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What Is the Most Environmentally Beneficial Way to Treat Commercial Food Waste? James W. Levis* and Morton A. Barlaz Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, North Carolina 27695-7908, United States
bS Supporting Information ABSTRACT: Commercial food waste represents a relatively available high-quality feedstock for landfill diversion to biological treatment. A life-cycle assessment was performed for commercial food waste processed through aerobic composting systems of varying complexity, anaerobic digestion, and landfills with and without gas collection and energy recovery, as well as a bioreactor landfill. The functional unit was 1000 kg of food waste plus 550 kg of branches that are used as a bulking agent. For each alternative, global warming potential, NOx and SO2 emissions, and total net energy use were determined. Anaerobic digestion was the most environmentally beneficial treatment option, leading to 395 kg net CO2e per functional unit. This result was driven by avoided electricity generation and soil carbon storage from use of the resulting soil amendment. The composting alternatives led to between 148 and 64 kg net CO2e, whereas the landfill alternatives led to the emission of 240 to 1100 kg CO2e. A traditional landfill with energy recovery was predicted to have lower emissions than any of the composting alternatives when a fertilizer offset was used. There is variation in the results based on uncertainty in the inputs, and the relative rankings of the alternatives are dependent on the soil amendment offset that is used. The use of compost to offset peat has greater emission offsets than the value of compost as a fertilizer.
’ INTRODUCTION Over 30 million metric tons (Mg) of food waste were generated in the United States in 2009, representing 14.1% of municipal solid waste.1 There are many opportunities to collect source-separated food waste from groceries and other commercial establishments.2 When buried in a landfill, food waste decomposes to form methane, a greenhouse gas with a global warming potential (GWP) 25 times greater than CO2 on a 100year time scale.3 Landfill methane is often used as an energy source, but landfills are estimated to be the second largest source of anthropogenic methane in the U.S. due to fugitive emissions and emissions prior to installation of gas collection systems.4 Because food waste decomposes rapidly relative to paper, some of the methane generated from its decomposition will be released prior to gas collection system installation. As an alternative to disposal in landfills, source-separated food waste can be aerobically composted or used to generate methane by anaerobic digestion (AD). There has been limited research on emissions associated with organic waste management. Diggelman and Ham5 used a lifecycle methodology to analyze various food waste management alternatives including in-vessel composting and landfilling with energy recovery, but they did not consider windrow composting which is more common, or landfills without gas collection or energy recovery. Lundie and Peters6 also used a life-cycle methodology to consider landfilling and centralized composting. Their study did not consider energy recovery at a landfill, or offsets from avoided mineral fertilizer production due to land r 2011 American Chemical Society
application of the compost product. Boldrin et al.,7 determined the variability in greenhouse gas (GHG) emissions from open and in-vessel composting alternatives, and compared the use of compost with the use of peat as a growth media.8 The objective of this study was to compare the emissions and energy implications of managing food waste in several composting alternatives, AD, and four landfill scenarios including a landfill without gas collection (LWOC), a landfill in which gas is collected but the gas is flared (LWOER), a landfill with energy recovery from the collected gas (LWER), and a bioreactor landfill (LFB) with energy recovery. The focus of this analysis is on food waste generated at commercial and industrial facilities (e.g., restaurants, food processing plants, etc.) as these facilities provide a large source of concentrated food waste. Four composting alternatives that reflect varying levels of technology were considered including (1) windrows, (2) aerated static piles (ASP), (3) Gore composting system, and (4) in-vessel systems.
