From waste-to-worth: energy, emissions, and nutrient implications of manure processing pathways

Transcription

1 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: /bbb.1496; Biofuels, Bioprod. Bioref. 8: (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) pathway 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 kg CO 2 -eq, DFFS is 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 ammonia volatilization, which could be reduced by injecting manure. Ratios of fossil energy (FER AD ) and energy return on investment (EROI AD ) of 3.7 and are determined for AD pathways, compared to FER of 0.29 and EROI of 0.27 for grid electricity. When allocating results to specific outputs, variability can be reduced by applying system subdivision and allocation. Sensitivity analyses highlight the importance of reducing emissions during manure storage and the influence of changes in fertilizer and sand bedding 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. The most common disposal method for manure and agricultural by-products is land application, which produces significant atmospheric greenhouse gas (GHG) emissions, consumes land and fossil energy resources, and can result in soil nutrient build-up. Agriculture accounts for approximately 50% of the methane (CH 4 ) and 80% of the nitrous oxide (N 2 O) global anthropogenic emissions, being manure Correspondence to: Horacio A. Aguirre-Villegas, Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall B23, Madison, Wisconsin 53706, USA. aguirreville@wisc.edu Society of Chemical Industry and John Wiley & Sons, Ltd

2 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann management responsible for 7% of both agricultural CH 4 and N 2 O emissions. 1 Volatilization of ammonia (NH 3 ) from animal manures during collection, storage, and after land application reduces the nitrogen content available to plants and contributes to negative environmental impacts. In its gaseous form, NH 3 can travel long distances before being deposited and further transformed to indirect N 2 O emissions or infiltrated into aquatic and terrestrial ecosystems. 2 Animal agriculture operations can reduce their impacts to air and water quality by implementing selected manure management strategies. Confined animal operations 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 byproducts increases costs and requires significant 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 efficiency of their operations while maintaining economic profitability. Increasing the value of agricultural by-products or increasing efficiencies has been achieved through processes such as solid liquid separation (SLS) and anaerobic digestion (AD). The value of animal manures has been related to their nitrogen (N), phosphorus (P), and potassium (K) contents; but, depending on the soil and crop characteristics, the proportion of nutrients 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 addition, there are economic benefits 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 efficiencies are achieved with the latter. 3 Even though SLS adds flexibility for nutrient management, separating manure into liquid and solid fractions can affect posterior emissions and energy consumption. AD systems have contributed to achieving both climate change mitigation and energy independence by utilizing agricultural wastes, such as livestock manure, to produce biogas. This renewable gas is 50% to 65% methane and can be collected and used for direct burn applications, conversion to electricity, or compressed fuel. 4,5 Following digestion, the manure or digestate can still be land-applied as fertilizer and emits less CH 4 than non-digested manure during storage as it contains less carbon (C) and volatile solids (VS). 6 Through 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 NH 3 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 difficult to assess the overall impact without an accounting system. Wisconsin (WI) produces nearly 4.7 million dry tons of dairy manure annually. The concentration of dairy cattle in WI is a source of significant 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 currently operating on WI dairy farms, making it the leading state in digester installations and electricity production in the country. Despite this leadership and the significant 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 affect 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 different combinations of management and technology options. In addition, there is limited lifecycle inventory data to conduct a comprehensive manure processing study according to specific local contexts. This study aims at addressing these knowledge gaps by applying life cycle assessment (LCA) techniques to: (i) develop lifecycle inventory data representative of WI for alternative dairy manure processing pathways including manure land-spread, SLS, and AD; (ii) quantify lifecycle GHG emissions, NH 3 emissions, primary fossil energy consumption, and nutrients fate and form for each pathway; and (iii) identify the environmental trade-offs among these pathways. The results of this study will provide useful information to researchers, dairy operators, and policymakers on the environmental impacts and trade-offs of different manure processing techniques. Literature review Previous studies evaluating environmental impacts of dairy manure management have been limited in scope by 2014 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 771

