# Flocculation Harvesting Techniques for Microalgae: A Review
## 7 Conclusions and Future Perspectives  
The importance of microalgae research is increasing in parallel with increasing demands for food, animal feeds, pharmaceuticals, and biofuels. However, moving from lab-scale to commercial-scale applications still requires extensive developments for reliable, cheap, and eco-friendly algae cultivation and harvesting processes. Table 6 summarizes the mechanism and advantages/disadvantages of the five different flocculation methods mentioned above. The specific flocculation process should be carefully selected and optimized comprehensively in consideration of various key factors such as efficiency, environmental impact, operating cost, value-added utilization of whole biomass, characteristics of algal species, and culture conditions. Along with process optimizations such as scale-up, detailed understanding of the genetic, biochemical, and physiological characteristics of auto-floculating algae is essential for development of fast and efficient auto-floculation harvesting processes. Genetic and metabolic engineering of algal cells can be considered to improve their auto-floculation capability without adverse effects on growth or target metabolites (e.g., lipid and carbohydrate). The expression and temporal regulation of specific cell wall protein(s) causing cell aggregation in response to measurable environmental triggers (e.g., pH) and/or after reaching the desired growth stage, could also be important targets for further investigation. Utilization of other microorganisms for algae harvesting (called bio-floculation) can be considered to be one of the most promising and potentially sustainable approaches. Use of fungi, yeasts, bacteria, and/or self-floculating algal species to harvest target non-floculating algal species has been reported to have synergistic effects on overall biomass and lipid productivity. More studies are needed to elucidate the mechanism of microbial floc (or pellet) formation along with continued screening for efficient bio-floculants from natural environments. The biological flocculation process should be further optimized to both maximize harvesting efficiency and reduce operation cost. Potential contamination of the bio-flocculant on the algal product(s) and the aquatic environment should be carefully considered for large-scale applications. Albeit competitive in terms of cost and performance, the use of conventional inorganic chemical flocculants has not yet been recommended for food, feed or pharmaceutical applications. However, it might be considered for large scale applications such as biofuel and wastewater treatment if the adverse impacts of the applied chemical flocculants on the final products and water resources are avoided or minimized. The discovery of new organic flocculants (e.g., from natural sources) with low cost, high efficiency, and less toxicity should also be a focus of future research. Recently, algae harvesting using various functional nano/micro particles based on AC and magnetic materials has been intensively investigated. However, more studies are needed to reduce the high manufacturing costs of particle-based flocculants. In addition, to facilitate effective recycling, the design and engineering of flocculants with high-efficiency and multi-functionality for post-harvesting processes, such as lipid extraction and fuel conversion, can be the basis for overcoming the high cost of artificial particle use. For practical application of electrochemical flocculation processes, extensive studies should be carried out to develop optimal electrodes with efficient, non-fouling, and less-toxic properties. Scale-up and process optimization for reducing both equipment and electric costs should also be investigated further. Finally, synergistic combinations of different flocculation techniques could provide a powerful means of maximizing biomass recovery for large-scale algae-based process while minimizing operating costs and environmental impacts.