# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 4 Blocking and Blocking Mitigation in Submerged HF Modules  
Blocking, clogging, or sludging in submerged HF modules obstructs local flows in the fiber bundle and results in an uneven flow distribution; this is detrimental to the performance and can promote membrane fouling [58,107]. It should be noted that this section is targeted at the obstruction in the module rather than membrane fouling. The root cause has been attributed to the accumulation of solids, and the growth and merger of cake layers formed on the individual fibers in the module. In such cases it has been shown that the overall performance of a module packed with HFs is worse than that of a single HF [108]. This aforementioned blocking is distinct from two other forms of blocking that can occur in submerged HF modules: (i) the term 'blocking' can also refer to that of the aerators especially in MBR systems [104]; (ii) it also can refer to the blocking or closure of the pores of the HFs. Both of the latter forms of blocking are distinct from the blocking that occurs due to solids accumulation within the spaces between the fibers in the membrane module that is the focus here. The hydrodynamics within the module represents the key factor influencing the blocking phenomenon [107,109]. Stagnant regions caused by poor local flow lead to lower shear on the fibers, which in turn results in cake buildup and eventually local blocking [110]. The packing of the fibers within a module has been observed to be very different axially [111], which causes some non-uniformity in the flow. Due to the complexity of the hydrodynamics in the module [111, 112, 113, 114, 115, 116], the buildup of cake deposits is not likely to be uniform either among the fibers or along the fiber surface. The misdistribution of fouling deposits, which is acknowledged to be a direct function of the non-uniformity of the flow within the HF module [117, 118, 119, 120, 121, 122], can result in large variations in the performance of fibers at the same position in the module and in poorer performance of the fibers in the middle of the module [108]. The adherence of fibers to one another is traced to the buildup and eventual merging of cake layers, which in turn causes the fibers to foul more rapidly due to hindered local flow; hence, in time it results in blocking. It has been observed that some fibers tend to be held tightly together by the dense cake layers, while other fibers remain freely suspended [108]. The principal difference between constant-pressure and constant-flux operation is that the former is self-limiting whereas the latter is self-accelerating when fouling or blocking occurs [110]. The self-acceleration in the commonly used constant-flux operation is because (i) incipient blocking will reduce the local flow that thereby enhances the deposition [108], and (ii) the flux decline in some fibers has to be matched by a flux increase in other fibers to maintain a constant net flux that thereby accelerates blocking. Either local or global non-uniformities caused by the operating conditions or design parameters [104,109], including high packing densities, low cross-flow velocities, high feed concentrations, high TMP, or lack of means to promote unsteady-state shear [123] (e.g., bubbling [14]), contribute to a greater tendency for blocking. The control of fouling and blocking in practice is primarily through employing some or all of five strategies [109]: (i) pretreatment of the feed, (ii) physical or chemical cleaning protocols, (iii) flux reduction, (iv) aeration enhancement, and (v) chemical or biochemical modification of the feed. For (i), it is widely acknowledged that upgrading the pretreatment, in particular the screening, is pivotal to the successful retrofitting of an ASP (activated sludge process) or SBR (sequencing batch reactor) with an MBR [109]. Hair, rags, and other debris tend to aggregate at the top of the submerged HF module and become entwined with the filaments, thereby preventing their effective removal by backwashing [124].