Introduction

Global warming is a serious environmental issue caused by greenhouse gas emissions, including methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O)1,2. The evidence of global warming is well proved by the increase in weather temperature, melting of the Arctic Sea ice, weather pattern changes, and insects’ ecology change3,4. The estimation of the future emissions of greenhouse gases (GHGs) from developing countries suggests high values that would reach 64% and 76% of total global GHG in 2030 and 2050, respectively5. Methane and nitrous oxide have high global warming potentials (GWP) compared to carbon dioxide; the estimated GWP of methane and nitrous oxide are 28–36 and 264–298 times that of carbon dioxide for the next 100 years6. Although there are many sources of methane, nitrous oxide, and carbon dioxide, municipal solid waste (MSW) in landfills is one of the major sources of these gases7,8,9. For example, it has been reported that landfills were ranked as the third-largest source of methane in the United States in 202010,11. It should be mentioned here that MSW refers to all materials resulting from different daily activities such as residential, commercial, industrial, institutional, and construction12,13. It also should be mentioned that landfills are recognized as one of the most commonly used methods for biodegradable waste disposal in developing countries due to their management simplicity and affordability14,15. The anaerobic decompositions of organic waste in the MSW release methane and other GHGs into the atmosphere, and the mounts of the produced GHGs is governed by several factors, including landfill volume, temperature, organic content, moisture, and waste age16. Unfortunately, the number of landfills is constantly increasing in developing countries due to the increase in population and waste generation per capita, which in turn results in a significant increase in the production of MSW17.

Researchers from various regions have studied the MSW generation rates and generally concluded that the MSW generation rate increases with the increase of the income per person, see Fig. 112,18,19.

Figure 1
figure 1

MSW generation rate in different countries12,18,19.

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Like any other developing country, the use of landfills in Iraq is expanding quickly due to the rapid increase in the population, which recently reached 42 million people12,20. For example, Alsamawi, et al.18 reported that the solid waste generation rate in the capital of Iraq (Baghdad) rates increased from 0.63 kg/capita.day in 2006 to 0.74 kg/capita.day in 2010. Similarly, the solid waste generation in Mosul city, Iraq, in 2010 was 0.647 kg/capita.day, and it would increase within the next few years to reach 1.1 kg/capita.day by 202821. Unfortunately, solid waste management in Iraq is still unstable for many reasons, such as repeated wars and sanctions22. Additionally, population development has increased trash production, placing a significant burden on the facilities for handling and disposal23,24. Therefore, the expected emissions of GHGs from the landfills in Iraq are expected to be huge within the next few years, which in turn indicates the urgent need to explore efficient GHGs recycling methods.

A significant body of research, therefore, was carried out to minimize the effects of GHGs on the environment through several approaches25,26. For example, recycling of methane can be done by converting it into heat or electricity to control its emissions and support the responsible operation of landfills27,28. The selection of the best recycling approach depends on the generated amount of GHGs, which can be estimated using several methods. One of the most commonly used methods to estimate the emissions from landfills is the Land GEM developed by the United States Environmental Protection Agency (USEPA)29,31,31.

In this context, this research aims to estimate the total methane emissions from the local landfill in Al-Hillah City, Iraq, for the next 47 years (2023 to 2070) and the potential production of electrical power from the produced methane gas. The estimation process will be carried out using the Land GEM model.

The study area

Al-Hillah city is one of the major cities in Babylon Province (100 km south of the capital city of Iraq32), and it is located between latitudes 32°36′1″ and 32°8′45″ North and longitudes 44°14′9″ and 44°33′39″ East (Fig. 2), with a total area of 860 km212,23. This city is the home of about 993,000 people, according to governmental reports in 202033, with a population density of 1155 capita/km2, which is a relatively high density. Geographically, Al-Hillah city lies between the Tigris and Euphrates Rivers, making it rich in agricultural areas34. Typically, the average sunlight in the city is 12 h per day in summer and 6.8 h per day in winter. The city’s climate is dry in summer and cloudy in winter, with average annual relative humidity and rainfall of 45.8% and 102 mm, respectively, and an average annual wind speed of 7.2 km/h. The weather temperatures extremely vary in this city depending on the season and daytime, where it could be less than zero °C in winter and more than 50°C in summer12,35,36.

