Waste Management 34 (2014) 1179–1190

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Waste Management

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Mitigation of methane emission from an old unlined landfillin Klintholm, Denmark using a passive biocover system

http://dx.doi.org/10.1016/j.wasman.2014.03.0150956-053X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +45 45251561.E-mail address: pekj@env.dtu.dk (P. Kjeldsen).

Charlotte Scheutz a, Rasmus Broe Pedersen a, Per Haugsted Petersen b, Jørgen Henrik Bjerre Jørgensen c,Inmaculada Maria Buendia Ucendo a, Jacob G. Mønster a, Jerker Samuelsson d, Peter Kjeldsen a,⇑a Department of Environmental Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmarkb Ramboll Denmark A/S, DK-5100 Odense C, Denmarkc Klintholm I/S, DK-5874 Hasselager, Denmarkd FluxSense AB/Chalmers University of Technology, SE-412 96 Göteborg, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 July 2013Accepted 25 March 2014Available online 20 April 2014

Keywords:Greenhouse gasesMethane oxidationWhole landfill emission measurementCompostTemperate climate

Methane generated at landfills contributes to global warming and can be mitigated by biocover systemsrelying on microbial methane oxidation. As part of a closure plan for an old unlined landfill without anygas management measures, an innovative biocover system was established. The system was designedbased on a conceptual model of the gas emission patterns established through an initial baseline study.The study included construction of gas collection trenches along the slopes of the landfill where themajority of the methane emissions occurred. Local compost materials were tested as to their usefulnessas bioactive methane oxidizing material and a suitable compost mixture was selected. Whole site meth-ane emission quantifications based on combined tracer release and downwind measurements in combi-nation with several local experimental activities (gas composition within biocover layers, flux chamberbased emission measurements and logging of compost temperatures) proved that the biocover systemhad an average mitigation efficiency of approximately 80%. The study showed that the system also hada high efficiency during winter periods with temperatures below freezing. An economic analysis indi-cated that the mitigation costs of the biocover system were competitive to other existing greenhousegas mitigation options.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Landfills containing organic wastes produce biogas containingmethane (CH4) and carbon dioxide (CO2). Landfills are significantsources of atmospheric CH4 which contributes to climate changes(Bogner et al., 2008). At some landfills the landfill gas (LFG) isnot or cannot be utilized and the gas is either flared with risk ofproducing toxic combustion products or just emitted to atmo-sphere (Christensen and Kjeldsen, 1995). As an alternative gasemission reduction option, biocover systems may be establishedat landfills. Biocover systems use biological active materials, sup-porting microbial methane oxidation. Experiments have docu-mented that a very high CH4 oxidation rate can be obtained inbio-covers, high enough to significantly reduce the CH4 emissionfrom the landfill (Scheutz et al., 2009a). A few studies have beenmade on implementing biocover systems in large or full scale atlandfills (Barlaz et al., 2004; Stern et al., 2007; Cabral et al.,

2010; Einola et al., 2009; Huber-Humer et al., 2009; Scheutzet al., 2011a). Most of these studies have assessed the CH4 oxida-tion based on flux chamber measurements in some cases combinedwith analysis of stable carbon isotopes (Scheutz et al., 2011a). Aflux chamber based approach will in many cases be very uncertaindue to high spatial heterogeneity in surface LFG emission, but alsodue to other LFG emission pathways such as leachate collectionsystems (Fredenslund et al., 2010; Scheutz et al., 2011b). Only inone of the mentioned studies, the Fakse landfill study, documenta-tion of the CH4 oxidation efficiency of the full-scale biocover sys-tem was carried out by whole landfill emission measurementsusing the tracer dilution approach (Scheutz et al., 2011b). In thiscase whole landfill emission measurements were performed priorto and after the establishment of the biocover system to quantifythe mitigation efficiency of the biocover system (Scheutz et al.,2011a,c).

The biocover approach has been used either as a stand-alonetechnology in cases where the landfill has not been equipped witha gas extraction system or as a supplement to gas extraction. Manyolder landfills have been covered with low permeable clay soils to

Fig. 1. Description of the biocover system approach with the logical order of projectactivities.

1180 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

reduce the infiltration of precipitation to waste layers. The clay soilmay prevent even surface emissions of LFG and enhance hot spotformation, which can jeopardize the function of a fully coveringbiocover established on top of the clay soil. Besides, at old landfillswith low gas production a full coverage of the landfill area withbiocover materials may not be needed. An alternative could bethe so-called biowindows concept, which was chosen at the Fakselandfill (Scheutz et al., 2011a). The concept is to construct a biocov-er system incorporating the presence of the existing, low perme-able soil cover by establishing permeable regions (biowindows)in the clay cover using materials with much higher permeabilityto enhance gas transport into the biowindow area and to fosterhigh CH4 oxidation potential. The Fakse landfill study using thebiowindows concept showed that there might be several chal-lenges for the approach to work. At the Fakse landfill, largeamounts of clay soils have been disposed of together with otherwaste. The presence of the clay soil gave very complicated gas flowpatterns with significant horizontal gas transport, which made itdifficult to route the gas to bio-windows located on the top landfillsurface (Scheutz et al., 2011a). Besides, it was difficult to obtain aneven gas distribution over the area of a single biowindow eventhough that the bioactive compost materials used in the biowin-dows was underlain by a 0.15 m thick gravel distribution layer.The thickness of the gas distribution layer may have been toolow (Scheutz et al., 2011a).

A second-generation biocover system was developed at an oldunlined part of the Klintholm landfill, Denmark. The concept usedwas based on the lessons learned from the Fakse landfill study andother full-scale studies. Locally produced compost materials wereused as biological active filter material. A similar documentationmethodology as used at the Fakse landfill was used includingintensive measurements of the whole landfill CH4 emission priorand after establishment of the biocover system. The approach isshown in Fig. 1, which present the different project activities.The scope of this paper is to present (a) a new biocover systemdesign and construction approach based on a baseline study andtest result of local potential biocover materials, (b) to evaluatethe performance of the implemented biocover system for mitiga-tion of the landfill gas emissions and (c) analyze the economic via-bility of the biocover system.

2. Site characterization

2.1. Initial characterization

Klintholm landfill is an active landfill located in SvendborgMunicipality at the east coast of the island of Fuen, Denmark(Fig. 2). The landfill was established in 1978. The oldest part ofthe landfill, the so-called Cell 0 is established without bottom liner,leachate collection system or gas management system. The cell islocated in an area with a chalk quarry, which initially was filledwith coarse inert waste materials. The cell was active from 1980until 1996. The cell was covered with large volumes of soil from1997 and onwards. The top soil thickness is therefore between 3and 4 m – only on the slopes the waste is covered with thinner soil

layers (30–100 cm). Waste composition has varied through theyears but contained 40–60% combustible waste, 5–15% sludgeand 35–45% non-combustible waste. Cell 0 has an estimated totalwaste volume of 485,000 m3 and an area of approx. 4 hectares. Acomposting facility is situated northwest to Cell 0 while the newerparts of the landfill (Cell 1/2) are situated to the south and east(Fig. 2). These cells were active from 1993 until 2008. Cells 1.1–1.2 received 230,000 tons of mixed waste including combustiblewaste (40% combustible waste initially but decreasing throughthe period). Cells 2.1 and 2.2 have only received soils with loworganic content.

