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Correspondence to: Horacio A. Aguirre-Villegas, Department of Biological Systems Engineering, University of Wisconsin-Madison,

460 Henry Mall B23, Madison, Wisconsin 53706, USA. E-mail: [email protected]

770 © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

Modeling and Analysis

From waste-to-worth: energy, emissions, and nutrient implications of manure processing pathways Horacio Andres Aguirre-Villegas, Rebecca Larson, Douglas J. Reinemann, University of Wisconsin-Madison, WI, USA

Received January 9, 2014; revised and accepted April 4, 2014 View online May 12, 2014 at Wiley Online Library (wileyonlinelibrary.com); DOI: 10.1002/bbb.1496; Biofuels, Bioprod. Bioref. 8:770–793 (2014)

Abstract: Four manure processing pathways are evaluated to provide a system-level understanding of their impacts on different sustainability indicators. In particular we look at how solid-liquid separation (SLS), anaerobic digestion (AD), and AD+SLS affect depletion of fossil fuels (DFF), nutrient balances, global warming potential (GWP), and ammonia emissions when compared to the base-case (BC) path-way of direct land application. Lifecycle sustainability assessment techniques are applied to develop inventory data and model a Wisconsin dairy farm. For the BC, net GWP is 101.2 kg CO2-eq, DFFS is 106.1 MJ, ammonia emissions are 2.62 kg, and nitrogen availability is 2.45 kg per ton of excreted manure. Net GWP is reduced in all pathways compared to BC by 19% for SLS, 48% for AD, and 47% for AD+SLS. DFF is reduced by 43% for AD and 40% for AD+SLS, but increased by 13% for SLS. Ammonia emissions are increased in all pathways by 2% for SLS, 40% for AD, and 44% for AD+SLS. Nitrogen availability remains the same in SLS but decreases in AD and AD+SLS due to higher ammo-nia volatilization, which could be reduced by injecting manure. Ratios of fossil energy (FERAD) and energy return on investment (EROIAD) of 3.7 and 0.98–1.8 are determined for AD pathways, com-pared to FER of 0.29 and EROI of 0.27 for grid electricity. When allocating results to specifi c outputs, variability can be reduced by applying system subdivision and allocation. Sensitivity analyses highlight the importance of reducing emissions during manure storage and the infl uence of changes in fertilizer and sand bedding. © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd

Supporting information may be found in the online version of this article.

Keywords: GHG; energy; ammonia emissions; nutrients; anaerobic digestion; solid-liquid separation; dairy manure; LCA

Introduction

Waste management is a critical component for the economic and environmental sustainability of the agricultural industry. Th e most common

disposal method for manure and agricultural by-products

is land application, which produces signifi cant atmos-pheric greenhouse gas (GHG) emissions, consumes land and fossil energy resources, and can result in soil nutri-ent build-up. Agriculture accounts for approximately 50% of the methane (CH4) and 80% of the nitrous oxide (N2O) global anthropogenic emissions, being manure

© 2014 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 8:770–793 (2014); DOI: 10.1002/bbb 771

Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann

manure or digestate can still be land-applied as fertilizer and emits less CH4 than non-digested manure during stor-age as it contains less carbon (C) and volatile solids (VS).6 Th rough the digestion process, mineralization increases ammoniacal N, which is more readily available for plant uptake than organic N.7,8 However, there might be greater NH3 losses through volatilization as ammoniacal N increases.9 All these changes and losses contribute to the overall N availability when land applied and the resulting emissions during storage and application make it diffi cult to assess the overall impact without an accounting system.

Wisconsin (WI) produces nearly 4.7 million dry tons of dairy manure annually. Th e concentration of dairy cattle in WI is a source of signifi cant environmental concern as discussed, but is also an immediate and abundant biomass source for energy generation.10 As a result, more than 30 digesters with electrical generation equipment are cur-rently operating on WI dairy farms, making it the leading state in digester installations and electricity production in the country. Despite this leadership and the signifi cant amounts of manure generated per year, little is known about the lifecycle impacts of manure management and processing practices. Adopting a lifecycle approach is necessary when evaluating manure processing pathways as changes in physical and chemical characteristics of manure aff ect downstream operations and their related environmental impacts. In order to make comparisons between management choices, a methodology is required to quantify the overall system impacts for diff erent combi-nations of management and technology options. In addi-tion, there is limited lifecycle inventory data to conduct a comprehensive manure processing study according to specifi c local contexts. Th is study aims at addressing these knowledge gaps by applying life cycle assessment (LCA) techniques to: (i) develop lifecycle inventory data repre-sentative of WI for alternative dairy manure processing pathways including manure land-spread, SLS, and AD; (ii) quantify lifecycle GHG emissions, NH3 emissions, primary fossil energy consumption, and nutrients fate and form for each pathway; and (iii) identify the environ-mental trade-off s among these pathways. Th e results of this study will provide useful information to researchers, dairy operators, and policymakers on the environmental impacts and trade-off s of diff erent manure processing techniques.

Literature review

Previous studies evaluating environmental impacts of dairy manure management have been limited in scope by

management responsible for 7% of both agricultural CH4 and N2O emissions.1 Volatilization of ammonia (NH3) from animal manures during collection, storage, and aft er land application reduces the nitrogen content available to plants and contributes to negative environmental impacts. In its gaseous form, NH3 can travel long distances before being deposited and further transformed to indirect N2O emissions or infi ltrated into aquatic and terrestrial eco-systems.2 Animal agriculture operations can reduce their impacts to air and water quality by implementing selected manure management strategies. Confi ned animal opera-tions have more control to further process manure than pasture-based systems as they are able to collect almost all excreted manure on the barn. However, handling and transporting large volumes of manure and agricultural by-products increases costs and requires signifi cant energy, typically from non-renewable fossil sources.

In recent decades, global concerns such as climate change and energy security have challenged agricultural producers to reduce environmental impacts and increase energy effi ciency of their operations while maintain-ing economic profi tability. Increasing the value of agri-cultural by-products or increasing effi ciencies has been achieved through processes such as solid liquid separation (SLS) and anaerobic digestion (AD). Th e value of animal manures has been related to their nitrogen (N), phospho-rus (P), and potassium (K) contents; but, depending on the soil and crop characteristics, the proportion of nutri-ents required for crop production may not always be the proportion of nutrients available in manure, which could result in over- or under-application of these nutrients. To avoid this problem, SLS has been implemented as a processing technology to separate some of the nutrients, particularly P, along with the manure solids. In addi-tion, there are economic benefi ts from transporting and applying concentrated nutrients in the solid stream, while making the liquid manure easier to handle. SLS can be achieved using gravity-driven separation systems (i.e. sedimentation basins and ponds) or mechanical separators (i.e. screw press and centrifuges), but it has been argued that greater effi ciencies are achieved with the latter.3 Even though SLS adds fl exibility for nutrient management, separating manure into liquid and solid fractions can aff ect posterior emissions and energy consumption. AD systems have contributed to achieving both climate change mitigation and energy independence by utilizing agricul-tural wastes, such as livestock manure, to produce biogas. Th is renewable gas is 50% to 65% methane and can be col-lected and used for direct burn applications, conversion to electricity, or compressed fuel.4,5 Following digestion, the

772 © 2014 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 8:770–793 (2014); DOI: 10.1002/bbb

HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways

feedstocks for biogas production due to its N content, easy degradability, and low cost availability.5 Th ese SLS and AD studies constitute valuable sources of information, but the scope of each individual study is limited and they not take into account potential interactions with other parts of the agricultural production system. A lifecycle approach can be used to include all variables that could infl uence the environmental performance of a complex agriculture-based system.32

Adopting a lifecycle approach, when evaluating bioen-ergy and agricultural systems, has become common prac-tice. Proof of this is the increasing number of agriculture related LCA studies available in the literature, and the inclusion of lifecycle thinking in energy and environmen-tal policies such as the Renewable Fuel Standard (RFS2) of the Energy Independence and Security Act and the California Low Carbon Fuel Standard.33,34 In agricultural LCAs, manure has been a subsection of broader studies that have focused primarily on dairy products or biogas. Dairy LCA studies typically target the production of fl uid milk and other dairy products with little detail on specifi c manure management systems.35–37 Biogas LCA studies have centered on new processing technologies or on co-digesting manure with other biomass feedstocks. De Vries et al. evaluated the environmental impacts of reverse osmosis to concentrate nutrients in the manure liquid fraction and generating biogas from the solid frac-tion.38 Environmental consequences were higher when compared to land applying manure for the concentrated technology, but climate change and depletion of fossil resources were reduced with the addition of an AD system. Borjesson and Berglund and Berglund and Borjesson ana-lyzed diff erent AD scenarios for feedstock production and energy generation.39–41 Th e authors found that operating the biogas plant demands more than one third of energy consumption and that AD systems are energy positive for short distance transport of raw materials. Poeschl et al. described single-feedstock digestion and co-digestion systems in small and large biogas plants in Germany, and concluded that the most effi cient substrates in terms of environmental impacts were straw and corn silage within single-feedstock scenarios and combining municipal solid waste with agricultural and food industry residues within co-digestion scenarios.4,42

All of these biogas LCA studies have been conducted in Europe, where economic incentives have justifi ed the cost of transporting waste streams from various sources to cen-tralized digesters and to use food crops as feedstocks. Th ere is a need to conduct comprehensive assessments on manure management considering environmental sustainability

focusing only on individual impact indicators, one stage of the manure handling system, or one manure process-ing technology. Air emission studies have focused on manure storage and land application as they are the major contributors to CH4, N2O, and NH3 emissions.11–14 Barn fl oor activities also contribute to GHG emissions,15–18 but NH3 emissions have been the main focus of study at this management stage.19,20 Air emissions depend on local con-ditions (e.g. temperature) and manure characteristics (e.g. volatile solids), which are aff ected by factors proper to the system under study.7,8,21,22 Th is dependence makes it dif-fi cult to apply emission factors determined by the previous studies to other systems and highlights the importance of tailoring emission factors to local conditions. Energy consumption has been analyzed by studies that focused on quantifying the costs of handling dairy manure.23–25 Th ese studies targeted direct energy consumption rather than primary energy and reported costs rather than environ-mental impacts of manure practices.

