Wastewater treatment plants, which are essential for protecting water quality, can generate substantial greenhouse gas (GHG) emissions through biological treatment, energy consumption, and sludge management. This study applies life cycle and avoided-impact assessments to quantify GHG emissions and evaluate mitigation options for the Central Wastewater Treatment Plant in Ulaanbaatar, Mongolia, using 2024 operational data. The functional unit was 1 m³ of treated wastewater. Four scenarios were evaluated: continued long-term sludge storage (S1), a 20% reduction in electricity consumption (S2), anaerobic digestion with combined heat and power (CHP) generation (S3), and controlled composting (S4). The baseline climate change impact was 1.2969 kg CO₂-eq/m³, corresponding to approximately 90,137 t CO₂-eq attributable to wastewater received in 2024, including cumulative sludge-storage emissions over an assumed four-year period. Direct emissions accounted for 81.71% of the total impact. CH₄ emissions from sludge storage and wastewater treatment collectively contributed 61.58%, whereas electricity consumption contributed 17.32%. S2 reduced the impact to 1.2520 kg CO₂-eq/m³, a 3.46% reduction relative to S1. S3 achieved the greatest modelled mitigation by avoiding baseline sludge-storage CH₄ emissions and substituting grid electricity and coal-based heat with recovered biogas energy. Its net impact was 0.6379 kg CO₂-eq/m³, a 50.81% reduction. In contrast, S4 increased the net impact to 1.4473 kg CO₂-eq/m³, an 11.60% increase, because emissions from composting and subsequent compost application outweighed avoided sludge-storage emissions and fertilizer-substitution credits. Under the modelled assumptions, AD with CHP showed the greatest mitigation potential among the evaluated options. These results provide quantitative evidence to inform sludge management and climate mitigation planning in Ulaanbaatar.