Mechanism of hydraulic shear generation in activated sludge reactors, its impact on sludge cultivation, and the evolution of quantitative expression methods

Hydraulic shear has a significant impact on the formation, morphology, and properties of flocs and granular sludge in activated sludge reactors, making it a key parameter in related research. This paper first analyzes the microscopic mechanisms of hydraulic shear generation from the perspective of gas‒liquid‒solid three-phase fluid dynamics in reactors, and then discusses the parameters related to physical structures and flow patterns that affect the magnitude of hydraulic shear. Subsequently, a detailed summary of current research on the influence of hydraulic shear on the morphology, strength, and properties of activated sludge aggregates during cultivation is provided below. The similarities and differences in these research results concerning the effects of hydraulic shear on activated sludge aggregates are analyzed. Finally, the paper reviews the entire developmental history of hydraulic shear control and quantitative expression methods in activated sludge reactors. The main methods currently available for the quantitative control of hydraulic shear in reactors have been discussed. In the aspect of hydraulic shear quantitative expression, it has progressed from early simple expression methods to macroscopic average hydraulic shear, and finally to the direct quantitative measurement techniques for hydraulic shear and microscopic local expression methods based on computational fluid dynamics simulations. As a whole, this review provides a comprehensive and in-depth analysis of the role of hydraulic shear in activated sludge reactors and describes a detailed evolution route of its quantitative expression method for hydraulic shear, which may serve as guidance for subsequent research.