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Fouling Challenges in Ceramic MBR Systems
7 FOULING CHALLENGES IN CERAMIC MBR SYSTEMS Anastasios I. Zouboulis, Petros K. Gkotsis Laboratory of Chemical and Environmental Technology, Section of Chemical Technology and Industrial Chemistry, School of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece
1. Introduction Fouling is the general term given to the process in which a variety of species (components) present in water or wastewater can gradually increase the resistance of a membrane by their adsorption or deposition onto the membrane surface, or even by complete pore-blocking. Fouling leads to permeate flux decline, which in turn decreases time intervals for membrane cleaning and replacement, both resulting in higher operating costs. Although the Membrane Bio-Reactor (MBR) technology provides significant advantages over the (convenient) Activated Sludge Process (ASP), such as better effluent quality, reduced space requirements, lower production of excess sludge, etc., membrane fouling is regarded as its most serious drawback. In MBRs, fouling increases the operating cost, due to the increased energy consumption and the application of frequent membrane chemical cleaning and, eventually, membrane replacement. Therefore, there is a great interest in investigating the mechanisms, control methods, and challenges of membrane fouling in ceramic MBRs (Gkotsis et al., 2014).
2. Fouling Categories, Mechanisms, and Stages Fouling can be classified according to three different criteria: (1) the nature of the substances which are mainly responsible for membrane fouling (foulant agents), (2) the reversibility (recovery) of flux values after the application of a single cleaning operation, and (3) its specific mechanism. According to the nature of the Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-816822-6.00007-0 Copyright © 2020 Elsevier Inc. All rights reserved.
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foulants, fouling can be classified into: (1) bio-fouling, which designates the formation of bio-films due to the presence of microorganisms and other compounds, which are attached and growing on the membrane surface, (2) organic fouling, which is caused by the presence of organic compounds, such as polysaccharides, proteins, humic substances, grease, oil, surfactants, and other organic biopolymers, and (3) inorganic fouling, which refers to the deposition (“scaling”) of inorganic materials, like salts, metal oxides, etc. According to the reversibility of flux, fouling can be classified into: (1) reversible fouling, when it can be removed by the application of physical cleaning methods, such as aeration with coarse bubbling, backwashing, relaxation, and relevant techniques, (2) irreversible fouling, when it can be removed by the application of chemical cleaning methods, such as the use of NaOCl, NaOH solutions, etc., or (3) irrecoverable fouling, which cannot be removed (recovered) by the application of physical or chemical cleaning methods and results from the long-term use of membranes after a series of consecutive physical and chemical cleaning cycles (Gkotsis et al., 2017, 2014). It is generally accepted that membrane fouling takes place according to one of the following major filtration mechanisms: pore blocking, standard blocking, intermediate blocking, or cake filtration (Fig. 7.1). During the complete pore blocking, it is assumed that each particle reaching the membrane blocks a pore without superimposing over other particles. During the standard blocking, the particles deposit within the membrane pores and the pore volume decreases proportionally to the volume of deposited particles. During the cake filtration, the depositing particles do not block the membrane pores, either because the membrane is dense and there are no pores to block, or because the pores are already covered by other particles and therefore, they are not available to be blocked. Finally, during the intermediate blocking, it is assumed that some particles deposit on other particles, as in the case of cake filtration, while other particles block the membrane pores, as in the case of complete pore blocking (Wang and Tarabara, 2008).
Figure 7.1 Schematic illustration of the four major fouling mechanisms: (A) complete blocking, (B) standard blocking, (C) intermediate blocking, and (D) cake filtration (Wang and Tarabara, 2008).
Chapter 7 Fouling Challenges in Ceramic MBR Systems
In convenient MBR systems, applied for wastewater treatment, fouling usually proceeds by a three-stage process (Fig. 7.2): Stage 1: Conditioning fouling. During the initial stage, strong interactions take place between the membrane surface and the Extracellular Polymeric Substances (EPS) of the mixed liquor. In this stage, rapid irreversible fouling takes place and a passive adsorption of colloids and organic substances has been observed, even during operation under zero flux conditions. In addition, the hydraulic resistance is almost independent of the tangential shear stress and the initial adsorption may account for almost 20%e2000% of the clean membrane resistance, depending on the membrane pore size (Ognier et al., 2002a,b). However, it has been found that the contribution of conditioning fouling to the overall resistance becomes negligible once filtration takes place (Choi et al., 2005). These studies suggest that the initial adsorption of colloids onto cleaned or even new membranes and pore blocking may be expected in MBRs. The intensity of this effect depends mainly on the membrane pore size distribution, the surface chemistry, and the hydrophobicity (Ognier et al., 2002a). Stage 2: Slow/steady fouling. During the second stage, the deposition of EPS on the surface of membrane continues, while microbial bio-flocs also begin to accumulate. As a result, a gel layer is formed, which increases in size, leading to a dense layer of solids on the membrane surface. This layer is gradually compressed resulting in the long-term increase of Trans-Membrane Pressure (TMP), which may be of a linear or slightly exponential form (Judd, 2011).
