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ACCELERATED METHANE OXIDATION COVER SYSTEM TO

REDUCE GREENHOUSE GAS EMISSIONS FROM MSW LANDFILLS

IN COLD, SEMI-ARID REGIONS

CHRIS A. ZEISSEBA Engineering-Research

(Received 22 September 2005; accepted 28 April 2006)

Abstract. Many regional landfills for municipal solid waste (MSW) and industrial, commercial,institutional (ICI) wastes in cold, dry regions do not produce enough gas to support conventional gas

extraction, treatment, and utilization or flaring. Yet, some solution is required to reduce emissions

of methane and trace constituents to the atmosphere for the protection of the public and of the

global climate. Methane oxidation, as a natural biochemical process, offers an opportunity to reduce

methane emissions with a simple, passive alternative cover system. The goal of this article is to

develop an effective design of Methane Oxidation Covers to achieve superior methane management

performance while still producing equivalent closure conditions to conventional covers in semi-

arid, cold climates. Specifically, the goal is to reduce methane surface emissions by 50% to 80%,

with no significant increase in leachate production compared with conventional covers of clay and

topsoil.

A field pilot test of an alternative cover system with gas collection, methane oxidation and heat

extraction was conducted on an operating MSW/ICI waste landfill in Western Canada from August

2001 to February 2005. The cool, semi-arid region experiences cold winters (down to minus 40 C)

for up to 5 months of the year, and annual precipitation rates of 150 mm to 450 mm p.a., of whichone third to one half falls as snow.

The need to direct gas from large surface areas to gas control zones of minimal area led to the

configuration of the system of gas collection trenches connected to a central methane oxidation

(MethOx) bed. The need to keep the bed above 5C in winter required the development of a simple,

passive heat transfer system.

The maintenance of suitable moisture contents and the restriction of percolation were accomplished

by the choice of filter material and the layering of the bed over the gas percolation layer.

The test program was conducted in three phases from August 2001 to February 2005. In the first

test phase, a methane oxidation bed of yardwaste compost performed well during the summer, but

froze from November to April and did not resume oxidation until May. Oxygen was always present at

or above 3%(vol.) and the moisture content remained above 25%(vol.) in the lower layer of the bed.

The freezing temperature caused the most serious performance reduction. In the next phases of the

study, a passive heating system was installed in an accelerated methane oxidation bed. Heat exchangefrom inside the landfill to the filter raised the bed temperature to 14 to 18 C during the third winter of

the test. The moisture contents of 25% to 50% (v/v) in the bed were high, but the percolation rate was

only 7.3 mm/a, or about 2% of total precipitation. The methane oxidation performance increased with

the heating of the bed, from a 33% emission reduction in an unheated bed, up to 89% in a well heated

filter bed. The achievement of high oxidation performance (over 80%), the complete reduction of

surface emissions from the test area (to zero), and the low percolation rate through the filter bed (less

that 2%) constitute a proof of principle for MethOx covers in cool, semi-arid climates. The possible

improvement of the Alternative Cover Systems performance by adding vegetation to the filter bed is

currently being tested in the ongoing research project.

Water, Air, and Soil Pollution (2006) 176: 285306

DOI: 10.1007/s11270-006-9169-z C Springer 2006

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286 C. A. ZEISS

keywords: MSW landfill, landfill gas management, methane oxidation, alternative cover design,

heat extraction, heat recovery, greenhouse gas emission reduction

1. Methane Oxidation Covers in Cold, Semi-Arid Climates-Problem

Statement and Research Goal

Solid waste landfills in cold, semi-arid regions tend to produce landfill gas, and,in particular, methane at slower rates than necessary to support conventional gas

extraction and utilization systems. As a result, many regional landfills rely onconventional covers (with one metre of clay and a 0.3 m topsoil layer in the sim-plest configuration), to control gas emissions. These conventional covers oftendo not sufficiently reduce landfill gas emissions, because (even properly com-pacted) clay covers contain microcracks and can develop larger cracks whendesiccated.

Methane oxidation is a simple, natural process, which converts methane to CO 2in the presence of oxygen and under suitable conditions. Methanotrophs (methaneoxidizing organisms) are ubiquitous in the atmosphere and in many soils. Theyadjust to and function under many environmental conditions. The goal of thisstudy is to develop the engineered design of methane oxidizing alternative cov-ers to achieve high average oxidation rates in landfill covers subject to cold,

semi-arid climate conditions. Specifically, the objectives of the research are to

reduce methane surface emissions by 50% to 80% with no significant increasein leachate production, compared with conventional landfill covers of clay andtopsoil.

First, the basic mechanisms and essential conditions for methane oxidation(MethOx) in soil covers are compiled and summarized from the research litera-

ture and from previous test results. Then, typical conditions for landfill sites incold, semi arid climates are described. Third, the configuration and design of theAccelerated Methane Oxidative Alternative Cover Systemare presented. The fourthsection presents the results of the MethOx pilot test, and, in particular, the condi-tions and methane oxidation performance achieved during the 18-month test pe-riod from June 2003 to February 2005. The discussion of the Alternative MethOx

Cover System, its significance and perspective for future applications closes thearticle.

