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Geotextiles and Geomembranes 29 (2011) 298e312

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Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

An incremental model to assess the environmental impact of cap cover systemson MSW landfill emissions

Matthias J. Staub a, Giorgia Marcolina a,b, Jean-Pierre Gourc a,*, Raphaël Simonin a

a LTHE, Grenoble University, BP 53, 38041 Grenoble Cedex 09, FrancebUniversità degli Studi di Padova, Dipartimento di Ingegneria Idraulica Marittima Ambientale e Geotecnica, 35100 Padova, Italy

a r t i c l e i n f o

Article history:Received 21 July 2010Received in revised form26 November 2010Accepted 16 December 2010Available online 11 February 2011

Keywords:Landfill cap coversGeomembranesMethaneFugitive emissionsSustainable developmentModelling

* Corresponding author. Tel.: þ33 4 76 63 51 34; faE-mail address: [email protected] (J.-P. Gourc)

0266-1144/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.geotexmem.2011.01.013

a b s t r a c t

Landfill cap covers are designed to contain fugitive methane emissions and to prevent uncontrolledleachate infiltration. The objective of this paper is to propose a method for assessing their impact on themitigation of landfill greenhouse gas emissions using a new dedicated model (IMAGE-Landfill) that takesinto account the different steps of landfill management history as well as the incremental methaneproduction from the different cells. In particular, different collection and oxidation efficiencies areconsidered depending on the landfill operation, on its life step and on the chosen cap cover system. Twodifferent theoretical single cell case studies and one real-scale landfill were used to calibrate the IMAGE-Landfill model. The results highlight that, depending on the selected cap cover lining system, thepotential greenhouse gas emissions may be divided by a factor 2.5e7, even if no energy is recovered fromthe collected methane.

This model is a useful tool for landfill operators to model methane production, but also to select themost appropriate cover type for each step of the landfill lifetime. Finally, this research appeals for a betterconsideration of environmental impacts when designing landfill barriers.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Landfilling remains presently the first treatment option formunicipal solid waste (MSW) and other non-hazardous wastes inmost parts of the world. Different research programmes have beenlaunched in the past decades to optimize landfill operation and tomitigate its environmental impact as climate change and the claimfor alternative energy sources are more than ever present.

The environmental concerns about landfilling can be dividedinto two major parts:

- Migration of fugitive methane through the landfill’s cap cover,which is contributing to the greenhouse gases (GHG) emissionsto the atmosphere (Issue a);

- Leachate breakouts through the bottom liner and, conse-quently, groundwater pollution (Issue b).

Issue (b) has been far more investigated than issue (a) (e.g.Giroud, 1997; Rowe et al. 2004, 2007, 2009; Rowe 2005; Barroso

x: þ33 4 76 82 50 14..

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et al., 2006; Brachman and Gudina, 2008). The concerns of issue(a), i.e. the environmental control of fugitive emissions, can beaddressed by two different research themes:

- Issue (a-1) to deepen the knowledge of the bio-hydro-mechanical behaviour of municipal solid waste (MSW);

- Issue (a-2) to optimize the characteristics of the cap cover inorder to maximize biogas collection and energy recovery.

It is worth noting that the social acceptance of landfills isstrongly dependent on its image, which is conveyed by the engi-neers and operators: this paper intends to present an objectivemethod to assess the environmental impact of the cap coversystem quantitatively in order to control the fugitive emissions ofbiogas.

Landfill gas (LFG, or biogas) fugitive emissions are one of themajor environmental issues related to sanitary landfills. LFG isroughly composed of 60% methane (CH4), 40% carbon dioxide (CO2)and more than 150 trace compounds (Reinhart and Townsend,1997). Methane and carbon dioxide are both greenhouse gases(GHG) that contribute actively to global climate change. To comparethe influence of the different GHG, the Intergovernmental Panel onClimate Change (IPCC, 2007) has introduced the notion of global

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Notations

A, B, C, D steps of landfill managementBMP biomethane potential (Nm3 CH4 t�1 dry matter)CE collection efficiency (%)CH4 methaneCO2 carbon dioxideDM dry matterEE energy efficiency (%)GHG greenhouse gasGWP global warming potentialHcell(i) height of the cell (i) (m)i, j, m, n summation indexesk CH4 generation rate (years)LCA life-cycle assessmentLFG landfill gasMSW municipal solid wastemj gravimetric fraction for the component j of the dry

wasteq(i) annual mass of waste buried (wet t year�1)Q waste quantity (t wet matter)

Qcell(i) waste quantity buried in cell (i) (t wet matter)N number of landfill cellsOE oxidation efficiency (%)RE recovery efficiency (%)rh average humid density of waste (tm�3)s shape parameter (years�1)Scell(i) surface of the cell (i) (m2)t(i) time from the beginning of landfilling of cell (i) (years)tc(i) time of completion of a landfill cell (i) (years)T absolute time from the beginning of landfilling (cell

(i)¼ 1) (years)s relative time for a landfilled waste material, starting at

its disposal (years)s0B relative time for a landfilled waste material when

methane production starts (years)s1/2 relative time for a landfilled waste material when half

the methane has been produced (years)Vcell(i) volume of the cell (i) (m3)w water content (kg kg�1)Y instantaneous methane production (Nm3 t�1 dry

matter)

Table 1Landfill cell management steps and associated typical durations.

Steps A B C D

Period Operation Post-operation Aftercare Custodial careDuration 1 year 2 years 28 years 70 years

M.J. Staub et al. / Geotextiles and Geomembranes 29 (2011) 298e312 299

warming potential (GWP). As the GWP of carbon dioxide is equal to1, the one of methane is 25 for a time horizon of 100 years. In otherterms, 1 kg of CH4 is accounted for 25 kg of CO2 equivalent.Worldwide landfill emissions are difficult to quantify due to theabsence of data for numerous countries, but estimations range from3% to 4% of the overall methane emissions (Spokas et al., 2006).

Several previously conducted studies are based on LFGproduction models that are not suitable to describe the LFGproduction of an entire landfill site that includes several cells(Scharff and Jacobs, 2006). Moreover, predetermined LFG collectionefficiency values valid for the entire landfill at whatever time aregenerally considered. However, it is acknowledged that thecollection efficiency evolves during the landfill history (Spokaset al., 2006). Hence, this research proposes an incremental LFGproduction model to tackle these two serious drawbacks. Theemissions of each single cell are considered separately withdifferent cap cover efficiencies depending on the operation stepand type of liner.

A program based on the above hypotheses has been designed topredict global fugitive emissions from landfills. This program aimsat being a useful tool for landfill operators to model methaneproduction, to select the most appropriate cover type in relationwith the most suitable treatment for methane during all themanagement steps of the landfill.

