1. Introduction
Due to increasing population, rise in economic development, and rapid urbanization, resource consumption has intensified, and an increase in the global waste generation rate has been observed [
1]. Hence, environmental issues associated with waste management are raising serious concerns [
2]. Sustainable management of municipal solid waste (MSW) is one of the major challenges responsible authorities face in developing countries [
3,
4,
5]. The primary goal of solid waste management is to deal with the environmental, public health, resource, aesthetic, land-use, and economic issues related to inadequate waste management practices [
6]. Any single waste disposal approach cannot deal with waste materials in an environmentally sustainable way [
7]. Therefore, an integrated waste management approach is widely recommended for sustainable waste management [
8]. Integrated solid waste management can be described as selecting and applying suitable approaches and technologies to meet specific waste management objectives and goals through considering environmental and public health concerns [
9]. However, adaptation of the waste management and disposal strategies vary by the economic level of the countries [
10].
As a comparatively inexpensive technology for waste treatment and disposal, landfilling has been opted for globally, particularly in developing nations [
11]. Hence, in global waste management strategies, about 37% of MSW is disposed of in landfills, and 33% of MSW ends up at open dumps [
12]. Therefore, reducing landfill emissions is a fundamental goal in the waste management strategy that is also climate protective [
13], as the waste management sector is contributing up to 5% of global emissions [
14].
Like many developing countries, the waste management situation in Pakistan has also deteriorated due to political negligence, lack of finance and technology, public awareness and behavior, and administrative issues [
15,
16,
17]. Open disposal is the most common technique for MSW management due to the absence of sanitary landfills in Pakistan [
18,
19]. The major cities and small towns in Pakistan have become a showcase of negligence and mismanagement of MSW generated, causing a significant impact on environmental and social-life quality.
According to the Ministry of Climate Change of Pakistan [
20], the total GHG emissions in Pakistan for the year 2015 were 408.1 MtCO
2-eq. In the total quantity of GHG emissions in Pakistan, the contribution of the waste sector is 15.5 MtCO
2-eq. with methane (CH
4) 13.4 MtCO
2-eq. and nitrogen oxide (N
2O) 2.1 MtCO
2-eq, where disposal of MSW is causing 12.5 MtCO
2-eq. of CH
4 emissions [
20]. Hence, in the international context of CH
4 emissions from MSW handling and disposal, Pakistan is contributing 0.64% share in global CH
4 emissions associated with waste handling and disposal.
A study by Korai et al. [
21] reported that in Pakistan, 170 landfill sites are required to dispose of about 30.8 million tonnes of MSW generated annually throughout the country, achieving more than 90% of waste collection efficiency. Presently, Karachi does not have any adequately engineered landfill facility for municipal solid waste disposal, and all available waste landfilling sites are open dumps [
19,
22].
Reliable data about waste management are essential for a comprehensive, critical, and informative assessment of waste management options in every waste management programme [
23]. These fundamental data about MSW management are lacking in Pakistan [
24,
25]. Nevertheless, some studies regarding waste management in Karachi have been published in recent years (2015–2020), in which mainly the status quo of MSW management in the city and energy generation from MSW were reported [
24,
26,
27,
28,
29,
30]. Furthermore, the waste management situation and challenges in Karachi were reported by [
26,
28,
31].
However, in previous studies, GHG emissions from solid waste landfill (dump) sites in the existing situation (waste composition and climate) were not covered. Therefore, there is still a gap in comprehensive knowledge about the GHG emissions potential from waste disposed at landfill (dump) sites in Karachi. In this course, there is a pressing need for more comprehensive research to assess GHG emissions related to waste disposal and to propose strategies for their mitigation through the creation of new sustainable landfills for the city. This assessment can be done by comparison of different GHG mitigation approaches focusing on landfills. Hence, this research tackles the issue of missing data of GHG emissions from landfills in Karachi employing simulated landfills. The ultimate motivation behind this research is to propose an environmentally sustainable landfill strategy focusing on GHG emissions control and improved environmental behaviour.