2. Optimal Target Conditions and Ranges for Methane Oxidizing Covers

Optimal conditions to support methane oxidation (MethOx) are derived from

the research literature and previous tests to define the target values for keyparameters. The configuration, design, and operation of Alternative Methane

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ACCELERATED METHANE OXIDATION COVER SYSTEM TO REDUCE GREENHOUSE GAS 287

Oxidizing Cover Systems must focus on achieving and maintaining theseconditions.

2.1. BASIC METHANE OXIDATION MECHANISMS

Methane oxidation is a naturally occurring biochemical oxidation process per-formed by several groups of bacteria, the Methanotrophs. Oxidation occurs in

atmospheric air, in some soils, and in engineered porous covers. Methane oxidizersestablish themselves fairly quickly and are adaptable and resilient to changing con-ditions. Most of the oxidizing population and activity is confined to a narrow layerof 10 to 20 cms thickness at a depth of 40 cm to 60 cm (Nozhevnikova et al., 1993).

This active layer establishes itself at a depth where combined conditions of temper-ature, moisture content, and oxygen and methane concentrations are suitable, but

can adapt to changing conditions.

2.2. METHANE OXIDATION TEMPERATURES

Methanotrophic organisms perform at the highest rates in the optimal temperaturerange from 15C to 35C. Above 40C, methane oxidation rates decline and dropto zero by 50C. Methane oxidation slows noticeably at cooler temperatures, butpsychrophilic (cold tolerant) methane oxidizers show measurable activity downto temperatures of 2C to 5C (in lab studies, Scheutz et al., 2004; and field re-

search, Omelchenko et al., 1992; Vecherskaya et al., 1993; Borjesson et al., 2004;Christophersen et al., 2004). The oxidation rate responds to increasing temperaturevery strongly from the threshold temperature of 3Cupto25C.TheQ10 values (in-dicating the multiple increase in oxidation rate for a temperature increase of 10 C)for methane oxidation are high at between 1.9 to 7.26. Psychrophilic methanotrophsare predominantly of the type (Type I) that also favors low methane and high oxygenconcentrations.

2.3. MOISTURE CONTENTS AND OXYGEN AVAILABILITY

Soil moisture content is reported as either the most important or the second most

important environmental variable (after temperature) for methane oxidation. Soilmoisture provides a water film that acts as a protective layer against extreme con-centrations and as a diffusion transport film for oxygen and methane to the cells,and for CO2 and waste products (e.g., NH3) away from the cells.

Methane oxidation activity occurs in a soil moisture content range from 8%(w/w) up to 50% (i.e., saturation). The optimal moisture content range is reported

as 10% to 20% w/w (Whalen et al., 1990; Czepiel et al., 1996). A slightly higheroptimal moisture content is reported at between 20 to 30% (w/w) for an unspecifiedtype of soil (Scheutz et al., 2004), while Christophersen et al. (2004) reports some

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TABLE I

Soil and yardwaste compost - porosities and field capacities

Soil type Porosity n [-] Field capacity FC [-] Difference porosity - FC

Claya 0.451 0.419 0.032

Clay silta 0.452 0.411 0.041

Clayey sanda 0.400 0.366 0.034

Yardwaste compost in this study 0.75 0.32 0.43

aData source: Schroederet al., 1994.

variation of the optimum moisture contents between four (unspecified) soil types.Between 2C and 15C lower temperatures correlate with slightly higher optimalmoisture contents (by 2 to 3% w/w).

Zeiss and Bajic (2001) show that the optimal moisture content varies with soiltype andporosity. Coarse sand, forexample, hasthe lowest optimal moisture contentat barely 10% (w/w), while yardwaste compost has the highest at over 30% (w/w).

The change in oxidation rate with increasing moisture content also varies betweensoils with different porosities and moisture holding capacities (i.e., field capacities,see Table I). Sitaula et al. (1995) show a substantial decrease in methane oxidationrate by 35% to 50% with a 10% moisture content increase from 32% to 42% insilty sand forest soils. Nesbit (1992) reports a methane oxidation rate reduction of56% at saturation.

For identical increases by 10% (v/v) of moisture content over the respectiveoptima for three soil types, Zeiss and Bajic (2001) report oxidation rate reductionsof 90% in clay till (porosity n = 35%), 50% to 75% in sandy tills (porosity n= 25%), but of only 5% to 10% in mixed yardwaste compost soil (porosity n =60 to 80%). The explanation can be seen from inspection of Table I, where thePorosity and Field Capacity of four cover soils are shown (as v/v [-]). The small

differences for the first three soil types indicate that these soils have very little gasfilled porosity (3%) when at their moisture holding capacities (Field Capacity),whereas the yardwaste compost provides 42% gas-filled porosity for gas flow andas reserve moisture holding capacity.

Generally, optimal moisture contents of between 20% to 30% (w/w) (equivalent

to 15 to 20% v/v based on a bulk density of 0.75) are desirable for oxidation,but the soils with high porosity and low moisture holding capacity are best ableto tolerate fluctuating moisture contents without forfeiting oxidation rates and gaspermeability. This quality is essential to store the seasonally high infiltration duringfall and spring.

