Life cycle assessment of capital goods in waste management systems
Brogaard*, Line K. & Christensen, Thomas H.
Department of Environmental Engineering Building 115, Technical University of Denmark DK-2800 Kongens Lyngby, Denmark
* Corresponding author
Department of Environmental Engineering Building 115 Technical University of Denmark 2800 Kongens Lyngby, Denmark Phone: +45 4525 1488 Fax: +45 4593 2850 E-mail:
[email protected]
NOTE: this is the author’s version of a work that was accepted for publication in Waste Management. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Minor changes may have been made to this manuscript since it was accepted for publication. A definitive version is published in Waste Management. DOI: 10.1016/j.wasman.2016.07.037 1
Abstract The environmental importance of capital goods (trucks, buildings, equipment, etc.) was quantified by LCA modelling 1 tonne of waste treated in five different waste management scenarios. The scenarios involved a 240 L collection bin, a 16 m3 collection truck, a composting plant, an anaerobic digestion plant, an incinerator and a landfill site. The contribution of capital goods to the overall environmental aspects of managing the waste was significant but varied greatly depending on the technology and the impact category: Global Warming: 1-17%, Stratospheric Ozone Depletion: 290%, Ionising Radiation, Human Health: 2-91%, Photochemical Ozone Formation: 2-56%, Freshwater Eutrophication: 0.05-99%, Marine Eutrophication: 0.03-8%, Terrestrial Acidification: 2-13%, Terrestrial Eutrophication: 1-8%, Particulate Matter: 11-26%, Human Toxicity, Cancer Effect: 1092%, Human Toxicity, non-Cancer Effect: 1-71%, Freshwater Ecotoxicity: 3-58%. Depletion of Abiotic Resources – Fossil: 1-31% and Depletion of Abiotic Resources – Elements (Reserve base): 74-99%. The single most important contribution by capital goods was made by the high use of steel. Environmental impacts from capital goods are more significant for treatment facilities than for the collection and transportation of waste and for the landfilling of waste. It is concluded that the environmental impacts of capital goods should always be included in the LCA modelling of waste management, unless the only impact category considered is Global Warming.
Keywords: Capital goods, waste, incineration, landfilling, collection, transport, life cycle assessment.
1
Introduction
Environmental impacts from waste management systems have been evaluated many times by life cycle assessment (LCA) during the last few decades. However, in most studies the environmental costs of capital goods (buildings, machinery, etc.) have not been included. Frischknecht et al. (2007) used data from the Ecoinvent database (Ecoinvent, 2015), including capital goods for landfilling and incineration, and found that capital goods contributed significantly to the impact categories related to resource use (Mineral Resources and Land Use).
A few studies (Doka, 2009; Ecobalance, 1999; Menard et al., 2004; Schleiss, 1999; MartínezBlanco et al., 2010; Rives et al., 2010) have presented life cycle inventories for capital goods for single waste management technologies. In most cases the data were not well-documented, though, and so it is difficult to identify what is actually included. In recent studies we have provided details 2
about capital goods for a range of waste management technologies: waste collection and transport (Brogaard and Christensen, 2012), landfills (Brogaard et al., 2013a), incineration (Brogaard et al., 2013b) and composting and anaerobic digestion (Brogaard et al., 2015). The mentioned studies demonstrated that some materials are used in large amounts per tonne of waste treated, but it is not always the case that the production of these materials has a major environmental impact. Materials used in smaller amounts can have the greatest impacts per tonne of material from production. In the present study the waste management of capital goods will also be included, i.e. impacts caused by the treatment of the waste capital goods as well as savings made by substituting virgin materials.
The objective of this study is to assess the importance of capital goods in waste management LCA studies, by using the recently published data described above. LCA modelling includes the materials and energy used in the construction of capital goods, the actual waste treatment, including recovered materials and energy, and some capital goods being recycled at the end of their life. The goal is to provide a quantitative assessment of the need and importance of including capital goods in future LCA studies of waste management systems.
2
Approach and method
The LCA modelling in this paper has two flow systems: a capital goods flow system and a waste flow system. This is illustrated in Figure 1. The assessment of the capital goods system includes all life cycle phases: the extraction and production of materials and energy, the construction of capital goods in terms of plants and machinery, the maintenance of capital goods during use and their disposal, where they are recycled or subjected to other treatments. Transportation is included in the processes and between the life cycle phases. Capital goods data provided by Brogaard and Christensen (2012), Brogaard et al. (2013a), Brogaard et al. (2013b) and Brogaard et al. (2015) were expressed per tonne of waste treated, and the operational capacity, maintenance and lifetime of the various components of capital goods were taken into account. The waste management of such materials is included in the present assessments. Life cycle inventory data for the production of materials and energy for capital goods and all process data were obtained from relevant sources. References for all processes are presented in the Supporting Information Table S7.
The waste system considers waste as a “zero-burden” boundary, which means that the production of products and materials that end up as waste is not included in the LCA modelling. The products were produced and used for a purpose other than merely becoming waste. The waste system in3
cludes the collection and transportation of waste, the treatment of waste and the materials and energy recovered. Recovery creates environmental savings, as the waste management system is credited with environmental loads avoided by not producing materials and energy via virgin sources.