Figure 7.2 Changes in the spatial and temporal distributions of biopolymer compounds and live/dead cells in the membrane cake layer over the TMP development process (Meng et al., 2017).
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Stage 3: TMP jump. The third stage is characterized by a sharp increase in the TMP values. This increase has been observed in lab-scale MBR systems in which the experimental conditions were completely controlled and, thus, it cannot be attributed to possible abrupt changes in the operating conditions. During this stage, the membrane permeability is already small in the more fouled areas of membrane. As a result, filtration is promoted in the less fouled areas, where the local flux exceeds the critical flux, which is defined as the flux below which no or insignificant fouling is observed. Under these conditions, the fouling rate increases rapidly and exponentially with the increase of flux, leading to a sudden rise in the TMP values, also known as TMP jump. Several models have been proposed to explain the TMP development: a. Inhomogeneous fouling (area-loss) model: This model was proposed to explain the observed TMP profiles in nominally subcritical filtration of upflow anaerobic sludge MBR systems (Cho and Fane, 2002). The TMP jump appears to coincide with the measured loss of local permeability at different positions along the membrane, due to a slow fouling stage, as imposed by the presence of EPS. It is argued that the flux redistribution, which takes place in order to maintain a constant average flux, results in regions of subcritical flux and consequently in the rapid fouling and TMP rise. b. Inhomogeneous fouling (pore-loss) model: Similar TMP development has been observed for the crossflow filtration of a model biopolymer (alginate) in an MBR system (Ye et al., 2005). The obtained data have been explained by a model that involves the redistribution of flux among the open pores. In this case, local flux velocities in these pores may eventually exceed the critical flux of alginate solution, causing the creation of aggregates, which rapidly can block the pores. The model proposed by Ognier et al. (2004) was also based on this idea. While the previous “area-loss” model considers the macroscopic redistribution of flux in the membrane surface, the “pore-loss” model is based on a microscopic (pores’) scale. c. Critical suction pressure model: The two-stage profile of a gradual TMP rise, followed by a more rapid increase of respective values, has been observed in certain studies, based on the “dead-end” filtration concept (operation mode) of fine colloids by the application of an immersed
Chapter 7 Fouling Challenges in Ceramic MBR Systems
hollow fiber membrane module. At the critical suction pressure, it is suggested that coagulation or collapse occurs at the base of a created cake, as shown by relevant membrane autopsy evaluations (Chang et al., 2005). In this study, the rapid resistance increase, leading to the TMP jump, was attributed to a thin, but dense layer, which was observed close to the membrane surface. d. Percolation theory: According to the percolation theory, the porosity of fouling layer gradually decreases, due to the continuous filtration and material (components) deposition onto and within the already previously deposited layer. At the critical condition, the resistance of the cake layer is reduced, resulting in a rapid TMP increase. This model has been proposed for MBRs (Hermanowicz, 2004), but the suggested rapid change of TMP values (within minutes) cannot always be observed in practice. However, the combination of percolation theory with the inhomogeneous fouling (area-loss) model can account for the observed TMP development. e. Inhomogeneous fiber bundle model: Another manifestation of this TMP profile was observed for a model hollow fibers’ bundle, where the flux from individual fibers was monitored (Yeo and Fane, 2005). The bundle was operated under a constant flux, which was initially evenly distributed among the fibers. However, the average flux became less evenly distributed over the operation time, so that the standard deviation of the fluxes between the individual fibers started to increase rapidly. Consequently, the TMP values rose to maintain the average flux across the fiber bundle, reflecting the increase in the standard deviation of the fluxes. At some point, both TMP and standard deviation rose rapidly. This is believed to be due to flux maldistribution within the bundle, which leads to local pore and flow channel fouling. It was possible to obtain a steadier TMP pattern and standard deviation profiles, when the flow regime around the fibers was more rigorously controlled by applying higher liquid and/or air flow rates. More recently, the TMP jump was also attributed to the poor oxygen transfer within the fouling layer. As a result, bacteria in the biofilm layer can die, releasing extra levels of Soluble Microbial Products (SMP). Experimental data have shown an increase of SMP concentrations at the bottom part of the fouling layer, when the level of dissolved oxygen decreases (Gkotsis et al., 2014; Judd, 2011).