’ MODELING APPROACH Life-cycle assessment (LCA) was used as the analytical framework for comparison of food waste management alternatives. The approach used to estimate emissions and energy use for each Received: October 21, 2010 Accepted: July 27, 2011 Revised: May 15, 2011 Published: August 12, 2011 7438
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Environmental Science & Technology alternative is described in this section. For this study, GWP, SO2, NOx, and total net energy use (TEU) associated with each alternative were determined. GWP was chosen since it is a significant environmental concern and due to the relatively high amount of fugitive methane generated during landfilling. SO2 and NOx are leading criteria pollutants that are the primary cause of acidification, and TEU is a useful measure for resource conservation. TEU for the purposes of this study is defined as the total energy embedded in the coal, oil, natural gas, and uranium used in each alternative. This includes direct fuel use in equipment, fuel used to generate electricity, and the fuel used to acquire, refine, process, and transport fuels for direct or indirect use. Avoided energy use associated with the beneficial use of methane and compost is also included in TEU. The functional unit is 1000 kg of food waste plus 550 kg of branches. Although the focus of this study is food waste, shredded branches or wood chips are typically added to food waste as a bulking agent prior to composting. Thus, the management of food waste by composting includes, by necessity, some branches, and branches were added to the functional unit for all alternatives. The analysis does not include waste collection since the focus of the study is on commercial generators where large quantities of food waste are generated in nearly pure form. It is assumed that containers are collected when full and food waste containers are similar in size to mixed waste containers. In practice, the use of composting or AD likely necessitates some extra collection activity relative to a landfill where all wastes can be collected in one vehicle. Thus, the treatment of composting and AD may be slightly biased relative to a landfill and this bias is explored with the results. All of the alternatives use electrical energy and in the case of AD and landfills, energy is also recovered. When electrical energy is recovered, it is assumed to offset coal and natural gas generation at 72.5% coal and 27.5% natural gas, which represents the adjusted proportion of each fuel on the national grid.9 This leads to a CO2e offset of 1.02 kg CO2e per kWh. Methane was assumed to be converted to electricity using a heat rate of 11.6 MJ/kWh, which was developed from vendor literature. Aerobic Composting Model Description. Four composting alternatives of varying technological sophistication were considered including (1) windrows, (2) ASP, (3) Gore cover system, and (4) in-vessel systems. Windrows represent the simplest technology and were assumed to have no odor control in contrast to the other three technologies which utilized biofilters. The ASP system was assumed to use a positive pressure system and to control aeration based on pile temperature. The Gore cover system is similar to an ASP system, but the piles are covered with a breathable expanded polytetrafluoroethylene fabric. The Gore cover aeration system also uses positive pressure, but aeration is controlled based on the oxygen concentration. There is no mechanical odor control except at the tipping floor and over conveyors, but odorous compounds are dissolved in a condensation layer that forms on the inside surface of the cover, and the system has been shown to reduce volatile organic and ammonia emissions by over 90% relative to windrow composting of similar green wastes.10 The Wright in-vessel composting system was chosen as representative of typical in-vessel composting systems. In this alternative, substrates are mixed and aerated within a reactor. Odors are controlled by blowing air from the reactor through a biofilter. The energy required for aeration and odor control was calculated based on data from compost operators and HVAC equipment vendors. There are a number of factors
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Table 1. Properties of Materials Used in Food Waste Composting volumetric
bulk density
moisture
substrate
proportion
(kg/m3)
content (%)a,b
food waste shredded branches
0.25 0.50
540b 150b
70 50
screen rejects
0.25
150b
50
finished compost
N/A
700c
50
a
Defined as mass of water divided by the mass of wet material. b Adopted from Tchobanoglous et al., 1993.20 c Adopted from Boldrin et al., 2010.8
