Life cycle assessment of energy from solid waste—part 2: landfilling compared to other treatment methods

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Journal of Cleaner Production 13 (2005) 231–240

Life cycle assessment of energy from solid waste—part 2:landfilling compared to other treatment methods

Gsa Moberg a,1, Goran Finnveden a,b,�, Jessica Johansson a, Per Lind a

a Department of Environmental Strategic Studies, FOI (Swedish Defence Research Agency), SE 172 90 Stockholm, Swedenb Centre for Environmental Strategies Research - fms, KTH (Royal Institute of Technology), SE 100 44 Stockholm, Sweden

Abstract

In the present paper, the validity of the waste hierarchy for treatment of solid waste is tested. This is done by using the tool lifecycle assessment on recycling, incineration with heat recovery and landfilling of recyclable waste for Swedish conditions. A wastehierarchy suggesting the environmental preference of recycling over incineration over landfilling is found to be valid as a rule ofthumb. There are however assumptions and value choices that can be made that make landfilling more preferable. This is the casefor some waste fractions and for some of the environmental impacts studied when only a limited time period is considered. Whentransportation of waste by passenger car from the households is assumed for the other treatment options but not for landfilling,landfilling also gains in preference in some cases. The paper concludes that assumptions made including value choices with ethicalaspects are of importance when ranking waste treatment options. Uncertainties related to the assessment of toxicological impactscan also influence the conclusions.# 2004 Elsevier Ltd. All rights reserved.

Keywords: Landfill; Life cycle assessment; Waste; Hierarchy; Time perspective; Carbon sink; Transport

1. Introduction

Waste is generated as a consequence of most of our

daily activities. The appropriate way of taking care of

this waste in the most efficient way and with the least

negative impacts is a question of concern. A waste hier-

archy is often suggested and used in waste policy mak-

ing. Different versions of the hierarchy exist but in most

cases these suggest that landfilling has the lowest pri-

ority. Normally recycling has a higher priority than

incineration. The first priority is to reduce the amount

of waste, and it is in general accepted. However, the

remaining waste needs to be taken care of as efficiently

as possible and the hierarchy after the top priority is

often contested [1] and discussions on waste policy are

in many countries intense. The order of recycling and

incineration in particular is often discussed, but as will

be discussed later in this paper, the order of incineration

and landfilling may also change depending on assump-tions made and system boundaries set.The present paper summarises some of the results

from a study performed at the Environmental Strate-gies Research Group (fms) where different strategiesfor treatment of solid waste are evaluated based on alife cycle perspective [2] and the general methodologyand results are presented in an accompanying paper [3].The aim of this paper is to test the validity of the wastehierarchy, focusing on cases where the landfill optionmay be ranked higher. Assumptions and valuationsleading to these cases are also discussed.

2. Methodology and assumptions

2.1. Methodology

Life cycle assessment (LCA) is used to evaluate differ-ent waste treatment methods. LCA studies the environ-mental aspects and potential impacts throughout aproduct’s life (i.e. from cradle to grave) from rawmaterial acquisition through production, use and dis-posal [4]. LCAs focus on products, or rather functions

232 A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240

that products provide. Products can include not onlymaterial products but also service functions, forexample taking care of a certain amount of solid waste.This is an appropriate perspective when comparing dif-ferent options for waste management. In general, lifecycle assessment (LCA) methodology based on stan-dards and guidelines [4,5] is used. This methodology isalso applicable in LCAs of waste management [1,6]. Themethodology used is described in more detail by Finn-veden et al. [2,3].When comparing different options fulfilling a similar

function, it is important to consider the complete lifecycle and not only one phase, e.g. production or use.This is because environmental impacts and benefitsmay occur at different phases of the life cycle. Themost important phases may not be the same when twooptions are compared. Included in the systems analysismade here are also any additional functions producedby the studied system, e.g. production of recycledmaterials or recovery of heat from waste treatment sys-tems. In order to make a fair comparison of systemsproducing different additional functions, the systemsare credited for the resources used and emissions fromthe production of the additional functions in someother way, the so-called avoided production (cf. [7]).An example is that production of newsprint from woodwith all related resources used and emissions is creditedto the studied system recycling waste newsprint.An LCA consists of four different phases, which are