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3. Fouling Control and Mitigation Methods Notwithstanding the significant progress of MBR technology, membrane fouling remains the primary drawback for its universal and full-scale application, since it reduces the system’s productivity, increases the energy requirement for gas scouring and frequency of chemical cleaning, and results in higher membrane replacement costs. The methods which are applied for fouling control and mitigation in MBR systems can be distinguished into conventional methods, which have been implemented since the early years of MBR technology application and include the operation of the system under low fluxes and the application of simple physical or chemical methods, and innovative methods, which are implemented mainly in recent years and include the use of a series of different state-of-the-art materials and methods, such as the use of specific additives, the application of electric field, ultrasound, and/or ozone, the appropriate modifications of membrane surface, etc. It must be noted that the innovative methods are not applied individually, but they are usually used in combination with a conventional method, aiming to result in more effective fouling reduction.
3.1 Conventional Methods for Fouling Control The conventional methods, which are employed in order to achieve fouling control and mitigation, mainly include the optimal operation of MBR process, i.e., the operation under lower than critical permeate fluxes, and the application of various physical or chemical methods. MBR systems can operate under either a constant flux operation or a constant TMP mode, and fouling is indicated by the increase of TMP values or by the flux reduction, respectively. In the first mode, which is the most common for the MBR systems, it is generally accepted that keeping the flux at high levels favors membrane fouling and thus, must be avoided. This is logical, since the application of lower fluxes can delay the adsorption of foulants and prolong the membrane operating lifetime. On the other hand, the application of very low fluxes is not an economical procedure, because larger membrane surfaces would be required. For this reason, the respective critical flux is usually determined before the start-up of the reactor. Typical flux values in most MBR systems range between 10 and 50 L/m2$h. The most common physical methods include the operation of MBR systems by increasing the aeration rate, the application of backwashing, and the membrane relaxation. In general, the
Chapter 7 Fouling Challenges in Ceramic MBR Systems
increase of aeration rate favors the mitigation of fouling, due to the presence of high shear stresses, which are applied on the membrane surface and can lead to easier removal of deposited microbial bio-flocs. Typical aeration rates range between 0.18 and 7 m3/m2$h, depending on the membrane type and the system’s specific capacity. Backwashing (also known as backflushing) refers to the periodic reversal of the filtration flow in the opposite direction. It effectively removes a significant part of the cake layer from the membrane surface; however, its application is usually limited to hollow fiber or tubular membranes. Typical backwashing flow rates may range between 25 and 40 L/m2$h, although much higher values can be applied in the case of ceramic membranes’ use, which are particularly hydraulic resistant and able to operate even at 150 L/m2$h. Membrane relaxation is the periodic interruption of filtration for a short period of time, usually for a few seconds or minutes. The regular interruptions of filtration rate have shown that the deposited foulants can be diffused from the membrane surface back to the mixed liquor, resulting in the “relaxation” of the membrane and the increase of its operating lifetime. The application of chemical cleaning can remove effectively small particles and soluble components, which have been deposited on the membrane surface, or even inside the membrane pores. The chemical reagents, which are commonly used for chemical cleaning, usually belong to one of the following four categories: (1) acids, such as C2H2O4, C6H8O7, HNO3, HCl, H3PO4, H2SO4, etc., (2) bases, such as NaOH, KOH, Na2CO3, etc., (3) oxidants or disinfectants, such as NaClO, H2O2, O3, etc., and (4) other chemical reagents (e.g., surfactants, complexes, etc.), such as EDTA, DTPA, SDS, etc. The latest trend in chemical cleaning includes the use of chemical reagents from different categories in order to remove all different types of fouling (biofouling, organic, and inorganic fouling) potentially found in an MBR operating system (Yoon, 2016; Wang et al., 2014a,b).
3.2 Innovative Methods for Fouling Control Over the last few years, several novel strategies have been employed to prevent or control effectively the problem of membrane fouling. Most of them include the (bio)chemical mixed liquor modification of properties, such as the addition of specific chemicals (e.g., coagulants, adsorbents, and/or biofilm carriers), the application of ultrasound, electric field, and/or ozone, as well as various membrane surface modifications.
