Methane Oxidation in Landfill Cover Soils : A Review

Migration of methane (CH4) gas from landfills to the surrounding environment negatively affects both humankind and the environment. It is therefore essential to develop management techniques to reduce CH4 emissions from landfills to minimize global warming and to reduce the human risks associated with CH4 gas migration. Oxidation of CH4 in landfill cover soil is the most important strategy for CH4 emissions mitigation. CH4 oxidation occurs naturally in landfill cover soils due to the abundance of methanotrophic bacteria. However, the activities of these bacteria are influenced by several controlling factors. This study attempts to review the important issues associated with the CH4 oxidation process in landfill cover soils. The CH4 oxidation process is highly sensitive to environmental factors and cover soil properties. The comparison of various biotic system techniques indicated that each technique has unique advantages and disadvantages, and the choice of the best technique for a specific application depends on economic constraints, treatment efficiency and landfill operations.


INTRODUCTION
Methane (CH 4 ) gas is one of the most important greenhouse gases (GHGs).As a result of human activities, CH 4 emission concentrations in the atmosphere have increased from 715 ppb during the pre-industrial age to 1,732 ppb in the early 1990s and 1,774 ppb in 2005 (IPCC, 2007).Although the CH 4 concentration in the atmosphere is much lower than that of carbon dioxide (CO 2 ), its global warming potential is 25 times greater (IPCC, 2007).A study by Henckel et al. (2001) showed that the global CH 4 concentration is approximately 1.8 ppmv, which represents a doubling during the last 200 years.
Landfills rank as the third major anthropogenic source of CH 4 emissions after rice paddies and ruminant manure (Qingxian et al., 2007;Ritzkowski et al., 2007).A total of 40-60 metric tons of CH 4 are emitted from landfills worldwide, accounting for approximately 11-12% of the global anthropogenic CH 4 emissions (Ritzkowski et al., 2007).CH 4 gas migration from landfills to the surrounding environment negatively affects both humankind and the environment.Gas explosion disasters due to landfill gas (LFG) migration resulting from variations in atmospheric pressure were reported in the village of Loscoe in England in 1986 and at Skellingsted Landfill in Denmark (Christophersen et al., 2001).
Mitigation of landfill CH 4 emissions has been conducted using two approaches.The first approach uses gas collection systems for recovering or burning LFG, while the second approach seeks to reduce the emissions by various means, including waste recycling, composting and incineration.The first approach is more prevalent because it is cost-effective for large sanitary landfills.However, it is considered to be too costly and infeasible for older and smaller landfills whose CH 4 emission rates are much lower.Although major sanitary landfills utilizes gas collection systems, small quantities of LFG still escape into the atmosphere or migrate into the surrounding soil through the topmost layer of cover soil.Some researchers have found that conventional gas recovery systems only capture 50 to 90% of the CH 4 generated in landfills (Augenstein and Pacey, 1996).Therefore, the development and application of techniques for effectively reducing CH 4 emissions from landfills are required to minimize both the future global warming potential and the human risks associated with CH 4 gas emissions.
Microbial CH 4 oxidation in landfill cover soil may provide a means of controlling CH 4 emissions.Several studies have shown that the CH 4 oxidation process in landfill cover is an efficient method of CH 4 emission mitigation (Abushammala et al., 2013a;Huber-Humer et al., 2008;Stern et al., 2007;Huber-Humer, 2004;Hilger and Humer, 2003;Humer and Lechner, 1999).This process takes place in many natural systems and soils without human interference, due to the abundance of several groups of bacteria requiring oxygen (O 2 ) for the oxidation process.This process may be exploited to reduce CH 4 emissions at landfill sites where gas recovery systems are nonexistent or alongside existing gas collection systems to complement emissions control.A value of 0 to 10% of CH 4 oxidation has been recommended by the Intergovernmental Panel on Climate Change (IPCC) guidelines for national GHG inventories.However, laboratory and field studies indicates that the CH 4 oxidation capacity is between 0 and 100% (Jugnia et al., 2008).Conversely, Bogner et al. (1995) stated that landfill cover soil under certain conditions can be a sink for atmospheric CH 4 .Currently, there is insufficient information available regarding CH 4 oxidation capacity due to the lack of a standard method to determine the oxidation rate.
This study discusses the CH 4 oxidation process, which mitigates CH 4 emissions associated with LFG production.First, the mechanisms of CH 4 oxidation by methanotrophic bacteria in landfill cover soils are identified.Second, the key factors that control the CH 4 oxidation process in landfill cover soils are discussed.Finally, current techniques for mitigating CH 4 emissions using biotic systems are compared to investigate their key features and examine how they can be incorporated into the future design of landfill soil covers.