Figure 1: Flow diagrams of the LCA and the system boundary. Dark grey area shows the waste flow system, also described as traditional boundaries for traditional waste management LCAs. Light grey area indicates the capital goods flow system. Secondary materials and energy are connected to the substituted production of materials and energy.
2.1 Conceptual model The standard conceptual model for waste management system LCAs consists of potential environmental impacts (PEIs), which is equal to the impacts of handling of one tonne of waste, including the recovery of materials and energy (W). In the following the introduction of capital goods (CG) to the waste management system LCA will be presented. In order for the LCA to be comprehensive, potential environmental impacts from the total waste management system per tonne of waste handled (PEI) equate to the sum of the impacts of the two flow systems (CG and W). All the following considerations are per tonne of waste handled: PEI = CG + W
(Eq.1)
where:
PEI: potential environmental impact of the total waste management system per tonne of waste handled 4
CG: impact made by capital goods used to handle one tonne of waste
W: impact made by the handling of one tonne of waste, including recovery
The waste system can be further decomposed: W = Cw + Tw – mw·SM – ew·SE
(Eq.2)
where:
Cw: impact made by the collection and transport of one tonne of waste
Tw: impact made by the treatment of one tonne of waste
SM: impact made by saved materials, expressed as the impact made by similar production processes using virgin materials
mw: a factor expressing the amount of virgin material production avoided per tonne of waste handled. This factor is affected by technical and mechanical material losses.
SE: impact made by saved energy, expressed as the impact of similar energy production processes using other energy sources
ew: a factor expressing the avoided amount of energy produced per tonne of waste handled.
The capital goods system can be further decomposed; the parenthesis represents the disposal phase of capital goods: CG = ECG + CCG + (RCG + LCG + ICG – m·SMR,I – e·SEI,L)
(Eq.3)
where:
ECG: impact made by material extraction and production (including maintenance and lifetime considerations) per tonne of waste handled
CCG: impact made by the on-site construction of capital goods per tonne of waste handled. Transport to site included
RCG: impact made by the recycling of capital goods per tonne of waste handled
LCG: impact made by the landfilling of capital goods per tonne of waste handled
ICG: impact made by the incineration of capital goods per tonne of waste handled
SMR,I: impact made by saved materials from capital goods (direct recycling or after incineration), expressed as the impact made by similar production processes using virgin materials
5
m: a factor expressing the amount of virgin material production avoided per tonne of waste handled. This factor is affected by technical and mechanical losses of materials.
SEI,L: impact made by saved energy (from incineration or from landfilling), expressed as the impact made by similar production processes using other energy sources
e: a factor expressing the avoided amount of energy produced per tonne of waste handled.
Equation 1 can now be decomposed into: PEI = ECG + CCG + (RCG + LCG + ICG – m·SMR,I – e·SEI,L) + (Cw + Tw – mw·SM – ew·SE)
(Eq.4)
This equation is a conceptual and general description of the total environmental impact of the waste management system, including capital goods, and the fact that some capital goods can be recovered. The significance of each of them will depend heavily on the waste being considered – capital goods depend on the waste being treated and the impact made by the treatment, while recovery depends on the actual waste.
2.2 Life cycle assessment In order to assess the significance of capital goods in an environmental context, we developed five scenarios that addressed different waste types and different waste management systems. The scenarios are not comparable per se, but each of them can be used to assess whether or not the impact of capital goods matters or is insignificant. The geographical scope of all scenarios is Europe. For a few of the scenarios the scope is divided into two regions (like Scandinavia and Southern Europe). For all scenarios the best available data were used, although in some cases this would mean using regional data if the European average was not available. The life cycle assessments follow a consequential approach according to the identification of the context-situation framework for LCI modelling presented by Laurent et al. (2014). Thereby, the marginal production of materials and energy is substituted in relevant scenarios. The functional units of the five scenarios (two including two subscenarios) described in Section 2.4 are as follows: 1. Composting of 1 tonne of garden and park waste. The compost is used as a substitue for mineral fertiliser. Branches are made into woodchips and incinerated together with any reject from the composting process. 2. Anaerobic digestion of 1 tonne of organic waste (organic waste from households). Resulting gas is used for energy production and the effluent is spread on land. 6