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3.2.1 Use of Additives In recent years, the use of additives is the alternative method mostly applied for fouling control in MBR systems. Fouling mitigation is achieved via the modification of the mixed liquor’s characteristics, which can subsequently improve the filtration process and increase the membrane operation lifetime. The used additives can be classified into three general categories: (1) coagulants/flocculants, (2) adsorbents, and (3) biofilm carriers. The addition of appropriate (inorganic) coagulants can provide positive charges in a (usually) negatively charged biomass and enhance further the flocculation of microbial bio-flocs, which results in the increase of mixed liquor’s biomass filterability. As a result, the TMP rate is maintained at relatively lower levels and the membrane’s operating time is extended. The coagulants, which are usually used in most MBR systems in order to prevent or mitigate fouling, are simple metal Fe or Al salts, such as FeCl3, FeSO4, Al2(SO4)3, etc., as well as inorganic polymeric flocculants (IPF), such as polyferric chloride (PFCl), polyferric sulfate (PFS), polyaluminum chloride (PACl), etc., or organic polymers (flocculants), such as polyacrylamides, specific chitosans, tannins, etc. (Park et al., 2018; Iorhemen et al., 2016). The addition of adsorbents has also been shown to reduce fouling in MBRs. This is not only due to the formation of a porous, low-resistant layer on the membrane surface, but also due to the preliminary adsorption (and removal from the solution) of the substances, which are considered responsible for fouling. Powdered Activated Carbon (PAC), Granular Activated Carbon (GAC), and zeolites are the most commonly applied materials of this category (Nguyen et al., 2014). Bio-Film Carriers or Biocarriers (BFC) are specially designed, small-sized plastic materials, which are added in the mixed liquor of MBR systems at various filling ratios, aiming to mitigate fouling. Fouling mitigation takes place either by the application of high shear stresses near to the membrane surface, caused by their rigorous movement, due to aeration, or/and the decrease of SMP concentration via the enhanced adsorption (and removal) of proteins and polysaccharides content by the deposited biomass onto the biocarriers. Depending on the application, the filling ratio in the bioreactor ranges between 20% and 80% (Yoon, 2016).
3.2.2 Application of Electric Field, Ultrasound, and/or Ozone The applications which employ an electric field in MBRs can be divided into three categories: (1) Electro-coagulation (EC),
Chapter 7 Fouling Challenges in Ceramic MBR Systems
(2) Electrophoresis (EP), and (3) Microbial Fuel Cell (MFC). In the first two applications, which are most commonly found in MBRs, an external electric field is applied, while in the MFC, the electrical energy is in situ generated within the cell. During EC, the produced cations form aggregates with the negatively charged components of the biomass, resulting in the improvement of biomass filterability. During EP, the negatively charged components of biomass are led toward the positively charged anode, and thus, drawn away from the membrane surface (Hua et al., 2015; Zhang et al., 2015). Ultrasound has been also employed to mitigate fouling in MBRs. It is reported that the tiny bubbles, which are formed by the acoustic waves, can produce high shear stresses on the membrane surface, resulting in the delay or even the removal of the developed cake layer. The most important advantages of this method are that it can be easily combined with other fouling mitigation methods to improve effectiveness and therefore, the use of chemical reagents can be avoided. Ozone is a powerful oxidant that breaks down large polymeric compounds, and thus, it can be used to mitigate fouling in MBRs by reducing (breaking down, disintegrating) at least partly the colloidal or soluble organic substances’ content. In addition, it was reported that ozonation can increase the size of microbial flocs, due to changes in the zeta-potential values, resulting in the increase of sludge filterability (Yu et al., 2012; Liang et al., 2014).