METHANE OXIDATION BACTERIA
The CH 4 oxidation process in landfill cover soils is facilitated by a group of methanotrophic bacteria that live in landfill cover soil (Huber-Humer, 2004).For simplicity, previous studies have reported that the CH 4 oxidation process in landfill cover soils is accomplished by methanotrophic bacteria (Abushammala et al., 2012;Huber-Humer et al., 2008;Albanna et al., 2007;Stern et al., 2007;Kettunen et al., 2006).Methanotrophic bacteria (Fig. 1) are a group of obligate aerobes that have the ability to oxidize CH 4 under natural conditions to produce CO 2 , water (H 2 O), and microbial biomass (Eq.1).Other organic compounds in LFG, such as aromatic and halogenated hydrocarbons, can be partially or fully degraded by methanotrophic bacteria that have the ability to co-metabolize substrates other than CH 4 (CLEAR, 2009;Scheutz and Bogner, 2003).

microbial biomass
(1) There are several complex enzymatic pathways for CH 4 oxidation.Methanotrophs are divided into three types: type I methanotrophs follow a ribulose monophosphate (Ru MP) pathway, type II methanotrophs follow a serine pathway, and type X methanotrophs follow both pathways (Bogner, 1996).These classifications are based on their carbon assimilation pathways, intracytoplasmic membrane arrangements, cell morphology and the specific protein content of their DNA.In general, all three types of methanotrophs possess the CH 4 monooxygenase (MMO) enzyme, which assists them in oxidizing CH 4 for energy yield (Fig. 2) (Bogner, 1996).MMO can be found in two forms: particulate CH 4 monooxygenase (pMMO) and soluble CH 4 monooxygenase (sMMO).Most methanotrophic bacteria are known to express themselves as pMMO, while a few of them express themselves as sMMO, and some have the ability to express themselves in both forms (Lee, 2008).However, methanotrophic bacteria have broad differences with respect to their responses to different CH 4 concentrations (Reay and Nedwell, 2004), and they can be classified accordingly as high-affinity or low-affinity methanotrophic bacteria.High-affinity methanotrophic bacteria are characterized by low CH 4 oxidation capacity, which enables them to begin oxidation at low CH 4 concentrations (0.8-280 nmol L -1 ) (Huber- Humer et al., 2008).High-affinity methanotrophic bacteria exist in soils temporarily exposed to CH 4 concentration.Low-affinity methanotrophic bacteria exhibit a high oxidation capacity, with CH 4 levels in the range of 0.8-66 μmol L -1 (Huber- Humer et al., 2008).Low-affinity methanotrophic bacteria are more prevalent in landfill cover soils than are the high-affinity variant (Kightley et al., 1995).
Methanotrophic bacteria can use substrates other than CH 4 under certain conditions, resulting in a reduction in the CH 4 oxidation rate and oxidation of ammonia (NH 4 + + ) to nitrite and nitrous oxide, due to the non- specific nature of MMO (Knowles, 2005).Bogner (1996) documented inhibitions of methanotrophic activity due to nitrogen cycle processes that occur when hydroxylamine is produced by the oxidation of NH 4 + + by MMO, which inhibits MMO enzyme activity, when nitrite inhibits other enzyme activity necessary for CH 4 oxidation, and finally, when methanol is present in addition to NH 4 + + .