The aim of this study is to propose amethod to link landfill coverdesign to the complex mechanisms of landfill methane emissions.Hence, this research may help to quantify the possible emissionmitigation strategies for landfills with different types of cap coversand to assess the sensitivity of their environmental performance onlandfill design and operation. In this context, geosynthetics couldplay a key role. The conception of a model that can simulate landfillmanagement history, using time depending variables, in particularthe evolution of the cap cover collection efficiency of methaneduring landfill life, is innovative.

This study is not a life-cycle assessment (LCA). In a LCA, all thewaste flows, resource consumption and environmental emissionsfrom waste management systems are considered. Here, focus isgiven to the GHG emissions fromwaste degradation, not taking intoaccount all the other sources of emissions.

2. Material and methods

2.1. Different periods in the landfill cell life and associated biogascollection systems

Landfill cap covers constitute the active barrier which could stopor rather mitigate the fugitive methane emissions. Therefore, capcovers are a key component of modern landfills. Their role iscomplex: they must ensure a safe biogas collection, but also anefficient drainage of rainfall infiltration while enabling leachateinjections.

As far as fugitive emissions are concerned, different periods inthe life of a landfill cell should be considered. In fact, the best liningsystem depends upon the biodegradation state of the waste. Itseems impossible for now to find a unique cover fulfilling thedifferent conditions during all the life of a landfill cell. Four differentsteps should be considered for the landfill cell life (Table 1):operation (step A), post-operation (step B), aftercare (step C) andlong-term custodial care (step D) (Table 1). Corresponding dura-tions are proposed, based on legislation, operational practices andthe authors’ expertise (Gourc et al., 2010a,b). However, furtherdiscussion on this issue may be required. For each landfill step, thecover installed is characterized by a different value of collectionefficiency (CE e see Eq. (1)). This parameter represents the part ofmethane that is actively collected by the LFG collection system.

CE ¼ Collected CH4

Produced CH4(1)

Regardless of the considered cap cover, it is acknowledged thatfugitive methane passing the cap cover system is partially oxidized(Scharff and Jacobs, 2006; Spokas et al., 2006) according to thechemical reaction CH4 þ 2O2/CO2 þ 2H2O.

1 2 3 4 5 6 7 8 0

0.5

1

1.5

2

Time (years)

Typi

cal s

ettle

men

t (m

)

Permanent cover placement Permanent cover

0

Waste settlement

Temp. cover

Step B

Step C

Fig. 1. Landfill settlement and cap cover implementation in the first years afteroperation.

Fig. 2. Geosynthetic temporary cover (step B) on a French landfill.

M.J. Staub et al. / Geotextiles and Geomembranes 29 (2011) 298e312300

This passive methane oxidation will be considered in theassessment of GHG fugitive emissions. Similarly, an oxidation effi-ciency (OE) is considered for each step of landfill management.

OE ¼ Oxidised CH4

Uncollected CH4(2)

With the uncollected methane calculated by:

Uncollected CH4 ¼ Produced CH4$ð100%� CEÞ (3)

Eqs. (2) and (3) enable to deduce the CH4 fugitive emissionsfrom the CH4 production (Eq. (4)):

Fugitive CH4 ¼ Produced CH4$ð100%� CEÞ$ð100%� OEÞ (4)

while Eq. (4) gives the general formula to assess the fugitiveemissions from landfills, the major difficulties come from theevaluation of the CH4 production as well as from the OE and CEthroughout the landfill cell’s lifetime. These parameters depend onthe landfill management step. In addition, the CH4 productiondepends on the disposed waste, on time, on the landfill manage-ment practice as well as on the environmental conditions. The nextsubsections describe the main features of each landfill manage-ment step.

2.1.1. First step (A): landfill cell in operationDuring disposal and compaction of the waste, there is no actual

cover. The average duration for the filling operation of one cell istypically one year (Table 1). Apart from the addition of daily cover orof a light geosynthetic layer, the cell is open to transfers of biogas andrainfall water. In modern landfills, partial collection of biogas isdifficult but possible, using horizontal and vertical drain pipesinstalledduringoperation.However, their collection efficiency (CEA)remains low compared to the other landfill management steps.

During construction, due to the lack of a top cover, no oxidationis considered (OEA¼ 0). However, some attempts to use specificgeosynthetics covering the waste surface during operation arereported in the literature, such as “biotarps”which would have thepotential to oxidize methane emissions (Huber-Humer et al., 2008).

2.1.2. Second step (B): post-operational periodMSW is generally very compressible, with a settlement rate

decreasing with time (Dixon and Jones, 2005; Gourc et al. 2010b).Not only average top settlements but also differential settlements,observed by means of profile meter probes or landmarks, aresubstantial (strains exceeding 20%), increasing significantly the riskof the development of severe cracks in the cover system (Gourcet al., 2010a).

The final cap cover very often includes a 1-m layer of fine-graded soil. Large-scale experimentations demonstrate that sucha fine soil layer subjected to differential settlement displayscracking for very small extensions (1e2%) due to bending (Campet al., 2009, 2010; Gourc et al., 2010a).

To avoid cracking due to the settlement of the waste mass, theimplementation of the “permanent” cover can be postponed (Fig. 1;Gourc and Staub, 2010). The updated technique for top liningconsists in using a geosynthetic temporary cover during the firstperiod of high settlements rates (around two years) (Fig. 2). Hence,settlements corresponding to the two first years of operation occurwhile this temporary cover is in place.

2.1.3. Third step (C): aftercare periodAfter about two years an important fraction of the superficial

settlement has been completed and a permanent cover isimplemented. In order to prevent the slowing or the end of

biodegradation, a minimum moisture content of the waste shouldbe maintained. This is the origin of the two proposed structures forthe cap cover (Staub and Gourc, 2010) (Fig. 3):

Scenario 1, “Semi-permeable barrier”: a “semi-permeable” barrieris installed. This complies especially with the current Frenchlegislation. The barrier allows a limited rate of infiltrating waterinto the waste body, but presents the disadvantage of allowing theemission of landfill biogas to the atmosphere as well. This “semi-permeable” cover is generally implemented immediately after theend of operation on conventional landfills (Step 1). There is a lack ofdata concerning long-term collection efficiency of this kind of capcover, which certainly decreases with time.

