From waste to wealth: coupling different nutritional modes of Scenedesmus obliquus for waste remediation and algal product development

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.