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Waste Management 26 (2006) 399–407

Performance of a passively vented field-scale biofilter for themicrobial oxidation of landfill methane

J. Gebert *, A. Grongroft

University of Hamburg, Institute of Soil Science, Allende-Platz 2, 20146 Hamburg, Germany

Accepted 18 November 2005Available online 4 January 2006

Abstract

An upflow biofilter system was operated on a passively vented landfill for the treatment of residual landfill methane. Biofilter methaneemissions as a basis for determining methane removal rates were assessed by manual and automated chamber measurements, by mea-suring methane concentrations in the top layer gaseous phase in combination with gas flow rates, and by evaluating the methane load inthe reverse gas flow following the change of landfill gas flux direction as governed by the course of barometric pressure. Methane removalrates were very high with maximum values of 80 g h�1 m�3. For the observed cases, the limit of biofilter methane oxidation capacity wasnot reached and absolute removal rates were thus linearly correlated to the amount of methane entering the filter. The analysis of meth-ane loads flowing back from the biofilter following phases of longer, continuous and non-oscillating landfill gas emission, however,revealed that in these situations biofilter performance is restricted by deficient oxygen supply. At the oxygen-restricted capacity limit,removal rates are influenced by temperature (positively), methane influx (negatively) and flow rate (negatively) as a measure for the dis-placement of oxygen. These situations, however, account for only 12% of all emission phases. The investigated biofilter capacity, asderived from laboratory analyses of methanotrophic activities, is sufficient to oxidise 62% of the methane load emitted annually. Fieldand laboratory data provide a stable basis for the dimensioning of filters in future applications.� 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Methane-oxidising bacteria (MOB) or methanotrophsuse methane as the sole carbon- and energy source, thusplaying an integral role in global carbon cycling (Hansonand Hanson, 1996; Le Mer and Roger, 2001). Besides theirrecognised influence on the carbon balance of naturalmethane-influenced habitats such as marshland soil,swamps, lakes or tundra soil, methanotrophs increasinglygain attention for the abatement of anthropogenic methaneemissions, e.g., from landfills. The microbial oxidation ofmethane in biofilters is considered an alternative for thetreatment of landfill methane emissions that do not meetgas flow rate and methane content requirements for utilisa-tion or flaring (Figueroa, 1996; Streese and Stegmann,

0956-053X/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2005.11.007

* Corresponding author. Tel.: +49 40 42838 6595; fax: +49 40 428382024.

E-mail address: j.gebert@ifb.uni-hamburg.de (J. Gebert).

2003). Methane is aerobically oxidised to carbon dioxide,thereby serving as the sole carbon- and energy source forthe methane oxidising bacteria. Biofiltration as a meansfor the abatement of anthropogenic methane emissionsmay be applied at landfills in the initial phase of operation,old landfills or sites containing material of low gas genera-tion rate. The latter field of application will prove increas-ingly relevant in Europe when the EC landfill directive(1999/31) becomes effective in 2005, which stipulates thatonly material of low biological activity may be deposited.

Within the framework of a cooperative research projecttitled ‘‘Microbial Reduction of Methane Emissions’’, a bio-filter plant embedded in the recultivation layer has beenoperated on a passively vented landfill for dredging materialin Hamburg, Northern Germany, since 1999. The aim of thestudy was to test the applicability of biofilters for the abate-ment of landfill methane emissions in a field-scale approach.Results on landfill emission behaviour, on the suitability ofthe chosen filter materials, on batch kinetics and on met-

400 J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407

hanotroph community structure as investigated during theproject have been reported before. The investigated systemis characterised by highly dynamic passive landfill gas emis-sion in reaction to changes in the course of barometric pres-sure (Gebert and Grongroft, in press). The direction of gasflux changes on average every 20 h, causing a subsequentvariable methane influx into the biofilter of 0–247 g CH4 h

�1 m�3. The biofilter contains a dense meso-philic type II-dominated methanotroph community (Gebertet al., 2004) characterised by high values both for the half-saturation constant KM (15.1 lM) and for the maximummethane uptake rate vmax (2.6 lmol CH4 g dw�1; Gebertet al., 2003). This paper presents biofilter in situ methaneremoval rates and their factors of influence.

2. Materials and methods

2.1. Biofilter setup

Fig. 1 schematically illustrates the arrangement of gaswells, emission monitoring unit, and the biofilter, as oper-ated from October 1999 to date.