3. Incineration of 1 tonne of residual household waste (without paper, cardboard and glass, source-segregated by households), (3a) Nordic incinerator with combined heat and power production and (3b) a European incinerator producing electricity only. 4. Landfilling of 1 tonne of waste. (4a) Mixed household waste after source segregation of paper and glass with an organic content of around 50% and (4b) mixed waste with a lower content of organic waste (around 14%), which could represent bulky inert household waste. 5. Collection and transportation of 1 tonne of household waste. Table 1: Modelling specificities for the five scenarios with sub-scenarios. Features
Technology
Specific technology features
Type of waste treated
Scenarios 1
2
3a
3b
4a
4b
5
Composting
Anaerobic digestion
Incineration
Incineration
Landfilling
Landfilling
Collection and transportation
Simple windrow composting
Danish plant as representative for European plants
Garden and park waste
Wet flue gas cleaning system
Wet flue gas cleaning system
Gas and leachate collection
Leachate collection
240L high density polyethylene bin 3 and 16m waste collection truck
Waste with a content of organic waste of 14%, which could represent bulky inert waste from households
Household waste
Organic household waste
Household waste
Household waste
Household waste after source segregation of paper and glass with an organic content of 50%
Southern Europe
Europe
Europe
Europe
Electricity (24.2%) and heat (5.5%)
Electricity (25%) and heat (60%)
No energy recovery
Not relevant
Geographical representativeness for waste and technology
Europe
Europe
Nordic countries/ Northern Europe
Energy features
Not relevant
Electricity (39%) and heat (46%)
Electricity (22%) and heat (73%)
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Table 2: Environmental impact categories and normalisation references used for the assessment. CTU: Comparative Toxic Unit, e: Ecotoxicity, h: human, AE: Accumulated exceedance (European Commission, 2011). Impact category
Abbreviation
Method
Reference year
Normalisation reference
Unit
Non-toxicity related impact categories
Global Warming
GW
IPCC 2007
2010
8096
kg CO2eq/person
Stratospheric Ozone Depletion
SOD
WMO 1999
2010
4.14*10-2
kg CFC11eq/person
IR
Frischknecht et al. 2001
2000
1.33*10
POF
ReCiPe midpoint, Goedkoop et al. 2008
2000
56.7
kg-NMVOCeq/person
Freshwater Eutrophication
FE
ReCiPe midpoint, Goedkoop et al. 2008
2000
0.62
kg P-eq/person
Marine Eutrophication
ME
ReCiPe midpoint, Goedkoop et al. 2008
2000
9.38
kg N-eq/person
Terrestrial Acidification
TA
Accumulated Exceedance Seppälä et al. 2006
2000
49.6
AE/person
Terrestrial Eutrophication
TE
Accumulated Exceedance Seppälä et al. 2006
2000
115
AE/person
Particulate Matter
PM
Humbert et al. 2009 and 2011
2000
2.76
kg PM2.5/person
Ionizing Radiation, Human Health Photochemical Ozone Formation
3
kBq U-235 aireq/person
Toxicity related impact categories Human Toxicity, Cancer Effect
HTc
USEtox, Rosenbaum et al. 2008
2010
5.42*10
-5
CTUh/person
Human Toxicity, non-Cancer Effect
HTnc
USEtox, Rosenbaum et al. 2008
2010
1.10*10-3
CTUh/person
ET
USEtox, Rosenbaum et al. 2008
2010
665
CTUe/person
Depletion of Abiotic Resources-Fossil
Rf
CML v.4.2, 2013
2000
6.24*10
Depletion of Abiotic Resources-Elements (Reserve base)
Re
CML v.4.2, 2013
2000
3.43*10-2
Freshwater Ecotoxicity
Resource impact categories
8
4
MJ/person
kg Sb-eq/person
The study followed ISO standard 14044 (ISO, 2006) for LCA studies. The waste management LCA software tool EASETECH (Clavreul et al., 2014) was used. This tool was developed specifically for the modelling of waste management and contains all data needed for an assessment of waste treatment: collection, transport, landfilling, incineration, anaerobic digestion, composting and material recovery. All inventory data can be found in the Supporting Information Tables S1-S6. The specific references for all process data can be found in Table S7 in the supplementing information.
Life cycle assessment impact categories recommended by the European Commission (2011) were used for the assessment. The categories are listed in Table 2 and includes the USEtox methodology (Rosenbaum, 2008) for assessing toxic impact categories: human toxicity related to cancer (HTc), human toxicity non-cancer related (HTnc) and ecotoxicity (ET). The applied normalisation references for the used impact categories are based on Blok et al. (2013). The results are presented in unit person equivalents (PE) representing impacts as an average value for one person relating to the total impact of all activities in life in a specific area in the reference year.
2.3 Life cycle inventory data Inventories used for capital goods were found in Brogaard and Christensen (2012), Brogaard et al. (2013a), Brogaard et al. (2013b) and Brogaard et al. (2015). These papers compiled data from a range of sources and presented inventories per tonne of waste handled, including considerations on the maintenance and lifetime of individual components. Data representing the anaerobic digestion plant, the two types of landfills, the waste collection truck and the bin were used for this study as presented in the original source. The composting plant chosen from Brogaard et al. (2015) is designed for treating garden and park waste, and it is used herein as an example. Data for the incinerator were compiled as an average from the data presented in Brogaard et al. (2013a), in order to cover the highest number of capital goods parts specified for five similar plants. The data used for each technology are presented in Supporting Information Tables S1-S6 and described in the following section.