3 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways focusing only on individual impact indicators, one stage of the manure handling system, or one manure processing technology. Air emission studies have focused on manure storage and land application as they are the major contributors to CH 4, N 2 O, and NH 3 emissions Barn floor activities also contribute to GHG emissions, but NH 3 emissions have been the main focus of study at this management stage. 19,20 Air emissions depend on local conditions (e.g. temperature) and manure characteristics (e.g. volatile solids), which are affected by factors proper to the system under study. 7,8,21,22 This dependence makes it difficult 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 These studies targeted direct energy consumption rather than primary energy and reported costs rather than environmental impacts of manure practices. Hjorth et al. and Burton conducted reviews of separation efficiencies of different mechanical and chemical SLS technologies for livestock and other manures. 3,26 The authors concluded that centrifugation has the highest separation efficiency, followed by sedimentation, and non-pressurized and pressurized filtration. In general, SLS technologies had low removal efficiency for N when compared to total solids (TS). Wu focused on the recovery and distribution of nutrients of screw press separated dairy manure for bedding and fertilizer. 27 The author found that most of the P and N remained in the liquids fraction after the separation process, and that the efficiencies were even lower when adjusting the separator to produce drier solids suitable for bedding. Moller et al. studied SLS through centrifugation, chemical precipitation, and flocculation. 28 The authors found that separation efficiencies for N and TS depended on the manure s TS content contrary to P, which was not affected by this factor. Manure AD studies have primarily targeted energy inputs and biogas production according to specific aspects of digester design and feedstock s performance. Gebremedhin et al. developed a heat transfer model to predict energy requirements to operate a plug-flow digester. 29 Wu found that methane yield remained unchanged with increasing mixing power in complete-mix digesters, but that energy output increased with heat additions in plug-flow 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 feedstocks for biogas production due to its N content, easy degradability, and low cost availability. 5 These 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 influence the environmental performance of a complex agriculturebased system. 32 Adopting a lifecycle approach, when evaluating bioenergy and agricultural systems, has become common practice. 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 environmental 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 fluid milk and other dairy products with little detail on specific manure management systems 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 fraction. 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 analyzed different AD scenarios for feedstock production and energy generation The 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 efficient 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 justified the cost of transporting waste streams from various sources to centralized digesters and to use food crops as feedstocks. There is a need to conduct comprehensive assessments on manure management considering environmental sustainability Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

4 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann indicators and processing technologies according to local conditions and practices in the USA. This study aims to fill this gap in the literature by bringing together individual pieces of the manure handling systems and analyzing different environmental indicators of alternative manure processing technologies with a lifecycle approach. Methods Scope This work applies LCA and process-based concepts to evaluate four environmental sustainability indicators: (i) global warming potential (GWP), (ii) ammonia (NH 3 ) emissions, (iii) depletion of fossil fuels (DFF), and (iv) nutrient form and fate. These indicators are calculated and tracked through each step of a manure management system at a typical modeled dairy farm in WI where manure is collected, stored, and land applied. This sequence of steps, or unit-processes, constitutes the base-case (BC) pathway of this study, as it is the most simplified dairy manure disposal method. Three specifically defined manure processing pathways are compared against the BC pathway to identify potential environmental tradeoffs. These pathways are defined 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 lifecycle concepts are used to compare the marginal changes between the BC and the remaining pathways. The study is conducted at the farm level for a time unit of one day. The four pathways are modeled with the software GaBi 5 43 for a farm with 1000 lactating cows, 605 growing heifers (<21 months), and 286 mature heifers and dry cows. The size of the farm has been defined 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 greatest 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 Figure 1. System boundaries of the four pathways analyzed in this study Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 773

5 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways application. Animal husbandry and cultivation processes for cow feeding are not included in this analysis. Embedded and cumulative energy, GHGs, and NH 3 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 capital goods (i.e. machinery and buildings) is excluded as its contribution to the measured outputs is marginal. 45 The functional unit (FU) is defined as one metric ton (ton) of excreted manure as it reflects 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 The four sustainability indicators assessed in this study are highly relevant to manure systems. NH 3 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 CO 2 -eq). Characterization factors used for GHGs other than CO 2 are 298 kg CO 2 -eq for N 2 O and 25 kg CO 2 -eq for CH 4 based on the CML 2001 method. N 2 O emissions resulting from NH 3 emissions and leaching are accounted for using IPCC s definition that 0.01 of volatilized N as NH 3 is converted to N 2 O-N emissions and of leached N is converted to N 2 O-N emissions. 47 Emissions from manure are considered biotic emissions (CO 2(b), N 2 O (b), CH 4(b) ) and are evaluated separately from fossil fuel emissions (CO 2(f), N 2 O (f), CH 4(f) ) in this study to identify major emission sources and to account for the CO 2(b) recycling process that takes place during plant growing as it is assumed that the carbon contained in excreted manure has been previously captured as CO 2 by the crops that constitute the dairy diet. Depletion of fossil fuels (DFF) is defined as the energy consumed at the site plus the energy consumed in the production and delivery of that energy product. The fossil energy ratio (FER) and the energy return on investment (EROI) ratio can be used to determine the efficiency 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)). FER AD = Usable energy out (1) Fossil energy in EROI AD = Usable energy out (2) Total energy in The AD subscript is added to both ratios given that the concept of dairy manure as a resource to produce energy is conceptually different from traditional primary fuels (e.g. coal, natural gas, or oil), which are specifically extracted as energy resources. 48 These 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 processbased approach to capture flows in and out each unit-process of the manure lifecycle. Material, energy, and emission flows are related to system parameters and local conditions of WI (e.g. volatile solids and temperature). In addition, the electricity 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). The necessary 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. The survey identifies 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 field study has been conducted to characterize data (e.g. total solids, volatile solids, N, P, and K) about manure flows before and after AD and SLS. 49 Third, 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 affects excreted manure characteristics and composition. 37 Finally, representative literature review and verifiable 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 five weather stations on each of the regions of WI for the years 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 aggregated manure of the herd. 49 It is assumed that 50% of the VS are degradable. 52, Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