that influence the net emissions from each process including feedstock preparation, emissions from biodegradation, mixing and material handling, postprocessing, nutrient offsets, and administrative functions. The parameters used to model composting energy consumption and emissions are given in Table S2 of the Supporting Information (SI). Mass flows through each step of the composting processes are given in Figure S1. Each of the alternatives begins with the placement of substrates at the tipping floor. Branches must be shredded prior to arriving at the tipping floor. After the substrates are blended, they are put into piles in the windrow, ASP, and Gore alternatives. For the in-vessel alternative, materials are placed in a hopper and fed to the in-vessel reactor. The main differences in the composting alternatives occur in the active composting phase, which takes place either in piles or within the in-vessel reactor. Active phase processing times range from 20 to 70 days depending on the level of technology used (Table S2). In-vessel and ASP systems have the shortest processing times and windrows have the longest. After the active composting phase, materials are screened. The materials that pass through the screen go through a vacuum to remove plastic films and are then placed in curing piles. Screen rejects are either recycled into the initial blend, or buried in a landfill. After curing, the finished compost is ready for use. The model considers three different substrates (food waste, shredded branches, and screen rejects). These substrates are blended in volumetric ratios to obtain a desired carbon to nitrogen (C:N) ratio, moisture content, and free air space. The C:N ratio for the assumed mixture is 31.8. Food waste is the main substrate and nitrogen source, with shredded branches and screen rejects being used as bulking agents and carbon sources. The default mix in the model was 25% food waste, 50% shredded branches, and 25% screen rejects by volume, which is equivalent to 550 kg of branches and 1000 kg of food waste. Feedstock weights were determined based on the properties presented in Table 1. The mass of food waste to be managed was identified first, after which requirements for the other substrates were calculated. Anaerobic Digestion Model Description. AD results in methane generation and the methane was assumed to be converted to electrical energy. Electricity not used to power the AD facility is sold to the electric grid. A continuous single-stage, highsolids, mesophilic digester was assumed as this configuration is typical for organic waste management.11 Figure S2 shows the AD process flow diagram and mass flows. On-site electricity usage of 50 kWh Mg 1 day was derived from data on the Brecht II plant in Belgium.12 After digestion, shredded branches are added to the dewatered digestate and the mixture is cured aerobically to create 7439
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Environmental Science & Technology a soil amendment. The water from dewatering is recycled into the reactor and the excess is treated in a wastewater treatment plant. Emissions from transporting and treating the wastewater are included in the model. The parameters used to model AD energy use and emissions are given in Table S3. Landfill Model Description. An estimate of the energy and environmental emissions attributable to the disposal of materials in a landfill requires consideration of landfill operations, final cover placement, gas and leachate management, and long-term maintenance and monitoring. An existing landfill life-cycle model formulation was utilized13 and is described in the SI. Four landfill scenarios were considered (LWOC, LWER, LWOER, LFB). The parameters used to model landfill gas generation and collection are presented in Table 2. Methane oxidation was assumed to reduce emissions by 10% of the uncollected methane. Avoided Peat and Fertilizer Production Offsets. Both composting and AD lead to the production of a soil amendment which provides numerous benefits including weed suppression, moisture retention, soil carbon addition, and nutrient addition.14 Many of the benefits of compost use are difficult to quantify, but their highest value uses are avoiding the use of peat and mineral fertilizers. These offsets were not applied simultaneously, as it was assumed that the compost product would not typically be simultaneously used for its nutrient content and as a peat substitute. The fertilizer offset model assumes that there is a market for all of the available nitrogen in finished compost. This assumption will provide positively biased emissions offsets because in reality not all compost is used to offset nitrogen. This offset analysis was intended to provide an upper limit on the benefits of compost.
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Table 2. Landfill Gas Modeling Parametersa material
decay rate (yr 1)b
methane yield (m3/dry Mg)c
food waste
0.144 (0.432)d
300.7
branches
0.015 (0.045)d
62.6
a
Landfill gas collection efficiencies were determined as described in Levis and Barlaz, 2011.13 The gas collection schedule assumes no gas collection for the first 2 y of cell operation (6 mo for bioreactors), 50% collection efficiency for years 3 5 (0.5 3 y for a bioreactor), 75% for years 6 15 and 95% thereafter as explained in the SI. b From De la Cruz and Barlaz, 2010.21 c From Eleazar et al., 1997.22 d Decay rates in parentheses are for the bioreactor landfill alternative.
