This work aims to economically evaluate centralized and decentralized Sewage Treatment Plants (STPs) for a metropolitan city. For the same, centralized and decentralized wastewater treatments for a catchment area in Sharjah city (United Arab Emirates) were evaluated by Net Present Value (NPV) method. Three options meeting local government regulations were considered for the catchment. Options 1 and 2 were centralized, and Option 3 was decentralized. Option 1 was about connecting the catchment to the existing centralized STP location, Option 2 to connect the catchment to the new proposed location which may be situated at the lowest point around the catchment area, and Option 3 involved constructing three decentralized STPs. The results showed that Option 3 had the lowest NPV of costs including CAPEX and OPEX for 25 years. There was a saving in Option 3 of around 44% compared to Option 1, and 23% compared to Option 2. This finding is useful for engineering managers for infrastructure planning for a metropolitan city like Sharjah city.