Evaluation of environmental impacts from municipal solid waste ...

Copyright © ISWA 2006

Waste Manage Res 2006: 24: 16–26 Printed in UK – all right reserved

Waste Management & Research

ISSN 0734–242X

Evaluation of environmental impacts from municipal solid waste management in the municipality of Aarhus, Denmark (EASEWASTE) A new computer based life cycle assessment model (EASEWASTE) was used to evaluate a municipal solid waste system with the purpose of identifying environmental benefits and disadvantages by anaerobic digestion of source-separated household waste and incineration. The most important processes that were included in the study are optical sorting and pre-treatment, anaerobic digestion with heat and power recovery, incineration with heat and power recovery, use of digested biomass on arable soils and finally, an estimated surplus consumption of plastic in order to achieve a higher quality and quantity of organic waste to the biogas plant. Results showed that there were no significant differences in most of the assessed environmental impacts for the two scenarios. However, the use of digested biomass may cause a potential toxicity impact on human health due to the heavy metal content of the organic waste. A sensitivity analysis showed that the results are sensitive to the energy recovery efficiencies, to the extra plastic consumption for waste bags and to the content of heavy metals in the waste. A model such as EASEWASTE is very suitable for evaluating the overall environmental consequences of different waste management strategies and technologies, and can be used for most waste material fractions existing in household waste.

Janus T. Kirkeby Harpa Birgisdottir Trine Lund Hansen Thomas H. Christensen Environment & Resources, Technical University of Denmark, Lyngby, Denmark

Gurbakhash Singh Bhander Michael Hauschild Department of Manufacturing Engineering and Management, Technical University of Denmark, Lyngby, Denmark

Keywords: Solid waste, solid waste management, life cycle assessment, environmental system analysis, EASEWASTE, wmr 855–1 Corresponding author: J. Kirkeby, Environment & Resources, DTU, Building 113, DK-2800 Kgs. Lyngby, Denmark. Tel: +45 45251600; fax: +45 4593 2850; e-mail: [email protected] DOI: 10.1177/0734242X06062598 Received 4 April 2005; accepted in revised form 28 November 2005

Introduction This paper presents the results of an environmental assessment of the municipal solid waste system in the municipality of Aarhus, Denmark. The municipality is the second largest in Denmark with approximately 300 000 inhabitants. The aim of the assessment was to compare two alternative strategies for the treatment of organic household waste. The first strategy is the current practice in which household waste is source separated into green plastic bags and collected together with residual waste in black bags. Collected source-

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separated organic household waste in green bags is separated from the black bags at an optical sorting facility and routed to a pre-treatment facility where plastic and inorganic material is removed. The pre-treated organic waste is routed to a co-digestion plant that produces electricity and supplies district heating. The digested residue is distributed to nearby farmers for use on arable land. The second and alternative strategy for treatment of the organic household waste disregards the optical sorting and the biogas plant. Instead, organic

Environmental impacts from MSW management in Aarhus, Denmark

household waste is directed to incineration together with the residual municipal solid waste. Life cycle assessment (LCA) is a standardized, systematic and holistic approach for evaluating environmental exchanges, namely resource input and emissions to air, water and soil, from a core system as well as from upstream and downstream activities (the entire life cycle of the system). Environmental assessments of solid waste systems may alter the ranking of waste treatment strategies compared with the ranking of the waste hierarchy that enforces waste minimization, reuse and recycling, incineration with energy recovery above landfilling (Council of European Communities 1991). The waste hierarchy was developed to ensure environmentally sustainable solutions, but the ranking may not be valid for all materials and in all regions. A new model has been developed with the purpose of conducting LCA on municipal solid waste system. The name of the model is EASEWASTE, which is an abbreviation of Environmental Assessment of Solid Waste Systems and Technologies. The model enables comparisons between different waste management strategies, different waste treatment methods and different technologies for a given waste treatment method. The model can track the potential environmental impacts to the source of waste processes and to the waste material fraction (Kirkeby et al. 2004). For the Aarhus case study, data have been collected systematically from the waste treatment plants in the municipality and from the waste collection company. All energy and material consumptions, emissions to air, water and soil and waste flows have been investigated, as well as the waste composition which is determined partly from waste samples collected in the municipality (Tønning 2003) and partly from a national waste composition survey (Petersen & Domela 2003). First, a brief introduction is given to the EASEWASTE model that has been used in the project. Second, the solid waste management system is described including all relevant waste treatment processes in the municipality of Aarhus. Finally, the two alternative strategies and their environmental consequences are evaluated.

