Estimation and modeling of gas emissions in municipal landfill (Case study: Landfill of Jiroft City)

Fig. 1. Geographic map and location of Jiroft in Kerman Province

Annual rainfall in Jiroft is equal to 82 mm with an average annual temperature of 23.5 °C. Thus, in climatic categorization, Jiroft belongs to arid and semi-arid regions (Waste Management Master Plan of Jiroft, 2013). According to the latest census, Jiroft's population was 111,034 in 2013. According to Waste Management Organization of Jiroft, in average 131 tons of waste were produced and transported to the landfills in 2013. Given the predicted population of Jiroft (according to equation 1 or exponential growth function) in 2013, being 120,746, per capita waste generation of Jiroft was 1.08 kg/day person.

where P_n is population in the target year, P₀ is  population in the beginning year, ris annual population growth rate, and n is number of years between the first year and the target year.

Table 1 shows the physical analysis of Jiroft waste. About 80.65% of total waste of Jiroft is putrescible. Food wastes are highly decomposable, with other urban wastes not easily decomposed, the small rate of paper wastes being an exception. In Table 1, all decomposable urban wastes are divided into three categories: high-speed decomposition rate (69.31%), average-speed decomposition rate (2.16%), and low-speed decomposition rate (9.18%).

LandGEM is a tool, based on Microsoft Excel, which is used to estimate the rates of landfill gases as well as their associated emissions. One of the advantages of this model is that it can predict the amount of produced gases under certain circumstances of landfill and in the absence of information about a specific landfill, its database contains series of default information that can help. Default LandGEM information is based on two sets of landfill criteria: Clean Air Act (CAA) is one of the landfills requirement, including New Source Performance Standards/Emission Guidelines (NSPS/EG) and National Emission Standards for Hazardous Air Pollutants (NESHAP); the other criterion is based on Agency’s Compilation of Air Pollutant Emission Factors (AP-42), called EPA.

This study has used the latest LandGEM version (3.02). LandGEM is a first order equation (EPA, 2005).

where Q_CH4 is the anticipated annual production of methane, i is the increase within the studied years, n is difference between the predicted and first year of waste disposal, j is 0.1 (increase within the studied years), k is methane production rate (year^-1), L₀ is methane production potential (m³/Mg), M_i is waste mass in the i^th year (Mg or ton), and t_ij is the j^th section’s age of M_i  waste mass in the i^th year (decimal year, for example 2.3 years)

The model is in the form of an Excel workbook with several sheets. The input, required from the user, are design capacity of landfill, annual rate of waste disposal to landfills, methane generation rate (k), potential methane generation capacity (L₀), and number of years for the wastes to get accepted at landfills.

Methane generation rate (k) indicates methane production rate, thus speeding up biodegradation of the organic matter. As the rate increases, decomposition occurs in a shorter time. Landfill gas generation rate depends on four parameters: moisture content of waste, ability of microorganisms to decompose wastes to methane and carbon dioxide, waste pH, and waste temperature. Considering the arid and semiarid climate of the study area, CAA and LandGEM (version 3.02) generally approximated landfill gas generation rate as equal to 0.02. But according to the World Bank recommendations, this rate is obtained more accurately as indicated in Table 1. The obtained k value is equal to 0.027 1/year. This amount is determined based on moisture (rain) and decomposition rate of waste components and is probably more accurate than the proposed CAA (Rezaee., 2014). Methane production potential (L₀) depends only on the type and composition of landfills’ waste, e.g., the more the amount of cellulose in the waste, the greater the potentiality for methane production. Depending on the type of the climate, proposed by CAA, for arid and semi-arid regions, methane production potential equals to 170 m³/ton. As mentioned above, this is an approximate value, because, due to the type and composition of waste, we cannot consider a fixed value for all regions. Thus, the rate is calculated considering the type of waste and World Bank instruction (Table 2). The lowest rate of L₀ equals 198 m³/ton, while the highest rate is not taken into consideration due to lack of credibility (Rezaee, 2014).

NMOC concentration of landfill gas depends on the type of waste in landfill along with the reactions of different combinations of anaerobic decomposition wastes. NMOCconcentration is measured by parts per million volume (ppmv) unit. According to CAA, its default value equals 4000 ppmv, like hexane. Based on LandGEM (version 3.02) instruction, if it is in compliance with CAA, the amount of methane, produced at landfill, should be 50% (equal to the amount of carbon dioxide). By having the amount of produced methane (Q_CH4) and methane rate (P_CH4), the amount of produced carbon dioxide (Q_Co4) can be obtained, using Equation (3).