Landfill gas (LFG) from two gas wells venting differentsections of the landfill (1, 2) was combined (3) and thensplit up again to be fed to the two biofilter chambers (4,5). The biofilter is an upflow system consisting of five layers(top to base) inside a 15 m3 polyethylene container with aninclined base: A, humic topsoil (loamy sand, 10 cm) cov-ered with grass vegetation; B, sand (1.5 cm); C, gravel(1.5 cm); D, crushed porous clay (67 cm); E, gravel forwater drainage (10–30 cm). The purely mineral porous claymaterial constituting most of the filter volume is character-ised by a very high bulk pore volume of 83%, as well as by ahigh share of pores with a diameter of >50 lm (71%), pro-viding for a high gas permeability. The pH of the biofilterleachate is around 7, thus indicating favourable conditionsfor the activity of methanotrophic bacteria. Drawbacks ofthe porous clay material concern its high salt load, result-ing from the production process, which may impair met-hanotrophy in the initial phase of biofilter operation

Fig. 1. Gas well and biofiltration

(Gebert et al., 2003). Washing of the filter material priorto use for biofiltration purposes is therefore recommended.

The biofilter container is subdivided into chamber 1 at asize of 6 m3 and chamber 2 at a size of 9 m3, which can beoperated independently. Landfill gas is distributed via hor-izontal and parallel slotted gas supply pipes embedded inthe top centimeters of the drainage gravel layer (E). Atmo-spheric oxygen enters the biofilter by diffusion and advec-tion via the biofilter surface. As the biofilter is embeddedin the landfill recultivation layer without supplementaryheating, biofilter temperature follows ambient temperature.Humidification of the filter material relies on precipitationand landfill gas humidity only.

Parameters of landfill gas emission and biofilter opera-tion, monitored at a resolution of 10 min by an automateddata collection system, include: landfill gas composition(CH4, CO2, O2), flow rate, temperature and pressure asmeasured in Section 3 of the biofilter supply pipe(Fig. 1); atmospheric pressure and temperature; tempera-ture, humidity and pressure within the biofilter. Sensorspecifications are given in Table 1.

2.2. Methane emission and oxidation

Methane removal was calculated from the differencebetween methane emissions from the biofilter surface andmethane load onto the biofilter. The latter was monitoredcontinuously via automated collection of data on gas com-position and gas flow and was therefore well known. Meth-ane emissions from the biofilter were studied using thefollowing methods: (1) surface emissions as calculated frommanual and automated closed chamber measurements andgas composition at a 5 cm depth in combination with gasflow rate and (2) analysis of the mass methane backflowfrom the filter into the landfill after LFG flux reversal.The influence of the following parameters on the calculatedmethane removal rates was checked: temperature at 35 cmbeneath the biofilter surface and corresponding methano-trophic activity as derived from a laboratory experiment(compare the mesophilic temperature reaction presented

unit setup. 1–5, A–E: see text.

Table 1Range, error, resolution and logging frequency of sensors used to characterise landfill gas emission and biofilter operational parameters

Parameter Sensor Range Error Resolution Loggingfrequency

LFG pressure XCX 0.3 DN, SENSPECIAL CO. �20 to +20 hPa ±0.5% of range 0.001 hPa 10 minLFG, atm. andbiofilter temperature

PT100, DRIESSEN UND KERN CO. �10 to +80 �C ±0.3 �C 0.01 �C 1 h

LFG flow rate TSI� 8475-075 0.05–2.5 m s�1 ±1.6% 0.001 m s�1 10 min

LFG composition NDIR-Gas Analyser BE-4000, BERNT CO. CH4: 0–100 vol.% CH4, CO2: ±2% of range 0.01 vol.% 10 minCO2: 0–100 vol.% O2: ±1% of rangeO2: 0–25 vol.%

Atm. pressure PTB 101C, VAISALA CO. 900–1100 hPa ±0.25 hPa 1 hPa 10 minBiofilter humidity(water potential)

Tensiometers with pressure gauge XCX 15 DN,SENSPECIAL CO.

�850 to +150 hPa ±5 hPa 0.25% 30 min

Landfill Gas (LFG) pressure, temperature, flow rate and composition were measured in section 3 of the emission monitoring unit (see Fig. 1). Sensor errorsare given as declared by the manufacturers.

