Application of low-cost algal nitrogen source feeding in fuel ethanol production using high gravity sweet potato medium

Protein-rich bloom algae biomass was employed as nitrogen source in fuel ethanol fermentation using high gravity sweet potato medium containing 210.0 g l−1 glucose. In batch mode, the fermentation could not accomplish even in 120 h without any feeding of nitrogen source. While, the feeding of acid-hydrolyzed bloom algae powder (AHBAP) notably promoted fermentation process but untreated bloom algae powder (UBAP) was less effective than AHBAP. The fermentation times were reduced to 96, 72, and 72 h if 5.0, 10.0, and 20.0 g l−1 AHBAP were added into medium, respectively, and the ethanol yields and productivities increased with increasing amount of feeding AHBAP. The continuous fermentations were performed in a three-stage reactor system. Final concentrations of ethanol up to 103.2 and 104.3 g l−1 with 4.4 and 5.3 g l−1 residual glucose were obtained using the previously mentioned medium feeding with 20.0 and 30.0 g l−1 AHBAP, at dilution rate of 0.02 h−1. Notably, only 78.5 g l−1 ethanol and 41.6 g l−1 residual glucose were obtained in the comparative test without any nitrogen source feeding. Amino acids analysis showed that approximately 67% of the protein in the algal biomass was hydrolyzed and released into the medium, serving as the available nitrogen nutrition for yeast growth and metabolism. Both batch and continuous fermentations showed similar fermentation parameters when 20.0 and 30.0 g l−1 AHBAP were fed, indicating that the level of available nitrogen in the medium should be limited, and an algal nitrogen source feeding amount higher than 20.0 g l−1 did not further improve the fermentation performance.

► Bloom algae biomass was employed as nitrogen source in fuel ethanol fermentation using high gravity sweet potato medium. ► Fermentation rate and ethanol yield increased with increasing feeding amount. ► 20.0 g l−1 AHBAP was the optimal feeding amount in both batch and continuous fermentation. ► Amino acids hydrolyzed from protein served as the available nitrogen nutrition for yeast growth and metabolism.