Quantitative analysis of the methane gas emissions from municipal solid waste in India

Increased emissions of greenhouse gases have altered the global ambient temperature and adversely affected global climatic conditions. The municipal solid waste (MSW) generated by households is considered the third largest anthropogenic source of methane (CH4) emissions, constituting 11% of all global CH4 emissions. The current study derived total MSW CH4 emission estimates using the IPCC default method (DM), modified triangular method (MTM) and first order decay method (FOD). The estimated CH4 emission was higher for the DM than the other methods, and was comparable to estimates from other studies. This study observed that the net annual emission of CH4 from landfills in India increased from 404 Gg in 1999–2000 to 990 Gg and 1084 Gg in 2011 and 2015, respectively. We also found that CH4 emissions were highly correlated (R2 = 0.8) with the gross state domestic product (GSDP) of states and the gross domestic product (GDP) of the country, which is an indicator of human well-being. The MSW management policy of India needs to be reviewed in a current policy context, as the management and efficient utilization of MSW technologies might help increase the use of CH4 as an energy source and thereby improve its sustainable and cost-effective management.

90 million tons of solid waste is generated by the country, which is eight times higher than the amount of solid waste that was generated in 1947 27 . Metropolitan cities have the largest contribution to the generation of waste compared to smaller and less developed cities. At present, India produces 16 Mg CO 2 equivalent of CH 4 per year, and the annual value is expected to reach 20 Mg CO 2 equivalent by 2020. It is also estimated that CH 4 contributes 29% of the total GHG emissions from the country, which is higher than the global average of 15% 27,28 . Studies suggest that the MSW generated in India mostly consists of a large fraction of organic wastes (40-60%), 3-6% paper waste, and 30-40% ash and fine earth material waste, with smaller fractions of plastics, glass and metal wastes (all <1%). The moisture content of the wastes is 47%, with an average calorific value of 6.8-9.8 MJ/kg and a C/N ratio of 800-1000 Kcal/kg 21,23,26 .
In India, due to inadequate data availability, a large amount of uncertainty related to MSW management and emissions has been observed, which makes it difficult to estimate the accurate value for the landfill GHG emission potential. The International Panel on Climate Change 29 has established a method for the estimation of GHGs emitted from landfills that has been widely used by researchers 9,27,30,31 .
Studies have been conducted by using different methods, i.e., the stoichiometric method 32 , default method (DM) 9,33 , first order decay method (FOD), modified triangular method (MTM) 33,34 , in situ closed chamber methods 35,36 and the landfill gas emission method (Land GEM) 27 . India is the largest producer of MSW, and landfills are third largest contributors to the total CH 4 emission value of the country. To reduce the total CH 4 emission value, a strong policy narrative is required in India. The CH 4 emitted from landfills can be utilized as a potential source of renewable energy. With the above background, this study aimed to estimate the potential emissions of CH 4 from landfills at the national scale, which would support sustainable management practices in a quickly evolving economy.

Results
Temporal CH 4 emission estimates with the DM, MTM and FOD. The CH 4 emission values calculated using the DM, MTM and FOD are given in Fig. 1. Using the DM and TM, Kumar et al. 9 estimated the net annual methane generated in India to be 502.46 Gg and 400.66 Gg, respectively, in the year 1999 9 . However, this estimation was done for Class I and Class II cities. The current study estimates the CH 4 emission for the entire country, except for the states of Nagaland, Sikkim, Uttaranchal, Arunachal Pradesh, the Andaman and Nicobar Islands, Chhattisgarh, Goa, Daman and Diu, Lakshadweep and Gujarat for the year 1999. Out of these, three states were created in November 2000. Gujarat, a major contributor of CH 4 emissions, was not included in the study due to an absence of data for the year 1999. Our estimate with the DM, which is 404.86 Gg, used a fraction of degradable organic carbon (DOC) of 0.114 instead of 0.16, the value used by Kumar et al. 9 , based on the fractions depicted in the CPCB, 2013. However, when the DOC value of Kumar et al. 9 was used, the DM produced a CH 4 emission estimate of 625.05 Gg/Y. This study had different values compared to those of Kumar et al. 9 , because it considered the total MSW contribution at a national scale instead of only considering major cities. Similarly, the MTM of this study estimated a lower value, 297.52 Gg/Y, than the triangular method used by Kumar et al. 9 . The FOD yielded a CH 4 emission value of 402.39 Gg/Y for 1999. The total CH 4 emission value of three landfill sites in Delhi was calculated by Chakraborty et al. 34 , for the year 2009 using the DM (45.7 Gg), MTM (41.4 Gg) and FOD (31.1 Gg) 34 . However, these estimates were for the MSW received by three landfill sites, not the entire MSW generated in Delhi. The values obtained in our study were 57.35 Gg, 42.14 Gg and 52.15 Gg for the same year using the DM, MTM and FOD methods, respectively, for the entire amount of MSW generated in Delhi rather than the MSW of the three landfill sites considered in Chakraborty et al. 34 . Thus, we found that the DM of the IPCC and the FOD estimates of the CH 4 emissions agree with estimates from earlier studies.
The amount of CH 4 emissions increased approximately 2.5 times in a span of 10 years (1999-2009), reaching a total emission value of 1084.03 Gg/Y by 2015. An increase of 245% is observed from the year 1999 to 2011, while a total increase of 109% was found from 2011 to 2015 (Fig. 1).