Hjorth et al. and Burton conducted reviews of separation effi ciencies of diff erent mechanical and chemical SLS tech-nologies for livestock and other manures.3,26 Th e authors concluded that centrifugation has the highest separation effi ciency, followed by sedimentation, and non-pressurized and pressurized fi ltration. In general, SLS technologies had low removal effi ciency for N when compared to total sol-ids (TS). Wu focused on the recovery and distribution of nutrients of screw press separated dairy manure for bed-ding and fertilizer.27 Th e author found that most of the P and N remained in the liquids fraction aft er the separation process, and that the effi ciencies were even lower when adjusting the separator to produce drier solids suitable for bedding. Moller et al. studied SLS through centrifugation, chemical precipitation, and fl occulation.28 Th e authors found that separation effi ciencies for N and TS depended on the manure’s TS content contrary to P, which was not aff ected by this factor.

Manure AD studies have primarily targeted energy inputs and biogas production according to specifi c aspects of digester design and feedstock’s performance. Gebremedhin et al. developed a heat transfer model to predict energy requirements to operate a plug-fl ow digester.29 Wu found that methane yield remained unchanged with increasing mixing power in complete-mix digesters, but that energy output increased with heat addi-tions in plug-fl ow digesters.30 Comino et al. investigated biogas yield from co-digesting cattle manure with cheese whey and found that up to 65% whey can be co-digested.31 Appels et al. reviewed several feedstocks used in AD and concluded that animal manure is one of the most suitable



manure disposal method. Th ree specifi cally defi ned manure processing pathways are compared against the BC pathway to identify potential environmental trade-off s. Th ese pathways are defi ned according to the manure processing technology: (i) mechanical SLS, (ii) AD, and (iii) AD+SLS. Attributional lifecycle concepts are used to evaluate each individual pathway and consequential life-cycle concepts are used to compare the marginal changes between the BC and the remaining pathways.

Th e study is conducted at the farm level for a time unit of one day. Th e four pathways are modeled with the soft ware GaBi 543 for a farm with 1000 lactating cows, 605 growing heifers (<21 months), and 286 mature heifers and dry cows. Th e size of the farm has been defi ned this way as current commercial manure digesters are more economically feasible for large dairy operations and the average dairy size of farms holding digesters is approximately 2000 animals in WI.44 As result, the more than 200 CAFOs in WI represent the great-est potential for manure processing in the short term.

Figure 1 shows the system boundaries that encompass all unit-processes from manure excretion to manure land

indicators and processing technologies according to local conditions and practices in the USA. Th is study aims to fi ll this gap in the literature by bringing together individual pieces of the manure handling systems and analyzing dif-ferent environmental indicators of alternative manure processing technologies with a lifecycle approach.

Methods

Scope

Th is work applies LCA and process-based concepts to evaluate four environmental sustainability indicators: (i) global warming potential (GWP), (ii) ammonia (NH3) emissions, (iii) depletion of fossil fuels (DFF), and (iv) nutrient form and fate. Th ese indicators are calculated and tracked through each step of a manure management sys-tem at a typical modeled dairy farm in WI where manure is collected, stored, and land applied. Th is sequence of steps, or unit-processes, constitutes the base-case (BC) pathway of this study, as it is the most simplifi ed dairy

Figure 1. System boundaries of the four pathways analyzed in this study.



conceptually diff erent from traditional primary fuels (e.g. coal, natural gas, or oil), which are specifi cally extracted as energy resources.48 Th ese indicators are included in this study given that they provide valuable information to make comparisons across multiple energy systems.

Inventory data

A life cycle inventory (LCI) was developed using a process-based approach to capture fl ows in and out each unit-process of the manure lifecycle. Material, energy, and emission fl ows are related to system parameters and local conditions of WI (e.g. volatile solids and temperature). In addition, the elec-tricity matrix used in this study represents the mix of fuels that are part of the electric grid of WI and is presented in Table S1 of the supplementary information (SI). Th e neces-sary data for constructing the LCI include various sources. First, on farm data has been collected from an online survey sent to dairy producers in WI. Th e survey identifi es the most common manure processing techniques and manure management practices for collecting, transporting, storing, and applying manure (Table S2). A total of 178 producers responded to the survey out of 2000 randomly selected farms across the 5 regions of WI: northern, west central, south central, northeast, and southeast. Second, a fi eld study has been conducted to characterize data (e.g. total solids, volatile solids, N, P, and K) about manure fl ows before and aft er AD and SLS.49 Th ird, the Energy Intensity and Environmental Impact of Integrated Dairy and Bio-Energy Systems in WI model has been applied to determine data about animal husbandry and crop production for dairy diet, which aff ects excreted manure characteristics and composition.37 Finally, representative literature review and verifi able databases have been used for material and energy inputs, including the National Renewable Energy Laboratory US LCI dataset, PE International Professional database, and EcoInvent.43,50,51 Year-to-year temperature variation is captured by averaging daily data from fi ve weather stations on each of the regions of WI for the years 2007–2011.

Manure characterization

Manure excretion depends on the characteristics of the herd and the diet (Tables S3 and S4). Manure mass and TS are determined using Eqns (S1)–(S5) and nutrient and carbon contents are calculated for each cow type and on a daily basis (Table 1). Manure pH is 7.55, volatile solids (VS) constitute 80% of excreted TS, and ammoniacal nitrogen represents 45% of excreted nitrogen for the aggre-gated manure of the herd.49 It is assumed that 50% of the VS are degradable.52,53

application. Animal husbandry and cultivation proc-esses for cow feeding are not included in this analysis. Embedded and cumulative energy, GHGs, and NH3 emissions associated with the production of material and energy inputs (i.e. diesel and electricity) are included in the system boundaries; however, the production of capi-tal goods (i.e. machinery and buildings) is excluded as its contribution to the measured outputs is marginal.45 Th e functional unit (FU) is defi ned as one metric ton (ton) of excreted manure as it refl ects the technical utility of the product and the function of the system, which is disposing the manure generated by the herd.46

Environmental sustainabilty indicators

Th e four sustainability indicators assessed in this study are highly relevant to manure systems. NH3 emissions and nutrients (N, P, and K) are reported in kilograms. GWP is characterized for a 100-year time horizon and measured in kilograms of carbon dioxide equivalents (kg CO2-eq). Characterization factors used for GHGs other than CO2 are 298 kg CO2-eq for N2O and 25 kg CO2-eq for CH4 based on the CML 2001 method. N2O emissions resulting from NH3 emissions and leaching are accounted for using IPCC’s defi nition that 0.01 of volatilized N as NH3 is converted to N2O-N emissions and 0.0075 of leached N is converted to N2O-N emissions.47 Emissions from manure are considered biotic emissions (CO2(b), N2O(b) , CH4(b)) and are evalu-ated separately from fossil fuel emissions (CO2(f), N2O(f), CH4(f)) in this study to identify major emission sources and to account for the CO2(b) recycling process that takes place during plant growing as it is assumed that the carbon con-tained in excreted manure has been previously captured as CO2 by the crops that constitute the dairy diet.

Depletion of fossil fuels (DFF) is defi ned as the energy consumed at the site plus the energy consumed in the production and delivery of that energy product. Th e fossil energy ratio (FER) and the energy return on investment (EROI) ratio can be used to determine the effi ciency of a sustainable energy production. If the FER is ≤ 1, the amount of usable energy is less than the fossil energy expended to obtain that energy. If the EROI ratio is ≤ 1, the amount of usable energy is less than the total energy expended to obtain that energy (Eqns (1) and (2)).

FERAD = Usable energyout ______________ Fossil energyin

(1)

EROIAD = Usable energyout ______________ Total energyin

(2)

Th e AD subscript is added to both ratios given that the concept of dairy manure as a resource to produce energy is



and energy conversion for the AD and AD+SLS path-ways, and mechanical separation for the SLS and AD+SLS pathways.

Manure collection

All manure from the herd is collected daily with a scrape system requiring a 30 kW (40 HP) skid steer. An average travel distance of 0.88 m cow–1 is calculated based on dif-ferent barn designs (Table S5). Aft er collection, manure mass and composition are recalculated to account for C and N volatile losses. Th e production of sand is not included in the analysis, but the sand that is collected along with manure is quantifi ed as it aff ects downstream unit-processes. On average, 14 kg of sand are replaced daily per 1000-pound animal.55 It is assumed that sand has a TS content of 95% and does not contribute to emissions or nutrient fl ows. CO2 and CH4 emissions from excreted manure in the barn prior to collection depend on ambient temperature and surface area exposed to manure. Barn emissions are calculated using Eqns (S6) and (S7). Average surface area (alley area) used in this study is 1.44 m2 for growing heifers and 2.15 m2 for milking cows, mature heifers and dry cows (Table S5). An emission factor (EF) of 5.4E–5 g N2O kg–1 manure is used to estimate barn emis-sions considering average yearly seasonal temperatures in WI and cow body weight.16 NH3 emissions depend on ammoniacal N, pH, temperature, and surface area exposed to manure, and are calculated according to Eqn (S8).

Sand recovery

It is challenging to transport and handle sand-laden manure as it is approximately 35% TS (Wedel, pers. comm.). Sand separation is increasing in use at dairy farms because it reduces wear on equipment and bedding costs when sand is recycled. In addition, dairy producers which use sand bedding and have an AD system typically separate sand as it has no biogas generating potential and can cause severe clogging and build-ups in the digester. Sand separation requires electricity and water for opera-tion. In this study, a separation system of 19 kW (25¾ HP) operating at 70% full load is considered for the size of the herd.56 Th e components of this system include a 7.5 kW (10HP) sand-manure separator, a 11 kW (15HP) inclined auger, and a 0.6 kW (¾ HP) air compressor. With a calcu-lated fl ow rate of 12,700 kg hour–1 for the modeled farm, this system recovers 87% of the incoming sand contain-ing up to 2% organic solids. Recycled water is added to dilute the sand-laden manure and to facilitate sand set-tling according to a 1:1 ratio of the incoming sand-laden

Marginal changes in sand beddiing and nutrients

Th is analysis considers marginal changes of material and energy inputs when comparing the BC to the SLS, AD, and AD+SLS pathways. Additions or reductions of mate-rial inputs happen with bedding and synthetic fertilizers. Sand is a typical bedding material used by dairies in WI and marginal changes in sand requirements occur when a more effi cient sand separation system is installed in a dairy farm with a digester to avoid clogging problems. To account for sand marginal changes, it is assumed that additional sand is extracted from the Maiden Rock under-ground sand mines in WI and transported 336 km to the farm with a 35 ton capacity diesel truck. Th is is the average distance between the mine and the center point of each of the fi ve regions in WI. Urea, diammonium phosphate, and potassium chloride are the synthetic sources of N, P, and K to supplement crop production when needed. Since it is assumed that all manure is land-applied in the BC path-way, marginal changes in synthetic fertilizer application occur due to reductions or increases in manure’s nutrient availability for plant uptake in pathways with digester and separator. Availability of nutrients in manure and diges-tate reaching the land is 80% for P and K, 100% for ammo-niacal nitrogen, and 43% for organic nitrogen (25% the fi rst year, 12% the second year, and 6% the third year).54

Description of pathways, unit-processes, and LCI development

Th e four basic manure unit-processes that are considered in all pathways are: (i) manure collection, (ii) sand recov-ery, (iii) manure storage, and (iv) manure land application (Fig. 1). Additional unit processes are biogas production

Table 1. Excreted manure characteristics per cow type.

