Flocculation properties of several microalgae and a cyanobacterium species during ferric chloride, chitosan and alkaline flocculation

•High variation in optimal dosages between species and flocculation methods.•Chitosan was ineffective for harvesting marine species.•Species selection for low-cost separation is important. Flocculation holds great potential as a low-cost harvesting method for microalgae biomass production. Three flocculation methods (ferric chloride, chitosan, and alkaline flocculation) were compared in this study for the harvesting of 9 different freshwater and marine microalgae and one cyanobacterium species. Ferric chloride resulted in a separation efficiency greater than 90% with a concentration factor (CF) higher than 10 for all species. Chitosan flocculation worked generally very well for freshwater microalgae, but not for marine species. Alkaline flocculation was most efficient for harvesting of Nannochloropsis, Chlamydomonas and Chlorella sp. The concentration factor was highly variable between microalgae species. Generally, minimum flocculant dosages were highly variable across species, which shows that flocculation may be a good harvesting method for some species but not for others. This study shows that microalgae and cyanobacteria species should not be selected solely based on their productivity but also on their potential for low-cost separation.