Megacities face special challenges in the development and implementation of Municipal Solid Waste Management (MSWM) systems due to their high population density and large waste volumes. These systems need to be environmentally friendly and align with climate and resource protection goals. Existing and potential future MSWM systems must be evaluated based on these criteria. In this context, there is a lack of an assessment system for megacities, taking into account the current requirements of climate and resource protection, including the European Union's Green Deal. Additionally, this assessment should consider not only the criteria of Life Cycle Assessment (LCA) but also the avoided emissions of fossil greenhouse gases at the regional level through recycling and energy recovery from waste. Unlike previous works, this thesis aims to differentiate material flows, considering collection rates, sorting rates, and residual waste rates at the collection level and within defined facility systems and combinations. This differentiation will be demonstrated in the case of the Istanbul metropolitan area (Türkiye), where approximately 6,6 million Mg of municipal solid waste are generated annually (based on ISTAC 2020 data). Currently, 83% of this waste is deposited, 11% is composted, and 6% is recycled. The research questions focus on the ecological assessment of the existing Municipal Solid Waste Management (MSWM) system in the Istanbul metropolitan area and scenarios for future MSWM systems. Basic scenarios are to be developed with the aim of intensifying the recycling and energy recovery of waste. At the same time, the greenhouse gas emissions, primarily caused by landfilling, are to be significantly reduced. In this context, the target goals of the EU regarding recycling and landfill rates are also to be pursued. In combination scenarios, transitional scenarios are to be considered, taking into account the current Municipal Solid Waste Management (MSWM) system in Istanbul and existing plans. In this thesis, a methodical approach is taken towards mass and energy balances (input-output analyses) using material flow analyses (MFA) (inventory analyses). Here, particular emphasis is placed on the greenhouse gas emissions from waste treatment processes, and for the first time, the substitution of fossil energy sources through waste management measures is examined based on Türkiye energy mix. Additionally, an assessment is conducted using the life cycle assessment (LCA) method. The functional unit is selected as 1 Mg MSW (1 Mega gram of Municipal Solid Waste), and the characterization results are determined at the midpoint level. Within this thesis, the following impact categories are considered: Global warming potential (GWP), stratospheric ozone depletion (SOD), freshwater ecotoxicity (FE), terrestrial ecotoxicity (TE), particulate matter formation (PMF), human carcinogenic toxicity (HCT), and land use (LU). Ten different Municipal Solid Waste Management (MSWM) scenarios are developed, categorized as basic and combination scenarios. Basic scenarios: • B.S-1: Landfilling of the total MSW • B.S-2: Thermal treatment (incineration) of the total MSW with energy recovery • B.S-3: Metal recycling + thermal treatment (incineration) with energy recovery B.S-4(a) and B.S-4(b) include waste separation as dry and wet waste at the source: • B.S-4(a): Recycling + composting + anaerobic digestion + landfilling of residual waste • B.S-4(b): Recycling + composting + anaerobic digestion + thermal treatment (incineration) of residual waste with energy recovery B.S-5(a) and B.S-5(b) align with the waste management goals of the EU for 2030, including the separation of organic waste, recycling waste, and residual waste at the source: • B.S-5(a): Recycling + composting + landfilling + thermal treatment (incineration) with energy recovery • B.S-5(b): Recycling + anaerobic digestion + landfilling + thermal treatment (incineration) with energy recovery Combination scenarios: C.S-1 represents the current situation of the MSWM system in Istanbul: • C.S-1: Recycling + composting + landfilling C.S-2 includes the goals set by the Istanbul municipal government for the year 2023: • C.S-2: Recycling + anaerobic digestion + landfilling + thermal treatment (incineration) with energy recovery C.S-3 is a transitional scenario that incorporates 50% of the EU targets (2030) for recycling and landfilling: • C.S-3: Recycling + composting + landfilling + thermal treatment (incineration) with energy recovery Based on the calculations of net greenhouse gas (GHG) emissions and net energy generation from the scenarios, the worst scenario is B.S-1 (all of the waste is sent to landfill), which has the highest net GHG emission (908,6 kg CO2 eq) and the lowest net energy production (92,2 kWh). The second-worst scenario is C.S-1 (current MSWM system), which has the second highest net GHG emission (725,8 kg CO2 eq) and the second lowest net energy production (364,9 kWh). In contrast, B.S-4(b) has the lowest net GHG emission (50,5 kg CO2 eq), and it is followed by B.S-5(b) (58,1 kg CO2 eq) and B.S-5(a) (76,4 kg CO2 eq). B.S-3 (metal recycling + incineration) has the highest net energy generation (2.040 kWh) and simultaneously the lowest net GHG emission (264,9 kg CO2 eq) after B.S-4(b), B.S-5(a), and B.S-5(b), respectively. Taking into account the Türkiye energy mix, the greenhouse gas (GHG) emission coefficient is 0,702 kg CO2 eq/kWh for electrical energy and 0,219 kg CO2 eq/kWh for thermal energy. When considering the avoided GHG based on the net heat and electrical energy production according to Türkiye energy mix, only B.S-2, B.S-3, B.S-4(b), B.S-5(a) and B.S-5(b) result in an overall reduction of GHG emissions. B.S-3 performs the best with (240,8 kg CO2 eq), followed by B.S-4(b) (226 kg CO2 eq), B.S-2 (197,7 kg CO2 eq), B.S-5(b) (156,7 kg CO2 eq), and B.S-5(a) (89,8 kg CO2 eq), respectively. According to the LCA calculations results, B.S-5(a) is the best scenario in five out of seven impact categories, which are GWP (-67,74 kg CO2 eq), FE (66,95 kg 1,4-DCB), TE (-551,08 kg 1,4-DCB), HCT (-14,22 kg 1,4-DCB), and LU (-119,72 m2a crop area). B.S-5(b) follows B.S-5(a) and has the second-best effect in GWP (-26,28 kg CO2 eq), FE (74,7 kg 1,4-DCB), TE, (-520,64 kg 1,4-DCB), PMF (-0,91 kg PM 2,5 eq), and LU (-118,66 m2a crop area) categories. In contrast, B.S-1 is the worst scenario in all impact categories except for SOD category. C.S-1 follows B.S-1 and has the second-worst effect in GWP (651,3 kg CO2 eq), SOD (-2 x 10-4 kg CFC-11 eq), PMF (-0,19 kg PM 2,5 eq), and LU (-31,78 m2a crop area). In summary, both from the perspective of greenhouse gas and energy considerations, as well as life cycle assessment, C.S-1 (Current MSWM System of Istanbul) shows the worst result, followed by B.S-1. B.S-5(a) and B.S-5(b) emerge as the best scenarios in these regards. However, it is noteworthy that these scenarios are only feasible in megacities in emerging countries with the establishment of suitable conditions and require a significant lead time. Significant investments in collection systems and waste treatment technologies are necessary to achieve high rates of recycling, energy substitution, and a significant reduction in greenhouse gas emissions. Furthermore, citizens must be actively engaged through intensive public awareness campaigns during the introduction of separate collection systems. Essential prerequisites also include the establishment of appropriate legal frameworks and their enforcement. In this context, the implementation of scenario B.S-4(b) could represent a suitable solution for Istanbul and other megacities in developing countries. In this scenario, separation into wet and dry waste occurs at the source, followed by treatment in biological treatment facilities, sorting facilities, and thermal treatment of residual waste. It is important to consider the aforementioned conditions in this context as well. The environmental impacts of landfilling are significantly reduced, and greenhouse gas emissions are minimized. The developed scenarios, evaluated from an ecological standpoint, demonstrate, using the example of the greater Istanbul area, that waste management measures can contribute not only to resource conservation but especially to climate protection. Simultaneously, these measures can replace fossil emissions. The chosen approach is also transferable to other megacities, especially in emerging countries.