FACTORS AFFECTING METHANE OXIDATION
The CH 4 oxidation capacity of landfill cover soils varies within and among landfills due to many factors that affect the oxidation process, such as seasonal variations (Abushammala et al., 2013b;Einola et al., 2007;Maurice and Lagerkvist, 2003;Börjesson et al., 2001), physical and chemical heterogeneities of landfill cover soils (Tecle et al., 2008;Albanna et al., 2007;Visvanathan et al., 1999), and the CH 4 concentrations in landfills (Boeckx et al., 1996).According to Mosier et al. (2004), the major factors controlling CH 4 oxidation are potential biological demand and diffusion.The biological demand is regulated by both the physical and chemical environments, while the CH 4 diffusion rate is regulated by physical factors only.The reported values of landfill cover soils' CH 4 oxidizing efficiency vary widely in the literature.Albanna et al. (2007) reported that increasing the soil layer thickness from 15 to 20 cm increased the CH 4 oxidation values from 29% to 35% for a soil with 15% moisture content without nutrient addition, from 34% to 38% for a soil with a 30% moisture content without nutrient addition, and from 75% to 81% for a soil with a 30% moisture content with nutrient addition.However, in investigating the effect of bio-cover on CH 4 oxidation at the Leon landfill in Florida, Stern et al. (2007) found that the efficiency of CH 4 oxidation can reach 64% with biocover utilization, while only 30% efficiency was reported for the control cell.Abichou et al. (2009) reported that at the same landfill, an average of 79% of CH 4 was oxidized in the bio-cover system and 29% was oxidized in the control cell.These wide variations can be attributed to the previously mentioned factors.
The major controlling environmental factors governing the CH 4 oxidation process in landfill cover soils, such as soil texture, organic content, moisture content, temperature, pH, nutrients, and O 2 and CH 4 concentrations (Wilshusen et al., 2004a;Börjesson et al., 2001;Boeckx et al., 1996) are briefly discussed in this section.Applying knowledge about these controlling factors can optimize the process of mitigating CH 4 emissions from landfills.

1 Soil Texture
Soil texture affects LFG transport and atmospheric O 2 penetration.It therefore controls both CH 4 emission and oxidation rates.The CH 4 oxidation capacity in soils of various textures was investigated by Kightley et al. (1995), and it was found that higher oxidation efficiency occurs in coarse sand (61%) than in fine sand or clay (40-41%).Boeckx et al. (1997) concluded that coarse soils have higher oxidizing capacities than fine soils.Gebert and Grongroft (2009) recommended the use of coarse-textured soils with more than 17% air-filled pores by volume, such as sands, loamy sands, sandy loams and some of the coarsely textured loams, for use as CH 4 oxidizing bio-cover.

2 Soil Organic Content
The CH 4 oxidation rate increases with increasing soil organic content (Humer and Lechner, 2001;Christophersen et al., 2000;Visvanathan et al., 1999).Through soil incubation tests, Christophersen et al. (2000) found that soils containing more organic matter more effectively mitigate CH 4 emissions through oxidation.They also found a relationship between the optimal soil moisture content and the organic matter content.The water content provides optimal oxidation increases with increasing organic matter content.Visvanathan et al. (1999) found that higher soil organic contents resulted in higher CH 4 oxidation rates in column assays.High-organic-content materials, such as compost, are widely used in landfill cover systems to enrich their CH 4 oxidation capacity (Abichou et al., 2009;Huber-Humer et al., 2008;Gebert and Grongroft, 2006a;Wilshusen et al., 2004a;Streese and Stegmann, 2003).Materials with high organic contents, high levels of nutrients, and high porosity have been proven to have high CH 4 oxidation capacities, which in some cases, tends to oxidize atmospheric CH 4 .However, De Visscher et al. (2001) found that adding compost materials enhanced CH 4 oxidation, after a brief period of inhibition.

3 Moisture Content
There are several sources of water in landfill soil cover, including surface water infiltration, precipitation, water from manmade sources (leachate recirculation) and the decomposition reaction within the soil cover.As reported previously, a high moisture content in landfill soil cover reduces the available pore space for gaseous transport and diffusion.A high moisture content also reduces O 2 penetration into the soil cover, which is the main reactor for the CH 4 oxidation process.A low soil moisture content reduces the biological activity in soil cover and results in a reduction in CH 4 oxidation capacity (Tecle et al., 2008).The combination of soil drying due to low moisture content and the heat generated by CH 4 oxidation are likely to reduce the pore water content of soil, which may facilitate LFG transport through the shallow soil cover and reduce the oxidation capacity, due to the inhibition of microbiological activities that require a certain amount of water (Maurice and Lagerkvist, 2003).The desirable moisture content for high CH 4 oxidation activity is in the range of 11-25% by volume (Tecle et al., 2008).Boeckx et al. (1996) studied the effect of the soil moisture content on the CH 4 oxidation capacity of a landfill soil cover 30 cm thick.In his laboratory test, the moisture content of the soil was tested at 5, 10, 15, 20, 25 and 30% by weight, and the optimum moisture content was found to be between 15.6 and 18.8% by weight.Visvanathan et al. (1999) reported ideal moisture contents of 15% and 15 to 20% for maximum CH 4 oxidation in column and batch experiments, respectively.They stated that a negligible amount of CH 4 oxidation might occur at a 6% moisture content and that zero oxidation would occur at a 1.5% moisture content.Lee et al. (2009) found that the highest CH 4 oxidation rates occurred at a moisture content of 5% in a sandy landfill soil cover, with CH 4 oxidation rates decreasing as the moisture content increased.
Four sandy soils from two landfills in Denmark were investigated in batch experiments by Christophersen et al. (2000) to determine the effects of soil moisture on CH 4 oxidation.The results showed that the optimum moisture content range from 11 to 32% in all samples.It was also found that both moisture content and CH 4 oxidation increased as the organic matter content increased.More recently, work has been conducted by Park et al. (2002) to test the effect of the moisture content of loamy sandy soil on CH 4 oxidation capacity.They found that 13% by weight was the optimum moisture content for CH 4 oxidation in this soil.Another study conducted by Park et al. (2005) concluded that moisture content is the most important factor controlling the CH 4 oxidation rate is a sandy soil landfill cover.Mor et al. (2006) found that the effect of the soil moisture content on CH 4 oxidation in various types of compost was time-dependent and that the optimum moisture content ranges between 45 and 110% (dry weight basis).