Introduction to EASEWASTE Applying life cycle assessment to evaluate environmental impacts from solid waste management systems has become a widely accepted approach due to the systematic procedure and holistic approach applied by the method. The EASEWASTE model considers the total solid waste management system for households from the point where household waste is source separated and collected to the point of final treatment, recovery and disposal of the waste and all arising residues (Kirkeby et al. 2004). A high degree of detail is possible in

the specification of the composition of the solid waste which can be described in terms of 48 material fractions, each defined by 40 physical and chemical properties. The model is developed with a database including all treatment, recovery and disposal options, as well as external processes that can occur either upstream or downstream of a solid waste management system. The user sets up a model of the solid waste management system by choosing types of source separation, collection methods and treatment, recovery and disposal technologies for the collected waste and all arising residues. Valuable products such as recycled materials or energy arising from the waste management system are considered as substitutes for virgin materials or energy. Emissions to air, water and soil and resource consumptions that are avoided on this account, are subtracted from the other emissions and resource consumptions from the waste system. The model calculates emissions to air, water and soil and any consumption of resources. For translation of these exchanges into environmental impacts, the model applies the life cycle impact assessment method from the EDIP97 methodology (Wenzel et al. 1997).

Scope of waste management system Approximately 300 000 inhabitants reside in the municipality of Aarhus, approximately half of them in apartments and half in single-family houses in suburbs and smaller towns. In 2001 the municipality implemented an optical sorting facility that sorts out green bags used for organic household waste for subsequently opening and screening for any inorganic material. The total amount of municipal solid waste was approximately 81 000 tonnes year–1 of which 18 000 tonnes year–1 is source-separated paper and 4500 tonne year–1 is sourceseparated glass. Approximately 6000 tonnes year–1 organic material was sorted out at the optical sorting plant and after a subsequent pre-treatment directed to anaerobic digestion. The residual waste from the sorting was taken to the incineration plant located next to the sorting facility. Experience at the plant showed that sorting was more effective if the residual waste at the household was put in black bags handed out by the municipality and not any random bag from supermarkets and other sources. It was also observed that collection tended to destroy some of the green bags also handed out by the municipality and the use of thicker and stronger bags were thus implemented, and waste collection trucks decreased the degree of waste compaction. This led to an increased consumption of plastic bags whereas shopping bags and other random bags could not be used. The resulting extra consumption of plastic bags in the municipality was estimated to be 211 tonnes year–1.


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J.T. Kirkeby, H. Birgisdottir, T.L. Hansen, T.H. Christensen, G.S. Bhander, M. Hauschild

Fig. 1: Waste flows in the Municipality of Aarhus.

Table 1: Waste generation data. Single-family houses

Multi-family houses

Units

Number of houses

58200

78200

houses

Persons per house

2.62

1.74

Persons house–1

Yearly waste generation

282

282

Kg person–1 year–1

Sources: (Tønning 2003) and (Petersen & Domela 2003).

Glass was collected in cubes that were placed centrally in all parts of the municipality. Paper and cardboard was collected at the source at multi-family and some single-family dwellings. In other areas with single-family houses, a drop-off system with centrally placed cubes was mostly applied. Both glass and paper were brought to material recovery facilities (MRF) where sorting in different quality fractions is conducted for subsequent remanufacturing. Figure 1 shows all possible routes modelled for the solid waste management system in the municipality of Aarhus. The waste generation was defined by the number of dwellings, persons per dwelling and a specific unit waste generation rate per person per year. Some of the assumptions for calculation of waste generation in the municipality are shown in Table 1.