performed in an iterative manner. They are, accordingto ISO [4], Goal and scope definition, Life cycle inven-tory analysis, Life cycle impact assessment and finallyInterpretation using results from all three previoussteps. Within the impact assessment phase, the follow-ing elements are performed: Selection of impact cate-gories, indicators and models, classification,characterisation and weighting. The classification is anassignment of the inventory data to selected impactcategories. The categories included in this study arepresented in Table 1.Following the classification, the contributions to the

impact categories are quantified in the characterisation.For this a number of established methods are used:

photo-oxidant formation [8], abiotic resources [9], glo-bal warming [10], acidification and eutrophication [5]and depletion of stratospheric ozone [11]. Two differentmethods are applied to characterise toxicologicalimpacts, the toxicity parts of the Danish EDIP method[12,13] and the Dutch model USES-LCA [14]. Boththese methods result in a number of subcategories forboth ecotoxicological impacts and human toxicologicalimpacts. Details are presented in [3]. The results fromthe characterisation are further processed by weighting.This means converting and aggregating results acrossimpact categories. For this a method based on Swedishtaxes, Ecotax 98, is used [15]. In order to trace the lar-gest spans we use three different combinations ofcharacterisation and weighting factors, one minimumcombination and two maximum combinations. Themaximum combinations are identical except for thecharacterisation methods used to assess toxicologicaleffects (EDIP and USES). The resulting sets of weight-ing factors are called USESmin, USESmax and EDIP-max. Details about the impact assessment arepresented in [2]. The results are interpreted using theoutcomes from all steps of the assessment.The different waste management options studied are

landfilling, incineration, recycling of paper and plasticfractions and digestion and composting of food waste[2,3]. The calculations are made for the unrealistic situ-ation that all waste included is treated with the samestrategy. A very high level of recycling may requiremodifications in processes and collection systems com-pared to those modelled here. The household wastefractions used as input to the systems are the combust-ible and recyclable or compostable materials; foodwaste, newsprint, corrugated cardboard, mixed card-board and five plastic fractions. Amounts and compo-sition of the waste fractions are presented elsewhere[2,3]. In this paper, the focus is on the paper and plas-tic fractions exemplified by newsprint and PET. Thewaste management options are studied in a base scen-ario, which is complemented with a range of ‘‘what-if ’’scenarios.Data for the materials recycling processes are from

literature and databases for newsprint [16] and for PET[17]. The incineration and landfill models are, some-what modified, from [18]. The incineration model isbased on a modern Swedish plant with flue gas conden-sation, ashes are landfilled. Energy generated at theplant is assumed to be used for district heating. Thelandfill model is representative of average Swedishmunicipal landfills. The model includes landfill fires[19,20] and treatment of 80% of the leakage water dur-ing the first 100 years [19,21]; leakage treatment sludgeis landfilled according to [18]. The landfill gas formedduring the methane phase is assumed to be collectedwith 50% efficiency and combusted, generating electricityand heat.

Table 1

Impact categories used in the study

Impact categories

Total energy S

Non-renewable energy N

Abiotic resources A

Non-treated waste A

Global warming N

Depletion of stratospheric

ozone

E

Photo-oxidant formation H

A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240 233

2.2. Time aspects

2.2.1. Time perspectivesOne important difference between landfilling and

most other processes in an LCA is the time frame.Emissions from landfills may prevail for a very longtime, often thousands of years or longer. The potentialemissions from landfilling have to be integrated over acertain time-period. It is important to determine whichtime period is of interest. There is currently no inter-national agreement on this question [22]. Using theLCA definition as a starting point, it can be arguedthat emissions should be integrated until infinity. Inpractice however, a shorter time frame (decades andcenturies) is usually chosen (see [6] for a review). Thechoice of the time period can have a significant influ-ence on the results for materials that are persistent (e.g.plastics) and for substances that only slowly leach out,e.g. metals from municipal solid waste and ashes [23].The choice of the time frame is clearly a value choice