Figure 2
figure 2

Location of Al-Hillah City.

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Classification of solid waste in Al-Hillah City

The generated solid wastes in Al-Hillah City are divided into nine categories, namely organic, metals, plastics, wood, glass, paper, textile, aluminum, and others37,38. Organic solid wastes are the predominant type of solid waste in Al-Hilla City (55% of the total solid waste in the city), while aluminum represents the lowest percentage (2%). Figure 3 shows the percentages of these nine categories as a function of the total amount of solid waste produced in the city12,23.

Figure 3
figure 3

Composition of municipal solid waste in Al-Hillah City37,38.

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Solid waste management in Al-Hillah City

Currently, solid wastes in Al-Hillah City are manually collected twice a day by the local authorities in the city. Then, the collected wastes are transported to the sorting stations by various waste-collecting vehicles, such as compactors, tractors, large dumpers, and mini-dumpers. The reusable wastes, such as aluminum, cans, and glass, are recovered in the sorting stations. Then, the remaining wastes, mainly organic solid wastes, are either burned or landfilled. Additionally, waste pickers often search wastes for valuable waste components, such as metals and cans, before the collection process to sell these components in the local markets32,37,38.

Generally, municipal waste management services at Al-Hillah City cannot manage the whole volume of solid waste; it has been reported that only 44% of the population is served by waste collection services due to the lack of collection sites (there are only four sites in the city)23,38. The first site is in the Al-Neel district (17 km to the North of Al-Hillah City), and it is used to collect trash from the Al-Neel and Abi-Ghraq districts. The second site is in the Al-Kifil district (35 km to the South of Al-Hillah City) and has not been effectively utilized. The third site is a transitory site used to gather waste from specific locations within the city before transporting it to the Al-Neel waste site. The fourth site is local and serves only the Al-Kifil district23,38.

Materials and methods

Calculation method

Initially, the future population of Al-Hillah City in the year 2070 was estimated using Eq. (1), which is necessary to estimate the future solid waste generation in 207012,23.

$${\text{P}}_{{{\text{ex}}}} = {\text{ P}}_{{\text{c}}} \left( {{1} + {\text{g}}} \right)^{{\text{y}}}$$
(1)

Pex is the estimated population for the selected year. Pc is the current population for the starting year. g is the rate of annual growth = 2.99%. y is the years number.

While the generation rate of solid waste (EGRSW) was calculated using Eq. (2)

$${\text{EGRSW }} = {\text{ GSW}}_{{({2}0{23})}} \left( {{1 } + {\text{ WGI}}} \right)^{{\text{y}}}$$
(2)

EGRSW is the calculated generation rate of solid waste for each year (kg/capita/day). GSW is the current generation rate of solid waste for the year 2023 (0.82 kg per capita per day). WGI is the annual increment rate of waste generation per year (1% according to the literature).

The main equation to calculate the quantity of solid waste (EQSW) generated for every year until the year 2070, based on Eqs. (1) and (2), is12:

$${\text{EQSW (for specific year) }} = { (}({\text{P}}_{{({\text{current}})}} ({1 } + \, 0.0{299})^{{\text{y}}} ) \, \times {\text{ (GSW}}_{{({\text{current}})}} ({1 } + \, 0.0{1})^{{\text{y}}} {) } \times { (365}/{1}000{))}$$
(3)

Land GEM Model

The US Environmental Protection Agency (USEPA) developed Land GEM software to estimate the rate of gas emissions from municipal dumpsites. The land GEM model is a first-order equation, Eq. (4), which yields the annual emission rate of the targeted gas after calculating two key parameters, namely k (decay rate) and L0 (generation potential). The latter parameters can be calculated using either assumptions or actual data. These parameters were calculated in the current study using actual data gathered during a field survey.