Scenario 2, “Impermeable barrier”: an “impermeable” barrier,which may include a geomembrane and prevents the penetrationof uncontrolled water into the waste mass, is installed. This struc-ture should be coupled with a system of water/leachate injection inorder to avoid a state of “dry tomb”. It has been demonstrated by insitu measurements that horizontal trenches induce a betterhomogeneous distribution of moisture into the waste mass thanvertical wells (Clément et al., 2010), but new geosynthetics, locatedbeneath the structure of the barrier (Fig. 3) could also be designedto play this role of liquid distribution into the waste mass. Newgeosynthetics could be added under the impermeable cover, inplace of the pipe depicted on Fig. 3b in order to improve thedistribution of leachate and consequently the homogeneous

MSW

CH4 + CO2 Semi-

permeable

cover

CO2 + CH4LFG production

Biocover CH4 + CO2CH4 + CO2

Fig. 4. Principle of a biocover (final cover for step D).

Natural soil

DL

DL

Waste

K<10-6 m/s

Natural soil

DL

DL

Waste

K<10-9 m/s

Impermeable cover Semi-permeable cover ba

Geomembrane/GCL

Biogas LeachateBiogas

Fig. 3. Typical permanent cover systems (step B and/or C). DL¼ drainage layer.


wetting of thewaste. It is worth noticing that it is not uncommon tofind landfills using impermeable cap covers without water/leachateinjection systems, which require a sophisticated technique. Thisconcept of cap cover does not comply with the objective ofsustainability, since the biodegradation process is uncontrolled andprematurely stopped.

Generally, a completion time of 30 years after the end of oper-ation is considered for the aftercare. This traditional waiting time iscurrently contested and international working groups are trying topropose different thresholds related to the remaining hazardouspotential of the waste. In fact, the criteria for the completion oflandfill aftercare usually depend on biogas quantity or leachatequality thresholds that still need to be defined (Stegmann et al.,2003).

2.1.4. Fourth step (D): long-term custodial care periodOnce the aftercare period is completed, an active monitoring of

the site is no longer required. The operator is no longer in charge ofthe facility, which is assumed to bear no significant residualharming potential for the environment or health. At present time,there are only very few controlled landfills in existence which havereached this custodial care period.

The values for the collection efficiency CED (Table 2) stronglydepend on the cap cover type which is installed on the landfill(Spokas et al., 2006). As the entire time span considered is 100years (arbitrary value), the custodial care period is assumed to last

Table 2Steps for gas collection efficiency (CE) and gas oxidation efficiency (OE) for one cellin the different scenarios.

Steps A B C D

Period Operation Post-operation Aftercare Custodial careTime of

completiontc tcþ 2 years tcþ 30 years 100 years

Landfill with semi-permeable cap coverCover type None Permanent

semi-permeablePermanentsemi-permeable

Final

CE CEA¼ 35% CEB¼ 65% CECa¼ 50% CED¼ 0%OE OEA¼ 0% OEB¼ 20% OEC¼ 20% OED¼ 30%

Landfill with impermeable cap coverCover type None Temporary Permanent

impermeableFinal


a As long as LFG collection is maintained, but drops to zero when LFG collection isstopped.

for 70 years. During this period, the integrity of the permanentcover may be altered, and so is its collection efficiency. There isa lack of data concerning long-term efficiencies of cap covers,which may decrease with time. For instance, their adaptability todifferential settlements is reported to be low (Gourc et al., 2010a).What is more, at the end of aftercare, biogas is no longer collected,which means that the collection efficiency drops to zero duringstep D. It should be noted however that the emission level after 30years of waste disposal is relatively limited.

In addition to the natural oxidation of methane, differentoptions corresponding to systems which are still at an experi-mental level, could be suggested to enhance the oxidation ofresidual methane fluxes during custodial care period:

- biocovers designed to cover the entire surface of the facility,replacing the upper part of the semi-permeable cover (Huber-Humer et al., 2008) (Fig. 4);

- biowindows, integrated to the cap cover (Fig. 5) or biofilters,which may be deported (Fig. 6), being placed at the end of thebiogas drainage system (Gebert and Gröngröft, 2006; Cabralet al., 2010).

These systems, which alterate the characteristics of the capcover, must not be introduced before the end of the aftercare period(step C). Biocover systems could be preferentially used with a finalsemi-permeable cover. In this case, methane can diffuse through allthe landfill’s surface, and it is necessary to have an oxidation layerthat covers the entire surface of the facility (Fig. 4). This layerusually consisted of a coarse gas distribution layer to balance gasfluxes placed beneath an appropriate substrate layer, in whichmicro-organisms can grow (Huber-Humer et al., 2008). Thesubstrate has to have sufficient air-filled porosity in order to allowdiffusion of oxygen in the oxidation layer. Moisture content isanother fundamental parameter affecting the biological oxidationof methane.

MSW

CH4 + CO2 Impermeable

cover

CO2 + CH4LFG production

Biowindow CH4 + CO2CH4 + CO2

Fig. 5. Principle of a biowindow (integrated in the final cover for step D).

MSW

Biofilter unit

Impermeable

cover

LFG production

LFG collection

CH4 + CO2

CO2 + CH4

Fig. 6. Operation principle of a biofilter (added to the cell during step D).


Biowindow or biofilter systems are used on landfills with animpermeable final cover and are designed to cover only some partof the landfill surface. Biowindows are built by locally removing theimpermeable cover and replacing it (Fig. 5). The materials are veryclose to the ones used for biocovers, but biowindows are muchthicker. Instead, biofilters use the existing cap cover and collectionpipes to direct biogas to one central biofilter unit that replaces theflare, which is no longer working during the custodial care period(Fig. 6). The filter bed itself is relatively small (from 1 m2 toapproximately 100 m2) (Zeiss, 2006). In this study, we will onlyconsider passive biofilters, with no active collection of biogas.Geosynthetics could be associated to these oxidation systems, butthese specific facilities are still under development.

2.2. Methane and LFG production and modelling

The production of landfill gas (LFG) is a consequence ofbiodegradation of the organic fraction of MSW that is caused by theaction of bacteria and other micro-organisms in wet conditions. Atthe time of waste placement, air is present in the voids, and thisaerobic phase is responsible for CO2 production from the organicsolids during the exothermic aerobic reaction. Oxygen depletionwithin the landfilled waste marks the onset of the anaerobicdecomposition phase, methanogenesis, which mainly results inCH4 and CO2 production. In this paper, only methanogenesis ismodelled. Consequently the methane production starts at thebeginning of this phase from a relative time called s0B. The LFGconsidered is composed by 60% methane (CH4) and 40% carbon

0

1000

2000

3000

4000

5000

0 5 10 15 20

Nm

3 C

H4 / y

Fig. 7. Instantaneous meth

dioxide (CO2) during methanogenesis, however, a value of 50% CH4and 50% CO2 over the entire lifetime of a landfill seems morerealistic to be considered in the study (El-Fadel et al., 1997).