Considering the disposal phase of the materials from capital goods, the materials were split into four streams – one for recycling materials, one for “other” treatment, one for materials left at site and one which was not counted for. The latter was not included in the assessment, since this was used to route materials for which no recycling process data were found. Some materials stayed on site, such as gravel and clay at the landfill or gravel at the composting plant. Table 3 presents the routing of 9
materials in the assessment. “Other treatment” covers the landfilling or incineration of building materials from the steel hall and administration buildings included in the process data from Ecoinvent (2015). Detailed routing of materials up to end of life for capital goods can be found in Supplementary Information Tables S8-S14. The recycled materials were concrete, all types of steel, cables and plastic. The substitution factors (factor m in Eq.3) for each of the processes were given by the datasets used. Concrete substitutes were gravel 1:1 (Butera et al., 2015), recycled steel substitutes were unalloyed steel in a ratio of 1:0.87 (Bernstad, 2007) and recycled plastic substitutes were virgin high-density polyethylene (HDPE) plastic at 0.97:1 (Bernstad, 2006). HDPE was used to represent all plastics used, since this was the main type employed. Cables were recycled as plastic and copper. The recycled copper substitutes were virgin copper 1:1 (Classen et al., 2009). The substitution rates used were viewed as high for steel, copper and plastic, but they were chosen for this study as provided by the original process data, since these were the best available.
Table 3: Routing of materials by end-of-life from each of the capital goods. Materials are routed to recycling, other treatment or materials left at site. A few materials are not counted for in the assessment, due to lack of data. Treatment technology and capital goods for collection
Unit
Sent for recycling
Other treatment
Left at site
Not counted for
Major material in the routing
Composting plant Anaerobic digestion plant Incinerator Landfill Landfill - without gas collection 2-wheeler container 0.24m3 Waste collection truck
% % % % % % %
16 89 98 0 0 85 92
18 4 0 0 0 0 0
66 2 0 100 100 0 0
0.3 5 2 0.1 0.1 15 8
Gravel left at site Concrete sent for recycling Concrete sent for recycling Gravel and clay left at site Gravel and clay left at site Plastic sent for recycling Steel sent for recycling
Data for materials and energy used during the use phase of waste management technologies were found in the EASETECH database (Clavreul et al., 2014). Process data for material production and recycling were obtained from several sources. All processes are listed in Table S7 in the Supporting Information, including the geographical location and year of origin of each process.
2.4 Technology data In the following sections each of the inventories for scenarios 1-5 is described. Scenario 5, representing the collection and transportation of waste, is described in Section 2.4.5 and is different from the treatment scenarios, since it only includes collection and transportation and not a specific waste treatment per se. For the use phase of all scenarios, only energy inputs/outputs and products such as
10
compost are shown in the inventory tables in Supplementary Information. An overview of the modelling specificities can be found in Table 1.
2.4.1 Composting The composting plant considered herein is an open windrow plant for garden and park waste with a capacity of 58,300 tonnes per year (with 8,300 tonnes of added sieving residues) and an estimated life span of 10 years. The physical and chemical composition of the garden and park waste was defined by Boldrin and Christensen (2010). This is the simplest technology for treating organic waste, but because of its wide used globally it was chosen for this study. It should be noted that a more advanced composting technology would require more materials for capital goods.
All detailed data for capital goods are found in Brogaard et al. (2013a). The main materials used for the construction of a composting plant were gravel and concrete stones for the pavement. No significant maintenance of the plant was needed; for example, drainage water pipe cleaning was considered insignificant and not included. Detailed data used for the assessment are presented for the composting plan in Supporting Information Table S1.
Transportation distance for materials to the construction site is assumed to be 50km. The same distance for all materials is chosen, because any variations in distance depend on the location of the plant. Specific locations and the transportation of goods were not the focus of this study.
All capital goods materials were treated after the end-of-life of the plant, except for PVC and asphalt, for which data were lacking. Gravel was also not included, as it was expected to stay at the site after end-of-life.
Energy consumption was included for garden and park waste treatment – 3 litres of diesel and 0.2 kWh of electricity were used per tonne of waste handled at the plant (Andersen et al., 2010). Outputs per tonne of garden and park waste were 776 kg of compost and 55 kg of woodchips (Andersen et al., 2010). The compost substituted conventional NPK fertiliser at a rate of 100% for P (0.8 kg P/tonne of compost) and K (8.4 kg K/tonne of compost) content and by 0% for N content according to Danish legislation on the substitution of N by compost from garden and park waste (NaturErhvervstyrelsen, 2016). The woodchips were incinerated in an energy recovery system, whereby electricity and heat were generated at 22% and 73% of the lower heating value, respectively. Mar11
ginal electricity produced from coal, and marginal heat produced from natural gas, was substituted by the energy produced from incinerating woodchips and residues from the sorting process. Capital goods were also included in the incineration of woodchips and residues following the sieving of the compost. It was assumed that the same capital goods would be needed for incinerating woodchips, residues and household waste. Data for incineration capital goods are described in the section on the incineration scenario. Process data for the incineration of woodchips and residues were modelled in EASETECH with the specific composition of these input fuels.
2.4.2 Anaerobic digestion The anaerobic digestion plant includes two primary thermophilic digesters with a retention time of 30 days and two secondary mesophilic digesters with a retention time of 38 days. Lifetime is estimated at 30 years, and capacity is 80,000 tonnes of mixed organic waste per year. For the LCA, only organic household waste dominated by kitchen organics was considered. The chemical waste composition was defined by Riber et al. (2009). Table S2, in Supporting Information, shows the consumption of materials for capital goods as well as the main consumption patterns for waste treatment.