J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407 401

in Gebert et al., 2003; Abb. 9 A; optimum temperature: 35–38 �C, activation energy: 74.5 kJ mol�1), mean methaneload, mean and maximum gas flow rate, mean LFG oxygenconcentration, and emission phase duration. For the anal-ysis of the mass methane backflow, the impact of mean gasflow rate and oxygen concentration of the reversed fluxwere assessed, as well as duration of flux reversal untilthe methane content in the biofilter supply pipe hadreached <0.1 vol.%. Values for the quoted parameters wereaveraged over exactly that period of time that was sufficientfor the gas flow to exchange the biofilter air filled porespace once (9.4 m3 at field capacity water content).

Statistical analyses were carried out using standard soft-ware Origin� version 6.1.

2.2.1. Surface methane emissions – manual chamber and gasprofile measurements

Acrylic glass chambers (0.1 m3) were used to measuremethane emission from the biofilter surface (100 · 100 ·10 cm, Fig. 2). Chambers were placed on permanentlyinstalled stainless steel frames with a u-profiled rim. Theframes were inserted 10 cm into the biofilter topsoil layer.To secure air tightness, the rim was filled with water. Dura-tion of chamber operation was 12 min, gas samples werewithdrawn every 2 min. The increase of methane concen-tration during this period always proved to be linear. Inorder to ensure mixing of the chamber atmosphere andtherefore the retrieval of representative gas samples, thechamber atmosphere was continuously circulated using amembrane pump.

Fig. 2. Setup of closed chamber measurements.

Chambers obstruct advective flow. The increase ofmethane concentration inside the chamber with time thusrepresents diffusive emission across the biofilter surface.By transforming Fick�s first law as given in Eq. (1), themethane concentration c in depth x1 beneath the biofiltersurface can be calculated from the diffusive flux j:

c1 ¼�x1j�Deff

þ c0 ð1Þ

where c1 is the methane concentration in depth x beneathbiofilter surface (mol m�3); c0 is the methane concentrationat biofilter surface (mol m�3); x1 the depth beneath biofiltersurface (m); Deff the effective diffusion coefficient (m2 s�1) =5.4 · 10�7 m2 s�1, determined empirically; and j is the diffu-sive flux (mol m2 s�1).

The calculations were carried out for x1 = 0.01 mbeneath biofilter surface. The methane concentrations thusdetermined were compared to gas flow rate as measured inthe biofilter gas supply pipe to yield a methane emissionsvalue which could be related to the methane load ontothe biofilter. In analogy, methane concentrations at a5 cm depth as determined regularly within the frame of ver-tical gas profile measurements were used to calculate emis-sions, using the gas flow into the biofilter at the time ofsampling.

2.2.2. Surface methane emissions – automated chamber

measurementsIn order to account for the highly dynamic LFG emis-

sions and the subsequently variable gas load onto the bio-filter, an automated chamber system was developed. Thesystem consisted of two chamber boxes and a mobile lid,which was pneumatically shifted from one box to the otherevery 12 min. Air tightness of chambers was achieved witha flexible rubber sealing fitted to the lower side of the lid.As with the manual chambers described above, the cham-ber atmosphere was continuously circulated. Gas composi-tion samples were automatically withdrawn from thecirculating flow and analysed for methane and carbondioxide every 3 min by a Bruel and Kjaer BK1302 photo-acoustic infrared spectrometer. The system was operated

402 J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407

by a programme developed under Power Basic�version 3.5

software. Data was processed to yield methane emissionsas described above for the manually generated data.

2.2.3. Methane load backflow following flux reversal

Due to the sensitivity of LFG emission to barometricpressure, gas flow direction reverses regularly with atmo-spheric air flowing into the landfill via the biofilter (Gebertand Grongroft, in press). Methane that is left in the biofil-ter at this moment will flow back into the landfill as well.As the reversed gas flow passes all of the sensors installedin the biofilter supply pipe (Fig. 1), composition and flowrate of the reverse fluxes are measured and may be usedto calculate the methane loads leaving the biofilter. If thisis related to the mass of methane that entered the biofilterin the preceding emission phase or to the mass methanepresent in the biofilter just prior to flux reversal, a methaneremoval rate can be calculated. This removal rate willaccount for methane oxidation occurring during that emis-sion phase, as well as for methane oxidation occurring afterflux reversal. Amounts of methane present in the biofilterregardless of microbial oxidation will be in equilibriumwith LFG methane concentration entering the biofilterand can thus be deduced from this value. The relationshipbetween LFG methane concentration and amounts ofmethane present in the biofilter was determined empiricallyby quantification of the methane load flowing back afterflux reversal shortly after the biofilter was put into opera-tion and the microbial community had not establishedyet. In general, the amount of methane that may be storedin the biofilter is governed by the volume of air filled porespace, sorption to the biofilter material and to microorgan-isms, as well as by solution in the aqueous phase. For theanalysis, data sets were chosen where LFG emission priorto flux reversal was characterised by relatively long phasesof continuous, non-oscillating gas flux at high methaneconcentrations.

