# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 2 Submerged Membrane-Filtration Applications and Benefits
### Membrane bioreactors (MBRs)  
Membrane bioreactor (MBR) technology, which combines conventional activated sludge treatment with low-pressure membrane filtration, is widely used for the treatment of wastewater [18; 19; 58]. The considerable growth of MBR is driven by the high quality of the water produced, increased water scarcity, and decreasing specific energy requirements [28; 59]. The anaerobic membrane bioreactor  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Application**} & \multirow{2}{*}{**Operation Mode**} & \multicolumn{2}{c}{**Intermittent**} & \multicolumn{1}{c}{**Is Bubbling**} & \multirow{2}{*}{**Advantages**} \\  & & **Fouling Control** & & **Implemented?** & \\ \hline \multirow{2}{*}{\begin{tabular}{c} **Surface-water** \\ **treatment** \\ \end{tabular} } & Dead-end with intermittent & Backwashing, relaxation, chemical & With or without & Less chemical requirements; \\  & & & & bubbling during & consistent quality of the \\  & & & & & filtrate [35; 36; 37] \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} **Pretreatment of** \\ **RO (reverse** \\ **osmosis**) \\ \end{tabular} } & Dead-end with & Backwashing, relaxation, chemical & With or without & Improved water quality, smaller \\  & & & & & footprint; less chemical \\  & & & & bubbling during & requirements; consistent quality \\  & & & & & of the filtrate; lowered energy \\  & & & & & cost for RO plants [38; 39; 40; 41] \\ \hline \multirow{2}{*}{
\begin{tabular}{c} **Membrane** \\ **bioreactors** \\ **(MBRs)** \\ \end{tabular} } & Cross-flow with tangential shear & Continuous bubbling, sometimes backwash & Continuous & Small footprint; complete & solid-liquid separation; high \\  & & and relaxation & bubbling & volumetric organic removal rate; \\  & & & higher effluent quality [19; 42; 20] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Submerged membrane-filtration applications and benefits.  
(AnMBR), a combination of an anaerobic bioreactor and membrane filtration, also is a promising option for anaerobic treatment of wastewater [60; 61; 62; 63].  
A small footprint, complete solid-liquid separation, high volumetric organic removal rate, and higher effluent quality are some of the key advantages of the MBR and AnMBR [64; 20; 65; 66]. In the submerged hollow fiber MBR, the membranes are directly immersed in the aeration tank. The results of bench-scale experiments, as well as many industrial and municipal operations, demonstrate high treatment efficiencies for chemical oxygen demand (COD), total suspended solids (TSS), and turbidity [67; 68; 19; 42; 63]. Although the MBR technology has been applied in many full-scale plants worldwide for treating municipal and industrial wastewater, membrane fouling and correspondingly increased energy consumption remain chief obstacles, as highlighted in a recent review [28]. Specifically, because membrane fouling diminishes productivity, fouling mitigation measures such as air scouring and frequent cleaning of the membrane are needed to restore the membrane permeability, which increases the energy requirement; furthermore, frequent cleaning shortens the membrane lifespan and results in higher membrane replacement costs. Aeration, bubbling, or gas sparging are the most common methods for mitigating membrane fouling; the important features of the interaction of bubbles with submerged hollow fibers (HF) are discussed in Section 5.2.