Estimation of Biogas Generated in Two Landfills in South-Central Ecuador

: The landfill is a final disposal technique to confine municipal solid waste (MSW), where organic matter is degraded generating leachate and biogas composed of methane gases (CH 4 ), carbon dioxide (CO 2 ) and other gases that contribute to global warming. The objective of the current research was to estimate the amount of biogas generated through the LandGEM 3.03 mathematical model to determine the amount of electrical energy generated and the number of homes that would be supplied with electrical energy from 2021 to 2144. As a result of the application, it was estimated that in the Pichacay landfill, the highest point of biogas generation in 2053 would be 76,982,177 (m 3 /year) that would generate 81,226,339.36 (kWh/year), and would supply 5083 homes with electricity. Similarly, in the Las Iguanas landfill, the highest point would be 693,975,228 (m 3 /year) of biogas that produces 73,223,5296.7 (kWh/year) and would supply electricity to 45,825 homes. Of the performed gas analyses in the Pichacay landfill in 2020, an average of 51.49% CH 4 , 40.35% CO 2 , 1.75% O 2 and 17.8% H 2 S was presented, while in the Las Iguanas landfill, for 2020 and 2021, we obtained an average of 51.88/CH 4 , 36.62% CO 2 , 1.01% O 2 and 187.58 ppm H 2 S. Finally, the biogas generated by being harnessed minimizes the impacts related to global warming and climate change and would contribute electricity to the nearby communities.


Introduction
Anthropic activities have caused the generation of greenhouse gases (GHG), increasing in environmental temperatures, leading to increased rainfall, thawing, altering the hydrological system and acidification of the oceans [1,2]. One of the predominant factors that contribute to climate change is the generation of municipal solid waste (MSW), hence the World Bank indicates that 2.01 trillion tons of urban solid waste is generated per year worldwide, of which only 33% is managed in an environmentally safe way. The per Capita Production of solid waste (PPP) worldwide oscillates at approximately 0.74 (kg/inhab/day).
In effect, global waste is expected to increase to 3.40 billion tons of annual waste in 2050, of which 19% would be distributed in high-income countries while 40% would occur in low-and middle-income countries [3]. Likewise, the World Biogas Association (WBA), estimated that worldwide, more than 105,000 billion tons of organic waste is generated occupied area for its operation is approximately 140 hectares and its construction was projected with a useful life of approximately 20 years. The second landfill, called "Las Iguanas", is characterized by a lack of a biogas system. It is located in the southern coastal area, in the province of Guayas, Guayaquil canton, Km 14.5 Vía Guayaquil-Daule, and serves 2,350,915 inhabitants. This landfill has been managed by the ILM-LAS IGUANAS Consortium (Guayaquil, Ecuador) since 1994 and consists of four sectors, where the first comprises 28.69 ha, the second serves for the deposit of inert material, the third occupies 13.63 ha, while the fourth extends to 40.71 ha, giving a total surface area of 190 hectares.

Methods
For the estimation of biogas from the Pichacay and Las Iguanas landfills, descriptive research was conducted consisting of a field visit to the landfills and the collection of information on the amount of MSW entering the landfills from 2004 to 2020, the projection of the amount of waste from 2021 to 2052. Moreover, 2052 will be the last year in which it is estimated that MSW will be deposited in the sanitary landfills, to obtain the percentage of CH4, CO2 and H2S as given by the gas analysis reports of EMAC EP and the ILM-LAS Consortium in 2020. Based on these considerations, the mathematical model, Landfill Gas Emission Model version 3.03 of the United States Environmental Protection Agency (EPA), was selected. Likewise, the model is limited to calculate the amount of gases up to 2144, where the concentration will depend on the amount of organic waste disposed of in the landfill and the climatic conditions [16,[25][26][27]. Finally, the amount of electricity generated from the total biogas and the number of houses that would be supplied with the amount of electricity produced will be estimated, considering the use of cogeneration by means of internal combustion engines, as illustrated in Figure 2, which describes the methodology used to estimate biogas and electricity production [5,25,28].

