es froe exo inife/Pringayewe

parameters. This fact must be included when considering the techniques and economics of projects

logical behaviour proved to have an important role in minimising gas emissions to the atmosphere. Cap-

1. Introduction

The municipal solid waste (MSW) situatps arental cthe pogativene, car

2eq

potential (MCT, 2010). The evaluation of the gas generation poten-tial under specic conditions is extremely important to guaranteemore realistic predictions and increase investor condence. Unfor-

region is themost humid part of the country with an annual rainfallof approximately 2500 mm and very low evaporation rates(

ManF.J. Maciel, J.F.T. Juc /Waste(INMET, 2010). It is important to mention that the population isconcentrated in the coastal and southeastern regions of Brazil.Hence, the MSW generation is greatest in those regions.

The composition of MSW depends on the social, economic, andcultural aspects of the population. Local economic development isan important factor that inuences waste composition in Brazil. Atypical waste gravimetric composition (wet basis) varies from 45%to 60% organic matter, 12% to 23% paper/cardboard, 6.5% to 20%plastics, 1.1% to 3.9% glasses, and 1.8% to 4.3% metals, among othermaterials (Maciel, 2009). In relation to the cover layers, Brazilianlandlls generally have a monolithic compacted soil cover withsupercial vegetation. However, in most open dump sites, thereis no cover system to minimise gas emissions and other environ-mental concerns. Therefore, the waste composition and nal coverlayer characteristics are very different from those of the landlls ofother developed countries, and this difference should be consid-ered when evaluating waste biodegradation and LFG production.

The purpose of this paper is to present the results and method-ology used to evaluate the generation and emissions of gases in alarge-scale Experimental Cell in Brazil. The study included analyses

Fig. 1. Brazilian precipitation, evaporation, and ambient temperature maps and theagement 31 (2011) 966977 967of gas production and collection efciency parameters, geotechni-cal and biological cover layer behaviour, and estimates of CH4emissions into the atmosphere. This study helps to advance ourunderstanding of the landll gas emission situation in Brazil andcould support future projects developed in regions with similar cli-mates and operational conditions.

2. General site description

The Muribeca Landll is located in a tropical coastal area of theMetropolitan Region of Recife/PE in the northeastern region of Bra-zil. This landll has been monitored since 1994 and occupies anarea of 65 hectares (Juc et al., 1998). Over 2400 t/d of domesticwaste were deposited there until 2009, when this landll reachedits nal storage capacity and was closed. There is over 15 milliontons of waste in this area, and the maximum landll thickness isabout 70 m. A passive gas ventilation system was installed in onlythe last 2030 m of the landll and consists of 50 vertical gas wells.

A research project that involved the construction of a large-scale Experimental Cell close to the Muribeca Landll disposition

main climate parameters from Recife/PE (Period: 19611990; INMET, 2010).

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

area was initiated in 2007 to investigate the biodegradability char-acteristics of the waste and the gas and energy production associ-ated with the local climate and operational conditions. Thedimensions of this cell were 65 m long 85 m wide 9 m thick,and it was divided into two waste layers with thickness of 6.0 m(bottom) and 3.0 m (top). The slope inclination of the cell basewas 30. Fig. 2 illustrates the layout of the Experimental Cell anda frontal view of the cell before and after its construction. The baselayer of the cell consisted of compacted clayey soil with a thicknessthat varied from 0.40 to 0.90 m. A containment wall was designedaround the cell with a height of 1.04.0 m (45 slope inclination) tominimise the percolation of the leachate outside the cell and tofacilitate waste compaction during the waste lling period. Theleachate drainage system was constructed in the cell base layer(414 m of drains with a gravel layer of 0.50 0.40 m transversalarea). The gas drainage system consisted of ve vertical gas wellswith external diameters of 0.70 m and with internal PVC tubes(/ = 0.11 m). The vertical gas wells were then connected to a pilotpower unit (20 kW) via a horizontal gas collection system.

The total surface area of the Experimental Cell was 5881.4 m2,of which 1625.3 m2 corresponded to the plateau area where thethree experimental covers of a methanotrophic layer (590.2 m2),

a capillary layer (500.3 m2), and a conventional layer (534.8 m2)were implemented. Fig. 3 illustrates the geometric characteristicsof the experimental cover layers. Except in these experimental lay-ers, the nal cover layer consisted of a compacted clayey soil with athickness ranging from 0.40 to 0.90 m. The cell did not have anintermediate waste cover layer.