Low-alloy steel, stainless steel and concrete are the main materials used for the construction of an anaerobic digester. Electricity consumption for pumps and reactor heating is included in the assessment together with diesel consumption for machinery used to spread the digestate on land. The digestate substitutes conventional NPK fertiliser at a rate of 100% for P (1.6 kg P/tonne of digestate) and K (3.3 kg K/tonne of digestate) content and by 40% for N (4.6 kg N/tonne of digestate) content according to Danish legislation on the substitution of N by digestate from organic household waste (NaturErhvervstyrelsen, 2016).The produced biogas is used for electricity and heat production, substituting for marginal energy. The energy recovery efficiency of the gas is 39% for electricity and 46% for heat. The produced energy substitutes for marginal electricity from coal, while marginal heat replaces natural gas.
Concrete, cables, all steel and plastic are expected to be sent for recycling after demolition and are modelled likewise herein. The demolition and disposal of administration buildings and building for the generator at the plant are included in the Ecoinvent (2015) datasets. The demolition process for the anaerobic digestion plant is not included, due to lack of data.
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2.4.3 Incineration The incinerator assessed in this study is a grate furnace incinerator with a capacity of 120,000 tonnes per year and a lifetime of 30 years. More plants with different capacities were included in the paper by Brogaard et al. (2013b), and the one assessed was chosen to cover and include as many parts of capital goods as possible from the data presented in Brogaard et al. (2013b). The waste incinerated is residual household waste, from which paper, glass and cardboard have been partially removed by source segregation. Physical and chemical waste composition was defined by Riber et al. (2009). Supporting Information Table S3 shows the flow of materials for capital goods as well as the main flows for waste treatment. The main materials for the capital goods of the incineration plant are concrete and reinforcing steel (modelled as low-alloy steel) as well as high-alloy steel for any machinery. Fiberglass is included, due to the wet flue gas cleaning system.
Of the capital goods, all steel and concrete are considered recycled. All machinery is treated as steel because of a lack of data, though other materials would also be found in machinery. Fiberglass from the flue gas cleaning system is not treated, because of lack of data for disposal options.
Waste treatment at the incineration plant is modelled via two sub-scenarios, namely Scenario 3a, with high energy recovery (95%) representing Nordic conditions, and Scenario 3b, representing conditions in Southern Europe with lower energy recovery (29.7%), due to less use of generated heat. Efficiencies were based on Turconi et al. (2011). The two sub-scenarios were chosen to assess the importance of capital goods in relation to the influence of energy production. The two plants are therefore modelled in the same way. Energy substituted in scenarios 3a and 3b is marginal electricity produced from coal and marginal heat produced from natural gas. All emissions from the incineration were accounted for, i.e. direct emissions (CO, dioxins, HCl, HF, NOx, SO2 and particulates) as well as waste-specific emissions. Incineration process data for the waste treatment process (used in both 3a and 3b) originate from the study by Riber et al. (2008).
2.4.4 Landfilling The landfill is a modern landfill with extensive leachate and gas controls. The data represent a landfill with a capacity of 3,500,000 tonnes and a filling period of 10 years. The aftercare period is 30 years. All data for the construction of capital goods were obtained from Brogaard et al. (2015). Ta-
13
ble S4 in the Supporting Information shows the flow of materials for capital goods as well as the main flows for waste treatment. The waste landfilled in Scenario 4a is mixed household waste, after the source segregation of paper and glass, with an organic content of around 50%. The physical and chemical waste composition was defined by Riber et al. (2009). Scenario 4b assesses the impacts from landfilling 1 tonne of mixed waste with a lower content of organic waste (around 14%), in order to represent landfills in countries with small amounts of organic waste going to landfill. The waste modelled herein is equal to inert bulky waste from households and is based on compositional and physical characteristics from Møller et al. (2010). Capital goods in Scenario 4b do not include landfill gas collection and management, since the amount of landfill gas is much smaller (25m3 CH4/tonne of waste during 100 years, based on assessment calculations in EASETECH), due to the low organic content in the waste.
Gravel, clay and concrete are the dominant capital goods materials. Most of these materials are used underground as liners and as part of the drainage system and are not disposed of for subsequent recycling or other treatment. This makes the landfill different from the other technologies, since only a few capital goods materials are recovered through recycling at end-of-life.
Energy consumption for the operation of the landfill is part of the Tw in the conceptual model. Diesel for machinery, for spreading and compacting waste and the transportation of a daily cover of soil (0.016 l/tonne of waste), as well as electricity for leachate pumps (0.08 kWh/tonne of waste), were included in the inventory for the waste treatment (Tw).