for the inventory analysis of an LCA. It is related toethical views about impacts on future generations [24].It is clearly a question that deserves more attention.Important aspects to discuss include the possibilitiesand consequences of different choices as well as theethical discussion, which apparently cannot be avoided.A similar situation may occur for different parts of thelife cycle impact assessment. The choice made by theSETAC-Europe working group on life cycle impactassessment is to consider first the infinite time period,then a short time period of 100 years and finally ifwanted other time periods [25,26].Here, a hypothetical infinite time period is used

when inventorying emissions, which is considered to bein line with the precautionary principle. This may beseen as a ‘‘worst case’’, assuming complete degradationand spreading of all landfilled material [27]. To evalu-ate the effects of choosing another, shorter, time per-spective, this is also tried. A limit in time is then setafter the so-called surveyable time period. This is theperiod until the landfill has reached a pseudo steadystate, a time period corresponding to approximatelyone century. For municipal solid waste landfills, this isdefined as the time it takes for the landfill to reach thelater part of the methane phase when gas production isdiminishing and this time is approximated to be onecentury [27]. For landfilling of incineration ashes, thesurveyable time period is defined as the period duringwhich the soluble chloride salts are leached out [28].

2.2.2. Carbon sinkCommon practice in LCAs is to disregard biotic

CO2-emissions. This can be motivated from differentperspectives [29]. One includes an expansion of the sys-tem boundary to include also the uptake of the CO2 inthe growing tree. This expansion is often done as a

thought experiment rather than an actual modelling.Another perspective can be the assumption that when

biotic resources are harvested, new resources are plan-ted that will take up an equivalent amount of CO2.Again, this modelling is normally not done explicitly.Yet another perspective is the assumption that if thebiotic resources, e.g. trees, had not been harvested,they would have been left in the forest and degradedthere. This degradation can however be quite slow,and the time frame has to be extended to several cen-turies before all biotic materials have been degraded[30].The biological carbon is thus seen as part of a cycle,

where carbon is sequestered by and released fromrenewable sources continuously. However, if the sur-veyable time period is set as a boundary in time this

cycle is interrupted. Then, one may consider landfills tobe carbon sinks keeping carbon from being released tothe atmosphere. With this perspective, the landfillingoption may be credited for the avoidance of the globalwarming potential the trapped biological carbon wouldhave had as carbon dioxide in the atmosphere. This isdone by subtracting carbon dioxide emissions corre-sponding to the amount of biological carbon trapped[2]. However as noted above, this concept is a valuechoice neglecting potential effects on future genera-tions.

2.3. Assumptions, boundaries and scenarios

In the base scenario, the following major assump-tions and system boundaries are used (the choices arediscussed elsewhere [2,3]):

. distances for transportation of waste are moderate,

. heat production, which is credited to waste treat-ment systems where heat is produced, is from incin-eration of forest felling residues,

. electricity is produced from hard coal,

. recycled material is credited to waste treatment sys-tems using data for production of virgin material of

the same kind and. the time perspective is a hypothetical infinite time

period.

Several ‘‘what-if’’ scenarios are used to discoverparameters of importance for the outcome of the study[2]. The following are discussed here:

. the natural gas scenario, where heat production cred-ited to waste treatment systems with heat recovery isfrom natural gas,

. the surveyable time period scenario, where a limit intime regarding landfill emissions is set after approxi-mately one century,

234 A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240

. the carbon sink scenario, which has the same timelimit as the surveyable time period scenario, but alsocredits landfills for the biological carbon, which isnot emitted—the landfill is regarded as a carbonsink,

. the increased transports scenario, where longer trans-port distances by truck to incineration and recyclingfacilities are assumed, and

. the passenger car scenario, where waste for recyclingand incineration is source separated and transportedby car to collection points. This variant is tried bothfor recycling and incineration due to the possibledevelopment towards separate incineration of differ-ent waste fractions for better efficiency and alsotowards small-scale and co-incineration using spe-cific fractions, even though these incineration techni-ques are not specifically modelled here.

