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Oleaginous microalgae are considered as promising sources of biofuels and biochemicals due to their high lipid content and other high-value components such as pigments, carbohydrate and protein. This study aimed to develop an efficient biorefinery process for utilizing all of the components in oleaginous microalgae. Acetone extraction was used to recover microalgal pigments prior to processes for the other products. Microalgal lipids were converted into biodiesel (fatty acid methyl ester, FAME) through a conventional two-step process of lipid extraction followed by transesterification, and alternatively a one-step direct transesterification. The comparable FAME yields from both methods indicate the effectiveness of direct transesterification. The operating parameters for direct transesterification were optimized through response surface methodology (RSM). The maximum FAME yield of 256 g/kg-biomass was achieved when using chloroform:methanol as co-solvents for extracting and reacting reagents at 1.35:1 volumetric ratio, 70 °C reaction temperature, and 120 min reaction time. The carbohydrate content in lipid-free microalgal biomass residues (LMBRs) was subsequently acid hydrolyzed into sugars under optimized conditions from RSM. The maximum sugar yield obtained was 44.8 g/kg-LMBRs and the protein residues were recovered after hydrolysis. This biorefinery process may contribute greatly to zero-waste industrialization of microalgae based biofuels and biochemicals.

This chapter summarizes the methods for harvesting microalgal biomass and conversion of microalgal lipids into biodiesel. The fundamental harvesting methods, including centrifugation, gravity sedimentation, filtration, flotation, and flocculation, are reviewed. The recently developed biological methods for efficient harvesting of microalgal biomass are addressed. The conventional extraction and transesterification of microalgal lipids into biodiesel are summarized. The advantages of simultaneous extraction and transesterification, so-called direct transesterification, of microalgal lipids into biodiesel are highlighted.

Marine microalgae are used in aquaculture as a suspension to feed young fishes and shrimps. Marine Chlorella sp. is one of the most attractive marine microalgae because in addition to pigments it can accumulate lipids at a high content (>30 %) and release exopolymeric substances (EPSs) into the culture medium. This study aimed to evaluate the optimal cultivation of marine Chlorella sp. as potential sources of lipids, EPSs and pigments. Among the culturing regimes tested, mixotrophic cultivation mode produced the highest yields of biomass, lipids and EPSs. Several factors affecting mixotrophic cultivation were optimized. An increase in light intensity up to 65 μmol m−2 s−1 and carbon dioxide up to 10 % v/v in air enhanced both biomass and product formation. An increase in glucose concentration up to 1 % w/v enhanced biomass and EPS yields but decreased lipid and chlorophyll contents. A semi-continuous cultivation under optimal conditions produced microalgal biomass of 2.76 g L−1 with a high lipid content of 44.9 % and EPS of 1.46 g L−1. This study has also shown that the microalgal lipid and EPS have potential to be used as biodiesel feedstocks and bioflocculants, respectively. The concomitant production of these valuable products together with the microalgal biomass would be a potential way to offset the production cost and contribute greatly to developing industrialized microalgae cultivation.

This study aimed to optimize flocculation efficiency of lipid-rich marine Chlorella sp. biomass and evaluate its composition in different cultivation modes. Among three flocculants including Al(3+), Mg(2+) and Ca(2+) tested, Al(3+) was most effective for harvesting microalgal biomass. Four important parameters for flocculation were optimized through response surface methodology. The maximum flocculation efficiency in photoautotrophic culture was achieved at pH 10, flocculation time of 15 min, Al(3+) concentration of 2.22 mM and microalgal cells of 0.47 g/L. The flocculation in mixotrophic culture required lower amount of Al(3+) (0.74 mM) than that in photoautotrophic and heterotrophic cultures (2.22 mM). The biomass harvested from mixotrophic culture contained lipid at the highest content of 42.08 ± 0.58% followed by photoautotrophic (32.08 ± 3.88%) and heterotrophic (30.42 ± 1.13%) cultures. The lipid-extracted microalgal biomass residues (LMBRs) contained protein as high as 38-44% and several minerals showing their potential use as animal feed and their carbohydrate content were 16-29%.