Prediction of Greenhouse Gas Emissions in Municipal Solid Waste Landfills Using LandGEM and IPCC Methods in Yazd, Iran

ORIGINAL ARTICLE Introduction: The increase in greenhouse gas (GHG) emissions has changed the global temperature and had a negative impact on global climate conditions. Landfill gas is one of the major GHG contributors. With the knowledge of GHG inventory, it is possible to carry out disaster prevention measures. Materials and Methods: In this study, tow Landfill Gas Emissions Modeling (LandGEM) and Intergovernmental Panel on Climate Change (IPCC), were used to determine the GHG quantity of the Yazd county landfill sector using from 2000 to 2020. Results: During this period, by the IPCC model, the total level of methane emissions from the Yazd county landfill was 23.17 Giga gram/y (Gg/y), while based on the LandGEM model, the total value of methane emissions from the Yazd county landfill was 5.74 Gg/y. The total amount of CO2 in the Yazd county landfill of the years 2000–2020 is estimated to be 15.75 Gg/y in the LandGEM model. There is the potential to generate 11.88 MWh/year electricity for the Yazd county landfill in 2020. Conclusion: The results of the present study can be employed to plan and implement a system for collecting methane gas and control the emission of GHG to landfills. Article History: Received: 22 September 2020 Accepted: 20 November 2020


Introduction
Rapid urbanization, the unregulated population and economic growth have resulted in a municipal solid waste (MSW) in developing nations 1 . About 67 % of the total solid waste produced in the world is disposed in sanitary landfills or discarded in unsanitary landfills or dumps 2 . Processes of the physical, chemical, and microbial that take place in solid waste causes landfill gas (LFG) 3 .
The most important parameters of LFG are methane (CH 4 ) (usually for the 45%-60% limit), carbon dioxide (CO 2 ) usually for the 40-60% limit), and multiple other materials compounds 4 . The main greenhouse gases (GHGs) include CH 4 and CO 2 , which global warming potential of CH 4 (GWP) is 20 -28 times higher than a CO 2 for a period of one hundred years 5 . The global emissions and GHGs accumulation in our atmosphere are serious concerns; so many countries have pledged to reduce their emissions 6,7 . About 3.5-7 % emissions of universal CH 4 are related to emissions from landfills 8,9 . It is estimated that in the United States, the wastes sector is the third-biggest source of emissions following the production of fossil fuel and intensive livestock 10 .
30% of evaluated emissions of anthropogenic CH 4 in Europe are originated from landfills 8 . In 2010, CH 4 emitted from the landfills were estimated for almost 11% of the total volume of CH 4 emitted 4 . Researchers evaluated that the share of methane to climate warming will attain 50% in 2030 when it becomes the major greenhouse gas 11 . Due to the high population density, Asian countries are one of the most important producers of MSW. Therefore, control of fugitive CH 4 emissions from landfills should be focused on waste management procedures 12 .
There are various methods to evaluate methane emissions, such as field testing, location evaluation, and mathematical models 13 . Modeling methods are used due to the problems in the precise control of the CH 4 emissions of whole landfill sites 14 . The most widely used models include Intergovernmental Panel on Climate Change (IPCC) method, First-Order Decay (FOD) process, Triangular method (TM), and Landfill Gas Emission Model (LandGEM) version 3.02 method 15,16 .
LandGEM is developed the equation of the first-order dissociation rate for calculating emissions of the landfill waste in MSW 17 . The software offers a quite simple method to quantifying emissions of landfill gas 13 . IPCC extended a multiphase model in the year 2006 to estimate CH 4 production from all the countries in the world 18 .
Literature shows that different researchers have computed the GHG emission potential of landfill sites using several models. The results for Sadeghi et al. presented that in 2023, 2028, and  2033 gas generation will reach 411, 549, and 671  m 3 /h, respectively 19 . Sharma et al. studied methane production from the landfill of Panki in India using FOD method, landGem, and IPCC Default model. The average annual methane emission levels from the Panki landfill were reported 24.27, 25.14, and 197.33 Gg by FOD, IPCC and LandGEM method respectively, from 2010 to 2030 20 .
Determining CH 4 emission is very serious and urgent, which can provide more awareness on real emissions from landfills, that could then be applied for GHG emission reporting 21 . The shortage of information about measuring methane gas emitted from Yazd county MSW landfill, this study pointed to characterize the Land GEM and IPCC model for gas emitted in Yazd county MSW the landfill with the evaluation results of CO 2 and CH 4 production.

Material and methods
Study area Yazd County is located at 31.89° N and 4.35° E (Figure 1), has a landfill that it receives than 382 tons of waste per day. In this region, there is no opportunity for the recovery of methane. Yazd county landfill site located in the northeast of the county, the area of almost 222 hectares began in 1981. The depth of the Yazd County Landfill is about 18 meters. Then the disposed waste is hidden with a substrate of soil at a depth of 30 to 40 cm. The soil composition of the landfill is sandy-Lumi.

