Actual data of greenhouse gas emissions from Ontario’s large landfills were compared to modeled data generated by USEPA LandGEM model. Different scenarios were developed and compared based on their associated greenhouse gas (GHG) emissions. The life cycle modelling was made for municipal solid waste landfilled till 2015 and the environmental aspects were evaluated for a 100-year period after disposal. The data utilized in the calculations to model the municipal solid waste landfilling scenarios make extensive use of site-specific data from the landfills provided by Ontario Ministry of Environment and Climate Change (MoECC). The assessment of different scenarios showed that reduction of landfilled organic waste can lead to significant reduction in the potential environmental loads with respect to global warming when compared to the current MSW landfilling. Conventional landfilling with gas collection for electricity generation showed a better performance than the flaring scenario. Landfill gas collection and power generation resulted in a reduction in GHG emissions of up to 78% relative to methane flaring scenario. Utilization of soil top covers to oxidize methane (use of methanotrophs) has been demonstrated to drastically reduce environmental burdens.
Keywords: Municipal solid waste, landfill gas, life-cycle assessment, waste to energy, greenhouse gas emissions
The most common disposal option in Canada is landfills. An estimated 33 million metric tons (Mt) of municipal solid waste (MSW) was generated in Canada in 2016, 76.5% of which (26 Mt) was discarded in landfills (Statistics Canada, 2016). In 2015, landfill emissions accounted to 27% of Canada’s emissions. Ontario has the highest waste disposal amongst other Canadian provinces, followed by Quebec, Alberta and British Columbia. Statistics Canada reported that solid waste is generated at a rate of 936 kilograms per person per year and 729 kilograms of the generated waste go to the 2000 operating landfills in the country (CCME, 2014). Ontario Waste Management Association (OWMA, 2013) reported that the total waste generated in Ontario in 2013 was 12.5 million metric tons. There are 880 landfills in Ontario and 28 of them have landfill gas (LFG) recovery system. Recently, most Canadian provinces adopted the strategy of substituting all small unlined landfills with larger lined landfill (CCME, 2014). Ontario has distinguished the benefits of using the generated landfill gas as a good waste management practice, which could become a major renewable energy source. Recent studies confirm that any investment in LFG will definitely yield returns in spite of any barriers to LFG collection and utilization (Sullivan, 2010).
Since methane is produced only during the anaerobic decay of organic matter, the diversion of organic waste from landfills to composting will help reduce methane production (Lou and Nair, 2009). However, collected landfill gas can be used to heat nearby industrial operations or to produce electricity that can be sold to the power grid which provides a source of revenue, replaces fossil fuel use, and reduces greenhouse gas emissions (Thompson and Tanapat, 2005). Most provinces in Canada, as well as many other countries in the world have not taken action to ban organics from landfills or require landfill gas recovery.
Landfill gas production rates must be accurately quantified in order to evaluate appropriate methane reduction strategies. Reliable emission forecasts for project feasibility and environmental compliance are crucially required for the design and operation of landfill gas extraction and utilization projects (Huitric and Soni, 1997; Oonk and Boom, 1995). Due to the high uncertainty in estimating gas production rates and total gas yield, municipalities and companies are usually reluctant to invest in methane recovery projects. Landfill gas models serve the function and generate forecasted methane production rates for landfills.
Landfill gas models estimate methane generation over time by analyzing the complex changes occurring during landfill decomposition. First order kinetic equation is generally used for modelling methane generation at landfills based mainly on annual waste amounts and decomposition rates (Bogner and Matthews, 2003; Thompson et al., 2006, 2008). In first order models, the rate of methane production decreases proportional to the degradation of organic matter in any given year and the remaining fraction of organic matter from previous years (Borjesson et al., 2000). Landfilled waste follows an exponential decrease trend in gas production until it is completely degraded (Huitric and Soni, 1997). First order models, like LandGEM, TNO, and Belgium, are currently used by the United States, Denmark, and the Netherlands, respectively, to describe the behavior of landfills with respect to LFG generation (Thompson et al., 2007, 2008).
The main objectives of this work are to apply LandGEM model in order to quantify the LFG currently generated in the landfills in Ontario and to evaluate proposed scenarios that are believed to enhance the environmental performance of the landfill sector.