Manure excretion characteristics

Lactating cows

Growing heifers

Dry cows and mature heifers

kg animal–1 day–1

Manurea 68.2 27 40.2

TSa 8.75 4.59 4.32

Cb 3.67 1.66 0.85

Kb 0.32 0.07 0.15

Nb 0.44 0.16 0.18

Pb 0.05 0.02 0.02a Calculated according to Eqns (S1)–(S5). b Calculated according to Reinemann et al.37



effi ciency is calculated based on a 35.77 MJ m–3 Lower Heating Value (LHV) of methane.61 A 90% capacity factor is assumed for the digester and engine to allow for shut-downs and maintenance. Digestate has three main diff er-ences when compared to incoming manure, 45% reduction of degradable VS, 68% increase in ammoniacal N content due to mineralization, and 3% increase in pH.49 GHGs are emitted from leaks in the AD system and CH4 combustion during electricity generation. A CH4 leakage factor of 2.8% is included for digester constructed from steel, lined con-creted, or fi berglass.62 Stoichiometry was used to calculate CO2(b) emissions from CH4 combustion (Eqn (S12)) where 2.75 kg of CO2 are produced from the complete combus-tion of 1 kg of CH4.

Storage

Manure effl uent from the processing stage(s) is stored in a rectangular clay-lined open basin. Th e storage is sized to hold the effl uent for a period of six months based on Eqn (S13). A storage surface area of 5113 m2 is calculated to hold a daily manure volume of 213 m3. For the BC, a natu-ral organic crust is assumed to form on the surface as the TS are greater than 7%.7 Seventeen percent of the organic N is converted to ammoniacal N at the end of each storage period, where most of the mineralization occurs during the summer.7,12 Agitation of storage prior to application consumes an average electricity rate of 0.25 kWh ton–1 manure according to the manure survey.

CH4(b) emissions are calculated daily from the manure entering the storage aft er collection plus the accumulated manure being stored up to that point (Eqn (S14)). Few studies have quantifi ed CO2 emissions from storage and no process-based equations have been developed. In this study, an EF of 0.012 kg CO2 m–3 for stored manure with crust formation is used.13,63 N2O emissions from manure storage have been cited to be directly related to ammo-niacal N content.8 Th is relation cannot be captured with IPCC EFs as they do not diff erentiate among N forms;64 therefore, an EF of 13.46 g N2O kg–1 of NH3-N is used.11 Indirect N2O emissions result from both NH3 and leach-ing losses. NH3 emissions are calculated with Eqn (S8) and 0.4% of stored N is lost as leaching.65

For the SLS and AD+SLS pathways, storage following the separation process is altered due to the formation of a solid and liquid stream resulting from a screw press separator. Th e liquid manure stream from the SLS process is stored for six months in an open basin as in the BC pathway. Th e liquid volume is reduced to 185 m3 and the overall surface area is also reduced to 4437 m2 at 5% TS when compared to the BC pathway. Th e reduced TS content eliminates the

manure.56 Separated sand, emissions from electricity consumption, and manure effl uent at 7.7% TS are the three outputs of this unit-process. It is assumed that no biotic emissions occur at this stage due to the short reten-tion time. For pathways with a digester, the sand recovery system requires the addition of a cyclone, which increases the total system capacity to 27 kW (35¾ HP) (Wedel, pers. comm.). Th e addition of the cyclone improves the sand recovery to 95%. Of the remaining 5% which is not recov-ered, 2% is discarded and 3% passes through the digestion system exiting with the digestate.

Mechanical separation

Th e mechanical separation unit-process is added to the SLS and AD+SLS pathways to divide manure or digestate into solid and liquid streams, facilitating nutrient manage-ment (Table S6). A screw press requiring 0.5 kWh tonne–1 of incoming manure is used for the separation process.57

Biogas production and energy conversion

Th e AD and AD+SLS pathways consider mesophilic diges-tion using a plug-fl ow digester and converting biogas to electricity using a generator. It is assumed that a portion of the produced electricity is used by the manure manage-ment operations. Th e remaining electricity is available to be used on-farm or injected to the grid. Th e inputs to the anaerobic digester include manure following sand separa-tion, electricity, and heat. Manure is re-characterized aft er barn collection and sand separation to account for losses and changes in form. Based on the manure production and a hydraulic retention time (HRT) of 28 days, a 4840 m3 digester volume is required to handle the inputs from the design farm. Electricity is used at a rate of 7 kWh ton–1 of wet manure for pumping and monitoring operations.58 In addition, a 3.6 kW power and 5600 m3 day–1and fl ow rate gas processing unit is required for removal of hydrogen sulfi de (H2S), which damages equipment. Heat is required to maintain the digester temperature at 38 °C for bacte-rial growth (Eqn (S9)) and to compensate for heat losses through digester walls (Eqn (S10)). In this work, heat is provided by combusting a portion of the biogas in a heat exchanger with 85% effi ciency.59

It is assumed that biogas is composed of 65% CH4 and 35% CO2, and less than 1% of other gases including H2S. Methane production is calculated using bacterial growth kinetics (Eqn (S11)). In this study, biogas is used to pro-duce electricity in a reciprocating internal combustion engine with 35% electricity effi ciency and 50% thermal effi ciency aft er the biogas cleaning process.60 Energy



Table 2. Summary of assumptions considered in each unit-process and pathway.

Pathway Unit process Assumptions

BC Collection Daily collection of sand-laden manure from milking cow, heifer, and dry cow barns

30 kW (40 HP) diesel fueled skid steer

Alley area is exposed to manure

Sand recovery Continuous sand separation

19 kW/35,960 l (25¾ HP) mechanical separator

87% sand recycling effi ciency

Storage Six month storage

Rectangular clay-lined open basin

Organic natural crust formation

16.5% ammoniacal N increase due to mineralization

Agitation before unloading

Land application Broadcast two times a year (spring and fall)

179 kW/ (240 HP/9,500 gallon) capacity diesel tanker

P is the limiting nutrient for application

SLS Collection Same as BC

Sand recovery Same as BC

Mechanic SLS Screw press

Separation effi ciencies: 14% (N), 28% (P), and 10% (K)

Storage Manure liquids: clay-lined open basin, no crust formation

Manure solids: concrete fl oor storage

Six month storage for both streams

Land application Application of solids and liquids as organic fertilizer

Liquids are applied to closer areas and solids to further areas

Marginal changes for synthetic fertilizers are considered

AD Collection Same as BC

Sand Recovery Addition of a cyclone to the separation system

27 kW (35¾ HP) total power

95% sand recycling effi ciency

Marginal changes for fresh sand are considered


Plug-fl ow digester

28 day retention time

No heat recovery

H2S removal

2.4% biogas leakages

35% and 50% electrical and thermal effi ciency

68% ammoniacal N increase due to mineralization

90% capacity factor

Storage Digestate storage under same assumptions as BC

Land application Digestate application under same assumptions as BC

AD+SLS Collection: same as BC; sand recovery and biogas production and energy conversion: same as AD; mechanic SLS, storage, and land application: same as SLS

formation of a natural crust on the stored manure, and thus, surface aeration. CH4(b) emissions are increased by 40% when no organic crust is formed. An EF of 0.041 kg

CO2 m–3 stored manure without crust formation is used.63 Th ere are no N2O emissions from storage in the SLS, AD, and AD+SLS pathways as there is no crust formation.7,8,47



cropland locations as they have a higher nutrient density, this management strategy reduces overall diesel consump-tion. Transportation distance can vary signifi cantly in actual practice depending upon fi eld layout, but is this case the land is assumed to be contiguous. Biotic emissions from manure liquids and solids are calculated based on the equations and EFs of the BC pathway, except for N2O from manure solids, which behave as untreated solid manure and thus, result in higher N2O emissions during storage. Nearly 1.97% of ammoniacal N aft er NH3 emissions from manure solids is emitted as N2O-N when land applied.8

A summary of the assumptions, calculated material and energy inputs, and calculated emission outputs for each unit-process and each pathway analyzed in this study can be found in Tables 2–4.

Allocation

Th is paper considers two approaches to analyzing the envi-ronmental impacts of the four pathways presented in Fig. 1. Th e fi rst approach considers whole-system environmental impacts to identify and compare environmental trade-off s among pathways. Th e second approach considers multi-functional systems, where the environmental impacts are partitioned among the diff erent products of each pathway (Table 5). Th e four products include: (i) manure only in the BC pathway; (ii) manure solids and manure liquids in the SLS pathway; (iii) electricity and digestate in the AD path-way; and (iv) electricity, manure solids, and manure liquids in the AD+SLS pathway. Common strategies to address the multi-functionality of the second approach include system subdivision, system expansion, or allocation.70 Th is paper adopts the allocation approach and compares it with the approach explained in Aguirre-Villegas et al., by fi rst sub-dividing the system and then applying allocation wherever subdivision is no longer possible.71 Th is approach is adopted as it has shown to reduce the variability in results regardless of the allocation ratio applied. Two diff erent ratios of eco-nomic value (EV) and total solids (TS) are assessed in order to evaluate the methodological decision related to allocation according to Eqn (3). A TS ratio is selected because as biogas production depends on volatile solids destruction.