Methane oxidation is fairly insensitive to oxygen concentration as long as itis above 3% v/v (Czepiel et al., 1996; Christophersen et al., 2004). This limit is

usually easily maintained, as it is throughout the three phases of this study, downto one metre depth.

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2.4. SOIL CHARACTERISTICS

Methanotrophs are neutrophilic, preferring an optimal pH range of between 6.5to 8.0 (Sitaula et al., 1995). Oxidation activity seems to continue up to pH8.5 to 9.0. Some Methanotrophs, however, are tolerant of low pH values, downto 3.0 (or lower, as reported from some northern peat bogs). Indeed, Sitaulaet al. (1995) report enhanced methane oxidation in soils with low, mediumand high levels of ammonia when irrigated with acidic precipitation of pH

3.0. pH conditions of slightly below neutral (6.5 to 6.8) seem to be the besttarget.

High soil organic content generally increases the oxidation rates (Christophersen

et al., 2004), and increases the optimal soil moisture content (by a few%). Whilesoils with organic contents of 1 to 10% show moderate oxidation potential, biowastecompost and other materials with high organic content (of up to 35% w/w) show

10 to 100 fold higher oxidation potentials. Moreover, such materials also havehigher moisture holding capacity and larger porosities (at uncompacted densitiesof 0.5 to 0.75 tonnes/m3). Low C:N ratios (12 or below) tend to produce highand inhibitory concentrations of ammonia (NH4

+) and nitrite (NO2) (Boeckx

and Van Cleemput, 1996), whereas higher C:N ratios of between 25 to 97 pro-duce minor amounts of ammonia (NH4

+) and nitrite (NO2), which cause no

inhibition. This is in part due to limiting nitrogen content, as well as the co-nitrification of ammonia to nitrate along with methane oxidation. De Visscheret al.

(1999) concluded that the ammonia produced by the mineralization of organicmaterial is not inhibitory. Thus, possible sources of inhibitory levels of ammonia(NH4

+), and its intermediate product nitrite (NO2), are due to high ammonia

content of (or additions to) the soil. The inhibitory effects are not completely re-

versible. The effects of ammonia on methane oxidation rates have been quantifiedby Park et al. (2004) as a factor of 0.13519 NH4

+ on oxidation rate OR asmol/m2 day.

The MethOx cover medium should be structurally stable and settle very littleto maintain its porous structure. There is a need to prevent the passage of heavyequipment, cattle, vehicles and other activities that may compact MethOx cover

areas. Fences or vegetative access control for the small MethOx filter beds (lessthan 3% of total area) may be necessary.

These specifications are applied as targets for the MethOx Cover System.

3. Landfill Site and Climate Conditions

The patterns of gas generation and surface emissions, the climate conditions and thematerials available at landfills constitute three important parameters for the coverdesign.

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3.1. LANDFILL GAS GENERATION AND SURFACE EMISSION PATTERNS

MSW landfill gas generation and surface emissions follow temporal and spatialpatterns. The temporal patterns include long-term lifecycle patterns, medium-termseasonal fluctuations, and short-term variations.

Gas generation rates over the lifecycle of an MSW landfill typically show a rapidincreasing period leading to an early peak (early phase), followed by a slow expo-nential decline over several decades (operating phase). Conventional gas extraction

systems commonly collect a fraction of the gas generated during the middle phase.In the late phase, gas generation decreases asymptotically over several decades sothat gas extraction is usually discontinued and most of the generated gas escapes

to the atmosphere (less a small fraction oxidized in the cover soil).Methane emission rates show distinct seasonal patterns, with emission rates

varying by as much as ten up to one hundred-fold between cold and warm seasons

(Zeiss and Bajic, 1999; Borjesson and Svensson, 1997; Park and Shin, 2001). Thefluctuations between seasons, however, can vary in opposite directions. Borjessonand Svensson (1997), for example, report a tenfold increase in methane emissionrates from a low (average) of 0.29 g CH4/m

2-day in summer to 2.6 g CH4/m2-

day in winter. The difference is explained by the lower soil temperature and loweroxidation activity in winter. Conversely, Park and Shin (2001) report the lowest

emission rates of 4.2 g CH4/m2-day in winter, rising to 8 g CH4/m

2-day in spring,and to 22.9 g CH4/m

2-day in summer. The trend from low emissions in winter tohigher emission rates in summer is correlated with higher air temperatures. Zeiss(2005) reports a third pattern of the highest emission rates occurring in fall and inspring, with lower emissions in winter due to declining methane generation, and insummer due to higher soil temperatures and higher methane oxidation rates.

The spatial distribution of landfill methane emissions usually consists of a veryfew, very small high emission areas interspersed with large surface areas withno or very low methane emissions. In one study, e.g., Borjesson and Svensson(1997), four monitoring points produced the five highest emission rates of over9.6 g CH4/m

2-day, while the other 67 observations from the rest of the site were allbelow 3 g CH4/m

2-day. Further, Czepiel et al. (1996) reported a CV of+/ 326%

of methane emission rates on a landfill site with an average methane emission rateof58gCH4/m

2-day. Their highest detected emission rate was 1,495 g CH4/m2-day.