The biocover project was part of the final closure plan of Cell 0where a final soil cover was constructed to manage surface waterat the cell. The local authorities also demanded a mitigation planfor the gas emissions from the cell as part of the overall closure plan.

2.2. Baseline study

The baseline study has been presented in Kjeldsen et al. (2009)where details in respect to materials, methods and results aregiven. The surface emission of CH4 was screened by a handheldFID detector measuring air concentrations of CH4. The screeningrevealed about 20 emission hot spot areas with an estimated totalarea of 500 m2. Most hotspots were found along the southern andwestern slopes of Cell 0. It has previously been observed that thegas formed in landfills moves horizontally out through slopesprobably due to the presence of horizontal layering of the waste,which prevent a more vertical gas flow (Scheutz et al., 2011a).No hot spot areas were observed on top of the landfill probablydue to the presence of thick layers of clayey soil. The hot spotswere further investigated by measuring CH4 emissions using fluxchambers. Methane emissions from hot spots at the slopes werein the range of 1 and 600 g CH4/day. Two open gas wells, whichwere identified on the cell, could potentially emit significant quan-tities of CH4. The emission from the two gas vents were quantifiedby the method given by Fredenslund et al. (2010). The resultsshowed that the CH4 emission through this pathway (total of0.2 kg h�1) was much lower than emissions through the identifiedhot spots (2.3 kg h�1).

Due to the lack of a bottom liner and the presence of a clay soilcover, the LFG may potentially migrate out of the landfill bottomand up through the surrounding areas. To investigate the possiblemigration process, a series of gas probes were installed at the edgeof the landfill. This revealed elevated concentrations of CH4 andespecially CO2 in the soil air. The highest concentrations werefound at the northern edge, while for the other edges only elevatedCO2 concentrations were found. This indicated that the CH4 wasoxidized in the soil. As part of the baseline study, the whole landfillemission was measured by the tracer dilution technique. This datawill be included in this paper (integrated into the result section) toobtain a comparison with measurements done after the establish-ment of the biocover system.

3. The biocover system – design and construction

On the basis of the performed measurements of the total CH4

emissions (as described in Sections 4.2, and 5.2) and the chosencompost mixture’s ability to oxidize CH4 (Sections 4.1 and 5.1),the required total area of biocover was calculated. A safety factorwas included; the dimensioning CH4 emissions were set to10 kg h�1 (about double of the measured emission in the baselinestudy – confer Section 5.2 and Table 3) and the CH4 oxidationcapacity of the compost mixture to 50 g m�2 day�1 (50% of thedetermined capacity – Table S1), which gives a necessary biofilterarea of 4800 m2.

Compostingfacility

Cell 1.1Inactive.

Temporarycovered

Uncon-taminated

soil

Cell 2.1Contaminated

soil

Cell 2.2Contaminated

soil

Cell 0

Cell 1.2 Inactive.Temporary

covered

Cell 1.3Active.Mixedwaste

Klintholm

b

c

a

Fig. 2. Overview of the Klintholm landfill. (a) Location of the landfill in Denmark. (b) Arial photo of the site. (c) Map with the location of the different landfill cells andcomposting facility.

Table 1Characterization of the compost types used in the batch incubation experiments.

# Compost typea Originb Age Mesh (mm) Water content Loss of ignition

(per 100 g DMc)

1 GW-15K K 6–8 months <15 32.8 28.42 GW-45K K 6–8 months <45 41.0 27.53 KW/GW-15K K 6–8 months <15 65.0 21.64 KW/GW-45K K 6–8 months <45 84.2 46.75 Mixture GW-15K and KW/GW-45K (1:1) K – <15 57.5 31.7

<456 KW/GW-15K K 5 years <15 26.5 ± 0.4 15.3 ± 1.27 KW/GW-15K K 6 months <15 39.0 ± 2.2 22.1 ± 1.1

2–3 years8 GW-15S S <15 43.4 ± 1.6 16.9 ± 0.4

9 Mixture GW-15S and KW/GW-15K (6:1) S/K – <45 39.9 ± 4.1 18.8 ± 2.010 Mixture GW-15S and KW/GW-15K (7:2) S/K – <45 35.2 ± 0.6 18.8 ± 0.3

For material # 9 and 10 are the numbers in parentheses the weight-based material mixing ratio.a GW: garden waste, KW: kitchen waste.b K: Klintholm, S: Svendborg.c DM: dry matter.

C. Scheutz et al. / Waste Management 34 (2014) 1179–1190 1181

Table 2Results from the batch incubation tests.

# Compost type1 Rates – methane oxidation tests(ug gDM�1 h�1)

Rates – respiration tests(ug gDM�1 h�1)

Rates – methane potential tests(ug gDM�1 h�1)

CH4 O2 CO2 O2 CO2 CH4 CO2

1 GW-15K �3.6 ± 0.2 �23.3 ± 0.6 21.2 ± 0.03 �16.4 ± 0.7 20.7 ± 2.5 0.2 ± 0.0 3.3 ± 0.12 GW-45K �0.6 ± 0.2 �36.5 ± 2.9 49.3 ± 0.3 �55.7 ± 0.1 74.7 ± 0.6 2.6 ± 0.01 8.1 ± 0.33 KW/GW-15K �109.4 ± 15.4 �305.5 ± 39.5 174.6 ± 43.0 �8.9 ± 1.0 11.0 ± 1.4 �0 �04 KW/GW-45K �0.8 ± 0.5 �30.8 ± 3.0 38.6 ± 4.7 �33.8 ± 1.7 45.8 ± 2.4 5.3 ± 0.3 14.9 ± 1.25 Mixture GW-15K and KW/GW-45K (1:1) �1.2 ± 0.1 �23.4 ± 0.2 22.0 ± 2.2 �58.2 ± 4.9 74.8 ± 5.0 0.8 ± 1.0 6.3 ± 3.46 KW/GW-15K �8.5 ± 0.5 �24.6 ± 1.4 12.5 ± 0.6 �1.0 ± 0.1 0.9 ± 0.0 �0 �07 KW/GW-15K �60.5 ± 0.7 �221.6 ± 11.1 141.2 ± 6.7 �31.8 ± 2.6 40.3 ± 4.3 0.1 ± 0.0 29.3 ± 5.38 GW-15S �66.9 ± 11.4 �172.4 ± 58.3 73.9 ± 13.1 �14.0 ± 2.0 21.9 ± 3.4 �0 7.2 ± 0.99 Mixture GW-15S and KW/GW-15K (6:1) �75.7 ± 12.6 �208.5 ± 16.9 107.1 ± 6.1 �14.0 ± 2.0 21.9 ± 3.4 �0 3.9 ± 0.210 Mixture GW-15S and KW/GW-15K (7:2) �31.4 ± 12.5 �98.2 ± 29.9 53.1 ± 25.4 �10.0 ± 0.2 18.1 ± 4.4 �0 6.4 ± 0.1

1 GW: garden waste; KW: kitchen waste; 15: material sieved on 15 mm mesh; 45: material sieved on 45 mm mesh; K: Klintholm; S: Svendborg

Table 3Overview of whole site CH4 emission measurements including the two campaigns made prior to the biocover system.

Campaign date Ambienttemperature (�C)

Tracers useda Number of plumetransects

Total whole site CH4

emission (kg/h)CH4 emission from cell 0(kg/h)

Emissions reduction bybiocover system (%)

Before establishment of biocover system02.04.2008 10 3N, 1Cb 15 14.0 ± 1.6 4.3 (31%) n.a.05.08.2008 16 4N 12 16.0 ± 2.0 6.4 (40%) n.a.Average 15.0 5.4 n.a.

