From waste to wealth: coupling different nutritional modes of Scenedesmus obliquus for waste remediation and algal product development
Alla Silkina, José Ignacio Gayo-Peláez, Fleuriane Fernandes, Claudio Fuentes-Grünewald, Rahul Vijay Kapoore, Kam W. Tang
Abstract
Abstract This study exploited the different combinations of nutritional modes (autotrophic, mixotrophic and heterotrophic) of the green alga Scenedesmus obliquus in a two-stage cultivation process to remediate wastes as well as maximise the production of high-protein algal biomass. The alga was first cultivated autotrophically in Stage-one, using 0.5% of digestate as a nutrient source and natural light to support the growth; afterward, the culture was concentrated and used in Stage-two cultivation under the different nutritional modes. 2.5% of anaerobic digestate from food waste was used to provide inorganic nutrients; waste sugar recovered from a confectionery manufacturer was used as the organic carbon source to support mixo- and hetero-trophic growth. In Stage-one, the algae removed ammonium and phosphate at 9.1 mg L −1 day −1 and 3.8 mg L −1 day −1 , respectively, and the algal yield approached a carrying capacity of 1 g dry weight per liter. In Stage-two, the algal population increased faster in mixotrophic and heterotrophic modes, reaching a final dry weight concentration of 7.2–10.5 g L −1 , than in autotrophic mode. Ammonium and phosphate were consumed at about twice the rate as in autotrophic mode, and 90–100% of the waste sugar was removed within 48–72 h. The final algal biomass had a protein content of 44% in mixotrophic mode, compared to only 32% in autotrophic mode. This study demonstrated that a two-stage cultivation process coupling autotrophic and mixotrophic growth of S. obliquus can be used to remediate both inorganic and organic wastes and boost the production of protein-rich algal biomass, which can be beneficial to a sustainable circular bioeconomy.