The Experimental Cell was lled from April 2007 to January2008. The waste amount was registered by balancing all vehiclesentering the cell, and in total, 36,659 Mg of waste were disposedof (see Fig. 4). The average waste lling rate of 122 Mg/d corre-sponds to a waste production of 100,000200,000 Brazilian inhab-itants. The nal mean waste density was 10.41 kN/m3.

3. Materials and methods

3.1. Physicalchemical characterisation of the waste

Fifteen fresh waste samples were collected during the Experi-mental Cell lling period for evaluation of the gravimetric andvolumetric composition, water content, volatile solids (VS) con-tent, pH, and contamination by adhering particles. The chemicalcharacterisation of the waste (carbohydrates, lignins, proteins,

d so

he E

968 F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 96697775% soil + 25% compost0.3-0.45m

0.3m

WasteCompacted soil

0.1-0.2m

0.3m

Methanotrophic layer

0.75-0.85m Compacte

Conventional layer

Fig. 2. Layout of tWaste

Fig. 3. The types of the nal cover layCompacted soil

gravel ( 0.10 m)

0.55 - 0.7m

0.2m

Waste

geotextile

0.3 - 0.55m

0.2m

Capillary layer

il 0.5-0.65m

xperimental Cell.ers used in the Experimental Cell.

usuarioRealce

usuarioRealce

usuarioRealce

usuarioNotaleachate - lixiviado

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

usuarioRealce

sep/

ont

ed

MSWPrec

and

Manand lipids) was performed in four of these samples. Seven monthsafter the end of the lling period, 18 samples were obtainedthrough a Standard Penetration Test procedure (SPT; NBR 6484,2001), which on average aged less than 1 year. The same physi-calchemical tests were performed on the SPT samples and on vesamples of old waste (1215 years) from the Muribeca Landll.

The gravimetric composition of the waste was determined by ahand-sorting procedure into 13 separate categories. To calculatethe composition on a dry basis, the water content was determinedon subsamples (100200 g) of each waste fraction. Then, the con-tamination levels of the samples (adhering particles) were evalu-ated by washing the fractions separately to remove the adheringsoil and food particles and oven-drying them at 105 C to a con-stant weight. The paper fraction was cleaned by hand. The wastecontamination test allowed the waste gravimetric composition tobe obtained on a dry-clean basis. The volumetric compositionwas determined using a manually operated press compactor(0.135 m3 waste capacity). The volumetric composition was deter-mined separately at 500 kPa compaction pressure for the freshwaste and its fractions.

0

2000

4000

6000

apr/07

may/

07jun/

07 jul/07

aug/0

7

M

MSW

mas

s di

spos

Fig. 4. Evaluation of MSW disposition in8000

10000

of (

t)

F.J. Maciel, J.F.T. Juc /WasteThe water content of the waste was determined on a wet basisusing the gravimetric method. Each waste sample (approximately0.51.5 kg) was placed in a drying oven at 105 C until a constantmass was achieved. The nongrindable fractions (metal, plastic,glass, textile/rubber/leather, stone, bone, and ceramic) were re-moved prior to the determination of the VS, and the remainingmaterial was dried and triturated before homogenisation in anindustrial cutter. Next, 5 g of each sample was placed in a mufeat 550 C for 4 h. This analysis was conducted in triplicate. ThepH of the waste was analysed by shaking 25 g of waste in 100 mlof deionised water and monitoring with a pH meter for 2 h, or untila constant value was obtained. The lignin content was analysedaccording to ASTM D271-48 (1956), and the carbohydrate, lipids,and protein contents were determined using the adapted foodgravimetric and volumetric methodologies described by Maciel(2009).

3.2. Biochemical Gas Potential (BGP)

The BGP was tested in triplicate with laboratory-scale anaerobicbatch experiments adapted from Hansen et al. (2004). The sampleswere tested in 250 ml coloured reactors, whose caps were de-signed with two valves that allowed gas pressure readings(100 kPa analogue manometer, resolution = 2 kPa) and samplingof the headspace gas. The valves also allowed the circulation of aN2/CO2 (80%/20% vol./vol.) mixture inside the reactor before theincubation period. In the laboratory, the waste sample was rstblended in a large industrial cutter (Siemens CR-4L) to reducethe particle size and homogenise the sample; inorganic waste frac-tions were removed prior to the procedure. Water was not added.After this stage, distilled water was added to the cutter to obtain adiluted mixture of 0.2 g of waste per millilitre. Then, 12.5 ml of thisdiluted mixture (2.5 g of waste on a wet basis = 12.5 0.2 g/ml)and 50 ml of domestic sludge (thermophilic inoculum) were placedinside the reactors. These reactors were maintained at 37 C, andgas production was monitored over 75 days. The volume of gasproduced in the reactor was calculated using Eq. (1) based on theIdeal Gas Law. The results were further converted to StandardTemperature and Pressure (STP) conditions.