Solid waste management through landfilling requires estimations of landfill gas, particularly methane emissions, to evaluate compliance with regulatory air quality standards [
32]. However, the quantity of GHG emissions from waste disposed at landfill sites in Karachi is unknown. Therefore, to evaluate the environmental footprint of waste dumped, it is vital to know the amount of GHG emissions, especially methane released from waste disposal sites under prevailing conditions (climate and waste composition).
The aims and objectives of this study are to assess the GHG emissions potential of the waste disposal sites in view of the waste management (quantity and composition) and specific climate conditions in Karachi and compare different GHG mitigation approaches with a focus on landfills (anaerobic landfills and aerated landfills) for Karachi. This study can contribute to future research, planning, and design of new sustainable landfills in Karachi, Pakistan, and in the region.
4. Results and Discussion
4.1. Production and Composition of Landfill Gas during Anaerobic Operations
The landfill gas (LFG) production rate from reactors R3-MOD and R4-MOD (with leachate recirculation) was significantly higher than for reactors R1-ACT and R2-ACT at the beginning of anaerobic operations. The rate of LFG production sharply increased in R3-MOD and R4-MOD and reached the maximum values of about 0.18 and 0.16 L/kg DM/h, respectively, during the initial 28 days of anaerobic operations.
Afterward, the gas production showed a continuous decline, and the weekly gas production rate declined to <0.5% of cumulative gas production in 133 days. According to another study [
76], this rapid decline in biogas production shows quick depletion of organic carbon due to the flushing effect. In contrast, an initial decline was noticed in biogas production rate in reactors R1-ACT and R2-ACT during the initial 28 days as shown in
Figure 6. Subsequently, gas production was gradually increased in the reactors and reached maximum values only of 0.06 and 0.04 L/kg DM/h in R1-ACT and R2-ACT, respectively. The level of landfill gas production < 0.5% of cumulative gas production in reactors R1-ACT and R2-ACT took, on average, 26% more time to reach (189 and 373 days, respectively) in contrast to R3-MOD and R4-MOD.
The leachate recirculation and excess water addition in reactors R3-MOD and R4-mod accelerated the biological processes, resulting in higher biogas production. According to [
67], a more conducive environment can be provided to the microorganisms in landfill by leachate recycling and adding excess moisture.
However, as shown in
Figure 7, this initial lag (for 28 days) in gas production from reactors R1-ACT and R2-ACT (operated as traditional landfills) was noticed due to hydrolysis and formation of organic acids [
54]. In the anaerobic decomposition phase, the cumulative landfill produced from the reactors was recorded as follows: R1-ACT 159 L/kg DM, R2-ACT 187 L/kg DM, R3-MOD 184 L/kg DM, and R4-MOD 157 L/kg DM.
The waste degradation phases can also be differentiated with landfill gas production and the fraction of CO
2 and CH
4 in biogas [
77]. The methanogenic phase in landfill production is defined by a methane concentration of approximately 50–60% and carbon dioxide approximately 40–50% in biogas [
78]. According to the results obtained, it was estimated that the methanogenesis phase in reactors R3-MOD and R4-MOD was reached after 20 days of anaerobic degradation, whereas the methanogenesis phase in R1-ACT and R2-ACT was reached in 50 days.
Figure 7 shows the graphical view of cumulative landfill gas produced from different reactors during anaerobic operation.
The average composition (v/v) of CH4 and CO2 in landfill gas produced during anaerobic operation in R1-ACT was noted as 56.2% CH4 and 43.8% CO2, in R2-ACT 57.1% CH4 and 42.9% CO2, in R3-MOD 64.2% CH4 and 35.8% CO2, and in R4-MOD 67.6% CH4 and 32.4% CO2. Moreover, from analysis of these results, it is possible to determine that reactors equipped with a leachate recirculation facility (R3-MOD and R4-MOD) reached the methanogenesis phase in 60% less time than the reactors operated without this facility (R1-ACT and R2-ACT).
Similar outcomes are reported [
77] where the methanogenesis phase was reached in 60% less time in reactors due to leachate recirculation. At the end of the anaerobic operation, CH
4 and CO
2 concentrations noted in LSRs are shown in
Figure 8. The average CH
4 concentration in the gas produced from LSRs simulating bioreactor situation was 23% higher (with 80% CH
4 concentration) than LSRs simulating conventional/open landfill conditions where CH
4 concentration achieved up to 66% at the end of the anaerobic phase.