ARx _____________________ ARp1 + ARp2 + … +ARpn (3)

where: AR = allocation ratio that can take the form of

economic value or total solids p1…pn = indicate the products that the system is

producing x = target product to assign environmental burdens

Th e solid manure stream following mechanical separa-tion has 31% TS, is produced at a daily rate of 66 m3 for the whole herd, and stored on a closed area with concrete fl ooring. CH4(b) emissions from manure solids are calcu-lated with Eqn (S15). An EF of 0.2 kg CO2 m–3 for stored manure solids is used.63 As opposed to manure liquids, the mixed aerobic and anaerobic conditions during manure solids storage promote N2O(b) emissions;8 therefore, an EF of 23.15 g N2O kg–1 of NH3-N is used.11 In terms of NH3 emissions, a total 20% of ammoniacal N volatilizes as NH3-N during storage of manure solids.66

Land application

It is assumed that manure is surface spread two times a year (spring and fall) with a 179 kW and 35,960 l (240 HP/9500 gallon) capacity diesel tanker.24 Manure contains N, P, and K that partially displaces synthetic fertilizers that require high fossil fuel energy inputs.54 WI regulates manure application according to crop requirements for P; therefore, the application area in WI is typically dependent on manure P concentration. In this study, it is assumed that manure is applied to crops that constitute the dairy cows’ diet (Table S4) (aside from cottonseed as this seed is not produced in WI). A total area of 3.6 Ha day–1 is needed to apply all manure produced from the modeled herd based on the prescribed diet. Nutrient requirements are obtained from the crop enterprise budgets for common cash and for-age crops grown in WI developed by the University of WI’s Center for Dairy Profi tability (CDP).67 Diesel consumption for manure application is calculated based on time and motion analysis and equations for fuel consumption.24,68

Fossil GHGs are emitted from diesel production and combustion.69 CH4(b) emissions are calculated according to Eqn (S16) and occur primarily during the fi rst days aft er application. Th is study adopts a conservative approach as it does not consider carbon sequestration aft er manure is land-applied; thus, it is assumed that all carbon (aft er CH4 emissions) is emitted as CO2 aft er application. N2O(b)-N is emitted at a rate of 1.76% of applied NH3-N aft er ammonia emissions.8 NH3 emissions aft er application are calculated with Eqn (S17). It is assumed that 30% of total applied N is leached in regions (including WI) where the sum of rain in rainy season minus the sum of potential evaporation in the same period exceeds the soil water holding capacity.64

For the SLS and AD+SLS pathways, land application fol-lowing the separation process is altered due to the forma-tion of a solid and liquid stream resulting from a screw press separator. Both liquid and solid streams are land applied on-farm. In this analysis, manure liquids are applied to nearby cropland and manure solids are transported to farther



Table 3. Material and energy inputs, expressed in terms of the functional unit, calculated for each unit-process and according to each pathways of this study.

Unit-process Pathway Description

Value ton1 of excreted

manure

Collectiona BC, SLS Excreted manureb 1 metric ton

Diesel fuelc 0.17 kg

Fresh sandd 44.2 kg

Recycled sande 293 kg

AD, AD+SLS

Fresh sand 16.5 kg

Recycled sand 321 kg

Sand recovery

BC, SLS Sand-laden manure 1,334 kg

Electricity 4.91 MJ

Recycled water 1,284 kg

AD, AD+SLS

Sand-laden manure 1,334 kg

Electricity 7.18 MJ

Recycled water 815 kg

Storage BC Manure from sand recovery

2,262 kg

Electricity agitation 2.03 MJ

SLS Manure liquids 2,021 kg

Manure solids 238 kg

Electricity liquids 1.82 MJ

AD Digested manure 1,789 kg

Electricity agitation 1.61 MJ

AD+SLS Digested manure liquids

1,672 kg

Digested manure solids

117.0 kg

Electricity agitation liquids

1.50 MJ

Land applicationg

BC Stored manure 2,256 kg

Dieself 1.42 kg

SLS Manure liquids 2,017 kg

Manure solids 238 kg

Diesel liquids 1.27 kg

Diesel solids 0.16 kg

Synthetic N addition 0.01 kg

Diesel addition 1.7E–5 kg

AD Stored digested manure

1,787 kg

Diesel 1.12 kg


Synthetic P addition 6.0E–5 kg

Synthetic K addition 0.03 kg


Table 3. (Continued)Land applicationg

AD+SLS Stored digested liquids

1,670 kg

Stored digested solids

116 kg

Diesel liquids 1.05 kg

Diesel solids 0.08 kg


Synthetic P addition 6.0E–5 kg

Synthetic K addition 0.03 kg


Mechanical SLS

SLS Manure from sand recovery

2,259 kg

Electricity separation 4.07 MJ

AD+SLS Digested manure 1,789 kg

Electricity separation 3.22 MJ


AD, AD+SLS

Manure from sand recovery

1,841 kg

Electricity digester 46.4 MJ

Electricity biogas cleaning

2.64 MJ

Heat digester 182 MJa Excreted manure and diesel fuel inputs for the collection unit-process are the same for all pathways (BC, SLS, AD, and AD+SLS).b Manure from milking cows, heifers, and dry cows calculated with Eqns (S1)–(S3).c Based on an average travel distance of 0.88 m per freestall and 30 kW (40 HP) skid steer.d The impacts of producing and transporting fresh sand are not considered in this study but sand use is shown for completeness and to calculate marginal changes of sand recovery when com-pared to the base case pathway. e Based on information provided by Wedel.56

f Based on Hadrich et al. and ASABE.24,68 Diesel requirements are 0.17 l to load the 9500 gallon truck, 0.13 l to travel a distance of 2 km to and from the crop area, and 0.36 l to spread manure. Assuming diesel has 43 MJ kg–1 LHV and 0.83 kg l–1 density.g Addition of synthetic P and K are a result of losses during the sand recovery process and the installation of a cyclone. Synthetic N addition is a result of NH3 volatilization.

EV allocation accounts for market prices to assign value to outputs and partition the environmental burdens among product and co-products. Price values for P, K, and N fertilizers are taken from the crop enterprise budgets for common cash and forage crops grown in WI for the year 2012,67 and electricity price is assumed to be 0.107 $ kWh–1.72 TS allocation partitions the burdens according to the TS in each end product, where TS reductions are used to allocate impacts for electricity in the AD and AD+SLS pathways.



Table 4. Calculated emission outputs, expressed in terms of the functional unit, from each unit-process and pathway of this study.

Unit-process

Pathways

Gasa BC

SLS

AD

AD+SLS

Liquidb Solid Liquidb Solid

g ton–1 manure excreted

Plant uptakeb CO2(b) –194,023 –194,023 – –194,023 –194,023 –

Collectiond CO2(b) 3,084 3,084 – 3,084 3,084 –

CO2(f) 590 590 – –46.3 –46.3 –

CH4(b) 25.3 25.3 – 25.3 25.3 –

CH4(f) 0.83 0.83 –0.07 –0.07 –

N2O(b) 4.78 4.78 – 4.78 4.78 –

N2O(f) 4.0E–3 4.0E–3 – –6.0E–3 –6.0E–3 –

NH3 362 362 – 362 362 –

Sand recoverye,f CO2(b) – – – 1,801 1,801 –

CO2(f) 1,122 1,122 – – – –

CH4(f) 0.87 0.87 – – – –

N2O(f) 1.6E–2 1.6E–2 – – – –

Storagef CO2(b) 27.7 81.8 139 72.3 67.3 68.1

CO2(f) 463 414 – – – –

CH4(b) 2,500 1,922 122 199 55.7 62.2

CH4(f) 0.36 0.32 – – – –

N2O(b) 37.3 11.7 3.06 22.8 26.2 4.80

N2O(f) 6.6E–3 5.9E–3 – – – –

NH3 199 890 54.0 1,738 2,006 68.0

Land application CO2(b) 183,879 105,283 79,693 98,647 57,353 41,504

CO2(f) 5,199 4,650 607 4,287 3,848 591

CH4(b) 34.1 20.8 0.24 44.7 26.2 0.22

CH4(f) 7.6 6.79 0.98 6.90 5.6 2.03

N2O(b) 80.6 64.1 7.09 79.6 73.5 7.40

N2O(f) 0.12 0.11 0.30 1.86 0.1 3.4

NH3 2,057 1,160 194 1,579 1,088 241

Mechanical SLS CO2(f) – 928 – – – –

CH4(f) – 0.72 – – – –

N2O(f) – 1.32E–2 – – – –

Biogas and energyf CO2(b) – – – 88,512 88,512 –

CH4(b) – – – 426 426 –

a Gas emissions are differentiated from biotic (b) and fossil (f) sources. b Liquid for collection and sand recovery refl ect the emissions of manure before separated into liquid and solid streams.c Plant uptake considers that the carbon contained in excreted manure has been previously captured as CO2 by the crops that constitute the dairy diet.d Negative fossil emissions result from a more effi cient sand recovery process and the avoided production and transportation of fresh sand.e CO2(b) emissions from AD pathways come from manure lost along with discarded sand in the cyclone. f No fossil emissions from electricity consumptions are shown in AD pathways since electricity is provided by the AD system.



presented for the entire system and as allocated to each product for each of the four defi ned pathways outlined in the methods section. Th ese products are: (i) manure (BC); (ii) electricity and digestate (AD); (iii) manure solids

Results and discussion

Results of GWP, NH3 emissions, DFF, and N availability are expressed per ton of excreted manure. Th e results are

Table 5. Multi-functionality issues through the system and approaches to deal with them by subdivision and allocation strategies in the pathways of the study.

Pathway System type Products Subdivision and allocation strategy

Pathway 1: BC Single-output Manure N/A

Pathway 2: SLS Multifunctional Manure solids and manure liquids

Subdivision: storage and land application for manure liquids and solids

Allocation: collection, sand recovery, and mechanical separation

Pathway 3: AD Multifunctional Electricity and digestate

Subdivision: storage and land application for digestate, and biogas production and energy conversion for electricity

Allocation: collection and sand recovery

Pathway 4: AD+SLS

Multifunctional Electricity, manure solids, and manure liquids

Subdivision: storage and land application for manure liquids and solids, and biogas production and energy conversion for electricity

Allocation: collection and sand recovery (electricity, manure liquids, and manure solids), and mechanical separation (manure liquids and manure solids)

Figure 2. C and N mass balance for each unit-process of the BC pathway.



(in the form of CH4) during storage, but also how com-busting biogas to produce electricity introduces C emis-sions (in the form of CO2) sooner in the manure lifecycle. Since this study assumes no C sequestration, all land-applied C is eventually emitted to the atmosphere in a period of 100 years. During AD pathways, more than half of this C is emitted as CO2 immediately aft er combustion. Even though there are implications of this time diff erence in GWP, its quantifi cation is limited by the current LCA methodology. Th e N balance shows the diff erent forms of N through the manure lifecycle and how these are parti-tioned in individual unit processes. Both AD and AD+SLS pathways show an increase in ammoniacal N during stor-age and anaerobic digestion. At the same time, emissions are increased during storage and reduced during land application in AD pathways when compared to the BC and SLS pathways.

and liquids (SLS); and (iv) electricity, manure solids, and manure liquids (AD+SLS).

System results

Mass balance

A mass balance for C and N identifying the magnitude and sources of emissions throughout the manure lifecycle is presented in Figs 2 and 3 for BC and AD+SLS and Fig S1 and S2 for SLS and AD. Th e magnitude of C emissions aft er land application shows the importance of initiatives that quantify and improve carbon sequestration rates of raw manure, digestate, separated manure, and separated manure aft er digestion. Th is is especially true in the BC and SLS pathways where more C reaches the land applica-tion process compared to the AD pathways. In addition, the balance shows how AD pathways reduce C emissions

Figure 3. C and N mass balance for each unit-process of the AD+SLS pathway.



liquid fraction. Th is is because 42% of the C is within the solid fraction.