According to the IPCC calculation principle to assess the GHGemissions, as noticed in Section 1, carbon dioxide that is emittedfrom biogenic sources (i.e. emitted during the aerobic degradationphase aswell asCH4 converted intoCO2byflaringoroxidation) is notincluded in theemission calculation. Instead,methaneemissions areincluded in the calculation of emissions, due to the high GWP ofmethane, even though theyareof biogenicorigin. This iswhy focus isgiven on the methane, and not on the entire LFG production.

To estimate the methane production from a typical landfill cell,the SWANA model is considered here. It was also chosen by otherauthors to assess GHG emissions from landfills (Camobreco et al.,1999). This model describes the instantaneous methane produc-tion according to a first-order production rate based on Eq. (5):

YðsÞ ¼ BMP$Q

1þw$k$e�k$ðs�s0BÞ$

kþ ss

$�1� e�sðs�s0BÞ

�(5)

with s the relative time from the beginning of waste landfilling, andY(s)¼ 0 for s� s0B, s0B being the relative time when methaneproduction starts for the corresponding waste (years). In the aboveequation, Y(s) is the methane yield in Nm3 years�1, BMP is thebiomethane potential in Nm3 t-1 DM, Q is the mass of waste buriedin the cell inwet tons,w is the average gravimetric water content ofthe waste (kg kg�1), k is the CH4 generation rate in years�1, s is theshape parameter in years�1 and s0B is the time when methano-genesis starts in years. The BMP is the maximum volume ofmethane which could be produced by a ton of dry waste. It can beassessed by a biochemical standard test or using a stoechiometricapproach, taking into account the different organic fractions.

Fig. 7 shows the instantaneous methane production Y(s), andFig. 8 represents the cumulative methane production for thousandtons of dry waste in one cell using the SWANA model.

The delay between the time when biodegradation and meth-anogenesis start is short in comparison with the active lifespan ofa real landfill cell (here, 100 years). Here, s0B is equal to a couple ofmonths.

The value for the CH4 generation rate k can be determined usingthe half-life value s1/2 when half of the expected CH4 has beenproduced (USEPA, 2005) and s0B is the time when methanogenesisstarts:

25 30 35 40 45 50

ane production Y(s).

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40 45 50

Nm

3 C

H4 / t D

M

BMP

BMP /2

τ (in years)

τ1/2

τ0B

Fig. 8. Cumulative methane production.


k ¼ lnð2Þs1=2 � s0B

(6)

For the following calculations, values for the CH4 productionparameters are proposed based on literature data and personalcommunications from operators.

2.3. Methane treatment

Fig. 9 shows the fate of methane from its production to itsemissions or collection. The collected methane is either flared orused to generate electricity and/or heat:

- flares simply convert methane to carbon dioxide and hencereduce potential methane emissions drastically. The consid-ered burning efficiency of 100% is disputable but no data attestsfor lower values provided that the flare works under normalconditions;

- here, energy recovery is considered as long as it is technicallyand economically viable. It is considered that 80% of thecollected methane is used for energy recovery as long as theincoming flow is sufficient for the energy recovery plan andthe remaining 20% are flared (personal communication). The

MSW

Atmosphere

Top soil

Biogas leakage

Collected

Methane

Uncollected

Methane

Flaring and/or

co-generation

(CO2)

Oxidation (CO2)

(CO2) + CH4

CH4

Fugitive

Fig. 9. Fate of methane from its production to its emission, oxidation or recovery.

energy recovery plant is assumed to achieve a high perfor-mance due to cogeneration technology (electricity generatedfrom a turbine as well as heat generation). An energy efficiency(EE) of 60% is considered for the cogeneration plant, taking intoaccount the variability in quality and quantity of the resource(FNADE, 2007).

The thresholds corresponding to the minimum possible flow totreat considered in this study are 125 Nm3/h of methane for energyrecovery and 35 Nm3/h for the flare; both values are based onFrench recommendations for an average landfill size (INERIS, 2005).Of course, these systems are generally used for a group of landfillcells. All collectedmethane is eitherflared, used for energy recovery,or both, in accordance with French and European legislation.

The uncollected methane may be partially oxidized in the topsoil of the permanent or the final cap cover, or using biocovers,biowindows or biofilters (Section 2.1). Natural oxidation or atten-uation of methane emissions in the temporary or permanent capcover, occurring throughout the landfill’s lifetime, are distinguishedfrom enhanced oxidation during the last management step thanksto biocovers, biowindows or biofilters, as explained in Section 2.1.

2.4. Methane fugitive emissions depending on landfill cap coverdesign

One of the scopes of this study is to compare the environmentalperformance of two different types of landfills: landfills equippedwith semi-permeable and impermeable covers. These two casesdiffer in both CE (during the whole landfill lifetime) and also OE(during the custodial care period) (Table 2):

- for a landfill with a semi-permeable cover, the CE-value isexpected to be 65% when LFG collection starts (Spokas et al.,2006). The permanent semi-permeable cover remains on thelandfill during the aftercare period. The top cover can besubject to damages, as cracking is due to highwaste settlement,which happens preferentially in the first years of wastedisposal (Fig. 1). As a consequence, during aftercare, thecollection efficiency of the lining system is lower, here 50%;

- for a landfill with an impermeable cover, a temporary cover isinstalled during post-operation and a permanent impermeable

tc(i)/20 t(i)

V(i)

0tc(i)

Operation (A) Post-Operation (B)

Vcell(i)

Simplified filling

process

Realistic filling process

Fig. 10. Real and simplified filling cell procedures for a given cell (i).


cover is placed during aftercare, with a CE of 65% and 90%respectively (Spokas et al., 2006). Finally, in the custodial careperiod, the monitoring of the site is stopped and consequentlythe CE is equal to 0, because of the absence of an active systemto collect and treat remaining emissions.

These different values for CE are average values from the liter-ature, or come from the own knowledge of the authors andpersonal communications.

Concerning the oxidation efficiency of the top cover, in bothcases an OE-value of 20% during post-operation is considered. Thisvalue remains the same during the aftercare period for the landfillwith a semi-permeable cover, but for the landfill with an imper-meable cover, a value of 10% is assumed (Table 2). The reason for thatis this type of cover is characterized by punctual emissions (hot-spots), in theweakness points of the geomembrane (welding points,punctual holes.) (Scharff and Jacobs, 2006). Due to these localizedhot-spots with high flowrates, the OE is assumed to be lower.

