Lipid Accumulation and Nitrogen Removal for Chlorella vulgaris and Scenedesmus Obliquus Using Wastewater

^a*Ruiz-Marín Alejandro, Canedo-López Yunuen ^a

^*a Facultad de Química, Universidad Autónoma del Carmen. C. 56 #4, Av. Concordia, Col. Benito Juárez, C.P. 24180. Ciudad del Carmen, Campeche, México. Email: aruiz@pampano.unacar.mx; ycanedo@pampano.unacar.mx

            Nitrogen limitation has been widely proposed as a method to increase lipid content of microalgae in biodiesel-oriented processes. However, this is typically accompanied by a reduction on the growth rate, and as a result, the overall lipid productivity does not necessarily increase. In this study a novel multi-stage nitrogen-reduction process is proposed, in order to promote a balance between growth rate and lipid accumulation which could result in a net increase of lipid-productivity in microalgae, while simultaneously reducing nitrogen concentrations in wastewater. Chlorella vulgaris and Scenedesmus obliquus were grown initially in nitrogen-rich (90 mg L^–1) artificial-wastewater medium, followed by sequential dilutions (50% v/v) in fresh medium with N-NH₄ concentrations of 60, 40, and 20 mg L^–1, respectively. The overall lipid productivity was compared to those obtained in various two-stage nitrogen reduction processes, wherein the nitrogen-rich culture was followed by a 50% v/v dilution in fresh medium containing 30, 20, or 10 mg L^–1 N-NH₄ in the second stage. Increased net lipid-productivity was observed for both species in the two-stage mode, although nitrogen depletion was not achieved in these cases. On the other hand, in the sequential mode only C. vulgaris exhibited a net lipid-productivity increment. The highest lipid productivities occurred in the two-stage mode for both S. obliquus and C. vulgaris (194.9 and 133.5 mg L^–1 d^–1, respectively). The lipid productivities achieved in this study are among the highest reported in the open literature to date, and the fatty-acid profiles are adequate for biodiesel production.

Keywords: Nitrogen limitation, lipid productivity, microalgae, biodiesel, multistage cultivation.