Scenarios Description of scenarios The environmental assessment is based on two pairs of scenarios, where the first pair (scenarios A and B) consider the

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total amount of municipal solid waste, including glass and paper from households in the municipality comparing an incineration alternative and a biogas alternative for the organic material. The other pair of scenarios (scenarios C and D) considers only the organic household waste that was expected to be sorted correctly in the green bags, excluding residual waste, glass and paper. A set of sensitivity analyses was conducted on the scenarios for organic waste, where environmental differences appear more significant. • Scenario A considers approximately 81 000 tonnes year–1 of which 6000 tonnes is led to anaerobic digestion. 4500 tonnes is led to glass remanufacturing and 18 000 tonnes to paper remanufacturing. The remaining approximately 52 500 tonnes year–1 is incinerated with both district heating and electricity generation. An extra consumption of approximately 211 tonnes year–1 of plastic bags is assumed to achieve a proper sorting at the optical sorting facility. • Scenario B considers the same amounts of glass and paper to remanufacturing and approximately 59 000 tonnes combusted at the incineration plant. There is no source


Table 2: Treatment and disposal of waste and residues in all scenarios (tonnes). Scenario A

Scenario B

Scenario C

Scenario D

Total

81582

81372

17211

17000

Optical sorting and pre-treatment

58317

–

17211

–

Anaerobic digestion

5947

–

5992

–

Use of biomass

5160

–

5326

–

Incineration

52370

57264

11219

17000

Bottom ash treatment

10196

10784

954

1455

Reuse of bottom ash

7545

7894

688

1060

Landfilling

2076

2179

242

335

Remanufacturing of iron scrap

1004

1051

88

141

Material recovery facilities: paper

18706

18706

–

–

Paper remanufacturing

18706

18706

–

–

Material recovery facilities: glass

4559

4559

–

–

Glass remanufacturing

3419

3419

–

–

Glass bottle reuse

1140

1140

–

–

separation and sorting of the organic fraction and hence no extra plastic consumption, while the fuel demand for collection is a little less than in scenario A due to a higher degree of compaction that enables waste collection trucks to have larger loads. • Scenario C considers 17 211 tonnes of source-separated organic household waste that is led to the optical sorting plant. The extra plastic consumption constitutes 211 tonnes of this amount and 6000 tonnes is led to anaerobic digestion, and 11 211 tonnes is combusted. • Scenario D considers 17 000 tonnes organic household waste that potentially could have been source separated, but is combusted at the incineration plant.

manufacture are subtracted from emissions occurring in the waste management system. External electricity and district heating is assumed to be based on coal because the combined heat and power plant supplying Aarhus mainly uses coal as fuel. Figure 2 shows the environmental impacts caused by scenario A where it can be seen that incineration of waste is the main contributor to impacts as well as avoided impacts in the system. Paper remanufacturing is a major contributor to the savings of emissions of greenhouse gasses. Figure 2 also shows that approximately half of the contribution to poten-

Table 2 shows all treatments of waste and residues for the four scenarios.

Results The results for all four scenarios are calculated as normalized potential impacts and resource consumption according to the EDIP97 method (Wenzel et al. 1997). Primary energy consumption is normalized against a reference representing the average annual electricity and heating consumption of a person for use of electrical household appliances, etc. in Denmark expressed as primary energy (Danish Energy Authority 2003). The resulting environmental impacts and resource consumption may have a negative value indicating that the system in the scenario leads to avoidance of an impact or resource consumption due to displacement of external virgin materials or energy such as electricity, district heating, paper and glass. When these products are substituted, the emissions to air, water and soil that would have occurred during their

Fig. 2: Normalized potential impacts for scenario A, treating all MSW.