Table 1. Estimated rates of urban landfill gas production in Jiroft based on physical analysis of waste and rainfall (82 mm/year) (Rezaee, 2014)

Table 2. Estimation of methane production potential (L₀)based on Jiroft waste decomposition

Table 3. Parameters and specifications of the landfills studied in LandGEM model

Screen View software, developed by USEPA, has been also used in this study to determine the concentrations of landfill gases, produced around Jiroft. Using Screen View software, the concentrations of methane gas, carbon dioxide, and NOMC'S are estimated according to the information, obtained from LandGEM, the assumptions, related to landfill, and the data input, requested in accordance with Table 4 in the final disposal year (2045). Given that the gases arise from the landfill, surface diffusion is taken into consideration. Hypothetical length and width of the landfill, based on waste production in the next 30 years, are considered 1250 m and 850 m, respectively. Also, given that the landfill is outside the city, the considered area is assumed to be rural.

Table 4. Screen view input data

 RESULTS AND DISCUSSION

According to the United Nations' report, per capita waste production in developing countries is 500-900 g/day.person, whereas, in Iran, its average is 850 g/day.person. (Rezaee, 2014), suggesting that per capita waste generation in Jiroft (1.08 kg per day) is higher than Iran and other developing countries. It is 27% more than the average per capita waste generation in Iran. In Table 5, Jiroft population has been estimated for 30 years, between 2016 and 2045, using Equation (1). According to the estimations, the city's population in 2016 is 142,788 and, with its current population growth rate in place, the city’s population in 2045 will be 481,551. The amounts of waste produced in 2016 and 2045 are 565, 44 and 190,692 tons per year, respectively. Total waste, produced during these 30 years will be equal to 3,324,274 tons.

Table 5. Population growth and estimated urban waste, produced in Jiroft from 2016 to 2045

The amounts of urban waste in landfill (the last column in Table 5) are considered as inputs to LandGEM model. Based on the outputs of this model, the highest rate of landfill emissions belongs to 2045, i.e. one year after the last year of waste disposal in Jiroft landfill. Total amounts, obtained for produced gas, methane, carbon dioxide, and NMOCs in 2045 will be 32,994, 8813, 24,181, and 378.8 ton/year respectively; while these amounts for Jiroft landfill in 2045 will be 26,420,149, 13,210,075, 13,210,075, and 105,680.6 m³/year, respectively. According to LandGEM model assumptions, demonstrated in Table 3, volumes of methane and carbon dioxide will be equal. Since 69.31% of the gases, produced at landfills, are organic matters, the amount of landfill gases is very large, signifying that it is an essential issue to collect landfill gases, given the type of waste produced at Jiroft. 

Figure 2 shows the volumetric production rates of emissions, during 140 years, since the onset of waste disposal, having considered the LandGEM output. As the gradient of the amount of waste, buried in the landfill graph, is increased, gas production gradient rises until the closure of landfill. The maximum amount of gas production is witnessed in the years after the landfill is closed. Since 2045, as no waste is going to be disposed in the landfill and the food for decomposition will be decreased each year, gas production rate will also decline. Such a decreasing trend in Figure 2 will be continued until 2156. As indicated, since the assumed amount of the volume of methane in LandGEM is 50%, the volume of methane and carbon dioxide produced are equal and their charts, similar. Yet, according to Figure 3, the mass of carbon dioxide is greater than methane. Mass of production carbon dioxide in 2045 will be 2.74 times more than methane mass in the same year. In other words, in the landfill, methane is higher in place than carbon dioxide, so it is more volatile. 

Fig. 2. Volume of produced gas emissions in Jiroft landfill from 2016 to 2156 (LandGEM estimation)

Fig. 3. Mass of produced gas emissions in Jiroft landfill from 2016 to 2156 (LandGEM estimation)

NMOC gases are chemical compounds with similar behavior in the atmosphere but different chemical structures. They also contribute to the production of tropospheric ozone. Figure 2 and 3 show the amounts of produced NMOC gases, though the total amount of these gases is much less than methane and carbon dioxide (about 1%). Thus, Figure 4 illustrates the chart of annual NMOC production in Jiroft landfill, separately. The importance of non-methane organic gases is that although they are lower, they leave more adverse effect on human health. A number of these compounds are known as the main causes of cancer. About 46 non-methane organic compounds, produced in Jiroft landfill and estimated by LandGEM, are shown in Table 6.

