Recovery of microalgal biomass and metabolites: process options and economics
Introduction
Microalgae can be used to produce numerous high-value bioactives Borowitzka, 1986, Bubrick, 1991, Pulz et al., 2001, Li et al., 2001, Banerjee et al., 2002. Production of microalgae-derived metabolites requires processes for culturing the alga Ben-Amotz and Avron, 1987, Molina Grima, 1999, Molina Grima et al., 1999, Sánchez Mirón et al., 1999, Tredici, 1999, Borowitzka, 1999, Pulz, 2001, Pulz et al., 2001, recovery of the biomass, and further downstream processing to purify the metabolite from the biomass. As with many microbial processes for producing bioactives, the downstream recovery of algal products can be substantially more expensive than the culturing of the alga. This review examines some commercially relevant options for recovering microalgal products. A case study is used to illustrate the economics of recovery of eicosapentaenoic acid (EPA), an essential fatty acid from microalgae. EPA is an established neutraceutical and evidence is emerging for its therapeutic benefits in disease management Peet et al., 2001, Peet et al., 2002.
Section snippets
Production of microalgal biomass
Production of microalgal biomass can be carried out in fully contained photobioreactors or in open ponds and channels. Open-culture systems are almost always located outdoors and rely on natural light for illumination (Terry and Raymond, 1985). Closed photobioreactors may be located indoors or outdoors Sánchez Mirón et al., 1999, Pulz, 2001, but outdoor location is more common because it can make use of free sunlight. Design and operation of the microalgal biomass production systems have been
Recovery of biomass
Harvesting of biomass requires one or more solid–liquid separation steps. Biomass can be harvested by centrifugation, filtration or in some cases, gravity sedimentation. These processes may be preceded by a flocculation step. Recovery of biomass can be a significant problem because of the small size (3–30 μm diameter) of the algal cells. Culture broths are generally relatively dilute (<0.5 kg m−3 dry biomass in some commercial production systems) and hence large volumes need to be handled to
Dehydration of biomass
Harvesting generally results in a 50- to 200-fold concentration of algal biomass. The harvested biomass slurry (5–15% dry solids) must be processed rapidly, or it can spoil within a few hours in a hot climate. The specific postharvest processing necessary depends strongly on the desired product. Dehydration or drying of the biomass is commonly used to extend the shelf-life of the biomass especially if biomass is the final product. Drying methods that have been used for microalgae include spray
Process economics: a case study of EPA production
Here, we discuss the economics of producing EPA from the marine microalga P. tricornutum, as a representative case study for producing high-value intracellular products from microalgae. The case study is useful in identifying potential bottlenecks to commercializing microalgae-derived products.
Conclusion
Several options exist for recovering and processing microalgal biomass to obtain intracellular metabolites produced by microalgae. For commercial recovery of high-value products, centrifugation appears to be the preferred method of recovering the biomass from the broth. Centrifugation may be preceded by a flocculation step to improve recovery. When centrifugal recovery is not feasible, for example when the alga being recovered is fragile, microfiltration can be a suitable alternative. To the
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