During custodial care, afinal cover corresponding to an alterationof theprevious permanent cover is installed.Hence, a different valueof OE is assumed, according to the different type of top cover:

- in the landfill with a semi-permeable cover, the methane maypass through the entire top cover and be oxidized. In thisphase, the biogas flow rate is lower than in the other operationsteps, and a biocover may be installed. As a consequence, theoxidation efficiency is improved and it has been set to 30%;

- in the landfill with an impermeable cover, a passive system toenhance methane oxidation may be installed. Typically, thesefacilities, such as biofilters or biowindows have an improvedoxidation efficiency of 60% (Humer and Lechner, 1999;Hettiaratchi et al., 2000).

All the characteristics of the two theoretical considered casesare reported in Table 2. The times considered for each step are inagreement with Table 1.

2.5. Incremental modelling of landfill emissions e the IMAGE-Landfill model

For a landfill cell, the waste is disposed in layers during theoperation period. The SWANAmodel was calibrated for the averagemethane production on landfill cells, and hence integrates theproduction of the several waste layers which have been disposedone after the other. For the methane generation parameters,a typical French MSW is considered (see Section 3.1.1).

A landfill includes N cells which are assumed to be successivelycompleted. An absolute time T is considered for the globalconstruction of the landfill with T¼ 0 at the beginning of theconstruction of the first cell (i¼ 1).

The geometry of the cells has been simplified in this paper tocorrespond to a cuboid of horizontal surface Scell and height Hcell.This was chosen in order to simplify the cell’s filling history. Thecell’s (i) typical filling operation is shown in Fig. 10: the operationtime of cell (iþ 1) starts when the cell (i) is totally filled. A cell (i) iscompleted when its volume Vcell(i) is entirely filled with theincoming waste, for a time tc(i):

VcellðiÞ ¼ ScellðiÞ$HcellðiÞ ¼ QcellðiÞrh

(7)

where Vcell(i) is the volume of the cell (m3), Scell(i) and Hcell(i) arerespectively the surface and the height considered for the cuboid,and rh is the average humid density of the waste (rounded at1.1 tm�3 for the case studies considered here).

The time of construction tc(i) is obtained by Eq. (8):

tcðiÞ ¼ QcellðiÞqðiÞ

(8)

where q(i) is the annual mass of waste buried (wet tonne/year), tc(i)is the time of construction (years) for the whole cell.

Fig. 10 shows the realistic and the chosen simplified construc-tion process for one landfill cell (i), which starts at t(i)¼ 0 and endsat t(i)¼ tc(i). The CH4 production model SWANA is used for eachsingle cell (i), starting after an initial lag-time of s0B after the wastedisposal. The CH4 production for a given cell (i) is calculated as if thecomplete filling was instantaneous at time t(i)¼ tc(i)/2.

The timewhenmethanogenesis starts for thewhole cell is notedat t0B. According to these simplifications, methane production forthe entire cell (i) is expected to start at time:

t0BðiÞ ¼ tcðiÞ2

þ s0B (9)

The value for s0B used in the present study is 2 months (i.e. 0.17year) as indicated in Section 2.2.

The entire CH4 production corresponds to the integration of theCH4 production from each single cell. CH4 production of cell (i) canbe inferred at any time T:

T ¼ tðiÞ þXi�1

m¼1

tcðmÞ (10)

The total instantaneous CH4 production for the landfill at time Tfrom the beginning of landfilling is equal to:

YðTÞ ¼Xi

m¼1

YcellðmÞ�tðmÞ

�(11)

To assess CH4 production and diffuse GHG emissions of landfills,an “incremental model for the assessment of greenhouse gasemissions from landfills” (IMAGE-Landfill model) has beenproposed. A program was written in MS Visual Basic because of itscapacity to interact withMS Excel. The structure of the program canbe summarized by the following:

1. The user enters the input data of the site:a. Geometrical parameters (N, Hcell(i), Scell(i), q(i));b. Waste physical parameters (rh, w);c. Biogas parameters (BMP, k, s, t1/2);

2. The program calculates the variables in subsequent steps:a. The instantaneous and cumulative biogas production for

each cell;


b. The quantity of collected biogas for each cell;c. The quantity of biogas used for energy recovery and amount

of energy generated;d. The environmental impact of landfill: GHG emissions;

3. Output data and the related graphs are automatically created ina spreadsheet.

3. Modelling results: application to theoretical and real casestudies

3.1. Application of the IMAGE-Landfill model to a theoretical singlecell

3.1.1. Hypotheses for a theoretical single cellIn France, landfilled waste is composed of approximately 50%

municipal solid waste (MSW) and 50% non-hazardous industrialwaste. Here, a combination of typical French MSW and averagenon-hazardous industrial waste is considered (Fig. 11). The bio-methane potential (BMP) is the total (potential) production ofmethane when the entire biogenic carbon is fully degraded. Thecalculation of BMP takes into account the BMPj of each component(j) of the waste, according to Machado et al. (2002). Eq. (12) showsthe methodology of the calculation:

BMP ¼Xn

j¼1

BMPj$mj (12)

where BMP is the biomethane potential for the total waste, BMPj isthe biomethane potential for the component j of waste, mj is thegravimetric fraction of the component j of the drywaste and n is thenumber of components in which the waste is divided. The value ofBMPj for each component j of waste is taken from the literature(Machado et al., 2002 and references therein). The BMP value canalso be inferred from standard tests. This waste has an estimatedBMP of 80 m3 CH4 t�1 DM. From the different fractions of Fig. 11,only the normally and slowly biodegradable componentscontribute to the potential methane emissions, the readily biode-gradable materials (putrescibles) degrading during the first aerobicstage of biodegradation (Reinhart and Townsend, 1997).

To estimate the CH4 production, two different hypotheses weremade concerning the operation:

- for the landfill with semi-permeable cover, a value ofk¼ 0.1 years�1, i.e. a half-life value s1/2¼ 7.5 years is proposed;

0%

4%

8%

12%

16%

20%

Putres

cible

waste

Fine w

aste

Papers

, card

board

Textile

s

Wood a

nd co

mbusti

bles

Pro

po

rtio

ns (%

DM

)

Fig. 11. Gravimetric fractions mj of every component j of the

- for the landfill with impermeable cover, it has been assumedthat active leachate injection was made to maintain highmoisture contents (bioreactor principle), so a value ofk¼ 0.2 years�1, i.e. a half-life value s1/2¼ 4.5 years is proposed.

The theoretical case study is a single cell of a French landfill ofaverage size (q¼ 165,000 t wet matter year�1). Regulation imposesa cell area of 5000 m2, and landfill depths usually reach around30 m. The average humid density considered for the waste isrh¼ 1.1 tm�3. This means that the filling time is exactly one yearhere. In both cases, methane production is assumed to start att0B¼ 0.67 years (Eq. (9)) after the first waste layer is disposed in thecell.

