Effect of hydrodynamics on autoflocculation and gravity sedimentation of Chlorella vulgaris

Abstract 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 ɛ of 0.45 × 10−3 m2/s3, the average floc size and settling velocity achieved their maximum at 373.5 ± 36.4 μm and 2.17 ± 0.29 m/h respectively. However, after shear strength exceeded 0.0115 Nm−2 and energy dissipation efficient exceeded 1.25 × 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.