6 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann Table 1. Excreted manure characteristics per cow type. Manure excretion characteristics Lactating cows Growing heifers Marginal changes in sand beddiing and nutrients Dry cows and mature heifers kg animal 1 day 1 Manure a TS a C b K b N b P b a Calculated according to Eqns (S1) (S5). b Calculated according to Reinemann et al. 37 This 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 material 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 efficient 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 underground sand mines in WI and transported 336 km to the farm with a 35 ton capacity diesel truck. This is the average distance between the mine and the center point of each of the five 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 pathway, 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 digestate reaching the land is 80% for P and K, 100% for ammoniacal nitrogen, and 43% for organic nitrogen (25% the first year, 12% the second year, and 6% the third year). 54 Description of pathways, unit-processes, and LCI development The four basic manure unit-processes that are considered in all pathways are: (i) manure collection, (ii) sand recovery, (iii) manure storage, and (iv) manure land application (Fig. 1). Additional unit processes are biogas production and energy conversion for the AD and AD+SLS pathways, 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 different barn designs (Table S5). After collection, manure mass and composition are recalculated to account for C and N volatile losses. The production of sand is not included in the analysis, but the sand that is collected along with manure is quantified as it affects 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 flows. CO 2 and CH 4 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 m 2 for growing heifers and 2.15 m 2 for milking cows, mature heifers and dry cows (Table S5). An emission factor (EF) of 5.4E 5 g N 2 O kg 1 manure is used to estimate barn emissions considering average yearly seasonal temperatures in WI and cow body weight. 16 NH 3 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 operation. 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 The 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 calculated flow rate of 12,700 kg hour 1 for the modeled farm, this system recovers 87% of the incoming sand containing up to 2% organic solids. Recycled water is added to dilute the sand-laden manure and to facilitate sand settling according to a 1:1 ratio of the incoming sand-laden 2014 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 775

7 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways manure. 56 Separated sand, emissions from electricity consumption, and manure effluent 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 retention 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.). The addition of the cyclone improves the sand recovery to 95%. Of the remaining 5% which is not recovered, 2% is discarded and 3% passes through the digestion system exiting with the digestate. Mechanical separation The 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 management (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 The AD and AD+SLS pathways consider mesophilic digestion using a plug-flow digester and converting biogas to electricity using a generator. It is assumed that a portion of the produced electricity is used by the manure management operations. The remaining electricity is available to be used on-farm or injected to the grid. The inputs to the anaerobic digester include manure following sand separation, electricity, and heat. Manure is re-characterized after 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 m 3 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 m 3 day 1 and flow rate gas processing unit is required for removal of hydrogen sulfide (H 2 S), which damages equipment. Heat is required to maintain the digester temperature at 38 C for bacterial 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% efficiency. 59 It is assumed that biogas is composed of 65% CH 4 and 35% CO 2, and less than 1% of other gases including H 2 S. Methane production is calculated using bacterial growth kinetics (Eqn (S11)). In this study, biogas is used to produce electricity in a reciprocating internal combustion engine with 35% electricity efficiency and 50% thermal efficiency after the biogas cleaning process. 60 Energy efficiency is calculated based on a MJ m 3 Lower Heating Value (LHV) of methane. 61 A 90% capacity factor is assumed for the digester and engine to allow for shutdowns and maintenance. Digestate has three main differences 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 CH 4 combustion during electricity generation. A CH 4 leakage factor of 2.8% is included for digester constructed from steel, lined concreted, or fiberglass. 62 Stoichiometry was used to calculate CO 2(b) emissions from CH 4 combustion (Eqn (S12)) where 2.75 kg of CO 2 are produced from the complete combustion of 1 kg of CH 4. Storage Manure effluent from the processing stage(s) is stored in a rectangular clay-lined open basin. The storage is sized to hold the effluent for a period of six months based on Eqn (S13). A storage surface area of 5113 m 2 is calculated to hold a daily manure volume of 213 m 3. For the BC, a natural 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. CH 4(b) emissions are calculated daily from the manure entering the storage after collection plus the accumulated manure being stored up to that point (Eqn (S14)). Few studies have quantified CO 2 emissions from storage and no process-based equations have been developed. In this study, an EF of kg CO 2 m 3 for stored manure with crust formation is used. 13,63 N 2 O emissions from manure storage have been cited to be directly related to ammoniacal N content. 8 This relation cannot be captured with IPCC EFs as they do not differentiate among N forms; 64 therefore, an EF of g N 2 O kg 1 of NH 3 -N is used. 11 Indirect N 2 O emissions result from both NH 3 and leaching losses. NH 3 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. The liquid manure stream from the SLS process is stored for six months in an open basin as in the BC pathway. The liquid volume is reduced to 185 m 3 and the overall surface area is also reduced to 4437 m 2 at 5% TS when compared to the BC pathway. The reduced TS content eliminates the Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