With 22%, CH4(b) is the second contributor to GWP in the BC. Th is trend is maintained in the SLS pathway as CH4(b) from the liquid fraction contributes 18% to GWP. Methane emissions are higher from manure liquids due to the anaerobic conditions that are created on the liquid storage pond. Gross CH4(b) emissions from storage are signifi cantly reduced in the AD and AD+SLS pathways since methane is captured and combusted to produce elec-tricity, converting part of the CH4(b) emissions that would have been emitted during storage to CO2(b). GWP from CH4(b) emissions at the digestion unit process in both AD and AD+SLS pathways are a result of leakages from the AD system. N2O(b) emissions from application is the third source of GWP in the BC representing 8% of this impact category. Th ese emissions are consistent among pathways, but represent a higher percentage during land applica-tion of AD and AD+SLS (10% of gross GWP) due to the reduced total GWP of these pathways. In the SLS pathway, N2O(b) emissions from land applying the liquid fraction are higher (7% of gross GWP) than those from the solid fraction (1% of gross GWP) because most of the readily available ammoniacal N stays with the liquid fraction aft er the separation process.

Global warming potential (GWP)

GWP is presented in gross and net terms. Th e gross approach shows the contributions to GWP from each unit-process across all pathways, including captured and emitted CO2(b). Th is approach is useful to guide manage-ment practices and improvements that seek to reduce GHG emissions from specifi c manure unit-processes. Captured CO2(b) is calculated as -194 kg CO2-eq ton–1

excreted manure for all pathways. Gross GWP is 295.2 kg CO2-eq ton–1excreted manure for the BC pathway, 276.3 kg CO2-eq ton–1excreted manure for the SLS pathway, 246.4 kg CO2-eq ton–1excreted manure for the AD path-way, and 247.7 kg CO2-eq ton–1excreted manure for the AD+SLS pathway. Figure 4 shows that CO2(b) emissions are the major contributors to gross GWP in all pathways, but they occur from diff erent unit processes. For example, CO2(b) from land application is responsible of 62% of GWP in the BC pathway, while CO2(b) from biogas combustion is responsible for 36% of GWP in the AD and AD+SLS pathways. In the SLS pathway, CO2(b) come mostly from manure liquids application (47% of gross GWP) and sol-ids application (30% of gross GWP). Emissions from the separated solid and liquid fractions are relatively similar even though nearly 90% of the total manure mass is in the

Figure 4. Contribution GWP from each unit-process and pathway according to the type of GHG.



these processing pathways are limited by two factors: the elimination of a natural crust during manure storage (which creates anaerobic conditions and promotes CH4(b) emissions), and the CH4 leakages during the digestion process. Finally, CO2(f) emissions are reduced in both AD and AD+SLS pathways because all manure handling elec-tricity needs are provided by the system, but are increased in the SLS pathway due to the installation of the mechani-cal separator. Figure 5 also shows an increase in GWP from CO2(b) emissions, which can be explained by the fact that some of the carbon emitted in the form of CH4(b) in the BC pathway is being emitted as CO2(b) in the SLS, AD, and AD+SLS pathways.

Ammonia (NH3) emissions

NH3 emissions increased for all pathways in comparison to the BC. NH3 emissions per ton of excreted manure are 2.6 kg NH3, 2.7 kg NH3, 3.7 kg NH3, and 3.8 kg NH3 for the BC, SLS, AD, and AD+SLS pathways, respectively. In all pathways, NH3 emissions occur during collection, storage, and land application (Fig. 6). During manure collection, NH3 emissions are equivalent among path-ways as there are no technology or management changes. When manure is stored, the formation of a crust in the BC prevents the volatilization of ammoniacal N in the BC pathway. As a result, 79% of total NH3 emissions occur during manure land application. Manure does not form a natural crust when it is digested or separated. Th e lack of a crust formation following digestion or separation and the mineralization of nitrogen through the digestion proc-ess further increases NH3 emissions during storage. As a result, storage accounts for 33%, 47%, and 53% of total NH3 emissions in the SLS, AD, and AD+SLS pathways respectively. Ammoniacal N is more susceptible to volatili-zation during land application if it is not rapidly injected or incorporated, practices that are assessed in the sensitiv-ity analysis section. In spite of this, NH3 emissions aft er land application are reduced in the SLS, AD, and AD+SLS pathways because TS are also reduced when compared to the BC, facilitating manure infi ltration into the soil more readily. In pathways with a solid-liquid separator, the liq-uid stream is responsible for 77% of total NH3 emissions as ammoniacal N remains in the liquid portion following separation. Even though NH3 emissions are increased dur-ing storage of liquid manure in the SLS pathway, total NH3 emissions are similar to those of the BC pathway. Th is is mostly attributed to low NH3 emissions from manure solids storage and reduced NH3 emissions from land appli-cation due to reduced TS in the liquid stream (facilitating the infi ltration of ammoniacal N into the soil).

Contribution to GWP from each pathway is expressed with a net GWP approach, which subtracts the CO2 uptake by the crops produced for cow feed. Th is net approach is useful to compare the changes in environmental impacts across pathways. Net GWP is 101.2 kg CO2-eq ton–1

excreted manure for the BC pathway, 82.3 kg CO2-eq ton–1

excreted manure for the SLS pathway, 52.4 kg CO2-eq ton–1 excreted manure for the AD pathway, and 53.7 kg CO2-eq ton–1 excreted manure for the AD+SLS pathway. Biotic emissions represent 92%, 89%, 91%, and 90% of net GWP in the BC, SLS, AD, and AD+SLS pathways respec-tively (the remaining portion is from fossil emissions).

Figure 5 shows that overall the three processing path-ways (SLS, AD, and AD+SLS) have reduced net GWP when compared to the BC pathway. Th e AD pathway presents a 48% reduction in GWP, just above the AD+SLS reduction of 47%, and doubling the reduction of the SLS pathway of 19%. Th ese reductions are attributable prima-rily to lower N2O(b), CH4(b), and CO2(f) emissions. During storage, the BC pathway forms a natural crust due to the high TS content. Th e remaining pathways do not form a crust as the TS content is reduced, creating anaerobic conditions and avoiding any direct N2O(b) emissions through nitrifi cation. Th e net N2O(b) reduction is 4.6, 9.5, and 1.8 kg CO2-eq ton–1excreted manure for the AD, SLS, and AD+SLS pathways. Reductions are lower in AD and AD+SLS pathways because there are still indirect N2O emissions from NH3 volatilization, which is higher in pathways with a digester. Most GWP reductions come from CH4(b) during storage due to the reduction of VS aft er digestion. Th ese reductions go from 12.9 kg CO2-eq ton–1

excreted manure in the SLS, 51.2 kg CO2-eq ton–1excreted manure in the AD, to 54.4 kg CO2-eq ton–1excreted manure in the AD+SLS pathways. CH4(b) reductions in

Figure 5. Comparison of net GWP change between the SLS, AD, and AD+SLS pathways with respect to the BC pathway, and according to the type of GHG.



improvement when compared to the electricity from the Grid that has an FER of 0.29 and an EROI of 0.27.

Th e relative low EROIAD as compared to other renew-able energy systems can be explained by several factors. First, the low energy density of manure and the energy involved in handling this high water content biomass source. Second, the nearly 3% methane losses that are

Depletion of fossil fuels (DFF)

DFF is much greater in the non-AD pathways as expected (Tables 6). DFF is increased by 13% during the SLS path-way given that electricity to operate the mechanical sepa-rator comes from the Grid. DFF is reduced by 43% and 40% during the AD and AD+SLS pathways respectively given that electricity is provided by the AD system. All DFF comes from diesel fuel in these pathways. Material energy is a result of the marginal changes in embedded energy of sand bedding and synthetic fertilizers. Positive numbers indicate an addition of material requirements and negative numbers indicate a reduction of material requirements in that pathway when compared to the BC pathway.

Electricity is produced at a rate of 221 MJ ton–1 excreted manure for the AD pathway and 218 MJ ton–1 excreted manure for the AD+SLS pathway. To achieve this produc-tion rate, 31 m3 of methane ton–1 of excreted manure are generated before accounting for methane losses, conver-sion effi ciencies, and electricity requirements of the sys-tem. Fossil energy ratio (FERAD) is 3.7 for the AD pathway and 3.5 for the AD+SLS pathway, while the energy return on investment (EROIAD) ratio is 0.98 for the AD path-way and 0.94 for the AD+SLS pathway. Th is is a major

Figure 6. NH3, direct N2O, and indirect N2O emissions from each unit-process and pathway.

Table 6. DFF according to each unit-process and pathway.

Unit-process

Depletion of Fossil Fuel (MJ ton–1 manure excreted)

BC SLS AD AD+SLS

Collection 8.7 8.7 –1.33 –1.33

Sand recovery 17.2 17.2 – –

Mechanic SLS – 14.3 – –

Liquid storage 7.1 6.4 – –

Liquid application 73.1 65.4 61.0 59.7

Solid application – 8.2 – 4.0

Total 106.1 120.5 59.7 62.4

Electricity 24.3 37.8 – –

Diesel 81.8 82.3 66.6 66.8

Material – 0.44 –6.9 –4.45



not converted to usable energy. Th ird, the ineffi cient use of energy during the conversion process as all ther-mal energy (55%) is lost to the environment as heat. Th e EROIAD can be increased to 1.3 if the digester is heated with recovered waste heat from the generator, and can be further increased to 1.8 if all heat is recovered. Th e EROI increases to 5 if biogas is used directly for combustion. Further increases in the EROIAD can be achieved with the addition of feedstocks with higher biogas production potential, but those scenarios were not evaluated in this analysis.

Nutrients

N, P, and K are tracked throughout the unit processes as shown in Table 7. P and K had only marginal losses that occur during sand recovery with the cyclone in the AD pathways. Final P and K availability for crop production is 80% of these land-applied quantities. SLS is the only proc-ess that aff ects P and K distributions by partitioning these nutrients between the solid and liquid streams. Evaluating the fate of N is more challenging because it volatilizes as NH3, is lost as N2O(b), and changes forms due to minerali-zation during digestion and storage. Mineralization aff ects the organic and ammoniacal fractions thereby impacting fi nal availability for crop production. As shown in Fig. 7, N availability for crop production is almost the same for all SLS, AD, and AD+SLS pathways when compared to the BC pathway, despite the diff erence in total N being land-applied. N availability mostly depends on NH3 emissions aft er manure broadcast and the form of N reaching the land (organic N is not as readily available as ammonia-cal N). More total N reaches the land in the BC pathway than in any other pathway, but NH3 emissions during land application are higher in this pathway as well as its organic N fraction. Even though there is mineralization during digestion and storage, NH3 volatilization reduces fi nal

N availability in AD pathways. Th ese nitrogen losses in the form of NH3 emissions could be reduced by adopting dif-ferent manure management practices such as covering the storage and injecting manure instead of surface broadcast-ing as examined in the sensitivity analysis section.