2. Case Study
Waste Management and landfilling in Ontario
Ontario residents and businesses generate on average 12 million tons of waste annually. Although there was quite an advancement in Ontario waste diversion programs; such as the Green Bin and Blue box programs, yet the waste diversion framework is deficient. Since 2008, the waste diversion has been under 25% and the majority of waste is being disposed of at landfills (OWMA, 2013). Doubtless, this is a loss in possible alternative energy sources and economic opportunities. In addition, this is occurring at a time when Ontario is experiencing waste diversion stagnation because waste disposal capacity is more than the landfill capacity that resulted in the disposal of waste in the United States. This is an unsustainable process because of the greenhouse gas (GHG) emissions resulting from waste transportation.
The typical content of residential waste in Canada is composed of 40% organics, 10% bulky goods, 40% recyclable material and 10% other materials (Statistics Canada, 2016). Food waste quantity in residential solid waste is relatively constant all year round, but yard waste fluctuates according to the season and the type of area (i.e., urban areas generate more food waste, and rural areas generate more leaf and yard waste). A common household generates on average between 150 and 200 kilograms (kg) per person of organic waste annually. Organic waste is also generated in large quantities by the industrial, commercial, and institutional sectors which include food packaging and distribution companies, restaurants, cafeterias and convention centers, and supermarkets.
In this work, LandGEM model, developed by US-EPA, is used to quantify the LFG generated in the landfills under investigation. LandGEM is an automated tool for estimating emissions generated from landfills including: landfill gas, methane, carbon dioxide, nonmethane organic compounds (NMOCs). It can either use default parameters or site-specific data if available. LandGEM is based on a first-order decomposition rate equation for estimating quantity of emissions generated from the decomposition of waste in landfills. The methane generation estimates from LandGEM model were compared to methane recovery rates for 30 large landfills in Ontario. Information regarding the total waste delivered to landfills and the amount of collected methane was obtained from the Ontario Ministry of Environment and Climate Change.
The first order kinetic equation used in the model is described in Equation 1 (LandGEM Version 3.02)
Q = methane production (ton/yr)
M = waste generation (ton/yr)
k = decay rate (yr-1)
Lo = methane generation potential (kg/ton)
t = Time of waste disposal (yr)
Based on equation 1, the methane production depends mainly on three factors: waste amount, methane generation potential and decay rate. As for waste amounts, the site-specific yearly waste quantity from the opening of the landfill to year 2015 were applied to the model. The methane generation potential (Lo) represents the amount of methane produced per ton of waste landfilled and can be calculated using Equation 2 (IPCC 2001).
Lo= F ×DOC ×
Lo = Methane generation potential (kg/ton)
MCF = Methane correction factor (fraction; default = 1.0)
DOC = Degradable organic carbon (Ontario Ministry of the Environment, 2004 default 210 kg/ton)
DOCf = Fraction of assimilated DOC (IPCC, 2006 default = 0.77, mostly used default for reliable estimates = 0.5)
F = Fraction of methane in landfill gas (0.5 default)
16/12 = molecular weight ratio CH4/C (ratio)
The DOC of the waste represents the waste portion available for microbial degradation into landfill gas. The IPCC (2001) recommended a methane generation potential (Lo) between 100–200 kg of methane/ton of waste. As recycling and composting programs in the 1990s reduced organics going to landfills, Environment Canada (2006a, 2006b) applied higher Lo before 1990 (165 kg of methane/ ton of waste) compared to1990 (117 kg/ton). The DOCs for Ontario was calculated from waste composition values by Natural Resource Canada (NRCan) and the DOC value for the period 1990-2004 in Ontario was assumed to be constant at 210 kg/ton. Accordingly, and based on Equation 2, Lo is calculated to be 102 m3/ton. If the upper limit of DOCf is used (0.77), Lo will be 158 m3/ton.
Decay rate (k) in Equation 1 is the biodegradation half-life of organic material in a landfill. The decay rates range from one to 50 yr-1 and even longer in landfills located in dry and cold climates. Although many different environmental conditions act upon decay, typically only precipitation is considered to have an effect on the decay rate “k” (USEPA, 2004; Maurice and Lagerkvist, 1997). Moisture is essential for bacterial growth, metabolism, and nutrient transport. A linear relationship between moisture and decay rate has been observed in field and laboratory studies (McDougall and Pyrah, 1999). Decay rates are determined based on precipitation rates in the following Equation 3 based on USEPA (2004).
k=3.2 × 10-5x+0.01 ……………………….(3)
k = decay rate (yr-1)
x = annual average precipitation (Ontario mean precipitation 902 mm) (Environment Canada Weather, 2006).