The annual methane gas emissions were determined by the following land GEM model39:

$${\mathrm{Q}}_{\mathrm{CH}4}=\sum_{\mathrm{i}=1}^{\mathrm{n}}= \sum_{\mathrm{j}=0.1}^{1}\mathrm{k }{\mathrm{L}}_{0} ({\mathrm{W}}_{1}/10) {\mathrm{e}}^{-{\mathrm{kt}}_{\mathrm{ij}}}$$
(4)

QCH4 is the annually produced quantity of CH4 gas (m3/year). n is the number of years used in the model for calculating. i is the period increment (1 year). j is the period increment (0.1-year). Lo is the potential generation capacity of CH4 gas (m3/Mg). k is the rate of methane (CH4) production (1/year). Wi is the accepted waste quantity for interval year (ith) (ton). tij is the age of waste mass (Wi) for the part (jth) in the year (ith).

Governing equations

The capacity to generate methane at a rate of L0 (m3/Mg) from the landfills in Al-Hillah City depends on the type and contents of solid waste39.

Land GEM is built on a first-order decomposition rate equation to estimate emissions from the degradation of landfilled MSW. The model offers a comparatively straightforward method for calculating landfill gas emissions. Methane yield is determined by the model using essential inputs, which are39:

  1. (i)

    The amount of waste dumped in landfills during the studied period.

  2. (ii)

    Degradable organic content (DOC).

  3. (iii)

    The form of organic waste.

  4. (iv)

    Decomposition rate.

$${\text{DOC }} = {\text{ S}}_{{1}} \left( {\text{A}} \right) \, + {\text{ S}}_{{2}} \left( {\text{B}} \right) \, + {\text{ S}}_{{3}} \left( {\text{C}} \right) \, + {\text{ S}}_{{4}} \left( {\text{D}} \right) + {\text{ S}}_{{5}} \left( {\text{E}} \right)$$
(5)

DOC represents decomposable organic carbon. A represents paper and cardboard trash in MSW. B represents textile waste in the MSW. C represents food waste in the MSW. D represents wood, bones, and straw waste in the MSW. E represents plastic and rubber waste in the MSW. S1, S2, S3, S4, S5 represent organic carbon that is degraded for each fresh waste type (%).

The decay rate (k) is the half-life of biodegradation of organic waste in landfills, expressed in (1/year). The considerable degree of ambiguity and mistake related to k is acknowledged by the IPCC (2006)40. The decay rates in dumps located in arid, chilly areas can be from 1 to 50 years, and sometimes even longer. The following equation is used to estimate the decay rate:

$${\text{k }} = { 3}.{2 } \times { 1}0 \, {-}{ 5 }\left( {\text{x}} \right) \, + \, 0.0{1}$$
(6)

k represents a decay rate (year−1); x represents the yearly mean rainfall for the relevant time for the region where the location of the landfill is.

The value of L0 increases with the cellulose waste content. Methane generation potential (L0) has a value between 6.2 and 270 m3/Mg of waste. The EPA sets L0 to a default value of 170 m3/Mg waste39.

$${\text{L}}_{0} = {\text{ MCF }} \times {\text{ DOC }} \times {\text{ DOC}}_{{\text{f}}} \times {\text{ F }} \times { 1}.{334}$$
(7)

Lo represents the possible capacity of methane production (kg/ton); MCF represents a correction factor of methane, where its default value is equal to 1; DOC represents a degradable organic carbon (kg/ton); DOCf represents an assimilated fraction DOC, where (default of IPCC, 1996 = 0.77; default of IPCC, 2006 = 0.50; F represents the methane gas fraction in a landfill with a default value of 0.5 (Eq. 7); (1.334) represents a conversion factor of methane (molecular weight ratio) to carbon39.

By applying the equation of the Land GEM model (Eq. 4), the annual methane gas emissions can be determined for a specific period.

The required input data into the Land GEM model included the values of methane rate (k) = 0.02 (year−1), potential methane generation capacity (L0) = 100 (m3/Mg), methane content = 60 (% by volume), and NMOC concentration = 600 (ppmv as hexane).

Results and discussion

The Land-GEM model stands as a critical tool for ensuring the proper management of landfills as it provides a realistic estimation of the generated amount of methane, which provides an accurate picture of the amount of methane and the necessary collection systems. Consequently, this model reduces the risk of explosions and leakage of landfills, and it provides an estimation of the generated heat and power from the harvesting of methane.