The fate of methane has been extensively discussed in Section2.3. The general thresholds on a whole landfill are considered to be125 Nm3/h of methane for energy recovery and 35 Nm3/h for theflare, as exposed earlier (INERIS, 2005). Based on the assumptionthat the collection and recovery systems are shared by 10 cells,a minimum threshold of 12.5 Nm3/h of CH4 was defined for energyrecovery for one cell. Similarly, once the methane flow has reachedthis minimum limit, a simple flare is maintained onsite during theaftercare period until methane production decreases to 3.5 Nm3/hof CH4.

3.1.2. Results for a single cellFig. 12 shows the methane production and collection, the

quantity used for energy production and the quantity of flaredmethane. Fig. 12a shows the results for the landfill with semi-permeable liners, and Fig. 12b shows the results with impermeableliners. CE-values are taken from Table 2. All the graphs are reportedwith a methane flux in Nm3/h, because for the design of the landfillcomponents, this measure unit is more significant.

The durations for energy recovery and flaring are also high-lighted. The effect of the cover type scenario on the methanetreatment strategy is remarkable (Table 3). The duration of energyrecovery is the same in both cases (15 years), but the respectivequantities of methane utilised for energy production are different.

In the semi-permeable landfill (Fig. 12a), there is a discontinuityin the methane collection curve due to the different CE-values ofthe semi-permeable cover. In the impermeable landfill (Fig. 12b),the main production of biogas takes place in the first steps oflandfill management, and the presence of an impermeable coverwith higher collection efficiency associated to moisture addition inthe form of leachate allows using a higher quantity of methane for

Plastic

sGlas

s

Metals

Inert M

ateria

ls

Specia

l Was

te

Readily biodegradableBiodegradableSlowly biodegradableInert fossilInert

waste considered for the evaluation of GHG emissions.

Table 3Time and flow thresholds for one cell for the flare and the energy recovery plant.

Time threshold Methane flow threshold

Landfill with semi-permeable cap coverEnergy recovery deadline 15 years 12.5 Nm3/hFlare deadline 30 years 3.5 Nm3/h

Landfill with impermeable cap coverEnergy recovery deadline 15 years 12.5 Nm3/hFlare deadline 22.5 years 3.5 Nm3/h


energy production than in the other case. Hence, obviouslyimpermeable covers might be seen as incentives for enhancedrecovery rates, which, in turn, may benefit financially to the oper-ators as the sale of the generated energy may reduce operationcosts on the long-term. The flare is stopped onsite 30 years after thebeginning of waste disposal for the semi-permeable landfill cell and22.5 years after waste disposal for the impermeable landfill cell(Table 3). Once the flare is stopped, the methane is no longercollected (CEc¼ 0%) and only methane oxidation occurs.

The quantity of methane that is oxidized by the action ofbacteria in the top cover and the fugitive methane for bothscenarios is shown in Fig. 13.

The cumulative emissions over the entire landfill lifetime (here,100 years) are given in Fig. 14. The potential emissions are calcu-lated from the BMP of 80 Nm3 CH4 t�1 DM (¼57 kg CH4 t�1 DM),and given that the GWP of CH4 is 25 times the one of CO2(¼1429 kg CO2-eq t�1 DM). The use of a biogas collection andtreatment system divides potential emissions by a factor 3 witha semi-permeable cover and by approximately 7 with an

0

10

20

30

40

50

60

70

80

90

0 5 10 15

Absolute tim

Nm

3 C

H4 / h

CE=65% CE=50%

0

20

40

60

80

100

120

140

160

0 5 10 15Absolute tim

Nm

3 C

H4 / h

CE=65% CE=90%

b

a

Fig. 12. Methane production and fate for the landfill wi

impermeable cover. The net balance is even better if the avoidedemissions from energy recovery are considered.

When the collected methane is used to generate energy, thisenergy substitutes the production of energy from other productionsources (substitution principle). Hence, methane recovery allowssubtracting from the overall GHG balance the emissions associatedwith the production of the same amount of energy (using thenormal electricity grid) (Camobreco et al., 1999). To account for

20 25 30 35 40

e (T) in years

Semi-permeable cover

CH4 ProductionCH4 Collection

CH4 Used for energy recovery

CH4 Flared

20 25 30 35 40e (T) in years

Impermeable cover

CH4 Production

CH4 Collection

CH4 Used for energy recovery

CH4 Flared

th semi-permeable (a) and impermeable (b) cover.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25 30 35 40

Absolute time (T) in years

Nm

3 C

H4 / h

Semi-permeable coverFugitive CH4Oxidised CH4Collected CH4

Fugitive CH4Oxidised CH4Collected CH4

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30 35 40Absolute time (T) in years

Nm

3 C

H4 / h

Impermeable cover

a

b

Fig. 13. Fate of methane in the landfill with semi-permeable (a) and impermeable cover (b).


these substituted emissions, a lower heating value of 35.8 MJ/Nm3

for methane was considered. Avoided emissions are taken intoaccount for 551 g CO2-eq/electric kWh (Lombardi et al. 2006;FNADE, 2007) and 242 g CO2-eq/thermic kWh (RTE-ADEME,2007; FNADE, 2007), with 1 kWh¼ 3.6 MJ.

Considering the total reduction of emissions after treatment,93% of the initial potential emissions may be mitigated when animpermeable cover and a biowindow or a biofilter is installedduring the custodial care period. However, this figure drops to only60% when a semi-permeable cover and a biocover are installedduring custodial care (see Fig. 14).

3.2. Application of the model to a large-scale case study on a Frenchlandfill

3.2.1. Hypotheses for the large-scale case studyThe considered case study is a French privately owned site with

a waste load of q¼ 50,000e415,000 wet tonnes year�1 (40%municipal waste, 40% commercial waste, 20% bulky waste andother inert materials). This site corresponds to a well-operatedconventional landfill with active LFG recovery and it is still in

operation. Data is available from the operator since the opening ofthe landfill, with the total expected operation time of the landfillbeing 21 years and N¼ 57 cells. Table 4 summarizes the main sitecharacteristics.

Fig. 15 shows the cumulative volume V(T) of waste disposed forthe whole landfill, similarly to Fig. 10 for one single cell, calculatedby:

VðTÞ ¼XN

i¼1

VcellðiÞ (13)

according to Eqs. (7) and (10).The waste disposed in this landfill has an average BMP of

64 Nm3 t�1 DM (Eq. (12)). To assess the methane production, theother parameters used were k¼ 0.2 year�1, i.e. s1/2¼ 4.5 yearsbecause landfill operation was targeting high initial moisturecontents (USEPA, 2005).