VLFG P VV CR T 1

where VLFG is the landll gas volume (L), P is the internal gas pres-sure of the reactor (mbar), VV is the empty volume of the reactor (L),

07oc

t/07no

v/07

dec/07 jan/

08

hs

0

100

200

300

400

500

Prec

ipta

tion

(mm)

mass

ipitation

precipitation at the Experimental Cell.

agement 31 (2011) 966977 969C is the molar volume (22.41 L/mol), R is the universal gas constant(83.14 L mbar/mol K), and T is the reactor temperature (K).

3.3. Characterisation of the cover soil and CH4 emission

The geotechnical characterisation of the cover soil involveddetermining the granulometric composition (NBR 7181, 1984),Atterberg limits (NBR 6459, 1984 and NBR 7180, 1984), standardcompaction (NBR 7182, 1986), water content (NBR 6457, 1986),in situ density (NBR 7185, 1986), and water permeability (NBR14545, 2000) of the soil using Brazilian standard methods (NBR).

The supercial emission of CH4 was determined using the staticux chamber method described by Maciel (2003). The sides of theux chamber were constructed of galvanised metal with a top ofclear acrylic. A high-density polyurethane foam was used betweenthe acrylic and the metal sides to ensure a good seal and preventgas from leaking. Fig. 5 presents the ux chamber conguration.The internal volume of the chamber was 8.347 L (area = 0.395 -0.395 m and height = 0.0535 m). Prior to the chamber being setinto the cover of the Experimental Cell, the surface area coveredby the chamber was carefully levelled using a manual hoe. Then,the outer frame of the chamber was placed into the soil (permittingdirect contact between the chamber and soil) thus guaranteeingthat the gas passed only through the internal area (0.156 m2). This

Local climate data were obtained from the Muribecal Landll

me

anouter frame actually served to force the chamber into the soil. Withthis method, the internal gas concentrations (CH4, CO2, and O2),temperature, and pressure were measured in the chamber usinga portable multigas detector. The ambient temperature and atmo-spheric pressure were also monitored. All measurements were ta-ken every ve minutes during the test. The test duration variedfrom 30 to 60 min, and the gas ux was determined by the initialslope of the plot of the CH4 mass-versus-time. The following equa-tions were used to determine the volumetric and mass CH4 uxrates:

QCH4 VA DCCH4

Dt 273:15273:15 T

Patm1:000

2

JCH4 QCH4 qCH4 3

where QCH4 is the volumetric supercial ux rate of CH4 (NL/s m2),

JCH4 is the mass supercial ux rate of CH4 (g/s m2), V is the internal

chamber volume of 8.34 L, A is the cover soil area of 0.156 m2,DCCH4 /Dt is the CH4 concentration increase (% vol.) with time (s),T is the gas temperature inside the chamber (C), Patm is the atmo-spheric pressure (mbar), and qCH4 is the specic mass of CH4 (g/L).

After the measurements were taken, the chamber was carefullyremoved from the cover layer, and a soil sample was taken with avolumetric ring for density and water content analysis. Then, ametallic tube sampler (0.04 m in diameter) was used to collect soilsamples at 0.10 m intervals in the cover depth until the waste massinterface was reached. These samples were analysed for pH, watercontent, and volatile solids. The gas concentrations and soil tem-perature were also measured directly in the borehole.

The static ux chamber monitoring program consisted of (i) 30

temperature

Cover layer

0.05

m0.

1m

0.1m0.395m0.1m

LFG production

Outflow connection

Fig. 5. Flux chamber sche

970 F.J. Maciel, J.F.T. Juc /Waste Mtests performed on the top cover layer of the Experimental Cell (10tests on each experimental cover) and (ii) 18 ux tests performedon the slopes (lower and upper) and the edge of the ExperimentalCell. The results of each ux chamber test were interpolated usingthe Kriging method for 2D mapping analysis to estimate the totalCH4 emissions from the Experimental Cell to the atmosphere.