Substituting for x value in Ontario, k derived value for Ontario is 0.037. USEPA (2004) suggested a gas recovery rate of 75% for clay final covers to predict the amount of gas that can be recovered from the total generated gas. Information regarding the total waste delivered to landfills and the amount of collected methane was obtained from the Ontario Ministry of Environment and Climate Change. LandGEM was applied to quantify the landfill gas generation at the landfills in Ontario. For this purpose, data for landfills larger than 1.5 million cubic meter capacity was collected from the MoECC. This capacity threshold was selected based on the current regulation in Ontario, Ontario Regulation 232/98 (“O. Reg. 232/98”) that stipulates that landfills larger than 1.5 million cubic meter should incorporate landfill gas collection system. MoECC data also contained information about collected LFG in few landfills. Accordingly, 30 large landfills in Ontario fulfill the criteria of this research work, whereas only 24 landfills have data on collected LFG amounts. Accordingly, the 30 landfills were modelled using LandGEM Version 3.02 to estimate the methane generation rates. To measure the accuracy of the model generation rates, the actual collected gas from 24 landfills were compared to the modelled generation rate for the same landfills. Absolute percent error was used to measures the percent difference between the observed and the modelled values.
The global warming potential resulting from methane generated from landfills and its impact on the environment is usually assessed using the Global warming GW index (IPCC, 2007). This is a measure of how much a mass of greenhouse gas contributes to global warming relative to carbon dioxide (Table 1). It is a function of both the chemical species and its residence time in the atmosphere. The units of global warming potential are metric tons of carbon-dioxide equivalents (MTCO2-Eq).
Four different scenarios (S1 to S4) were proposed as alternatives to investigate means to improve current management strategy. These scenarios are summarized below:
The flaring scenario which corresponds, into some extent, to the current MSW management system in Ontario, where 9.7 million tons of waste disposed into landfills according to Statistics Canada 2016. LFG recovery is about 3.5% (CCME, 2014). In this scenario, it was assumed that all the collected LFG was flared in enclosed, high-temperature flares. Flaring efficiency is set to 95%. One of the recent regulations adopted by the Ontario government requires new and existing landfills to install a system for methane collection if the landfill releases more than 1.5 million cubic meters annually to reach the provincial target of reducing GHG emissions by over 4 million tons annually (Edie, 2008).
S2 Low organic & flaring
To evaluate the extent to which the out-phasing of organic waste from landfills affects the environmental performance of a landfill, a low organic scenario of conventional household waste landfill was created using the German Guidelines for low organic landfills. The main differences compared to the low-organic landfill scenarios were the CH4 potential of the waste (now set to 102 m3/ton , in comparison with 20 m3/ton set for the low-organic waste), the LFG collection efficiency (higher for household waste landfill scenarios), and the compositions of the generated LFG. The higher LFG collection efficiency for the household waste landfill compared to the LFG collection efficiency for low-organic-waste landfill was supported by a higher porosity of the household waste. This was successfully implemented at low-organic waste landfills in European Union. An example is the Nauerna landfill in The Netherlands, where the methane production potential reached 13 m3/ton (Manfredi et al., 2009). Some Canadian provinces have reduced organics going to landfills and some other provinces like, Prince Edward Island (PEI) and Nova Scotia (NS), have banned organics in all landfills to increase waste diversion of organics to composting facilities (Thompson et al., 2008).
S3 Power Generation
The reference scenario considered that all the collected LFG was utilized for electricity and heat generation in a Combined Heat and Power CHP plant. The electricity produced was considered utilized in a national power grid substituting for electricity produced in coal-fired power plants, which represents 17% of the Canadian power mix. From a life cycle assessment (LCA) perspective, the utilization of the energy recovered from LFG saved emissions to the environment, because emissions that would have occurred if the same amount of electricity/heat produced from LFG was instead produced with the marginal electricity production technology (here assumed to be coal-fired power plants) were avoided. Therefore, in the LCA-calculations, the saved emissions were credited to the LFG utilization system. Fifty-two landfills in Canada either recover methane to produce electric power or heat or alternatively flared the landfill gas to reduce methane to CO2 (Thompson et al., 2007, 2008). One of the successful examples is Centre de Tri et d’Élimination des Déchets, which generates 25 MW electricity and powers 8200 single detached houses at an initial cost of CAD $37 million with a payback period of only 5 years (McKirdy,1999).
S4 Methane oxidation & flaring
Non-collected methane goes through landfill covers and is partially oxidized through biological methane oxidation. Studies conducted to determine the oxidation factor ranged from 11 to 89%. In this scenario, the oxidation factor is set at the mean value of 36% and flared collected gas. In other words, this scenario is a combination of flaring and methane oxidation to optimize results.
