In large cities, several hundred cubic meters of municipal wastewater is produced every day, which because of high levels of pollution, needs to be treated either for disposal or secondary use. Some of the most important indicators and pollutants that should be reduced to the standard range are VOC, TSS, TDS, BOD 5 , nitrogen, phosphorus, heavy metals, and pathogens. Wastewater treatment plants, which have different capacities and procedures depending on the type and characteristics of the infl uent and effl uent, have large budgets of several hundred million for their life cycle. These costs include construction, maintenance, operation, chemical and biological materials, energy, and amortization. The costs associated with these environmentally essential infrastructures vary according to their treatment processes. This issue leads environmental engineers and policymakers to economic evaluation of various methods. This study with the help of CapdetWorks2.5 software has discussed three different methods of Complete Mixed Activated Sludge (CMAS), Aerated lagoon, and Oxidation Ditch in wastewater treatment plants in terms of operating costs. Evaluation and implementation of the decision-making system were conducted using Analytic Hierarchy Process (AHP). The results of the analysis showed that aeration ponds are the most optimal option in terms of operating costs with a value of 47.3%. The other two studied methods, namely CMAS and Oxidation Ditch, have a value of 24.9% and 27.9%, respectively.