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Table 3: Environmental impacts from scenarios A and B. Scenario A PE

Scenario B PE

–6860

–6921

–

Acidification

–180

–155

Biogas marginally better

25

Photo chemical ozone

318

283

–

–35

Global warming

Difference in PE –61

Nutrient enrichment

–237

–226

–

11

Hum.Tox, water

3576

3506

–

–70

Hum.Tox, air

–25

–17


8

Hum.tox, soil

1085

607

Incineration marginally better

–478

59

77


19

–114

–93


21

Ecotox, water chronic Ecotox, water acute Ecotox, soil

0

0

0

Ozone depletion

0

0

0

PE, person equivalents after normalization of the environmental characterization.

Table 4: Normalized resource consumptions for scenarios A and B.

Natural gas Crude oil Coal Lignite

Scenario A PE

Scenario B PE

19586

19123


–463

453

163


–291

–65733

–65113


620

–833

–807

–

26

Difference in PE

Water

–30

–30

–

0

Aluminum

328

314

–

–14

–8876

–9296


–421

–29

–30

–

–2

–21670

–21792


–122

Iron Manganese Primary energy *

*Normalization factor for primary energy corresponds to the annual need for heat and electrical appliances per person in Denmark.

tial human toxicity via soil is caused by the use of digested biomass on arable land. The environmental impact for acute eco-toxicity is probably irrelevant, since this is a short term impact for direct surface water emissions, which are of less importance in most waste management systems. The differences in environmental impacts for scenario A and B (Table 3) are relatively small for most impacts due to the fact that only 6000 tonnes of the overall approximately 81 000 tonnes is treated differently in the two scenarios. However, the potential impact for human toxicity via soil is significantly greater for the biogas alternative due to the use of digested biomass on arable land. The use of digested biomass contributes to the emission to soil of heavy metals and especially arsenic contributes to the potential human toxicity. This is because the addition with the digested biomass is larger than the amount that would be added with the substituted commercial fertilizer. The substitution of fertilizer includes production and emissions to soil from use of the commercial fertilizer. The high values for human toxicity via water are in both cases mainly caused by the air emission of

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mercury from the incineration plant. For global warming potential and photochemical ozone formation, incineration is marginally better, whereas acidification may be marginally better for the biogas option. Resource consumption shows higher savings with respect to natural gas and oil for the incineration alternative whereas the biogas alternative has a higher saving for coal. Considering the total primary energy consumption the incineration scenario is more advantageous (Table 4). Results from scenario C (Figure 3) show that the global warming potential arising from the optic sorting and pretreatment including the consumption of extra plastic is approximately equal to the savings arising from the anaerobic digestion of the recovered organic waste. The results also show that the potential human toxicity impacts via water and via soil are of major importance and are both partly caused by the use of digested biomass. The differences between scenario C and D (Figures 4 and 5) are more pronounced than between A and B, but still differences are marginal for most of the environmental impacts.


Fig. 5: Normalized resource consumption for scenario C and D, organic MSW only.

Sensitivity analysis Fig. 3: Normalized potential impacts for scenario C, treating organic MSW only.

A series of alternative scenarios, in which central parameters are changed were examined. The purpose was to perform a sensitivity analysis of the scenarios described above with the aim of evaluating the robustness of the results and conclusions. The parameters examined were chosen either because there was a certain degree of uncertainty in the data or because it was assumed that a parameter may have had a significant impact on the overall results. The sensitivity scenarios were all based on scenarios C and D, and all sensitivity parameters concerning the biogas or the incineration plant are described below. The results for the sensitivity scenarios are shown in Figure 6. Sensitivity scenarios Sc. C1: Methane potential of food waste increased by 11%

Fig. 4: Normalized potential impacts for scenario C and D, organic MSW only.

The results for C and D show that incineration of organic household waste is marginally better than anaerobic digestion as regards global warming and human toxicity via water, and significantly better considering the potential human toxicity via soil, due to the content of arsenic in the organic waste. For acidification, nutrient enrichment, and photochemical ozone formation and for all ecotoxicities there are no significant differences between the two scenarios.