After establishment of biocover system – 201009.04.2010 10 3Nc 11 9.1 ± 1.2 3.2 (35%) 4125.05.2010 6 4N 8 7.0 ± 1.0 1.3 (19%) 7614.06.2010 8 3Nc 11 6.1 ± 0.7 1.0 (16%) 81Average 7.4 1.8

After establishment of biocover system – 2011/1219.12.2011 4–6 3N,2A, 1S 6 4.8 ± 0.3 0.7 (15%) 8719.12.2011e 4–6 3A 6 3.3 ± 0.3 0.9 (27%) 8317.01.2012e 5 3A 14 1.6 ± 0.4 0.6 (37%) 89Average 3.2 0.7Overall average 76–83d

a Number refer to release points, N = N2O, C = CO, A = C2H2, S = SF6.b Tracer release point representing the composting facility source.c Emission from the composting facility was not included on this date.d The high value obtained by omitting the first value (9 April 2010).e Measured by the CRDS based system.

Biofilter

Gas collectionpipe (90 mm)

Gas distributionpipe (90 mm)

Gas sampling pipe

Soilembankment

Soilembankment

Waste lift

Waste lift

Waste lift

Waste lift

Soilembankment

Waterdischarge

trench

Compost (70 cm)Crushed concrete (30 cm)

10 meter

100 cm80 cm

80 cm

Vegetation layer 20 cm)Soil fill

LDPE liner(0.5 mm)

Gas collectionpipe (90 cm)

b

a

A

A

A A

Fig. 3. (a) Sectional view of the Cell 0 showing trenches, new soil cover of slopesand biocover sections located on top of landfill. (b) Cross sectional view through agas collection trench. The gas collection trench is continuous the whole way up tothe gas sampling pipe.

1182 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

The initial site investigation showed that the landfill top wascovered with a very thick low permeable clay soil layer, so it wasdecided not to base the biocover system on the ‘‘biowindows’’ con-cept, which also had limited success at the Fakse landfill (Scheutzet al., 2011a). The baseline study showed as already mentionedthat most gas emission was related to hot spots on the slope areas.Therefore it was decided to dig several 1.5 m wide trenches intothe preliminary soil cover on the slopes to get direct contact tothe waste mass (see Fig. 3a). The trenches were placed along allsloping areas, except the eastern slope where cells 2.1 and 2.2are placed. These cells have bottom liners, which are placed ontop of the eastern slope of Cell 0. The distance between two adja-cent trenches was about 15 m. In total 33 trenches were con-structed. In the bottom of each trench a smaller trench (80 cmwide and 30 cm deep) was dug. This small trench was filled withcoarse gravel (32–64 mm grain size) and a 92 mm diameter slottedgas collection pipe. The small trench was covered with a LDPE linerand the larger trench back-filled with the excavated soil (seeFig. 3b). After all trenches were constructed, the slopes were cov-ered with a low permeable soil. A group of adjacent gas collectionpipes (between 2 and 6 for most biocover sections) were connectedwith a horizontal gas distribution pipe on the top of the landfillplaced in the middle of each of the nine biofilters – see Fig. 4.

15.0

15.017.5

17.5

20.0

10.012

.515.017

.520.0

22.5

22.5

20.0

17.5

17.5

20.0

20.0

17.5

15.0

12.5

20.0

12.5

15.0

17.5

17.5

20.0

20.0

22.5

22.5

15.0

12.5

Gas collection trench

12

3

4

5

6

7

8

9Site 10

Site B Site

7

Site

6

Site

5

Site4Site

3Site 2

Site 1

Site

9Site 8

SiteA

Measuring site withgas probes and flux chamberMeasuring site withgas probe, flux chamberand temperature probes

1 2 3 4

5 6 87

9 10 11 12

Placementof measuringpoints

10 m

5 m

Flux chamber location inintensive campaignExisting gas well

Gaswell

Fig. 4. Plan view of the biocover system with indication of the 33 gas collection trenches and the nine biocover sections placed on the top of the landfill. Numbering of thenine biocover sections is given. Besides, overview of the location of measuring station, temperature/moisture probes, gas well, and intensive flux chamber campaign is shown.

C. Scheutz et al. / Waste Management 34 (2014) 1179–1190 1183

For two of the biocover sections (Sections 6 and 8) the gas load wasonly relying on a vertical gas transport into the biocover through adeep trench dug through the thick soil cover. The horizontal gasdistribution pipe was placed in a 80 cm wide trench filled withcrushed concrete (20–60 mm grain size) overlain by a 30 cm gasdistribution layer of the same crushed concrete and a 70 cm layerof the chosen compost mixture. Each horizontal gas distributionpipe was equipped with a vertical gas sampling pipe – closed atthe top and fitted with a sampling valve (see Fig. 3a). Each biocoversection was 10 m wide and of variable length (see Fig. 4). The totalarea of the nine biocover sections was 4800 m2. Excess water infil-tration through the biocover sections was collected at the bottomof the gas distribution layer, which was sloping towards the perim-eter of the landfill and was further drained away through the gascollection trenches and into a water discharge trench placed atthe foot of the slopes (see Fig 3a). The water discharge systemwas equipped with a water lock at the far end to prevent gas fromescaping through this route. The system was completely passivelymanaged; no water or gas pumps were used. The constructionwork was carried out between April and September 2009. The bio-cover system was started up in October 2009.

4. Material and methods

4.1. Testing biocover materials

Compost was produced at the Klintholm site as well as at anearby composting facility in Svendborg. At the Klintholm siteboth garden waste compost and compost based on a mixture of

kitchen waste and garden waste was produced, while only gardenwaste compost was produced at the Svendborg facility. Initially theCH4 oxidation capacity of the different types of compost was testedin simple batch incubation tests.

4.1.1. Batch incubation testsBatch incubations were performed to determine potential CH4

oxidation rates, respiration rates, and CH4 production potential inseven materials. In total ten compost samples were tested. Charac-terization of the ten compost samples can be seen in Table 1. Thecompost materials were between 6 months and 5 years old. Fromthe raw compost two types of compost were made by sievingthrough a 15 mm or a 45 mm mesh, respectively. The batch incu-bations were constructed by adding 70 g of moist compost mate-rial to 1000 mL infusion bottles. The bottles were sealed withgas-tight rubber septa, and pre-equilibrated with CH4 overnightto acclimate the bacteria to the presence of CH4. Subsequently,the bottles were ventilated for 20–30 min, resealed and an atmo-sphere of 15% (vol.) CH4 and 35% (vol.) O2 was established. Allexperiments were done in duplicate with two coarse sand controls(Dansand 6, Denmark) at room temperature (22 �C) and at a mois-ture content of 42–44% (dry matter basis). CH4 oxidation rateswere calculated by the linear part of the curve (zero order degrada-tion, r2 > 0.98). Duration of the experiments was between 17 and480 h depending on the activity.

The compost respiration test was set up using batch incubationsin the same way as the CH4 oxidation test, except that only atmo-spheric air was supplied in the headspace. The duration of the res-piration tests were between 400 and 800 h. Also a series of batch

1184 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

incubation tests were set up to test the CH4 production potential.For these test the batches were flushed with pure N2 to create anoxygen free atmosphere in the batches. The duration of CH4 pro-duction potential tests was 1300 h.

