Mechanism of rapid settling algal flocs formation in a novel fluidized bed flocculator

Abstract

Coagulation-sedimentation has been widely used for the emergency removal of harmful algal blooms (HABs). The technical challenge of how to construct special structured flocs in a targeted manner to achieve ideal settling performance remains unsolved, especially for algae with gas vesicles, such as Microcystis. In this study, a novel fluidized bed flocculator (FBF) was proposed to construct nearly spherical and compact algal flocs with its multi-stage velocity gradient flow field characteristics, thus enhancing the continuous sedimentation performance of Microcystis flos-aquae flocs, compared with a conventional mechanical stirring flocculator (MSF). The results revealed that the harvesting efficiency in the FBF reached 93 % after 30 s of settling at a dose of 30 mg L−1 FeCl3 and 1 mg L−1 PAM, whereas the efficiency in the MSF was only 47 %. Although the flocs generated in the FBF and MSF were similar in size, the former exhibited a settling velocity of 22.4 m h−1, which was 2.7 times higher than that of the latter. This difference can be attributed to the significantly higher fractal dimension of the flocs in the FBF (1.80) compared to the MSF (1.62). The settling velocity of the flocs was determined by both the floc size and fractal dimension, and flocs with large, compact and spherical shape structure yielded faster settling performance than small, loose, and multi-branched floc. The velocity, turbulent kinetic energy, and turbulent kinetic energy dissipation rate in the FBF decreased uniformly and gradually along the direction of floc formation, resulting in the formation of compact and spherical flocs and consequently rapid settling performance. This study may provide a novel solution for the rapid sedimentation of microalgae and enhance our understanding of the flow field characteristics required for an effective flocculator.