Measured and predicted floc size of cohesive sediment in the presence of microalgae

In aquatic environments, biological factors significantly influence the flocculation process of cohesive sediments, thereby impacting sediment transport dynamics. Due to its complexity, the mechanism of biological flocculation still remains unknown. Here, we conducted laboratory experiments to investigate how living microalgae (Skeletonema costatum) affects the flocculation of mineral clay under various shear rates and suspended sediment concentrations (SSC) in saline water. The microalgae (Skeletonema costatum) and SSC both have positive influences on the increase in floc size. However, the shear rate (G) shows dual effect. Specifically, there exists a critical shear rate, G^*, at which the floc size increases with G when G≤G^* and decreases with G when G>G^*. More importantly, G^*is affected by SSC and exhibits no dependence on microalgae content. The microalgae (Skeletonema costatum) has a dominant effect on both floc shape and floc size of microalgae-mineral flocs compared to shear rate under the present experimental conditions (SSC: 700 mg/L, chlorophyll-a concentration: 0∼13.76 µg/L, shear rate: 10∼90 s⁻¹). Additionally, the elongated-rod flocs are more easily formed in microalgae-mineral clay suspensions, whereas the plate-stacked flocs are more abundant in pure mineral clay suspensions. The promoting effect of microalgae is obvious under low shear rate conditions (G≤40 s⁻¹), while at high shear rate (G>40 s⁻¹), this effect is significantly attenuated, with a reduction by nearly half. Finally, a new bioflocculation model was proposed to predict the equilibrium median floc size for both conditions with and without microalgae. The model was well validated through comparisons with laboratory measurements.