Table 3. Agricultural Nutrient Demands and Compost Requirements average
nitrogen phosphorus potassium
nutrient
nutrient
content
demand
(kg nutrient/
mineral
compost
(kg/ha/
dry Mg
fertilizer
required
year)a
compost)
93 76.5
1.85 0.56
0.4 1.0
123.5
1.32
1.0
ratio to
equivalent (Mg/ha/yr) nitrogenb 126 137 93.6
1.0 1.0 0.74
a
Average nutrient demand for corn and soybeans.15 b Demand for phosphorus and potassium for each kg of nitrogen in the compost. All of the applied phosphorus and 74% of the potassium will count toward a fertilizer offset. It is assumed that the rest of the applied potassium is unnecessary, and therefore no avoided emissions are counted.
Figure 1. GWP, SO2, NOx, and total energy use for treatment alternatives. All data are based on the functional unit of 1000 kg of food waste plus 550 kg of branches and the inclusion of fertilizer offsets for composting and AD. Composting results with peat offsets are given in Tables S4 S8. 7440
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Figure 2. GWP, SO2, NOx, and total energy use by process from composting alternatives. All data are based on the functional unit of 1000 kg of food waste plus 550 kg of branches.
As is typical, nitrogen was assumed to be the controlling nutrient for fertilizers and the demands of phosphorus and potassium were determined based on the total amount of nitrogen that can be applied. Any phosphorus or potassium applied above the demand does not receive offset credit. Nitrogen in compost is not as available to plants as nitrogen in mineral fertilizers, so a mineral fertilizer equivalent of 0.40 was applied.7 This means that 2.5 times as much nitrogen in compost is required compared to mineral nitrogen fertilizer. Because soybeans and corn are the leading crops in the U.S., and since soybeans must be rotated, the model assumes an annual rotation between soybeans and corn. The model thus uses the average nitrogen, phosphorus, and potassium demand of soybeans and corn developed from USDA.15 Table 3 illustrates how the ratio of phosphorus and potassium to nitrogen was determined. The benefits associated with increased moisture retention, soil carbon, and weed suppression were not quantified. Soil amendment with compost leads to increased soil carbon storage by two mechanisms. The first is from the carbon content of the compost as some carbon will remain after 100 years, and is thus considered stored. A carbon storage factor of 10 kg C per Mg applied compost was adopted from Boldrin et al.7 Compost addition to soil may also lead to incremental humus formation and resulting carbon storage. An estimate of 170 kg C stored per
Mg applied compost due to incremental humus formation was developed from U.S. EPA.16 Peat production requires preparing the land, excavating the peat, transporting the peat, and peat decomposition.17 The benefits of avoided peat production were adopted from Boldrin et al.8 and are presented with the results.
’ RESULTS AND DISCUSSION Model Results. Figure 1 presents the emissions and energy use from each of the food waste treatment alternatives. The contributions of individual processes to the composting and AD results are given in Tables S4 S8. Figure 2 shows the emission breakdown by process for each of the composting alternatives. The AD alternative leads to the largest reductions in all environmental emissions and energy use. For SO2, NOx, and TEU, this is mainly due to the offset emissions from avoided electricity generation, but the negative GHG emissions associated with AD are almost equally from soil carbon storage and the electricity offsets (Table S6). The LWER and LFB alternatives receive electricity and carbon storage offsets, but their overall methane collection efficiency, defined as methane collected divided by methane produced over 100 yr, is 66% and 56%, respectively. Even with offsets, the bioreactor actually leads to greater GHG 7441
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Table 4. Minimum and Maximum Values Used in Parametric Sensitivity Analysis current
minimum
maximum
composting food waste moisture content (% wet weight basis)a
70
50
80
incremental humus formation (kg-C/dry Mg compost)b
170
130
210
mineral fertilizer equivalent for nitrogen (%)c
40
20
60
percent initial carbon emittedc
58
40
83
1.85
0.9
2.8
percent nitrogen in final compost (kg/Mg dry compost)c AD food waste moisture content (% wet weight basis)a
70
50
80
gas turbine heat rate (MJ/kWh)d ultimate methane yield of food waste (L/dry-kg)e
12.1 300
10.3 218
13.2 400
incremental humus formation (kg-C/dry Mg compost)b
170
130
210
percent coal included in offset calculations (%)
72.5
0
100
landfill food waste moisture content (% wet weight basis)a
70
50
80
methane yield of food waste (L/dry-kg)e
300
218
400
carbon storage factor of branches (kg C/dry-Mg)f decay rate of food waste (yr 1)e collection efficiency from years 11 100 (%) percent coal included in offset calculations (%)
380
361
456
0.144
0.096
0.229
(0.432)g 95
(0.289)g 85
(0.688)g 95
72.5
0
100
a
Tchobanoglous et al., 1993.20 b From U.S. EPA, 2010.16 c From Boldrin et al., 2009.7 d Variation in Caterpillar gas turbines. e From De la Cruz and Barlaz, 2010.21 f Experimental carbon storage factors23 provide a lower bound, so values were varied by 5% and +20%. g Decay rates in parentheses are for the bioreactor landfill alternative.