4.1.2. Column experiment to determine CH4 oxidation potential.Based on the results obtained from the batch incubation tests,two of the most suitable compost materials were selected for fur-ther investigation in column experiments. The two selected com-post materials was a garden waste compost (#8 – confer Table 1)and a mixed garden waste/kitchen waste compost (#10 – conferTable 1). A column filled with coarse and dry sand (Dansand 6,Denmark) was used as a control. The column experiments con-sisted of three identical PVC columns (1 m high, 25 cm inner diam-eter) closed at both ends. A mixture of 50% v/v CH4 and 50% v/v CO2

was pumped into the bottom of the column at a constant rateresulting in CH4 loads of 215 g/(m2 day). Atmospheric air was usedas sweep gas, which was pumped through the head space chamberat the top of the column with a rate of 60 mL min�1. Gas was sam-pled from the outlet and analyzed for methane content. Averageflow rates were used for calculation of mass balances. The durationof the experiment was 85 days. More details of the experimentalmethod can be seen in Pedersen et al. (2011).

4.1.3. Gas analysisThe samples taken in the batch incubations and column exper-

iments were analyzed for CH4, CO2, oxygen (O2) and nitrogen (N2)via gas chromatography using a thermal conductivity detector.More details are given by Pedersen et al. (2011).

4.2. Evaluation of biocover system performance

The performance of the biocover system was evaluated by sev-eral field activities. Most of the field measurements were carriedout in the period from October 2009 until June 2010, except thewhole landfill emission measurements, which were continued intoJanuary 2012.

4.2.1. Whole landfill emission measurementsThe whole site CH4 emission from the Klintholm landfill area

was measured using a tracer dilution technique, that combinescontrolled tracer gas release from the landfill with time-resolvedconcentration measurements of the tracer and CH4 plume down-wind the landfill using a high precision instrument mounted in avan for analysis of tracer(s) and CH4. The method is described inScheutz et al. (2011c). Prior to establishment of the biocover sys-tem as part of the baseline study, two measurement campaignswere carried out to determine the whole site CH4 emission. Thecampaigns were carried out on 2nd of April 2008 and 5th of August2008. Both measurement campaigns were performed under stableweather conditions with a western wind direction so that it waspossible to use a road along the beach for driving the measuringvan. Under such wind conditions, it was possible to measure threeCH4 plumes from the landfill site originating from three emittingsources: a composting facility, Cell 0 and Cell 1.1/1.2 (conferMønster et al., 2014). All later measuring campaigns were madeat stable weather conditions and with a western wind with mea-surement at the beach road. In total 8 measurements campaignswere performed in the period 2008–2012 either by a FTIR absorp-tion spectroscope based system (details in Scheutz et al., 2011c) ora Cavity Ring-Down Spectroscopy (CRDS) based system (PicarroInc., USA) (details in Mønster et al., 2014). In all cases, 3–4 tracerrelease points were chosen with locations on each emitting source(Cell 0, Cell 1.1, Cell 1.2 and the composting facility). In some casesthe sources, Cell 1.1 and 1.2, were represented by a single tracerrelease point. With a western wind the chosen locations of the

release points made it possible to split up the plumes from thesources and quantify the release alone from Cell 0. For some datesthe CH4 plume from the composting facility could easily be differ-entiated from the other sources and no tracer release was estab-lished for the composting facility. The method (tracerconfiguration, tracers (N2O, CO, C2H2 and SF6) and analyticalinstruments) was documented and validated at a field campaign(19th December 2011). The results are presented in Mønsteret al. (2014).

4.2.2. Surface screening and visual inspectionSurface screenings of landfill soil cover and the biofilter sections

were performed by measuring near surface concentrations of CH4.A Photovac MicroFID portable flame ionization detector (FID)(Photovac Inc., Massachusetts, USA) was used for these measure-ments. More details of the method are given in Scheutz et al.(2011a). Besides, the surfaces of the biocover sections were contin-uously visually inspected for cracks, damaged parts or areas withdifferent or lacking vegetation cover.

4.2.3. Surface flux measurementsSurface emission rates of CH4 and CO2 were determined using

static flux chambers. The flux chambers were made of stainlesssteel and equipped with sampling ports and a manually operatedfan securing that the air inside the chamber was totally mixed dur-ing sampling. The emission rates of CH4 and CO2 were measured bytaking a time series of gas samples. An Innova 1312 photoacousticmulti gas monitor (LumaSense Technologies A/S, Denmark) wasused to measure concentrations of CH4 and CO2. More details ofthe used materials and method are given in Scheutz et al.(2011a). Flux chamber measurements were carried out seventimes at 12 fixed measuring stations (sites 1–10, sites A and B)placed at different locations of biocover area (see Fig. 4). Besides,intensive campaigns were carried out on June 3rd 2010 at threeof the biocover sections, where elevated CH4 concentrations bythe FID screening had been identified (see Fig. 4). At each locationa grid of 12 measuring locations were selected where flux mea-surements were carried out at each location.

4.2.4. Concentration profiles in the biocover sectionsFor the purpose of measuring gas transport and CH4 oxidation

profiles in the upper part of the waste, twelve sets of shallow gasprobes were installed at different depths in the biocover sections(locations coincided with sites for flux chamber measurements asshown in Fig. 4). The probes consisted of steel tubes (6 mm in innerdiameter) that were closed at the bottom and slotted over thelower 3 cm. The steel probes were hammered into the ground atdepths of 10, 20, 40, 60, and 70 cm below ground surface and sam-pled in evacuated sample vials (Exetainer�, Vial 819 W, 6 mL, Lab-co Ltd., UK) for analysis of CH4, CO2, O2, and N2. For furthersampling method details confer Scheutz et al. (2010).

4.2.5. Gas composition in gas distribution system and interior oflandfill

Gas was sampled from the vertical gas sampling pipes, whichconnect to the gas distribution layer in the 9 biocover sections(Fig. 3a) to determine the gas composition in the gas distributionsystems. Measurements were carried out in five campaigns overthe project period. Prior to sampling the pipes were purged for30 min using a purge flow of 10 l/min. Testing of the concentrationdevelopment during purging (data not shown) showed basically nochanges in gas composition in any of the gas sampling pipes overthe 30 min period. The sampling was carried out as for the gasprobes.

To evaluate the composition of the raw gas in the interior of thelandfill an existing gas well (location – see Fig. 4) was sampled and

g/m

2 /d)

200

175

150

C. Scheutz et al. / Waste Management 34 (2014) 1179–1190 1185

analyzed for main components (CH4, CO2, N2 and O2). The well wasscreened 2–10 m below the top surface of landfill. The well waspurged for an hour prior to sampling using a vacuum pump (10 l/min). Sampling and analysis were carried out as for the gas probes.

Met

hane

oxi

datio

n ra

te (

125

100

75

50

25

0

4.2.6. Temperature profiles in biocover sectionsCompost temperature was measured with ECH2O EC-TM

probes and logged with the EM50 datalogger (Decagon Devices,USA). Profiles were measured at two locations, sites A and B(Fig. 4) at 10, 20, 40, 60 and 70 cm depth in the biocover compostmaterial (placed next to two of the gas profile locations). Measure-ments were logged with a minimum 1/2 h frequency from Decem-ber 2009 to June 2010.

ControlGarden waste compostMixed garden/kitchen waste compost

300 10 20 40 50 60 70 80 90

Time (days)

Fig. 5. CH4 oxidation rates (in gCH4 m�2 day�1) as a function of experimental timedetermined in the column experiment.

