Covalorization of RO Reject and Yeast Extract for Enhanced Microalgal Biomass and Bioactive Compound Production

The generation of a large volume of nutrient-rich effluent during the water purification (e.g., reverse osmosis, RO) process is emerging as a serious environmental concern. While microalgal technology provides a new avenue for generating high-value commodities and recovering resources from waste streams. This study provides a circular bioeconomy strategy for recycling the RO reject for Chlorella pyrenoidosa biomass production. Additionally, the effect of yeast extract (YE)-enriched RO reject on microalgal biomass and its composition was also investigated. Interestingly, at 8 g L–1 YE, microalgae achieved a 28% higher biomass yield (1.34 ± 0.006 g L–1) on the sixth day, compared to the final day biomass in BG11 (1.05 ± 0.015 g L–1). In addition to biomass (2.91 ± 0.018 g L–1), the highest protein yield (588.75 ± 1.38 mg g–1) was also recorded at 8 g L–1 YE. Moreover, a low YE dose (2 g L–1) was found to be an ideal condition for obtaining polysaccharide-rich biomass (35.01 ± 1.74% TS), while a high YE concentration (12 g L–1) induced the synthesis of lipids (32.89 ± 0.89% TS) and exopolymeric substances (EPSs; 972.85 ± 29.85 mg g–1). Overall, this study highlights the valorization potential of RO reject and YE as a sustainable alternative to freshwater and synthetic nutrient media for microalgae cultivation.