Sustainable cultivation of Nannochloropsis gaditana microalgae in outdoor raceways using flue gases for a complete 2-year cycle: a Circular Economy challenge

Abstract

This research was undertaken in the framework of the Circular Economy. Its aim is to assess the technical feasibility and sustainability of a semi-industrial process for the outdoor cultivation of Nannochloropsis gaditana, using flue gases, on demand, from a coal-fired power generation plant. Within this main aim, three secondary objectives were defined: (i) to produce an accurate forecast of the annual average biomass production; (ii) to assess how the production rate and the biochemical composition of the biomass vary with the weather conditions over the course of a year; and (iii) to identify suitable markets for the biomass produced, in accordance with Spanish legislation. The study was carried out over 2 years, and an extra year was taken to analyze the results. The importance of this research is that it covers the lack of current knowledge, based on the available literature, regarding experimentation with microalgae culture outdoors, on a semi-industrial scale of 10,000 L, for a period of two full years. Our results show that, firstly, productivity changes from 0.025 ± 0.010 g (dry weight) L⁻¹ day⁻¹ (g d.wt L⁻¹ day⁻¹) to 0.160 ± 0.030 g d.wt L⁻¹ day⁻¹, as irradiation increases, and secondly, that the content of polyunsaturated fatty acids decreases, from 4.770 ± 0.770% dry weight (% d.wt) to 3.220 ± 0.535% d.wt, as irradiation increases. The maintenance of the culture during a complete cycle of 2 years, under the stated conditions, demonstrates the technical feasibility and sustainability of the process. In accordance with current Spanish regulations, the results of biochemical analysis of the harvested biomass confirm that it is suitable for use as animal feed.