# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 6 Techniques for Fouling Control in Dead-End Submerged Membrane Systems
### Backwashing  
Backwashing is commonly practiced in most HF filtration systems to limit fouling in both dead-end and cross-flow applications. Typical TMP profiles for dead-end filtration with intermittent backwashing for cake removal are shown in Figure 7. Two modes of operation are usually practiced, either (i) using a fixed cycle time (\(t_{c}\)), whereby backwashing is implemented after a designated filtration time, thereby causing the maximum TMP to increase with each cycle if residual fouling occurs (Figure 7a), or (ii) operating to achieve a fixed TMP\({}_{\text{max}}\), whereby backwashing is implemented whenever the TMP reaches a predetermined value, thereby requiring the frequency of backwashing to increase with each cycle if residual fouling occurs (Figure 7b). Backwashing is usually effective in reducing the TMP, but some deposits tend to remain attached and contribute an additional residual resistance to the filtration in subsequent cycles. Therefore, other than backwashing alone, cycling between backwashing and chemical cleaning (Section 8) is also a common practice to reduce the minimum TMP (TMP\({}_{\text{min}}\)) attainable. Figure 8 illustrates a typical TMP profile with intermittent backwashing and chemical cleaning with a fixed cycle time. Although backwashing loosens and detaches the fouling cake from the membrane surface so that the foulants can be removed easily by cross-flow or air bubbles [6; 25; 90; 93; 288; 289], some drawbacks  
Figure 8: Typical profile of the TMP as a function of time showing the effect of intermittent backwashing and chemical cleaning for a fixed cycle time. Figure 7: Typical TMP profiles with intermittent backwashing; (**a**) operation to achieve a fixed TMP\({}_{\text{max}}\); (**b**) operation for a fixed cycle time. also exist. In cases for which the cake layer serves as a secondary layer to protect the membrane from internal fouling by macromolecular components, over-frequent backwashing can provide more opportunity for macromolecules to enter the membrane pores [290] or change the chemical composition and/or structure of the fouling layer (e.g., from a mixed cake layer of particulates and macromolecules to a fouling structure dominated by the macromolecules after several filtration/cleaning cycles [291]) and consequently the fouling patterns [26]. Generally, the first few cycles of backwashing lead to more significant irreversible fouling, after which the percentage of irreversible fouling with respect to total fouling becomes constant. The reason for the augmented vulnerability to irreversible fouling of new membranes relative to a used membrane is the greater probability of blocking the larger pores in the pore-size distribution, which can be the dominant fouling mechanism in the first few cycles [26; 90; 91; 292]. Overall, an increased backwashing flux was found to be slightly more effective than increased backwash duration when the same amount of backwash volume was used [26; 91; 93]. Similarly, [PERSON] et al. [90] reported that excessive backwash duration and strength resulted in permeate loss, severe pore blocking, and high specific energy consumption. [PERSON] et al. [26] investigated the effect of filtration duration (from 1200 to 5400 s per cycle) on membrane fouling during real seawater filtration while the other operating parameters were kept constant. It was found that the final TMP after 16 h of filtration and the percentage of reversible fouling that can be removed by backwashing did not increase when the filtration duration increased from 1200 to 3600 s, while a further increase in the filtration duration from 3600 to 5400 s promoted membrane fouling due to a more compact cake layer that was more irreversible. [PERSON] et al. [41] reported that, for a pilot-scale pressurized HF module, prolonging the duration of backwashing was found to be more effective than air scouring in controlling membrane plugging. Studies by [PERSON] et al.