## 2 Characteristics of Biofilm Growth in Porous Media  
The morphology and structure of biofilms as well as the factors influencing the structure of biofilms in fluid have been widely studied. The interaction of fluid hydrodynamics and mass transport contributes to the diverse heterogeneous structure of biofilms. Biofilm development is a dynamic process that involves the surface adsorption and proliferation of microbes, the secretion of EPS, and the partial desorption of biofilms into the fluid [2; 21; 22]. Under an environment that suffers from shear force, detachment of bacterial cells and EPS fragments occurs during each process. The processes are more complicated when biofilms develop on a surface in a porous aqueous environment due to the complexity of pore networks in porous media and the variety of pores sizes and shapes within porous media. The related processes of biofilm evolution in a pore are split into four main stages as shown in the schematic in Figure 1.  
There are common descriptions of characteristics for the biofilm development either in pure fluids or porous media. Microscopic observations have revealed that biofilms are heterogeneous and have structures such as pores, voids, and cavities [4; 10; 23; 24]. Therefore, biofilms can introduce an additional pore structure to porous media. We infer that this is one of the reasons for the lowest permeability in bioclogging porous system. As shown in Figure 2, the microscopic morphology and the adhesion of mature biofilm are affected by the substrate composition and flowing condition [24; 25; 26; 27; 28]. For example, the biofilms under high shear condition are ribbon-like, have stronger adhesion, and are more homogeneous with lower porosity [26; 27], whereas the biofilm under low shear stress is m  
Figure 1: Biofilm development processes in a pore. Researchers have found that biofilms formed under non-flow are significantly thicker than those formed under shear stress [30]. The porosity and density of biofilms are also affected by the concentration and loading rate of the substrate (Figure 2) [24; 25; 31]. Local detachment occurs when the substrates are almost completely consumed or the fluid shear stress is satisfied [21; 32; 33; 34; 35]. Furthermore, there are many gaps and flow channels in the biofilm matrix, so the flow in the matrix will form pressure or resistance which is responsible for the detachment of biofilms [29]. With microbe desorption into the fluid, the biomass will be redistributed. There are still some differences in the characteristics for biofilm growth in porous media. For instance, the biomass distribution is likely to be concentrated at sites of nutrient injection [9; 36; 37] and depends on the concentration of the inoculated microbe, nutrient injection rate, and pore distributions. The development of biofilms will clog the pore space and reduce the permeability of the porous media [17; 38]. The detached biofilm segments induced by shear stress may accelerate the clogging of the pore space. Furthermore, the thickness of the final biofilm was found to be directly influenced by the size of the pore space and has no obvious relationship with the pore structure [37]. In addition, it has been proved that the clogging of pore space will give rise to the local mass transport abnormity [39]. The fluid hydrodynamics and mass transport influence the structure of biofilms. The growth and rough surface of biofilms also lead to heterogeneity of the local fluid field, which results in distribution of local substrates concentration inhomogeneity [17; 40; 41; 42; 43; 44; 45; 46]. Therefore, the structure and distribution of biofilm in porous media is the net result of the interaction between biofilm accumulation and fluid hydrodynamics. Non-Invasive and In Situ Measurement of Mass Transport and Fluid Hydrodynamics of Bioclogging System in Porous Media  
Many analytical methods have been employed to identify the biofilm-related mechanisms (biomass formation and distribution, permeability reduction, substrate use, hydrodynamic dispersivity, etc.)