Study on the kinetic mechanism of hydrodynamically enhanced modified clay removal of Aphanizomenon flos-aquae: From laboratory to field engineering applications

Modified clay is considered to be the most direct and effective method for emergency mitigation of harmful algal blooms (HABs), and it has been widely applied. However, its application on Aphanizomenon flos-aquae blooms has not been reported. Additionally, the hydraulic characteristics of modified clay in settling algal cells and the mechanism of its influence by flow fields have not been well studied, which is detrimental to the effective application of modified clay in alleviating HABs. Therefore, this study investigates the feasibility of using modified clay to alleviate Aphanizomenon flos-aquae blooms through laboratory-scale experiments and field engineering applications, and analyzes the keinetic mechanism of algal cell settling by modified clay based on flow field characteristics. The results indicate that PAC-modified clay can effectively alleviate Aphanizomenon flos-aquae blooms, achieving a removal efficiency of over 90 %. Differences in mineral composition, particle size, mineral-to-PAC ratio, and hydraulic shear significantly affect the removal performance of algal cells. The flow field characteristics of the water not only affect the removal efficiency of algal cells, but also influence the settling speed of the cells. By artificially increasing hydraulic shear and enhancing the distribution of velocity and turbulence kinetic energy in the water, the interaction between algal cell-coagulant-clay particle is promoted, resulting in the formation of larger and denser flocs, yielding faster settling performance. In the practical engineering application at Changshuiyan Reservoir, the use of boat propellers to agitate the water also verified this reinforcing effect. Aphanizomenon flos-aquae blooms were effectively alleviated, and transparency increased from the initial 0.5 m to 5.5 m. This study, for the first time, combines flow field simulation of clay particle settling to analyze the kinetic mechanism of hydraulic shear in enhancing the efficiency of modified clay in alleviating HABs from a kinetic perspective.