2.2.4. Biofilter methane oxidation efficacy as derived from

laboratory batch experiments

In order to track the growth and activity of the methan-otrophic population following first operation, the biofilterwas regularly sampled across the entire depth. Methaneoxidation activity was measured as described in Gebertet al. (2003). The depth-related methane oxidation activi-ties were summed to give potential methane removal ratesfor the entire biofilter volume. According to the tempera-ture function of methane oxidation given in Gebert et al.(2003), a temperature-dependent methane removal capac-ity was calculated for each filter temperature as automati-cally recorded in 10 min intervals at five depths beneaththe filter surface. From that methane removal capacity,the simultaneous methane influx into the biofilter, equallyautomatically recorded in 10 min intervals, was subtracted,yielding a distribution of values for methane removal ratesas related to in situ temperature and in situ methane influx.Calculations were carried out to cover a period of 1 year.

2.3. Gas chromatography

Analyses of samples from the manual chamber and gasprofile measurements for CH4, CO2, O2 and N2 were car-ried out using a Carlo Erba Vega Series 6000 gas chro-matograph. The GC was equipped with an FID, as wellas with a temperature conductivity detector (TCD). Thedetection parameters were: columns Haye Sep D, 20 ft,mesh 100/200 and Molecular Sieve 5 A, 7 ft, mesh 60/80.Injection-, oven and detector temperatures were 100 and70 �C, FID 200 �C and TCD filaments 180 �C.

3. Results and discussion

3.1. Biofilter temperature and water balance

Temperature is one of the most relevant factors of influ-ence for all biochemical transformations. If all other fac-tors are in their optimum, biofilter temperature will thusdetermine the rate of methane oxidation. As the biofilteris integrated into the landfill cover system and is not exter-nally heated, the temperature regime of the investigatedbiofilter in general follows ambient temperature, with theextent varying according to the thermal conductivity ofthe chosen filter materials.

As expected, the variations in ambient temperatures, asmeasured by Pt 100 sensors, are increasingly levelled offwith increasing depth from the filter surface (Fig. 3). At54 and 71 cm depths, the daily dynamics as induced bysolarisation are no longer detectable. During summer2001, the temperature at a 20 cm depth even exceededambient temperature as the vegetation cover was still littleand the dark, humic topsoil material absorbed the heat.Due to the supply of landfill gas and heat generated bymicrobial activity, the biofilter can be considerably warmerthan ambient temperature during winter times (Fig. 3B).Table 2 summarises characteristic values of the biofiltertemperature regime.

The average annual temperature increases with increas-ing depth from the filter surface but in general is very sim-ilar across the filter profile (10.5–13.5 �C). However, thetemperature amplitude varies pronouncedly between layersclose to the surface and deeper layers, with a maximum of39.4 �C close to the surface and only 16.8 �C at the 71 cmdepth. This is in line with the levelling off of temperaturevariations with increasing depth as reported above. There-fore, methane oxidation in the top layer is subject to consid-erably higher, but also to considerably lower temperatures,presumably inducing higher and lower oxidation rates, thanin the deeper layers. These are characterised by a muchmore constant temperature regime and are thereforeassumed to show more even methane oxidation rates.

Microbial activity is bound to the availability of waterand is therefore influenced by the pore size distribution,i.e., by the water potential of the material colonised bythe microorganisms. In the case of methane oxidation,rates may also be strongly influenced not only by the avail-

11.6 12.6 13.6 14.6 15.6 16.6 17.60

5

10

15

20

25

30

2001

5 cm 19 cm36 cm 54 cm71cm air

1.1 2.1 3.1 4.1 5.1 6.1 7.1-10

-5

0

5

10

Tem

pera

ture

[˚C

]Te

mpe

ratu

re [˚

C]

2002

A

B

Fig. 3. Exemplary course of the variation of biofilter temperature indifferent layers in summer (A) and winter (B).

