1. Introduction
The waste management sector contributes a significant amount of greenhouse gas (GHG) emissions, primarily  carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). In terms of GHG emissions and climate change, municipal solid waste (MSW) management is also considered  a global issue. The alternatives and decisions of MSW management approved by local governments can affect the release of GHG emissions globally [1]. The IPCC's Fourth Assessment Report stated emission from the waste sector has contributed 2.8% of the total global GHG emission from anthropogenic sources [2]. Consequently, this amount affects global warming and climate change. However, MSW management presents potential options for GHG emission reduction and has links to other sectors (e.g., energy, industrial processes, forestry and transportation) with further GHG reduction opportunities [3]. Developing countries are able to reduce national GHG emissions by around 5% and eventually by 10%  through  adopting integrated municipal solid waste management [4].
2. Material and methodology
2.1. Material
This study was conducted in Hanoi, the fastest economic growing city of the country [5]. According to  Hanoi URENCO's report, the MSW generation in Hanoi city was 6,500 tons/day (about 2,372,500 tons/year) [6]. This amount accounted for about 11% of total MSW generation of whole country. The amount of MSW is predicted to increase by 15% annually [7]. The waste collection rate was estimated to be 95% in the inner city and 60% in suburban areas. Overall, collection of MSW was 85% for whole city by kerbside collection system. The MSW is characterized by high organic component described  in Fig. 1. Treatment practice applied in Hanoi is landfill without landfill gas capture system. Landfill received 84.4% of collected MSW. Composting just reached very low amount for 2%.
2.2. Methodology
Life cycle assessment of municipal solid waste
This study aims to estimate GHG emissions and investigate potential of reduction of existing municipal solid waste management in the metropolitan city of Hanoi. It used life cycle model to estimate the emissions associated with the current situation and several MSW management scenarios. The GHG emissions of all stages of the MSW management system including collection, transportation, composting, recycling, landfill, etc. were included through a life cycle assessment (LCA) approach. In general, LCA of environment studies evaluates the environmental interventions and potential impacts associated with products from "cradle to grave", from raw material acquisitions through production, use and disposal [9].
Integrated waste management model
There have been several models developed for estimating GHG emissions from the waste sector within the discipline of life cycle assessment. This study applied the Integrated Waste Management, version 2.5 (IWM-2) designed and developed by Andrew J.D. Richmond. It was developed from IWM-1 (1994) produced by P.R White, M. Franke and P. Hindle.  The model technique used for IWM is a life cycle inventory of municipal solid waste [10]. The IWM is a spreadsheet-based model including several input screens relevant to MSW management. The outputs of the model are presented in the form of an excel spreadsheet including the total life cycle emission of GHGs by individual gas and source, CO 2 equivalence and charts.
Scenarios studied
This study proposed five scenarios to estimate and  compare the impacts of MSW management alternatives on GHG emissions. The scenarios are based on the current situation and future expectation of MSW management, particularly source separation aspect. The scenarios are assumed to use the same amount of waste and include the same composition, therefore the results obtained are comparable among scenarios studied. SO - Baseline: composting: 2%; biogas: 0%; recycling: 8.2%; incineration: 5.4%; landfill: 84.4% S1 - Composting system upgrade: 30% of total collected waste sent for composting (composted product is used for agriculture); biogas: 0%; recycling: 8.2%;  incineration: 5.4%; landfill : 56.4% S2 - Biogas system application: 30% of  total collected sent for anaerobic digestion (biogas generated is used to produce electricity); composting: 2%; recycling: 8.2%; incineration: 5.4%; landfill: 54.4% S3 - Landfill gas (LFG) capture system application: Based on S0; landfills in Hanoi are equipped LFG capture system; LFG capture efficiency is 90%;  the captured CH4 is flared. S4 - Integrated MSW management: composting: 20% of total collected waste (composted product is used for agriculture); biogas: 10% of  total collected waste (biogas generated is used to produce electricity); incineration: 10%; recycling: 10%; landfill: 50%; energy recovery system from incineration applied; LFG capture system (50% capture efficiency) and the captured CH4 is flared.
3. Results
3.1. Waste flow of scenarios studied
Waste flow of scenarios studied is presented in Table 2. As mentioned in above section, this study assumed the same amount and composition of MSW as the baseline for year  of 2011 to make obtained results comparable among scenarios. The S0 - baseline and S3 - LFG capture application present the highest amount of waste sent for landfill at 1,701,630 tons/year, followed by S1 at around 1,136,792.5 tons/year. The scenario S4 - integrated management had the smallest amount of waste sent to landfill at 1,008,312.5 accounting for 50% of total collected waste in 2011. The amount of waste treated by composting, biogas, recycling and incineration has been increased as s oown in  Table 1.

