus loading. For Lake Champlain, the EPA set standards for phosphorus loading in the various lake watersheds (Phosphorus TMDLs for Vermont Segments of Lake Champlain, 2015). These allocations differ depending on the lake segments as some segments have greater problems with managing phosphorus runoff and different major sources of phosphorus. Additionally, in many

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of these segments, high reductions in phosphorus loading from agricultural land must be made in order to meet these TMDL standards (Phosphorus TMDLs for Vermont Segments of Lake Champlain, 2015). As a result of these regulations, the new TMDL may make liquid injection and other conservation management practices more appealing in the agricultural sector. The TMDL might also encourage the development of more phosphorus monitoring stations, which could measure the difference in phosphorus runoff attributed to various agricultural practices.

iii. Nutrient Trading Schemes Lake Champlain is not the only water body that has struggled with phosphorus loading;

the Chesapeake Bay has also struggled with water pollution from nutrients including phosphorus. The Chesapeake Bay also has a TMDL cap on nutrient loading into its watershed and policymakers there have explored the idea of nutrient trading programs in the region. Not only does a nutrient ‘cap and trade’ program help the region reach its TMDL goal, but it also has the potential to reduce cleanup costs. This market based approach to cleanup benefits polluters who adopt cleaner practices, including farmers who can sell excess permits if they adopt conservation agricultural practices. The ability of farmers to profit from the reduction in their phosphorus loading to waterways could reduce some of the economic stress farmers feel meeting TMDL requirements. The nutrient trading approach has been one response in some areas to meet water quality standards (Quinlan, 2012).

Vermont has also considered nutrient trading to reduce phosphorus pollution in Lake Champlain. However, this type of plan requires strong regulations that demand polluters to clean up their pollution. Currently, Vermont still does not have any trading plans in place but is in the process of considering an initiative that would incorporate a nutrient trading scheme to meet Lake Champlain TMDL requirements (Hirschfeld, 2014; Vermont Agency of Agriculture, Food and Markets and the Vermont Department of Environmental Conservation, 2014).

4.3 Future of Phosphorus Reduction

4.3.1 Phosphorus Separation Technologies

Currently, across the U.S., TMDLs, conservation management practices, and liquid injection techniques are being used to reduce total phosphorus runoff from agricultural processes. However, additional technologies exist that can be used in tandem with manure digesters on farms to further reduce phosphorus over application and runoff. These technologies involve the separation of phosphorus from other nutrients present in manure. This separation would result in a phosphorus product that provides the farmer with significantly more control over its application on fields. These technologies could reduce the overloading of phosphorus in agricultural soil due to more exact application. Additional phosphorus that does not get applied to a farmer’s fields could then be sold to other regions where farmers lack phosphorus. This

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creates a new market for separated nutrients. Phosphorus can be removed from the manure through chemical, physical, or biological separation methods. We have explored the potential of a few of these technologies and their implications for use along with a manure digester project.

Interestingly, the digester project at the Goodrich Farm already employs a method of physical separation of manure into solid and liquid components through the use of a screw press (Figure 10). This process in itself can reduce phosphorus content in the liquid component by around 20% as the separated solids which then become bedding for the cows capture these nutrients (Focus on Vermont Farm Projects: Single and Two-Stage Dairy Manure Separation, 2015). However, additional types of phosphorus separation can further reduce the quantities of phosphorus present in the manure and liquid fertilizer by-product of separation.

i. Decanter Centrifuge Separation Once the manure has been physically separated with a screw press, Native Energy

proposes the use of a decanter centrifuge to further separate phosphorus. In this process, the liquid component of manure separation enters the centrifuge where it is spun at a high speed until the particles containing phosphorus stick to the barrel surface. This process has the ability to remove about 70% of the remaining phosphorus in the liquid and the centrifuge output is dry phosphorus “cake” that can be used as needed by the farmer after spreading the remaining liquid manure or sold to other farms (Focus on Vermont Farm Projects: Single and Two-Stage Dairy Manure Separation, 2015). According to Native Energy’s assessment of a two-stage separation process using a decanter centrifuge, a farmer with 500 cows will make over half a million dollars over 10 years after the implementation of such a project. These benefits take into account the operating and upfront costs of the project as well as the profits made through the sale of excess phosphorus cakes (Vermont Farm Projects: Single and Two-Stage Separation, 2014).

