# 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
[26] showed that increasing the backwash duration from 10 to 30 s led to the final TMP and fouling rate decreasing by more than 50% as well as a slight increase in the percentage of fouling removed by backwashing. However, a further increase in the duration beyond 30 s did not result in any additional improvement, but instead slightly reduced the percentage of fouling removed; this indicates that an excess backwash volume may cause membrane blockage or change the structure of the fouling cake due to impurities in the backwash flux. [PERSON] et al. [160] studied the effect of backwashing on the pore size of hollow fiber ultrafiltration membranes by using the evapoporometry [91; 293; 294] technique. They reported the following: (i) backwashing can enlarge the pores of a membrane with a greater effect on the larger pores for operation at the same TMP; (ii) pore enlargement due to backwashing was larger for amorphous (PVDF fibers) relative to glassy polymers (PAN fibers) due to the lower modulus-of-elasticity of the former; (iii) cyclic filtration and backwashing at constant flux could more effectively remove foulants both on and within the larger membrane pores compared to the small pores; and (iv) increasing the backwashing flux could remove foulants from smaller pores. Results for seawater showed that the lowest final TMP and the maximum percentage of fouling removed by backwashing after 16 h of filtration was for the case for which the backwash flux was 1.5 times the filtration flux [26], which suggests the existence of an optimum backwash flux for fouling mitigation. The observation that a further increase in the backwash flux to twice that of the filtration flux led to an increase in the final TMP and a reduction of foulant removal implies that backwashing changes the fouling rate during the filtration cycle. Similar to excessive backwash duration, it seems that an excessive backwash flux also causes convection of impurities to the membrane pores or a residual fouling layer that results in less reversible fouling and a higher fouling rate. The existence of an optimum backwash flux for fouling mitigation was also reported by [PERSON] et al. [41], who found that an increase in the backwash flowrate up to twice that of the permeate flowrate resulted in a process improvement, but no further benefits were observed for a further increase in the backwash flowrate. Compared to the duration or interval of backwashing, the effect of backwashing flux was found to be more significant for fouling mitigation [288]. It has been reported that air scouring during backwashing can assist fouling removal and improve backwash efficiency [289; 295]. While the backwashing is expected to detach the cake layer from the fibers, air scouring loosens the deposits and carries them from the membrane surface into the bulk fluid [289; 295]. The impact of aeration during backwashing on membrane fouling during seawater filtration was investigated by [PERSON] at al. [26]. Their results showed that backwashing with a moderate air flowrate had a lower final TMP and also slowed down the fouling rate during the filtration. However, a high air flowrate limited the benefits of air scouring and did not improve the reversibility.