Fig. 4. Mass of produced NMOC’sin Jiroft landfill from 2016 to 2156

Table 6. NMOC gases produced at Jiroft landfill in 2045 (LandGEM estimation)

This study has modeled the emissions around Jiroft landfill, using Screen View software. In order to model the concentration of emissions, produced by Jiroft landfill, data from Table 6 (LandGEM output) were required, based on which, emission rates of methane, carbon dioxide, and NMOC are 0.000263 g/s.m³, 0.000721 g/s.m³, and 0.0000113 g/s.m³, respectively. The type of the landfill region is rural and the emission rate in the direction of the wind has been considered for 2045. Figure 5 shows the concentrations of each of the emissions with respect to their distance from Jiroft landfill. As shown in this figure, the concentration of methane at a distance of 10 m from the landfill is 30,940 ton/m³, which increases up to 700 m, where it reaches 40,590 ton/m³. From there on, proportionate to its distance from the landfill, the concentration plummets; however, there are still some emissions even at 15 km from the landfill, meaning that if you do not properly locate a landfill, its emissions can even reach the city, affecting its air. The same goes for concentrations of carbon dioxide and NMOCs, whose maximum concentrations reach to 112,700 and 1765 ton/m³, respectively.

Fig. 5. Relationship between methane, carbon dioxide and NMOC’s concentrations with distance from the landfill

In a study, conducted in 2013 in the city of Sanandaj, LandGem software was used to estimate the amount of landfill gas. There 50% content of methane, the methane production rate constant of 0.045 1/year, and gas production potential constant of 200 m³/ton were taken into consideration. The calculated amount of landfill gases such as methane, carbon dioxide, and NMOC over 20 years, were 23,150, 6184, 16,970, and 266 tons/year, respectively (Rezaee., 2014).

Rezaee (2014) also concluded that the maximum gas production of landfill reaches its maximum rate one year after the closure of the landfill. The time of landfill gas production, is similar to this study. The methane generation rate (k), likewise the decay rate, is within the range of 1/year 0.02-0.7 and the amount is 0.02 and 0.7 for dry areas and wet areas respectively.

As the numerical amount of k increases, the rate of methane production will increase, too. Potentiality of gas production, suggested by USEPA and used in a number of papers, is 170 m³/ton, but in this respect the waste in this study has a higher percentage of corruptive waste, with the gas production potentiality, obtained as 198 m³/ton, close to other studies (Rezaee, 2014). The gas production potentiality is estimated in a number of other studies and reported to be 100 m³/ton (Chalvatzaki & Lazaridis, 2010; Aydi, 2012; Alexander et al., 2005; Tchobanoglous, 1993).

In most previous studies the proposed USEPA was used for methane production rate and potential gas production and the results were obtained on the basis of these parameters (Talaiekhozani & Nasiri, 2016; Kalantarifard & Yang, 2012; Talaiekhozani et al., 2016). It should be noted that the numbers, intended for these parameters, are estimated for the conditions of the location and type of waste, produced in the USA. Hence, for accurate estimation of the required parameters (k and L₀) for the purpose of calculating the amount of gas production of landfill, other methods such as the World Bank must be used.

Due to the growing population and the consequent increase in waste production in future, along with global warming, caused by increasing greenhouse gases, high levels of emissions around the landfill of Jiroft, risk of explosion of methane produced in landfills, and economic feasibility of using methane, measures have to be taken to collect produced gases around Jiroft landfill, in near future. These emissions can also be carried by the wind miles away and leave adverse effects on the environment as well as human health.

Conclusion

The main reason behind the formation of landfill gas is biological activity. The main landfill gases are CO₂ and CH₄, both of them, among the greenhouse gases. In addition the risk of explosion in landfill is very high, due to the presence of methane gas. High heating value of methane gas could attract national interest, as a result of energy management, particularly in the developing countries. Thus, it is very important to predict landfill gases and model their release. LandGem model, which is based on first-rate equation analysis, has been used to quantify the amount of emissions from urban waste landfills. In this paper, the production rate of landfill gas (methane generation rate) (k) with respect to the decomposition rate of the waste components and water content was 0.027 1/year and lowest potentiality for methane production (minimum potential methane generation rate) (L₀), depending on the type and composition of waste buried in the landfill of Jiroft, was 198 m³/ton. NMOC concentration was 4000 ppmv such as hexane; and the waste production per capita in the city of Jiroft is intended 1.08 kg/day. This study estimated the amount of gases, produced in Jiroft landfill from 2016 to 2045, using LandGEM model. It also modeled concentrations of produced emissions at various distances, using Screen View, concluding that with the current rate of waste production, in 30 years, Jiroft landfill will be potential of producing gases such as methane, carbon dioxide, and NMOCs, up to 8813, 24,181, and 378.8 tons/year, which will be emitted to long distances, even farther than 15 km. Thus, in case of constructing a landfill at Jiroft, it is necessary to collect its emissions in order to prevent it from polluting the environment and also overcome the risk of explosion. Also, due to high heating value of methane, its trade will be profitable. It is evident that all data output generated by the software can be related to other factors, as well.