4 Temperature
CH 4 oxidation in landfill soil cover is a biological process, and soil temperature is an important factor affecting this process (Streese and Stegmann, 2003).The methanotrophic community structure changes due to temperature variations, rather the quantity of type II methanotrophs decreasing with increasing temperature and precipitation (Horz et al., 2005).Several studies have reported on the optimum temperature for CH 4 oxidation in soil cover.Castro et al. (1995) found that soil temperature is an important factor in CH 4 oxidation at temperatures between -5� C and 10� C but has no effect on CH 4 oxidation at temperatures between 10� C and 20� C. Visvanathan et al. (1999) documented inhibition of CH 4 oxidation at temperatures higher than the optimum temperature, which they found in laboratory experiments to be in the range of 30 to 36� C. De Visscher et al. (2001) confirmed these results in reporting that 35� C was found to be the optimum temperature for CH 4 oxidation activity in a sandy loamy soil from a landfill in Belgium.They also concluded that soil temperatures in excess of 30� C for long periods can lead to a reduction in CH 4 oxidation activity.Scheutz and Kjeldsen (2004) reported that CH 4 oxidation increased exponentially (with R 2 ¤0.91) with increases in soil temperature from 2 to 25� C. The maximum CH 4 oxidation rate occurred at 30� C, and the oxidation rate started to decline at 40� C. The effect of temperature on CH 4 oxidation in various types of compost was studied by Mor et al. (2006), who found that the effect of temperature on CH 4 oxidation is time-dependent and that the optimum temperature range is between 15 to 30� C. Borken et al. (2006) found that in forest soils, summer drought may increase CH 4 oxidation.
On the other hand, it has been reported that there is an interdependency between the effects of soil temperature and water content on CH 4 oxidation.Visvanathan et al. (1999) found that a sufficient moisture content combined with an appropriate temperature (approximately 20� C) could result in higher CH 4 oxidation.However, Castaldi and Fierro (2005) found that CH 4 oxidation rates were maximized when the water content was very low and the temperature was high.Einola et al. (2007) have reported an interdependency between soil temperature and water content, the most important factors controlling CH 4 oxidation capacity, and their effects on CH 4 oxidation.

5 pH
Variation in the pH value of a landfill soil cover affects CH 4 oxidation activities (Hutsch et al., 1994).According to Whittenbury et al. (1970), all types of methanotrophic bacteria can grow in pH values ranging from 5.8 to 7.4, with the optimum pH value being in the range of 6.6 to 6.8.However, Saari et al. (2004), found the optimum pH for CH 4 oxidation to vary from 4 to 7.5 in tests of CH 4 oxidation capacity in different type of soils with pH values ranging from 3 to 7.5.They also found that for some soils, the optimum pH for CH 4 oxidation is greater than the natural pH.The optimal pH value for CH 4 oxidation in soil samples collected from the Skellingsted Landfill in Denmark was found by Scheutz and Kjeldsen (2004) to be 6.9.
Methanotrophic bacteria are sensitive to the acidification of surrounding soils.Mer and Roger (2001) observed that the oxidation rate of non-fertilized permanent grassland at the Rothamsted experimental station in England decreased from -67 to -35 nL CH 4 .L -1 .h - (nL= =nanoliter) when the pH of the cover soil at the site decreased from 6.3 to 5.6.Others have reported that the CH 4 oxidation decreases to zero at pH values between 5.6 and 5.1 (Huetsch et al., 1994).According to Hanson and Hanson (1996), methanotrophic bacteria cannot grow at pH values below 5. Numerous attempts to isolate or obtain enrichments for methanotrophic bacteria that would grow at pH values below 5.5 from acidic peat samples have failed.