The manure-derived phosphorus cakes can help reduce agricultural runoff from fields when applied instead of raw dairy manure. A study comparing the runoff potential of dairy manure and biosolid cakes found even with the same phosphorus loading rates, the concentrations of phosphorus in runoff was significantly higher when manure was applied as opposed to the cake treatments (Brandt and Elliott, 2003).

Currently, Green Mountain Power is working with Native Energy on the implementation of a manure separator project on Vermont dairy farms (“Community Energy & Efficiency Development Fund 2015 Annual Plan,” 2014). Additionally, Green Mountain Power’s proposed Community Digester in St. Albans will explore the potential of dry nutrients (such as the phosphorus cakes) to be applied precisely or exported to other parts of the country where farmers lack access to sufficient quantities of phosphorus (“Linking Community and Technologies to Capture and Convert Nutrients to improve water quality in Lake Champlain,” n.d.).

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ii. Dissolved Air Flotation In addition to the centrifuge as a phosphorus separating option, David Dunn from Green

Mountain Power also suggested dissolved air flotation as a potential technique to reduce phosphorus (Dunn, 2015). One of the downsides that Dunn mentioned regarding the centrifuge was its high-energy load. In comparison, the dissolved air flotation option has a lighter energy load. This technology removes suspended solids from the liquid by using air bubbles to float the solids to the surface and scrapes them away ("About Dissolved Air Flotation," 2015).

In cases where this technology has been used to treat polluted water from wastewater treatment plants, average reductions of total phosphorus were between 55 and 81% (Kolvunen and Heinonen-Tanski, 2008). While applications of dissolved air flotation are more common with wastewater treatment in treatment lagoons, this technology can also be used to target the nutrient filled runoff from agricultural fields at certain points throughout the year (“Algal Removal Using Dissolved Air Flotation,” 2015). Companies that specialize in nutrient recovery from waste have also explored the use of dissolved air flotation to capture nutrients from manure in dairy operations ("Dissolved Air Flotation," N.d.). DAF systems are also often smaller and have lower installed costs than other water clarifying technologies (Ross and Valentine, 2008).

These separator options could help provide another alternative to traditional manure spreading techniques. Options like Dissolved Air Flotation and centrifuge technology can be used in tandem with the digester and would allow a farmer to take full advantage of the manure produced from his or her animals. One thing standing in the way of further use and development of these options is likely the cost of such projects. High upfront or user costs might make phosphorus separation less feasible for the smaller scale farms that are found throughout the state of Vermont.

4.4 Middlebury College and Phosphorus Reduction

Middlebury College’s investment in the Goodrich Farms anaerobic digester project not only has the potential to help the college reach its goal of carbon neutrality, but also encourages the reduction of phosphorus loading by changing agricultural practices. The liquid byproduct of anaerobic manure digestion and separation encourages dairy farmers to change their fertilizer application techniques in favor of a practice that can lead to better manure management. Furthermore, it would reduce some of the pressures on farmers from TMDL standards as the separation process reduces manure phosphorus content and allows for an injection method that reduces runoff. Middlebury’s support for and encouragement in the development of the project demonstrates the college’s concern for its surrounding environment and the desire to be a good community member. Middlebury’s investment in this type of project will hopefully spread information about the potential for additional manure digester projects in Vermont.

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4.5 Results and Discussion

4.5.1 Models After exploring the potential for phosphorus loading reductions with the use of a manure digestion and separation project on dairy farms, we sought to determine where a digester project would have the greatest impact on phosphorus loading. In order to determine where Vermont watersheds suffered the most from agricultural phosphorus runoff, we modeled phosphorus loading into Lake Champlain. In addition, we wanted to support our research on the co-benefits of manure digestion we examined various phosphorus loading scenarios that use different best management practices to reduce total phosphorus loading. In order to model where agricultural phosphorus runoff reductions would have the greatest impact in Lake Champlain, we used data from the Vermont Center for Geographic Information to map Vermont’s watersheds feeding into Lake Champlain, and to determine where in Vermont land was used for agricultural purposes. Then, to model the possible phosphorus loading from Vermont’s agricultural lands in each of these watersheds, we used coefficients representing agricultural runoff from Troy et al.’s “Updating the Lake Champlain Basin Land Use Data to Improve Prediction of Phosphorus Loading” (2007) in ArcMap. We applied these coefficients to the areas where land use was defined by agriculture in order to map where phosphorus runoff was the most problematic. Our resulting analysis (Figure 13) models annual phosphorus loading in kilograms per acre from land used for agricultural purposes in the Champlain Basin. The areas with the greatest phosphorus output came from the Missisquoi, Pike, and Otter Creek watersheds. This map also has the potential to suggest sites for future manure digester locations. The watersheds with higher contributions of phosphorus to Lake Champlain would be great sites to place manure digesters. If dairy operations existing within these watersheds were located near a waterway, a manure digester could significantly reduce the runoff from surrounding farms.