Trends in methane emissions from landfills and populations.
In the past few decades, India has witnessed rapid population growth in urban areas; thus, the overall population has grown (Table 1).
Migration from rural areas to the urban areas has increased the generation of MSW. According to the census of India, during the period from 2001 to 2011, the population of the country increased by 181 million, which is In 2015, a total of 1084 Gg CH 4 was released, with the maximum contribution, 208 Gg, emitted from Maharashtra, followed by Uttar Pradesh (148 Gg) and Tamil Nadu (112 Gg). The results of this study are similar to those of Kumar et al. 9 , who reported net annual CH 4 values of 400 Gg and 500 Gg in 1999 using the direct and first order decay (FOD) method, respectively ( Table 2).
The generation of MSW is often linked with per-capita income and development. Our finding shows a very high correlation between the MSW methane emissions and the GSDP of the states, which is considered an indicator of social well-being (Fig. 4).
Based on the available data for the periods 1999-2000, 2009-2010 and 2014-15, along with the GSDP data for the same years, it has been observed that the generation of CH 4 is very much related to the population and GSDP of the states (Fig. 5). The positive association, based on R 2 , between the GSDP and the CH 4 emissions from landfills was 0.88, 0.68 and 0.80 for 1999-2000, 2009-10 and 2014-15, respectively. A high GSDP indicates high levels of daily human activities and consequently a high amount of MSW generation, leading to CH 4 emissions.

Discussion
Methane Emission, Population Growth and Economic Development. We found that the CH 4 emissions were highly correlated with economic development and population growth. Economic development drives the population to move to cities where basic infrastructure and amenities are available. This diversion of the population subsequently leads to changes in the overall lifestyle and living standards and thereby increases the per capita generation of MSW. We found that the higher the population density, the more MSW was generated. Similarly, for the regions, higher GDPs indicated higher human activity levels and consequently higher MSW generation rates. The generation of MSW is often linked with per-capita income and development. The generation of MSW is considered high in developed countries compared to developing countries 30 . Our findings show a high correlation between the MSW methane emission level and the GSDP, which is an indicator of social well-being, of the states. Along with their development level, the lifestyles of the communities also change accordingly, which further enhances the generation of MSW. CH 4 from landfills: A potential source of energy. India, one of the fastest growing economies, is still improving its existing technological capability to generate power sources and energy from MSW. Most of the gases emitted from landfills have the potential to be utilized as biogas, which is a source of clean/green energy. Landfills directly emit GHGs into the environment, which affects the global temperature and puts the environment at risk. The open dumping of wastes leads to significant reductions in nutrients, i.e., nitrate, potassium and phosphorus; reduces the ability to recycle and reuse waste; and increases the risk of groundwater pollution by leaching. Controlled or engineered landfills with a proper covering and collection system can enable the sustainable disposal of solid wastes and their use as an energy source. According to one estimate, only 20-25% of landfill gases can be recovered, while the rest of the gases escape into the atmosphere. If properly designed landfills are established 38 , 30-60% of the gases can be utilized from these controlled landfills 39 . These recovered landfill gases can be used as a direct source of energy and for power generation or can be upgraded into vehicle fuel 27 .

Status of and challenges for MSW management in India.
The emission of CH 4 is directly related to the amount of waste generated from the households or the society in a region. In India, there is a huge amount of climatic, cultural and demographic variability based on the different food habits of its communities. According      to the 2011 census, the urban population in India increased from 285 million in 2001 to 377 million in 2011, and approximately 70% of this population resides in 366 cities of the country. In the present study, we observed that over the last fifteen years, the total MSW has increased by three times compared to the amount that was This law is intended to make the community aware of ways to reduce waste generation and to involve locals in the proper management of waste and innovations of technology for recycling and energy recovery purposes. The huge amount of waste generated in the country has a potential use as renewable energy, as landfill gases have high a calorific value, and can consequently minimize the impacts of greenhouse gas emissions on the climate.