Table 2Characteristic values of the temperature course across the biofilter profileover 1 year, derived from two sensors per layer

Depth beneathfilter surface (cm)

Avg.(�C)

Max.(�C)

Min.(�C)

Amplitude(�C)

Cases forT < 0 �C (%)

5 10.5 33.7 �5.7 37.4 11.419 11.0 29.0 �0.9 29.9 3.836 11.9 25.5 1.7 23.8 054 13.0 23.1 4.9 18.2 071 13.5 23.0 6.2 16.8 0

Resolution: 1 measurement per hour, n = 8496.

J J A M J J A-100

0100200300 80 cm

Wat

er p

oten

tial [

hPa]

2001 2002

-900

-600

-300

0

50 cm

-900

-600

-300

0

20 cm

Fig. 4. Course of filter material water potential in 20, 50, and 80 cm depthbeneath biofilter surface during the summers of 2001 and 2002 as revealedby two parallel sensors. Axis break: values erased due to defective sensors.

M J J A S O N D J F M A M J J A S

0

10

20

30

40

50

influx

emission

Met

hane

load

[g h

-1 m

-3]

2001-2002

Fig. 5. Methane influx and emissions as determined from surfaceemissions (manual chambers and gas profile data, see Section 2.1).

J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407 403

ability of water, but also by water content as both sub-strates of methane oxidation, methane and oxygen, arepoorly water-soluble. The humidity status of the biofilterthus is crucial, especially in the light of the fact that theinvestigated system is not irrigated but relies on precipita-tion and landfill gas humidity only.

Fig. 4 shows that during the critical summer months thefilter material in general was well humidified, as indicatedby the continuously low water potential. Only in the sum-mer of 2001 a noticeable desiccation occurred in the top lay-ers, causing higher water potentials of up to �650 hPa. In

the expanded clay material, this corresponds to water con-tents of about 5 vol.%. As experiments confirmed (Gebertet al., 2003), even relatively low water contents in this mate-rial do not impair methane oxidation activity.

3.2. Methane removal as determined from surface emissions

Fig. 5 shows methane influx and emission rates as deter-mined from surface emissions, while Fig. 6 compares abso-lute methane removal rates to methane influx for the samedata set. The data indicate that the absolute methaneremoval rate increases with an increasing influx of methaneinto the biofilter. For the observed cases, the limit of filterperformance in terms of absolute methane removal rateswas therefore not reached and consequently its methaneoxidation potential not fully tapped. Correspondingly,there was no relationship between absolute removal ratesand relative removal rates (% of methane influx), but rather% removal rates were very high, and independent of theabsolute amounts of methane oxidised. The maximum

0 10 20 30 40 50

0

10

20

30

40

Met

hane

rem

oval

rat

e [g

h-1 m

-3]

y = -0.34 + 0.77 xR2 = 0.77n = 34p < 0.0001

Methane influx [g h-1 m-3]

linear regression

100 %

Fig. 6. Methane removal rates as determined from surface emissions(manual chambers and gas profile data). The 100% line indicates completeoxidation of landfill methane entering the biofilter.

3 4 5 6 7 8 9 10 11 12 13 1410-5

10-4

10-3

10-2

10-1

100

101

102

A

Me

htena

aol d

g[ h

1-m

3-]

June 2002 [day]

July 2002 [day ]1 2 3 4 5 6 7 8 9 10 11

influx

emission

10-5

10-4

10-3

10-2

10-1

100

101

102

B

Me

htena

aol d

g[ h

1-m

3-]

influx

emission

Fig. 7. Methane influx and emissions as measured with the automatedchamber system. Note the logarithmic y-scaling.

1 2 3 4 5 6 7 8

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Met

hane

rem

oval

rat

e [g

h-1 m

-3]

Methane influx [g h-1 m-3]

100 %

Fig. 8. Methane influx and methane removal rate as determined frommass methane backflow after LFG flux reversal. The 100% line indicatescomplete oxidation of landfill methane entering the biofilter.

404 J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407

absolute methane removal rate observed for this data setwas 37.3 g h�1 m�3. Neither absolute nor relative methaneremoval rates could be related to any other of the potentialfactors of influence tested (see Section 2.2). Rather, meth-ane oxidation was primarily determined by the availabilityof methane itself. The correlation coefficient (R2 = 0.77)however indicates that mass methane entering the biofilteraccounted for the largest part but not for the entire datasetvariability and that therefore there are additional factors ofinfluence determining the extent of methane removal. Thetemperature range for the dataset depicted in Fig. 6 cov-ered a wide span from 3 to 24 �C but could not be shownto influence methane removal rates for this dataset.