8 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann 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 efficiency 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 efficiencies: 14% (N), 28% (P), and 10% (K) Storage Manure liquids: clay-lined open basin, no crust formation Manure solids: concrete floor 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 efficiency Marginal changes for fresh sand are considered Biogas production and energy conversion Plug-flow digester 28 day retention time No heat recovery H 2 S removal 2.4% biogas leakages 35% and 50% electrical and thermal efficiency 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. CH 4(b) emissions are increased by 40% when no organic crust is formed. An EF of kg CO 2 m 3 stored manure without crust formation is used. 63 There are no N 2 O emissions from storage in the SLS, AD, and AD+SLS pathways as there is no crust formation. 7,8, Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 777

9 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways The solid manure stream following mechanical separation has 31% TS, is produced at a daily rate of 66 m 3 for the whole herd, and stored on a closed area with concrete flooring. CH 4(b) emissions from manure solids are calculated with Eqn (S15). An EF of 0.2 kg CO 2 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 N 2 O (b) emissions; 8 therefore, an EF of g N 2 O kg 1 of NH 3 -N is used. 11 In terms of NH 3 emissions, a total 20% of ammoniacal N volatilizes as NH 3 -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 forage crops grown in WI developed by the University of WI s Center for Dairy Profitability (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 CH 4(b) emissions are calculated according to Eqn (S16) and occur primarily during the first days after application. This study adopts a conservative approach as it does not consider carbon sequestration after manure is land-applied; thus, it is assumed that all carbon (after CH 4 emissions) is emitted as CO 2 after application. N 2 O (b) -N is emitted at a rate of 1.76% of applied NH 3 -N after ammonia emissions. 8 NH 3 emissions after 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 following the separation process is altered due to the formation of a solid and liquid stream resulting from a screw press separator. Both liquid and solid streams are land applied onfarm. In this analysis, manure liquids are applied to nearby cropland and manure solids are transported to farther cropland locations as they have a higher nutrient density, this management strategy reduces overall diesel consumption. Transportation distance can vary significantly in actual practice depending upon field 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 N 2 O from manure solids, which behave as untreated solid manure and thus, result in higher N 2 O emissions during storage. Nearly 1.97% of ammoniacal N after NH 3 emissions from manure solids is emitted as N 2 O-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 This paper considers two approaches to analyzing the environmental impacts of the four pathways presented in Fig. 1. The first approach considers whole-system environmental impacts to identify and compare environmental trade-offs among pathways. The second approach considers multifunctional systems, where the environmental impacts are partitioned among the different products of each pathway (Table 5). The 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 pathway; 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 This paper adopts the allocation approach and compares it with the approach explained in Aguirre-Villegas et al., by first subdividing the system and then applying allocation wherever subdivision is no longer possible. 71 This approach is adopted as it has shown to reduce the variability in results regardless of the allocation ratio applied. Two different ratios of economic 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. AR x (3) AR p1 + AR p2 + +AR pn 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 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

10 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann 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 ton 1 of excreted manure Collection a BC, SLS Excreted manure b 1 metric ton Diesel fuel c 0.17 kg Fresh sand d 44.2 kg Recycled sand e 293 kg AD, Fresh sand 16.5 kg AD+SLS Recycled sand 321 kg Sand BC, SLS Sand-laden manure 1,334 kg recovery Electricity 4.91 MJ Recycled water 1,284 kg AD, Sand-laden manure 1,334 kg AD+SLS 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 Electricity agitation liquids kg 1.50 MJ Land BC Stored manure 2,256 kg application g Diesel f 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 Synthetic N addition Synthetic P addition Synthetic K addition Diesel addition 1.12 kg 0.08 kg 6.0E 5 kg 0.03 kg 1.48E 5 kg Table 3. (Continued) Land AD+SLS Stored digested application g liquids Mechanical SLS Biogas production and energy conversion SLS Stored digested solids Diesel liquids Diesel solids Synthetic N addition Synthetic P addition Synthetic K addition Diesel addition Manure from sand recovery 1,670 kg 116 kg 1.05 kg 0.08 kg 0.14 kg 6.0E 5 kg 0.03 kg 2.43E 4 kg 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 Electricity digester Electricity biogas cleaning Heat digester 1,841 kg 46.4 MJ 2.64 MJ 182 MJ a 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 compared 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 NH 3 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 $ kwh 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 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 779