3.2.3 Membrane Surface Modification Several of the membrane surface characteristics, such as hydrophilicity, charge, and roughness, are known to be strongly related to fouling, because they determine the interaction between the membrane and the foulant agents. The majority of commercially available membranes for pressure-driven processes are made from hydrophobic polymers with relatively high thermal, chemical, and mechanical stabilities. However, because of the hydrophobicity of these materials, they are prone to adsorption/interaction with the fouling substances, which are partly hydrophobic. It has been well documented that membranes with hydrophilic surfaces are less susceptible to fouling. Therefore, an increase in the hydrophilicity of membrane surface is often a key goal to reducing membrane fouling by the presence of organic pollutants and/or microorganisms. The membrane surface charge is also another important consideration for reducing membrane fouling. Usually, it is appropriate to employ membranes with a negative surface charge, because the
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biomass is negatively charged as well. As a result, the electrostatic repulsion created between the foulants and the membrane can prevent their deposition on the membrane, thereby to mitigate fouling. Over the last few years, several membrane surface modification methods have been applied in order to increase hydrophilicity and the negative surface charge, and thus, to mitigate fouling in MBRs. These methods include the physical coating/adsorption or grafting on the membrane surface, the use of specifically patterned membranes, the plasma treatment, or other chemical reactions on the membrane surface, or the surface modification by nanoparticles: • The physical coating of a thin layer of water-soluble polymers or surfactants is a flexible technique to optimize the hydrophilicity, smoothness, and surface charge of the membrane surface. Physical coating can take place via filtration, adsorption, or casting techniques. • Membranes with a patterned surface have also become recently an attractive option for fouling mitigation in membrane processes applied for water or wastewater treatment. The origin of this enhanced antifouling behavior is attributed to enhanced shear-induced diffusion, whereby the presence of submicron surface patterns increases the effective shear rate near the membrane surface. • Over the last 2 decades, plasma treatment has been intensively studied, attempting to increase the hydrophilicity and to induce the low-fouling properties for membrane surfaces. Usually, plasma treatment of the membranes can be carried out in three different modes: (1) with non-polymerizable gas molecules, (2) with polymerizable vapors, and (3) with plasma-induced grafting of the polymer chains to the membrane surface. • The introduction of charged groups on the membrane surface can be also a useful approach to mitigate membrane fouling, caused by charged organic compounds. In this type of modification, various chemical reactions may be used for creating different functional groups, such as eSO3 or eCOOH, on the membrane surface. The number of the introduced functional groups and the thickness of the modified surface layer depend mainly on the treatment time, temperature, and concentration of modification agent. • The use of nanoparticles has received particular attention, especially during the last few years, attempting to enhance flux rates and to reduce fouling. Two different methods have been used for preparing modified nanoparticle-based membranes: (1) the deposition of nanoparticles on the membrane
Chapter 7 Fouling Challenges in Ceramic MBR Systems
surface through dipping of a porous support in an aqueous suspension of nanoparticles and (2) the entrapment of nanoparticles in a polymer matrix via a phase inversion method by the addition of the nanoparticles to the casting solution (Gkotsis et al., 2014; Hilal et al., 2012; Kochkodan et al., 2014).
4. Fouling Control in Ceramic MBRs Several research studies have suggested the use of ceramic membranes as an alternative substitute of conventional polymeric membranes, due to their higher stability against chemical cleaning reagents and their structural ability to withstand higher temperatures and mechanical forces, than the respective polymeric membranes, already in wider use for the MBR treatment systems. Nevertheless, and despite their great potential, membrane fouling still remains the major bottleneck for the widespread application of ceramic MBRs, as it increases substantially the operating and capital costs. Therefore, a meticulous study and understanding of membrane fouling in such systems is essential in order to find the most appropriate control methods, which will deal effectively with future challenges. In view of possible damages that the addition of several chemical cleaning agents, such as NaClO, NaOH, ozone, etc., could bring to the polymeric membranes, the ceramic membranes are preferred for their greater stability and integrity under chemical attacks. Zhang et al. (2018) treated high-strength dyeing wastewater by using an anaerobic flat sheet ceramic MBR. Membrane fouling was controlled by the implementation of backwashing as a physical method, while the ceramic membranes were chemically cleaned for 1e2 h with 1000 mg/L of NaOCl, when the TMP was reached 30 kPa. The results showed that the addition of NaOCl was an effective method for fouling control, since the TMP was restored to the initial value after this specific chemical cleaning. During the operation of a MBR system, the TMP values were less than 10 kPa, indicating higher permeability of the flat sheet ceramic membrane. This enabled the system to operate also by using lower energy consumption. Wang et al. (2014a,b) developed a new method, which allowed the backwashing of ceramic membranes at much lower NaClO concentrations (Cbackwashing ¼ 0.05e1.5 mg/L), although it has to be applied more frequently, in order to eliminate the need for a more intensive thorough chemical cleaning (Fig. 7.3). The results showed that there is an optimal backwashing NaClO concentration (0.2 mg/L), which effectively reduced fouling, combining
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Pure water backwashing
NaClO solution backwashing
Permeate
Permeate
Air
Air
Figure 7.3 Schematic illustration of the aerobic ceramic MBR setup (Wang et al., 2014a,b).
