Soy whey and brewer's spent grain hydrolysates wholly replace conventional medium for microalgae growth: Process performance and economic considerations.

To enhance circularity in heterotrophic microalgal bioprocesses, this study completely substituted glucose and Bold's basal medium (BBM) with brewer's spent grain (BSG) and soy whey (SW) hydrolysates. Mild acid hydrolysis conditions of BSG (0.2 M HSO, 130 °C, 36 min) and SW (0.1 M HCl, 95 °C, 30 min) were optimised for glucose release, and their hydrolysates were optimally mixed (15 % SW-85 % BSG) to obtain a medium that best supported Auxenochlorella protothecoides growth. Maximum biomass production (X) and productivity (P) obtained in the hydrolysate medium containing 50.75 g/L endogenous glucose (X: 22.17 g/L; P: 7.06 g/L/day) were comparable to that in BBM containing 50.44 g/L exogenous glucose (X: 20.02 g/L; P: 6.34 g/L/day). Moreover, estimated hydrolysate medium production costs were within an order of magnitude to BBM. Overall, the integrated approach of tailored hydrolytic treatments and complementary side-streams presents a promising technical and economic feasibility, with applications extending beyond A. protothecoides.