11 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways Table 4. Calculated emission outputs, expressed in terms of the functional unit, from each unit-process and pathway of this study. Pathways SLS AD+SLS Gas a BC Liquid b Solid AD Liquid b Solid Unit-process g ton 1 manure excreted Plant uptake b CO 2(b) 194, , , ,023 Collection d CO 2(b) 3,084 3,084 3,084 3,084 CO 2(f) CH 4(b) CH 4(f) N 2 O (b) N 2 O (f) 4.0E 3 4.0E 3 6.0E 3 6.0E 3 NH Sand recovery e,f CO 2(b) 1,801 1,801 CO 2(f) 1,122 1,122 CH 4(f) N 2 O (f) 1.6E 2 1.6E 2 Storage f CO 2(b) CO 2(f) CH 4(b) 2,500 1, CH 4(f) N 2 O (b) N 2 O (f) 6.6E 3 5.9E 3 NH ,738 2, Land application CO 2(b) 183, ,283 79,693 98,647 57,353 41,504 CO 2(f) 5,199 4, ,287 3, CH 4(b) CH 4(f) N 2 O (b) N 2 O (f) NH 3 2,057 1, ,579 1, Mechanical SLS CO 2(f) 928 CH 4(f) 0.72 N 2 O (f) 1.32E 2 Biogas and energy f CO 2(b) 88,512 88,512 CH 4(b) a Gas emissions are differentiated from biotic (b) and fossil (f) sources. b Liquid for collection and sand recovery reflect 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 CO 2 by the crops that constitute the dairy diet. d Negative fossil emissions result from a more efficient sand recovery process and the avoided production and transportation of fresh sand. e CO 2(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 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

12 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann 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 Subdivision: storage and land application for manure liquids and solids manure liquids Allocation: collection, sand recovery, and mechanical separation Pathway 3: AD Multifunctional Electricity and digestate Pathway 4: AD+SLS Multifunctional Electricity, manure solids, and manure liquids Subdivision: storage and land application for digestate, and biogas production and energy conversion for electricity Allocation: collection and sand recovery 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) Results and discussion Results of GWP, NH 3 emissions, DFF, and N availability are expressed per ton of excreted manure. The results are presented for the entire system and as allocated to each product for each of the four defined pathways outlined in the methods section. These products are: (i) manure (BC); (ii) electricity and digestate (AD); (iii) manure solids Figure 2. C and N mass balance for each unit-process of the BC pathway Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 781

13 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure 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. The magnitude of C emissions after land application shows the importance of initiatives that quantify and improve carbon sequestration rates of raw manure, digestate, separated manure, and separated manure after digestion. This is especially true in the BC and SLS pathways where more C reaches the land application process compared to the AD pathways. In addition, the balance shows how AD pathways reduce C emissions (in the form of CH 4 ) during storage, but also how combusting biogas to produce electricity introduces C emissions (in the form of CO 2 ) sooner in the manure lifecycle. Since this study assumes no C sequestration, all landapplied 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 CO 2 immediately after combustion. Even though there are implications of this time difference in GWP, its quantification is limited by the current LCA methodology. The N balance shows the different forms of N through the manure lifecycle and how these are partitioned in individual unit processes. Both AD and AD+SLS pathways show an increase in ammoniacal N during storage 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. Figure 3. C and N mass balance for each unit-process of the AD+SLS pathway Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

14 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann Global warming potential (GWP) GWP is presented in gross and net terms. The gross approach shows the contributions to GWP from each unit-process across all pathways, including captured and emitted CO 2(b). This approach is useful to guide management practices and improvements that seek to reduce GHG emissions from specific manure unit-processes. Captured CO 2(b) is calculated as -194 kg CO 2 -eq ton 1 excreted manure for all pathways. Gross GWP is kg CO 2 -eq ton 1 excreted manure for the BC pathway, kg CO 2 -eq ton 1 excreted manure for the SLS pathway, kg CO 2 -eq ton 1 excreted manure for the AD pathway, and kg CO 2 -eq ton 1 excreted manure for the AD+SLS pathway. Figure 4 shows that CO 2(b) emissions are the major contributors to gross GWP in all pathways, but they occur from different unit processes. For example, CO 2(b) from land application is responsible of 62% of GWP in the BC pathway, while CO 2(b) from biogas combustion is responsible for 36% of GWP in the AD and AD+SLS pathways. In the SLS pathway, CO 2(b) come mostly from manure liquids application (47% of gross GWP) and solids 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 liquid fraction. This is because 42% of the C is within the solid fraction. With 22%, CH 4(b) is the second contributor to GWP in the BC. This trend is maintained in the SLS pathway as CH 4(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 CH 4(b) emissions from storage are significantly reduced in the AD and AD+SLS pathways since methane is captured and combusted to produce electricity, converting part of the CH 4(b) emissions that would have been emitted during storage to CO 2(b). GWP from CH 4(b) emissions at the digestion unit process in both AD and AD+SLS pathways are a result of leakages from the AD system. N 2 O (b) emissions from application is the third source of GWP in the BC representing 8% of this impact category. These emissions are consistent among pathways, but represent a higher percentage during land application of AD and AD+SLS (10% of gross GWP) due to the reduced total GWP of these pathways. In the SLS pathway, N 2 O (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 after the separation process. Figure 4. Contribution GWP from each unit-process and pathway according to the type of GHG Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 783