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Figure 13. GIS analysis of annual phosphorus loading from agricultural lands by each Vermont watershed flowing into Lake Champlain. Data from Vermont Center for Geographic Information and Troy et al., 2007. To further support this recommendation, we sought to determine how great of an impact

the liquid fertilizer byproduct of manure digestion could make on resulting phosphorus runoff when the fertilizer is injected into the soil rather than spread over the surface. We were unable to find previous studies that modeled these various manure applications on Vermont soils, however, Eric Smeltzer, from the Vermont Agency of Natural Resources provided us with the Lake Champlain BMP Scenario Tool developed for the EPA to help model different phosphorus loading scenarios. This multifaceted tool compares the results of various field management practices if used on land within major Lake Champlain watersheds to determine their impacts on phosphorus loading in nearby waterways. This tool describes how various agricultural management practices impact field runoff and helped us make an informed recommendation for dairy farmers in Vermont regarding their agricultural practices.

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We were able to use this scenario tool to explore the phosphorus reduction potential of a few manure management practices we explored earlier in our research. Tables 1, 2, 3, and 4 document the results of this tool when calibrated for two different watersheds using two different best management practices. We decided to use this tool as a way to model and compare the potential benefits of two different management strategies that could be implemented with the liquid fertilizer byproduct of manure digestion.

We chose to focus our scenario modeling on the Otter Creek and Missisquoi River watersheds based on the results of our phosphorus loading model (Figure 13). These areas were highlighted in our analysis due to the significantly higher modeled phosphorus loading from agricultural land in these areas. We selected land use types from the available options based on agricultural land types that would be most likely to exist on Vermont dairy farms and therefore would have the potential to see the benefits of the liquid fertilizer byproduct from a manure digester. This narrowed our selection down to corn-hay rotation and continuous corn. However, these two land use types occur on various types of soil or have different hydrologic soil groups (HSG). The HSG is used to describe the type of soil and runoff potential. We also provided the phosphorus loading from these specific land groups before the implementation of a best management practice (BMP). If the original phosphorus loading from the land type was insignificant, we did not consider it in our analysis. For both the Otter Creek and Missisquoi Watersheds we focused on two BMP strategies for which we provide the effectiveness of the strategy and the potential phosphorus reductions that result from their use.

In Tables 1 and 3, we focused on the management practice of manure injection with a reduced phosphorus fertilizer. This practice describes the fertilizer application practices we are recommending with the use of a manure digester and separator. The columns in each table labeled BMP (Best Management Practice) Efficiency and TP (Total Phosphorus) Reduction are the main foci of the scenario tools. In each scenario involving liquid injection with reduced phosphorus manure, the best management practice reduced the total phosphorus loading. The positive, though small, impact on phosphorus loading demonstrates further potential for the use of manure digester and separation technologies in phosphorus reduction.

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Table 1. Changes in phosphorus load for different agricultural land types in Otter Creek with application of manure injection with reduced phosphorus manure as a best management practice.

Land Use Type HSG TP Load (kg/yr)

BMP Efficiency

TP Reduction (kg/yr) with BMP

Corn-hay rotation on non-clayey soils A 724.76 5% 33.89

Corn-hay rotation on non-clayey soils B 4,909.27 3% 170.14

Corn-hay rotation on non-clayey soils C 1,972.69 3% 63.08

Corn-hay rotation on clayey soils D 34,306.24 3% 1,149.88

Continuous corn on non-clayey soils B 796.85 9% 70.31

Continuous corn on non-clayey soils C 374.55 9% 32.06

Continuous corn on clayey soils D 5,399.27 5% 264.79 Data from Lake Champlain BMP Scenario Tool

Table 2. Changes in phosphorus load for different agricultural land types in Otter Creek with application of cover crop, conservation tillage and manure injection as a best management practice.


BMP Efficiency



Corn-hay rotation on non-clayey soils B 4,909.27 43% 2,094.62



Continuous corn on non-clayey soils B 796.85 43% 339.99

Continuous corn on non-clayey soils C 374.55 46% 173.23

Continuous corn on clayey soils D 5,399.27 64% 3,464.53 Data from Lake Champlain BMP Scenario Tool

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Table 3. Changes in phosphorus load for different agricultural land types in Missisquoi River with application of manure injection with reduced phosphorus manure as a best management practice.