Zeiss and Bajic (2000 and 2001) reported similar patterns, with a maximum surfaceemission rate of 2,002 g CH4/m

2-day in one small area accounting for over 90%of methane emissions of a regional MSW landfill in western Canada. They alsonote that most of the high emission areas seem to occur on the side slopes and the

escarpments of landfill cells.In comparison with emission rates, the highest methane oxidation rates reported

in the literature are up to 500 g CH4/m2-day (albeit in lab studies, Zeiss and Bajic,

1999),anduptoapprox.150to200gCH4/m2-dayinfieldstudies.Asaconsequence,

MethOx Cover Systems must be able to collect and oxidize extremely high methane

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fluxes in a few small areas, and, conversely, low fluxes from very large cover areas.The maximum achievable methane oxidation rate for field application is probably

no more than between 100 and 200 g CH4/m2-day (based on the reported lab and

field studies), but may be less in winter.

3.2. TEMPERATURES , PRECIPITATION AND PERCOLATION RATES THROUGHLANDFILL COVERS IN A COLD, SEMI-ARID CLIMATE

Prairie regions of Canada and the northwestern US extend roughly from the west-ern end of the Great Lakes to the Rocky Mountains north of the 46th parallel.In Canada, they include the provinces of Manitoba, Saskatchewan and Alberta.

The climate is characterized by long, cold winters and moderately warm sum-mers, with a significant fraction of precipitation falling as snow, ice or frozen rain(Oxford 2003). January average temperatures from minus 10C to minus 30Ccontrast with July average temperatures of 15 to 20C. The growing season asthe number of days with average temperature of 5C or more is usually between170 to 140 days per year and the heating degree days (HDDs) range from 3,000

to 3,500 p.a. The total precipitation is between 200 mm/year to 600 mm/year, ofwhich between 100 mm/year to 200 mm/year (of 33% to 50% of total precipita-tion) falls as frozen precipitation. Fall rain and spring thaw events create periodsof high infiltration of cold water, leading to high moisture contents and low soiltemperatures.

Alternative cover designs (that omit a conventional barrier) raise the concern

of increased percolation and leachate generation. Although there is no universallyaccepted percolation performance limit (in mm/year), recent tests under the Al-ternative Cover Assessment Project (ACAP, Albright et al., 2004) provide goodranges of percolation performance for conventional soil and FML barrier coversand for alternative covers (i.e., capillary barriers). The researchers find percolationrates through conventional covers with soil barriers of between 52 to 195 mm/yr

(or 6% to 17% of precipitation, albeit in humid climates). Most of the percolationis attributed to preferential flow through cracks in the clay soil barriers.

Percolation rates through similar covers in semi-arid climates are not reportedin Albright et al. (2004) due to a scarcity of data. In contrast, average percola-tion rates of 33 to 160 mm/yr (6% to 18%) through alternative covers in humid

regions, and of less than 2.2 mm/yr (0.4%) in arid, semi-arid and sub humid climesare reported. One half of these alternative covers transmitted less than 0.1 mm,but the two that allowed 26.8 mm/yr and 52 mm/yr percolation were subject tospring snow melt events. It may be inferred that relative percolation rates of 0.4%to 6% of precipitation represent a good performance range. This range is usedhere as a performance target for percolation through Alternative Oxidative Cov-ers. At a total annual precipitation of 300 mm, the maximum threshold of 6% is

equivalent to 18 mm/yr. Methane oxidizing covers would need to be designed withadequate thickness and layering to keep percolation below this threshold, even to the

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292 C. A. ZEISS

point of requiring thickening in the oxidation area to provide sufficient percolationresistance.

3.3. SOIL AND BYPRODUCT MATERIALS FOR USE IN ALTERNATIVE COVERS

The environmental siting criteria require most modern landfills to have clay till,and clay materials available (from cell excavation, or at the cost of excavation) onthe site. Many of these sites, in contrast, lack topsoil and gravel. Synthetic covers(FMLs) cost over $10 per square metre. Washed gravel can cost $12 per tonne andmore, which, at an in-place density of 1.9 tonnes per cubic metre, equates to a priceof about $23 per cubic metre for gas drain material. Other byproduct materials are

alternatives to achieve similar cover qualities and become interesting because ofthe advantages of lower cost and better availability, particularly if the byproductmaterials can be processed on-site with available equipment. Simple yardwastecomposting is usually easily done, as is glass crushing. Tire chipping requiresspecial mobile shredders, but these are now better available in many jurisdictionsthrough tire recycling programs. Old carpeting can be collected and cut and used

as protection layers over FMLs, and as filter layers over drain layers.Yard waste composting, or co-composting of yard waste with suitable by prod-

ucts (shredded wood chips, pulp sludge, etc.), can produce a very good organic soil,which can be customized to serve as porous filter medium.

4. Configuration and Design of the Accelerated Methane Oxidizing Cover

System

The design of the accelerated methane oxidation cover system must accommodate

low surface emission rates (of 0 to 50 g CH 4/m2-day) from large cover areas, and

very high emission rates (of up to 2,000 g CH4/m2-day) from very small hot spots

in the landfill cover. An extensive cover design consists of an equally thick layer offilter medium over the entire cover area. This approach has been used in Europe,Scandinavia, and elsewhere (see e.g., Humer and Lechner, 1999). This extensive

approach requires large volumes of filter soil (about 1 metre times the area), drainmaterial (about 0.3 m times the cover area), and air space (about 1.3 m times thecover area). It also might increase leachate generation and restrict the allowable

post-closure land uses on the entire surface area. A different design concept isrequired for the site and climate conditions described above and to reduce thematerial volumes and their associated costs.