Results from other scenarios including effects onchoices of the energy system and alternative materialproduction are discussed in the larger report [2] andthe accompanying paper [3].

3. Results and discussion

In the following presentation only a selection of resultsare shown, for a full presentation of results see [2].

3.1. General results

The results of the LCA of the base scenario indicatethat landfilling is in general the least preferred option,results for some of the impact categories are presentedin Fig. 1. As the systems are credited for producingadditional functions by subtracting emissions andresources used that would have resulted from avoidedproduction, the results presented here may be negative.Negative results are thus avoided impacts. The resultspresented are for waste newsprint and PET (polyethy-lene terephthalate) representing paper and plasticwaste. In Fig. 1 exceptions, where landfilling is notlowest ranked, can also be seen for non-renewableenergy use and NOx. The total weighted results givethe ranking recycling before incineration before land-filling, which is exemplified by results from the Ecotax98/USESmax method in Fig. 1.Looking at the total energy balance, landfill is

ranked as the least preferable option. However, thenon-renewable energy balance is dependent on assump-tions concerning the origin of the heat production avoi-ded when heat is produced from waste incinerators orlandfill gas combustion. In the base scenario, the pri-mary energy source for heat is forest felling residues.When this is changed to natural gas, a non-renewablefuel, the ranking is changed, lowering landfilling to

least preferred option [3]. This is a consequence ofmore energy being recovered through incineration thanthrough landfilling.In Fig. 1, three different results are presented for the

category ecotoxicological impacts. The differences arein characterisation method used, EDIP or USES, andin weighting values, USESmin and USESmax, usingdifferent weights within the Ecotax 98 weightingmethod. As can be seen, the rankings are dependent onwhich method is used. This is a way of illuminating thelarge uncertainties related to the toxicological impactcategories. For some of the other waste fractions theranking of the landfill option may also change in thiscategory, depending on method used.

3.2. Time perspective

In the surveyable time period scenario, a large partof the metals, and also most of the fossil carbon, e.g. inwaste plastics, are not emitted from the landfill at all.A total cut-off after the surveyable time, approximatelyone century, has passed is made. The incinerationoption is also affected by the assumption of a shortertime perspective, since ashes are landfilled and e.g. themajor part of the metals within the ashes is not emittedin this scenario.When a shorter time perspective is used for landfills

the resulting rankings of the different waste manage-ment strategies may change compared to the base scen-ario. For newsprint and PET, this happens for theglobal warming and ecotoxicological impact categories.For all the plastic fractions, landfilling becomes pref-

erable to incineration concerning global warming whena short time perspective is used, but recycling is stillranked as the most preferable option. This is illustratedwith PET as an example in Fig. 2. Emissions of carbonto air from landfilling of plastic waste mainly occursubsequent to the surveyable time period, and are thusomitted in this scenario. When plastic waste is inciner-ated, all carbon is immediately emitted as carbon dioxideand thus landfilling of plastic waste contributes less toglobal warming during the surveyable time period. Inthe case of landfilling newsprint and other paper frac-tions, the major contribution to the global warmingimpact category is from emissions of methane during thesurveyable time period and only a smaller difference isseen which, as shown in Fig. 4, does not change theranking of the treatment options.The other impact category affected by this change in

time boundary is ecotoxicological impacts. In this case,changes are dependent on how the ecotoxicologicaleffects are modelled in the characterisation methodsused. When the EDIP and USESmin methods are used,the results for incineration are dominated by avoidedterrestrial emissions of metals from wood ash. Sincethese metals are modelled to be emitted subsequent to

A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240 235

the surveyable time period, the avoided emissions are

drastically reduced in the surveyable time perspective.