6 Nutrients
Aside from the carbon substrate from CH 4 oxidation, bacteria in landfill soil cover require other nutrients for their cellular metabolism.The addition of nutrients to a soil cover system results in activation of methanotrophic bacteria, thus enhancing the CH 4 oxidation rate and oxidation efficiency (Lee et al., 2009;Albanna et al., 2007;Börjesson et al., 1998).Albanna et al. (2007) found that soil moisture and the addition of nutrients have a combined effect on CH 4 oxidation in soil cover, and they reported that adding nutrients to incubated soil with a 32% average moisture content doubles the oxidation efficiency.However, adding nutrients to a soil with a low moisture content (15%) was found to have a negative effect on the oxidation efficiency.Lee et al. (2009) found that the CH 4 oxidation capacity of sandy soil cover increased by approximately 60% with the addition of 100 mg-N NH 4 + + per kg of soil.
Vegetation might affect on the growth and activity of methanotrophic bacteria in a variety of ways (Wang et al., 2008).Vegetation roots assist the process of transporting O 2 from the atmosphere into deeper soil layers (Fig. 3) (Tanthachoon et al., 2007).Furthermore, exudates that are supportive nutrients for methanotrophic bacteria are released to the root zone, which enhances CH 4 oxidation (Tanthachoon et al., 2007).Therefore, vegetation on the surfaces of landfill covers encourages methanotrophic activities throughout the soil depth profile.However, vegetation might compete with microorganisms for nutrients and water, which might result in an overall decrease in CH 4 oxidation (Hilger and Humer, 2003).Bohn and Jager (2009) found that the CH 4 oxidation rate can be enhanced by at least 50% by vegetation growth on landfill cover soils.
In engineered biological treatment systems, nitrogen and phosphorous is added in the form of NH 4 + + and orthophosphate.Adding NH 4 + + reduces the CH 4 oxidation capacity due to NH 4 + + inhibiting the activities of methanotrophic bacteria (Reay and Nedwell, 2004;Wang and Ineson, 2003;Hanson and Hanson, 1996).However, as discussed previously, the oxidation of NH 4 + + produces nitrite, which has an inhibitory effect on the MMO enzyme.Bosse et al. (1993) found that the CH 4 oxidation rate decreases at NH 4 + + concentrations ›4 mM (mM= =millimolar) and is completely inhibited at NH 4 + + concentrations ¤20 mM.Keller et al. (2006) reported that nutrients (nitrogen and phosphorus) are important in the control of peat land microbial carbon cycling and that the roles of these nutrients differ with short-and long-term incubation.

7 Oxygen Concentration
Oxygen is one of the main reactors and limiting factors controlling the CH 4 oxidation process in landfill cover soils (Berger et al., 2005).The O 2 concentration Landfill Methane Oxidation varies with the depth of soil cover and is influenced by many variables, including gas characteristics, meteorological conditions, the microbial CH 4 oxidation rate, the soil texture and the cover thickness.Soil porosity controls the depth of O 2 penetration into soil (Humer and Lechner, 1999).The overlapping of the gradients of the CH 4 and O 2 concentrations in a soil profile occurs at the point of maximum CH 4 oxidation, and the depth at which this overlapping occurs is the optimum depth for maximum CH 4 oxidation.Several researchers have found different maximum CH 4 oxidation zones.Visvanathan et al. (1999) found that maximum oxidation occurs at depths of 15 to 40 cm, while Börjesson and Svensson (1997) found that 50 to 60 cm is the optimum depth for maximum CH 4 oxidation.A study conducted by Jones and Nedwell (2006) stated that the maximum CH 4 oxidation occurred at depths from 10 to 30 cm, while Jugnia et al. (2008) stated that 0-10 cm is the optimal depth for CH 4 oxidation activity.William and Zobell (1949) reported that O 2 concentrations between 10 to 40% produce the highest range of CH 4 oxidation rates (Table 1), with an increase or decrease in O 2 concentration outside this range decreasing the CH 4 oxidation rate.

