# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 6 Techniques for Fouling Control in Dead-End Submerged Membrane Systems
### Relaxation  
Relaxation, the intermittent cessation of permeation, has been incorporated in many membrane bioreactor (MBR) designs and some other submerged HF systems as a standard operating protocol to control membrane fouling [93; 94; 288]. For example, the investigation of an MBR found that relaxation was still beneficial even when the relaxation necessitated periods of higher flux to give the same production of permeate [288]. Different relaxation conditions resulted in distinctly different temporal TMP profiles, but all the runs that incorporated relaxation displayed a lower final TMP than the continuous mode. Relaxation was found to be more favorable than backwashing for this MBR application, because, while performances were similar, backwashing may have resulted in membrane pore clogging [288].  
The protocol for intermittent filtration/relaxation can be optimized in terms of the ratio of the durations in each cycle (ratios between 0.5-50 were tested) to be more beneficial for fouling removal [94; 296], but were not necessarily beneficial for retarding the TMP increase as filtration progressed. This suggests that the relaxation duration and interval should be carefully managed to achieve the best outcome in terms of reducing the fouling resistance during relaxation and retarding the TMP increase during filtration.  
[PERSON] et al. [26] investigated the effect of relaxation on the performance of dead-end (i.e., without bubbling) filtration using membranes with two different porosities and a seawater feed. Relaxation was confirmed to limit membrane fouling compared to continuous filtration, but was more effective for the membrane with a higher porosity. The difference in results due to porosity was hypothesized to be that, although the relaxation removed part of the foulant cake for membranes with lower porosities, when the filtration flux resumed the cake was reorganized to a more compact structure. Table 2 lists fouling control methods for submerged HF systems.  
\begin{table}
**Operation Mode** & **Fouling Control Technique** & **Important Parameters** & **Applications** & **Benefits** \\ \hline \multirow{4}{*}{Cross-flow} & \multirow{4}{*}{Bubbling} & Bubble characteristics, gas & \multirow{4}{*}{MBR, AnMBR} & Unsteady or transient shear stress; \\  & & flowrate, bubbling modes & (intermittent or continuous) & \\ \cline{2-4}  & & Vibration amplitude & AnMBR, MF, UF, & Low energy cost; surface shear; \\  & & and frequency & MD, FO & effective cake removal; facilitates \\ \cline{2-4}  & & & & & separation of macromolecules \\ \cline{2-4}  & & & & & \\ \cline{2-4}  & & & & & \\ \cline{2-4}  & & & & & \\ \cline{2-4}  & & & & & \\ \cline{2-4}  & & & & & \\ \hline \hline \end{tabular} \
\begin{tabular}{c c c} \hline \hline \multirow{4}{*}{Dead-end} & Backwashing & Backwash flux, backwash duration, backwash frequency & All HF systems & Internal fouling control; can be applied with air scouring \\ \cline{2-4}  & & Relaxation duration, & All HF systems, & \\ \cline{2-4}  & & relaxation frequency & especially MBR & - \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{AnMBR: anaerobic membrane bioreactor; MF: microfiltration; UF: ultrafiltration; MD: membrane distillation; FO: forward osmosis; AFBR-AFMBR: anaerobic fluidized-bed bioreactor-anaerobic fluidized-bed membrane bioreactor; IAFMBR: integrated anaerobic fluidized-bed membrane bioreactor.} \\ \end{tabular}
\end{table}
Table 2: Fouling control methods in submerged HF systems [18; 297; 298; 299].