emissions than the LWOER alternative (Figure 1). This is due to the rapid decay rate assumed for food waste in a bioreactor landfill, and the 25% gas collection efficiency assumed for the first 2 years. Of course, the LWOC alternative leads to the highest GWP due to the uncollected gas from food waste decomposition. However, NOx is highest for the LWOER alternative due to the emissions attributable to the flaring of landfill gas. The negative GWP results for the composting alternatives are mostly driven by soil carbon storage (Figures 1 and 2, Tables S4 S8). The differences in the emissions and energy use associated with composting alternatives are mostly driven by the high electricity use associated with odor control in the ASP alternative (Table S5) and the reactor in the in-vessel alternative (Table S7). The significance of GWP from biological degradation during composting is based on published CH4 and N2O emission factors (Tables S2 and S3). All of the composting alternatives lead to lower GHG emissions than all of the landfill alternatives except the LWER. The GWP per functional unit from composting ranges from 148 kg CO2e to 64 kg CO2e ( 280 kg CO2e to 190 kg CO2e with peat offset) compared to 240 kg CO2e to 1100 kg CO2e for the landfilling alternatives. The LWER and LFB alternatives both lead to reduced SO2, NOx, and TEU relative to all of the composting alternatives due to electricity consumption during composting. Although odorous and volatile organic emissions were not quantified, their emissions will be highest in the windrow composting and landfill alternatives. Neglecting waste collection will not significantly affect the results, since collection is estimated to emit 3.6 to 8.1 kg CO2e per Mg18 and the slight differences among collection alternatives would have even less of an effect on the results. The trends described here are based on fertilizer offsets. The use of peat offset leads to 126 kg of additional CO2e reduction compared to the
fertilizer offset, and the SO2, NOx, and total energy use from the peat offsets are also greater than the respective fertilizer offsets (Tables S4 S8). Sensitivity Analysis. The sensitivity of the results to key parameters was evaluated using a parametric sensitivity analysis. Parameters were selected based on preliminary work and judgment. Uncertain input parameters and their ranges are given in Table 4 and the results for GWP in the composting, and AD and landfill alternatives are presented in Figures 3 and 4, respectively. Analogous results for SO2, NOx, and TEU are presented in Figures S3 S8. For the composting and AD alternatives, increasing moisture content increases emissions because materials are processed on a wet weight basis, but the offsets associated with carbon storage, methane generation, and energy recovery decrease as the moisture content increases and dry mass decreases. One exception is that in the ASP alternative, increasing moisture content decreases the SO2, NOx, and TEU because the reduced amount of dry mass reduces the necessary odor control and forced aeration electricity use. Increasing moisture content results in decreased GHG emissions for the landfill alternatives as methane generation decreases due to the lower dry mass. At 80% moisture, the LFER has a lower GWP than AD when using a fertilizer offset. SO2, NOx, and TEU increase with moisture content for the LFER and LFB alternatives because less gas is being produced, which decreases the electricity offset. For the composting alternatives, the nonlinear behavior of the GWP at the upper end of the range of moisture contents explored may be due to the fact that the amount of screen rejects had to be reduced from 25% to 20% at a moisture content of 80% to maintain a realistic mass balance. The percent carbon emitted also had a relatively large effect on the results. An increase in this factor leads to increased methane emissions and decreased 7442
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Figure 3. Sensitivity results for GWP from composting alternatives. The x-axis shows percent distance between the minimum and maximum values for each parameter (Table 4). MC: moisture content; HFF: humus formation factor; %C Emit.: fraction of the initial C that is released as CO2; %N in FC: nitrogen content of the final compost product. The MFE for nitrogen was also tested. The line is directly under the %N in FC line because both control the same offset mechanism and were varied by the same relative amount.
