Bio-Flocculation of Microalgae: Status and Prospects

Background: Potential of algal biomass for fuels, food, feed, and value-added chemicals through autotrophic and heterotrophic metabolic pathways has long been recognized. Existing downstream processing techniques, however, are not mature enough to overcome either techno-economic barriers or sustainability concerns, among which, energy-intensive microalgae harvesting is one of the biggest challenges. Biological microalgae harvesting has been demonstrated as the most promising technology in the near future that are economically viable and environmentally friendly at the same time. Methods: This paper critically reviews the current status and prospects of bioflocculation for microalgae harvesting. Approaches for microalgae bio-flocculation are summarized, factors that influence bio-flocculation of microalgae are indicated, and strategies for scaled-up applications are also discussed. Results: Microalgae bio-flocculation approaches include auto-flocculating algae-based, bacteria-based, filamentous fungibased, and plants extracts-based bio-flocculation. Type of flocculating microorganism, strain of target microalgae, and various co-culturing conditions could all influence microalgal bio-flocculation efficiency. Utilization of the bioflocculated algal biomass for fuel, food, feed, valued-added products, and for wastewater treatment are the two major strategies for scaled-up application of microalgae bio-flocculation in the future. Conclusion: Bio-flocculation of microalgae is a promising low-cost algae harvesting technology with unique advantages over traditional harvesting methods. Future studies should not only focus on improving bio-flocculation efficiencies under specific conditions, but should also investigate strategies for scaled-up applications. More attention should be paid on the disposal and utilization of the harvested mixture of algal and flocculating microbial biomass.