Sedimentation and Rheological Study of Microalgal Cell (Chlorella sp. HS2) Suspension

Microalgae (Chlorella sp. HS2) have a high potential as a new biomass filler resource. Microalgae suspension is investigated depending on pH condition, focusing on microscopic sedimentation and a rheological behavior in order to understand in-depth the behavior of Chlorella sp. HS2 for harvesting process design. In terms of sedimentation analysis, it is found that Chlorella sp. HS2 cells settle down due to high density of 1.56 gcm−3. Meanwhile due to its small size and dilute concentration, the settling velocity is too slow for harvesting by natural sedimentation. Chlorella sp. HS2 cells undergo weak aggregation in the medium depending on pH condition. When the Chlorella sp. HS2 suspension (pH 5.4) is adjusted at pH 2.5, the surfaces of the microalgal cells turn neutral and cells are aggregated by van der Waals force between cells, leading to relatively faster sedimentation compared to Chlorella sp. HS2 cells without pH adjustment. The aggregation of Chlorella sp. HS2 cells depending on pH condition is reflected in rheological properties of the suspension. At pH 2.5, shear viscosity of the Chlorella sp. HS2 suspension increases and the suspension shows shear thinning behavior, meaning that the neutralized surface of Chlorella sp. HS2 makes cells aggregation. However, the aggregation of microalgal HS2 cells is easily dissociated and aligned along shear flow. Therefore, for the successful harvesting of biomass Chlorella sp. HS2, the flow and colloidal condition must be considered along with coagulation for rapid harvesting of cells.