When the USESmax method is used, the results for

incineration and landfilling are dominated by aquatic

emissions of metals from landfilled ashes and waste.

Since a large part of these metals are modelled to leach

diagrams show the rankings of waste treatment options for the waste fractions newsprint and PET (polyethylene terep

a selection of impact categories. Total weighted results of one of the impact assessment methods are also presented. The energy categories are pre-

sented as MJ and the other categories as SEK. This is because weighted figures are used, but this does not affect the relationships within each

impact category. Three different versions of the impact category eco-toxicological impacts are presented. The differences are due to different

characterisation methods, EDIP and USES, and also to different weighting factors, USESmin and USESmax.

236 A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240

out subsequent to the surveyable time period studied

in the surveyable time scenario, the landfill option is

better off here concerning ecotoxicological impacts.

However, there are emissions of importance using the

toxicological impacts assessment methods presented

earlier, which are counted also in the surveyable time

scenario, for example emissions from vehicles used and

from landfill fires. The emissions from landfill fires that

are of most importance are dioxins, but also to some

extent PAHs. Ranking of waste treatment options con-

cerning ecotoxicological impacts gives landfill the rank-

ing second or last depending on the characterisation

method used for newsprint. In Fig. 3, the results for

the waste newsprint fraction are presented, while chan-

ges for PET are similar.

3.3. Carbon sink

If, with the limited time perspective, the landfill is

considered to be a carbon sink, an additional advan-

Fig. 1b. (continued)

A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240 237

tage for the landfilling option is gained. The additionalfunction of trapping biological carbon leads to morepreferable results for the landfilling option concerningglobal warming for newsprint. The resulting ranking isrecycling before landfilling before incineration. As canbe seen in Fig. 4, the difference between landfilling andincineration is small here. The plastic fractions are notaffected since their carbon content is of fossil origin.

3.4. Transportation of waste

Different distances for transportation of waste bytruck to treatment facilities do not influence the rank-ings of treatment options much [2]. However, transpor-

tation of waste from the household by passenger car tocollection points may influence the results significantly.This can be seen in a scenario where passenger cars areassumed to be used for transportation of sorted wastefor recycling and incineration. Major alterations in theresulting rankings are seen for the impact categoriesphoto-chemical oxidant formation and for human andecotoxicological impacts. In the toxicological cate-gories, landfilling is ranked as the most preferred alter-native in several cases, when the other options areburdened with passenger car use. In Fig. 5, the effectsof transportation in waste newsprint management areshown for the ecotoxicological impact category. It canclearly be seen that longer transportation by truck does

ategory global warming for waste PET. The results are shown for the base scena

period scenario.

ults for the impact category eco-toxicological impacts for the waste newsprint fractions using three different im

methods. The results are presented for the base scenario and for the surveyable time period scenario.

238 A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240

not affect the other options much, but when passengercar is used the ranking is altered. Similar changesappear for all waste fractions studied.In cases where new ways of landfilling waste are con-

sidered, e.g. biocells, source separation of differentwaste fractions may also be relevant for this optionand in these cases possible increased transportationmust consequently be taken into account.

3.5. Uncertainties

Many of the effects of altering assumptions andboundaries as described above can be seen in the toxi-cological impact categories. It should be noted that anyconclusions based on the results for these categoriesshould be drawn with extra care. For example, theuncertainty is assumed to be substantially lower for theglobal warming impact category, mainly depending onfewer data gaps and differences in the reliability of thecharacterisation methods available. In the toxicologicalimpact categories, the uncertainties are large. Uncer-tainties include uncertainties of used data, data gaps,and methods for comparing different toxicologicalimpacts and also for estimating the impacts of different

emissions, including cumulative and synergistic effects.Since emissions from landfills are also spread overlarge periods of time, actual emissions are not possibleto measure and models and assumptions used includeadditional uncertainties.Uncertainties in the data used can be large [31]. In

