# 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
#### 5.3.1 Vibrations  
Vibrations have been proven to be an effective way to induce shear on a membrane surface and consequently reduce concentration polarization (CP) and fouling [123; 218; 219]. Various modes of vibration are applicable for different membrane systems. A submerged HF system can be vibrated longitudinally or axially (Figure 3a), transversely (Figure 3b), and rotationally (Figure 3c). Although anaerobic systems are gaining momentum in the wastewater industry due to their potential for energy production, bubbling by recycled biogas has some challenges; hence, the vibration approach is attractive for fouling mitigation in anaerobic MBR (AnMBR) systems. In particular, transverse vibration has been proven to be an effective way to mitigate the fouling in AnMBR applications [220; 221]. Many studies have been carried out to probe the effectiveness of the different modes of vibration (i.e., longitudinally, transversely, or rotational). [PERSON] et al. [222] found that the use of vibrations slowed the flux decline for the submerged HF system that they investigated, and that longitudinal oscillation outperforms rotational oscillation. [PERSON] et al. [223] showed that vibration was more effective for a bentonite suspension compared to a washed yeast solution that may cause internal fouling; hence, vibration is more effective primarily for cake removal but not for the mitigation of internal fouling, which was corroborated by [PERSON] et al. [220]. It was also shown that transverse vibration decreases the fouling rate much more effectively than longitudinal vibration [223]. [PERSON] et al. [224] reported that adding transverse vibration to longitudinal vibration (by using chess-patterned vanes) resulted in an almost doubling of the critical fluxes at the same frequency; adding coagulants further elevated the critical flux of the vibrating system, although floc breakup at higher frequencies (namely, 10 Hz) tended to reduce the critical flux. [PERSON] and [PERSON] [225] also found that vibration facilitates the separation of macromolecules (e.g., BSA) and larger components (e.g., yeast cells) at sub-critical fluxes by loosening and removing the built-up cake in the filtration of a mixed suspension for a membrane with pores larger than the macromolecular components. Fiber spacing and looseness were found to be important parameters to improve the benefits of vibration with respect to the turbulence kinetic energy and eddy length scale [226]. A recent study investigated rotating instead of vibrating the HF membranes and found membrane rotation to be more effective than gas scouring [227]. Figure 4 shows that both higher amplitudes and higher frequencies for longitudinal vibration contribute towards fouling mitigation, with an observed reduction in the fouling rate by as much as 90% [23]. [PERSON] et al. [224] found that the critical flux has a stronger dependency on frequency at higher frequencies but a weaker dependency at lower frequencies, presumably due to a change of the flow regime in the vibrating system. [PERSON] et al. [228] used high-frequency vibration in a pilot-scale submerged MBR and found it promising with respect to energy savings compared to conventional air-cleaning systems. The unsteady-state shear induced by vibration can be related to performance enhancement to provide a quantitative assessment of the beneficiation [123]. [PERSON] and coworkers [229; 230; 231] applied  
Figure 4: Effect of vibration amplitude and frequency on the fouling rate expressed as the time rate-of-change of the TMP (transmembrane pressure) for filtration of a 4 g/L bentonite suspension [23] for a constant flux of 30 LMH. Reproduced with permission from [23], Copyright Journal of membrane science, 2013. Figure 3: Schematic of an HF (hollow fiber) module with different modes of vibration: (**a**) longitudinal vibration; (**b**) transverse vibration; and (**c**) rotational vibration.