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Over the past two decades, the membrane bioreactor has emerged as one of the innovative technologies in advanced wastewater treatment. MBRs are being increasingly used for wastewater treatment that requires excellent effluent quality, e.g., water reuse or water recycling (Judd, 2008; Wang et al., 2008). Membrane bioreactor (MBR) technology combines the biological process by activated sludge and membrane filtration with a direct solid-liquid separation. With micro or ultrafiltration membrane technology (with pore sizes ranging from 0.05 to 0.4 Âµm), MBR systems permit the complete physical retention of bacterial flocs and almost all suspended solids within the bioreactor. The MBR system has many advantages over conventional wastewater treatment methods. These include small footprint and reactor requirements, high effluent quality, good disinfection capability, higher volumetric loading and less sludge production (Judd, 2006). As a result, the MBR process has now become a striking sustainable option due to their rising numbers and capacity for the treatment and reuse of industrial and municipal wastewaters. At present, thousands of MBR plants are operated all over the world, and its current market has been estimated to value around US$ 216 million and to rise to US$ 363 million by 2010 (Atkinson, 2006). However, membrane biofouling decreased the MBR filtration performance with filtration time. This is due to the bacterial cell's deposition, growth onto and into the membrane (Xiong & Liu, 2010). Soluble microbial products (SMP) and extracellular polymeric substance (EPS) secreted by bacterial also play the important role in biofouling (Ramesh et al., 2007). This major drawback and process limitation has been under investigation since the early MBRs, and remains one of the most challenging issues facing further MBR development (Yang et al., 2006).
During the last few years, Le-Clech et al. (2006) and Meng et al. (2009) reviewed MBR fouling by focusing on almost all the fouling factors; namely, they provided a very comprehensive review on sludge characteristics, operational parameters, membrane materials and feed water characteristics.
In recent years, a considerable number of papers was published on biofouling, e.g., the number of annual publication was 1450 in 2009 (Table 1).This This review paper is mainly focused on biofouling of membrane bioreactor: fundamentals of biofouling of membrane bioreactor, biofouling factors, control stragies and some future prospects. It is expected that this review may serve as a seed for further development and application of biological methods towards effective control of microbial attachment and membrane biofouling.
TABLE 1: Annual publications on membrane fouling
Number of Publications
Membrane fouling is a major obstacle that hinders faster commercialization of MBRs. As shown in Figure 1, membrane fouling in MBRs can be attributed to both membrane pores clogging and sludge cake deposition on membranes, which is usually the predominant fouling component (Lee et al., 2001). Membrane fouling results in a decrease of flux or an increase of transmembrane pressure (TMP) depending on the operation mode.
FIGURE 1. Membrane fouling: pore blocking and biofilm formation
Biofouling is the deposition, growth and metabolism of bacteria cells or flocs on the membranes, which have a significant concern in membrane filtration (Pang et al., 2005; Wang et al., 2005). For a low pressure membrane such as microfiltration and ultrafiltration for treating wastewater, biofouling is a major hitch because most foulants (microbial flocs) in MBRs are much larger than the membrane pore size. Biofouling may start with the deposition of an individual cell or cell cluster on the membrane surface, after which the cells multiply and form a biocake. Many researchers propose that soluble microbial products (SMP) and extracellular polymeric substance (EPS) secreted by bacteria also play important roles in the formation of biological foulants and cake layer on membrane surfaces (Liao et al., 2004; Ramesh et al., 2007). Biofilm structure is known to be highly variable ranging from patchy, discontinuous colonies to thick, continuous films and affects functional characteristics such as mass transport phenomena, resistance to antimicrobial depending on environmental conditions (Lewandowski, 2004).
