# Flocculation Harvesting Techniques for Microalgae: A Review
## 1 Introduction  
Microalgal biomass has attracted much attention in the academic and industrial fields due to its various industrial applications such as animal/aquaculture feeds, food supplements, nutraceuticals, and pharmaceuticals [1; 2]. Recently, petroleum-fuel scarcity as well as global warming associated with greenhouse gas emissions (e.g., CO\({}_{2}\)) are obliging scientists and engineers to actively look for new and renewable sources of transportation fuels [3]. Various liquid and gaseous biofuels, such as diesel, aviation fuel, ethanol, butanol, hydrogen, and methane, can be produced from algal biomass through biological and thermochemical transformation technologies [4; 5]. Microalgae can utilize CO\({}_{2}\) as an inorganic carbon substrate using light energy and can be grown using diverse water resources, including freshwater, seawater, and even industrial/domestic wastewater. They can be also cultivated at a large-scale using different bioreactor systems such as open ponds and photobioreactors [6; 7]. However, due to the low concentration (\(\sim\)5 g/L) in culture, small size (\(\sim\)5 \(\upmu\)m) and negative surface charge (\(\sim\)-20 mV) of algal cells, external energy and/or chemicals aregenerally required to accelerate their recovery from base water [8; 9]. Furthermore, other morphological and physiological characteristics of algal cells such as shape, cell wall structure, and extracellular organic matter (EOM) change significantly depending on the nutritional and environmental conditions including medium composition, light, temperature, pH, culture duration, and bioreactor type [10]. The algae harvesting costs are generally estimated at 20-30%, with the occasional rise to 60%, of the total biomass production cost, depending on the algal species and culture process used [11; 12]. Therefore, the development of a high-efficiency and cost-effective harvesting process is key to achieving commercial scale algae-based process. Algal biomass harvesting has been extensively studied with particular focus on centrifugation, gravity sedimentation, screening, filtration, air flotation, and flocculation techniques. However, there is no single universal harvesting method for all algal species and/or applications, which is both technically and economically viable [13; 14]. For instance, centrifugation is based on a mechanical gravitational force that allows for efficient harvesting of suspended cells in a short time. However, due to the intensive energy requirement, it is recommended only for high-value algal products such as in foods and pharmaceuticals [14; 15]. In the filtration process, micro-sized algal cells can be passed through a suitable membrane under high pressure to obtain a thick paste of algal biomass [16]. This size-exclusion method may be useful and scalable for algae harvesting only if problems in membrane blocking can be minimized or prevented [17; 18]. The air flotation (or inverted sedimentation) harvesting process is based on the generation of up-rising gas bubbles that bind to algal cells and induce their flotation to the liquid surface [19]. However, due to differences in the surface hydrophobicity of algal cells, harvesting efficiency varies greatly depending on the species of algae [20]. It should also be noted that the high operation cost for producing small air bubbles can limit large-scale commercialization. Flocculation refers to the aggregation of unstable and small particles through surface charge neutralization, electrostatic patching and/or bridging after addition of flocculants. Flocs formation allows for separation (or recovery) by simple gravity-induced settling or any other conventional separation method [21; 22]. The flocculation process is simple and efficient, and has been extensively investigated as a promising strategy for harvesting various algal species [9; 23]. Figure 1 shows the flocculation harvesting process of algal cells for algal biorefinery.