Due to the low biomass concentration of microalgae (about 0.5 g L-1 in open ponds) and the small size of microalgal cells (usually 5-50 pm), harvesting mic­roalgal biomass is a major challenge (Uduman et al. 2010). Centrifugation is an efficient method for harvesting microalgae; however, this is too energy-intensive for most low-value applications (i. e., biofuels). Options such as flocculation are a promising approach to reduce harvesting costs (Vandamme et al. 2013). During flocculation, individual cells form larger aggregates that can easily be separated from the culture medium by gravity sedimentation, flotation, or enhanced settling in an inclined lamella separator. Using flocculation, the biomass can be concentrated from a dilute culture with a dry matter content of about 0.05 % to a sludge with a dry matter content of 0.5-5 %. Mechanical techniques such as centrifugation or a filter press are required to remove the remaining extracellular water and to obtain a thick paste with a dry matter content of 20 %.

Most HRAPs used for wastewater treatment today contain mixed consortia of microalgae rather than pure cultures. Usually, these communities are dominated relatively large, colony-forming chlorophytes such as Pediastrum, Microctinium, Scenedesmus, Dictyosphaerium, and Coelastrum (Benemann et al. 1980; Park et al. 2013). Possibly, these species are favored by the flow regime generated by the paddle wheel in high-rate algal ponds. These relatively large colonial microalgae often flocculate spontaneously, a process that is referred to as bioflocculation (Park et al. 2011a). Bioflocculating microalgae may form aggregates with other non — bioflocculating species (Salim et al. 2011), and bioflocculated microalgae have high settling rates and can be relatively easily concentrated to a slurry of 1 -3 % dry matter by simple gravity sedimentation (Sheehan et al. 1998). By recycling part of the harvested biomass, the dominance of these bioflocculating microalgae can be maintained (Benemann et al. 1980; Park et al. 2011b, 2013). Bacteria present in the wastewater may also play a role in bioflocculation (Su et al. 2011). Bacteria grow on organic matter present in wastewater, and research by Lee et al. (2008) and Lee et al. (2012) showed that the presence of bacteria in cultures of Chrysotila and Chlorella resulted in flocculation of the microalgal cells. In both studies, it appeared that extracellular polymeric substances produced by the microalgae were involved in the flocculation process. Van den Hende et al. (2011) showed that a sufficient supply of organic matter is important to sustain mixed algal-bacterial flocs.

The high pH that is typical of microalgal cultures can induce precipitation of Ca or Mg salts and can also induce flocculation of microalgal cells; a process that is referred to as autoflocculation. Ca phosphates precipitate at a relatively low pH of about 8.5-9 and can induce flocculation of microalgae. Such pH levels are regularly encountered in outdoor microalgal cultures when irradiance levels and temperatures are high. Flocculation by Ca phosphate precipitation requires relatively high Ca and phosphate concentrations in the wastewater and is therefore only applicable in hard waters with excess phosphate levels (Sukenik and Shelef 1984; Sukenik et al. 1985). While autoflocculation by Ca phosphate works well in laboratory conditions, it often fails in large-scale systems, even when Ca and phosphate concentrations are sufficiently high (Nurdogan and Oswald 1995). This may be due to autoflocculation by Ca phosphate is inhibited by the presence of organic matter in microalgal cultures (Beuckels et al. 2013). Bioflocculation and autoflocculation have been studied in the past 30 years in laboratory conditions and pilot systems. It appears that their performance depends strongly on species and cultivation conditions, yet, the reliability of these methods remains to be proven in long-term and large-scale operations (Benemann et al. 2012). More details on the recent developments on harvesting and dewatering can be found in Chaps. 12—14.