Fig. 7 shows methane influx and emissions as deter-mined by the automated chamber measurement. Methaneemissions from the biofilter during the summer of 2002were continuously more than two orders of magnitudelower than methane input into the biofilter. Methane oxi-dation rates were thus close to 100%. Maximum inputand therefore maximum absolute methane removal rateswere around 80 g CH4 h

�1 m�3. Methane emissions in gen-eral only traced LFG emission dynamics in case of higherinfluxes and stronger influx variability.

3.3. Methane removal as determined from the methane load

backflow

Fig. 8 shows methane influx and methane removal ratesas determined from the mass methane backflow after LFGflux reversal. From the results it is concluded that underconditions of longer and steady gas emission, biofilter per-formance is low, despite the fact that before flux reversalmethane influx into the biofilter was a lot less than forthe cases shown in Fig. 6. As for the cases reported meth-ane concentrations entering the biofilter were on average ashigh as 35 vol.% (maximum: 50 vol.%); it can be ruled outthat the low performance is a result of unfavourable kinetic

conditions with respect to methane (KM = 15.1 lM, equals1.2 vol.% in the gaseous phase at field capacity water con-tent, see Gebert et al., 2003). Negative values for methane

0 20 40 60 80 100

0

20

40

60

80

100

Met

hane

rem

oval

rat

e m

eas.

[%]

Methane removal rate calc.[%]

Fig. 9. Relative methane removal rates in the summers of 2001 and 2002,measured versus calculated data. Multiple regression function for thecalculated data: see text.

0 5 10 15 20 250

20

40

60

80

100 y = 8.1 + 3.4 x

n = 11

r = 0.74, p < 0.01

Met

hane

rem

oval

rat

e [%

]

Temperature [˚C]

Fig. 10. Methane removal rates in the period 05.12.2001–29.07.2002 asdependent on biofilter temperature (measured in the centre of the filterprofile).

J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407 405

removal rates most probably result from uncertainties (inthis case an underestimation) in calculations of the massof methane that entered the biofilter prior to LFG fluxreversal. As explained in Section 2.2, the mass of methaneentering the filter was calculated for the period of time thatwas sufficient for the gas flow to exchange the biofilter airfilled pore space once. For these calculations, field capacitywater content and therefore a constant pore volume wasassumed. Depending on the exact humidity status of the fil-ter material this may, however, vary to some extent.

The failing methane oxidation performance, as shown inFig. 8, is assumed to be due to insufficient oxygen supply tothe biofilter methanotrophic organisms, occurring when thediffusive oxygen flux is lower than the counterdirectiveadvective LFG flow or insufficient during longer phasesof steady, continuous LFG emissions lacking the flow oscil-lations as observed for most other phases. Using laboratorycolumns filled with biofilter material, diffusion coefficientsfor oxygen as well as oxygen profiles developing against dif-ferent gas flow rate were measured (manuscript in prepara-tion, data not shown here). The data was used to simulatevertical oxygen concentrations developing under particularLFG influx conditions using a simple box model. Resultsshowed that the average LFG flow rate prior to flux rever-sal for the dataset shown above (0.014 m3 h�1 m�3) allowsfor an oxygen concentration of only 8–10 vol.% across thefilter profile, assuming a continuous flow for a period of24 h. However, for the investigated landfill this situationoccurs very rarely. The phases selected for the interpreta-tion of the mass methane backflow account for only 12%of all phases. Rather, LFG emission at the investigated siteis characterised by a very frequent reversal of gas flow direc-tion. More than half of all emission and air influx phasesare shorter than 10 h. As opposed to the data sets presentedin Section 3.2, methane removal rates as limited by oxygenavailability could be distinctly related to biofilter tempera-ture (Fig. 10) and to the methane influx (Fig. 9). The rela-tionship to temperature could be described by a simplelinear regression (for regression parameters, see box insideFig. 10), whereas the relationship to methane influx wasbest reflected by the following multiple regression:

y ¼ 190� 44� CH4-stored� 793� flow rate

R2 ¼ 0:99; p < 0:001; n ¼ 7ð2Þ

where CH4-stored = methane present in the biofilter priorto flux reversal and flow rate = mean gas flow rate priorto flux reversal.

Both parameters are statistically significant at the 1%level.