LandGEM Biogas Model Version 3.03
The LandGEM biogas model version 3.0, is a Microsoft Excel application that uses a first-order equation to estimate the total volume of biogas, CH4, CO2 and other NMOC gases generated [18,[29][30][31][32][33][34]35]. The equation used by the model is detailed below in Equation (1): (1) where: = annual methane generation in the year of calculation (m 3 /year). I = 1-year increments. n = (year of time calculation)-(initial year of waste acceptance). j = 0.1-year time increment. k = methane generation rate (1/year). L0 = potential methane generation capacity (m 3 /mg). Hence, if the value of k is higher, the CH4 rate will increase and then decline with time. The value of k depends on four factors: moisture content of the waste mass, availability of nutrients for microorganisms, the pH, and the temperature (T°) of the waste mass. In Tables 1 and 2, the indices for the k and L0 values are indicated. The model assumes that emissions from landfills are composed of 50% CH4 and 50% CO2 and other atmospheric pollutants. Therefore, if the CH4 content is outside the 40 to 60 percent range, the use of the LandGEM model is not recommended [29][30][31].

Estimation of the Electricity Production Potential
In order to calculate the power to produce electrical energy, the principle of cogeneration using internal combustion engines (ICM) was used, due to its low cost per kW/USD and high efficiency [15,20,32]. It has a biogas flow rate of approximately 300 to 1100 cfm, and it is calculated by the following equation [20][21][22]32,33]: where: Edispo. = available electrical power. PCI biogas = internal calorific value of biogas. Qb.r. = recoverable biogas flow (m 3 /year). = biogas energy efficiency, 38% of energy per m 3 of biogas was considered, an assumed yield of 50%, which depends on the technical specifications of the ICM. Therefore, to determine the number of homes that would be supplied with the amount of electricity obtained from biogas, average energy consumption of 143,730.00 (kWh/year) was considered, which is the annual consumption of an average family per month.

Amount of Municipal Solid Waste Disposed of in Sanitary Landfills
From 2004 to 2020, a total of 2,131,077.23 tons were disposed of in the Pichacay landfill, with a monthly average of 10,895.04 tons (MSW). This corresponded to 12.03% of waste from markets, 6.41% from industry, 80.75% of homes, 0.32% from sterilized biohazard, 0.09% from organic, and 0.39% from recycled. On the other hand, in the Las Iguanas landfill from 1994 to 2020, 28,714,041.14 tons of waste were disposed of, of which 92% was organic waste, and 8% inert waste.

Characterization of Municipal Solid Waste
The Pichacay sanitary landfill has performed the characterization of the MSW during the years 1985,1990,1995,2001,2007,2012,2015 and 2018, where the physical composition of the household solid waste was identified. The characterization was conducted by stratifying the population by electrical energy consumption that allowed to distinguish between the generation of solid waste per household per capita and per capita of urban solid waste at 0.47 (kg/inhab/day) [19,34,35]. Hence, in recent years the characteristics of the MSW of the Cuenca canton, presented 60.91% organic matter, amongst others, as detailed in Table 3. Likewise, the ILM Consortium performed the characterization of the MSW of the Las Iguanas landfill in 2012, 2016 and 2017, which, depending on the economic conditions, established five groups with per capita production (PPC) of 0.87 (kg/inhab/day) conditions: economic low, medium-low 0.96 (kg inhab/day), medium 0.98 (kg/inhab/day), medium-high 1.27 (kg/inhab/day) and high 1.56 (kg/inhab/day) [9,36]. Therefore, the characterization conducted in 2017 indicates 67% organic matter, amongst others, as detailed in Table 4.

Production of Biogas Generated from the Sanitary Landfill
The main gases of biogas are CH4 and CO2, where CH4 presents the global warming potential (GWP) from 28 to 36 years for 100 years. It is a precursor of ozone, as CO2 emissions cause an increase in atmospheric CO2 concentration which lasts thousands of years [14,37,38]. There are also several sources of obtaining biogas such as animal, plant, human, agro-industrial, forestry, aquatic crops and wastewater sources that can be used by reducing the factor 21 of CH4, one of the most potential gases responsible for the warming by greenhouse gases. Hence, the importance of taking advantage of these gases in the production of electrical energy is demonstrated in the studies performed at the global and national level as detailed in Table 5 [9,[39][40][41]. Therefore, to minimize the impacts related to global warming and climate change, the biogas generated in landfills may be used as an energy source to produce electricity [22,32,33]. Therefore, regarding the biogas produced from the decomposition of MSW, it is fundamental to calculate the amount and concentration of biogas, through the application of the LandGEM Landfill Gas Emission model version 3.03, in order to estimate the volume and the concentration of biogas generated in the Pichacay and Las Iguanas sanitary landfill over a period of 32 years. This generates information that will allow for calculating the production of electrical energy considering its available power and to publicize the importance of reducing GHG, using biogas as a raw material in the production of electrical energy.