The waste generation rates and cumulative waste in place for the waste sites in Al-Hillah City in units of (tons/year) for the period of (2023–2070) were estimated in the Land GEM model. Table 1 shows the estimated weight of solid wastes (tons) for 48 years based on the population in the study area during 2023–2070. Based on Eq. (3), the calculated quantity of EQSW generated for every year until 2070 was calculated using the present generation rate of GSW for 2023 (0.82 kg/capita. day), and WGI equals 1%. Additionally, the predicted population for every year from 2023 until 2070 (Pex) with an annual growth rate of 2.99% using Eq. (1).

Table 1 Waste generation and cumulative waste in the waste sites in Al-Hillah City (2023–2070).
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The methane estimation (ton/year) from landfills in Al-Hillah City for the selected periods (2023–2070) is shown in Table 2. In the Land GEM model, the annual generated rate of total methane was 1.32 × 10–3 tons per 1 tan of solid waste from the landfills in Al-Hillah City.

Table 2 Total methane emissions from landfills in Al-Hillah City for the studied period.
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According to Al-Rawi (2013)41, 6280 tons of methane gas can generate 11,304 kWh, which was used in the equation below to estimate the producible electricity power (GEP) (kW/year) from methane gas generation (MGG) in 2023:

$${\text{GEP}}_{{{2}0{23}}} \left( {{\text{kW}}/{\text{year}}} \right) = {\text{ MGG}}_{{{2}0{23}}} \left( {{455}.{8}} \right) \, \left( {{\text{ton}}/{\text{year}}} \right) \, \times { 113}0{4 }\left( {{\text{kWh}}} \right) \, \times { 24 } \times { 365 } = { 7187}0{\text{54 kW}}/{\text{year}} = { 7187}.0{\text{54 MW}}/{\text{year}}$$

Table 3 shows the generated methane gas from the landfills in Al-Hillah City in the next 47 years and the producible electrical power in MW/year. The estimated amount of accumulated waste in the next 48 years is about 50,227,808 tons, which equals an average of 1,046,413 tons per year. This amount of waste produces 875,217 tons of methane gas, which equals an average of 18,234 tons of methane per year. Regarding energy production, about 13,797,190 MW/year of electricity could be produced from the methane gas. According to a statement from the Department of Electricity in Babylon, the current need for emergency electric power is projected to reach 100 MW in 2022, which means that recycling up to 65% of the emitted methane gas from landfills will supply 20% of the electric power used in the city. Thus, the idea of re-utilizing the methane emissions in Al-Hillah City is overwhelmingly beneficial in both environmental and economic aspects. Not only would this process be immensely economical, but it would also help the environment in numerous ways by reducing the amount of pollutants in the air. It is noteworthy to highlight that the obtained results about utilizing the GHGs from landfills to produce energy are agreed with the final conclusions of a wide body of literature, such as the studies of Yi et al.42, Ghosh et al.43, Yaman44, Tan et al.45, Dace et al.46 and Nabavi-Pelesaraei et al.47.

Table 3 GEP (MW/year) for 2023 – 2070 using the emitted methane from the landfills in Al-Hillah City.
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In conclusion, it could be said, according to the results of this study, this simple solution could pave the way for more efficient and sustainable energy production in the future.

Conclusions

Solid waste accumulation and the shortage of electrical power generation are two critical issues Iraq faces. In an effort to address them, this study was conducted to find a sustainable solution to these issues. The study has indicated the importance of properly managing MSW, which is essential for environmental protection, while the development of alternative energy sources can help fill the increasing gap between electricity production and consumption.

According to the obtained results from the Land GEM model, the amount of accumulated waste over 47 years is 50,227,808 tons, with an annual average of 1,046,413 tons. The generation of this amount of waste has also led to a massive amount of methane emission (about 875,217 tons in total and an annual average of 18,234 tons). The good news is this amount of gas can produce 13,797,190 MW/year of electricity. That’s enough energy to power a large number of emergency services, hospitals, and businesses. In general speech, it can be said that harvesting 65% of the emitted methane gas from the landfills in Al-Hillah City can cover up to 20% of the electricity used for emergency power lines in the city. Not only would this drastically reduce energy expenses, but it could also help purify the air by reducing the amount of pollutants.

To further assess the impacts of the model’s estimates on the environment, data regarding local policies and regulations should be incorporated. This additional information may support the implementation of strategies to successfully improve landfill management and reduce future emissions in Al-Hillah City.