15 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways Contribution to GWP from each pathway is expressed with a net GWP approach, which subtracts the CO 2 uptake by the crops produced for cow feed. This net approach is useful to compare the changes in environmental impacts across pathways. Net GWP is kg CO 2 -eq ton 1 excreted manure for the BC pathway, 82.3 kg CO 2 -eq ton 1 excreted manure for the SLS pathway, 52.4 kg CO 2 -eq ton 1 excreted manure for the AD pathway, and 53.7 kg CO 2 -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 respectively (the remaining portion is from fossil emissions). Figure 5 shows that overall the three processing pathways (SLS, AD, and AD+SLS) have reduced net GWP when compared to the BC pathway. The 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%. These reductions are attributable primarily to lower N 2 O (b), CH 4(b), and CO 2(f) emissions. During storage, the BC pathway forms a natural crust due to the high TS content. The remaining pathways do not form a crust as the TS content is reduced, creating anaerobic conditions and avoiding any direct N 2 O (b) emissions through nitrification. The net N 2 O (b) reduction is 4.6, 9.5, and 1.8 kg CO 2 -eq ton 1 excreted manure for the AD, SLS, and AD+SLS pathways. Reductions are lower in AD and AD+SLS pathways because there are still indirect N 2 O emissions from NH 3 volatilization, which is higher in pathways with a digester. Most GWP reductions come from CH 4(b) during storage due to the reduction of VS after digestion. These reductions go from 12.9 kg CO 2 -eq ton 1 excreted manure in the SLS, 51.2 kg CO 2 -eq ton 1 excreted manure in the AD, to 54.4 kg CO 2 -eq ton 1 excreted manure in the AD+SLS pathways. CH 4(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. these processing pathways are limited by two factors: the elimination of a natural crust during manure storage (which creates anaerobic conditions and promotes CH 4(b) emissions), and the CH 4 leakages during the digestion process. Finally, CO 2(f) emissions are reduced in both AD and AD+SLS pathways because all manure handling electricity needs are provided by the system, but are increased in the SLS pathway due to the installation of the mechanical separator. Figure 5 also shows an increase in GWP from CO 2(b) emissions, which can be explained by the fact that some of the carbon emitted in the form of CH 4(b) in the BC pathway is being emitted as CO 2(b) in the SLS, AD, and AD+SLS pathways. Ammonia (NH 3 ) emissions NH 3 emissions increased for all pathways in comparison to the BC. NH 3 emissions per ton of excreted manure are 2.6 kg NH 3, 2.7 kg NH 3, 3.7 kg NH 3, and 3.8 kg NH 3 for the BC, SLS, AD, and AD+SLS pathways, respectively. In all pathways, NH 3 emissions occur during collection, storage, and land application (Fig. 6). During manure collection, NH 3 emissions are equivalent among pathways 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 NH 3 emissions occur during manure land application. Manure does not form a natural crust when it is digested or separated. The lack of a crust formation following digestion or separation and the mineralization of nitrogen through the digestion process further increases NH 3 emissions during storage. As a result, storage accounts for 33%, 47%, and 53% of total NH 3 emissions in the SLS, AD, and AD+SLS pathways respectively. Ammoniacal N is more susceptible to volatilization during land application if it is not rapidly injected or incorporated, practices that are assessed in the sensitivity analysis section. In spite of this, NH 3 emissions after land application are reduced in the SLS, AD, and AD+SLS pathways because TS are also reduced when compared to the BC, facilitating manure infiltration into the soil more readily. In pathways with a solid-liquid separator, the liquid stream is responsible for 77% of total NH 3 emissions as ammoniacal N remains in the liquid portion following separation. Even though NH 3 emissions are increased during storage of liquid manure in the SLS pathway, total NH 3 emissions are similar to those of the BC pathway. This is mostly attributed to low NH 3 emissions from manure solids storage and reduced NH 3 emissions from land application due to reduced TS in the liquid stream (facilitating the infiltration of ammoniacal N into the soil) Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

16 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann Figure 6. NH 3, direct N 2 O, and indirect N 2 O emissions from each unit-process and pathway. 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 pathway given that electricity to operate the mechanical separator 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 production rate, 31 m 3 of methane ton 1 of excreted manure are generated before accounting for methane losses, conversion efficiencies, and electricity requirements of the system. Fossil energy ratio (FER AD ) is 3.7 for the AD pathway and 3.5 for the AD+SLS pathway, while the energy return on investment (EROI AD ) ratio is 0.98 for the AD pathway and 0.94 for the AD+SLS pathway. This is a major Table 6. DFF according to each unit-process and pathway. Depletion of Fossil Fuel (MJ ton 1 manure excreted) Unit-process BC SLS AD AD+SLS Collection Sand recovery Mechanic SLS 14.3 Liquid storage Liquid application Solid application Total Electricity Diesel Material improvement when compared to the electricity from the Grid that has an FER of 0.29 and an EROI of The relative low EROI AD as compared to other renewable 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 2014 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 785