8 Methane Concentration
The influence of the CH 4 concentration on the CH 4 oxidation capacity can be described using the Michaelis-Menten equation (Eq.2): where V is the actual CH 4 oxidation rate (m 3 m -3 s -1 ), V max is the maximum CH 4 oxidation rate (m 3 m -3 s -1 ), K M is the Michaelis constant for CH 4 (%) and C is the CH 4 concentration (%).Many researchers have reported the effect of the CH 4 concentration on the CH 4 oxidation capacity (Pawlowska and Stepniewski, 2006;Visvanathan et al., 1999;Bogner et al., 1997).Pawlowska and Stepniewski (2006)

BIOTIC SYSTEMS FOR CH 4 OXIDATION
LFG treatment using a variety of types of biotic systems, including bio-washers (Figueroa, 1996), biomembranes (Figueroa, 1996), bio-filters (Huber-Humer et al., 2008;Gebert and Grongroft, 2006a;Wilshusen et al., 2004b;Streese and Stegmann, 2003;Figueroa, 1996), bio-windows (Huber- Humer et al., 2008), biocovers (Shangari and Agamuthu, 2012;Huber-Humer et al., 2008) and bio-tarps (Huber- Humer et al., 2008), has been discussed in the literature.The first two types of systems (bio-washers and bio-membranes) for landfill emissions treatment are not discussed in this section because of their limited use.Biotic systems such as bio-filters, bio-windows, bio-covers and bio-tarps are discussed in more detail as they are the most widely used types of systems.
Biotic systems are economical options for controlling low levels of CH 4 emissions from landfills.Biotic systems can be used in many applications in landfills, in addition to gas collection systems for trapping CH 4 emissions at old landfills, at small landfill sites at which gas collection systems are not economical options and during landfill site postclosure and aftercare processes.
Biotic systems used for CH 4 emissions mitigation are described in the following sections in terms of their key features and their incorporation into the design of future landfill cover soils.

1 Bio-Filter
Bio-filters were first used for contaminated gas treatment in the USA in 1966 to deodorize sewage sludge digestion gas.Recently, the application of bio-filters has expanded to CH 4 oxidation of LFG in addition to odor elimination.The first application of bio-filters for LFG treatment on a laboratory scale to investigate deodorization and the degradation of both H 2 S and CH 4 was in 1979.Aerobic degradation of CH 4 in LFG using bio-filters was first investigated in 1986 (Figueroa, 1996).
Several laboratory and field experiments have been conducted to investigate bio-filter designs, media and gas flow.The filters are operated either in open or fully contained beds.A bio-filter consists primarily of a filter material that influences the performance of purification by its physical, chemical and biological properties (Figueroa, 1996).This filter material is considered to be the most important part of a bio-filter system because it supports bacteria cultures and is capable of sorption of contaminated gas.Bio-filter materials are primarily of biological origin, such as peat, compost from bio-waste, heather, shredded bark and sawdust (Huber-Humer et al., 2008;Figueroa, 1996).Bio-filters have high water storage capacity and sufficient nutrients to facilitate biological processes.Admixtures such as expanded clay, polystyrene, lava and active carbon can be added to improve the structure of the filter material and increase its purification efficiency.LFG passively vented through the pressure gradient  between the landfill and the atmosphere (Gebert and Grongroft, 2006a) can be directed through the filter in either of two modes (Huber-Humer et al., 2008): upflow or downflow (Fig. 4).
The CH 4 degradation process in a bio-filter is highly dependent on the retention time of LFG inside the filter (i.e., gas flow).Figueroa (1996) found 50 g CH 4 m -3 h -1 removal at a surface load of 5 m h -1 and complete CH 4 removal at a surface load of 0.5 m h -1 .However, several environmental conditions affect the filter efficiency (Figueroa, 1996), such as water content, temperature, pore volume or residence time and filter resistance.Good control of these environmental factors results in high filter efficiency and a positive effect on the functions of microorganisms.
CH 4 oxidation rates in the range of 20-60 g m -3 h -1 have been observed in a variety of laboratory column studies of bio-filters (Wilshusen et al., 2004a;Streese and Stegmann, 2003;Park et al., 2002), including studies up to one year in length.Wilshusen et al. (2004a) studied several types of compost filter material using column experiments conducted over periods up to 220 days on a laboratory scale to compare their CH 4 oxidation potential.They observed that a maximum of 400 g CH 4 m -2 day -1 CH 4 oxidized over a period of 100 days, followed by a decrease in rate to approximately 100 g CH 4 m -2 day -1 over the next 120 days.Various bio-filter materials for LFG treatment were tested by Streese and Stegmann (2003).They found that a mixture of compost, peat, and wood fibers exhibited a stable CH 4 oxidation rate of approximately 20 g m -3 h -1 for a CH 4 concentration of 3% by volume over a period of one year.On the other hand, finegrained compost used as a bio-filter material was reported by the same authors to result in a CH 4 removal rate of up to 63 g m -3 h -1 in the first three months of the experiment for a CH 4 concentration of 2.5% by volume.Later, in the fifth month of the experiment, the decrease in the CH 4 oxidation rate was monitored.Both Wilshusen et al. (2004a) and Streese and Stegmann (2003) attributed the reduction in the CH 4 oxidation rate after reaching its maximum level to extracellular polymeric substances (EPS) formed by methanotrophic microorganisms.
EPS formation is a serious problem with bio-filters (Huber- Humer et al., 2008;Gebert and Grongroft, 2006b;Wilshusen et al., 2004a;Streese and Stegmann, 2003).These substances can block the pore space of the filter material and delay the substrate supplementation to the microorganisms inside the filter material, resulting in the deceleration of methanotrophic activity.EPS formation occurs primarily as a consequence of prolonged use of an active gas feed system (Wilshusen et al., 2004a;Streese and Stegmann, 2003).Passive bio-filters tends to receive gas in an intermittent manner.However, by controlling the inlet flux rate to a landfill bio-filter, it may be possible to mitigate or prevent EPS formation (Huber-Humer et al., 2008).Nonetheless, usage of additional gas distribution layers in bio-filter material optimizes mass transfer of gas components, thus reducing EPS formation (Streese and Stegmann, 2003).Hilger et al. (2009) reported that a nutrient imbalance could promote EPS formation in a bio-filter system.

