# The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review
## 1 Introduction  
Low-pressure membrane processes, such as microfiltration (MF) and ultrafiltration (UF), are popular technologies in the water industry due to their proven efficiency in removing particles, colloids, and high molecular weight organics [1; 2]. MF and UF membranes either are contained in a closed pressurized module or incorporated in an uncontrained module that is submerged (immersed) in a tank. In submerged systems, the feed enters the tank at atmospheric pressure and the permeate is removed by applying suction on the permeate side of the membrane, which limits the transmembrane pressure (TMP) to \(<\)1 atmosphere and more typically to \(<\)0.5 atmosphere. The submerged concept is now well-established in the water industry, with applications in surface-water treatment, pretreatment prior to reverse osmosis (RO) in desalination and water reclamation, and membrane bioreactors(MBRs) [3]. Submerged membranes are either in a flat sheet (FS) format (vertically aligned) or hollow fiber (HF) form either horizontally aligned or more typically vertically aligned, often with suction applied at both ends. Submerged FS membranes are used only in MBRs, whereas submerged HFs cover a range of applications and consequently are much more common. The submerged HF concept has the advantages of a higher packing density, the ability to induce movement by mechanisms such as bubbling, and the feasibility of backwashing [4; 5; 6; 7; 8]. The focus of this review is the submerged HF concept. Submerged membrane systems have to deal with fouling, which represents a major drawback that restricts the application of membrane processes in the water industry [8]. In general, fouling is a widespread and costly problem that affects membrane performance and is a complex function of the feed characteristics, membrane properties, and operating conditions [9; 10; 11]. Fouling mechanisms include physical and chemical adsorption, precipitation of sparingly soluble salts, the growth of biofilms, and the deposition of suspended matter onto or into the membrane [12]. Inadequate pretreatment, poor fluid management (process hydrodynamics), extreme operating conditions, and improper membrane selection are factors that exacerbate fouling [13; 14]. The key parameters influencing fouling deposition in submerged HF membranes are the membrane characteristics (e.g., membrane material, the structure of membranes/fibers, fiber diameter, length, and tautness), feed properties (e.g., foulant characteristics, concentration, viscosity), operating conditions (e.g., temperature, flux), and hydrodynamic conditions (e.g., surface shear, air flowrate) [15; 16]; these parameters are discussed in detail in this review. For feeds with a high solids concentration, such as membrane bioreactors (MBRs), cross-flow operation is required for the constant application of surface shear to mitigate concentration polarization and fouling deposition [14; 17]. A common practice for submerged membranes in MBRs is two-phase bubbly flow [18; 19; 20]; other approaches could include mechanical vibrations [21; 22; 23] and particle fluidization [24], all of which are discussed in this review. An important development that coincided with the introduction of submerged HFs was the realization that dead-end filtration was attractive for dilute feeds (surface waters, RO pretreatment). In this case the filtration is operated in cycles, with the dead-end forward flux interrupted by backwashing and periodic surface flushing [25] or relaxation [26]. The submerged HF module makes backwashing feasible for polymeric membranes. As HF membrane performance continues to improve, submerged HF systems are increasingly becoming more attractive for water treatment, particularly in membrane bioreactor (MBR) applications [27; 28]. In spite of the importance of submerged HF processes and the extensive research literature on advancing such systems, a comprehensive review summarizing the current state-of-the-art systems with respect to performance and fouling control remains a gap in the literature.