Methane production through anaerobic digestion of residual microalgal biomass after the extraction of valuable compounds

Abstract

Biorefinery concepts that combine several valorization pathways, including both bioproducts and bioenergy/biofuels, are of high interest in order to maximize the economic value of microalgal cultivation systems. In this study, the residual biomass obtained from the microalga Chlorella vulgaris following the chemical extraction of a valuable fraction consisting of either chlorophyll, proteins, or lipids was used as feedstock for biogas production in a laboratory-scale biochemical methane potential (BMP) assay performed in batch mode. The main aim of this study is to assess the effect of chemical extraction and removal of valuable fractions on the methane yield of residual microalgal biomass. The dry weight ratio of residual biomass to total biomass was 0.54 g/g after lipid extraction, 0.62 g/g after chlorophyll extraction, and 0.42 g/g after protein extraction. The methane yields of raw microalgae as well as residual microalgal biomass after lipid, chlorophyll, or protein extraction and removal were in the range 207–237 mL CH4/g volatile solids. Differences in the methane yields were not statistically significant. Hence, the recovery and removal of valuable compounds did not have a significant impact on the final methane yields of microalgal biomass by anaerobic digestion. Nevertheless, the methane production kinetics of residual microalgal biomass after lipid extraction displayed a diauxic pattern, possibly due to the inhibition of methanogenic bacteria by chloroform used as extraction solvent. This suggests that solvent toxicity must be considered when applying anaerobic digestion as a downstream process in biomass-based biorefineries.