4. Results & Discussion
4.1 Total emissions in Ontario landfills
LanGEM model was used to quantify the amount of LFG generated in the 30 big landfills in Ontario. The results are shown in Figure 1. The data collected from the MoECC contained annual amount of landfilled waste and design capacity of each individual landfill. These data were used as input data to the LandGEM model. The earliest data modeled in Figure 1 are dated back to 1970. As previously discussed, LO and k used to run the LandGEM model were 0.037 and 102 m3/ton, respectively. Figure 1 shows the results for LFG and methane as well as the expected collected methane based on the assumption that only 75% of methane can be collected. Figure 1 shows that annual methane generation is maximum in 2027 with 3.2 x 108 m3. Methane generation thereafter slowly decline but continue for more than 125 years after the peak. The model was also run based on the assumption that 95% of the collected LFG are flared. Methane emissions will reach a maximum of 1.2 x 107 m3 in 2027.
Figure 1. Annual modelled CH4 for the 30 big landfills in Ontario.
4.2 Model Calculation
In order to investigate the validity of the model in Ontario and if the model parameters (Lo, k) are accurately estimated, the model was run for the 24 landfills, for which measured quantities of collected methane are available. Figure 2 shows the methane generation as estimated by LandGEM compared to the actual data reported by the MoECC for 24 landfills. The LandGEM model consistently underestimated methane generation when Lowas calculated based on DOCf default value 0.5 (Lo = 102 m3/ton), which is the mostly used value by researchers for reliable estimates (Oonk and Boom, 1995; Scharff, 2005). However, when the Intergovernmental Panel on Climate Change (IPCC) default value of 0.77 for DOCf was used (Lo = 158 m3/ton), the model overestimates methane generation (IPCC, 2006).
Figure 2. Comparison of modeled and actual CH4 generation rates
To measure the percent difference between the observed and the modelled values, absolute percent error was calculated. Percent error calculated for Loat 102 m3/ton was found to be 3.7%, while it was 59% for Lo at 158 m3/ton.
Although the IPCC provided a default value of 0.77 for DOCf, it appears that this value is overestimated based on recent investigations (Oonk and Boom, 1995). Experimental values of 0.5-0.6 have been used by researchers in the Netherlands and proved to give reliable estimates of landfill gas generated and recovered (Scharff, 2005).
4.3 Environmental Impact potential of the different scenarios
The status quo of landfill management in Ontario can be improved by implementing the suggested scenarios previously discussed. These scenarios are modelled using LanGEM to compare between their environmental impact, specifically global warming impact, according to IPCC characterization. Clearly, the best management scenario, is the one that has the least CH4 emissions. The overall GHG emissions of CH4 calculated as MTCO2Eq for all scenarios are shown in Figure 3.
Figure 3. Comparison of GW potential for different scenarios
Figure 3 shows that scenario 2 of low organic content achieve better environmental performance than the other waste scenarios due to lower organic content of waste. Power generation scenario (S3) was found to reduce emissions by 78 % relative to the flaring scenario (S1). Oxidation and flaring scenario reduced emissions to 1,024,600 MTCO2Eq as it decreases the amount of non-collected methane released to the atmosphere by oxidation through the top soil cover. Further discussion on each individual scenario is detailed below.
Scenario 1 assumed that all the collected LFG are flared in enclosed, high-temperature flares. It is a common understanding that flaring converts all methane gas to CO2 by complete combustion. EPA’s Emissions Factors and AP-42 (USEPA, 1995), which is the most widely used reference for flaring efficiency, recommends using a value between 98-100%. AP-42, Compilation of Air Pollutant Emission Factors, has been published since 1972 as the primary compilation of EPA’s emission factor information. It contains emissions factors and process information for more than 200 air pollution source categories. However, applying this default value generally can lead to underestimating the GHG emissions (IPCC, 2007; Checkel and Handford, 2010). This scenario considered the flaring efficiency to be 95%, based on the research work of the University of Alberta’s (UoA’s) Flare Research Group that developed an emissions calculation tool “the Flare Efficiency Estimator (FEE)” (Kostiuk et al., 2004). The methane emissions after flaring was calculated to be 1.4 x 107 m3. By considering the non- collected methane, the total global warming potential (GWP) of methane emissions is 1,491,524 MTCO2Eq.