As for the theoretical case study (Section 3.1), at the end ofoperation step for every cell a temporary cover is placed beforea permanent cover is installed onsite. The permanent cover isimpermeable, but the assessment of its collection efficiency (CE)

1429

583

1429

232

-53 -108

-200

0

200

400

600

800

1000

1200

1400

1600

Potential emissions Fugitive emissions afteroxidation

Avoided emissions (energyrecovery)

kg

C

O2 eq

./t D

M

Semi-permeable coverImpermeable cover

Fig. 14. Simulated overall emissions and substitutions for both single cell scenarios.


will be made by calibration on recorded methane collection ratesand modelled methane production. Data related to the biogascollection is known for the 12 first years of operation.

3.2.2. Results: assessment of methane production and IMAGE-Landfill model calibration

As explained earlier, cap cover placement is made successivelyfor every cell in 4 different steps (Table 1). The aftercare period isconsidered finished simultaneously for all N cells when no longermonitoring of any cell is required. In order to calibrate the modeland to assess the collection efficiency of the permanent cap cover,precise measurements of diffuse emissions were provided (Spokaset al., 2006). The collection efficiency of the permanent cover iscover-dependent and hence site-dependent.

The procedure to validate the model can be summarized in twosteps:

- First, the actual CH4 production is, up to now, impossible todetermine by any other means than modelling. The methaneproduction is modelled using the parameters given fromlandfill operators (Fig. 16). The parameter k (or s1/2) is esti-mated. The values for q, w, rh, Scell(i) and Hcell(i) are directlytaken from the site. The values for t0B and BMP are deducedfrom Eqs. (9) and (12) respectively. All the parameters of Eq. (5)being determined, the methane production of the site can becalculated;

- Second, the modelling of methane collection is conducted. Inorder to find the best fit between the model simulation ofmethane collection and the measured data (12 first years ofoperation), different values of collection efficiency for thepermanent cover are tested. For the two initial steps (operationand post-operation), previous values proposed in Table 2 areconsidered: CEA¼ 35% and CEB¼ 65% (due to the presence ofa temporary cover). When CEC¼ 90%, a good agreementbetween measurements and modelling is observed, which isconfirming onsite fugitive emission measurements that

Table 4Site characteristics.

N Hcell(i) (m) Scell(i) (m2) Qcell(i) (1000 m3) q(i) (1000 m3 y�1) tc(i) (years)

57 7.6e25.8 5000 42e129 55e456 0.2e1.22

estimated the efficiency of the permanent cover to be aroundCE¼ 90% (Spokas et al., 2006).

The permanent impermeable cover having a high CE duringaftercare, the values for the OE and CE during the entire landfilllifetime were assumed to be the same as in the theoretical singlecell landfill with an impermeable cover (Section 3.1). Hence, thecorresponding values for the large-scale case study are given inTable 5.

In Fig. 16 the result of the model calibration is shown. It showsthe modelled CH4 production and collection, as well as data aboutCH4 collection from the site for a given CEC of 90% and consideringthe filling of N¼ 57 cells in Tc¼ 21 years (Table 5).

3.2.3. Results: greenhouse gas emission assessment from the large-scale case study

Fig. 17 shows the CH4 production, collection and recovery fromthe real case study. The volumes of collected and recovered CH4 arealso indicated. The calculation is made for a time horizon of 100years, i.e. 79 years after the end of operation (Table 5). For everycell, CE and OE values are taken from Table 2 (landfill with imper-meable cap cover).

The total buried wastemass after the end of operation is 4539 ktwet matter. Consequently, according to the hypotheses made onthe BMP, 291 millions of cube meters of methane will be producedduring the entire lifespan of the landfills (100 years).

In order to assess the global performance of the cap coverdisplacement procedure, the global collection efficiency (CEG) iscalculated:

CEG ¼ Total collected CH4

Total produced CH4(14)

The site has a CEG of 85% (Eq. (14)). This efficiency is lower than thecollection efficiency of the permanent cover system (90%) becausethe overall efficiency also comprises the temporary cover systemsthat are installed at the beginning of operation. It is worth notingthat the gas tightness of the final cap covers and the durability of itsfunction over the entire lifetime considered for the landfill mayhave a major environmental impact. However, it can be said thatthis CEG is very satisfactory.

As in the theoretical case studies, the quantity of methane usedfor energy recovery is 80% of the total methane collected and the

0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

0 2 4 6 8 10 12 14 16 18 20 22 24

Absolute time (T) in years

Vo

lu

me (m

3)

V(Tc)

Tc

Data from the operator Prognosis

Fig. 15. Filling process of landfill: quantity of waste disposed for each year of operation.


burning efficiency of the flare is 100%. The methane collected isused both for electricity and heat generation. The energy recoveryplant is expected to achieve a high performance thanks to cogen-eration technology and its EE is 60% (25% electricity, 35% heating).As mentioned above, the thresholds are 125 Nm3/h of methane forenergy recovery and 35 Nm3/h of methane for the flare.

The total operation time of the landfill is 21 years in the simu-lations. Energy recovery is made during 25 years and flares aremaintained onsite for 41 years (Fig. 17), in accordance with theproposed threshold and with the imperative that flares are stoppedat the end of the aftercare, 30 years after the closure of the last cell(end of operation). The fluctuation in methane flows, especially atthe end of the landfill lifetime, requires adapting the flaring system,as it is usually done by operators.

The global recovery efficiency (REG) was calculated fromEq. (15):

REG ¼ Total recovered CH4

Total produced CH4(15)

0

400

800

1200

1600

2000

2400

0 5 10 15 20 25

Absolute ti

Nm

3 C

H4 / h

Fig. 16. Calibration of the collection efficiency CEC basing on m

REG is equal to 60%. This efficiency corresponds to an average yearlyproduction of more than 17 electric GWh and 24 thermal GWh,which is far from negligible. However, the potential energyrecovery lies high above these values, as 40% of the total methaneproduction is wasted. These figures highlight that it is still possibleto improve energy recovery on landfills, and this should beencouraged by incentives at a national or European level.

The fugitive emissions without considering the avoided emis-sions are equal to 127 kg CO2-eq t�1 DM, with a reduction of 89% ofthe potential emissions (Fig. 18).

When the final emissions are considered, the site has a netconsolidated balance of 41 kg CO2-eq t�1 DM (127e86 kg CO2-eqt�1 DM). This value is very satisfactory and it is due exclusively tothe high collection efficiency of the site, and shows that landfillsmay ultimately reach the state of carbon neutral facilities.

This means that a special effort has to be made first to enhancethe collection rates, prior to any effort in improving the recoveryefficiency. By improving collection rates, the economic viability ofenergy recovery will also be enhanced.

30 35 40 45 50 55 60

me (T) in years

CH4 Production (model)CH4 Collection (model)CH4 Collection (data)

odelled and observed methane collection from the site.