2 Bio-Window
A bio-window is a system for mitigating landfill CH 4 emissions to the atmosphere.Composted materials with adopted environmental conditions are usually used as bio-window media to attain maximum CH 4 oxidation efficiency through enhanced microbial activity by CH 4 oxidation bacteria.The bio-window (Fig. 5) is integrated with the landfill soil cover in small regions of a landfill where high CH 4 emissions are observed.Measurements of the spatial variability of CH 4 emissions from landfill cover soils using the flux chamber technique and geo-statistical analysis are used to identify CH 4 emission hot spots within a landfill.Incorporation of a bio-window system into a landfill soil cover in these zones greatly mitigates the CH 4 emissions of the entire landfill.This technique is useful when the use of full-expanse compost materials is not economically feasible and when no gas collection system is available to feed a bio-filter system (Huber- Humer et al., 2008).A bio-window receives passively vented LFG from the underlying waste, thereby offering flexible routes for gas movement.

3 Bio-Cover
In 2009, Huber-Humer et al. defined a landfill biocover as a top cover that optimizes the environmental conditions for methanotrophic bacteria and enhances biotic CH 4 consumption.A typical bio-cover system consists of a highly porous gas distribution layer above the waste, often gravel or crushed glass, followed by a compost-amended layer.The thickness of the gas distribution layer usually ranges from 10 to 30 cm (Jugnia et al., 2008;Stern et al., 2007), while the compost layer in the upper part is thicker, up to 100 cm or more, to attain high oxidation capacity.The gas distribution layer above the waste results in uniform LFG fluxes to the bio-cover layer, which permits biological activity to occur in a typical manner (Fig. 6).Many researchers have attempted to reduce landfill CH 4 emissions to the atmosphere using bio-cover systems (Shangari and Agamuthu, 2012;Bogner et al., 2010;Abichou et al., 2009;Huber-Humer, 2009;Jugnia et al., 2008;Stern et al., 2007;Bogner et al., 2005;Huber-Humer, 2004;Hilger and Humer, 2003;Humer and Lechner, 2001;Humer and Lechner, 1999).Their results show high CH 4 oxidation capacity in diverse, mature and well-structured compost materials, in both laboratory investigations (Abichou et al., 2009;Stern et al., 2007) and field trials (Bogner et al., 2005;Huber-Humer, 2004;Humer and Lechner, 2001;Humer and Lechner, 1999).Shangari and Agamuthu (2012) found that CH 4 oxidation can reach 100% when a bio-cover of brewery spent grain and compost materials is used at a ratio of 7 : 3. Abichou et al. (2009) found that 100% CH 4 oxidation capacity can be achieved using compost bio-cover as a landfill cover.Humer and Lechner (2001) reported that the CH 4 oxidation capacity of compost landfill cover can reach 100% under optimum conditions of proper design and compost quality.Berger et al. (2005) found that in cover soil consisting of two layers, a mixture of compost plus sand (0.3 m) over a layer of loamy sand (0.9 m), the CH 4 oxidation capacity ranged from 98% to 57%.A system consisting of 50 cm of pre-composted yard biocover placed over 10-15 cm of crushed glass, utilized as a gas distribution layer, over a 40-100 cm interim cover, was used by Stern et al. (2007) to investigate its landfill CH 4 emission reduction and CH 4 oxidation capacity.They found that the bio-cover cells reduced CH 4 emissions by a factor of 10 and doubled the percentage of CH 4 oxidation relative to control cells.