Analysis of results obtained (LFG production and CH
4 fraction of reactor R2-ACT) from this study and waste (moisture content and quantity) disposed at official dumpsites in Karachi revealed that waste dumpsites are potentially emitting 3.9 MtCO
2-eq. methane annually.
Table 6 presents the detailed calculations for greenhouse gas emissions estimated for the waste disposed of annually at dumpsites in Karachi.
4.2. Gas Formation Potential Assessment (GP21)
The average value of biogas formation potential from the fresh waste sample utilized in this study was 252 L
N/kg DM. The graphs of net specific gas formation from five replicated fresh waste materials (WM) are illustrated in
Figure 9. As regulation for stabilization criteria for municipal solid waste (e.g., GP
21) is not available in Pakistan, the test results were compared with proposed limits of waste stabilization reported in the literature (according to the German regulation). In the German regulation for landfilling of pre-treated waste material, the proposed target value for residual gas formation potential (GP
21) is ≤20 L
N/kg DM for waste acceptance in landfills for final disposal [
58]. However, an assessment is based on the GP
21 value achieved from fresh waste material (WM); the waste dumped in landfill sites in Karachi has about 92% higher emissions potential than the limit value prescribed in German regulation.
After completion of the experiment, the residual gas potential (GP
21) value noted from the waste material sampled from reactor R1-ACT was well below the landfill aeration completion criteria of ≤10 L
N/kg DM proposed by authors [
80] (as shown in
Figure 10) due to active post-aeration operation in the reactor. The residual gas formation from this reactor was reduced to 97.5% from the initial value of gas formation potential from the fresh waste sample loaded in the reactor [
59]. This significant decrease in gas formation shows that the organic substance available for the anaerobic digestion process had already degraded to a large extent [
58].
The waste sampled from reactor R3-MOD barely met the target limit for residual gas potential GP
21 with 9.34 L
N/kg DM [
59]. In comparison, the waste samples from reactors operated under completely anaerobic conditions throughout the test duration (without post aeration operation) showed a higher residual gas potential from the limit value during the GP
21 test. The waste sampled from reactor R2-ACT produced 19.01 L
N/kg DM, and waste sampled from reactor R4-MOD produced 14.84 L
N/kg DM [
59].
Figure 10 shows the residual gas potential from waste sampled from different landfills regarding suggested GP
21 criteria for waste stabilization.
4.3. Respiration Activity (RI4 and RI7)
The initial respiration index of fresh waste material (WM) for four days (RI
4) was determined as 81.8 mgO
2/g DM and for seven days (RI
7) was 116.7 mgO
2/g DM. The evolution in respiration activity of fresh waste material is shown in
Figure 11.
It is assumed from the respiration index value achieved from fresh waste material that the level of respiration index in waste is being disposed of at waste disposal sites in Karachi is 94% higher than the reference limit value of ≤5 mgO
2/g DM given for waste acceptance in landfills in the German regulation for landfilling of pre-treated waste material [
58,
63].
After the LSRs experiment was completed, the respiration activity of waste sampled from each reactor was determined in order to assess the biological stability of the waste degraded under different landfilling approaches. The seven day respiration index (RI7) decreased significantly from the initial level in all LSRs. However, the biological activity in waste sampled from R1-ACT was lower than waste sampled from all other reactors.
The value of respiration index (RI
7) achieved in R1-ACT was ≤2.5 mgO
2/g DM, which is in line with proposed value for ending of active in-situ aeration of waste in landfills reported in literature [
80]. The test results show that a significant amount of residual organic material was degraded in the waste material sample from R1-ACT. Furthermore, it is assumed that up to 90% reduction in biodegradable organic carbon (BOC) was achieved in the waste material at the end of the experiment (post-aeration operation) in the reactor [
80].
The respiration index (RI7) level of waste sampled from R3-MOD was higher than the limit value for the completion of landfill aeration with a value of 4.5 mgO2/g DM; even the post aeration phase was realized in the reactor. This phenomenon proves that the conditions in R3-MOD were not favorable for aerobic degradation (due to active aeration) of waste in the reactor. The one the factors involved in this phenomenon would be presence of significant moisture in waste mass as the reactor R3-MOD was simulating bioreactor landfill conditions with excess water addition and leachate recirculation.