3.4. Landll gas production

The LFG volumetric ow rate, composition, and temperaturewere determined in ve vertical gas wells of the Experimental Cell.These measurements were taken weekly without suction (passivegas ventilation system). The volumetric ow rate was determinedusing a hotwire thermoanemometer (Unity, model 208, 0.220m/s, resolution 0.1 m/s and precision 3.0%). The anemometer wasplaced in three different positions in the transverse section of thetube (i.e., in the centre and 23 cm from each side of the tube),and an average gas ow velocity was obtained. Then, the gas volu-metric ow rate was calculated using the internal tube area multi-Meteorological Station. Fig. 6 illustrates the climatic water balanceduring the investigation. The measured annual hydric excesses of1057 mm in 2007 and 1202 mm in 2008 were consistent withthe historic data (see Fig. 1). The average monthly ambient temper-ature varied from 20.5 C to 27.1 C, the wind velocity varied from1.3 to 2.8 m/s, and the relative humidity varied from 68.9% to86.1%. The difference between the maximum (1016.2 mbar) andminimum (1010.0 mbar) atmospheric pressure was approximately6.2 mbar (0.62 kPa). The hydric balance, temperature, and humid-ity in this site were favourable for accelerating the biologicaldecomposition.plied by the average ow velocity. The total gas ow rate of theExperimental Cell was obtained by summing the gas ow ratesof each well.

The LFG composition was determined using a portable detector(Drager X-am 7000). The gases measured were CO2 (0%-100% vol./vol., precision 2.0%), CH4 (0100% vol./vol., precision 5.0%), O2(025% vol./vol., precision 1.0%), and H2S (0500 ppm, precision5.0%) Themeasurements weremade at three-minute intervals un-til a constant value was obtained. Finally, the LFG temperature wasobtained using a digital thermometer Appa Mt-520 model (50 Cto 1300 C), whose thermocouple terminal (k-type) was positionedthrough the gas ux until a constant value was obtained.

4. Results and discussion

4.1. Climate conditions

on the nal cover layer.

agement 31 (2011) 9669774.2. Physicalchemical characterisation of the waste

The average water content of the waste during the Experimen-tal Cell lling period was 52.3 9.7%, which was a favourable ini-tial condition for the biological decomposition processes becausethe breakdown of complex substances is governed by the hydroly-sis process. The water content range of the waste (40% to 60%) iscommon for the Brazilian climate and operational conditions (Bi-done and Povinelli, 1999). Similar water contents in wastes wereobserved in Greece and Spain, whereas in most parts of the UnitedStates, the values ranged from 15% to 40% (Alcntara, 2007).

The water content for each MSW fraction is presented in Table1. The rapid-to-moderate biodegradable matter (such as organicmatter and paper/cardboard) presented high water content values(>46.2%), and the inorganic fractions (such as glass, rubber, andmetals) presented very low values (

200

300

400

vapo

ratio

n (m

m)

ont

Cell closure

nce

po

F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 966977 971-200

-100

0

100Pr

ecip

itatio

n (-)

e

M

Fig. 6. The climatic water bala

Table 1Physical characterisation of the waste.

MSW fractions Water content (%) Gravimetric com(4560%) (Maciel, 2009) and different from the data reported byAlcntara (2007) for most developed countries, including Canada(33.9%), the United Kingdom (20%), Switzerland (30%), Japan(22.2%), France (25%), and the United States (11.4%). This fact, asso-ciated with the high water content, should inuence the level ofgas production in the initial decomposition stages. Also, 61.8% ofthe waste mass (on a dry-clean basis) was rapid-to-moderate bio-degradable matter (organic matter and paper/cardboard). Consid-ering the volumetric basis, these fractions represented 45.5% ofthe total volume that would be shortly transformed into the gasand liquid phases. It is important to mention that the plastics frac-tion represented only 16.3% of the weight but almost 30% of thevolume. On a volumetric basis, the percentages of plastic and or-ganic matter were very similar. This analysis is also important inunderstanding the long-term geoenvironmental behaviour of thelandll because the elasticplastic and biodegradability propertiesof the waste vary with time.

Wet basis Dry

Organic matter 46.2 44.4 42.Paper/cardboard 52.3 14.2 12.Plastic (exible) 36.9 16.9 18.Plastic (hard) 17.4 3.2 4.Styrofoam 30.4 0.4 0.Wood 37.4 1.8 2.Textiles 46.2 4.4 4.Rubber/letter 8.7 1.6 2.Metals 17.1 1.7 2.Glass 3.0 0.6 1.Diapers 3.4 Coconut 64.1 2.7 1.Others 11.9 4.7 7.