4 Bio-Tarp
There are two types of cover that are used in landfills before final capping.The first type is referred to as a daily cover and the second type is referred to as an intermediate cover.On an operational landfill site, a daily cover is used to cover the in-place waste at the end of each working day.An intermediate soil cover is used after a cell is completed and is awaiting final capping.
The daily cover functions to prevent interaction between the waste and air, thereby reducing odors.Furthermore, the daily cover is important to prevent windblown litter, minimize the risk of fire within the site, and discourage scavengers and flies.Most landfills use a 15-cm soil layer as a daily cover (Hilger et al., 2009;Huber-Humer et al., 2008).Alternative daily cover (ADC) materials, such as green and brown waste, sewage sludge, water slurries or commercial products such as foams and canvas, can also be used.The use of ADC materials are appropriate at some sites where local soils are unavailable and additional air space is required.Tarps are one type of ADC that maximizes airspace and thereby minimizes the required volume required of any other daily cover.Tarps are placed at the end of the working day and removed the next day to allow for further waste deposition.The filling of an active landfill cell may take a long period of time, during which no CH 4 collection occurs.In this case, the use of a bio-tarp (Fig. 7) is a good strategy for mitigating CH 4 emissions via methanotrophic bacteria impregnated in its material.Adams et al. (2011) found  Humer et al., 2008).
that the use of multiple layers of water-absorbent geotextiles as bio-tarps removed 16% of CH 4 , while adding landfill cover soil, compost or shale amendments to the bio-tarp increased the CH 4 removal by up to 32%.Unlike bio-filters, bio-windows and bio-covers, bio-tarps can be removed and re-activated and can serve as a portable emissions reduction strategy.A comparison of the aforementioned biotic systems is provided in Table 2.   (Table 2) in which the various types of biotic systems are compared, biofilters appear to be appropriate at landfills where LFG collection is in operation because of their high CH 4 uptake capacity.Bio-covers offer the advantage of covering an entire landfill while simultaneously providing good water-holding capacity and porosity for vegetation and evapotranspiration.Bio-windows can be used at landfill hotspots.Bio-tarps can be appropriate alternative daily covers for use in mitigating CH 4 emissions during landfill operations at times when no CH 4 collection occurs.Each type of biotic system has advantages and disadvantages, and the choice of which method to apply depends on economic constraints, treatment efficiency and landfill operations.

Fig. 4 .
Fig. 4. Various integrated design of bio-filters and landfill soil cover: (a) upflow mode and (b) downflow mode.
documented a significant influence of CH 4 concentration on the CH 4 oxidation capacity through a bio-filter model assay.They found 6Asian Journal of Atmospheric Environment, Vol.8(1), 1-14, 2014

Table 1 .
CH 4 consumption at various O 2 and N 2 concentrations over a 6-day assay period at 32� C.
This study discusses the CH 4 oxidation process,

Table 2 .
Comparison of different biotic system techniques.mitigates CH 4 emissions associated with LFG production.Many factors affect the CH 4 oxidation capacity of landfill soil cover.The most important factors are environmental factors and the properties of the cover soil.Special consideration must be given to those factors to enhance the CH 4 oxidation process and to mitigate landfill CH 4 emissions.Biotic systems are economically feasible options for controlling low levels of CH 4 emissions from landfills.Based on the summary table which