Wastewater treatment and biodiesel production by Scenedesmus obliquus in a two-stage cultivation process

•Wastewater treatment with microalgae produced biomass to be used as biodiesel source.•Wastewater cultured S. obliquus increased lipid content in response to stress factors.•CO2, light and salt factors acting isolated increased lipids.•Salt presence in darkness increased lipids avoiding the use of photobioreactors.•ω-3 eicosapentaenoic acid content of biomass slightly exceeded EU biodiesel normative.

The microalga Scenedesmus obliquus was cultured in two cultivation stages: (1) in batch with real wastewater; (2) maintaining the stationary phase with different conditions of CO2, light and salinity according to a factorial design in order to improve the lipid content. The presence of the three factors increased lipid content from 35.8% to 49% at the end of the second stage; CO2 presence presented the highest direct effect increasing lipid content followed by light presence and salt presence. The ω-3 fatty acids content increased with CO2 and light presence acting in isolation, nevertheless, when both factors acted together the interaction effect was negative. The ω-3 eicosapentaenoic acid content of the oil from S. obliquus slightly exceeded the 1% maximum to be used as biodiesel source (EU normative). Therefore, it is suggested the blend with other oils or the selective extraction of the ω-3 fatty acids from S. obliquus oil.