It is evident from the result of respiration activity investigations that the supplied air in the reactor was not well distributed, and the assimilation of the air was limited by water coating the waste material [
59]. As a result of limited air distribution, residual organics in the waste material were not sufficiently oxidized, and targeted waste stabilization was not achieved [
59]. Therefore, the air distribution and assimilation should be optimized by draining out the supplementary water available in landfills prior to the start of in-situ aeration operation [
59].
The respiration activity in both reactors R2-ACT and R4-MOD was higher than landfill stabilization criteria, with RI7 value of 5.3 mgO2/g DM and 4.8 mgO2/g DM, respectively. Both reactors were operated under anaerobic conditions throughout the experiment operation time. However, the lower value of RI7 in the waste sampled from the reactor R4-MOD than the value in the waste sampled from the reactor R2-ACT is a result of the operation conditions in the reactors.
The difference above shows that more decomposition of organic material was achieved due to leachate recirculation and optimal moisture content in the waste. The comparison of seven-days respiration index (RI
7) of all reactors with respect to the limit value for landfill stabilization (completion of aeration) is shown in
Figure 12.
For comparison, the results of different parameters analysed for the fresh synthetic waste sample prepared for this research and degraded waste samples collected from all landfill simulation reactors after the experiment are summarized in
Table 7.
4.4. Carbon Balance
The carbon balance is conducted by analyzing TOC contained in the waste sample before and after the experiment, monitoring the TOC concentration in leachate sampled, and gas flow rate and composition (CH4 and CO2). All reactors analyzed the initial amount of organic carbon as 413 GC/kg DM. The highest carbon discharge in the liquid phase (leachate) was observed from reactor R2-ACT, where 39 GC/kg DM was mobilized in leachate during the 448 days of anaerobic operation.
The lowest value of carbon discharge in leachate was noted in reactor R4-MOD with a value of 16 GC/kg DM, followed by R3-MOD, where 19 GC/kg DM carbon was mobilized in the liquid phase. In reactor R1-ACT, 21 GC/kg DM carbon mobilized in leachate, the second-highest mobility of carbon in the liquid phase. The highest carbon gasification was observed in R1-ACT with an 88 GC/kg DM value, followed by R3-MOD with a 75 GC/kg DM carbon discharge rate. Both reactors were aerated after the anaerobic phase.
The lowest level of carbon discharge through the gas phase was recorded from R2-ACT operated under anaerobic conditions throughout the experiment, where 65 GC/kg DM was mobilized with biogas. In reactor R4-MOD, the total quantity of carbon gasification was determined as 71 GC/kg DM.
Figure 13 shows the carbon discharge through liquid and gas phases during the pre-aeration, anaerobic, and post-aeration operations conducted during the experiment in respective landfill simulation reactors. A study [
76] reported similar observations regarding carbon discharge, where the lowest carbon gasification occurred in the reactor column operated under continuous anaerobic conditions during the test, and the highest carbon gasification was observed in the aerobic column. Moreover, the authors also reported that the anaerobic reactor column showed the highest carbon discharge through leachate as analogously observed in reactor R2-ACT in this study.
The higher carbon reduction in the solid waste was observed in reactors R1-ACT and R3-MOD with total reduction of 109 gC/kg DM and 49 gC/kg DM, respectively, where the post aeration phase was realized. Whereas in reactors operated under anaerobic conditions R2-ACT and R4-MOD, only 16 gC/kg DM and 12 gC/kg DM carbon was reduced. According to Ritzkowski and Stegmann [
81], carbon conversion rate is significantly influenced by the ecosystem surrounding microorganisms (including oxygen concentration, pH, temperature, and moisture content) and presence of biodegradable organic matter in waste mass.