BMP Efficiency



Corn-hay rotation on non-clayey soils B 3,047.29 3% 105.61


Corn-hay rotation on clayey soils D 6,435.00 3% 215.69

Continuous corn on non-clayey soils B 1,087.63 9% 95.97

Continuous corn on non-clayey soils C 2,208.51 9% 189.05

Continuous corn on clayey soils D 1,990.34 5% 97.61 Data from Lake Champlain BMP Scenario Tool

Table 4. Changes in phosphorus load for different agricultural land types in Missisquoi River with application of cover crop, conservation tillage and manure injection as a best management practice.


BMP Efficiency

TP Reduction (kg/yr)


Corn-hay rotation on non-clayey soils B 3,047.29 43% 1,300.18

Corn-hay rotation on non-clayey soils C 9,527.40 46% 4,406.42


Continuous corn on non-clayey soils B 1,087.63 43% 464.06

Continuous corn on non-clayey soils C 2,208.51 46% 1,021.43

Continuous corn on clayey soils D 1,990.34 64% 1,277.13 Data from Lake Champlain BMP Scenario Tool

Tables 2 and 4 look at the same two watersheds, but we ran the scenario tool using a

different BMP. This BMP involves a series of management practices that we have explored through our research: cover crop, conservation tillage, and manure injection. The resulting efficiency and total phosphorus reduction for this combination of management practices were significantly greater than the scenario solely focusing on manure injection and reduced phosphorus fertilizer.

Although the scenario tool offered many more options for BMPs, we chose these scenarios to highlight the potential of manure injection. While the sole use of manure injection and reduced phosphorus manure does not make a huge impact on phosphorus load reduction, it still is an additional benefit to farmers who currently work with manure digesters. Additionally, the second scenario that employed a trio of management practices suggests that farmers using a liquid injection fertilizer application method with a byproduct of manure separation should also explore the conservation management practices described above. The combination of these

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efforts significantly increases the phosphorus reductions from agricultural land used for different purposes with varying soil types. Not only do these data encourage the use of various conservation management practices, but they also highlight the potential co-benefit of manure digesters in that they can provide farmers with an easily injectable fertilizer source. The large phosphorus loading reductions modeled in this scenario tool from the combination of cover crop, conservation tillage, and manure injection, highlight how significantly these practices can help farmers comply with TMDL requirements in their watershed.

4.5.2 Recommendations

Based on our models, we believe that the co-benefits of anaerobic manure digestion through the production of a liquid fertilizer make a strong case for the future siting of manure digester projects in the Champlain Basin and in other regions where phosphorus runoff has become an issue. A digester project would encourage farmers who put manure in the digester to use the liquid byproduct as a fertilizer on their fields and reduce their environmental impact both through the capture of methane and through the mitigation of phosphorus runoff. In Vermont, we recommend the siting of digester projects located in Otter Creek and the Missisquoi and Pike watersheds where runoff has been particularly problematic. If there aren’t any large farms in these areas, we recommend the construction of a community digester where farmers could truck their manure off site but could still reap the benefits of phosphorus mitigation through the use of the liquid fertilizer byproduct.

Additionally, when the technology becomes more economically feasible, we recommend the addition of phosphorus separators such as the decanter centrifuge to digester projects. These nutrient separators would further reduce dairy farm impacts on nearby watersheds in Vermont. The resulting dry nutrients produced in the separation project could also profit farmers if excess nutrients were sold to areas that had phosphorus shortages.

Finally, we would recommend the addition of monitoring stations in Lake Champlain tributaries to make more accurate assessments regarding problem watersheds in the basin. These stations could also support the modeling data if they show a significant decline in phosphorus with an increase in the number of manure digesters and separators.

5. Conclusion As 2016 draws nearer, Middlebury College has partnered with an anaerobic digester project at the Goodrich Farm to facilitate some of the final GHG emissions reductions needed to reach net zero. In 2013, with the addition of the biomass plant and various efficiency projects, emissions were reduced from ~30,500 MTCD to 13,848 MTCD, a 55% reduction. With the digester project, the college will switch from the use of #6 fuel oil to RNG, producing a 53% reduction from 2013 emissions. In addition, an average of baseline methane emissions modeled

