It is widely acknowledged that one of the major bottlenecks of bioenergy production from microalgae lies in the concentration step. The selected studies assess a large variety of tech­nologies to achieve concentration, dewatering, and sometimes drying of the algal biomass. The final dry-matter content (DM) before biofuel production depends also on the transforma­tion process. For instance, anaerobic digestion of bulk microalgae requires a low DM content, from 5% (Collet et al., 2011) to 14% (Clarens et al., 2011). DM content for biodiesel production varies from 14% (Clarens et al., 2011) in the case of wet extraction to 90% (Lardon et al., 2009) in the case of dry extraction and from 50-98% for direct combustion. Table 13.7 summarizes harvesting and conditioning technologies in regard to the biomass transformation option se­lected in the various studies.

Several studies suggest a first step of flocculation/sedimentation to concentrate the bio­mass (Table 13.7). It was supposed to be done by pH adjustment with lime (Lardon et al., 2009; Brentner et al., 2011) or by addition of aluminium sulphate (Clarens et al., 2010; Stephenson et al., 2010; Brentner et al., 2011), chloride iron (Hou et al., 2011; Khoo et al., 2011), or chitosan (Brentner et al., 2011). For some species, harvesting can be done by passive sedimentation. This first step results in algal slurry with a DM content varying from 2% (Lardon et al., 2009) to 14% (Clarens et al., 2011). An important issue for the char­acterization of this step is the determination of the settling velocity and the ratio of biomass staying in the supernatant. Still, the concentration of the algal slurry after settling is not high enough to allow efficient down-processing. The most classical way to further increase the biomass concentration is centrifugation, even though this method is considered one of the most energy consuming (Molina Grima et al., 2003). Collet et al. (2011) use data from a spiral plate centrifuge, which is reputed to consume less energy; other authors rely on rotary drums (Lardon et al., 2009). Finally, solar drying was used in one study (Kadam, 2002), which led to an important decrease of the energy consumption of this step.