Allocated results

Whole-system allocation and subdivision/allocation ratios are applied to net GWP, NH3 emissions, and DFF. Th e same whole-system allocation rates are calculated to all sustainability indicators. As shown in Table 8, this approach adds signifi cant variability to the model as it interchanges results in magnitude and direction depend-ing on which allocation strategy (TS or EV) is applied to partition the environmental burdens among co-products of each pathway. For example, the TS strategy assigns 42% of the burdens to manure liquids in the SLS pathway, but when the EV strategy is applied, only 13% of the burdens are assigned to this same product. Th is same variability happens with the AD+SLS pathways, where 81% of the burdens are assigned to electricity with the EV strategy, but only 30% with the TS strategy.

Subdivision/allocation ratios are calculated for indi-vidual sustainability indicators. As shown in Table 9, the variability is signifi cantly reduced when applying this approach, as the partitioning of environmental impacts among co-products is consistent no matter what alloca-tion strategy is applied. With this approach, the contri-bution of manure liquids to GWP is 90–91% in the SLS pathway and 67–68% in the AD+SLS pathway. Digestate contributes 81% to GWP in the AD pathway. Th is same trend applies to NH3 emissions and DFF. As shown in Fig. 4, the majority of net impacts are related to storage

Figure 7. Organic N, ammoniacal, and N availability after land application for each pathway.

Table 7. P, K, and N availability after land application for the solid (sol) and liquid (liq) streams of each pathway.

Pathways

Nutrient availability after land application (kg ton–1 excreted manure)

Pliq Psol Kliq Ksol Nliq Nsol

BCa 0.58 – 3.48 – 2.45 –

SLS 0.42 0.16 3.14 0.35 2.17 0.27

ADa 0.58 – 3.45 – 2.38 –

AD+SLS 0.42 0.16 3.11 0.35 2.07 0.25a P, K, and N for the BC and AD pathways are presented in the liquid column.



and land-application. Th e subdivision/allocation approach assigns the burdens of these specifi c unit-processes to the products that are responsible for them (i.e. digestate stor-age is not related to electricity production), avoiding over-weighting of the environmental impacts to co-products.

Th is is one of the fi rst LCA studies to focus on the manure management system rather than the milk produc-tion system. Th us, there are no similar studies available in the literature to compare results for all pathways and

sustainability indicators under the same assumptions. GWP has been the most studied impact in the LCA lit-erature, but manure GWP has been aggregated to total results and the functional unit has usually been kilograms of milk. Reinemann et al. reported 0.19 kg CO2-eq kg–1 milk attributed to manure handling, which is equivalent to 71 kg CO2-eq ton–1 manure.37 Th oma et al. reported a US average of 1.23 kg CO2-eq kg–1 milk, with manure responsible for 22% of these emissions.35 Th is is equivalent

Table 8. Whole-system allocation rates for GWP, NH3 emissions, and DFF.

Pathway Allocation Ratio

Manure Solids Manure Liquids Electricity Digestate

System allocation rates for all sustainability indicators (%)

SLS TS 42 58 – –

EV 13 87 – –

AD TS – – 30 70

EV – – 81 19

AD+SLS TS 29 41 30 –

EV 2 17 81 –

Table 9. Subdivision/allocation rates for GWP, NH3 emissions, and DFF.

PathwaySystem GWP (kg CO2-eq ton–1 excreted manure)

Allocation Ratio


Subdivision/allocation rates for GWP (%)

SLS 78.2 TS 10 90 – –

EV 9 91 – –

AD 59.3 TS – – 19 81

EV – – 21 79

AD+SLS 57.3 TS 13 68 19 –

EV 13 67 20 –

PathwaySystem NH3 (kg ton–1

excreted manure)Allocation

Ratio


Subdivision/allocation rates for NH3 emissions (%)

SLS 2.6 TS 15 85 – –

EV 11 89 – –

AD 3.5 TS – – 3 97

EV – – 8 92

AD+SLS 3.7 TS 11 86 3 –

EV 8 83 9 –

PathwaySystem DFF (MJ ton–1

excreted manure)Allocation

Ratio


Subdivision/allocation rates for DFF (%)

SLS 119.4 TS 21 79 – –

EV 11 89 – –

AD 55.5 TS – – 1 99

EV – – 2 98

AD+SLS 59.7 TS 8 90 2 –

EV 7 88 5 –



As a result, N2O(b) emissions aft er land application have a signifi cant infl uence on GWP in the AD and AD+SLS pathways. GWP sensitivity to NH3 emissions indicates the importance of indirect N2O emissions in this model as well as CH4 leakages in pathways that involve a digester. Oppositely, some other parameters that infl uence results are negatively related to GWP. For example, an increase of TS in AD pathways results in a decrease in GWP because biogas production is related to TS content. GWP is also negatively related to parameters that increase nutrient availability, such as the number of milking cows and nutrient content (N, P, and K) in excreted manure, because the model considers that any increase in manure nutrients would replace the production of synthetic fertilizers and their related lifecycle impacts. Th is trend is not linear since manure application is limited due to nutrient application constraints.

A 10% change in nitrogen availability, diesel fuel, and sand recovered would have the biggest infl uence on DFF, which indicates the amount of primary energy that is embedded in the production of nitrogen fertilizer and diesel fuel, and the signifi cant quantities of sand that are used for cow bedding. DFF is sensitivity to N volatilization because additional synthetic fertilizer is needed to com-pensate for those N losses. Parameters that increase nutri-ent availability (i.e. number of cows) reduce DFF because of replacement of synthetic fertilizers. Energy require-ments, including electricity for solid-liquid mechanical separation, sand recovery, and diesel for manure collection and application; impact DFF in BC and SLS pathways. Th ese energy requirements are directly related to the amount of manure mass that is processed; therefore, DFF is sensitive to parameters that increase manure quantities (i.e. water used in sand recovery). In AD pathways, DFF is inversely sensitive to TS (more TS less DFF), especially to those TS that get into the digester because they are directly related to renewable energy production.

NH3 emissions are most sensitive to volatilization aft er land application in BC and SLS pathways, and during storage in AD and AD+SLS pathways. For pathways that involve SLS, volatilization from manure liquids is more infl uential than from manure solids. As N in excreted manure increases, downstream NH3 emissions also increase. Inversely, as more organic N is available when reaching land application, NH3 emissions are reduced. Finally, N availability is sensitive to excreted N (mainly by milking cows), ammoniacal N (since it is more avail-able), fi nal availability of organic N, and N volatilization as it decreases its availability. NH3 emissions are reduced by 40% if manure is injected instead of being surface

to 102.8 kg CO2-eq ton–1 manure if this number expressed in terms of the functional unit of this paper. Although these results can be compared to the 101.2 kg CO2-eq ton–1 manure of the BC pathway, assumptions, management practices, and climatic conditions are diff erent. Th e Dairy Greenhouse Gas Model v 1.2 was used to simulate the con-ditions of the BC pathway for WI.73 However, results are not comparable because manure GWP is reported together with barn and cropland GWP in that model. Th e only unit process that could be compared was storage with results of 80 kg CO2-eq ton–1 manure vs. 74 kg CO2-eq ton–1 manure from this paper.

Results from the AD and AD+SLS pathways can be compared to biogas LCA studies available in the litera-ture; however, most of these studies consider diff erent practices (i.e. co-digestion with other biomass feedstocks) and report results in terms of energy produced. Poeschl et al. reported –23.2 kg CO2-eq ton–1 manure for a refer-ence pathway where biogas was produced from cattle manure.4 Th e authors accounted for the credits of avoiding the production and use of fossil electricity and synthetic fertilizers, which precludes the comparison among results. Borjesson and Berglund reported 15 g CO2-eq MJ–1 of heat and power for animal manure when conducting system expansion to partition the environmental impacts of the system between digestate and energy.40 Aft er applying the subdivision/allocation approach (and including heat as part of the products to make results comparable) this paper determines that 10 g CO2-eq MJ–1 are emitted to produce electricity.

Senstivity analysis

A sensitivity analysis is performed by varying each of the parameters used to determine GWP, NH3 emis-sions, DFF and nutrient availability by ±10%. Results for AD+SLS pathway are presented in Fig. 8 and in the SI for the remaining pathways (Figs S3–S5). A cut-off criterion is adopted to include those parameters that infl uence the results by more than ±1%. Sensitivity to nutrient avail-ability is presented for N only as it is the most dynamic nutrient. GWP is most sensitive to changes in CO2(b) emissions aft er application in all pathways because it is assumed that all carbon in manure is eventually emitted as CO2 aft er application, which highlights the importance of future studies that quantify the carbon sequestration of manure application in the long term. CH4(b) emis-sions during storage have a bigger infl uence on GWP in the BC and SLS pathways since CH4 has been captured and transformed to energy during the digestion process.



land-applied in the pathways with a digester. Th is reduc-tion increases nitrogen availability by 50% in AD and AD+SLS, and 46% when compared to BC without injec-tion. Th e overall eff ects on GWP and DFF of these and other management practices will be analyzed in future work.

Conclusions and implications

Th e manure processing pathway has a substantial infl u-ence on the estimates of environmental impacts, which highlights the importance of assessing diff erent sustain-ability indicators to identify trade-off s among pathways. Th is study uses a process based approach to develop manure specifi c inventory data and applies LCA tech-niques to quantify and compare GHGs, NH3 emissions, DFF, and nutrients between land applying manure, which

is the most traditional disposal method in WI, with three other pathways that use SLS and AD technologies. Findings show that AD pathways have the largest reduc-tions in GWP and DFF, but the largest increase in NH3 emissions. Reductions in GWP come mostly from avoid-ance of CH4(b) and N2O(b) emissions during storage and land application, and reduction of fossil CO2 emissions due the production of electricity from biogas. Th e AD+SLS and AD pathways present the largest increase in NH3 emissions with 44% and 40% respectively, while the SLS pathway presented no change. Th is signifi cant increase is a result of higher ammoniacal N in manure aft er both storage and digestion, the inexistence of a natural crust on top of the storage, and the land-spread application method (broadcast) since all of these factors create the conditions for N volatilization. Th ese losses could be mitigated by adopting diff erent management practices such as manure

Figure 8. Change in GWP, NH3 emissions, DFF, and N availability for a ±10% change of individual parameters of the AD+SLS pathway.



injection. When compared to grid electricity, the FERAD improved by a factor of 12 and the EROIAD by a factor that ranges from 3 to 7 depending if heat is recovered in AD pathways. Th ese ratios would have been higher by recov-ering the thermal energy that is currently lost during the energy generation process. Land applied total P and K are similar to excreted P and K for all pathways, with the dif-ference that they are divided between the solid and liquid streams during SLS pathways. Even though the minerali-zation that occurs in the digester increases ammoniacal N (which is more available for crop production) during AD pathways, the overall net N availability aft er land applica-tion is very similar in all pathways.