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by RGGI and CARB at the Goodrich Farm representing the maximum offset potential estimates an additional 35% reduction with the implementation of the digester and subsequent capture and destruction of methane. These two aspects of the digester project therefore put projected 2016 reductions at 94.5% from 2008, with about 1,661 MTCDE still left to be offset. Although this project may not completely get the college to net zero, it plays a major role in that reduction and has the potential to provide more offsets in the future. Methane accounting is still a new and developing process that is ultimately determined by site-specific variables. As such, the way protocols assess and implement variables can change estimates drastically and represent different offset results. The more detail available about on-site practices, the more accurately offsets can be estimated; but it is important to note these variations within the context of the abstract world of carbon accounting. Different levels of emissions evaluations are necessary for different scales, however, and coarser protocols like the EPA model should be applied to larger scale projects while more specific calculations (RGGI and CARB) can be used for the finer scale. The digester project also offers the Goodrich Farm a way to reduce potential phosphorus overload and contamination in the Otter Creek watershed. Digester and phosphorus separation technologies can develop in tandem and contribute to more holistic conservation management practices. Overall, the digester project is a multifaceted way to reduce the carbon footprint for both the college and Goodrich Farm. It reduces the output of three different pollutants: CO2 from fuel oil, methane from open manure lagoons and phosphorus from spreading. This combination reflects how complex pollution and emission issues are and how remediation efforts can effectively reflect that complexity. Solutions like manure digesters provide yet another pathway to reduce emissions that can work congruently with other efficiency projects. Middlebury College’s partnership with the digester project also supports the development of digesters as a valuable energy efficiency strategy in Vermont. With some fine-tuning, it has the potential to develop into a standard of practice on farms across the state. It also shows that benefits generated with digester technology are not confined to the farm: offsets and energy efficiency benefits can be distributed to the larger community through the distribution of RNG and injectable phosphorus fertilizer. As technology continues to improve in the renewable energy market, more integrative techniques have also become more cost effective and encourage partnerships within communities. With this project, the college continues to work towards a carbon neutral goal and also encourages the use of a relatively new technology that supports the farming community and watershed ecosystems of Vermont as a whole. In the end, although carbon neutrality is a goal specific to campus, the energy efficiency solutions are a team effort.

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Acknowledgements

We would like to extend a thank you to our community partners Jack Byrne, the Directory of Sustainability Integration at Middlebury College, and Dan Smith, the CEO of LincolnRNG. Both helped immensely with our understanding of the proposed project, the goals of their organizations, and our success would not have been possible without them.

Additionally, thank you to David Dunn and Patrick Wood who provided us with further information on the manure digestion process and models used to account for methane emissions reductions from these types of projects. We would also like to extend a thank you to William Hegman of the Middlebury College Geography Department for his assistance in creating our GIS model and to Eric Smelter for providing us with the EPA’s Scenario Tool for Lake Champlain, which gave us a more precise idea of the impacts of liquid injection technology on phosphorus runoff.

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Appendix A: EPA

Methodology for estimating methane emissions from dairy manure management was taken from the U.S. Greenhouse Gas Inventory Report: 1990-2013 Annex 3. Following annual data was collected for using the model:

Animal population data: This includes determining animal type e.g. lactating dairy cows, heifers, etc., which accounts for differences in methane generation among livestock type. This was site-specific data obtained from the Public Service Board of Vermont. Typical animal mass (TAM) by animal type: The TAM refers to the annual average live weight of an animal based on livestock type. The EPA estimate of the average TAM in the State of Vermont was used. The data was obtained from the U.S. Greenhouse Gas Inventory Report: 1990-2013, Annex 3, Table A-204 (EPA, 2014). Waste management system type (WMS): The WMS refers to the types of systems used to manage manure. This includes determining manure storage facility and type e.g. dry or liquid storage and the amount of manure spread annually. This was site-specific data obtained from the Public Service Board of Vermont.

Volatile solids (VS) production rate by animal type: The VS content of manure is the fraction of the diet consumed by cattle that is not digested and thus excreted as fecal material, which combined with urinary excretions constitutes manure. The EPA estimate of the average VS production rate in the State of Vermont was used. The data was obtained from U.S. Greenhouse Gas Inventory Report: 1990-2013, Annex 3, Table A-206 (EPA, 2014). Methane producing potential (B0) of the volatile solids by animal type: The B0 is the maximum amount of methane that can be produced from a given quantity of manure per animal type. The EPA estimate of the average B0 in the State of Vermont was used. The data was obtained from the U.S. Greenhouse Gas Inventory Report: 1990-2013, Annex 3, Table A-204 (EPA, 2014). Methane conversion factors (MCF) by manure management system: The MCF is the maximum amount of methane per animal type and WMS that would be created if the volatile solids were completely converted into methane. The EPA estimate of the average MCF in the State of Vermont was used. The data for the state of Vermont was obtained from the U.S. Greenhouse Gas Inventory Report: 1990-2013, Annex 3, Table A-210 (EPA, 2014).