the direct membrane cleaning with the indirect impact on microbial activities. Yue et al. (2018) also employed the NaClO-assisted backwashing (Cbackwashing ¼ 0.05, 0.25, 1, and 10 mg/L) for the mitigation of fouling in an anaerobic ceramic MBR, which treated domestic wastewater. The results showed that the addition of NaClO at the concentration of 1 mg/L was capable to mitigate the membrane fouling. Mei et al. (2017) used a ceramic membrane to elucidate the role of NaOH backwashing in membrane fouling mitigation and its impact on the bioreactor performance (Fig. 7.4). Ex-situ cleaning tests showed that the addition of NaOH at concentrations between 0.05 and 1.30 mmol/L had a positive impact on the anaerobic biomass, while higher concentrations (>1.30 mmol/L) provoked toxic effects. However, the in-situ cleaning tests revealed that the membrane fouling rate was significantly reduced after backwashing with 10e20 mmol/L of NaOH and the methanogenic activity was also slightly improved. In addition, economic analysis showed that 12 mmol/L was the optimal concentration for fouling control. Sun et al. (2018a,b) investigated the membrane fouling potential of the Dissolved Organic Matter (DOM) and of the halogenated by-products, which were generated during the on-line chemical cleaning of a ceramic MBR by the application of ozone (Fig. 7.5). The results revealed that the released DOM could considerably cause irreversible membrane fouling, with the humic acids fraction having about 500 Da molecular weight, being mainly responsible for the observed fouling. This study offered a better understanding of membrane fouling associated with DOM, which was generated by the addition of ozone during the on-line chemical cleaning, enabling further the design of a ceramic MBR system toward a long-term operation stability.
Chapter 7 Fouling Challenges in Ceramic MBR Systems
Figure 7.4 Schematic illustration of an anaerobic ceramic MBR setup (Mei et al., 2017).
Apart from their remarkable performance in terms of compatibility with aggressive chemical (cleaning) reagents, several studies have shown that the filtration with ceramic membranes was improved in terms of TMP reduction, or of normalized flux increase (i.e., at constant flux conditions or at constant pressure operation mode, respectively) (Jeong et al., 2017). For example, Lee et al. (2013) have found that ceramic membranes present lower fouling propensity than the polymeric membranes, due to the weaker bonding between foulants and membranes. Therefore, not only the required physical and chemical cleaning times can be reduced, but also certain laborious maintenance tasks, such as repair and replacement of polymeric hollow fibers (membranes), can be also eliminated. € ppenbecker et al. (2017) introduced a novel AnMBR system Du with an external cross-flow ceramic membrane, which employed fluidized glass beads as turbulence promoters to mitigate fouling (Fig. 7.6). The reactor treated raw municipal wastewater and three ceramic membranes (two ultrafiltration membranes made from ZrO2 or Al2O3 and one microfiltration membrane made from TiO2) were evaluated in terms of fouling behavior, mechanical resistance, and pollutants’ removal performance during 46
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(A)
Ozone generator
Mass flow controller Ozone destroyer
Oxygen concentraor
Ozone water tank Ozone sensor Pump
Reactor
PC
(B) Permeate pump
Crossflow cell
P
P Pressure Influent pump
transducer
Flow meter
Magnetic stirrer
PC
Digital balance
Figure 7.5 Schematic illustration of the (A) ozone generation and application system for membrane fouling mitigation, and (B) a cross-flow membrane filtration system (Sun et al., 2018b).
Figure 7.6 Schematic illustration of an anaerobic ceramic MBR system (D€ uppenbecker et al., 2017).
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operation days. The results showed that the use of ceramic membranes with the fluidized glass beads content can significantly reduce the membrane fouling. In addition, the mean soluble COD removal rates for the ZrO2, Al2O3, and TiO2 membranes were 50%, 55%, and 41%, respectively. The aforementioned studies underline the superior performance of ceramic membranes in terms of fouling mitigation and removal performance, as compared to most common polymeric membranes in use. Nevertheless, despite their great potential in municipal and industrial wastewater treatment, there are still important challenges that need to be dealt for their full-scale widespread application and promotion.