When allocating the sustainability indicators to spe-cifi c system outputs, it is demonstrated that adopting the traditional whole-system allocation approach introduces substantial variability to the results. Th is variability can be signifi cantly reduced by applying a combined subdi-vision and allocation approach. A sensitivity analysis shows the importance of the N balance through manure management since it aff ects all sustainability indicators simultaneously. In addition, since C is a major component of manure TS, it is important to understand the dynamics of manure C when it reaches the land and how to improve carbon sequestration to reduce GWP. Besides direct emis-sions, such as CH4 and N2O during storage and land appli-cation, indirect emissions from NH3 emissions have a big infl uence on GWP. Even though marginal changes do not have a major contribution to environmental impacts in the modeled pathways, sensitivity shows that an increase in synthetic fertilizers and bedding materials would increase DFF. Th is highlights the importance of promoting prac-tices that increase nutrient availability since the produc-tion of these materials is so energy intensive.

Th e approach adopted and the model developed in this paper can be applied to a broad range of sustainability indicators and systems that have not been studied in the LCA literature. Th is approach is useful to compare marginal changes among alternative pathways that are a result of improvement decisions and technologies, and to identify and quantify the environmental trade-off s from a system oriented perspective regardless of the diff erence in fi nal products. Th is paper combines data from diff er-ent sources and integrates multiple simulation models to fi ll the current knowledge gaps in the areas of dairy manure management, conventional LCA, and environ-mental sustainability to inform businesses, farmers, and governments and promote science-based responsible deci-sion making. Th is paper is only a fi rst step in the process of developing a more comprehensive study which will

involve addressing some important challenges. First, more research is needed to characterize and understand GHG and NH3 emissions from digestate and separated manures. Th is study only diff erentiates N2O and NH3 emissions based on ammoniacal content, but other variables such as C:N ratio and particle size could infl uence GWP. Second, the decision to include the environmental impacts related to the agricultural phase of manure needs to be analyzed. LCA studies have so far considered that manure is a waste and thus, has no environmental impacts associated to its production (all environmental impacts are assigned to the main product of milk). However, as manure gains more importance as a source of nutrients for crop pro-duction and becomes a major input for renewable energy generation, it will turn from being a waste to a valuable co-product of the dairy system. Considering manure as a co-product will add complexity and further challenges to the model due to the additional allocation and variables that will have to be included in the analysis. Th ird, alter-native strategies should be analyzed to improve the envi-ronmental sustainability of manure processing pathways. For example, diff erent manure management practices (i.e. alley scraper collection and land-injection), co-digestion of manure with other locally available biomass resources (i.e. corn stover and cheese whey), and exploring diff erent biogas conversion technologies besides electric energy (i.e. compressed gas for transportation). Finally, economic and social considerations should be included to complete the three sustainability pillars that constitute a thorough Life Cycle Sustainability (LCSA) study.

Acknowledgement

Th is material is based upon work support by the National Institute of Food and Agriculture, United States Department of Agriculture, under ID number WIS01604.

References 1. United States Environmental Protection Agency (USEPA),

Global Anthropogenic Non-CO2 Greenhouse Gas Emissions: 1990-2020. EPA 430-R-06-003. [Online]. Available at: http://nepis.epa.gov/Adobe/PDF/2000ZL5G.PDF [October 13, 2013].

2. Michel J, Weiske A and Möller K, The effect of biogas diges-tion on the environmental impact and energy balances in organic cropping systems using the life-cycle assessment methodology. Renew Agric Food Syst 25(3):204–218 (2010).

3. Hjorth M, Christensen KV, Christensen ML and Sommer SG, Solid-liquid separation of animal slurry in theory and practice. A review. Agron Sustain Dev 30(1):153–180 (2009).

4. Poeschl M, Ward S and Owende P, Environmental impacts of biogas deployment – Part I: Life cycle inventory for evalu-ation of production process emissions to air. J Clean Prod 24:168–183 (2012).



5. Appels L, Lauwers J, Degrève J, Helsen L, Lievens B, Willems K et al., Anaerobic digestion in global bio-energy production: Potential and research challenges. Renew Sust Energ Rev 15(9):4295–4301(2011).

6. United States Environmental Protection Agency (USEPA), Protocol for Quantifying and Reporting the Performance of Anaerobic Digestion Systems for Livestock Manures. AgSTAR Program. [Online]. Available at: http://www.epa.gov/agstar/documents/protocol.pdf [October 13, 2013].

7. Rotz CA, Corson MS, Chianese DS, Hafner SD, Jarvis R and Coiner CU, The Integrated Farm System Model (IFSM) Reference Manual. [Online]. Pasture Systems and Watershed Management Research Unit, Agricultural Research Service, United States Department of Agriculture (USDA). Available at: http://www.ars.usda.gov/Main/docs.htm?docid=8519 [September 26, 2013].

8. Chadwick D, Sommer S, Thorman R, Fangueiro D, Cardenas L, Amon B et al., Manure management: Implications for greenhouse gas emissions. Anim Feed Sci Technol 166/167:514–531(2011).

9. Petersen SO and Sommer SG, Ammonia and nitrous oxide interactions: Roles of manure organic matter management. Anim Feed Sci Technol 166/167:503–513 (2011).

10. Radloff G, Du XS, Porter P and Runge T, Wisconsin Strategic Bioenergy Feedstock Assessment. [Online]. WI Bioenergy Initiative. Available at: http://www.epa.gov/agstar/lib/index.html [September 5, 2013].

11. Amon B, Kryvoruchko V, Amon T and Zechmeister-Boltenstern S, Methane, nitrous oxide and ammonia emis-sions during storage and after application of dairy cattle slurry and infl uence of slurry treatment. Agr Ecosyst Environ 112(2/3):153–162 (2006).

12. Sommer SG, Petersen SO, Sørensen P, Poulsen HD and Møller HB, Methane and carbon dioxide emissions and nitrogen turnover during liquid manure storage. Nutr Cycl Agroecosystems 78(1):27–36 (2007).

13. Sneath RW, Beline F, Hilhorst MA and Peu P, Monitoring GHG from manure stores on organic and conventional dairy farms. Agr Ecosyst Environ 112(2/3):122–128 (2006).

14. Flessa H and Beese F, Atmospheric pollutants and trace gases – Laboratory estimates of trace gas emissions follow-ing surface application and injection of cattle slurry. J Environ Qual 29:262–268 (2000).

15. Ngwabie NM, Jeppsson K-H, Gustafsson G and Nimmermark S, Effects of animal activity and air tem-perature on methane and ammonia emissions from a naturally ventilated building for dairy cows. Atmos Environ 45 (37):6760–6768 (2011).

16. Wheeler EF, Adviento-borbe MA, Topper PA, Brown NE and Varga G, Ammonia and greenhouse gas emissions from dairy freestall barn manure, in Proceedings of the ASABE Annual International Meeting, June 29–July 2, 2008. ASABE, Providence, RI, USA (2008).

17. Laurens JP, Naveen NK and A MM, GHG emissions from manure in a naturally ventilated, freestall dairy barn, compar-ing sand and sawdust bedding, in Proceedings of the ASABE Annual International Meeting, August 1–4, 2004. ASABE, Ottawa, Ontario, Canada (2004).

18. Jungbluth T, Hartung E and Brose G, Greenhouse gas emis-sions from animal houses and manure stores. Nutr Cycl Agroecosystems 60:133–145 (2001).

19. Harper LA, Flesch TK, Powell JM, Coblentz WK, Jokela WE and Martin NP, Ammonia emissions from dairy production in Wisconsin. J Dairy Sci 92(5):2326–2337 (2009).

20. Moreira VR and Satter LD, Effect of scraping frequency in a freestall barn on volatile nitrogen loss from dairy manure. J Dairy Sci 89(7):2579–2587 (2006).

21. Sommer SG, Petersen SO and Moller HB, Algorithms for cal-culating methane and nitrous oxide emissions from manure management. Nutr Cycl Agroecosyst 69(2):143–154 (2004).

22. Webb J, Estimating the potential for ammonia emissions from livestock excreta and manures. Environ Pollut 111(3):395–406 (2001).

23. Sweeten JM and Reddell DL, Time-motion analysis of feedlot manure collection systems. Trans ASABE 22(1):138–143 (1979).

24. Hadrich JC, Harrigan TM and Wolf CA, Economic comparison of liquid manure transport and land application. Appl Eng Agric 26(5):743–758 (2010).

25. Wiens MJ, Entz MH, Wilson C and Ominski KH, Energy requirements for transport and surface application of liquid pig manure in Manitoba, Canada. Agric Syst 98(2):74–81 (2008).

26. Burton CH, The potential contribution of separation tech-nologies to the management of livestock manure. Livest Sci 112(3):208–216 (2007).

27. Wu Z, Phosphorus and nitrogen distribution of screw press separated dairy manure with recovery of bedding material. Appl Eng Agric 23(6):757–762 (2007).

28. Moller HB, Hansen JD and Sorensen CAG, Nutrient recovery by solid-liquid separation and methane productivity of solids. Trans ASABE 50(1):193–200 (2007).

29. Gebremedhin K, Wu G, Gooch C, Wright P and Inglis S, Heat transfer model for plug-fl ow anaerobic digesters. Trans ASABE 48(2):777–785 (2005).

30. Wu B, Integration of mixing, heat transfer, and biochemi-cal reaction kinetics in anaerobic methane fermentation. Biotechnol Bioeng 109(11):2864–2874 (2012).

31. Comino E, Riggio VA and Rosso M, Biogas production by anaerobic co-digestion of cattle slurry and cheese whey. Bioresource Technol 114:46–53 (2012).