4.1. BASIC CONFIGURATION OF THE ACCELERATED METHANE OXIDATIONCOVER SYSTEM

The basic system consists of two new components: (a) networks of gas collec-tion trenches centred on, (b) the central methane oxidation filter beds. A third

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Figure 1. Gas Collection Area with Trenches and Central MethOx Bed.

component, the heating of the beds, is required to sustain methane oxidation duringcold seasons. The trench system directs the landfill gas from a large area (of about

5,000 m2 to 10,000 m2 each) to a relatively small central MethOx bed (from 1 m2 toapproximately 100 m2 area each, see Figure 1). The size of the collection area andof the filter bed can be adapted to the range of surface emission distributions, i.e.,low emission rates from a large area to very high emission rates from a small area.The former situation requires long trenches to cover a large area, with a relativelysmall filter bed; the latter case requires a small collection area with a relatively large

filter bed.

4.2. GAS COLLECTION TRENCH SYSTEM

The study area exhibited moderate surface emission rates of between 0 and 44grams CH4/m

2 day, and an average methane emission rate of 0.7 grams CH4/m2

day from the test area of 4,608 m2 on the southeast corner of an operating MSWlandfill in western Canada. The gas collection system in this study consisted ofsix trench segments with a total length of 212 m. The trenches were located so

that no part of the test area was more than 20 m from a trench. The trenches wereall excavated down to the waste, and filled with a drain layer of 0.3 m to 0.45 mof gravel, and covered by one metre of clay till soil. The five ends of the trench

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294 C. A. ZEISS

segments were connected to the base of the central oxidation bed located on theupper edge of the slope (see Figure 1). The effectiveness of this design was tested

by the measurement of methane surface emission rates at eight fixed monitoringlocations (TA-2 to TA-7, TB-1 and TB-3 in Figure 1).

4.3. CENTRAL OXIDATION FILTER BED

The Central MethOx Bed of about 10 m by 10 m by 1 m depth was constructed over

the ends of the collection trenches and over the heat well. The existing cover andtopsoil were removed down to the top surface of the solid waste. Then, about 30 m3

of crushed rock was placed as a 0.3 m deep gas percolation layer on the waste. Thegas trenches were ramped up and connected to the drain layer underneath the bed.A 4 mm polypropylene geotextile was placed on the drain layer. About 90 m3 ofyardwaste compost were placed loosely as a 0.9 to 1.0 m deep layer on the geotextile

(see Figure 2). The heat exchanger was installed in the bed on a five degree slopeat depths between 0.4 m (at the northwestern corner) to 0.8 m (at the southeasterncorner).

Thebed wasinstrumentedwith fivesets of gasprobes, moisture sensors, andther-misters. Three sensor locations each contain two sets of sensors at 0.1 m and 0.9 mdepths. Two sensor locations each consist of five sets of sensors at 0.2 m increments

from 0.9 to 0.1 m depths. Two moisture content sensors (time-domain reflectrom-eters, TDRs) were installed at 0.8 m and at 0.2 m depths. All instrumentation was

Figure 2. Central MethOx Bed Cross Section with Heat Pipe.

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connected to multiplexers and a datalogger in the central instrumentation cabinetpowered by solar panel and programmed to acquire data automatically and regu-

larly. The percolation volumes and rates were measured with a lysimeter of onesquare meter area installed in the gas drain layer underneath an identical filter bed.

The initial conditions of the filter bed consisted in August 2003 of (a) an in-situbulk density of 0.77 Mg/m3, (b) a porosity of 0.65, (c) initial bed temperatures of25C at the surface and of up to 38C in the core (at 60 cms) depth. The initialmoisture contents were 61% (v/v) at 80 cm depth, and 40% (v/v) at 20 cm depth.

Thus, the characteristics achieved were close to the optima, except for slightlywarmer temperatures in the core, and higher than optimal moisture contents.

4.4. HEAT EXTRACTION WELL AND HEAT EXCHANGE IN THE METHOX FILTERBED

A heat extraction pipe and heat exchanger pipe were installed to extract heat frominside the landfill and to transfer it up into the MethOx Bed during the winter.

A thermal well consisting of a vertical 50 mm diameter pipe was installed in aborehole to 18 m depth at the southeast corner of the MethOx Bed. The boreholewas backfilled with sand and sealed with two bentonite layers at 0.2 m and 1.5 mdepths from surface. The vertical pipe leg was connected to a square heat exchangerof 5 m by 5 m (50 mm diameter steel pipe) placed at a five-degree slope in theMethOx Bed (see Figure 2). The heat well was instrumented with four thermisters

located at 18 m, 13 m, 8 m and 3 m depths.

