# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 5 Parameters Affecting the Performance of a Submerged Hollow Fiber System
### Shear Stress on Membrane Surface by Non-Bubbling Techniques
#### 5.3.2 Particle Scouring  
Particle scouring is one of several unsteady-state shear techniques useful in membrane processes for fouling mitigation and improving the mass-transfer coefficient [123]. In practice, the solid particles are brought into close contact with the membrane surface via fluidization, which is the process whereby the particles are dispersed and suspended by the liquid such that they behave like a fluid [234; 235]; thereby, the fouling layer on the membrane surface is mechanically scoured by the particles. A recent review assessed the mechanical cleaning concepts in membrane filtration [27]. [PERSON] et al. [204] concluded that particle fluidization is similar to cleaning via bubbling in terms of (i) the momentum of both bubbles and fluidized granular activated carbon (GAC) correlates more strongly with the critical flux, rather than to the velocities or concentrations; and (ii) an increase in energy input increases the critical flux. In contrast to bubbling, particle fluidization is different with respect to (i) the local critical flux decreasing instead of increasing with height; (ii) its optimization involves a complex interplay of particle size, concentration, and liquid flowrate, instead of simply involving increasing the gas flowrate. As early as the 1970s, the beneficial impact of the fluidization of particles for membrane processes was recognized as being attributable to both the mixing action of the particles to reduce the solute concentration gradient, and the mechanical action of the particles to both vibrate and clean the membrane surface [236; 237; 238; 239]. These studies predate the submerged HF. However, various types of inert solids subsequently have been shown to be beneficial for the mitigation of membrane fouling in submerged HF applications via the scouring mechanism [240; 241; 242; 243; 244], although negative effects such as the break-up of sludge flocs [240] and poor filterability of the activated sludge suspension [241] also have been noted. Another potential effect is membrane damage [238; 242], which necessitates careful selection of scouring conditions. The use of powdered activated carbon (PAC) is primarily targeted for mitigating organic accumulation, biological degradation, and reducing the cake-resistance (e.g., [245; 246; 247; 248; 249; 250; 251; 252; 253]), all of which contribute towards improving the permeate flux. Almost as an afterthought, PAC also was recognized as being beneficial for inducing fouling-mitigating shear on the membrane [254; 255; 256]. Because the increased inertia associated with larger-sized particles can lead to more effective scouring, granular activated carbon (GAC), whose mean diameter is an order-of-magnitude larger than that of PAC, has recently gained interest [24; 257; 258; 259; 260; 261; 262; 263; 264]. Although PAC is more effective than GAC in terms of adsorption capability [257], it has been claimed that GAC is more effective at the higher concentrations encountered in practice and in the longer term [24]. Several reports of the apparent success in the use of GAC in submerged HF membrane systems (namely, the fluidized-bed membrane reactor) for low-cost, sustainable operation have appeared in recent years; hence, a closer look is warranted. The first report was on the use of a two-stage AFBR-AFMBR (i.e., anaerobic fluidized-bed bioreactor-anaerobic fluidized-bed membrane bioreactor) for sustainable control of membrane fouling [24]. Extensive tests subsequently have been carried out [24; 258; 259; 260; 261; 262; 263; 264; 265; 266; 267; 268; 269; 270; 271; 272; 273; 274; 275; 276; 277; 278], especially in view of the potentially lower energy cost than that of bubbling [24; 204] and suitability for the anaerobic MBR.