The methane potential of organic waste is an uncertain value as reported in different surveys examining biogas potential. This scenario increases the methane potential from 450 N m3 tonne–1 volatile solids (VS) to 500 N m3 tonne–1 VS. However, the results do not change the conclusion since the ranking between incineration and biogasification does not change. Incineration is still marginally better than anaerobic digestion even though the savings in global warming potential increase by approximately 5%. Sc. C2: The extra consumption of 211 tonnes of plastic for waste bags is neglected

This scenario examines the assumption that the optical sorting plant could work without using stronger and thicker plastic bags. Neglecting the extra consumption of


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Fig. 6: Normalized potential impacts for sensitivity scenarios, organic MSW only.

211 tonnes of plastic makes the biogas scenario as good as, or slightly better than, the incineration scenario with respect to global warming and primary energy consumption. This is due to a decrease in oil consumption for plastic and a decrease in CO2-emission from production and subsequent incineration of the plastic bags. The savings of global warming potential increase by approximately 25% and for primary energy the saving is approximately 33% higher (see Figure 7).

Sc. C3: The energy consumption for optical sorting and pre-treatment is half of value in scenario C

The optical sorting plant was very new when data on energy and material consumption was collected, so the data were uncertain. Thus, the optical sorting plant may improve the operation and reduce energy consumption. The energy consumption at the optical sorting and pre-treatment of the organic waste seem to be of minor importance compared to the remaining part of the waste management system and to the energy consumption required by the production of plastic bags. Sc. C4: The total energy efficiency at the biogas plant increased from 70 to 88%

Fig. 7: Primary energy savings for sensitivity scenarios for approximately 17 000 ton year–1, organic MSW only.

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The energy efficiency at the biogas combustion had a high degree of uncertainty due to an unreliable gas flow meter, and the energy recovery from the use of biogas has an important impact on the overall environmental impacts due to displacement of coal-based energy production. The increase of energy production from combustion of the produced biogas makes the biogas scenario equivalent to incineration with respect to most environmental impacts. Of especial note is the fact that the increase in electricity production contributes to avoided emissions from an external coal-powered heat and power plant.


Sc. C5: The unburned methane emission is decreased from 3 to 1% of the produced methane

The amount of unburned methane is a very uncertain value. The amount used in the analysis was not measured at the actual combustion plant but was found in the literature (Nielsen & de Wit 1997). This sensitivity scenario was conducted because newer technologies may reduce the emission. The decrease in the incomplete combustion of methane from the biogas plant affects only the global-warming potential. Methane is considered to be an important greenhouse gas, but overall a reduction of uncombusted methane seems to have only a minor effect. Sc. C6: The content of heavy metals is half of what it was in scenario C, content of Mn is set to zero

Heavy metals in the organic waste were shown to have a significant impact on human toxicity potential. The data in the literature varies and therefore there is a degree of uncertainty associated with the results in scenario C. The content of heavy metals (As, Cd, Cr, Cu, Hg and Mo) in the organic waste material fractions is set to half of what was assumed in scenario C. Simultaneously the content of manganese is set to zero, because there is a lack of data for manganese emissions during production and use of the commercial fertilizer, which is assumed substituted by the nutrients in the organic waste. The reduction of the heavy metal content has a very significant impact on the potential human toxicity via water and soil, which is more than halved compared to scenario C. Sc. C7: Biogas scenario in which the electricity generation efficiency at the incineration is increased whereas heat generation efficiency is decreased

This scenario should reflect the energy production in the future after having made improvements on the existing incineration plant. The scenario should be compared with scenario D1. During this assessment the municipality of Aarhus was upgrading the boilers at the incineration plant in order to increase the electricity efficiency up to 20% whereas the district heating efficiency would probably decrease from 69 to 60%. This improves the environmental impacts, but in comparison with scenario D1, incineration is still better with respect to most environmental impacts and resource consumptions. Sc. D1: Incineration scenario where the energy efficiency is as in scenario C7