Microbial EPS and SMP
Microbial biofim on membrane surface decreases the flux but it can be controlled by backwash. In general, there are two substances produced during biological activities causing fouling, that is, extracellular polymeric substances (EPS) and soluble microbial products (SMP EPS.). EPS is an insoluble macromolecule polymerized by microorganisms or substances like capsule, gel and humic acid. SMP is produced by cell metabolism or self-digestion, which can be considered to be soluble, large molecules (Tarnacki et al., 2005). They form colloidal substances and gradually accumulate in the membrane hole, decreasing the effective pore size of the membrane.
TECHNIQUES USED TO STUDY THE BIOFOULED MEMBRANE
The biofouled membrane can be visualised by techniques (Table 2) such as scanning electron microscopy (SEM) (Miura et al., 2007; Meng et al., 2010), CLSM (Bjorkoy & Fiksdal, 2009; Lee et al., 2010), atomic force microscopy (AFM) (Mi & Elimelech et al., 2010) , and direct observation through the membrane (DOTM). DOTM and CLSM have been widely used to characterize membrane biofouling (Li et al., 2003; Jin et al., 2006; Zhang et al., 2006a; Hwang et al., 2007; Lee et al., 2007; Meng et al., 2010). The DOTM approach was developed to record the deposition behaviour in simple cases of latex particles and flocs (Li et al., 2003; Zhang et al., 2006a). Zhang et al. (2006a) used a DOTM to examine the interactions between the bioflocs and the membrane surface. The images showed that the bioflocs could move across the membrane surface by rolling and sliding. More recently, CLSM has become a powerful approach for characterization of membrane biofouling, which can not only identify the deposited cell, but also present the 3D structure of the fouling layer. Ng et al. (2006b) applied CLSM to visualise the bacterial distribution on the membrane surface, and found that bacteria were widely present on the fouled membrane. The combination of CLSM and image analysis can visualise or quantify the architecture of bio-cake layer (Lee et al., 2008). Yun et al. (2006) characterised the biofilm structure and analysed its effect on membrane permeability in MBR for dye wastewater treatment. They found that membrane filterability was closely associated with the structural parameters of the biofilms (i.e., porosity, biovolume). The visualisation of biofouling using these techniques is helpful for understanding of the floc/cell deposition process and the microstructure or architecture of the cake layer.
In addition, a few investigations have been performed to study the microbial community structures and microbial colonization on membranes in MBRs (Jinhua et al., 2006; Zhang et al., 2006c; Miura et al., 2007; Bereschenko et al., 2010). The microbial community structures can be investigated using microbiology methods such as polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) and Fluorescence In Situ Hybridization (FISH) (Miura et al., 2007). Zhang et al. (2006c) reported that the microbial communities on membrane surfaces could be very different from the ones in the suspended biomass. They provided a list of bacteria that might be the pioneers of surface colonisation on membranes. Bereschenko et al. (2010) used a combination of molecular (fluorescence in situ hybridization [FISH], denaturing gradient gel electrophoresis (DGGE), and cloning) and microscopic (field emission scanning electron, epifluorescence, and confocal laser scanning microscopy) techniques to analyze the abundance, composition, architecture, and three-dimensional structure of biofilm communities. The results of the study showed a unique role of Sphingomonas spp. in the initial formation and subsequent maturation of biofilms on the RO membrane and feed-side spacer surfaces. Jinhua et al. (2006) reported that g-Proteobacteria more selectively adhered and grew on membranes than other microorganisms, and the deposited cells have higher surface hydrophobicity than the suspended sludge. The high shear stress induced by aeration can select the deposition of cells. Some cells can be detached easily by the shear stress, but other ones still adhere to membrane surface tightly. The selective deposition of the cell relies on the affinity of cells to membranes. And, due to the anoxic condition in the cake layer, the temporal change of microbial community structure would take place. We can see that some of the bacteria in the sludge should play an important role in membrane biofouling. The understanding of deposition behaviour of bacterial cell/flocs and mechanisms of cell attachment in MBRs will be vital for the development of appropriate biofouling control strategies in the future.
TABLE 2: Bacterial biofilm visualized by different techniques