Beneficial use of the aqueous phase generated during hydrothermal carbonization of algae as nutrient source for algae cultivation

Hydrothermal carbonization converts algal biomass to hydrochar , but also generates a waste aqueous phase rich in nitrogen and other elements that need to be treated before discharge into water bodies. At the same time, cultivation of microalgae requires significant amounts of nutrients, such as nitrogen, phosphorous, and trace metals. Utilization of aqueous phase in algae cultivation can potentially reduce algae production cost, while mitigating waste generation, leading to a circular process that enhances the sustainability of algal biofuels and bioproducts . In the present study, we assessed the potential of aqueous phase generated during hydrothermal carbonization of lipid-extracted algae to support algal biomass and lipid production from Chlorella vulgaris . The aqueous phase was found to contain high concentrations of nitrogen (2.4 g/L), phosphorus (1.75 g/L), organic carbon (20.9 g/L), and trace elements. Among several aqueous phase dilutions in non-sterilized water (2.5–7.5% v/v) tested for algae cultivation, maximum lipid productivity that was 1.7-fold higher than with standard cultivation media was achieved when C. vulgaris was grown in 5% aqueous phase. Time course studies showed significant nutrient removal of 95% nitrate, 60% ammonia, 47% phosphate, and 30% organic carbon by C. vulgaris over the course of a 10-day cultivation. Moreover, when compared with standard media, utilization of the aqueous phase resulted in augmentation of both storage and structural algal lipids that can be used for biofuel and nutraceutical production, respectively. Subsequent bench-top photobioreactor cultivation validated that the aqueous phase is a promising cultivation medium for algal product manufacturing. • Chlorella vulgaris tolerated up to 7.5% aqueous phase (AP) in water. • Optimal lipid productivity, achieved in 5% AP, exceeded that in standard media. • AP enhanced synthesis of phospholipids and neutral lipids. • AP from hydrothermal carbonization is an effective nutrient source for algae.