As in scenario C7, an increase in electricity generation efficiency improves the environmental impacts significantly, because production of external electricity has a higher impact than production of external district heating due to the allocation between the two energy forms for a combined heat

and power plant. The allocation of input and emissions from a combined heat and power plant takes into consideration the quality of the energy products and the fact that power has a higher quality than heat. This is associated with greater resource consumption and emission from production of power than from production of district heating. Other sensitivity parameters Carbon dioxide emissions from incineration are not included in the current version of EASEWASTE, with the exception of emissions arising from incineration of the extra plastic from waste bags. This assumption leads to very significant savings in the overall emission of global warming gases when energy is recovered. The energy production in the solid waste management system is of major importance, and when the avoided external energy production is based on coal, which is CO2 heavy, the global warming potential for the overall waste system becomes negative. Consideration must be given to whether both electricity and district heating is produced in the waste management system and which energy sources are substituted. This can have some effects on the conclusions obtained. If only electricity is produced at both the biogas plant and at the incineration plant, the biogas scenario will come out more beneficial because higher electricity efficiencies can be obtained from biogas combustion than from waste combustion. If the energy source that is substituted has a lower CO2 content source than coal, the avoided global warming potential would decrease; however, the ranking between scenarios will not change given that the energy efficiencies are constant. The time horizon for emissions occurring from the landfill for incineration residues is defined to be 100 years in the reference scenarios. When modelling the emissions from the landfill, this time horizon is divided into four time periods, each with different leachate generation rates, collection efficiencies and leachate compositions. All these parameters are associated with some uncertainty. As a time horizon of 100 years may seem short for landfilling of incineration residues, a sensitivity screening has been made on the time horizon for the incineration scenario of organic waste. It showed that increasing the time horizon for landfill emissions from 100 years to 10 000 years creates differences in the potential ecotoxicity via water whereas the remaining impacts are not affected. Modelling the effects of carbon sequestration by the use of biomass on arable land is possible in EASEWASTE. Carbon sequestration takes into account the fact that carbon is bound in organic matter for a period of time thus avoiding an immediate emission of carbon dioxide. A choice must be made regarding the amount of the carbon spread on arable land that is assumed to be bound in the soil. The avoided


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global warming potential may increase by approximately 8%. Under the assumption that 15% of the total carbon is bound to the soil (Hogg 2002). Collection and transport of waste and residues constitute only a minor part of the environmental impacts when the energy content of the municipal solid waste is utilized to the extent described in this project. Thus, optimizing the collection system will have only a minor impact on the overall results. The collection and transport is controlled more by the economic costs than by environmental and other issues.

Discussion The results from the scenarios rely on a large set of data that was collected partly from the treatment facilities in the Municipality of Aarhus, and partly from literature and experiences. Some of the main problems in the environmental assessment are connected to the reliability of the data for waste composition, for the waste treatment options and for the impact assessment. Waste composition The waste composition was estimated from national and local surveys identifying several material fractions in singlefamily and multi-family dwellings. The chemical composition of each material fraction was collected from a series of foreign surveys (Chandler 1994, US EPA 1997, Beker & Cornelissen 1999, Marb et al. 2003). Combining material fractions from other surveys with the defined material fractions used in EASEWASTE created problems due to the application of different classifications of waste in all surveys. Furthermore, there was a great variation among surveys reporting on the heavy metal content of some waste material fractions. An improved source separation of waste fractions including significant amounts of heavy metals would lead to a decreased load of metals to incineration which again would lead to significant improvements of the environmental impact potentials given that the alternative treatment of hazardous materials has less potential impact. Therefore, an improved source separation may be of essential importance in order to improve the overall environmental achievements. Incineration Another uncertain parameter is the allocation of emissions and energy recovery for the incineration plant that combusts bulky waste, and commercial and industrial waste from a municipality in addition to household waste. This was solved by conducting an experiment where only household waste was fed into a specific combustion chamber at the actual incineration plant. In the experiment emissions and energy recovery were measured continuously over a week (Riber &

