A comparative evaluation of economic efficiency was performed for native polyculture microalgae oil production in an oxidation ditch (OD) process wastewater treatment plant (WWTP). A cost function was developed for the process. The operational cost per 1 m(3) of wastewater (w.w.) was 1.34 $/m(3)-w.w. in the existing scenario, 1.29 $/m(3)-w.w. in algal scenario A (no cost for CO2 and waste heat) and 1.36 $/m(3)-w.w. in algal scenario B (no cost for CO2). The conditions were set as follows: hydraulic retention time (HRT): 4 days, microalgal productivity: 0.148 g/L and daily treatment volume: 81.6 m(3)-w.w./d. The cost differences were related to the increase in polymer flocculants for algae separation (+0.23 $/m(3)-w.w), carbon credits from CO2 absorption (-0.01 $/m(3)-w.w), the sales of biocrude (-0.04 $/m(3)-w.w) and sludge disposal (-0.18 $/m(3)-w.w). Hence, the introduction of the algae scenario was the same cost-effective as the existing scenario. Microalgae oil production in an OD process WWTP can serve as a new energy system and reduce the environmental load in a society with a declining population.