Table 2Chemical characterisation of the waste.

Sample Age BGP (Nml/g) Volatile

Experimental Cell (4 samples) Fresh (15 days) 112.7172.4 47.4 9Experimental Cell (18 samples)

y = -6,77ln(x) + 26,4930

40

50

60

tile

solid

s (%

)

120

160

200

(Nm

l/g d

ry)

s (V

972 F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 966977crease of all parameters. However, the lowest decrease was ob-served for the lignin content. The lignin presented a distinct degra-dation behaviour due to its complex molecular structure and slowdegradation characteristics.

4.3. Evaluation of the cover layer and fugitive emissions

4.3.1. Geotechnical characterisation of the soilThe soil used in the nal cover layer of the Experimental Cell,

including the experimental layers, had a granulometric distribu-tion of 23% clay, 22% silt, and 53% sand (32% ne sand). Accordingto the Unied Soil Classication System (USCS), the soil was classi-ed as sandy clay (SC). The Atterberg limits were a liquid limit (LL)of 48% and plasticity index (PI) of 11.2%. An optimal water contentof 16.5% for a maximum dry density of 16.9 kN/m3 was achieved.The saturated water permeability (jw) was 2.0 108 m/s. Thistype of soil can be used in landll covers due to its low permeabil-ity coefcient, but the soil volumetric variations (contraction/expansion), which are related to the plasticity properties, shouldbe monitored to prevent cracks from forming. The use of soil, inassociation with alternative materials (for instance, compost),should be considered to improve the physical and biological char-acteristics of the cover and to minimise gas emissions. For thisinvestigation, we designed a methanotrophic layer that includeda mixture of yard waste compost and soil in the upper part. The ini-tial granulometric composition of the compost used in the layer in-cluded 34.7% particles from 0.075 to 0.42 mm, 52.4% particles from0.42 to 2 mm, and 7.3% particles from 2 to 4.8 mm. The clay frac-tion (

activity are CO2 and H2O. (Conrad, 1995, cited by Borjesson andSvensson (1997)). The cover soil oxidation activity was indirectlyevaluated in this investigation through the analysis of the coversoil parameters. Table 4 presents the average soil parametersdetermined along the cover layer depth. The average methano-trophic cover soil temperature (36.1 C) was higher than that inthe capillary layer (34.7 C) and the conventional layer (33.2 C).The literature is not conclusive as to the optimal temperaturerange for CH4 oxidation. Boeckx et al. (1996) reported that the opti-mum soil temperature varies between 20 C and 30 C. Borjesson etal. (2001) suggested that temperatures between 25 C and 35 Cmay promote 85% CH4 oxidation. Gebert et al. (2007) proved thatthe oxidation rates increased exponentially for temperatures upto 38 C, although the oxidation activity was identied for all tem-perature ranges analysed (345 C).

The volatile solids and water contents of the soil samples werealso higher in the methanotrophic layer, which was related to thepresence of organic matter in the compost/soil mixture layer. Bor-jesson and Svensson (1997) veried that the CH4 oxidation rateswere directly associated with the level of organic matter in the soil.The presence of organic matter also facilitated supercial vegeta-tion growth in the methanotrophic layer, as observed during themonitoring period. This vegetation provided nutrients for the

microorganisms and helped O2 enter the soil cover. The methano-trophic layer presented a neutral pH with an average value of 7.45,whereas the other layers tended to have an acidic pH. Generally,methanotrophic microorganisms are neutrophilic. Yoon et al.(2005) stated that the optimum pH to grow bacteria ranged from6.7 to 8.0, although other studies did not register a signicant var-iation in oxidation for pHs between 3.5 and 8.0.

4.3.4. Supercial CH4 emission mappingFig. 8 presents a map of the CH4 emissions in the Experimental

Cell. The isoux curve units are NL/m2 h. The total amount of gasreleased into the atmosphere was calculated as the sum of eachisoux contribution area. The value obtained was approximately45.0 Nm3/h and reected a dry season of the year (September toDecember 2008). In this period, the average CH4 collection ratefrom the vertical wells was approximately 48.3 Nm3/h (passivedrainage). These results allowed us to conclude that the Experi-mental Cell was emitting almost the same amount of CH4 intothe atmosphere as that collected in the gas wells. This situationis common in Brazilian landlls because the majority of the dis-posal sites do not have gas recovery systems or even cover layercontrol. If the methanotrophic or capillary layers had been usedin the nal cover layer of the whole Experimental Cell, the CH4emissions into the atmosphere could have been reduced due tophysical and biological effects.

