Toward a zero-waste microalgal biorefinery: complete utilization of defatted Chlorella biomass as a sole heterotrophic substrate for Chlorella sp. HS2 and an improved composite filler

Abstract

The concept of circular biorefinery has been promoted as a sustainable new approach for the nascent microalgae industry. In particular, solvent extraction of the lipid fraction of microalgal biomass is generally performed when aiming to recover marketable compounds from microalgae; the waste residual biomass generated by this process can provide new market opportunities for microalgae in a wide array of commercial sectors. Herein, the heterotrophic cultivation of Chlorella sp. HS2 was demonstrated using a hydrolysate recovered following the dilute acid hydrolysis of defatted Chlorella biomass (DCB). While HCl and H2SO4 in each case was found to be an effective catalyst capable of converting nearly 40% of DCB into fermentable monosugars, the results of microalgal cultivation in diluted hydrolysate indicated high cellular growth without the need for any supplemental nutrients. Notably, the highest microalgal growth was observed when neutralizing HCl- and H2SO4-treated hydrolysates with NaOH and Ca(OH)2, respectively. Furthermore, the fabrication of a polymer/residual composite using the residual material obtained after H2SO4-catalyzed hydrolysis and Ca(OH)2 neutralization suggested improved tensile capabilities, attributed to the improved dispersion of salt precipitates-containing residue in the hydrophobic polymer matrices. Considering that the leftover residual DCB could be better conditioned as an organic？inorganic filler for composite fabrication through a combined acid hydrolysis-neutralization process, the results here suggest new integrated utilization routes for underutilized byproducts from the microalgal industry. Further investigations are thus warranted with a special focus on bolstering the economic feasibility and scalability of the postulated zero-waste microalgal biorefinery.