Comparison of flocculation and cell properties for 10 microalgae species using ferric chloride, chitosan and alkaline flocculation

Flocculation holds great potential as a low-cost harvesting method for microalgae biomass production. However in most studies the flocculation potential has been tested only for a few model species so far, which makes comparison between studies very difficult. In this study, three commonly used flocculation methods (ferric chloride, chitosan, and alkaline flocculation) were evaluated for the harvesting of 10 different freshwater and marine microalgae species. Cell properties such as cell size, cell volume, surface to volume ratio, medium conductivity, and zeta potential were measured and linked to the flocculation behaviour. Flocculation using ferric chloride resulted in a separation efficiency greater than 90% with a concentration factor higher than 10 for all species. Alkaline flocculation was expected to perform better in marine medium than in freshwater because of the higher magnesium concentrations, but our results did not confirm this trend, suggesting that cell properties play an important role as well. Chitosan flocculation worked generally very well in freshwater microalgae, but not for marine species. Moreover, the concentration factor was highly variable between microalgae species. Generally, high variation in optimal dosages was noted between different species, which shows that flocculation may be a good harvesting method for some but not for others. Secondly, if one flocculation method was efficient, other methods worked as well, especially for freshwater species. This study shows that microalgae should not only be selected based on productivity but also based on their potential for low-cost harvesting.status: publishe