5. Oxidation Filter Bed Key Conditions and Performance Achieved

The study focuses on the conditions achieved through the design and configurationof the alternative cover system and on the systems performance as the surfaceemission reduction in the test area and the methane oxidation performance of thefilter bed. The three essential conditions for high average performance are 1.) warm

temperatures of 5C to 25C in the filter bed, and, particularly, in the bottom ofthe bed during winter, 2.) optimal moisture content of 15% to 25% (v/v) and low

percolation through the bed, and 3.) physically and chemically stable filter medium(soil) in the bed.

5.1. TEMPERATURES IN HEAT EXTRACTION WELL

The temperature in the lower half of the bed is the limiting factor for methane

oxidation during winter. The bed temperature is influenced by four factors: 1) theair temperature, 2) by the self-heating capacity of the yardwaste compost, 3) by thetemperatures in the heat well, and 4) by the heat transfer from the well into the bed.

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296 C. A. ZEISS

The temperature in the heat well at 8 metres is stable, but moderate, at approxi-mately 7C. The temperatures at 13 and 8 metres are only slightly warmer at 8C.

The temperature at the shallowest point (3 metres depth) is warmer at its lowestof 12C, in January 2004. This profile of increasing temperature with decreasingdepth remains consistent through the summer, fall and winter seasons, but changesslightly in late winter and early spring with slightly cooler temperatures of between9 to 12C at 3 metres depth. After the first year, the temperature profile repeats itsseasonal fluctuations, but with slightly higher temperatures of between 12C and

15C at 3 metres during the second winter.The heat exchanger temperatures do not follow the air temperatures (with the

coolest temperature in December and January). Rather, the heat exchanger reaches

its warmest temperatures between October and December, but declines slightly inearly spring, likely due to the spring infiltration of melted snow and freezing rain(see also the discussion of infiltration patterns below).

5.2. TEMPERATURES IN ACCELERATED METHANE OXIDATION FILTER BED

In the first phase of the field test, the unheated MethOx Bed cooled in fall withdeclining air temperatures from the top down to the base. The entire bed profilefroze by January, and stayed frozen until early April. The temperature minima wereas low as minus 5C to minus 7C at 20 cms and 40 cms depth. In spring, the bedwarmed gradually from the top down. By mid-May, the temperature depth profileshowed a virtually constant 6 to 8C. By summer, the temperatures recovered to

about 25C, with a slight local maximum of 28C at 40 cm depth. This behaviourrepeated itself in the following years and served as the baseline performance for aMethOx filter bed without heating.

In the second test phase (August 2003 to August 2004, see Figure 3), the Ac-celerated MethOx Bed with the passive heat exchange exhibited a similar sea-sonal pattern as the unheated test bed. After construction, the bed temperatures

reached high levels of 38C at 60 cm and 80 cm depths in August 2003.By October 2003, the bed began to cool in the top 50 cm. The bottom halfstill showed temperatures of over 35C with a local maximum of about 39C.By November 2003, the temperatures had dropped to below 20C except at thebottom centre, where they were still between 30C and 35C. By January 2004,

the temperature profiles show a typical declining trend due to cooling from theambient air. The base layer stayed above freezing at depths below 60 cms withtemperatures between 7C to 13C. By April 2004, the heated bed temperatureswere virtually constant at about 7C to 8C. The passive single-phase heat ex-changer kept the bed from freezing from the base (90 cms) up to 60 cms below thesurface.

In the summer of 2004 the heat pipe was charged with distilled water to inducemore intensive heat transfer. The temperatures at the base of the bed and near theheat pipe increased further during the following winter (2004 to 2005), while the

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Figure 3. Temparature vs. bed depth profiles for outer position 4 and central position 5 in October

2003, January 2004, and April 2004.

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298 C. A. ZEISS

Figure 4. Temperature depth profiles for outer bed position 4 and central bed position 5 in January

2004 (air phase heat exchange only) and January 2005 (with charged heat pipe). Arrow up shows

increase in bed tempareture of about 6C over heat pipe.

temperatures in the centre (farthest from the heat pipe) repeated their pattern of

the previous years. The temperature profile over time for the base (depth level 5)are shown in Figure 4 for Position 5 (bed centre) and Position 4 (over the heatpipe). At position 4-5, near the heat pipe, the minimum base temperature increasedto 14C to 18C during January and February 2005, while the base temperaturesat the central position declined by about 4C (from about 12C in January 2004to just below 8C in January 2005). By this time, the self-heating capacity of the

bed material was virtually nil, and therefore did not contribute to the temperatureregime. The temperatures at the heated locations in the second year (with water-charged heat pipe) increased by about 6C above the previous years. As a result,the base of the bed (below 60 cms depth) never froze, maintaining a temperature ofno lower than 8C to 12C during the winter. These temperatures were sufficient

to support methane oxidation, albeit at rates lower than the target of 100 gramsCH4/m

2 day .

5.3. MOISTURE CONTENT AND PERCOLATION

The moisture content in the MethOx bed was measured at two depths (20 cmand 80 cm). The moisture contents are shown in Figure 5 as volumetric (%v/v)moisture contents from August 2003 to February 2005. Also shown in Figure 5 are

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Figure 5. Precipitation on the test site. Bed moisture content and percolation through the bed.

the precipitation (in mm), and the percolation through the MethOx test bed (mm).As might be expected, the moisture contents at both depths reflect the seasonalprecipitation pattern. The moisture content in the shallow layer (minus 20 cms)shows the strongest response to infiltration, rising to 60% in September of 2004.The deeper layer (80 cms) also responds by increasing to 35%. After the fall season,

the moisture contents settle back to approximately 25% during the winter until latesummer.

