Effect of hydrodynamics on autoflocculation and gravity sedimentation of Chlorella vulgaris

Autoflocculation followed by gravity sedimentation is a promising method for harvesting of microalgae. However, the effect of hydrodynamics on spontaneous flocculation of Chlorella vulgaris is yet uncertain. In this study, where quantitative analysis was conducted to study the effect of hydrodynamics on autoflocculation of Chlorella vulgaris, it was found that hydrodynamic turbulence enhanced autoflocculation efficiency by 40-53.3%. Autoflocculation efficiency increased first and then decreased with the increase of turbulence strength. With shear strength of 0.0069 N/m2 and energy dissipation efficient epsilon of 0.45 x 10-3 m2/s3, the average floc size and settling velocity achieved their maximum at 373.5 +/- 36.4 mu m and 2.17 +/- 0.29 m/h respectively. However, after shear strength exceeded 0.0115 Nm- 2 and energy dissipation efficient exceeded 1.25 x 10-3 m2/s3, the Chlorella vulgaris flocs started to disaggregate under the shearing effect. Hydrodynamic turbulence not only increased the probability of contact between Chlorella vulgaris cells, but also the enhanced the accumulation of EPS, which serves as bridge for autoflocculation after binding with the bivalent cation presented in the aqueous medium. Under hydrodynamic conditions, Chlorella vulgaris flocs start to settle by gravity, with separation efficiency reaching up to 94.5 +/- 4.5%. Therefore, hydrodynamic control presents a new approach for low cost harvesting of algae products.

Automated summary

Hydrodynamic turbulence enhances the efficiency of autoflocculation in Chlorella vulgaris by increasing the probability of cell contact and accumulation of EPS. However, excessive shear strength and energy dissipation efficiency can lead to the disaggregation of algal flocs. This study demonstrates that hydrodynamic control can be a cost-effective approach for harvesting algae products with high separation efficiency.