Gas is collected in Scenario 4a with an efficiency of 35% during years 0-5, 65% during years 5-15 and 75% during years 15-40, thus corresponding to 39 m3 CH4 per tonne of waste landfilled during the first 40 years. Any gas not collected (total 42 m3 CH4, based on assessment calculations in EASETECH) undergoes oxidation in the daily soil cover (0-5 years), the intermediate cover (5-15 years) and in the final top cover (15-100 years), summing up to a total of 9.4 m3 CH4 oxidised in the top cover. Electricity and heat produced from the landfill gas are estimated to be at an efficiency of 85% (60% for the heat and 25% for the electricity) and substituting respectively marginal electricity from coal and marginal heat from natural gas.
14
2.4.5 Collection and transportation – bin and truck Data for the production of goods for collection and transportation were obtained from Brogaard & Christensen (2012). The flows of materials for capital goods are shown in Tables S5 and S6 in the Supporting Information.
For collection, a HDPE 240L plastic bin is included. It weighs 13 kg with a HDPE lid and two rubber wheels attached to a steel axle. No maintenance of the 240L is included, as this only involves washing with cold water and without detergents. The lifetime of the good is 20 years, and it is assumed that all steel and plastic are recycled at end-of-life. The waste collection truck has a 16m3 body, two axles and a capacity of 6 tonnes of waste. The main material used for the truck is steel, which is also assumed as the main material recycled at end-of-life. All aluminium, steel and plastic are recycled. The remaining materials are not considered treated, due to lack of data. Collection of waste with the 240L bin requires 3.27 litre diesel per tonne of residual household waste, and transportation of this waste requires 0.03 litres/tonne/km (Larsen et al., 2009). A driving distance of 50 km is used for the transportation.
3
Results
The results of the five life cycle assessments are presented in the following sections. The scenarios and technologies are not directly comparable, because the types of waste treated are different; however, within each scenario, the results reveal the importance of capital goods. The results for all scenarios are also presented in Table S15 in the Supporting Information.
3.1 Composting The results for Scenario 1, with the functional unit of composting 1 tonne of garden and park waste, are presented in Figure 2. Capital goods of the composting plant contributed more than 20% to SOD, IR, PM, HTc and Re. Impacts made by capital goods were caused by energy consumption involved in the production of concrete pavements and by the disposal of slags from steel production. The impact on SOD is much lower than potential impacts on the other impact categories. The large impact seen on the depletion of abiotic resources is mainly affected by the use of resources for steel production, used for machinery and the steel hall.
15
0.20
Person equivalents/tonne of waste
0.12 0.10
0.15
0.08 0.06
0.10
0.04 0.02
0.05
0.00 -0.02
0.00
-0.04 -0.06
-0.05 GW SOD
IR
POF
FE
ME
TA
TE
HTc HTnc
PM
ET
Rf
Re
ET
Rf
Re
Incineration of wood chips and foreign objects 100%
100% 80%
Composting treatment
80%
60%
Capital goods composting plant
60%
Capital goods incineration plant of wood chips and foreign objects 40%
40%
20%
20%
0% 0%
-20%
-20%
-40%
-40%
-60%
-60%
-80% -100%
-80% GW SOD
IR
POF
FE
ME
TA
TE
PM
HTc HTnc
Incineration of wood chips and foreign objects Composting treatment Capital goods composting plant Capital goods incineration plant of wood chips and foreign objects
Figure 2: Scenario 1 – Composting of 1 tonne of garden and park waste. Potential impact contribution in PE/tonne and in % of the total impact, including waste treatment at the composting plant and capital goods. Impacts caused by treating the waste were due primarily to the emissions of methane and ammonia from the composting process. Nitrate leaching into surface water, caused by the use of the compost on land, led to a high impact on FE and ME. The large saving on HTnc is caused by savings of conventional fertiliser and the process-specific emission of zinc especially from producing mineral fertiliser. The impact made by zinc on HTnc is very uncertain because of methodological issues with 16
the USEtox characterisation factors for zinc. There is therefore a strong need for further research in this regard (European Commission, 2011).
The composting plant is a simple technology compared to other composting technologies. If a more advanced composting technology were to be assessed, capital goods would be expected to cause higher environmental impacts than in this study. Capital goods should be included when assessing any impacts caused by composting systems.
3.2 Anaerobic digestion The results of Scenario 2, considering the functional unit of anaerobic digestion of 1 tonne of mixed organic waste, can be found in Figure 3. Capital goods used for anaerobic digestion contributed most to the potential impact on HTc and Re. Impacts on HTc and Re were caused by the use of stainless steel and related emissions of heavy metals from the disposal of steel production slags. Impacts caused on PM and Rf were caused by energy consumption involved in the production of stainless steel. Savings for GW were caused by the substitution of energy produced from biogas. Leaching of phosphate and nitrate to surface- and groundwater from digestate used on land caused impacts on FE and ME. The fertiliser substitution caused savings for HTnc, because of savings in emissions of cadmium and zinc, and on ET as a result of savings in copper and zinc emissions from avoiding mineral fertiliser production.
Capital goods contributed significantly to the total impact on PM, HTc, Rf and Re. The steel tanks contributed most of all, and these should be a major focus when including capital goods of anaerobic digestion in LCAs. A specific retention time for the anaerobic digestion process was used for this assessment, as using different retention times changes the amount of waste being processed per plant lifetime. As an example, a longer retention time would increase the importance of capital goods per tonne of waste treated, since less waste could be treated over the lifetime of the plant.