4.4. Landll gas production

The average CH4, CO2, and O2 concentrations were 54.3 2.7%,

Table 4Average of soil parameters along the cover depth.

Cover Temperature(C)

pH Watercontent (%)

Volatilesolids (%)

Methanotrophic 36.1 5.1 7.45 0.55 20.8 7.5 16.3 1.8Capillary 34.4 1.9 6.08 0.86 16.9 0.6 7.33 1.5Conventional 33.2 2.0 5.74 1.18 14.9 1.0 7.43 1.1

F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 966977 973Fig. 8. CH4 isoux as fugitive emis40.7 2.9%, and 1.2 0.9%, respectively, during the rst 18 monthsof the monitoring period (February 2008 to June 2009). The CH4concentration reached 50% immediately after the cell was closed.sions of the Experimental Cell.

Therefore, the initial waste decomposition phases (i.e., aerobic,transition, and acid phases) were not observed in this study. Thesephases most likely occurred during the waste lling period. Themain LFG constituents (CH4 and CO2) remained almost constant inthis period (standard deviation of 2.7% and 2.9%, respectively). Incontrast, there was an accentuated reduction in the gas production.This indicates that part of the CH4 produced could not be collectedby the drainage system and consequently, became concentratedand accumulated in the cell. The gas collection efciency was fur-ther estimated around 40% (passive ventilation) of the total gas pro-duced. This determination was made by considering the totalsupercial CH4 emission (45.0 Nm3/h), the amount of CH4 recov-ered in the gas wells (48.3 Nm3/h), and an estimated biologicalCH4 oxidation factor of 20% of the total gas produced in the cell,as recommended by Chanton and Liptay (2000) and cited byMahieuet al. (2005), for clay soil layers under tropical climate conditions.

Table 5 presents the LFG production rates in each vertical gaswell (DV-01 to DV-05) for the initial (January 2008) and nal (July2009) monitoring months. The LFG ow rates varied from 2.5 Nm3/h (DV-05) to 58.4 Nm3/h (DV-03). This variation was inuenced bythe time and the thickness of the waste. The gas production ratesdecreased abruptly over an 18-month interval with a reduction

amount of waste disposed of in the Experimental Cell (36,659 t).Willumsen and Bach (1991) collected data from 86 landlls in dif-ferent countries and veried that the gas collection rate rangedfrom 0.8 to 10.0 m3/t year, depending on the waste age, althoughvalues up to 20.0 m3/t year were mentioned. Cooper et al. (1992)reported values from 0.7 to 8.0 m3/t year, and El-Fadel et al.(1997) reported values from 1.0 to 14.0 m3/t year (dry basis). Knoxet al. (2005) conducted an investigation at ve experimental cells(40 25 20 m depth) in the United Kingdom and found thatthe maximum gas collection rate occurred 57 years after thewaste disposal, with values ranging from 13 to 22 m3/t year. Thevalues obtained at two experimental cells in France of 2000 m2

of area (each) and 10 m in depth varied from 27.0 to 35.0 m3/t year(Barina et al., 2005). Therefore, the gas collection rates obtained inthe Experimental Cell were in the same range or higher than thosereported in the literature for real scale landlls and pilot test cells.Among the factors that facilitated the production of gas at this site,the physicalchemical characteristics of the waste, the favourableclimate conditions, and the landll geometry (for instance, wastethickness) should be considered.

The theoretical production of LFG was estimated based on thedefault and adjusted scenarios using the USEPA (2005) Landgem

/09

n of

974 F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 966977of up to 89.8%. The lowest ow rates were observed in the shallow-est wells (DV-04 and DV-05), and these also presented the mostsignicant decreases in the gas ow rates. The atmospheric inu-ence on the waste mass was noticed especially in the rst 3.0 mof the cell depth in the Muribeca Landll (Maciel, 2003). It isimportant to compare the reduction in the gas production withthe BGP results that showed an average decrease of 77.9% in thesame period. The LFG temperature also decreased in all wells(33.143.6%), and this fact was probably related to the wastedecomposition transition phases because the degradable substratebecomes less available for biological decomposition. The averageambient temperature in this period ranged from 20.5 C to27.1 C. The LFG composition and temperature monitoring resultsled to the conclusion that the waste degradation proceeded rapidlyin the operational and climatic conditions of the Experimental Cell.