5. Conclusions and Future Trends Ceramic membranes have outstanding features over most polymeric membranes, because of their remarkable robustness, easier physical and chemical cleaning, and higher life span. In addition, their remarkable separation performance has proved their capability for a wide range of municipal and industrial wastewater treatment and desalination applications. However, there are still important challenges and questions that need to be carefully addressed in the future research in the field of ceramic MBRs, such as the improvement of antifouling properties, the optimization of investment cost by introducing new fabrication and coating technologies, and the improvement of selectivity, water permeability, and packing densities, due to the relevant fragility of ceramic materials (God and Ismail, 2018; Samaei et al., 2018). Given that the ceramic membranes are resistant to harsh environmental (or experimental) operation conditions, several more aggressive chemical cleaning protocols can be effectively used for fouling control in comparison with the polymeric membranes. Furthermore, in the submerged AnMBR configuration, where anaerobic conditions must be maintained, it is difficult to remove the membranes from the bioreactor for cleaning purposes, without influencing the anaerobic environment. Therefore, anaerobic ceramic MBR systems with relatively lower fouling potentials are a challenging issue, when treating municipal or industrial wastewaters. Nevertheless, the practical implementation of ceramic membranes in MBR systems has been limited, because the raw construction materials (i.e., Al2O3, TiO2, ZrO2, or SiC) are still quite expensive. For this reason, research is turning toward the development of cost-effective ceramic membranes, which are made of natural mineral-based
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materials, such as pyrophyllite, dolomite, kaolin, zeolite, or Moroccan clay. Considering that these ceramic materials are cheaper than for e.g., the purified alumina, the natural mineralbased ceramic membranes can be an effective way to reduce their manufacturing costs (Jeong et al., 2017). The use of ozone in order to minimize toxicity issues of several wastewaters, the development of ceramic membranes with high resistance to irradiation and high electrical conductivity with a view to their specific applications in photocatalytic or electrochemical systems, respectively, are also some interesting suggestions for future research. Past studies have also showed that the number of ceramic membrane applications in the mining and pharmaceutical sector is limited in comparison with other industrial sectors, probably due to the complex nature of acid mine drainage, or pharmaceutical wastewaters. Consequently, future research efforts should also focus on the further investigation for different pore sizes of ceramic membranes in combination with the appropriate physico-chemical techniques during the pre- or post-treatment of problematic mining or pharmaceutical effluents. The manufacturing of bio-ceramic materials with selective adsorption properties could also be a promising technique for future applications. These materials may be useful as biological filters, e.g., for air filtration or for wastewater treatment. The incorporation of bio-ceramic modules can be useful in treatment plants fed by industrial wastewaters, while they can contribute to the production of lower amounts of wasted bio-sludge/biosolids as well.
List of Acronyms AnMBR ASP BFC COD DOM DTPA EC EDTA EP EPS GAC IPF MBR MFC PAC PACl
Anaerobic Membrane Bio-Reactor Activated Sludge Process Bio-Film Carriers Chemical Oxygen Demand Dissolved Organic Matter Diethylenetriaminepentaacetic acid Electrocoagulation Ethylenediaminetetraacetic acid Electrophoresis Extracellular Polymeric Substances Granular Activated Carbon Inorganic Polymeric Flocculants Membrane Bio-Reactor Microbial Fuel Cell Powdered Activated Carbon Polyaluminum chloride
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PFCl PFS SDS SMP TMP
Polyferric chloride Polyferric sulfate Sodium Dodecyl Sulfate Soluble Microbial Products Trans-Membrane Pressure
References Chang, S., Fane, A.G., Waite, T.D., 2005. Effect of coagulation within the cakelayer on fouling transitions with dead-end hollow fiber membranes. In: Proceedings of the International Congress on Membranes and Membrane Processes. ICOM), Seoul, Korea, pp. 21e26. August. Cho, B.D., Fane, A.G., 2002. Fouling transients in nominally sub-critical flux operation of a membrane bioreactor. J. Membr. Sci. 209, 391e403. Choi, H., Zhang, K., Dionysiou, D.D., Oerther, D.B., Sorial, G.A., 2005. Effect of permeate flux and tangential flow on membrane fouling for wastewater treatment. Separ. Purif. Technol. 45, 68e78. € ppenbecker, B., Engelhart, M., Cornel, P., 2017. Fouling mitigation in Du anaerobic membrane bioreactor using fluidized glass beads: evaluation fitness for purpose of ceramic membranes. J. Membr. Sci. 537, 69e82. Gkotsis, P.K., Banti, D.C., Peleka, E.N., Zouboulis, A.I., Samaras, P.E., 2014. Fouling issues in membrane bioreactors (MBRs) for wastewater treatment: major mechanisms, prevention and control strategies. Processes 2, 795e866. Gkotsis, P.K., Mitrakas, M.M., Tolkou, A.K., Zouboulis, A.I., 2017. Batch and continuous dosing of conventional and composite coagulation agents for fouling control in a pilot-scale MBR. Chem. Eng. J. 311, 255e264. Goh, P.S., Ismail, A.F., 2018. A review on inorganic membranes for desalination and wastewater treatment. Desalination 434, 60e80. Hermanowicz, S.W., 2004. Membrane filtration of biological solids: a unified framework and its applications to membrane bioreactors. In: Proceedings of the Water Environment-Membrane Technology Conference, Seoul, Korea, June 7-10. Hilal, N., Khayet, M., Wright, C.J., 2012. Membrane Modification. Technology and Applications, Chapter 3 (Reduction of Membrane Fouling by Polymer Surface Modification). CRC Press, Boca Raton, U.S.A. Sawston. Hua, L.-C., Huang, C., Su, Y.-C., Nguyen, T.-N.-P., Chen, P.-C., 2015. Effects of electro-coagulation on fouling mitigation and sludge characteristics in a coagulation assisted membrane bioreactor. J. Membr. Sci. 495, 29e36. Iorhemen, O.T., Hamza, R.A., Tay, J.H., 2016. Membrane bioreactor (MBR) technology for wastewater treatment and reclamation: membrane fouling. Membranes 6 (2), 33. Jeong, Y., Hermanowicz, S.W., Park, C., 2017. Treatment of food waste recycling wastewater using anaerobic ceramic membrane bioreactor for biogas production in mainstream treatment process of domestic wastewater. Water Res. 123, 86e95. Judd, S., 2011. The MBR Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment, second ed. Elsevier, Oxford, U.K. Kochkodan, V., Johnson, D.J., Hilal, N., 2014. Polymeric membranes: surface modification for minimizing (bio)colloidal fouling. Adv. Colloid Interface Sci. 206, 116e140.
