Life-Cycle Cost and Greenhouse Gas Trade-Offs of Phosphorus-Removal Upgrading Pathways for China’s Municipal Wastewater Treatment Plants

China’s municipal wastewater treatment plants (WWTPs) increasingly require phosphorus-removal upgrading to meet the Class IA standard, creating trade-offs among chemical use, energy demand, cost, and greenhouse gas (GHG) emissions. This study compared six upgrading pathways for a 10,000 m3/d anaerobic–anoxic–oxic (AAO) WWTP by integrating GPS-X simulation, a life cycle assessment, process-based GHG accounting, and life-cycle costing. All pathways achieved the Class IA total phosphorus target but differed in polyaluminum chloride (PAC) demand, electricity consumption, and sludge production. Process-based GHG emissions ranged from 0.424 to 0.580 kg CO2-eq/m3, while discounted life-cycle costs ranged from 0.324 to 0.375 RMB/m3. P4, combining AAO with sand filtration (SF) and PAC dosing at the secondary clarifier, achieved the lowest cost, whereas P6, combining AAO, high-efficiency sedimentation (HES), and SF, exhibited the lowest GHG intensity. Optimizing the PAC dosing location reduced the chemical demand and associated burdens without additional infrastructure. At the national scale, Scenario 1 achieved both a GHG reduction and net cost savings, whereas Scenario 3 delivered the largest GHG reduction at the highest additional net cost. Sensitivity and uncertainty analyses supported the main cost–carbon trade-off while revealing uncertainty in the economic ranking of closely performing pathways. These findings support integrated environmental and economic decision-making for phosphorus-removal upgrading.