The LFG collection rates per tons of waste (wet basis) rangedfrom 12.4 Nm3/t year (July 2009) to 46.2 Nm3/t year (January2008). These values were calculated by dividing the total gas pro-duction ow rate (sum of DV-01 to DV-05 ow rates) by the

Table 5LFG collection rates and temperatures for each gas well of the Experimental Cell.

Vertical gas drain Drainage height (m) LFG collection (Nm3/h)

January/08 July

DV-01 7.0 53.4 16.0DV-02 7.0 41.0 13.1DV-03 8.0 58.4 17.8DV-04 3.0 25.6 2.6DV-05 4.5 15.1 2.5

Table 6IPCC model parameters for the default and adjusted scenarios.

Model parameters Degradable organic carbon (DOC) DOCf (fractio

Default Adjust Default

Organic matter 0.18 0.21 0.50Nappies 0.24 0.32 0.50Wood/coconut 0.43 0.46 0.50Paper/cardboard 0.40 0.45 0.50

Textiles 0.24 0.40 0.50

Note: Average values of food and garden waste; MCF (Methane Correction Factor) = 0.9and IPCC (2006) models. These models are based on rst-order de-cay kinetics and consider as input parameters, the amount of wastedisposed of against time, the physical composition of the waste,the collection efciency, and the local climate conditions (amongother variables). The predicted LFG curve was initially obtainedusing default values for tropical wet climate conditions, as recom-mended by the model guidelines. The Landgem default parametersused were (i) methane generation rate constants (k = ln2/t1/2)equal to 0.15 and 0.20, (ii) potential methane generation capacity(Lo) = 94.8 Nm3/h (calculated based on the physical compositionof the waste on a dry-clean basis Table 1), and (iii) the CH4 con-tent of the LFG of 50%. Table 6 presents the default parameters ini-tially obtained in the IPCC (2006) model guidelines for a moist andwet tropical climate. Fig. 9 presents the default scenario predictionand the measured gas recovery data from the Experimental Cell. Itis observed that the LFG experimental data for the gas productionshowed higher values than those predicted in the rst-order mod-els using the default parameters, although the gas collection ratetended to decrease faster with time.

LFG temperature (C)

Decrease (%) January/08 July/09 Decrease (%)

70.0 50.3 31.8 36.868.0 48.6 32.5 33.169.5 49.3 32.2 34.789.8 54.4 30.7 43.683.4 54.8 31.6 42.3

DOC dissimilated) Half-life time (t1/2) (d) Delay time (d)

Adjust Default Adjust Default Adjust

0.50 1058.5 164.3 180 600.80 1496.5 253.0 180 600.54 7227 1011.9 180 600.50 3613.5 595.3 180 60

0.50 3613.5 595.3 180 60

0 (both scenarios); F (CH4 fraction on LFG) = 0.50 (both scenarios).

11

ime

L

L

IP

E

Man0

40

80

120

160

200

240

2008

2009

2010

20

T

Lan

dfill

ga

s flo

w ra

te (N

m3 /h

)

F.J. Maciel, J.F.T. Juc /WasteThe Landgem Model was adjusted by considering (i) Lo =123.9 Nm3/t, as obtained through the extrapolation of the CH4generation experimental curve over a 10-year interval and (ii)different k values (however, the best value that t the experimen-tal data was 0.80, representing an average half-life of the waste of317 days). Relating to the IPCC (2006) Model, the selected adjustedparameters are also presented in Table 6. The DOC and DOCf valueswere the maximum values specied in the IPCC (2006) guidelines,whereas the half-lives of the waste fractions were 1/5 of the max-imum parameters. This factor (1/5) was adopted because the LFGcollection rates observed in the initial phase of the monitoring per-iod were up to ve times higher than predicted by the default mod-el simulation. The results obtained for the adjusted LFG collectionscenario are illustrated in Fig. 10. The decomposition velocity ofthe waste was four to ve times faster than that suggested bythe model guidelines. The average half-life of the waste wasapproximately 1 year. Therefore, the application of traditionalrst-order models should be calibrated with previous experimen-tal data (real or pilot scale studies) to obtain site-specic biode-gradability parameters and consequently, more realistic LFGproduction and emission estimates.