32. Pimentel D, Ethanol fuels – Energy security, economics and the environment. J Agr Environ Ethic 4(1):1–13 (1991).

33. Energy Independence and Security Act (EISA), Pub. L. no. 110-140. 110th Congress. 1st session. [Online]. Available at: http://www.gpo.gov/fdsys/pkg/BILLS-110hr6enr/pdf/BILLS-110hr6enr.pdf [September 5, 2013].

34. California Air Resources Board (CARB), Proposed Regulation to Implement the Low Carbon Fuel Standard,Volume I. [Online]. California Environmental Protection Agency. Available at: http://www.arb.ca.gov/fuels/lcfs/lcfs.htm [September 5, 2013].

35. Thoma G, Popp J, Shonnard D, Nutter D, Matlock M, Ulrich R et al., Regional analysis of greenhouse gas emissions from USA dairy farms: A cradle to farm-gate assessment of the American dairy industry circa 2008. Int Dairy J 31:S29–S40 (2013).

36. Rotz CA, Montes F and Chianese DS, The carbon footprint of dairy production systems through partial life cycle assess-ment. J Dairy Sci 93(3):1266–1282 (2010).

37. Reinemann DJ, Passos-Fonseca TH, Aguirre-Villegas HA, Kraatz S, Milani F, Armentano LE et al., Energy intensity and



environmental impact of integrated dairy and bio-energy sys-tems in WI, The Greencheese Model. [Online]. Available at: http://fyi.uwex.edu/greencheese/ [December 15, 2013].

38. De Vries JW, Groenestein CM and De Boer IJM, Environmental consequences of processing manure to produce mineral ferti-lizer and bio-energy. J Environ Manage 102:173–183 (2012).

39. Börjesson P and Berglund M, Environmental systems analysis of biogas systems—Part I: Fuel-cycle emissions. Biomass Bioenerg 30(5):469–485 (2006).

40. Börjesson P and Berglund M, Environmental systems analy-sis of biogas systems—Part II: The environmental impact of replacing various reference systems. Biomass Bioenerg 31(5):326–344 (2007).

41. Berglund M and Börjesson P, Assessment of energy perform-ance in the life-cycle of biogas production. Biomass Bioenerg 30(3):254–266 (2006).

42. Poeschl M, Ward S and Owende P, Environmental impacts of biogas deployment – Part II: life cycle assessment of multiple production and utilization pathways. J Clean Prod 24:184–201 (2012).

43. PE International, GaBi 5: The Software System for Life Cycle Engineering. PE-Europe GmbH and IKP University of Stuttgart, Stuttgart, Germany (2012).

44. Kramer J, Wisconsin Agricultural Biogas Casebook. Focus on Energy. [Online]. Available at: http://www.focusonenergy.com/fi les/Document_Management_System/Renewables/biogas09_casestudy.pdf [September 5, 2013].

45. Kim S, Dale BE, Jenkins R, Life cycle assessment of corn grain and corn stover in the United States. Int J Life Cycle Assess 14:160–174 (2009).

46. Reap J, Roman F, Duncan S and Bras B, A survey of unre-solved problems in life cycle assessment: Part 1. Goal and scope and inventory analysis. Int J Life Cycle Assess 13(4):290–300 (2008).

47. Intergovernmental Panel on Climate Change (IPCC). Emissions from livestock and manure management, in IPCC Guidelines for National Greenhouse Gas Inventories Vol 4. [Online]. Agriculture, forestry and other land use. Available at: http://www.ipcc-nggip.iges.or.jp/public/2006gl/vol4.html [October 23, 2013].

48. Murphy DJ, Hall CAS, Dale M and Cleveland C, Order from chaos: A preliminary protocol for determining the EROI of fuels. Sustainability 3(12):1888–1907 (2011).

49. Ozkaynak A and Larson RA, Nutrient Fate and Pathogen Assessment of Solid Liquid Separators Following Digestion, in Proceedings of the ASABE Annual International Meeting, July 29-August 1, 2012. ASABE, Dallas, TX, USA (2012).

50. National Renewable Energy Laboratory (NREL), US Life-Cycle Inventory (LCI) Database. NREL (2010).

51. Ecoinvent Centre, Ecoinvent Data. v2.0. Ecoinvent Reports No.1-25 Please advi. [Online]. Swiss Centre for Life Cycle Inventories. Available at: www.ecoinvent.org [October 23, 2013].

52. United States Environmental Protection Agency (USEPA), An evaluation of a mesophilic, modifi ed plug fl ow anaerobic digester for dairy cattle manure. AgSTAR Program. [Online]. Available at: http://www.epa.gov/agstar/documents/gordon-dale_report_fi nal.pdf [October 23, 2013].

53. Wilkie AC, Anaerobic Digestion of Dairy Manure : Design and Process Considerations. Dairy manure management: Treatment, handling and community relations (NRAES-176).

Natural Resource, Agriculture, and Engineering Service, Ithaca, NY, USA, pp. 301–312 (2005).

54. Laboski CAM and Peters JB, Nutrient application guidelines for fi eld, vegetable, and fruit crops in WI (A2890). University of Wisconsin-Extension, WI, USA (2012).

55. Midwest Plan Service (MWPS-7), Dairy Freestall Housing and Equipment, 7th ed. Iowa State University, Ames, IA (2000).

56. Wedel A, Sand-manure separation, in Proceedings of the Got Manure? Conference, March 27-29, 2012, Liverpool, NY, USA (2012).

57. Moller HB, Lund I and Sommer SG, Solid-liquid separation of livestock slurry: Effi ciency and cost. Bioresource Technol 74:223–229 (2000).

58. Han J, Mintz M and Wang MQ, Waste-to-Wheel Analysis of Anaerobic-Digestion-Based Renewable Natural Gas Pathways with the GREET Model. [Online]. Energy Systems Division, Argonne National Laboratory. Available at: http://greet.es.anl.gov/publication-waste-to-wheel-analysis [October 23, 2013].

59. Combined Heat and Power Partnership (CHP), Catalog of CHP technologies. [Online]. US Environmental Protection Agency. Available at: http://www.epa.gov/chp/technologies.html [October 23, 2013].

60. United States Environmental Protection Agency (USEPA), Technology Characterization: Reciprocating engines. [Online]. Available at: http://www.epa.gov/chp/documents/catalog_chptech_reciprocating_engines.pdf [December 15, 2013].

61. Jalalzadeh-Azar A, Saur G and Lopez A, Biogas resources characterization. 2010 Hydrogen Program Annual Merit Review. [Online]. National Renewable Energy Laboratory (NREL). Available at: http://www.nrel.gov/docs/fy10osti/48057.pdf [October 23, 2013].

62. United Nations Framework Convention on Climate Change (UNFCCC), Anex 32: Project and leakage emissions from anaerobic digesters (Version 1.0.0). Methodological tool. [Online]. Executive Board of the Clean Development Mechanism Meeting 66. Available at: http://cdm.unfccc.int/methodologies/PAmethodologies/tools/am-tool-14-v1.pdf [September 5, 2013].

63. Chianese DS, Rotz CA and Richard TL, Simulation of carbon dioxide emissions from dairy farms to assess greenhouse gas reduction strategies. Trans ASABE 52(4):1301–1312 (2009).

64. Intergovernmental Panel on Climate Change (IPCC), N2O emissions from managed soils, and CO2 emissions from lime and urea application, in IPCC Guidelines for National Greenhouse Gas Inventories, vol 4. [Online]. Agriculture, Forestry and Other Land Use. Available at: http://www.ipcc-nggip.iges.or.jp/public/2006gl/vol4.html [October 23, 2013].

65. United States Environmental Protection Agency (USEPA), Annex 3: Methodological descriptions for additional sources or sink categories, in Inventory of US greenhouse gas emis-sions and sinks: 1990-2011. [Online]. Available at: http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html [October 23, 2013].

66. Rotz CA and Oenema J, Predicting management effects on ammonia emissions from dairy and beef farms. Trans ASABE 49(4):1139–1150 (2006).

67. Center for Dairy Profi tability (CDP), Crop enterprise budgets and economics, in Budgets and Other Resources. [Online]. Available at: http://cdp.wisc.edu/ [December 15, 2013].

68. ASABE, ASAE D497.6 JUN2009 Agricultural Machinery Management Data. ASABE, Dallas, TX, USA (2009).



69. United States Environmental Protection Agency (USEPA), Emission factors for greenhouse gas inventories [Online]. Available at: http://www.epa.gov/climateleadership/docu-ments/emission-factors.pdf [October 23, 2013].

70. International Organization for Standardization (ISO), ISO 14040: Environmental Management – Life cycle assessment – Principles and Framework. ISO, Geneva, Switzerland (2006).

71. Aguirre-Villegas HA, Milani FX, Kraatz S and Reinemann DJ, Life cycle impact assessment and allocation methods development for cheese and whey processing. Trans ASABE 55(2):613–627 (2012).

72. Energy Information Administration (EIA), Electric power monthly statistics. [Online]. Available at: http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_06_a [September 5, 2013].

73. Rotz CA and Chianese DS, The Dairy Greenhouse Gas Model (version 1.2; computer software). [Online]. Pasture Systems and Watershed Management Research Unit, Agricultural Research Service, United States Department of Agriculture (USDA). (2009). Available at: http://www.ars.usda.gov/Main/docs.htm?docid=21345#Reference [Accessed September 5, 2013].

Horacio Aguirre-Villegas

Horacio Aguirre-Villegas is a PhD candidate in Biological Systems Engi-neering at the University of Wisconsin-Madison. His research lies in the fields of bioenergy, climate change, waste managemen,t and life cycle assess-ment (LCA). In particular, he focuses on evaluating the sustainability of agri-

cultural and food production systems and their integra-tion with bioenergy systems from a life cycle perspective, pinpointing areas of improvement.

Rebecca Larson

Becky Larson is an assistant professor and extension specialist in the Biologi-cal Systems Engineering Department at UW-Madison focusing on biologi-cal waste issues. Becky completed her BSc MSc, and PhD in Biosystems Engineering Department at Michigan State University. Her research and

extension interests include all areas of biological waste including manure management, handling and treatment of agricultural waste, diffuse source pollution, agricultural sustainability, and waste- to-energy technologies includ-ing biogas production from anaerobic digestion.

Douglas J. Reinemann

Douglas Reinemann is professor and Chairman of the Biological Sys-tems Engineering Department at the University of Wisconsin-Madison. His research and educational inter-ests include energy use and energy production in agricultural systems. He is a member of the sustainability group

of the UW Great Lakes Bioenergy Research Center examining environmental impacts of biofuels production systems. He also leads the UW ‘green cheese’ team who are investigating synergies between dairy and biofuels production systems in Wisconsin and has been actively involved with the Midwest Rural Energy Council .

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