Many of the available harvesting methods are not suitable for complete dewatering of the algal biomass and must be used in combination with other methods to achieve the cell concentrations necessary for the production of a feedstock for algal biofuel production. This has been referred to as either primary and secondary concentration or bulk harvesting and thickening. While the terminology is different, the processes are the same. Primary or bulk harvesting takes low concentration cultures typical in photoautotrophic mass algal culturing and brings them to roughly 2-7 % solids. For most downstream processing, a secondary or thickening step is required to increase the solid concentration. The final solid content needed varies for this secondary concentration step and ranges from 50 % solids for downstream processes that can tolerate significant moisture content (e. g., hydrothermal lique­faction and aqueous extraction) to >90 % solids for dry feedstocks needed for traditional solvent extraction.

While primary harvesting remains a huge challenge for photoautotrophically produced biomass (due to large volumes at low density), processes that bypass the higher energy secondary concentration step could provide an advantage for com­mercial production of biofuels. It is beyond the scope of this review to discuss methods for drying algae (for delivery of >90 % solids) and will focus on methods to deliver concentrated algal feedstocks at <90 % solids as well technologies to avoid harvesting altogether. In Table 14.1, the different primary harvesting methods are listed with a comparison of their properties and the current scale of deployment in the algal industry (not limited to biofuels).

If the technique has an advantage in a particular property, it is marked by a plus (+) sign under the applicable Properties column. For example, in settling/sedi- mentation, very little energy is required, so it has a (+) in that column, but since product stability is not maintained, it has a (—) in that column. If the process could have advantages for some biofuels applications and not work for another, a (±) is provided in that column. If there is not enough information to make a decision, a question mark is inserted in the column in Table 14.1. The current scale section of Table 14.1 provides an estimate of the applicability of the method to rapid com­mercial deployment.