Population prediction and the development of potential waste in Yazd County
An estimate of the population of the future is necessary to determine values of wastes to be generated 22 . The urban population of the Yazd county landfill was determined to be 656474 in 2016 from the National Portal of Statistics of Iran. According to Eq. 1 23 , the population for future waste production of this area was estimated from 2000 to 2020. P n = P 0 (1+ r) n Equation (1) In Eq.1, P n is the number of population Year n, P 0 is the number of population in the first year, r is the annual rate of population growth, and n is number of years. To evaluate solid waste generation rate in 2016, the total generated wastes this year were distributed by the population that was 580 g per person. Consequently, by multiplying the generation rate of wastes by the evaluated population, the value of generated waste in the desired years was computed.

Assumptions and Calculation
In two models, LandGEM and IPCC can be applied either with default values or site-special data, which is the site-specific used in this study.

LandGEM model
LandGEM is established a first-order decay rate equation given follow by 24 : (2) where: Q CH4 = annual of methane production in the year in which the estimate is made (Giga gram/y), i = increment of 1 year time, n = (year of calculation) -(early year of waste receipt), j = 0.1 increment of year time, k = methane production rate (year -1 ), L 0 = production potential of methane (m 3 /Mega gram), M i = processing potential for methane in the i th year (Mega gram), t ij = age of the j th part of waste weight M i admitted in the i th year (decimal years, for example., 3.2 years).

IPCC model
The determining of the CH 4 emissions from the landfill were conducted in Microsoft Excel 2007 based on the IPCC guidelines 25 26 . The IPCC approach is based on the following equation for the calculation of methane released from landfill 18 : (3) where: MSWT = Total value of waste produced (Gg/year), MSWF = Disposed waste fraction, MCF = Correction variable of the waste that produces landfill methane gas, DOC = part of Organic carbon that is biodegradable, DOCF = a part of Easily-accessible biodegradable organic carbon for decay, F = methane biogas fraction, OX = part of CH 4 gas that is oxidized into CO 2 ,  27 .
To estimate CH 4 emission using two models of landGEM and IPCC, specific hypotheses for each model are discussed in the relevant section as follows: (a) CH 4

Correction Factor (MCF):
The MCF for different categories of landfill/dumping sites is given in Table 1.

(b) Degradable of organic carbon (DOC):
It is determined the waste material and can be measured using the following formulas: Equation (4) Where A: Paper and textiles fraction; B: garden waste and park waste fraction or other organic non-food waste; C: food wastes fraction and D: MSW as wood fraction.
(c) DOC F : fraction of the DOC that can be decomposed. It represents the amount of organic and can be computed as EPA LandGEM index 24 : DOC F = (0.014×T) + 0.28 Equation (5) Where, T: The temperature of the area atmospheric. (

d) Recovery of CH 4 (R):
This is the quantity of CH 4 produced at the landfill that can be recovered as well as an energy source. The default amount of CH 4 recovery is discussed as zero as the recovery of CH 4 is not discussed in the current study.

(e) Oxidation parameter (OX):
It is the value of CH 4 from the landfill that is oxidized to the soil or to other substances of the waste. If OX is zero, it indicates that no oxidation has happened, and OX equals 1 indicates 100% CH 4 oxidation 29 .

(f) The CH 4 Fraction for landfill gas (F):
The CH 4 fraction (F) is generally considered as 0.5, but it could shift between 0.4 and 0.6, depending on the components of the waste and situations of the site 28 . It is presumed to be 0.5 for CH 4 for the Yazd County landfill site.

(h) Constant output value of CH 4 (K):
The value of waste degradation and production of CH 4 can be determined using the following formula: K=3.2×10 -5 × (R) +0.01 Equation (6) Where R: Total annual rainfall in mm. amount of K for Yazd county landfill in Table 2.

Electrical Energy production Potential
The electrical energy (kWh/year) from the CH 4 amount of gathered landfill gas can be computed by the following formula: 4 4 0.9 (8) where LHV CH4 is the Lower heater amount of CH 4 and is expressed as 37.2 MJ/m 3 , QCH 4 = annual methane production in the year of the computing (Gg /year), 3.6 is the alteration factor from MJ to kWh, where λ is collection output (60%) 30 , η is the electrical alteration output for internal ignition engine offered as 33% 31 .

Population and waste generation
According to the Statistical Center of Iran, in 2016, the population growth rate in Yazd county is 2.41% 32 . To estimate methane emissions quantity by LandGEM and IPCC models, the mass of wastes produced over various years of design period must be almost calculated. Based on Pitchel estimates, the per capita waste generation value over 20 years in developed countries depending on the annual population of 2 -5% annually 19 . Table 3 presents the population, the volume of solid waste manufactured and the quantity of disposed of solid waste in Yazd County during the plan period. Figure 2 shows the components of Yazd county municipal solid waste include 59% food waste, 11% paper, 16% plastic, 4% textile, 3% glass, 1% metals, 0.05% woods, and 5% other materials. The annual amount of solid waste disposal related to the growth of population. Figure 3 presented the trends of population growth and solid waste that was disposed of the landfill site from 2000 to 2020. The amount of disposed of MSW was nearly calculated 47198 tons/year in 2000, which increased to 142336 tons/year in 2020. The people of Yazd county landfill produced 382 tons of MSW daily in 2018. It has been found that the rate of solid waste generation in the Yazd county landfill is 560 g/cap/day (Table 3).