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To calculate the total baseline methane emissions in carbon dioxide equivalents the

following equation was used:

Where,

BE = Baseline emissions (metric tonnes of CO2 equivalent/year) A = Animal type WMS = Waste management system NA = Number of animals for animal type TAMA = Typical animal mass (kg) VSA = Volatile solids production rate (kg VS/1000 kg animal mass/day) DM = Distribution of manure by WMS for each animal type (%) 365.25 = Days per year B0 = Maximum CH4 producing capacity (m3 CH4/kg VS) MCF = Methane conversion factor for the animal type and waste management system

(%) 0.662 = Density t 25oC (kg CH4/m3 CH4) 0.001 = Conversion factor of kg CH4 per year to metric tonnes of CH4 per year GWP = The 100 year methane global warming potential for the conversion of CH4 to

CO2 equivalent

The CH4 emissions for each WMS and animal type were summed to determine the total baseline CH4 emissions (Table A1).

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Table A.1. Values used to calculate baseline methane emissions based on the EPA method.

* We were unable to acquire the exact percentages of the amount of manure distributed between different management systems at the Goodrich Farm, as such average manure distribution values provided by the EPA for Vermont were used. ** The latest GWP taken from IPCC Fifth Assessment Report (Myhre et al. 2013).

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Appendix B: CARB, COP

The procedure to determine the modeled project baseline methane emissions follows Equation 5.3, which combines Equation 5.3 and 5.4 of the protocol. The following inputs were used in Equation 5.3 based on site-specific information about the mass of volatile solids (VS) degraded by the anaerobic storage system and available for methane conversion.

Population and Livestock Category (PL): The procedure requires a designation of livestock category, e.g. lactating dairy cows, heifers, etc., which accounts for differences in methane generation among livestock type. This information was obtained from Table A.2 of Appx. A. The population figure is site-specific data that should be monitored each month and averaged for annual total population.

Annual Volatile Solids degraded in anaerobic manure (VSL): VS content of manure is the portion of animal intake that is not digested and subsequently excreted along with urine as manure expressed in a dry matter weight basis (EPA, 2014). VS values must be calculated for all livestock categories. Volatile solids degraded annually were calculated using livestock population (PL), estimated volatile solids produced by livestock category (obtained in Appx. A of the COP document), reporting days per month and the previous month’s available and degradable VS.

MassL: Mass refers to annual average live weight of animals based on livestock category (L). This value is necessary in the conversion of VS from kg/day/1000kg to kg/day/animal. Site-specific livestock mass information was not available, so value was obtained as Typical Average Mass (TAM) from Table A.2 in Appx. A.

Maximum methane producing capacity for livestock category ‘L’ (B0,L): B0,L

is the maximum methane producing capacity of an amount of manure. This information was obtained as a default from Table A.3 in Appx. A

Density and conversion factor of methane (MCF): MCF refers to the amount of potential methane produced given a certain management type. Methane production is a function of the anaerobic conditions present in a system, its temperature and the retention time of manure in that system. For anaerobic lagoons and slurries, this value requires a site-specific calculation of volatile solids degraded by anaerobic storage system. In this equation, this is expresses as ‘degraded volatile solids” or “VSdeg”. This value is equal to the monthly available VS multiplied by the van’t Hoff-Arrhenius factor (F), which converts total available VS to methane convertible VS based on the monthly temperature of the system.

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Once these variables were collected, the following equation was used to estimate baseline emissions:

BECH4,AS = ∑VSdeg,AS,L x B0,L x 0.68 x 0.001 x GWP

Where, BECH4,AS = Total annual project baseline methane emissions from anaerobic

manure storage systems, in carbon dioxide equivalent (t CO2e/yr) VSdeg,AS,L = Annual volatile solids degraded in anaerobic manure system ‘AS’ from

livestock category ‘L’ B0,L = Maximum methane producing capacity of manure for livestock category

‘L’ 0.68 = Density of methane 0.001 = Methane conversion factor from kg to metric tonnes (mt) GWP = The 100 year methane global warming potential for the conversion of CH4

to CO2 equivalent Table B.1. Values used to calculate baseline methane emissions based on the CARB method.

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Appendix C: RGGI CO2e (tons) = (Vm x M)/2000 x GWP

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