Conclusion
In the last few decades, the overall growth of the population, urbanization level and economy of India has increased the amount of municipal solid waste generated in the country. The disposal of these wastes into landfills results in the emission of gases with a high global warming potential such as CH 4 , CO 2 , and NO x . The results of this study indicate that a gradual increase in the CH 4 emissions has occurred over the last 15 years (1999-2015). This study also establishes a relationship between the emission of CH 4 and the well-being of the associated community, which is highly linked to the gross state domestic product (GSDP) of the states. Landfill gases have the potential to be utilized as sources of green energy, and if properly planned and engineered landfills are constructed at suitable sites, these gases can be recovered and utilized as a sustainable source of renewable energy.

Methods
Data acquisition and determination of waste characteristics. This study was conducted to determine the generation and composition of waste in various parts of the country. It has been observed that the composition of waste depends on various factors, including the food habits and socio-cultural practices of a community, the climate and the community income 41,42 . The source composition of MSW in most Indian cities consists of a fraction of organic matter varying from 40 to 60%, followed by fractions of earth and ash materials (30-40%), paper (3-6%) and plastic, glass and metals (1% each) 27 . Kumar et al. 9 , reported that the composition of municipal waste was 30-70% rapidly biodegradable organic matter, 0.6-31% paper and cardboard, and 1-16% plastic materials 9 . In this study, we assume that 70% of the MSW reaches a landfill. The collection efficiency in India is poor due to a number of reasons (e.g., poorly designed bins, open dumps, collection vehicles, and the frequency of waste collection). The average collection efficiency for MSW in Indian cities and states is approximately 70% 23,43,44  Calculation of CH4 emissions from landfills. Among the available methods for the estimation of CH 4 emissions from landfills, the simplest one was provided by Bingemer and Crutzen (1987) 45 and revised by the IPCC in 1996. It is a mass balance approach that provides the actual emissions from the landfill, and it is widely used when detailed data are not available. The CH 4 emissions from MSW were calculated in accordance with Kumar et al. 9 : where 1 Gg/yr = 1000 tons/yr.
In equation 1, MSW T = total amount of solid waste generated (Gg/yr), MSW F = the fraction of MSW that reaches the landfill sites (considered 70% for the current study, as the rest of the MSW was considered lost by recycling, reuse or other processes), MCF = Methane correction factor, and DOC = Degradable organic carbon, which can be calculated using the following equation: where A = paper, cardboard and rags, B = leaves, straw and others, C = fruit and vegetable wastes and D = wood, DOC F = the value of the dissimilated fill gas, which was considered 0.77 (at a temperature of 35 °C 9,10 ), F = fraction of methane gas, which was considered 0.5, R = recovered methane gas, which was considered 0 for this study, as recovery techniques for landfill gases have not been adopted in most of India 9 , and OX = oxidation factor, which was also considered 0 (as the default value) and accounted for the methane gas that was oxidized in the upper layer of the waste in the presence of oxygen.
The IPCC has identified two types of uncertainties in the model, but the model generates reasonable estimates for regions where the data on waste generation are limited.
The uncertainty in the model is as follows: First, the methodology assumes that a constant amount of waste is added each year and that the CH 4 generation is the same in each year. Thus, temporal increments at the landfills are not considered, which may cause the methane emission value to be overestimated. The amount of waste disposed is a highly sensitive parameter in the default methodology.
Second, uncertainties in the estimates of the MSW T and MSW F may cause greater uncertainties in the total methane emission estimate. The model is also sensitive to the waste composition; the DOC content is sensitive to small variations in the assumed values, which can change the overall methane emission estimate.
Modified Triangular Method: The MTM is best suited for regions where the waste characterization data are not appropriate/adequate. This method considers the waste degradations to occur in two phases: first, the waste degradation starts one year after the deposition of the waste, and the rate of gas generation reaches a peak within the first 6 years. In the second phase, the gas generation decreases linearly to zero by the 16 th year (Fig. 6). The amount of gas released is based on the FOD in triangular form, as shown in Fig. 6, for the emission estimate for Delhi. The total gas generation (G) during the period t + 1 to t + 16, where t is the year of the waste deposition, is given by: = . G 1 87A C t 0 where A t is the amount of the waste deposited in year t.  The FOD estimates the temporal change in methane emissions. The underlying assumption in this method is that the DOC decays steadily with CH 4 formation. Limited data on the MSW can be analyzed by the FOD. The methane emissions can be calculated as follows: where E CH4 represents the methane emissions from an MSW landfill, T is the inventory year for which the emissions are calculated, k is the reaction rate constant, t 1/2 is the half-life of methane, MSW is the total MSW disposed in the landfill, MCF is the methane correction factor, DOC is the fraction of degradable organic carbon, DOC F is the fraction of total DOC that degrades, F is the fraction of methane in the landfill gas, 16/12 is the conversion ratio (CH 4 /C), R is the methane recovered value, and OX is the oxidation factor. The values of the parameters used here were derived from the IPCC 8 .