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Christensen 2004). By measuring not only the air emissions, but also the composition of bottom ash and air pollution control (APC) residues, the transfer coefficients for all metals contained in the household waste to air, to bottom ash and to APC residues were determined. Furthermore, the actual energy recovery arising from the household waste was determined to be approximately 25% lower than for the mixed waste including bulky, commercial and industrial waste. Anaerobic digestion Similarly, at the anaerobic digestion plant there was an allocation problem in allocating the produced biogas between the organic household waste and other biowastes, such as liquid manure and industrial organic wastes. The household waste constituted only 5% of the total biomass treated at the plant. The problem was to define the biogas production arising from the household waste only. The methane potential was estimated from laboratory experiments with organic household waste, and from another set of experiments conducted in a small-scale pilot reactor that were used to assess the utilization of the methane potential (Christensen et al. 2003) and (Hansen et al. 2004). Evaluation of results The potential impacts that are calculated from the emissions are calculated without considering the site-specific conditions of the exposed environment. This means that some of the potentials may not come through as real effects, for example, in the case where exposure to the emitted substances is limited or absent due to local conditions. In this sense, the ecotoxicity and human toxicity potentials in particular may be seen as realistic worst-case scenarios. In some of the scenarios, the potential human toxicity via water was very significant but whether this potential is actually realized depends on the degree to which humans are exposed. The exposure of humans via surface water would take place through drinking water and ingestion of fish and seafood. Therefore, if the exposed surface waters are used for drinking, and if fishing is common in the nearby area, the exposure and the resulting actual toxicity impact on people may be high. In the case of Aarhus this may not be the case, since the city lies at the sea where dilution is larger than in most fresh waters, and surface water is not used for drinking purposes in the vicinity of the city. The actual toxicity impacts may therefore be less than the results indicate, also due to the uncertainty in heavy metal content in household waste. Comparison with earlier environmental assessments The results of the Aarhus case agree in most respects with results from similar environmental assessments of solid waste systems that include anaerobic digestion of organic house-


hold waste. Finnveden and co-workers, using the Dutch LCA tool, Simapro, (Finnveden et al. 2000) found similar results but with a few notable differences. They found a significantly higher, positive global warming potential for the incineration scenario, whereas there were savings by the anaerobic digestion scenario. This is due to the fact that CO2 emissions from incineration were considered to contribute to climate change and because heat from incineration was substituting bio-fuels, whereas, in the case of Aarhus, CO2 emission from incineration was assumed to be CO2 neutral. This assumption is reasonable for waste of biogenic origin but should not be the case for waste made from fossil fuels such as plastics. Currently a new version of EASEWASTE is being developed in which CO2 emissions are calculated separately for fossil carbon and biogenic carbon. A comparative scenario was made in the Swedish study where district heating was substituting natural gas and the net savings of global warming potential for anaerobic digestion and incineration were of the same order of magnitude. This indicates that the results depend very much on the energy sources that are being used and substituted. Furthermore, the Swedish study did not find the same strong impacts to human health by use of digested biomass from the anaerobic digestion. The Swedish model, ORWARE, has been used several times for evaluation of waste treatment systems, among others by Sundqvist and Sonesson (Sundqvist et al. 2002), who found similar results but with a significantly higher impact for acidification from anaerobic digestion compared to incineration. Furthermore, toxic impacts seemed higher from anaerobic digestion systems than from the incineration scenario, a result in agreement with findings in the case of Aarhus. Sonesson and co-workers found that the emissions of global warming gases were reduced in the anaerobic digestion scenario compared to incineration. This was partly due particularly to emission of nitrous oxide, N2O, from incineration (Sonesson et al. 2000), but this emission was not found to have any significant importance in the case of Aarhus.