Estimation of the Biogas Produced LandGEM Version 3.03
In order to estimate the amount of biogas produced in the Pichacay and Las Iguanas landfills, the LandGEM landfill gas model Based on the considerations and estimation of MSW, it was possible to calculate the total volume of biogas, CH4, CO2, and NMOC in (m 3 /year), hence, in the Pichacay landfill, it is estimated that the total biogas will reach its highest point one year after closure, that is, in 2053 with 76,982.177 (m 3 /year). Afterward, it will decrease exponentially as the amount of organic matter consumed decreases until 2144, which will generate 813.48 (m 3 /year) of biogas. In the case of CH4, the highest volume will be two years after decommissioning, that is, in 2054 when it will generate 41,007,519 (m 3 /year). The highest volume of CO2 will be in 2054 with 32,220.193 (m 3 /year) and the highest value of NMOC will be in 2053 with 46,189.306 (m 3 /year), as illustrated in Figure 3 [2,4,19,22]. In the same way, the total amount of biogas estimated for the Las Iguanas landfill will reach the highest point in 2053 with 693,975.228 (m 3 /year), one year after the closure of the landfill. Later it will decrease exponentially as the amount of organic matter consumed decreases until 2144 which will generate 416,385 (m 3 /year). In 2053, the highest value of CH4 will be with 402,505,632 (m 3

Estimated Electricity Production from Biogas
In order to determine the amount of electricity generated from the total biogas in (m 3 /year) and the number of homes that would be supplied with the amount of electricity produced, the years 2021 to 2144 were considered. The year 2144 is the estimated year where the landfill would cease to produce biogas, as calculated by applying Equations (2) and (3). The aforementioned conversion factors were applied, given that it is estimated that the Pichacay and Las Iguanas landfills will close in 2052. For explanatory purposes, it was categorized into two groups of analyses where the first group covered the period 2021 to 2052, and the second from 2053 to 2144. Thus, in the first group, the electricity available from the biogas generated in the Pichacay landfill in 2021 will reach 24,313,579.99 (kWh/year) that would supply 1522 homes as demonstrated in Figure 5. The second group corresponds to the period from 2053 to 2144. The largest energy production with 81226339.36 (kWh/year) will be in 2053, which would supply 5083 homes within the area of influence ( Figure 6).  Similarly, the amount of electricity generated in the Las Iguanas landfill in 2021 produces 229,272,368.53 (kWh/year), which is an amount of energy that would be supplied if biogas was used to feed 15140 homes in the first group (Figure 7). In the second group, in 2053, it will be the highest point of energy with 732,235,296.74 (kWh/year) that would supply electricity to 45,825 homes ( Figure 8).

