# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 8 Submerged HF Membrane Integrity and Failure
### Module Design  
The optimization of membrane-module designs in terms of the potting of the membrane fibers and the design of the membrane housing is usually performed to reduce membrane fouling and to maintain membrane integrity. In submerged HF modules, the fibers are located in a constrained geometry. For example, the GE-Zenon system has membranes assembled into cartridges and held in a supporting frame that connects to the aeration and permeate suction header. The ends of the fibers are potted into the permeate carrier. These features are typical of most submerged HF modules. More details can be found elsewhere [3; 58].  
The potting efficacy of the fibers can significantly affect the performance and integrity of the module. Current membrane modules consist of up to 20,000 hollow fibers held together with either an epoxy or urethane resin; depending on the manufacturing process, the resin can be cured under static or dynamic conditions. In the slower static curing method the resin is allowed to cure without heat or external forces acting on it. Membranes that are potted statically would have the resin wick up the edge of the fiber due to capillary forces, which leads to the development of a sharp edge that potentially can cause fiber breakage (see Figure 9a). As seen in Figure 9b, with this method an elastomer overlay is usually added on top of the potting material to minimize the sharp edges. On the other hand, for the dynamic curing method, centrifugal forces are used under elevated temperatures to cure the potting resin, thereby preventing resin wicking and avoiding the development of sharp edges [313] (see Figure 9c). Notably, with the implementation of air-scouring and external loads, the probability of fiber breakage at the potting site increases; thus, proper selection of the potting method and material is important. Moreover, an optimized arrangement of the fibers within the module has been reported to significantly improve process performance by as much as 200% [119; 190].  
The design of the membrane module and housing plays an equally important role in maintaining membrane integrity. Although there are many membrane housing designs that try to minimize  
Figure 9: Membrane potting methods employing different conditions: (**a**) Static Conditions, (**b**) Static with Elastomer Overlay, (**c**) Dynamic Conditions.  
excessive fiber movement, routine membrane filtration and backwashing carried out at pressures higher than the manufacturer's recommendations can lead to membrane failure via the rupturing of membrane fibers, damage to the membrane module housing, and degradation of the membrane module seals. Consequently, the tolerable stress load on a membrane module depends on the membrane material and structure, and the packing density of the HF membranes in the module. Pilot plant testing indicated that membrane symmetry, which affects the stress at the juncture between the potting material and the fiber, was more important than the potting technique for hollow fiber integrity [313]. At present there is a lack of fundamental data on the stresses experienced by fibers during the filtration and cleaning cycles in the presence of air scouring [320]. This, coupled with the difficulty in accurately measuring and calculating stresses in a multiple fiber system, has significantly limited the development of improved membrane modules. Therefore, for improved membrane-module, design a need exists for both a better understanding of membrane potting methods and the stress-strain forces acting on the membrane fibers.