3.2. GHG emissions from the MSW management in Hanoi
Three main greenhouse gases including CO 2 , CH 4 and N 2 O and their global warming potentials (GWPs) are considered in this study. According to the IPCC's report, the GWPs for the 100 - year time horizon of CO 2 , CH 4 and N 2 O are 1, 21 and 310, respectively [11]. To finalize the results,  the term of carbon dioxide equivalence (CO 2 e) is calculated by multiplying the amount of gas emitted by its associated GWP. The results of GHG emissions are summarized in the following figures. Fig. 2a presents GHG emissions from the five scenarios studied by gas. It shows that CH 4 is the main gas emitted for most scenarios excepting S3. The highest amount of CH 4 is from current practice accounting for 2,702,973 tons CO 2 e, followed by S2 and S3 scenarios accounting for 1,575,168 and 1,552,026 tons CO 2 e, respectively. The lowest emission of CH 4 is from S3 because in this case landfills are equipped with a LFG capture system and capture efficiency reaches 90% of total CH 4 emission. Even scenario S4 - integrated management scenario emitted more CH 4 than S3 because it was assumed that LFG capture efficiency just reaches 50%, however, S4 is the smallest contributor of total emission among the seven scenarios. This figure illustrates the visible high potential of reduction through collecting CH 4 generated from landfills. S3 generated higher amount of CO 2 than those emitted from others. This is because methane from landfill collected and flared. Fig. 2b illustrates GHG emissions by source from the MSW management system. All scenarios studied show that landfill is the biggest contributor of GHG emissions compared to other sources of the MSW management system. It contributes more than 90% of total emissions. Notably, the recycling stage of all scenarios contributes minus emission. This is because material recycling from recycling stage can produce secondary materials that can be used to replace the virgin raw material production of an equivalent amount of material. This point reflects that recycling is one of the key environmental benefits in terms of GHG emission reduction.
S0- Baseline
The scenario S0 presents the current MSW management system in Hanoi in 2011.  The total amount of emission was 3,034,128 tons CO 2 e, in which landfill contributed up to 98% of total emission. This is because the huge amount of waste (84.4% of collected MSW) sent for landfill was associated with a high biodegradable component and landfill was not equipped LFG capture system. The remaining sources just emitted about 2% of total emission. Remarkably, recycling was able to reduce 102,679 tons CO 2 e that would have been emitted through exploitation and production of virgin resources. The average emission was 1.5 tons CO 2 e/ton of waste.
S1 - Composting system upgrade and S2 - Biogas system application
These two scenarios switch the same amount of organic waste sent for composting and anaerobic digestion. The amount of GHG emissions from S1 and S2 are almost same, in which S1 generated 1,759,709 ton CO 2 e and S2 emitted 1,689,385 ton CO 2 e. These amounts are 44% and 42% lower compared to those from S0, respectively. Notably, the anaerobic digestion (S2) does not demonstrate much lower emission than the composting (S1), although methane generated is used for electricity production. Theoretically, the emission from anaerobic digestion should be much lower than that from composting because it can both reduce CH 4 emission from landfill and displace emission associated with the production of virgin raw material for electricity production. Average units generated from waste are 0.87 ton CO 2 e per ton of waste from S1 and 0.83 ton CO 2 per ton of waste from S2.
S3 - LFG capture system application
This scenario assumed that 90% of total GHG emission from landfill is captured and flared. Total emission decreases by 920,250 ton CO 2 e and it is 70% lower than that from S0- current situation. It is  clearly seen that the LFG capture system is one of the most important keys to reduce GHG emission from waste management. The LFG capture system application produces only 0.45 ton CO 2 per ton of waste, this amount is much lower average unit of emission compared to  SO. This scenario reflects that CH4 emission from landfill is the key contributor of GHG emissions and indicates high potential of GHG emission reduction from current management through LFG recovery.
S4 - Integrated MSW management
Scenario S4 illustrates total emission from the integrated MSW management for Hanoi. The targets are set based on existing treatment facilities, technologies and governmental plan for MSW management. The scenario considered to improve all aspects of treatment including composting, anaerobic digestion, recycling, incineration and landfill. The LFG capture system is also applied but capture efficiency is only 50%. This scenario shows the lowest emission among the five scenarios studied. It produces 654,376 ton CO 2 e and can reduce 78% of total emission compared to the current management system. In term of GHG emission reduction, this is the best option, however, to achieve the targets a huge amount capital might be required.
4. Conclusions and recommendation
The amount of MSW sent for landfills accounted for 84.4% of total collected waste in Hanoi in 2011. The current MSW management system contributes 3,034,128 tons CO 2 e and the average emission unit was about 1.5 tons CO 2 e per ton of waste. These values indicate the high global warming impact from MSW management. Among emission sources of the MWS management system studied, landfill is the main contributor that generates more than 90% of total GHG emissions. This study used an IWM model to evaluate emission and potential reduction among five scenarios. The results indicate that the LFG capture system contributes in reducing a huge amount of total emission by 70% compared to current management. Composting and anaerobic digestion (S1 and S2) are also appropriate approaches for MSW management in terms of GHG reduction. They can reduce total emission by 42% and 44%, respectively. The last scenario, integrated MSW management is the best option. It contributes the lowest emission of 654,376 tons CO 2 e (accounted for 22%) when integrated MSW management is applied.