The biofilm detachment modes and mechanisms in porous media

Biofilms are widely present in natural and engineered porous media, playing a crucial role in pollution control, resource recovery, and industrial applications; in which, the biofilm detachment induces the dynamic changes in flow paths within porous media, and significantly impact the permeability and hydrodynamic properties of porous media. In this review paper, we first introduce two types of biofilm detachment based on the detaching position: internal detachment occurs within the biofilm far from the flow path, creating a new flow path; and external detachment occurs outside the biofilm near the flow path, causing the flow path to widen. These two types of detachment work together in porous media, often leading to increased permeability and reorganization of flow paths. Then we systematically summarize two kinds of stresses influencing the biofilm detachment: shear stress generated by fluid flow on the biofilm, and cohesive strength inside the biofilm. Shear stress significantly influences biofilm dynamics, including growth, structure, and detachment, and is influenced by the pore structure characteristics of the medium and the physical properties of the biofilm. Biofilm cohesive strength, crucial for stability and adhesion, increases with depth and is influenced by EPS content and composition, pH value, and the presence of metal ions. The interplay between shear stress and cohesive strength determines biofilm stability and detachment, affects biofilm performance in various applications. Therefore, understanding biofilm detachment mechanism can better control and optimize physical and chemical properties of the porous media.