Amount of greenhouse gas emitted
In the current study, several methods like LandGEM and IPCC models were applied to calculate CH 4 emitted from the landfill of Yazd County (Figure 4). On the base of usable data of waste disposed in landfill, the methane emissions were calculated from 2000-2020 (Table 3). Based on the model suggestion, For the first year of MSW deposit, the model has no gas escape. The calculation of CH 4 emissions using the LandGEM model shows lower results than calculations with the IPCC model. The accumulative methane emissions from the Yazd county landfill were realized to enhance from negligible 0.02 and 0.28 Gg/y in 2001 to as great as 5.74 and 23.17 Gg/y in 2020 as estimate by and LandGEM and IPCC models, respectively. IPCC predicted the highest cumulative methane emissions during 2000-2020. Figure. 5 indicate that in 2001 and 2020, the CO 2 generation rate is estimated to be 0.04 and 1.77 Gg/y, respectively. The results of the cumulative calculation of the estimated CO 2 emission during 2000-2020 are equal to15.75 Gg/y by LandGEM.

Calculation of Electrical Energy production Potential
The electrical energy (MWh/year) of CH 4 value from Yazd county landfill in from 2000-2020 was 58.97% given in Figure 6. The amount of potential electrical energy has been generated in the 2001 Yazd countylandfill 0.3 MWh/year. In 2020, the recovered landfill methane would be generating 11.88 MWh/year.

Discussion
Variation of the population affects lifestyle changes and thus increases the production of solid waste per capita and finally increases CH 4 emissions 30 . The results of this study indicated that a larger population leads to higher waste generation (Figure 3). There is a high relation between the MSW production and the population growth and CH 4 emission level. The level of solid waste delivered to the landfill site rapidly elevated after 2009 because of the growth of population, more efficiency on solid waste collection, and governmental policy on solid waste management ( Table 3). In this study, the LandGEM and IPCC waste models are used to quantify the CH 4 and CO 2 generated, influenced by important parameters such as the methane potential (L 0 ) and the decadence rate (k) of landfill waste. These parameters significantly affect the quality of solid landfill waste and estimate the amount of gas produced by considering MSW information, degradable organic carbon (DOC), and the decadence rate of MSW. Therefore, the impact of quality and quantity of organic waste is more significant than other wastes.
We compared the CH 4 emission results from the current study with the results of other studies. In this study, the LandGEM model estimated the total CH 4 emission as 5.74 Gg/y, while Fallahizadeh et al. (2019) reported the generation of CH 4 gas in Yasuj city during the years 2009-2012 was reported to be 250-330 m 3 /h (219-289.08 Gg/y) respectively 13 . Talaiekhozani et al. studied the 114 million m 3 (11400 Gg/y) of CH 4, between 1997 and 2023, it will be published with the capacity to generate 188100 MW of electrical energy. 22 According to the studies, CH 4 is 21 as efficient in trapping heat than CO 2 11 , which the difference between the outputs of the two models is similar to our study.
Landfills release GHGs directly into the environment, which leads to global warming, which is destroying the climate and human health 33 . If properly planed landfills are created, recovered landfill gases can be worked as a main source of energy and for power production or can be enhanced into vehicle fuel. In the present study, there is the potential to generate 11.88 MWh/year electricity in 2020.

Conclusion
Yazd County's rapid urbanization, economic development, and population growth have dramatically increased its total waste generation over the past decade. In this study, the greenhouse gas emissions (GHG) have been estimated by using the LandGEM and IPCC models from the Yazd county landfill site for the period of 2000-2020. Results for the IPCC model illustrate a CH 4 emission potential of nearly 23.17 Gg/y during this period. The total CH 4 emission was evaluated at 5.74 Gg/y using the LandGEM model. It is also noticed that the LandGEM model is found the first degree equation, so measuring the reduction in MSW and hence emissions decrease over time, while the default IPCC model does not conceive. Accordingly, the LandGEM model results in the lowest annual landfill emissions of CH4 sites compared to the IPCC process. The results of the calculation of the estimated CO 2 emission for the period of 2000-2020 are equal to15.75 Gg/y. The potential for electricity generation in 2020 will be 11.88 MWh/year, which provides an opportunity to generate significant energy. Significant knowledge is given by the findings of this research that can be used for the enhancement of the country-specific factor of emissions for estimating the emissions of methane in the waste disposal class.