| Summary: | 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<sup>3</sup> of wastewater (w.w.) was 1.34 $/m<sup>3</sup>-w.w. in the existing scenario, 1.29 $/m<sup>3</sup>-w.w. in algal scenario A (no cost for CO<sub>2</sub> and waste heat) and 1.36 $/m<sup>3</sup>-w.w. in algal scenario B (no cost for CO<sub>2</sub>). 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<sup>3</sup>-w.w./d. The cost differences were related to the increase in polymer flocculants for algae separation (+0.23 $/m<sup>3</sup>-w.w), carbon credits from CO<sub>2</sub> absorption (−0.01 $/m<sup>3</sup>-w.w), the sales of biocrude (−0.04 $/m<sup>3</sup>-w.w) and sludge disposal (−0.18 $/m<sup>3</sup>-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.
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