Table 5Steps for gas collection efficiency (CE) and gas oxidation efficiency (OE) for the large-scale case study.

Steps A B C D

Period Operation Post-operation Aftercare Custodial care

Large-scale case study with impermeable cap coverEnd of period for cell (i)

Ptc(j¼ 1ei)

Ptc(j¼ 1ei)þ 2 years Tcþ 30 years e

End of period for whole siteP

tc(j¼ 1eN)¼ Tc¼ 21 years Tcþ 2 years Tcþ 30 years 100 years

Cover type None Temporary Permanent impermeable Final


a As long as LFG collection is maintained, but drops to zero when LFG collection is stopped.


4. Discussion e what solution for the management ofresidual emissions during the custodial care?

The IMAGE-Landfill model is a useful tool to assess the envi-ronmental impact of landfills and to compare possible mitigationstrategies of fugitive methane emissions. Although the operationperiod is well characterized, the same cannot be said of the after-care and custodial care periods. The importance of the custodialcare period should not be underestimated. During this period, thelandfill site is no longer monitored, but methane emissions shouldstill be dealt with, due to uncertainties about the long-term capcover integrity.

One of the key questions is what would be the best option tomitigate the remaining potential emissions during the custodialcare period. As no active methane management is viable, alterna-tive cover systems targeting the oxidation of the remaining emis-sions should be preferred. Biocovers and biowindows could fit thisobjective (Barlaz et al., 2004). However, they usually require analteration of the permanent cover: biocovers would be imple-mented in place of the superficial part of semi-impermeable liners,and biowindowswould require thewithdrawal of some parts of the

0

400

800

1200

1600

2000

2400

0 5 10 15 20 25

Absolute t

4

Nm

3 C

H/ h

Modethe fu

Model simulationof the past

Energy recovery

Natural oxidation

Fig. 17. CH4 production, co

cover. Alternatively, biofilters would seem best compatible with animpermeable lining system, as they enable a continued use of thecollection system, but in a passive way. Completely impermeableliners on the long-term would possibly create a “dry tomb” on thelong-term which is not sustainable, since it implies a temporarypause of biodegradation by lack of moisture, which could poten-tially restart later on. Aeration of waste may be an alternative toaccelerate waste carbon oxidation.

However, semi-permeable covers do not offer a serious alterna-tive, since important amounts of water may penetrate into thelandfill. If an impermeable bottom liner (geomembrane) is installed,thiswill lead to theconversionof the landfill cell intoa leachate “tub”.

This calls for a new, innovative design of liners to include thepossibilityof controlled liquidaddition, tomaximizemethane recoveryduring operation, and to be compatible with alternative treatmentoptions for the remaining emission potential in the long term. In thiscontext, geosynthetics could improve landfill sustainability:

(1) during operation, the requirement for high moisture contentsand the complete capture of methane emissions claims forinnovative solutions, because the actual situation does not

30 35 40 45 50 55 60

ime (T) in years

CH4 ProductionCH4 CollectionCH4 Used for energy recovery

l prediction ofture

Flares Enhanced oxidation

llection and recovery

-86

127

1143

-200

0

200

400

600

800

1000

1200

1400

Fugitive emissions afteroxidation

Potential emissions Avoided emissions (energyrecovery)

kg

C

O2-e

q / t D

M

Fig. 18. Overall emissions and substitutions for the large-scale case study.


enable the combination of these operations, or only at veryhigh cost;

(2) during the aftercare period, impermeable cap cover systemsare a sustainable solution, but new versatile geosyntheticsaiming at draining the biogas and injecting leachate or air couldbe proposed;

(3) during the custodial care period, if the “impermeable barrier”(Scenario 2) is kept, oxidation of biogas is concentrated ona few biowindows or biofilters (Figs. 5 and 6). This is onlyefficient if the collection of biogas is optimum beneath thebarrier with a remaining biogas flow naturally collected andsent to the biowindow or biofilter. Indeed, the biogas tightnessof the geomembrane should be maintained, at least until theend of waste degradation. This triggers to important issue ofgeomembrane ageing;

(4) in case of “semi-permeable” barriers (Scenario 1), the soil barrier(generally silt)will be often subjected to differential settlements.The addition of a geosynthetical liner for reinforcement of thesoil barrier inorder to limit cracking andconsequently the layer’spermeability is often tested successfully at the laboratory scale(geogrid, micro-reinforcement with short fibers.) (Gourc et al.,2010a) but rarely applied at site-scale.

More generally, the actual fugitive emissions of methanethrough the cap cover are, to now, difficult to assess. Consequently,the real collection efficiencies of the liners can only be estimatedsince the methane production is impossible to evaluate. This trig-gers that there is a crucial need for estimating the fugitive emis-sions, which directly impact the environmental sustainability oflandfills.

5. Conclusions

MSW landfills are responsible of a significant fraction of theanthropogenic methane emissions, contributing to global climatechange. The fugitive emissions of landfills are due to migrationthrough the cap barrier. The present work deals with, on the onehand, the quantification of these emissions during the landfill life,on the other hand, the available options to limit these emissions.The contribution of this paper is related to different sides of thisenvironmental problem:

- It proposes a comprehensive analysis and a formalization of thedifferent phenomena playing a role in greenhouse gas genera-tion. A strategy corresponding to four distinct periods isconsidered, operation, post-operation, aftercare and custodialcare, following the latest discussionson this topic. Parameters tocharacterize the behaviour of the cap cover are proposed,namely the collection efficiency (CE) and the oxidation effi-ciency (OE). On the other hand, energy recovery using a cogen-eration plant is characterized by an energy efficiency (EE).

- The influence of the selected structure for the cap barrier,depending on the period of landfill lifetime, is considered. Areflection must continue on the optimization of the engi-neering practices, and new applications of proper geo-synthetics are also to consider.

- An incremental model, IMAGE-Landfill, is presented. Based onthe above assumptions, it allows a quantification of the influ-ence of landfill cap cover on the environmental sustainability ofthe corresponding landfill. A basic application to a single cellwith the two types of cap barriers allows to highlight the fate ofevery fraction of the methane. Later, an application to a realmulti-cells landfill shows that the method, calibrated thanks tobiogas monitoring during the initial period of the landfill life,allows forecasting the emissions along the overall lifespan,depending on the selected technical solutions.

The presented method, which assesses quantitatively the envi-ronmental impact of the cap cover system for controlling thefugitive emissions of biogas, could contribute to the social accep-tance of landfills, which is strongly dependent of its image. Theoutcomes of the present case study demonstrate that realistictechnical improvements could potentially enable to target carbonneutral landfills in the near future.

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