The positive correlation to temperature plausibly indi-cates that the methane removal rate increases with temper-ature (Fig. 10, data for winter and summer conditions),whereas under fairly constant temperature conditions (astrue for the ‘‘summer only’’ data presented in Fig. 9)removal rates decrease with an increasing amount of load(CH4-stored, see Eq. (2)), indicating that under the givenconditions the biofilter is operated at its capacity limit. This

is in line with the fact that for the cases mentioned, abso-lute and relative methane removal rates correlate strongly.Eq. (2) further indicates that absolute methane removalrates are negatively correlated to the mean gas flow ratein the period prior to flux reversal. The gas flow rate is ameasure of the oxygen availability in the biofilter, as theadvective inflow displaces oxygen present in the filter andhampers diffusive influx.

Fig. 11 shows a cumulative curve for the frequency distri-bution of the difference between methane influx and simul-taneous methane oxidation capacity of the filter. In around70% of all cases the difference is positive, indicating cases ofexcess capacity. In these situations the methane load ontothe biofilter can be oxidised completely, and could even beincreased. In 30% of all cases, biofilter capacity is not suffi-

-60 -40 -20 0 20 40 60 80

0

20

40

60

80

100

capacity < influx

capacity > influxuS

ac m

]%[ ses

CH4 oxidation capacity minus CH

4 influx

[g m-3 h-1]

Fig. 11. Cumulative curve of biofilter methane oxidation efficacy ascalculated by subtracting methane influx from biofilter methane oxidationcapacity, n = 7965.

Table 3Methane removal rates by different actively vented biofilters

Author Filter material Temperature (�C) Methaneremoval(g h�1 m�3)

Mennerich (1986) Compost 30 38Sly et al. (1993) Glass tubes ? 21Figueroa (1996) Compost 43 50Streese andStegmann (2003)

Compost 30 65

Wilshusen et al.(2004)

Compost Roomtemperature

4.1 g h�1 m�2

Melse and Van derWerf (2005)

Compost/perlite 12 11–15

Table 4Methane removal rates as determined for various landfill covers

Author Material Temperature(�C)

Methaneremoval(g CH4 h

�1 m�2)

Whalen et al.(1990)

Loamy landfill cover 25 2

Jones andNedwell (1993)

Humic landfill cover 22 0.3

Kightley et al.(1995)

Sandy landfill cover 20 5–7

Humer andLechner (2000)

Sewage sludge andwaste composts

18 1–16

406 J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407

cient to remove LFG methane completely. Analysis of thedistribution of methane loads over 1 year reveals that over-all 62% of the annual methane load can be oxidised. Win-terly conditions in combination with high fluxes accountfor the share of methane that is not removed (38%). It hasto be noted, however, that the above calculations assumesufficient oxygen supply to the filter microorganisms, whichmay not always be the case. Removal rates thus may belower. On the other hand the calculation presumes constantLFG fluxes into the biofilter. As the emission behaviour ofthe landfill is highly dynamic (Gebert and Grongroft, inpress), however, this does not properly reflect field condi-tions. LFG flux frequently is stagnant or oscillates in reac-tion to barometric pressure changes, resulting in advectiveoxygen flux into the filter. The share of methane oxidisedmay thus also be higher than indicated in Fig. 11.

3.4. Comparison of biofilter performance to methane removal

rates found for other biofilters and landfill covers

Tables 3 and 4 present methane oxidation rates foractively vented biofilters under controlled laboratory condi-tions and methane oxidation rates as measured for differentlandfill covers, respectively. The performance of the investi-gated biofilter with amaximumof 80 g h�1 m�3 asmeasuredwith the automated chamber iswithin the sameorder ofmag-nitude as reportedbyother authors for biofilters.However, itmust be noted that the removal rates observed here wereachieved in the field and under in situ conditions, thus atlower temperatures (see Table 2) and also not under condi-tions of controlled methane and oxygen supply. As the bio-filter on average is 1 m in depth and removal rates maythus be related to area units as well, results may also be com-pared to those given byWilshusen et al. (2004) and to valuesgiven for the performance of landfill covers (Table 4), whichare 5–270 times lower. This is not surprising as landfill coversin general receive by far less methane than biofilters and arealso usually not optimised for the purpose ofmethane oxida-tion with respect to parameters such as gas distribution, gas

permeable pore space (i.e., mass transfer, methane and oxy-gen supply), or water content.