The percolation rate averages 7.3 mm per year for the entire test period. Thepercolation pattern follows the infiltration pattern, but is dampened by the highevaporation rates during summer. The substantial thickness of the MethOx Bed

(0.9 m to 1.0 m thick) and the high porosity provide high moisture storage ca-pacity in the MethOx filter layer. The capillary break between the filter mediumand the underlying drain material (gravel) further reduce the percolation from thebed. With an average percolation rate of 7.3 mm/a and an average total precipi-tation rate of 370 mm/y for the years from 2001 to 2004, the percolation is justunder 2% of total precipitation. This percolation rate is comparable to best per-

formance achieved by conventional and alternative covers in the ACAP program(Albright et al., 2004), but is achieved on the MethOx cover bed without veg-etation and transpiration. The leachate production from these beds may actually

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be less than that from an identical area of conventional clay cover. Because thebed constitutes only a small fraction (2.2%) of the cover area, the MethOx gas

control systems contribution to the total leachate production is predicted to benegligible.

5.4. METHANE OXIDATION FILTER MEDIUM

The filter bed consists of a source-separated yardwaste compost from a mid-sizedcity in Alberta. This filter medium was analyzed to identify possible changes inthe soil quality as methane oxidation proceeded, and, in particular, to determine

if the compost soil degrades with age and methane oxidation. The material was

composted for 12 months and then placed in the accelerated test bed. An initialsample was taken at installation in August 2003; subsequent samples were takenevery four months for lab analysis. The results for selected parameters are listed inTable II below.

Most of the parameters show very little change over the 13-month operatingperiod. Organic content, pH, and C:N ratio (although lower than optimal) holdstable. The compost stability and the self-heating capability decrease to belowdetectable limits within the first nine months (by May 2004). As a result, thetemperatures during the second winter of operation are not affected by any self-heating of the bed medium. Ammonia appears to be decreasing, while nitrate

increases slightly. In general, the soil characteristics are remarkably stable andshow no degradation that would lead to a decrease in ability to support methaneoxidation.

Similarly, the filter bed showed very low settlement over the 18-month testperiod. The average settlement was merely 5 cms from an initial depth of 0.9 m,for a total settlement of 0.06, or 6%.

5.5. ALTERNATIVE METHANE OXIDATION SYSTEMSURFACE EMISSIONREDUCTION AND FILTER BED PERFORMANCE

The initial surface emission rate of methane from the test area (of 4,506 m2) wasmoderate, at 0.7 grams CH4/m

2 day, and resulted in a total methane emission rateof 3,240 grams of methane per day in June 2003, before the construction of the test

system. The performance of the Accelerated MethOx Test System was evaluatedby three methods commonly used as the state of the art:

(a) comparison of emissions after installation of the oxidative cover system withthose before the study to evaluate the effectiveness of the trench collectionsystem;

(b) measurement of the methane and carbon dioxide surface emission rate andcalculation of the mass and mass fraction oxidized by mass balance of theMethOx filter bed, and

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(c) stable isotope analysis of influent and effluent methane and carbon dioxide, andcalculation of the fraction oxidized.

5.6. METHANE SURFACE EMISSION REDUCTION FROM THE GAS COLLECTIONAREA

The surface emissions of methane from the collection area declined from an averageof 0.7 grams CH4/m

2 day (and a range of 0 to 44 grams CH4/m2 day) before the

test (June 2003) to zero immediately after the installation of the collection trenchsystem in August 2003. The surface emissions at all eight locations (TA-2 to TA-7,TB-1 and TB-3 in Figure 1) remained nil throughout the test period.

The performance of the filter bed is reported as two sets of results. First, themethane fluxes into and the residual surface emissions out of the bed are shownin Figure 6 in relation to the bed temperature at a depth of 50 to 65 cms. The

surface emission rates are very low, particularly during the latter part of the study,when they never exceed 5 grams CH4/m

2 day. The highest value of 13 gramsCH4/m

2 day was encountered during the first winter (January 2004) when boththe bed temperature (6C) and the moisture content (22%) were at their minima.Conversely, the methane flux rates varied significantly and increased when eitherthe bed temperature or the moisture content or both were low. The maximum flux

of 18.6 grams CH4/m2 day in May 2004 occurred when the bed temperature

was moderate (at 10C, albeit warming), and the moisture content was moderate(at 28%).