5. Results and discussion

5.1. Biocover material selection

The results of the batch incubation tests can be seen in Table 2.In general, much higher CH4 oxidation rates were obtained for thecompost materials sieved by the 15 mm mesh which most likely isdue to a higher specific surface area resulting in a higher number ofattached microbes. It is also observed that materials containingkitchen waste based compost generally had higher oxidation rates,but also higher O2 consumption due to respiration. In order toobtain an adequate volume of compost for the biocover system,garden waste and kitchen waste based composts were mixed andfurther tested (samples 9 and 10 in Table 1). These compost mate-rials also showed high CH4 oxidation rates and average O2 con-sumption. The respiration tests (Table 2) showed much higherrespiration for the materials, which have been sieved through thelarger mesh (45 mm), probably due to a larger content of non-degraded wood pieces, which are further degraded during the test-ing. Similar observations were done in a previous study (Pedersenet al., 2011). The lowest respiration was – as expected – seen in theoldest (5 year) compost (sample 6). This sample showed, however,a low CH4 oxidation rate. Most of the samples had respirationlower than recommended for compost materials to be used in bio-cover systems (respiratory activity less than 8 mgO2/gDM over7 days – Humer and Lechner, 2001). The test for CH4 productionpotential showed that only the coarse compost materials (sievedthrough 45 mm mesh) had significant potential for CH4 production(confer Table 2).

Overall, the batch incubation test revealed that finer compostmaterials (sieved through 15 mm mesh) were preferable to thecoarser materials and that compost mixtures with a content ofkitchen waste based compost made out a suitable active materialfor the biocover system.

To test the compost mixture intended for use as the material tobe used in the biocover system under more realistic dynamic con-ditions a column test was set up. The fine garden waste based com-post from Svendborg (compost material 8 – confer Table 1), whichwas available in adequate quantities, was tested in the columnexperiment; the other column contained compost material 10. Ofthe compost mixtures, compost 10 was chosen in favor of compostmaterial 9 due to the lower oxygen demand of this material. Fig. 5shows the CH4 oxidation rates as a function of experimental time.The figure shows that the rates started for both materials on a highlevel – in the order of 150–200 gCH4 m�2 day�1, but graduallydecreased to a stable level. Table S1 in the supporting materialshows the stable values in respect to CH4 load, CH4 oxidation rate,CO2 production rate and O2 consumption. The table shows that thecompost mixture even at a high loading rate showed high oxida-tion rate in the order of 96 gCH4 m�2 day�1. Compost material 10was thus selected as the material for the full-scale biocover system.

5.2. Overall biocover system performance

Table 3 shows the result of the whole site CH4 emission mea-surements including the two baseline campaigns and six cam-paigns starting about six months after the startup of the biocoversystem. Table 3 shows that the emission from Cell 0 is significantlylower after the establishment of the biocover system. The CH4

emission reduction by the biocover system as comparing the esti-mated emission from Cell 0 on a specific date with the estimatedaverage CH4 emission from Cell 0 prior to biocover establishment(equal to 5.4 kgCH4 h�1) is shown in the last column of Table 3 –values are in the range of 41–89%. The method of comparing wholelandfill emission measurement done before and after the biocoversystem construction assumes that the overall methane loading tothe total landfill surface is not affected by the new constructions.Omitting the first measurement after establishment made on 9thof April 2010 (3.2 kgCH4/h) an average emission reduction of 83%is obtained. It is noted that two years after start-up the biocoveroxidation efficiency was still very high (>80%) (Table 3).

The extended campaign carried out on 19th of December 2011shows that the FTIR based system gave a larger emission(4.8 ± 0.3 kgCH4 h�1) as compared to the CRDS based system(3.3 ± 0.3 kgCH4 h�1) (Mønster et al., 2014). The reason to the devi-ation is probably due to the higher time resolution of the CRDSbased system (confer Mønster et al. (2014) for more details). Theuse of a different tracer (CO) for the Cell 1.2 clearly revealed thatmost of the total CH4 emitted from all cells originated from this celland that the emission from Cell 0 was low (data not shown).

5.3. Gas composition in gas distribution system and interior of landfill

Analysis of the raw gas from the interior of the landfill showedtypical landfill gas composition with CH4 concentration of 69% andCO2 concentration of 27% (CH4/CO2-ratio of 2.5) and N2 concentra-tion below 5%. Fig. 6 shows the gas composition in the gas distribu-tion systems of each section as measured on the 6th of May 2010.The figure shows that gas compositions in the gas distribution sys-tem differed strongly from the composition in the interior of thelandfill. Comparing compositions between campaigns prevailedthat slightly higher values for CH4 were found at decreasing baro-metric pressure (data not shown). Similar effects have beenobserved at many other landfills (Scheutz et al., 2009a). The datashown in Fig. 6 are from a campaign with decreasing barometric

Gas

con

c. (%

vol

.)

80

70

60

50

40

30

20

10

0Cell 1 Cell 2 Cell 6Cell 4 Cell 8Cell 3 Cell 7Cell 5 Cell 9

CH4 N2O2CO2

Gas distribution system - 6 May 2010

Fig. 6. Gas composition in the gas distribution system for the different biocoversections as determined on 6th of May 2010.

1186 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

pressure, so CH4 and CO2 concentrations were slightly higher thanaverage values. Data from the other campaigns can be seen in S2 inthe supporting material. Fig. 6 shows that there were large differ-ences in the gas composition between biocover sections. In Sec-tions 2, 4, 7, and 9 elevated concentrations of CH4 (7–30%) wereseen probably as a result of a higher gas load through the con-nected gas collection trenches. In all sections high concentrationsof N2 close to atmospheric concentrations were observed and alsomeasurable O2 (but lower than atmospheric concentrations). CH4/CO2-ratios in the sections with measurable CH4 concentrationswere in the range of 0.14–1.1 (based on average data), which weresmuch lower that the value for the raw gas (2.5). The data clearlyindicates that atmospheric air was transported (either by pressure

10.012.5

15.017.5

17.5

20.0

17.5

10.0

20.0

FID-screening 6th of May 2010

0-200 ppm at edge of biofilte~ 20 ppm on the whole surfaof the section.

50-10010 ppm10-15 p

10 ppm in whole area.

25-50 ppm atedge of biofilter.

Fig. 7. Result of the FID screening on 6th of May 2

differences or by diffusion) into the gas distribution systems andthat CH4 oxidation was possible already starting off in the gas dis-tribution layer. This has also been observed in other biocover sys-tems (Scheutz et al., 2011a; Geck et al., 2013). The gas distributionsystem was as mentioned equipped with a water lock in the farend, so it is likely that the air was entering through the biocoversections possible at locations with low gas loadings. It is wellknown that the CH4 oxidation process is reducing the gas volume(Kjeldsen, 1996) and can potentially create a low pressure whichmay suck in atmospheric air.

5.4. CH4 screenings and surface emission measurements

Screening the surfaces with a FID detector can reveal areas withelevated CH4 concentrations, which may indicate hot spot areaswith elevated emissions. The CH4 screenings generally showed sur-face concentrations close to background concentrations both onthe slopes and most of the biocover surfaces. However, on the bio-cover Sections 2, 4, 7 and 9, hot spot areas with elevated concentra-tions in the range of 20–200 ppm was observed in the parts closestto the landfill slopes. Fig. 7 shows the result of a screening from 6thof May 2010 during decreasing barometric pressure. The concen-trations measured here were generally higher in comparison tomeasurement from 7th of April during more stable barometricpressure (data not shown). The areas marked with red where thehighest concentrations were measured, corresponded with un-veg-etated areas of the biocover sections.