Optimised absolute methane removal depends on differ-ent factors, such as favourable kinetic conditions (zeroorder) and thus minimised mass transfer limitation, ade-quate water and nutrient supply to the methanotrophicorganisms as well as degradation-resistant and thus physi-cally stable filter material. The biofilters described by Stre-ese and Stegmann (2003) and by Melse and Van der Werf(2005) were operated with inlet methane concentrations of2.5 and 3 vol.%, respectively. For the biofilter presentedhere, these concentrations would be below zero order deg-radation rates (Gebert et al., 2003). Wilshusen et al. (2004)used very high inlet concentrations (10–70 vol.% methane),and oxidation zones with sufficient oxygen supply werethus restricted to the top few decimeters of the column.The authors report biofilter methanotrophic cell countsbetween 2.2 and 7.3 · 106 cells per g dry weight, which isbetween two and four orders of magnitude lower thanthose determined for the biofilter presented here. Streeseand Stegmann (2003), Wilshusen et al. (2004), Hilgeret al. (2000) and Humer and Lechner (2001) report aboutthe formation of extrapolymeric substances (EPS) in biofil-ters and landfill covers. These block pore space and hampersubstrate supply to the microorganisms, thus impairingoptimum methanotrophic activity. EPS were neverobserved in the biofilter investigated here. All authors pre-sented in Table 4, except for Sly et al. (1993), used compost

J. Gebert, A. Grongroft / Waste Management 26 (2006) 399–407 407

as filter material. As compost is an organic material, it is ingeneral prone to microbial degradation, and thus to the set-tlement and clogging of pore space. The organic nature ofcompost also allows for the swift formation of anaerobicniches and may thus induce methane formation if watercontents are not very carefully controlled (experimentalresults presented in Gebert, 2004). For these reasons purelymineral material with an extremely high gas permeablepore volume (71%, total pore volume = 83%) was preferredfor the biofilter investigated here.

4. Conclusions

In the investigated landfill, biofilter in situ methaneremoval rates as revealed by surface emission measurementswere shown to be governed primarily by methane supply asmeasured for methane influxes of up to 46.4 g h�1 m�3,biofilter capacity was thus not exhausted (Fig. 6).Maximumobserved methane removal rates were as high as80 g h�1 m�3. In cases of longer and constant periods ofLFG emission (12%of all emission phases), themethane oxi-dation capacity presumably is strongly limited by insufficientoxygen supply. In these situations, methane removal rateswere inversely related to methane influx and influenced bytemperature (positive correlation) and gas flow rate as amea-sure for the displacement of oxygen. For a given system theinfluence of temperature on the relative share of methaneremoved is thus only relevant when the biofilter is operatedat or close to its limit of methane oxidation.

With respect to oxygen supply, the satisfactory opera-tion of a passively vented biofilter, which depends oncounterdirective oxygen influx via the filter surface, isstrongly related to the dynamics of landfill gas emission.Although it could be demonstrated that at the investigatedlandfill methane removal rates are very high, potentiallyallowing for complete oxidation of 62% of the methaneload emitted annually, operating the biofilter in the samemanner may not be suitable at another site exhibiting moreconstant, unidirectional LFG fluxes and thus not allowingfor advective oxygen input. For those situations, biofilterperformance would have to be supported by an additional,active introduction of oxygen/atmospheric air across thefilter profile or by increasing the diffusion gradient throughan adjustment of the surface-to-volume ratio. If the diffu-sion coefficients of the filter materials are known, maxi-mum LFG flow rates that still allow for sufficientdiffusive oxygen influx can be calculated.

Investigations into the microbiology and the suitabilityof the filter material have shown that the studied biofilteris intensely populated with methanotrophs across the entirevertical profile. Batch methane uptake rates of the type II-dominated population are very high. The filter materialcombination allows for sufficient gas permeability whilesecuring enough humidity for microbial activity. The for-mation of extrapolymeric substances as reported for landfillcovers and biofilters was never observed during the regularfilter material sampling over the entire period of investiga-

tion. Overall, the investigated biofilter represents a robustand, due to the low operational and maintenance expenses,also a low-cost solution for the effective treatment of resid-ual LFG emissions that no longer meet methane contentand/or flow rate requirements for utilisation or flaring. Incombination with the previously published laboratory dataon the influence of temperature and water content (Gebertet al., 2003), the acquired field data provide a good basis forthe dimensioning of biofilters for future applications.

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