Flocculation properties of eight microalgae induced by aluminum chloride, chitosan, amphoteric polyacrylamide, and alkaline: Life-cycle assessment for screening species and harvesting methods

The flocculation performances of AlCl3, chitosan, amphoteric polyacrylamide (acrylamide-methacrylic acid ester-acrylic acid copolymer [ACPAM]), and NaOH for eight microalga species were investigated. AlCl3 had wide application for both freshwater and marine microalgae while NaOH could only flocculate marine microalgae. By contrast, chitosan was very effective to freshwater species with low dosage demand; ACPAM was only valid for three freshwater species. The settling times, concentration factors (CF), and capillary suction times (CST) were measured. The settling times with ACPAM (0.5-0.6 min) were the shortest while flocculation by NaOH had the longest settling times (13-40 min). The CF determined the volume of microalgae slurry, which in turn affected dewatering. The CST was used to evaluate polyacrylamide (PAM) demand in dewatering (filter press) using a model developed in this study. Life-cycle assessment identified the optimal harvesting methods for the eight species using data from this study and the literature. The analyses revealed that chitosan flocculation + filter press was the best approach for five species and AlCl3 flocculation + filter press or two-step centrifugation was optimal for the remaining three species. Coccomyxa sp. K.J., Chlorella vulgaris, and Tetradesmus obliquus were screened for biodiesel production in consideration of the environmental impact and their lipid contents.