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A comparative evaluation of energy requirement and CO₂ emission was performed for native polyculture microalgae oil production in a wastewater treatment plant (WWTP). The wastewater provided nutrients for algae growth. Datasets of microalgae oil production and their details were collected from the Minamisoma pilot plant. Environmental impact estimation from direct energy and material balance was analyzed using SimaPro^® v8.0.4. in two scenarios: existing and algal scenarios. In the existing scenario, CO₂ emission sources were from wastewater treatment, sludge treatment, and import of crude oil. In the algal scenario, CO₂ emission with microalgae production was considered using wastewater treatment, CO₂ absorption from growing algae, and hydrothermal liquefaction (HTL) for extraction, along with the exclusion of exhausted CO₂ emission for growing algae and use of discharged heat for HTL. In these two scenarios, 1 m³ of wastewater was treated, and 2.17 MJ higher heating value (HHV) output was obtained. Consequently, 2.76 kg-CO₂ eq/m³-wastewater in the existing scenario and 1.59 kg-CO₂ eq/m³-wastewater in the algal scenario were calculated. In the HTL process, 21.5 MJ/m³-wastewater of the discharged heat energy was required in the algal scenario. Hence, the efficiency of the biocrude production system will surpass those of the WWTP and imported crude oil.