The highest cumulative carbon reduction was noticed from reactor R1-ACT, where cumulatively 53% of TOC was reduced from the initial quantity. Second, reactor R3-MOD showed higher carbon mobilization with 35% total carbon discharge from the initial amount. In contrast to this, the total carbon reduction in R2-ACT and R4-MOD was noted as 29% and 24%, respectively, from the initial amount of carbon loaded in reactors. Similarly, study [
58] observed higher TOC reduction (31.2%) in solid waste from aerated LSR than from anaerobic LSR (21.5%).
In the overall comparison, the cumulative carbon discharge was higher in aerated landfill simulation reactors than anaerobic reactors due to fact that the metabolism rate in aerobic conditions is significantly higher than in anaerobic conditions [
82], resulting in an enhancement in the rate of carbon conversion and stabilization of organic content [
60]. The carbon balance and mobilization during landfill simulation reactor (LSR) operation is graphically illustrated in
Figure 13.
5. Conclusions
Based on the results obtained from landfill simulation reactor R2-ACT simulating the open dumpsite conditions in the situation (annual rainfall rate and MSW composition) in Karachi, it is estimated that solid waste disposed of at dumpsites has the potential to produced landfill gas of approximately 187 m3/tonne DM (dry mass) with average methane concentration of up to 57.1% (v/v). Furthermore, through analysis of these results and MSW disposal situation in Karachi (amount and moisture content), it is estimated that the quantity of MSW disposal annually at dumpsites in Karachi is contributing about 3.9 million tonnes CO2-eq. methane emissions (with specific methane potential of 1.8 tCO2-eq./tonne DM disposed).
Furthermore, the results of gas formation potential (GP21) and respiration activity (RI4) investigations of the fresh waste samples showed that the MSW directly disposed of at dumpsites in Karachi is above the recommended stabilization levels for waste material permitted to final disposal in the landfills. Comparing the GP21 and RI4 results with recommended German waste stabilization criteria for landfilling, it is discovered that the fresh waste samples produced 92% higher biogas than the suggested limit for GP21. Furthermore, the results of the respiration index analysis showed that the fresh waste has about 94% higher respiration activity than the suggested limit. After the experiment, residual gas potential and respiration activity of the waste samples obtained from each reactor were investigated. The gas generation potential and extent of waste stabilization of each landfill approach simulated in this study were compared with suggested criteria and target values of GP21 and RI7 for the completion of landfill aeration operation. The results showed that sanitary anaerobic landfill conditions with a post aeration phase represented by in R1-ACT reactor have higher residual GHG mitigation and waste stabilization potential. The GP21 value noted from waste the material sampled from R1-ACT reactor was 6.5 LN/kg DM, which is noticeably lower than target value of the GP21. Moreover, the target value for the respiration index RI7 was also achieved in the waste with 2.5 mgO2/g DM.
Second, bioreactor landfill conditions with post aeration represented by reactor R3-MOD show a higher GHG mitigation potential. The waste sample from the reactor R3-MO achieved a target value GP21 of 9.4 LN/kg DM. However, the respiration activity value for the waste was higher than the limit value, with RI7 value of 4.5 mgO2/g DM. The least GHG mitigation potential was noted in bioreactor landfills without post aeration, represented by reactor R4-MOD. The GP21 value was above the target limit with 14.8 LN/kg DM. The respiration activity in the waste sampled from the reactor R4-MOD was also higher than the proposed limit, with RI7 value of 4.8 mgO2/g DM. Finally, the waste sampled from reactor R2-ACT representing open dump conditions (without active control) showed high GP21 and RI7 as 19.1 LN/kg DM and 5.3 mgO2/g DM, respectively, significantly over the proposed model criteria. Based on the results obtained from this study, it is concluded that the reactor simulating the sanitary landfill with post aeration showed higher mitigation potential of residual GHG emissions and waste stabilization. In addition, the LSR with post aeration followed this trend. However, bioreactors with the post-aeration approach showed high and speedy gas production during the anaerobic phase, and higher waste stabilization was achieved due to post-aeration.
To improve the solid waste management situation and optimize the GHG mitigation potential of landfills, this study recommends employing an integrated solid waste management approach in Karachi with comprehensive financial, legal, administrative, and institutional support. Further pilot and field-scale studies must be conducted in the future to optimize GHG mitigation potential of the bioreactor and sanitary landfills with the post-aeration option in Karachi.