0

40

80

120

160

200

240

2008

2009

2010

2011

Time

Lan

dfill

ga

s flo

w ra

te (N

m3 /h

) LI

E

Fig. 10. Predicted landll gas collectio

Fig. 9. The theoretical and measured landll2012

2013

2014

2015

(years)

andGem - k=0.20, Lo=94.8 Nm3/h, EF=41.4%andGem - k=0.15, Lo=94.8 Nm3/h, EF=41.4%

CC - default parameters, EF=41.4%xperimental data - EF=41.4%

agement 31 (2011) 966977 9755. Conclusions

The comprehensive study conducted at the Muribeca Landllproved important to understanding and dening parameters forlandll gas emission reduction projects in Brazil. The methodolo-gies used were based on experiments and studies from developedcountries, whose operational, climatic, and waste conditions aredistinct from those found in Brazil. For instance, the two largestLFG projects in Brazil (at Bandeirantes and So Joo landlls) failedto predict the observed reduction in gas emissions (UNFCC, 2009).The results of this investigation allowed us to conclude that thedecomposition of waste in a large-scale Experimental Cell undertropical wet climate conditions was four to ve times faster thanthat predicted by traditional rst-order models. These modelsshould be used carefully under conditions similar to those of theExperimental Cell. The average half-life of the waste was approxi-mately 1 year. The biodegradability parameters of the waste con-rmed this rapid biodegradation because the volatile solidscontent and BGP decreased by 40% and 77.9%, respectively, afterthe cell had been closed for 7 months. The LFG collection rateswere also higher than reported in the literature for similar sites

2012

2013

2014

2015

(years)

andGem - adjusted parameters, EF=41.4%PCC - adjusted parameters, EF=41.4%xperimental data - EF=41.1%

n rates with adjusted parameters.

gas collection at the Experimental Cell.

sence of a gas recovery system during the tests facilitated gas es-cape through the cover layer. This situation is common in

The experimental evaluation of the cover layers allowed us toconclude that the CH4 fugitive emissions were higher in the con-

anventional layer than in the methanotrophic and capillary layers.The investigation of the soil parameters (i.e., temperature, pH,VS, and water content) along the cover depth indicated soil condi-tions that better support oxidation activity in the methanotrophiclayer. One of the factors that explained the low emission rates inthe methanotrophic cover was the biological factor arising fromthe presence of a compostsoil mixture layer, which improved bio-logical oxidation activity. The relevance of a high air-lled porosity,which enhances oxygen diffusion in the upper part of the coverlayer, is another important factor that should be considered in fur-ther investigations. The gravel layer was decisive in reducing gasemissions in the capillary layer because the concentration andpressure of the gas was better distributed under the cover beforeits percolation through the soil. A new cover layer design is pro-posed for future researchers, including a gravel layer on the bottomof the methanotrophic layer to minimise the CH4 emissions. Themethane oxidation quantication methods, in association withthe geotechnical characteristics of the nal cover layer, were alsointeresting and deserve further study.

Acknowledgements

The authors gratefully acknowledge the Emlurb and Chesf forsponsoring all monitoring and operational activities of the Experi-mental Cell at the Muribeca Landll.

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Felipe Juc Maciel Doctorate in Civil Engineering fromFederal University of Pernambuco (UFPE). Researcherand consultant at the Solid Waste Group (GRS/UFPE)since 1998. Technical coordinator of the ExperimentalCell Research Project at the Muribeca Landll/Brazil.Main research subject: landll gas and environmentalassessment of landll sites.

Jos Fernando Thom Juc Professor of the Civil Engi-neering Department at UFPE. Doctorate from the MadridPolytechnic University. Coordinator of the Solid WasteGroup (GRS/UFPE). Coordinator of the Muribeca LandllMonitoring Program and other research projects sup-ported by Brazilian ofcial institutions. Consultant forthe United Nations National Program (PNUD) and theEnvironmental and City Brazilian Ministries on thesubject of MSW.

F.J. Maciel, J.F.T. Juc /Waste Management 31 (2011) 966977 977

Evaluation of landfill gas production and emissions in a MSW large-scale Experimental Cell in BrazilIntroductionGeneral site descriptionMaterials and methodsPhysicalchemical characterisation of the wasteBiochemical Gas Potential (BGP)Characterisation of the cover soil and CH4 emissionLandfill gas production

Results and discussionClimate conditionsPhysicalchemical characterisation of the wasteEvaluation of the cover layer and fugitive emissionsGeotechnical characterisation of the soilSuperficial CH4 emissionsSoil indicators of CH4 oxidationSuperficial CH4 emission mapping

Landfill gas production

ConclusionsAcknowledgementsReferences