Measurements of surface emissions of CH4 and CO2 were car-ried out at the 12 measuring sites. Results from the 7 campaignsare shown in Table 4. At nearly all locations, CH4 emissions werebelow detection limit. After 5–6 months after the establishment,there was basically no CH4 emission detected at any measuringsites. In contrast, the CO2 emissions were high at all sites, espe-cially in the beginning of the monitoring period. This was probably

10.0

12.5

7.5

10.012

.515.017

.520.0

22.5

22.5

20.0

17.5

17.5

20.0

20.0

20.0

22.5

22.5

12.5

10.0

7.5

Winddirection

r.ce

50-150 ppm at edge of biofilter.10-30 ppm on the whole surfaceof the section.

ppm at the edge decreasing to 3 meters down the slope.pm on the surface of the section.

010 showing areas with elevated FID readings.

Table 4Measured emissions of CO2 and CH4 at the 12 measuring sites for all 7 campaigns.

Date Site (Section)

1 2 3 4 5 6 7 8 9 10 A BS3 S2 S2 S2 S1 S1 S1 S5 S4 S4 S5 S1

CO2 (g/m2 d)20-11-2009 1275 722 31 79 52 357 147 82 72 18 n.m. n.m.11-12-2009 123 294 122 105 53 341 149 15 97 18 n.m. n.m.18-02-2010 190 216 376 2 6 125 394 0 67 9 n.m. n.m.10-03-2010 24 208 13 14 9 9 24 3 54 0 n.m. n.m.07-04-2010 91 142 34 63 26 48 78 25 52 38 n.m. n.m.06-05-2010 11 162 0 8 17 46 56 6 23 44 43 1103-06-2010 41 19 33 74 34 53 23 11 15 20 38 22Average 250 252 87 49 28 140 125 20 54 21 41 17

CH4 (g/m2 d)20-11-2009 446 5 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. n.m. n.m.11-12-2009 b.d. 100 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. n.m. n.m.18-02-2010 b.d. 73 15 b.d. b.d. b.d. 6 b.d. b.d. b.d. n.m. n.m.10-03-2010 b.d. 35 b.d. b.d. b.d. b.d. b.d. b.d. 15 b.d. n.m. n.m.07-04-2010 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. n.m. n.m.06-05-2010 b.d. 3 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.03-06-2010 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.

n.m.: Not measured; b.d.: below detection limit.

C. Scheutz et al. / Waste Management 34 (2014) 1179–1190 1187

due to high respiration rates, which decrease with time as a resultof further maturation of the compost. The decrease of CH4 emissionwith time could be due to an increased biomass of methanotrophicbacteria in the compost layer in combination with less competitiveO2 consumption from the diminishing respiration process. It is alsoworth noting that the CH4 oxidation seemed to be very active alsothrough the winter months where ambient temperature was �5 to�10 �C for long periods (see also Section 5.6).

To further evaluate the spatial variability of emissions – espe-cially close to the edge of biocover sections where the FID screen-ing revealed possible higher emissions, three networks each with12 flux measuring points were established at three of the sectionswith high (Sections 2 and 4) to medium (Section 3) gas loading(locations shown on Fig. 4). The results of this campaign are shownin Table 5. This investigation clearly showed that CH4 emission wasonly observed in the row of flux chambers placed closest to theedge of the biocover section and with the highest emissions in bio-cover Section 4, which also had the highest CH4 concentrations inthe gas distribution system. In Section 3, having medium CH4 con-centrations in the gas supply, no CH4 emissions were observed. CO2

Table 5Emissions of CH4 and CO2 in the intensive emission campaign carried out 3rd of June2010 on Sections, 2, 3 and 4. The locations of the 12 measuring points of each grid areshown in Fig. 4. Numbers in bold are average numbers.

CO2 (g/m2 d) CH4 (g/m2 d)

Grid – Section 2 Grid – Section 2119.1 37.2 16.0 11.0 0.3 b.d. b.d. b.d.

77.7 32.6 22.8 11.3 1.4 b.d. b.d. b.d.170.6 17.9 33.1 15.8 3.4 b.d. b.d. b.d.122.4 29.2 24.0 12.7 1.7 b.d. b.d. b.d.

Grid – Section 3 Grid – Section 327.7 28.8 20.1 29.6 b.d. b.d. b.d. b.d.20.7 33.4 21.2 26.7 b.d. b.d. b.d. b.d.14.4 27.6 19.1 15.5 b.d. b.d. b.d. b.d.21.0 29.9 20.1 23.9 b.d. b.d. b.d. b.d.

Grid – Section 4 Grid – Section 463.3 13.0 5.6 7.0 20.4 b.d. b.d. b.d.

152.4 30.8 6.0 12.8 46.2 0.7 b.d. b.d.201.5 24.8 11.8 10.6 56.1 b.d. b.d. b.d.139.0 22.9 7.8 10.1 40.9 0.2 b.d. b.d.

b.d.: Below detection limit.

emissions were observed in all grid points due to the all presentrespiration process, but were decreasing with distance from theedge in the sections with high methane concentrations in the gassupply (Sections 2 and 4). This reflects the high local gas loadingrates close to the edges. The high loading rates close to the edgescould be due to the fact that the gas collection pipes in the trenchesget in contact with the biocover layer at locations close to theedges, creating a heterogeneously distributed gas load to the bio-cover. Similar problems of short-circuiting of gas have beenobserved in other biocover studies. (Parker et al., 2013; Gecket al., 2013). In addition, the edges are vulnerable to erosion dueto wind exposure reducing the biocover thickness in some areas.

5.5. Concentration profiles in biocover sections

Examples of the gas profiles from the measuring stations areshown in Fig. 8. The results in the figure supports the findingsfrom the adjacent flux chamber measurements (confer Sec-tion 5.4). Site 8 is located in Section 5, which had a low gas load– there was never measured CH4 in the gas distribution system.The profiles reflect this finding with CH4 below detection limitfor all depths and times of year. In the early time (dates20.11.2009 and 11.12.2009) decreasing concentrations of O2 withdepths and increasing concentrations of CO2 with depth wereobserved, which reflects an ongoing respiration process in thenewly established biocover sections. Later (dates 18.05.2010and 06.05.2010) concentrations of O2 was constant over depthand CO2 concentrations very low, reflecting that respirationhad significantly decreased probably due to maturation of thecompost material over time. Site 9 is located in Section 4, whichhad the highest CH4 concentration measured in the gas distribu-tion system of all sections. Here CH4 concentrations in the rangeof 10–20% in the deepest probe were seen, and decreasedtowards the surface. Concentrations of CO2 were higher thanCH4 for the first to dates, and O2 (3–20%) could be found at alldepths of the compost. This picture is typical for a location withactive CH4 oxidation (Scheutz et al., 2009a), and supports that noCH4 emissions were found (Table 4). For the last to dates(18.02.2010 and 06.05.2010) the indications of methane oxida-tion was not as strong since the CO2 and CH4 profiles were verysimilar and was not reflecting the observation of no CH4 emis-sions on those dates (confer Table 4) and the high CH4 concen-tration in the gas distribution system (confer Fig. 6).

