1. Introduction
Sustainable management of municipal solid waste (MSW) is one of the major challenges municipal authorities are facing in developing countries [
1,
2,
3]. Sustainable waste management practices can be defined as handling of waste generated by the means of collection, transfer/transport, reuse, recycling, disposing, and landfilling, concurrently considering the costs and effects on public health and ecosystems [
4]. However proper handling of increasing amounts of MSW is rising serious concerns worldwide [
5]. Landfilling predominates in global waste management strategies, as 37% of MSW is disposed of in landfills and 33% of MSW is still ending up at open dumps [
6]. According to the World Bank report, about 19% of waste material is recovered by recycling and composting and 11% of waste is treated by sophisticated incineration plants. The worldwide share of waste management strategies is illustrated in
Figure 1.
Mismanagement of MSW has severe negative impacts on the environment and public health [
8]. The adaptation of waste management and disposal strategies vary by the economic level of the countries [
9]. Landfilling is a comparatively low cost waste treatment and disposal technology and is being used worldwide, particularly in developing countries [
10]. The waste dumpsites/unmanaged landfills are impacting the environment at each level from local and regional to global [
11]. The common environmental impacts of dumpsites/unmanaged landfill sites are formation of landfill gas (CH
4 and CO
2), leachate, dust and particulate matter, odors, and fire hazards [
12]. Dumpsites have adverse impacts on soil, surface water bodies, and underground water [
13]. In most cases in lower income and developing countries, solid waste is being dumped along or in the rivers or sea, causing serious impacts on aquatic and coastal life [
14].
Methane has a significant contribution to the effects of global warming due to fact that CH
4 is 25 times more effective at heat trapping than CO
2 in over 100-year time frames [
15,
16]. In the latest estimations reported by [
15], landfills, old dumpsites, and other waste disposal activates globally, share up to 19% in global CH
4 emissions with a total 67–90 million tonnes of CH
4 per year. Therefore, landfills are ranked as the second largest anthropogenic source after ruminant livestock (87–94 million tonnes of CH
4 per year) [
15]. Therefore, prevention of landfill emissions is a major goal in waste management strategies [
17,
18].
It is estimated that the methane emissions from MSW in developing countries are likely to increase in coming years owing to the rise in population of urban areas and rise in per capita waste generation rate as a result of improvements in economic conditions [
19]. In contrast to this, methane emissions from MSW in the developed world are likely to decrease because of further improvement in MSW management conditions (recycling and reuse), enhanced waste legislation as well as recovery and extraction systems [
4]. The latest reported amount of methane emissions associated with MSW sector in developed and developing countries including European Union (EU-27) and USA is presented in
Table 1.
Environment-polluting waste disposal sites can be successfully transferred to energy generating projects by transforming them into sanitary landfills and equipped with landfill gas (LFG) capturing and power generating infrastructure [
20,
21]. According to the U.S. Department of Energy (DOE), the usage of waste material as a feedstock for energy and fuel production offers various advantages [
22], such as waste material being available at low price, or even offering revenue such as a tipping fee. The waste material can be collected by using existing waste collection and separation infrastructure, which further reduces the cost of energy products derived from waste [
23].
The organic waste disposed in landfills is mostly degrading under anaerobic conditions [
44]. Soon after the waste is placed in the landfill, aerobic decomposition starts, as oxygen is consumed by the microbial biomass. The aerobic processes may continue until all oxygen present in pores and voids of the waste is consumed [
44,
45]. Degradation of organic mass by a group of anaerobic bacteria results in the production of CO
2 and CH
4 [
46,
47]. Methane gas is produced from decomposition of organic waste under anaerobic conditions mostly observed in landfills and large dumpsites [
48]. A scheme of biochemical reactions during anaerobic digestion of organic fraction of MSW in landfill environment is illustrated in
Figure 2.
To reduce the greenhouse gas (GHG) emissions for the MSW management sector, different waste treatment technologies have been developed, such as compositing, incineration, anaerobic digesters, an-aerobic landfills with gas capturing facilities and refused-derived fuels (RFD) [
21,
49,
50]. An accurate estimation of landfill gas (LFG), mainly methane (CH
4), is important for conduction life cycle assessment (LCA) of waste to energy (WtE) pathways of landfills [
23]. Sustainable landfilling operation is described as controlled disposal of waste on land to minimize negative environmental repercussions through capturing landfill gas and managing leachate [
4,
51].
Figure 3 shows a scheme of sanitary landfill with landfill gas collection system and electrical energy (combined heat and power; CHP) generation facilities.
However, landfills intended to be used for energy generation should be sustainably planned considering the aftercare phase when energy generation from landfill gas is not economically possible/feasible [
52,
53,
54].