S2 low organic waste
In order to evaluate the extent to which the out-phasing of organic waste from landfills affects the environmental performance of a landfill, a scenario of conventional MSW waste landfill with lower organic content was created, namely ‘low organic’. The main difference compared to the three other landfill scenarios were the DOC value of the waste (Lo is 20 m3/ton), the LFG collection efficiency (higher for household waste landfill scenarios), and the compositions of the generated LFG.
For modern conventional low organic waste landfill, the average content of organic matter is estimated to be about 40 kg of organic carbon (OC) per ton wet waste (Mou et al., 2014). This results in a maximum methane potential of approximately 20 m3 methane (about 40 m3 landfill gas (LFG)) per ton of wet waste landfilled. This is at least five to six times lower than the typical methane potential for mixed municipal solid waste (MSW) and household waste landfills. This scenario is valid in light of the fact that landfilling of organic waste is declining globally. With the implementation of the EU Landfill Directive (CEC, 1999), European states took the initiative of gradual reduction of landfilling of organic waste. According to the result of the LandGEM model for Lo of 20, a total of 1.66 x 107 m3 of methane is estimated, which is equivalent to 230,400 MTCO2Eq.
S3 Power Generation
Scenario 3 considered that all amounts of collected LFG are utilized for electricity and heat generation in a Combined Heat and Power (CHP) plant. The energy recovery efficiency was set to be 40% as an average of different investigations (Dace et al., 2015; Karapidakis et al., 2010). The heat produced could be utilized in a local district heating network and the electricity produced could be utilized in a national power grid. From an LCA-perspective, the utilization of the energy recovered from LFG saves emissions to the environment. In other words, the emissions that would have occurred if the same amount of electricity/heat produced from LFG was produced from coal-fired power plants. Therefore, in the LCA-calculations, the saved emissions were credited to the LFG utilization system. Scenario 3 saves the environment by 981,707 MTCO2Eq as a result of generating energy from collected LFG instead of fossil fuels. Moreover, the non-collected methane emissions will result in net emissions saving of 315,268 MTCO2Eq.
Legislations in Canada require that landfills document their operational records including the emissions of LFG (Ontario Regulation 144/16). New legislations require stringent measures to be executed in order to fulfill the target of the Ontario Ministry of Environment and Climate Change of GHG emissions. This will definitely motivate landfill operators to maximize LFG utilization. Further incentives could be financial assistance for LFG projects. The focus of Canada is to improve waste diversion, which will reduce the amount of waste sent to landfills. This will help reducing the amount of methane emissions generated from landfills on the long run (GMI, 2013). Environment Canada expects a reduction of 6 million tons of CO2 annually if 50% of the LFG gases produced from landfills is combusted effectively (Reference). Canadian provinces and municipalities have been making advancements in the diversion of waste away from landfills. However, there will always be the need for some waste to be landfilled and it has to be done properly to ensure that energy is created from the utilization of LFG (Watts, 2009). Accordingly, this study can support decision making on the sustainable treatment and management of MSW for GHG mitigation.
S4 Methane oxidation
Landfill gas that is not collected is released to the atmosphere through landfill cover soils. Methanotrophic bacteria in the topsoil metabolize methane as a carbon source, where a portion of the methane is also incorporated into the biomass of the microbial cells (Hanson and Hanson, 1996). Different studies indicated that methane oxidation process reduces the emissions of methane from the surface of landfills (Borjesson et al.,2001;Scheutz and Kjeldsen, 2003; Huber-Humer et al., 2008). However, estimating the quantities of methane emissions causes uncertainties in estimating national or global methane emissions from landfills (Bogner and Spokas, 1993, Bogner and Matthews, 2003).
IPCC and USEPA suggested a default value for methane oxidation in landfill covers to be set between 0 and 10% of generated methane (IPCC, 2006, USEPA, 2004). Florida State University reviewed the prevalent default value, compiled literature results and compared methane oxidation rates in a variety of soil types and landfill covers. Across all studies, they found that the fraction of oxidized methane ranged from 11 to 89% and the overall mean fraction oxidized was 36% with a standard error of 6%. Therefore, based on recent technological advancements in the design of soil top covers, the default value of 10% should be updated (Chanton et al., 2009).
For the purpose of this investigation, the oxidation factor of 36% was used, which represents the mean value reported in the literature reviewed. LandGEM model was executed and net maximum methane emissions was then calculated and found to be 5.9 x 107 m3 in 2027 (Fig.4). To account for the non-combusted methane, the total GHG generated was then calculated and found to be 1,024,600 tons of CO2Eq. In further study, a sensitivity analysis was conducted to account for methane oxidizing range of 11-89%.