20.11.2009Soil gas conc. (% v/v)

11.12.2009Soil gas conc. (% v/v)

18.02.2010Soil gas conc. (% v/v)

06.05.2010Soil gas conc. (% v/v)

Site

8 (s

ectio

n 5)

Soil

gass

am

plin

g de

pth

(cm

.b.s

)Si

te 9

(sec

tion

4)So

il ga

ss a

mpl

ing

dept

h (c

m.b

.s)

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80

CH4 N2O2CO2

Fig. 8. Gas profiles measured in site 8 (biocover Section 5) and site 9 (biocover Section 4) as a function of time of the year.

1188 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

5.6. Temperature profiles in biocover sections

Results from the temperature probes are shown in Fig. 9. Thetemperature is a very important environmental factor for microbialmethane oxidation (Scheutz et al., 2009a). The figure shows datafor the two locations each having 5 probes at different depths inthe compost layer for the period December 2009 until June 2010.The ambient temperature is also shown. The two measuring sta-tions were located in areas with very little gas load (as identifiedby adjacent gas profiling and flux measurements). From the figureit is seen that the temperatures were significantly elevated in com-parison to ambient temperatures and that the temperatureincreased with depth. Even during winter with ambient tempera-tures down to �5 to �10 �C, the temperatures were high andreached near optimal temperatures for CH4 oxidation in the deeperprobes (Scheutz et al., 2009a). The temperatures were generallyhigher at location A (Section 5) than at location B (Section 1).The reason for this is not known, it is unlikely that heat productionby CH4 oxidation was the reason since no CH4 was observed in thesections – as mentioned both sites are located in sections generallylow in gas loading (confer Sections 5.3 and 5.4). The reason must bethe compost respiration in combination with heat transport frombelow (temperatures in the interior of the landfill have not beenmeasured but is expected to be much higher than normal soil tem-peratures (Coccia et al., 2013)). Fig. 9 also shows that during thespring the temperature difference to the ambient temperaturegradually decreases probably due to a lower heat generation fromthe continuously maturing compost (confer Sections 5.4 and 5.5).

In general, the observations are very interesting, because theyindicate that CH4 oxidation in compost based biocovers had closeto optimal conditions in respect to temperature – also during

strong winters, and could be the reason for the observed high effi-ciency of the biocover system during winter season – as supportedby the whole landfill emission measurements, as well as fluxchamber and gas probe campaigns. Measurements of the moisturecontent in the same profiles (data not shown) shows rather highvalues (water-filled porosities of the compost of the order of 30–40% in the methane oxidizing depth) indicating that desiccationis avoided at all times.

5.7. Analysis of the economic viability of the biocover system

The costs of the whole project, which includes activities for finalsoil covering, surface water management and LFG mitigation, was2.55 million DKK and includes project management, design, con-struction, inspection and surveying costs. It is estimated that 42%(approx. 1.06 mill. DKK) of the total costs can be allocated to thebiocover system. A more detailed breakdown on the biocover costsis shown in Table S3 in the supporting material. It is assumed thatthe initial project costs including a baseline study of LFG emissionpatterns and quantities was remunerated with an interest rate of6% p.a. and depreciated over 30 years. In the cost estimate anannual maintenance cost was included of about 30,000 DKK (about4,000€) for addition of new compost, repair of introduced hot spotareas, etc. To estimate an average mitigation price (in DKK/tonsCO2 equivalence.), it was assumed that the baseline CH4 initialemission was 5.4 kgCH4/h (Table 3), the initial CH4 emission afterbiocover establishment was 1.0 kgCH4/h (an average value fromTable 3 not accounting for the first campaign of 9th of April2010) giving an initial mitigation of 4.4 kgCH4/h. It was furtherassumed that the CH4 generation in Cell 0 diminishes to zero in

Tem

pera

ture

(ºC

)Te

mpe

ratu

re (º

C)

20

15

10

5

0

40

35

30

25

20

15

10

5

0

-5

-10

.5 D

ec.

12 D

ec.

19 D

ec.

26 D

ec.

2 Ja

n.9

Jan.

16 J

an.

23 J

an.

30 J

an.

6 Fe

b.13

Feb

.20

Feb

.27

Feb

.6

Mar

.13

Mar

.20

Mar

.27

Mar

.3

Apr.

10 A

pr.

17 A

pr.

24 A

pr.

1 M

ay.

8 M

ay.

15 M

ay.

22 M

ay.

29 M

ay.

5 Ju

n.

Time (days)

Temperature - Site B

Temperature - Site A

70 cm

70 cm

60 cm

60 cm

40 cm

40 cm

20 cm

20 cm

10 cm

10 cm

Atm. temp

2009 2010

Fig. 9. Temperature of the compost layer for different depths as logged at the twosites A and B (for location confer Fig. 4) and for the whole project period (December2009 until June 2010). Ambient temperature is also shown.

C. Scheutz et al. / Waste Management 34 (2014) 1179–1190 1189

30 years. With a Global Warming Potential of CH4 set to 25 (Forsteret al., 2007), this lead to a total mitigated CO2-equivalences overthe 30 years of 14,500 tons. The unit cost was then 165 DKK/tonsof CO2-equil. or 216 DKK/tons of CO2-equil. excluding or includingthe running monitoring and documentation costs, respectively. Thereason for this differentiation was that Danish legislation demandsmonitoring and documentation of the LFG emission as part of thelandfills post-closure plan, so this activity will happen independentof any LFG mitigation activity. A unit cost of 165 DKK/tons of CO2-equil. is relative cost-effective value in comparison to other CO2

mitigation initiatives carried out in society (Enkvist et al., 2007).

6. Conclusions and perspectives

A project protocol including several consecutive elements hasproven successful for establishing a biocover system at an oldunlined landfill for mitigation of methane emissions. An innovativesystem, which included gas collection by established trenchesalong the landfill slopes where most of the baseline emissions wereoccurring, was proven successful with an average mitigation effi-ciency of approximately 80%. Testing of local compost materialsintended for building the bioactive layer of the biocover systemshowed that this step is important since some of the tested com-post materials were found un-suitable as bioactive methane-oxi-dizing material. Whole site emission measurement based on thetracer dilution methodology was a central element of the projectprotocol for quantifying the mitigation efficiency of the biocoversystem. Several experimental activities proved that the biocover

system also was efficient during cold winter, probably due toself-heating of the compost layer. Possible contributing processesare heat generation by the methane oxidation, compost respirationand heat transfer from the underlying warmer waste layers. Thegeneral concern of the usefulness of biocover systems during win-ter in temperate climates was proven invalid in this case.

An innovative gas collection system and an improved gas distri-bution to the biocover sections as compared to the previously con-structed biocover system at Fakse landfill were established. Theuse of a thicker and coarser gas distribution layer may be a reasonto the improved functionality. However, still heterogeneous gasloading was observed, leading to hot spot methane emissions closeto the gas loading locations in the biocover sections. Besides, anuneven gas load between the biocover sections was observed,probably due to the very heterogeneous gas emission pattern iden-tified by the baseline study. This resulted in that four out of thenine biocover sections were basically inactive in respect to meth-ane oxidation as a result of lacking gas load. It is worth noticingthat two of the four biocover sections were not connected to gascollection trenches; the gas load was entirely depending on a ver-tical gas transport into the deep trench under the biocover (similarto a biowindow approach). There still remains some further devel-opment of gas distribution methodologies to obtain optimallyfunctioning biocover systems.