this case, the ecotoxicological impacts are dominatedby emissions of metals during the hypothetical infinitetime perspective. These emissions are calculated fromthe inputs and thus determined by analysis of metals inthe different materials, which can be done with reason-able accuracy. It is therefore likely that these emissionsare fairly reliable. Emissions of metals during the sur-veyable time period are more uncertain. It is howeverclear that only a small part of the landfilled metals areemitted during the surveyable time period [32]. Webelieve that data gaps are of greater concern. Uncer-tainties related to data gaps can be reduced by betterdatabases. However, especially for the toxicologicalimpact categories, there will probably always remainsignificant data gaps. This is because a comprehensiveevaluation is prohibited by the sheer number of chemi-cals used in society, combined with a lack of knowl-edge of the behaviour of all these chemicals in technicalprocesses (such as waste management processes) [33].Uncertainties in methods for comparing different toxi-cological impacts can be illustrated by using severalcharacterisation methods in parallel. As noted above,the two methods used here can, in some cases, give dif-ferent results, indicating this type of uncertainty.Despite the uncertainties, we believe it is better to tryto illustrate toxicological impacts in LCA than to leavethem out of the assessment. This is especially impor-tant since toxicological impacts in waste managementsystems are generally regarded as being of importance,and this is also illustrated in this case study (by com-paring the results from the ecotoxicological impactsand the total weighted results using USESmax in Fig. 1,it can be noted that the results for incineration andlandfilling are largely determined by the ecotox-icological impacts).

4. Conclusions

One basic difference in comparing landfilling ofwaste to other treatment strategies is that fewer co-functions are produced. Even though 50% of the land-fill gas is assumed to be collected and combusted withenergy recovery, this only makes up a part of thepotential resource that the waste may constitute iftreated by recycling or incineration. This is a drawbackfor the landfilling option.One conclusion of the results presented here is that

the waste hierarchy is valid as a rule of thumb. Thereare, however, certain assumptions and valuations that

Fig. 5. Results for the impact category eco-toxicological impacts for

newsprint, using the USES characterisation method with maximum

weighting according to the Ecotax 98 weighting method.

Fig. 4. The results for the impact category global warming for

waste newsprint. The results are presented for the base scenario, the

surveyable time period scenario and for the scenario where the land-

fill is regarded as a carbon sink.

A. Moberg et al. / Journal of Cleaner Production 13 (2005) 231–240 239

can lead to exceptions to this rule. Aspects of parti-

cular interest for the landfilling option are:

. The time perspective chosen. This concerns whichemissions are to be charged to landfilling of wasteand incineration ashes,

. With a limited time perspective, whether the landfillshould be credited for trapping biological carbon sofar not emitted to the atmosphere,

. Transportation of waste, if this is substantially lessin the case of landfilling compared to other wastetreatment options. This is in particular relevant fortransportation by passenger car.

A general conclusion is that assumptions and systemboundaries used, including value choices with ethicalaspects, are of importance when ranking waste treat-

ment options.It can also be noted that to improve the possibilities

for studying and assessing overall impacts of wastemanagement options, further work needs to be done,e.g. in the field of landfill modelling, especially concern-ing long-term processes, and on toxicological assess-ment in general. One aspect of possible concern is

emissions from landfill fires, which can have a potentialinfluence on ecotoxicological and human toxicologicalimpacts, especially in a shorter time perspective.

Acknowledgements

This work was financially supported by the SwedishNational Energy Administration. The full study [2]

including appendices is available on www.fms.ecology.su.se. These results were presented at the 1st Interconti-nental Landfill Research Symposium, 11–13 December2000 in Lulea, Sweden and at the International Work-shop on Systems Studies of Integrated Solid WasteManagement, Stockholm, April 2001. Comments given

on these occasions and by anonymous referees areacknowledged. Relevant parts are reprinted with per-mission.

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