To enhance the oxidation process, investigations on possible amendment to landfill cover have been carried out and different bio cover materials have been studied for landfill gas reduction. The presence and number of intermediate covers has a significant impact on gas flux within a landfill. Landfills with compost amended soil covers (often termed “bio covers”) contribute to methane oxidation by stimulating biological activity in the surface layers by methanotrophs. Among the various bio cover materials, biochars are capable of absorbing methane and hence improving methane oxidation and increasing abundances of methanotrophic bacteria (Xie et al., 2013). Moreover, biochar was found to improve carbon storage, and soil ability to hold onto nutrients (Lehmann and Joseph, 2009). It has also been shown to enhance the adsorption of methane on the surface of the biochar particles, allowing it to be more accessible and oxidized by the methanotrophic bacteria (Reddy et al. 2014).
Figure 4. Methane emissions after oxidation of the non-collected methane.
4.4 Sensitivity analysis: DOC and oxidation
As mentioned earlier, total methane emissions depend mostly on DOC for the given amounts of landfilled wastes. Total emissions are proportional to the DOC of a waste and accordingly differences in emissions are expected for different types of waste according to their organic content. Low organic waste landfills generate lower quantities of GHG emissions than MSW landfills to the lower organic content in the low organic waste.
Though, using the same type of waste will generate the same amount of methane emissions, categorical differences in the emissions are experienced depending on the amount of oxidized methane through LFG flaring or through oxidation by methanotrophic bacteria in soil top cover. Flaring and biological oxidation determine whether the carbon is released in the form of carbon dioxide or methane. However, this affects GHG emissions significantly because of the difference in GWP between carbon dioxide and methane.
Sensitivity analysis was conducted to show the impact of the main parameters (Lo and oxidation efficiency factor) on the estimated GHG emissions from landfills. The sensitivity analysis results are shown in Fig. 5. Different Lo and oxidation efficiency factors were modelled, and the results are compared against the baseline scenario. As previously mentioned, the baseline scenario (S1) was modelled using Lo of 102 m3/ton and oxidation factor of 0%. Fig. 5 shows the relationship between GHG emissions (in MTCO2Eq) and different values of Lo and oxidation efficiency factor. An increase of the oxidation factors up to 25% resulted in a decrease of GHG emissions by 33%. Similarly, a decrease in the organic content of the waste by 25% (i.e. Lo is 76.5 m3/ton) resulted in a decrease in the GHG emissions of about 40% This shows that methane production potential (Lo) is more effective than oxidation factor. More reduction in GHG emissions can also be achieved if both methane potential and oxidation factor are optimized.
Figure 5. Sensitivity analysis of GHG emissions using different Lo and Oxidation factor values
Of the GHG emission sources from MSW landfilling, emissions of non-collected methane are the highest contributor of GHG emissions on a CO2-Eq basis because of its high GWP compared to carbon emissions. The results of the assessment prove from the perspective of the landfill that the reduction of landfilling of organic matter (as prescribed by the EU landfill directive: CEC 1999) can be an approach leading to significant reduction in the potential environmental loads in several impact categories compared with conventional landfilling of MSW waste. Low content of organic matter in the waste that is landfilled leads to less gas generation, and, typically, lower levels of operation needed at the landfill site compared with the landfilling of waste with high content of organic matter.
Nonetheless, it should be pointed out that other landfilling approaches have been demonstrated to drastically reduce environmental burdens also as in the utilization of soil top covers of high oxidation factor. However, with respect to global warming potential, a conventional MSW waste landfill with significant gas collection and utilization systems, in which electricity is produced as a substitute for electricity produced from conventional power sources, is a more favorable option in case of abundance of methane quantities generated. Utilization of landfill gas for energy purposes contributes to significant environmental savings. This is especially true for GWP, where net environmental benefits (displayed by numerically negative impact potentials) are achieved. This means that the utilization of the collected gas leads to saved emissions and, thus, avoided (negative) impact potentials which are larger than the impact potentials from direct emissions. Using both active LFG collection and power generation, GHG emissions were reduced by 78% relative to the flaring scenario of the collected methane. The study showed that organic content and the oxidation factor significantly impact GHG emissions. Shifting to low organic waste scenario decreased GHG emissions by 88% and using an oxidation factor of 36% resulted in a reduction of GHG emission by 46%.
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Table 1. Global warming potential for three common greenhouse gases (IPCC, 2007).
|Common name||Chemical formulae||100-yr Global Warming potential|
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