Coagulation and dissolved air flotation as a harvesting method for microalgae cultivated in wastewater

Microalgae harvesting is the major bottleneck for biomass production on a large-scale. In this study, coagulation followed by dissolved air flotation (DAF) was used as a harvesting method for Chlorella sorokiniana cultivated in wastewater. The harvesting performance was evaluated using four coagulants (aluminum sulphate, ferric chloride, Tanfloc SG and Zetag 8185) usually used in water or wastewater treatment. All coagulants tested showed high flotation efficiency at different conditions of dosage, pH and flotation velocity. The optimal doses were 10 mg L-1 Zetag 8185; 75 mg L-1 Tanfloc SG; 500 mg L-1 Al-2(SO4)(3), and 1000 mg L-1 FeCl3 at pH 7, which showed the maximum efficiency of 98.4, 94.5, 95.4, 96.7% at 8 cm.min(-1), respectively. Moreover, the photobioreactor effluent quality significantly improved after harvesting tests using the optimal doses. High removal was reached for turbidity (93.7-96.2 %), apparent color (91.7-92.3 %), TKN (90.2-92.6%), total phosphorus (89.4-90.9 %) and TSS (88.6-92.5 %). The use of metal salts requires caution due to the high residual concentration of iron and aluminum in the final effluent.