Conclusions Conclusions for the municipality of Aarhus The results from the environmental assessment of the solid waste system in the Municipality of Aarhus showed that there were no major differences in most of the potential environmental impacts, nor in the consumption of resources whether the source-separated organic household waste was anaerobically digested or combusted at an incineration plant. The relative differences were very small, partly due to the very small amount of organic waste that was treated differently in the two scenarios. Only 6000 tonnes out of the total 81 000 tonnes was sorted out at the optical sorting plant and

transferred to the biogas plant. The results indicated that the incineration scenario could save the energy for heating and for providing power for the operation of electrical appliances for approximately 10 000 dwellings. Furthermore, in comparison with the biogas scenario, the differences in global warming potential correspond to the annual impact from 60–70 persons, and in primary energy consumption for heat and electrical appliances to the annual consumption of 60–70 dwellings. Both results favour the incineration scenario. Source separation and recycling of paper and glass saved resources and minimized emissions. However, no environmental assessment was performed for alternative waste management strategies for paper and glass, so no final conclusions can be made for these material fractions. The results from the scenarios considering only organic waste including the sensitivity analyses showed more clearly the differences between incineration and biogas scenarios. It was found that the consumption of extra plastic needed for collection is a major contributor to the differences. This is due to the high consumption of energy resources for production of plastic and the high emissions related to production as well as subsequent combustion at the incineration plant. One major difference in the two alternative treatment options is the potential human toxicity via soil that occurs at a significantly higher level in the biogas scenarios due to the content of heavy metals, especially arsenic, in the source-separated organic household waste. The digested biomass that is spread on arable land contributes arsenic to the soil in larger amounts than the commercial fertilizer that it would replace. The most important environmental impacts in relation to this case are the saved global warming potential due to energy recovery and the potential human toxicity via water and soil. The potential human toxicity via soil is due to the arsenic content in organic waste and the potential human toxicity via water is mainly caused by the air emission of mercury from the incineration plant. It is assumed that the mercury is deposited on soil and surface waters. Acidification, photochemical ozone formation and nutrient enrichment are environmental impacts that are less important in this case. The most important resource saved is coal since both external district heating and electricity is based on coal from the nearby coal-fired heat and power plant. Natural gas and crude oil are consumed but not in the same order of magnitude. In addition, iron is saved due to iron recovery from the incineration plant. The results show that regardless of the choice of anaerobic digestion or incineration as the waste treatment option, large energy and resource savings occur in the solid waste management system. However, the incineration scenario may supply marginally more dwellings with energy for heating and electricity than a biogas option. At the same time the incinera-


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tion scenario saves on the emission of greenhouse gases. The amount saved corresponds to the annual impact of approximately 60 persons more when compared with the biogas scenario. As this difference is due to different waste handling for an area with approximately 300 000 inhabitants, the differences between the scenarios are truly insignificant. The assessment did not include economical costs for the different waste management strategies but the system including optical sorting, pre-treatment and a biogas plant was considerably more expensive. Therefore, financial matters led to the closing of this system in the summer of 2004. Applications and perspectives of environmental models for solid waste systems The case study showed that the waste hierarchy, which would have proposed anaerobic digestion above incineration in this case, is not generally valid from an environmental perspective. The waste hierarchy is not scientifically based; it is probably based more on common sense, green faith and beliefs. The

waste hierarchy could be replaced or it could be supported by the use of LCAs or other systematic and holistic tools that incorporate environmental benefits and offset from solid waste systems and their upstream and downstream related activities. The EASEWASTE model can be used on all material fractions in municipal solid waste to assess whether reuse, recycling, biological treatment, thermal treatment or landfilling is environmentally preferable. As the waste material fractions have different properties and need different sorting and treatment to be recovered, following the ranking of the waste hierarchy may not identify the most environmentally beneficial alternative. EASEWASTE is a model for supporting decisions regarding solid waste management systems and strategies, but results should be supported by evaluation of technological, economical and social issues. A new version of EASEWASTE is currently being developed to include social and financial costs as well as an automotive module for making sensitivity analysis.

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