17 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways not converted to usable energy. Third, the inefficient use of energy during the conversion process as all thermal energy (55%) is lost to the environment as heat. The EROI AD 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. The EROI increases to 5 if biogas is used directly for combustion. Further increases in the EROI AD 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 process that affects 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 NH 3, is lost as N 2 O (b), and changes forms due to mineralization during digestion and storage. Mineralization affects the organic and ammoniacal fractions thereby impacting final 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 difference in total N being landapplied. N availability mostly depends on NH 3 emissions after manure broadcast and the form of N reaching the land (organic N is not as readily available as ammoniacal N). More total N reaches the land in the BC pathway than in any other pathway, but NH 3 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, NH 3 volatilization reduces final Table 7. P, K, and N availability after land application for the solid (sol) and liquid (liq) streams of each pathway. Nutrient availability after land application (kg ton 1 excreted manure) Pathways P liq P sol K liq K sol N liq N sol BC a SLS AD a AD+SLS a P, K, and N for the BC and AD pathways are presented in the liquid column. Figure 7. Organic N, ammoniacal, and N availability after land application for each pathway. N availability in AD pathways. These nitrogen losses in the form of NH 3 emissions could be reduced by adopting different manure management practices such as covering the storage and injecting manure instead of surface broadcasting as examined in the sensitivity analysis section. Allocated results Whole-system allocation and subdivision/allocation ratios are applied to net GWP, NH 3 emissions, and DFF. The same whole-system allocation rates are calculated to all sustainability indicators. As shown in Table 8, this approach adds significant variability to the model as it interchanges results in magnitude and direction depending 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. This 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 individual sustainability indicators. As shown in Table 9, the variability is significantly reduced when applying this approach, as the partitioning of environmental impacts among co-products is consistent no matter what allocation strategy is applied. With this approach, the contribution 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. This same trend applies to NH 3 emissions and DFF. As shown in Fig. 4, the majority of net impacts are related to storage Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

18 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann Table 8. Whole-system allocation rates for GWP, NH 3 emissions, and DFF. Manure Solids Manure Liquids Electricity Digestate Pathway Allocation Ratio System allocation rates for all sustainability indicators (%) SLS TS EV AD TS EV AD+SLS TS EV Table 9. Subdivision/allocation rates for GWP, NH 3 emissions, and DFF. System GWP (kg CO 2 -eq Allocation Manure Solids Manure Liquids Electricity Digestate Pathway ton 1 excreted manure) Ratio Subdivision/allocation rates for GWP (%) SLS 78.2 TS EV 9 91 AD 59.3 TS EV AD+SLS 57.3 TS EV System NH 3 (kg ton 1 Allocation Manure Solids Manure Liquids Electricity Digestate Pathway excreted manure) Ratio Subdivision/allocation rates for NH 3 emissions (%) SLS 2.6 TS EV AD 3.5 TS 3 97 EV 8 92 AD+SLS 3.7 TS EV System DFF (MJ ton 1 Allocation Manure Solids Manure Liquids Electricity Digestate Pathway excreted manure) Ratio Subdivision/allocation rates for DFF (%) SLS TS EV AD 55.5 TS 1 99 EV 2 98 AD+SLS 59.7 TS EV and land-application. The subdivision/allocation approach assigns the burdens of these specific unit-processes to the products that are responsible for them (i.e. digestate storage is not related to electricity production), avoiding overweighting of the environmental impacts to co-products. This is one of the first LCA studies to focus on the manure management system rather than the milk production system. Thus, 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 literature, 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 CO 2 -eq kg 1 milk attributed to manure handling, which is equivalent to 71 kg CO 2 -eq ton 1 manure. 37 Thoma et al. reported a US average of 1.23 kg CO 2 -eq kg 1 milk, with manure responsible for 22% of these emissions. 35 This is equivalent 2014 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 787

19 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways to kg CO 2 -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 kg CO 2 -eq ton 1 manure of the BC pathway, assumptions, management practices, and climatic conditions are different. The Dairy Greenhouse Gas Model v 1.2 was used to simulate the conditions 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. The only unit process that could be compared was storage with results of 80 kg CO 2 -eq ton 1 manure vs. 74 kg CO 2 -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 literature; however, most of these studies consider different practices (i.e. co-digestion with other biomass feedstocks) and report results in terms of energy produced. Poeschl et al. reported 23.2 kg CO 2 -eq ton 1 manure for a reference pathway where biogas was produced from cattle manure. 4 The 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 CO 2 -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 After applying the subdivision/allocation approach (and including heat as part of the products to make results comparable) this paper determines that 10 g CO 2 -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, NH 3 emissions, 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 influence the results by more than ±1%. Sensitivity to nutrient availability is presented for N only as it is the most dynamic nutrient. GWP is most sensitive to changes in CO 2(b) emissions after application in all pathways because it is assumed that all carbon in manure is eventually emitted as CO 2 after application, which highlights the importance of future studies that quantify the carbon sequestration of manure application in the long term. CH 4(b) emissions during storage have a bigger influence on GWP in the BC and SLS pathways since CH 4 has been captured and transformed to energy during the digestion process. As a result, N 2 O (b) emissions after land application have a significant influence on GWP in the AD and AD+SLS pathways. GWP sensitivity to NH 3 emissions indicates the importance of indirect N 2 O emissions in this model as well as CH 4 leakages in pathways that involve a digester. Oppositely, some other parameters that influence 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. This 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 influence on DFF, which indicates the amount of primary energy that is embedded in the production of nitrogen fertilizer and diesel fuel, and the significant quantities of sand that are used for cow bedding. DFF is sensitivity to N volatilization because additional synthetic fertilizer is needed to compensate for those N losses. Parameters that increase nutrient availability (i.e. number of cows) reduce DFF because of replacement of synthetic fertilizers. Energy requirements, including electricity for solid-liquid mechanical separation, sand recovery, and diesel for manure collection and application; impact DFF in BC and SLS pathways. These 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. NH 3 emissions are most sensitive to volatilization after 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 influential than from manure solids. As N in excreted manure increases, downstream NH 3 emissions also increase. Inversely, as more organic N is available when reaching land application, NH 3 emissions are reduced. Finally, N availability is sensitive to excreted N (mainly by milking cows), ammoniacal N (since it is more available), final availability of organic N, and N volatilization as it decreases its availability. NH 3 emissions are reduced by 40% if manure is injected instead of being surface Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb

20 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann Figure 8. Change in GWP, NH 3 emissions, DFF, and N availability for a ±10% change of individual parameters of the AD+SLS pathway. land-applied in the pathways with a digester. This reduction increases nitrogen availability by 50% in AD and AD+SLS, and 46% when compared to BC without injection. The overall effects on GWP and DFF of these and other management practices will be analyzed in future work. Conclusions and implications The manure processing pathway has a substantial influence on the estimates of environmental impacts, which highlights the importance of assessing different sustainability indicators to identify trade-offs among pathways. This study uses a process based approach to develop manure specific inventory data and applies LCA techniques to quantify and compare GHGs, NH 3 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 reductions in GWP and DFF, but the largest increase in NH 3 emissions. Reductions in GWP come mostly from avoidance of CH 4(b) and N 2 O (b) emissions during storage and land application, and reduction of fossil CO 2 emissions due the production of electricity from biogas. The AD+SLS and AD pathways present the largest increase in NH 3 emissions with 44% and 40% respectively, while the SLS pathway presented no change. This significant increase is a result of higher ammoniacal N in manure after 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. These losses could be mitigated by adopting different management practices such as manure 2014 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 789

21 HA Aguirre-Villegas, R Larson, DJ Reinemann Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways injection. When compared to grid electricity, the FER AD improved by a factor of 12 and the EROI AD by a factor that ranges from 3 to 7 depending if heat is recovered in AD pathways. These ratios would have been higher by recovering 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 difference that they are divided between the solid and liquid streams during SLS pathways. Even though the mineralization that occurs in the digester increases ammoniacal N (which is more available for crop production) during AD pathways, the overall net N availability after land application is very similar in all pathways. When allocating the sustainability indicators to specific system outputs, it is demonstrated that adopting the traditional whole-system allocation approach introduces substantial variability to the results. This variability can be significantly reduced by applying a combined subdivision and allocation approach. A sensitivity analysis shows the importance of the N balance through manure management since it affects 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 emissions, such as CH 4 and N 2 O during storage and land application, indirect emissions from NH 3 emissions have a big influence 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. This highlights the importance of promoting practices that increase nutrient availability since the production of these materials is so energy intensive. The 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. This 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-offs from a system oriented perspective regardless of the difference in final products. This paper combines data from different sources and integrates multiple simulation models to fill the current knowledge gaps in the areas of dairy manure management, conventional LCA, and environmental sustainability to inform businesses, farmers, and governments and promote science-based responsible decision making. This paper is only a first 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 NH 3 emissions from digestate and separated manures. This study only differentiates N 2 O and NH 3 emissions based on ammoniacal content, but other variables such as C:N ratio and particle size could influence 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 production 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. Third, alternative strategies should be analyzed to improve the environmental sustainability of manure processing pathways. 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24 Modeling and Analysis: Energy, emissions, and nutrient implications of manure pathways HA Aguirre-Villegas, R Larson, DJ Reinemann 69. United States Environmental Protection Agency (USEPA), Emission factors for greenhouse gas inventories [Online]. Available at: [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): (2012). 72. Energy Information Administration (EIA), Electric power monthly statistics. [Online]. Available at: 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: htm?docid=21345#reference [Accessed September 5, 2013]. Horacio Aguirre-Villegas Horacio Aguirre-Villegas is a PhD candidate in Biological Systems Engineering at the University of Wisconsin- Madison. His research lies in the fields of bioenergy, climate change, waste managemen,t and life cycle assessment (LCA). In particular, he focuses on evaluating the sustainability of agricultural and food production systems and their integration 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 Biological Systems Engineering Department at UW-Madison focusing on biological 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 including biogas production from anaerobic digestion. Douglas J. Reinemann Douglas Reinemann is professor and Chairman of the Biological Systems Engineering Department at the University of Wisconsin-Madison. His research and educational interests 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 Society of Chemical Industry and John Wiley & Sons, Ltd Biofuels, Bioprod. Bioref. 8: (2014); DOI: /bbb 793