Pichacay and Las Iguanas Landfill Gas Analysis
The company BGP ENERGY CEM is in charge of monitoring the biogas from the wells, located in the Pichacay landfill on a monthly basis. From the monitoring carried out in 2020, the gases present an average percentage of 51.49% CH4, 40.35% CO2, 1.75% O2 and 17.8% H2S. In the same way, the ILM Consortium monitors the gases that are generated in the Las Iguanas landfill, which present an average of 51.88% CH4, 36.62% CO2, 1.01% O2 and 187.58 ppm H2S as listed in Table 6. It was observed that the Pichacay and Las Iguanas landfills presented concentrations of CH4 staying outside the range from 40 to 60% as required by the LandGEM model, values that are highlighted in red in Table 6. Therefore, the model may not be valid since the concentrations are out of the given range. However, for future biogas estimates, we will compare with other models to develop our own model considering the climatic conditions, the characteristics of the waste and concentrations of the gases typical of the sector where the landfill is located [16,32,53,54].  -21, 200-20, 201-20, 202-20, 203-20, 30-20, 31-20. Hence, in the results of works performed in 2011, the potential for electrical production of biogas, generated in the Pichacay landfill, calculated through theoretical models conducted by the (EPA), assuming a concentration of 50% CH4 and 50% CO2 starting in 2009, it has been estimated that it will reach a higher point of biogas generation in 2019, with 1152 m 3 /h with a maximum capacity of 1.90 MW. Subsequently, it will decrease year after year until reaching 0.10 MW in 2012, when the sanitary landfill will produce approximately 864 m 3 /h of biogas, which is estimated to produce 2 MW of electrical energy [18].
Likewise, studies of the Las Iguanas landfill were performed in 2018 where it presented a biogas value of 1,542,119.39 tons of CH4, which would generate 110,376.00 MWh of electricity. This is a value that could cover the demand of 1512% of residential, 2.87% of commercial, 2.24% of public lighting, and some 9.10% with the electricity demand of the city of Guayaquil [22,55,56]. Therefore, they recommend that a system be implemented to take advantage of the biogas that would generate 14 MW [41,57,58]. That is why the importance of knowing the potential that a landfill has to generate biogas through theoretical models to estimate the production of biogas according to the national balance of electrical energy with a cut to March 2021. The installed power in the generation of renewable electrical energy reaches biomass of 176.05 GWh which corresponds to 0.69%, 44.14 GWh of biogas (0.17%), 34.25 GWh photovoltaic (0.15%), 70.53 GWh wind (0.27%), and 23,126.92 GWh hydraulic (91.31%) of all the electrical energy delivered to the public service.
The implication for theoretical models presenting biogas production estimates is that they do not consider the efficiency of the conversion equipment that depends on the altitude above sea level where the generation plant needs to be installed. The stoichiometric mixture of fuel and oxygen are not in the exact concentrations [7,59], as it lacks historical information on the MSW entered into the landfill. These would have been fundamental values for technical and economic viability for biogas training, since the LandGEM model was developed to estimate the amount of biogas generated in sanitary landfills based on approximate data of acceptance of the amount of MSW entered, for the subsequent characterization of waste and monitoring of biogas [42,43].
Based on the data obtained from the estimation of biogas generated at sanitary landfills, it is possible to demonstrate the technical feasibility of using MSW as feedstock for biogas production and electricity generation, and the number of homes that would be supplied with electricity from the generated energy, improving air quality and reducing risks to health and the environment. However, for the application of the model, the following parameters need to be considered, such as year of opening, closing, operating capacity, CH4 (k), CH4 generation power (L0), and CH4 percentage. Finally, the climatic variables of precipitation, temperature, humidity and others must be considered. For future research, it is proposed to determine the cost-benefit ratio of biogas use.

Conclusions
According to the data obtained, through the application of the LandGEM version 3.03 model, the opening year 2004 and the closing year 2052 were considered, with a methane generation index k = 0.05 (1/year), potential generation of methane L0 = 170 (m 3 /Mg), other NMOC gases with a concentration of 600 ppm, and CH4 concentration of 56% for the Pichacay landfill and 58% for the Las Iguanas landfill. Where it was estimated that in the Pichacay landfill, the highest point of biogas generation will be one year after its closure, that is, in 2053 with 76,982.177 (m 3 /year). In the same way, in the Las Iguanas landfill, the highest point is in 2053 with 693,975,228 (m 3 /year), one year after the closure of the landfill. Subsequently, it will decrease exponentially as the amount of organic matter consumed decreases.
Based on the calculated biogas, the amount of electricity from the years 2021 to 2144 (when it is estimated that the landfill will stop producing biogas) was determined, and the electricity available from the landfill of Pichacay in 2021 generated will be 24,313,579.99 (kWh/year) which would supply 1522 homes, while the largest energy production with 81,226,339.36 (kWh/year) would supply 5083 homes. In the case of the Las Iguanas landfill in 2021, it will produce 229,272,368.53 (kWh/year), which is an amount of energy that would be supplied if biogas was used for 15,140 homes. Likewise, 2053 will have the highest point of energy with 732,235,296.74 that would supply electricity to 45.825 homes.
Finally, the model may present limitations that may affect the accuracy of the projection in the adequate and economical design in projects for the recovery of electrical energy from biogas. Factors that can affect precision are considering inaccurate assumptions, limited data, poor model calibration, atypical residue composition, change in k or L0 values, and CH4 concentrations outside the 40-60% range.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.