Several activities proved that there was a very limited methaneemission from most of the landfill surface except the few identifiedhot spots close to the edges of three biocover sections. However,emissions of carbon dioxide were observed on all biocover sectionsmainly as a result of compost respiration (aerobic microbial break-down of organic matter) and for the gas loaded biocover sectionsalso as a result of the methane oxidation. The respiration basedemission of carbon dioxide was especially high in the beginningbut decreased probably as a result of maturation of the compostmaterial with time.

An economic analysis of the cost-efficiency of the biocover sys-tem for mitigation of emission of greenhouse gases showed equiv-alent cost to other greenhouse gas mitigation activities.

Acknowledgments

This study was financed by the Danish Environmental Protec-tion Agency and the landfill owner, Klintholm I/S. Thanks to TorbenDolin for graphical support and Ricardo Repetti, Morten Bang Jen-sen and Bent Skov for helping out in the field.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.wasman.2014.03.015.

References

Barlaz, M.A., Green, R.B., Chanton, J.P., Goldsmith, C.D., Hater, G.R., 2004. Evaluationof a biologically active cover for mitigation of landfill gas emissions. Environ.Sci. Technol. 38 (18), 4891–4899.

Bogner, J., Pipatti, R., Hashimoto, S., Diaz, C., Mareckova, K., Diaz, L., Kjeldsen, P.,Monni, S., Faaij, A., Qingxian, G., Tianzhu, Z., Mohammed, A.A., Sutamihardja,R.T.M., Gregory, R., 2008. Mitigation of global greenhouse gas emissions fromwaste: conclusions and strategies from the intergovernmental panel on climatechange (IPCC) fourth assessment report. Working group III (Mitigation). WasteManage. Res. 26 (1), 11–32.

Cabral, A.R., Capanema, M.A., Gebert, J., Moreira, J.F., Jugnia, L.B., 2010. Quantifyingmicrobial methane oxidation efficiencies in two experimental landfill biocoversusing stable isotopes. Water Air Soil Pollut. 209, 157–172.

Christensen, T.H., Kjeldsen, P., 1995. Landfill emissions and environmental impact:an introduction. In: Christensen, T.H., Cossu, R., Stegmann, R. (Eds.), Proc.Sardinia ’95. Fifth International Landfill Symposium, CISA, EnvironmentalSanitary Engineering Centre, Cagliari, Italy, vol. III. pp. 3–12.

1190 C. Scheutz et al. / Waste Management 34 (2014) 1179–1190

Coccia, C.J.R., Gupta, R., Morris, J., McCartney, J.S., 2013. Municipal solid wastelandfills as geothermal heat sources. Renew. Sustain. Energy Rev. 19, 463–474.

Einola, J., Sormunen, K., Lensu, A., Leiskallio, A., Ettala, M., Rintala, J., 2009.Methane oxidation at a surface-sealed boreal landfill. Waste Manage. 29 (7),2105–2120.

Enkvist, P.-A., Nauclér, T., Rosander, J. 2007. A cost curve for greenhouse gasreduction. The McKinsey Quarterly, <http://www.epa.gov/oar/caaac/coaltech/2007_05_mckinsey.pdf>.

Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D.W., Haywood,J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J.,Prinn, R., Raga, G., Schulz, M., VanDorland, R., 2007. Changes in atmospheric constituents and in radiative forcing.In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor,M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis.Contribution of Working Group I to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. Cambridge University Press,Cambridge, UK and New York, NY, USA.

Fredenslund, A.M., Scheutz, C., Kjeldsen, P., 2010. Tracer method to measure landfillgas emissions from leachate collection systems. Waste Manage. 30, 2146–2152.

C. Geck, C., Gebert, J., Röwer, I.U., Scharff, H., Pfeiffer, E.M., 2013. Assessment of theefficiency of a methane oxidation biocover test field. In: Proceedings Sardinia2013, Fourteenth International Waste Management and Landfill Symposium, S.Margherita di Pula, Cagliari, Italy; 30 September – 4 October 2013.

Huber-Humer, M., Röder, S., Lechner, P., 2009. Approaches to assess biocoverperformance on landfills. Waste Manage. 29, 2092–2104.

Humer, M., Lechner, P., 2001. Design of a landfill cover to enhance methaneoxidation-results of a two year field investigation. In: Christensen, T.H., Cossu,R., Stegmann, R. (Eds.), Eight International Waste Management and LandfillSymposium, CISA, Cagliari, Italy. pp. 541–550.

Kjeldsen, P., 1996. Landfill gas migration in soil. In: Christensen, T.H., Cossu, R.,Stegmann, R. (Eds.), Landfilling of Waste: Biogas. E. & FN Spon., London, GB, pp.87–132.

Kjeldsen, P., Scheutz, C., Samuelsson, J., Petersen, P.H., Jørgensen, J.H.B., 2009.Establishing a biocover system for mitigating methane emissions from and old

unlined landfill-baseline studies and biocover construction. In: Sardinia 2009,Twelfth International Waste Management and Landfill Symposium, 5–9October, Sardinia, Italy. Proceedings.

Mønster, J, Samuelsson J., Kjeldsen, P., Scheutz, C., 2014. Quantifying methaneemission from fugitive sources by combining tracer release and downwindmeasurements – test, verification and documentation of the method. WasteManagement (in press).

Parker, T., Childs, A., Pointer, P., Baker, D., Johnson, T., and Wilson, W., 2013. Lessonslearned from first full size methane oxidation biofilter in the UK. In:Proceedings Sardinia 2013, Fourteenth International Waste Management andLandfill Symposium, S. Margherita di Pula, Cagliari, Italy; 30 September – 4October 2013.

Pedersen, G.B., Scheutz, C., Kjeldsen, P., 2011. Availability and properties ofmaterials for the Fakse landfill biocover. Waste Manage. 31 (5), 884–894.

Scheutz, C., Bogner, J., De Visscher, A., Gebert, J., Hilger, H., Huber-Humer, M.,Kjeldsen, P., Spokas, K., 2009a. Processes and technologies for mitigation oflandfill gas emissions by microbial methane oxidation. Waste Manage. Res. 27(5), 409–455.

Scheutz, C., Fredenslund, A.M., Nedenskov, J., Kjeldsen, P., 2010. Release and fate offluorocarbons in a shredder residue landfill cell: 2. Field investigations. WasteManage. 30, 2163–2169.

Scheutz, C., Fredenslund, A.M., Chanton, J., Pedersen, G.B., Kjeldsen, P., 2011a.Mitigation of methane emission from Fakse landfill using a biowindow system.Waste Manage. 31 (5), 1018–1028.

Scheutz, C., Fredenslund, A.M., Nedenskov, J., Samuelsson, J., Kjeldsen, P., 2011b. Gasproduction, composition and emission at a modern disposal site receiving wastewith a low-organic content. Waste Manage. 31 (5), 946–955.

Scheutz, C., Samuelsson, J., Fredenslund, A.M., Kjeldsen, P., 2011c. Quantification ofmultiple methane emission sources at landfills using a double tracer technique.Waste Manage. 31 (5), 1009–1017.

Stern, J.C., Chanton, J., Abichou, T., Powelson, D., Yuan, L., Escoriza, S., Bogner, J.,2007. Use of a biologically active cover to reduce landfill methane emissionsand enhance methane oxidation. Waste Manage. 27, 1248–1258.