Synergistic effect and mechanisms of compound bioflocculant and AlCl3 salts on enhancing Chlorella regularis harvesting

The high energy input required for harvesting microalgae means that commercial production of microalgal biodiesel is economically unfeasible. In this study, we investigated the flocculation efficiency and synergistic mechanisms of novel coupled flocculants, AlCl 3 and compound bioflocculants (CBF), to overcome this difficulty. AlCl 3 flocculation was found to be very sensitive to pH, and flocculation efficiency increased from 55 to 95 % when pH increased from 4 to 10. CBF was environmental friendly, less reliant on pH, but had a relatively low flocculation of 75 % in optimum conditions. The harvesting efficiency of Chlorella regularis can achieve a satisfactory level of 96.77 % even in neutral conditions, with a CBF dosage of 0.26 g/L, AlCl 3 dosage of 0.18 g/L, and coagulant aid (CaCl 2 ) dosage of 0.12 g/L. Interestingly, compared with the use of single flocculant, the dosage of CBF, AlCl 3 , and coagulant aid (CaCl 2 ) were reduced by about 52, 49, and 66 %, respectively. Besides, the aluminum (Al) ion content of the supernatant decreased significantly to a residue of only 0.03 mg/L, therefore meeting the downstream process needs easily. Patching and bridging played key roles in coupled flocculant flocculation, in which AlCl 3 mainly carried out the electrical neutralization. This work provides new insight into an efficient, economical, and environmentally friendly protocol for microalgae harvesting.