Figure 4. Sensitivity results for GWP from AD and landfilling alternatives. The x-axis shows percent distance between the minimum and maximum values for each parameter (Table 4). MC: moisture content; MYFW: methane yield of food waste; HR: heat rate of gas turbine; HFF: humus formation factor; DRFW: decay rate of food waste; CSFB: carbon storage factor of branches; %coal: % coal included in electricity offsets. ColEf: Methane collection efficiency after final cover is in place. 7443
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Environmental Science & Technology carbon storage. Except where previously noted, SO2, NOx, and TEU follow trends similar to the parameters shown. Finally, for the composting alternatives, incremental humus formation had a relatively strong influence on the results (Figure 3). For the AD and landfill alternatives, the methane yield of food waste is the second most significant factor after food waste moisture content (Figure 4), followed by the food waste decay rate. As described above, the dry mass of food waste directly affects methane emissions and electrical energy potential. Implications. The results presented here are applicable to food waste and cannot be generalized to other components of MSW that decompose more slowly. AD is the most environmentally beneficial alternative for commercial food waste in every category considered. A fuller accounting of the environmental impacts of each alternative should consider the emission of odorous compounds, changes in eutrophication due to soil amendment use, and the total land use, and would provide additional insights into the environmental impacts of each alternative. AD and in-vessel composting are the most costly alternatives.19 More cost-effective designs for anaerobic digestion systems would increase their adoption which would lead to emissions reductions relative to landfills and composting systems. The results also indicate that it may be beneficial to develop hybrid landfill AD systems with accelerated degradation and aggressive gas collection to provide an optimal trade-off between environmental and economic objectives. Policies that penalize greenhouse gas emissions and encourage the recovery of biomass energy would also improve the economics of AD and landfill gas to energy projects.
’ ASSOCIATED CONTENT
bS
Supporting Information. Description of the landfill process model, detailed input tables, process level emissions tables, and sensitivity results for SO2, NOx, and TEU. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (919) 515-0887; fax: (919) 515-7908; e-mail: jwlevis@ ncsu.edu.
’ ACKNOWLEDGMENT This research was supported by the Delaware Solid Waste Authority. J.W.L. was supported in part by a Fiessinger Fellowship from the Environmental Research and Education Foundation. ’ REFERENCES (1) U.S. Environmental Protection Agency. Municipal Solid Waste in the United States: 2009 Facts and Figures; EPA530-R-10-012; Office of Solid Waste: Washington, DC, 2010. (2) DSM. Delaware Solid Waste Authority Statewide Waste Characterization Study, 2006 2007; Prepared by DSM Environmental Services for Delaware Solid Waste Authority: Dover, DE, 2007. (3) Intergovernmental Panel on Climate Change. Climate Change 2007: The Physical Science Basis; Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change; Solomon, S., Qin, D., Manning, M., Chen, Z. M. Marquis, M., Averyt, K. B., Tignor, M., Miller, H. L., Eds.; Cambridge University Press: New York, 2007.
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