These factors, in conjunction with the warmer temperatures in the waste, pro-duced the highest methane fluxes during summer, when the maximum value reached38.6 grams CH4/m

2 dayin August2003. This value, occurring over a filter bedarea

TABLE II

Methane oxidation filter bed medium characteristics

Soil parameter Aug 03 Jan 04 May 04 Sept 04

1 Organic content% 19.1% 19.8% 20.5% 16.8%

2 Compost stability as respiration of CO2 1.5 0.67

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302 C. A. ZEISS

Figure 6. Methane oxidation performance as % of influx into the bed filter (read on the right axis)

and bed temperature at the bottom of the bed (read to the left axis).

of 100 m2 results in a total methane flux of 3,860 grams CH4/day, slightly higherthan the estimated initial total surface emission rate of 3,240 grams CH4/day fromthe entire test area. Given the likelihood of some methane oxidation (of 10% to

20%) in the existing cover, these values are congruent. In the periods of transitionfrom warm to cold air temperatures, and during cold periods, the reduced fluxes of

methane as well as the continuing oxidation in the filter bed result in lower fluxesand surface emission rates. The performance improvements during the three stagesof MethOx Filter Bed configuration are shown in overview in Figure 7. The averageoxidation performance (as a percentage of methane influx), the average temperature

at the base of the filter bed, and the average moisture content are shown for thethree development phases.

The minimum temperatures at the base of the bed increased from minus 3 C inphase 1, to 8C in phase 2, to 12C in phase 3. Concomitantly, the average annualoxidation performance increased from 33% in phase 1 to 55% in phase 2, to 89%in phase 3.

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Figure 7. Accelerated methane oxidation performance as the difference between methane influx (read

on left hand axis) and surface emission rates(read on right hand axis) as a function of bed temperature.

6. Accelerated MethOx Cover Systems - Significance and Outlook

Engineered methane oxidation covers provide an alternative approach to conven-

tional landfill gas management for MSW and ICI landfills in cold climates, and as acomplementary emission reduction method for landfills in moderate climates. Theengineering design of the alternative cover system must create suitable conditionsfor biochemical methane oxidation under varying gas emission patterns and cli-mate conditions in cold, semi-arid regions. Freezing temperatures in winter, rapid

infiltration of cold water in spring and fall, and the use of available and inexpensivematerials are the challenges for the development of the oxidation cover system.

The Accelerated Methane Oxidation (MethOx) Cover System combines a gascollection trench network to channel methane from a large surface area (5,000 m 2

to 10,000 m2) to a central MethOx Filter Bed (of 1 m2 to 100 m2). A heat exchangerheats the MethOx filter with heat extracted from the landfill. This Alternative Cover

System was installed in a test area of 4,506 m2 as a trench system of 210 m length,a central MethOx filter bed of 100 m2 and a heat well of 18 m depth. The mostefficient heat extraction method maintained temperatures in the MethOx bed at 8

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304 C. A. ZEISS

to 14C during winter. The moisture contents were slightly high, at 27% to 35%(vol.), but the percolation rate through the bed averaged only 7.3 mm/a, or 2% of

total annual precipitation. The filter medium, a yardwaste compost, provided highporosity and organic content with good structural stability (i.e., little settlement)over the testperiod. As a resultof the good conditions in the filter, methane oxidationperformance accelerated from the initial conversion rate of 33% in the unheatedfilter to 89% in the accelerated, heated bed.

This performance improvement achieves emission reductions comparable to

those of a conventional landfill gas collection and control system with gas extractionwells, headers and blowers. The achievement of high oxidation performance (over80%), the complete reduction of surface emissions from the test area (to zero), and

the low percolation rate through the filter bed (less that 2%) constitute a proof ofprinciple for MethOx covers in cool, semi-arid climates.

The Accelerated MethOx System incorporates a simple design, uses small vol-

umes of available materials, and operates passively, that is, without moving parts,operating effort or regular maintenance. Moreover, the system can be constructedwith landfill equipment and operators (in about 3.5 days for the test system asshown here). The configuration of a final cover would have several of these gas col-lection fields (of one or more acres area each) around several MethOx filter beds.Except for the filter beds themselves, the surface area would consist of conventional

cover. The few MethOx beds (of 1 to 100 m2 each) will not interfere with end uses,will not increase moisture infiltration, nor leachate generation over a conventionallandfill cover. The oxidation of methane not only reduces the landfills greenhousegas impacts, but also limits the exposure of nearby residents and other users ofthe site to ground level methane concentrations. MethOx filters are also capableof co-oxidizing other trace and odiferous gasses with methane. The MethOx cover

avoids the visual disamenities of conventional gas systems, such as the view ofpipes, flares, wellheads and such objects.

This study was carried out with bare filter beds to avoid the effects of vegetation.The next phase of the research will test the influence of vegetation on methaneoxidation conditions and performance.

This development and test of a full-scale MethOx alternative cover provides

a Proof of Principle for methane oxidation covers in cold semi-arid climate con-ditions, under which it is not feasible for many landfills to manage landfill gas

emissions by conventional gas extraction. This research study develops a specific,viable design for an alternative final cover to reduce greenhouse gas emissionsfrom landfills. Its practical significance lies in the simple, passive, robust design,low maintenance requirements, its visual appeal and easy integration into final

cover and end use plans for MSW landfills in cold semi-arid regions, and withslight changes, to applications in developing countries and remote regions.

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Acknowledgments

and Disclaimer This research was partially supported by Climate Change CentralAlberta. This support is gratefully acknowledged. Several suppliers of scientificequipment cooperated by making special efforts to adapt and supply their equip-ment: Campbell Scientific of Canada and Primary Systems of Calgary.

The results and discussion presented here are for discussion purposes only andshould not be relied on in any specific situation or for any purpose without the

express written consent by the authors and the publisher.

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