17
Person equivalents / tonne of waste
0.8
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Capital goods anaerobic digestion plant
PM
Operation anaerobic digestion plant
Figure 3: Scenario 2 – Anaerobic digestion of 1 tonne of mixed organic household waste. Potential impact contribution in PE/tonne and in % of total impact, including waste treatment by anaerobic digestion and capital goods.
3.3 Incineration Two sub-scenarios were assessed for Scenario 3 with the functional unit incineration of 1 tonne of residual household waste. Scenario 3a was modelled with high energy recovery (95%), the results for which are presented in Figure 4. Scenario 3b included lower energy recovery (29%), and potential impacts are shown in Figure 6. The use of machinery steel contributed significantly to some capital goods impact categories, especially POF, HTc, HTnc, ET and Re. Impacts on the toxicity categories by capital goods were caused
18
by heavy metals emissions into the air and water. Steel is also one of the main materials used in conjunction with concrete, which itself is used in larger amounts, albeit the environmental impacts per kg of material are lower.
For Scenarios 3a and 3b savings on GW, IR, FE, TA, PM, Rf and Re were caused by the substitution of fossil electricity and heat. The savings seen in Scenario 3a were greater than for 3b, because more energy is recovered from the waste. System impacts on ME and TE were caused by processspecific emissions of nitrogen oxides (NOx).
Figure 5 presents a test of the influence on the results of energy substitution and also the recycling of steel and aluminium from ashes followed by the substitution of virgin materials. The results presented in Figure 4 show that the substitution of energy is dominant for impacts caused by incinerating waste. The shares of impacts from capital goods (when excluding the energy substitution and recycling processes) are higher for most impact categories than impacts following on from the operation of the incinerator. This test shows that capital goods for incineration are important and that energy substitution is important to assess, since it is dominant within an incineration system.
Capital goods should be included when assessing waste incineration, even when energy recovery is high. In particular, machinery steel used at the plant should be included in the LCA.
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Person equivalents / tonne of waste
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Capital goods incineration (Nordic)
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Operation incineration (Nordic)
Figure 4: Scenario 3a – Incineration with high energy recovery of 1 tonne of mixed municipal waste. Potential impact contribution in PE/tonne and in % of total impacts, including waste treatment at the incinerator and capital goods.
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Capital goods incineration (Nordic) Operation without substitution of energy and recycling of materials from ashes
Figure 5: Presentation of impact contribution in % of impacts related to incineration (Scenario 3a), excluding the substitution of energy and the recycling of materials leading to the substitution of virgin materials.
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Person equivalents / tonne of waste
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Capital goods incineration (EU)
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Operation incineration (EU)
Figure 6: Scenario 3b – Incineration with low energy recovery of 1 tonne of mixed household waste. Potential impact contribution in PE/tonne and in % of total impacts, including waste treatment at the incinerator and capital goods.
3.4 Landfilling Scenario 4 assessed the functional unit of landfilling 1 tonne of waste in two sub-scenarios: scenario 4a, including waste with a high organic content (50%), and Scenario 4b, including waste with a low organic content (14%) and no capital goods for gas management.
Figures 7 and 8 show the potential impacts from Scenarios 4a and 4b, including capital goods as well as the contribution in percentage terms in relation to the impact categories from capital goods
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and waste treatment. Scenario 4b caused lower potential impacts than Scenario 4a from waste treatment, since less gas is produced and emitted.
For Scenario 4a the impacts made by capital goods on HTnc, PM, Rf and Re were caused by the production of steel for machinery, containers, fences and the reinforcement of concrete tanks. Clay and gravel are used in large amounts, and their excavation and production leads to potential environmental impacts on PM and HTnc. The impacts on Rf are caused by the use of diesel or machinery employed to spread materials when constructing capital goods.
In Scenario 4b capital goods lead to a significant impact share for SOD, IR, FE and Re. With a share of more than 20% of the total impact we consider it important that we include capital goods in the system. Gravel is one of the materials used in largest amount and also contributed the most to non-toxicity-related impact categories, especially FE in Scenario 4b. This is due to the large amounts of gravel used, not only because the impacts are small per tonne of gravel, but also because no recovery of energy was included to cause savings to FE in Scenario 4a. The impacts are very low for SOD, IR and FE compared to the other impact categories, and this should be borne in mind when evaluating these elements. For Scenario 4b impacts on Re are caused by the use of steel in the same way as seen in Scenario 4a.
Savings seen for GW are due to carbon storage in the landfill and because emissions leading to a positive contribution on GW are low compared to Scenario 4a. In Scenario 4a the production of gas is higher due to the higher content of organic waste, which in turn leads to not only savings from the substituted energy, but also to emissions from the uncollected gas. Impacts on POF, ME, TA and TE are caused by the emission of nitrogen oxides from the process of spreading the waste with a frontloader. The leachate treatment leads to emissions of heavy metals, thereby causing the impacts seen for ET.
