# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 4 Blocking and Blocking Mitigation in Submerged HF Modules
Therefore, adequate removal of the solids before the submerged HF module is key to mitigating blocking. Methanogenic [125] and chlorinated [126] pretreatments have also been explored, as well as hybrid processes, both of which add an additional unit operation before the membrane module [127, 128, 129, 130]. With respect to (ii), physical cleaning tends to remove reversible fouling while chemical cleaning can remove some irreversible fouling. The primary physical cleaning parameters include duration, frequency, and backwash flux [131, 132, 88,107, 133, 107], while the type and concentration of reagents are the important parameters for effective chemical cleaning [133, 134, 135, 136, 124]. Regarding (iii), flux reduction is not as cost-effective as might be expected, but the attendant extension of membrane lifespan and smoother operation may counterbalancesome of the increased cost. Flux reduction is commonly implemented as a sustainable permeability operation (i.e., at a lower flux to maintain stable operation) or as an intermittent operation (i.e., at a higher operating flux with intermittent remedial measures such as relaxation and/or backwashing). As for (iv), aeration is acknowledged to be the main mixing mechanism [137; 138] that generates a large amount of momentum that in turn reduces dead-zones and short-circuiting [139]. More details on aeration are given in Section 6.2. In connection with (v), the nature of the biomass in MBRs can be partially controlled through the addition of coagulants or flocculants [140; 141; 142; 143; 144] and adsorbent agents (e.g., activated carbon [145; 146; 147; 148; 149; 150; 151]). In addition, the bioprocess parameters, such as the sludge retention time (SRT), influence the biomass and supernatant as well as the fouling [152]. A suitable mitigation means for blocking has to be determined for each MBR plant. For example, a Mitsubishi Rayon unit failed to resolve the blocking problem via measures such as overnight relaxation and intensive or regular chemical cleaning, but regular backwashing for 30 s during each permeate production cycle achieved a stable low permeation flux [104]. To limit blocking, the PURON(r) system is designed with free movement of the filaments at the top of the module. This allows for larger solids, such as hair and agglomerated cellulose fibers, to escape without causing clogging in this region. The fibers are reinforced by an inner braid, to withstand the lateral movement of the filaments that subjects them to mechanical stress [153]. A major difficulty in the mitigation of module blocking in submerged HFs is the lack of simple techniques for in situ monitoring of the phenomenon. One approach that has been developed [122] is based on the assumption that blocking within the module is initially localized, which would result in a localized drop in flux relative to the overall average flux. Using a simple array of flow detectors mounted in the permeate header, it is possible to measure local fluxes and identify fiber regions that become less productive. It was shown that shifts in the standard deviation of the local fluxes was much more sensitive to local blocking than shifts in the overall system TMP. In the aforementioned study, the flow detectors were based on constant temperature anemometry strips, but other low cost detectors could be used.