4. Materials and Methods
The default methodology proposed by the IPCC [
75] was used in this study to estimate methane (CH
4) emissions from organic portion of the municipal solid waste (i.e., paper and cardboard, food, grass and leaves, textiles and wood) disposed at landfills in major cities in Pakistan. This methodology has been used in various reported studies [
23,
60,
76,
77,
78] to estimate and evaluate the CH
4 emissions from the organic fraction of MSW. The IPCC default method is based on the theoretical gas formation by the mass balance equation and the methodology presumes that all possible emissions of CH
4 from the waste are released during the same year in which the waste is dumped [
75]. According to IPCC good practices guidelines [
79], through using this default method a reasonable annual estimations of methane emissions can be made, if the composition and amount of waste disposed remain constant or slowly changing over a period of several decades. However, in the case of waste amount and composition rapidly changes over this time; thus this default IPCC methodology will not give accurate trend of emissions [
79]. In this method, CH
4 emissions from waste disposal sites are estimated using the following equation:
where
MSWT is the total quantity of MSW generated (tonnes/year),
MSWF is the fraction of MSW disposed of to the disposal sites,
MCF is the methane correction factor,
DOC is the fraction of degradable organic carbon in waste,
DOCF is the fraction of
DOC that actually decomposes,
F is the fraction of methane in landfill gas, 16/12 used for the conversion of carbon to methane,
R is the methane recovery rate, and
OX is the oxidation factor (fraction). For the current study,
MSWT and
MSWF values are taken from
Table 1 and
Table 5 for waste generation and disposal in the major cities in Pakistan.
For
MCF, IPCC default values ranges from 0.4 to 1.0 depending upon the type of disposal site conditions. Since at MSW disposal sites in Pakistan conditions are unspecified and unmanaged, so for this study an
MCF value of 0.6 was taken, according to IPCC guidelines.
DOC for each city was calculated by substituting waste composition data from
Table 4 for respective city and default values of DOC content in wet waste given in
Table 7 by using Equation (2), given in IPCC guidelines. No methane recovery is taking place at any waste disposal sites in Pakistan, so
R was assumed as zero, as provided by IPCC 1996 for Southeast Asian countries [
80] and similar value is considered by studies from India [
81] and from Bangladesh [
82] in the estimation of methane potential from landfills.
OX was also considered to be zero according to IPCC default value [
75].
where
A,
B,
C,
D and
E represent the fraction of paper and cardboard, leaves, and grass, food waste, wood, and textiles present in MSW respectively.
DOCF is the degradable portion of the total degradable organic carbon (DOC) can be determined through Equation (3) provided by IPCC guidelines, where temperature
T is used the value given by IPCC is 0.77 [
77].
According to the IPCC guidelines, the temperature of the anaerobic zone of the landfill is supposed to persist constantly at about 35 °C [
75]. According to climatic conditions and the geographical location of Pakistan, the temperature value in the anaerobic zones of the landfill seems reasonable to the temperature given in IPCC guidelines [
60]. The IPCC default value for
DOCF is 0.77, which was used in this study. IPCC provided the default value for
F (0.5) was used. The molecular mass of methane and the atomic mass of carbon are 16 and 12 respectively [
60]. The CO
2 equivalent is estimated by multiplying yearly CH
4 emissions by 25, as global warming potential (GWP) of CH
4 is 25 times greater heat trapping effect the CO
2 over a 100-year time frame [
16].
For the estimation of power generation from the amount of methane produced in disposal sites, three scenarios of methane recovery are evaluated by methane recovery of 25% as minimum, 50% (assumed) as medium, and 75% (reported by United States Environmental Protection Agency [
83]) as maximum. Mostly internal combustion engines are used for power generation in landfill projects due to their low cost and high electrical efficiency (η
e 30–40%), however other technologies like gas turbine and micro turbine are also used for power generation, subject to the size of the landfill [
84]. Therefore, internal combustion engine technology was considered for power generation with the average electrical efficiency of (η
e) of 30%. The lower heating value of pure CH
4 33,906 kJ/m
3 [
85] and density of methane 0.66 kg/m
3 at room temperature [
86] was considered for the determination of thermal power (P
t). For the determination of electrical power (P
e), the generation potential Equation (3) reported by [
60] was used.
6. Conclusions and Recommendations
After analyzing the population statistics of the last census held in 2017 and MSW management data available in the latest literature and studies, this study estimated that about 31.18 million tonnes and 9.78 million tonnes of MSW were generated Pakistan overall and the top 10 most populated cities in the country respectively. It is also estimated that annually about 0.51 million tonnes (12.8 MtCO2-eq) of methane is released from MSW disposal sites in the most populated cities considered in this study. The waste disposal sites in Lahore and Karachi possess higher methane production potential due to the higher organic fraction in MSW and higher waste collection and disposal rates in both cities. As a result, MSW disposal sites in Lahore and Karachi share 66.8% and 26.6% of total methane emissions from waste disposal sites in the most populated cities in Pakistan respectively.
Furthermore, it is estimated that, overall 62.4 MWh of electric power can be generated if 25% of methane produced annually at waste disposal sites in the top major cities is be recovered and utilized for power generation. The estimated revenue generation through selling this power is 9.58 billion PKRs (61.1 Million USD) annually at the unit cost of electrical power considered in this study (17.54 PKRs/kWh). Moreover, it is also determined that, in the case of increased methane recovery rates from waste disposal sites at 50% and 75% the electric power generation capacity would be 142.7 MWh and 187.1 MWh respectively. The power generation capacity can be enhanced by increasing the MSW collection rate.
In the present MSW management situation in Pakistan it is challenging to generate energy from waste disposal sites. Additionally, on the one hand the waste management situation in Pakistan, especially in major urban areas, has deteriorated, on the other hand major cities and small towns in the country are facing an energy shortage. Through adopting integrated solid waste management systems and generating energy from waste disposal sites, an untapped resource from waste can be utilize to face both crises in the country.
Moreover, it is highly recommended that a comprehensive legal framework regarding integrated MSW management systems should be introduced and implemented in Pakistan, including that the waste collection fee should be collected by local municipal authorities not by so called “unions”.