It is recommended to include capital goods in waste LCA modelling when landfilling is a significant technology in a waste management system. It should also be noted that, per tonne of waste, impacts from capital goods are lower for the landfill than for the treatment technologies. However, if the focus is solely on GW, the importance of capital goods is not significant. This is due to sav-
23
ings made from waste management producing energy and other savings on GW caused by carbon
Person equivalents / tonne of waste
being stored in the landfill.
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Figure 7: Scenario 4a – Landfilling of mixed waste with high content of organic matter (50%). Potential impact contribution in PE/tonne of waste and in % of total impacts, including operation of the landfill and capital goods. Note the difference in the y-axis scale compared to Figure 8 for Scenario 4b.
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Person equivalents / tonne of waste
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Capital goods landfill (low organic content)
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Operation landfill (low organic content)
Figure 8: Scenario 4b – Landfilling of mixed waste with low content of organic matter (14%). Potential impact contribution in PE/tonne of waste and in % of total impact, including the operation of the landfill and capital goods. Note the difference in the y-axis scale compared to Figure 7 for Scenario 4a.
3.5 Collection and transportation Scenario 5 assessed the collection and transportation of 1 tonne of household waste. Capital goods are the bin and truck, and the operation is the collection and transportation of waste. These types of processes are relevant as a first step for all of the waste management systems included in this study.
The collection truck contributed significantly more than the 240L bin per tonne of waste to environmental impacts. Capital goods contributed more to SOD, IR, FE, PM, HTc, HTnc, ET and Re 25
than the waste collection and transport operations, as illustrated in Figure 9. The large contributions capital goods made to the impact categories mentioned above were caused by the choice of European energy input from the Ecoinvent database (2015), including a wide mix of energy sources. The potential impact on Re was caused by the rare earth metals used for the truck’s electronic system.
Collection had higher impacts than transportation on all impact categories, but this, of course, depended on the actual transportation distance, which we set for this study at 50 km. All potential impacts from the operation were caused by emissions from combusting diesel used for the truck.
Capital goods for collection and transportation should be included when performing an LCA looking at the collection and transportation of waste. The potential impact on most of the impact categories depends on the energy mix assumed for the construction of the goods, but the inclusion of capital goods will be relevant in most cases. Impacts caused by capital goods for collection and transportation are much smaller than those caused by the capital goods of treatment facilities, and they may thus be of less importance for a holistic system.
3.6 Sensitivity analyses The sensitivity of the assessments was tested via different approaches. First, the choice of marginal energy and how this affects the results is presented, following which the approach of only including some of the materials in the disposal phase by the end of life of capital goods is discussed. The choice of process data is also assessed, since this can be crucial when using data from databases where data are not produced primarily for the actual study. This was assessed by substituting the process with a process like the one used. The robustness of the results was tested by changing the amount of the main materials used by 10% for all technologies.
The effect of the change of marginal energy on the share of impacts from capital goods to total impacts was analysed. This was done by changing the electricity substituted in scenarios where energy is produced. In the main scenarios coal-based electricity was treated as marginal, and in the sensitivity analyses electricity produced from natural gas was assessed as being marginal. Changes in the share of impacts from capital goods were: composting: -8 - 3%, anaerobic digestion: 0%, incineration (Nordic scenario): -20 - 32% and landfilling: -35 - 14%. This shows that the choice of marginal energy has an impact in some of the scenarios assessed in this study, since anaerobic digestion was not affected because energy substitution has a low impact compared to impacts made by capital 26
goods. The difference in the share for each of the impact categories can be found in Figure S1 in the Supporting Information. It was also shown by Turconi et al. (2011) that changing the marginal for an energy recovery system could change impacts significantly; however, the change of share is not significant enough to change the conclusions of this study, because, by using another marginal, it would still be relevant to include capital goods in LCAs of waste management systems.
0.025
Person equivalents / tonne of waste
0.006 0.005
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0.004 0.015 0.003 0.01 0.002 0.005
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0 GW SOD IR POF FE
ME
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Operation collection and transportation
Figure 9: Scenario 5 – Collection and transportation of municipal solid household waste. Potential impact contribution in PE/tonne of waste and in % of total impact, including capital goods of a 240L bin and a 16m3 collection truck, as well as waste collection and transportation.
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Recycling of demolition waste was considered for the four types of capital goods (composting plant, anaerobic digestion plant, incinerator and landfill). The materials sent for recycling were concrete, all steel, plastics, cables and aluminium. These materials were considered because reliable and representative process data for the recycling processes were readily available. Data were not available for fibreglass, rockwool and asphalt, and they were excluded from the modelling after end-of-life. The recycling of capital goods materials for a composting plant, an anaerobic digestion plant and an incinerator contributed between 0.3-32% of the impacts from these capital goods to the nontoxicity-related impact categories. For the toxicity-related impact categories the contribution from recycling was 0.1-5% in this regard. Resource impact categories were affected by 0.1-21% from the recycling of materials. Only a few of the materials used for capital goods in landfilling were recycled by end-of-life, since they will stay in the landfill forever. Therefore the recycling of materials for landfilling contributed