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Lee, E.-J., Kwon, J.-S., Park, H.-S., Ji, W.H., Kim, H.-S., Jang, A., 2013. Influence of sodium hypochlorite used for chemical enhanced backwashing on biophysical treatment in MBR. Desalination 316, 104e109. Liang, S., Xiao, K., Wu, J., Liang, P., Huang, X., 2014. Mechanism of membrane filterability amelioration via tuning mixed liquor property by pre-ozonation. J. Membr. Sci. 454, 111e118. Mei, X., Quek, P.J., Wang, Z., Ng, H.Y., 2017. Alkali-assisted membrane cleaning for fouling control of anaerobic ceramic membrane bioreactor. Bioresour. Technol. 240, 25e32. Meng, F., Zhang, S., Oh, Y., Zhou, Z., Shin, H.-S., Chae, S.-R., 2017. Fouling in membrane bioreactors: an updated review. Water Res. 114, 151e180. Nguyen, N.L., Hai, F., Nghiem, L., Kang, J., Price, W.E., Park, C., Yamamoto, K., 2014. Enhancement of removal of trace organic contaminants by powdered activated carbon dosing into membrane bioreactors. J. Taiwan Inst. Chem. Eng. 45, 571e578. Ognier, S., Wisniewski, C., Grasmick, A., 2002a. Influence of macromolecule adsorption during filtration of a membrane bioreactor mixed liquor suspension. J. Membr. 209, 27e37. Ognier, S., Wisniewski, C., Grasmick, A., 2002b. Constant flux filtration in membrane bioreactors. Membr. Technol. 7, 6e10. Ognier, S., Wisniewski, C., Grasmick, A., 2004. Membrane bioreactor fouling in sub-critical filtration conditions: a local critical flux concept. J. Membr. Sci. 229, 171e177. Park, J., Yamashita, N., Tanaka, H., 2018. Membrane fouling control and enhanced removal of pharmaceuticals and personal care products by coagulation-MBR. Chemosphere 197, 467e476. Samaei, S.M., Gato-Trinidad, S., Altaee, A., 2018. The application of pressuredriven ceramic membrane technology for the treatment of industrial wastewaters - a review. Separ. Purif. Technol. 200, 198e220. Sun, H., Liu, H., Han, J., Zhang, X., Cheng, F., Liu, Y., 2018a. Chemical CleaningAssociated Generation of Dissolved Organic Matter And Halogenated Byproducts In Ceramic MBR: Ozone Versus Hypochlorite. Sun, H., Liu, H., Wang, S., Cheng, F., Liu, Y., 2018b. Ceramic membrane fouling by dissolved organic matter generated during on-line chemical cleaning with ozone in MBR. Water Res. 146, 328e336. Wang, F., Tarabara, V.V., 2008. Pore blocking mechanisms during early stages of membrane fouling by colloids. J. Colloid Interface Sci. 328, 464e469. Wang, Z., Ma, J., Tang, C.Y., Kimura, K., Wang, Q., Han, X., 2014a. Membrane cleaning in membrane bioreactors: a review. J. Membr. Sci. 468, 276e307. Wang, Z., Meng, F., He, X., Zhou, Z., Huang, L.-N., Liang, S., 2014b. Optimisation and performance of NaClO-assisted maintenance cleaning for fouling control in membrane bioreactors. Water Res. 53, 1e11. Ye, Y., Le-Clech, P., Chen, V., Fane, A.G., 2005. Evolution of fouling during crossflow filtration of model EPS solutions. J. Membr. Sci. 264, 190e199. Yeo, A., Fane, A.G., 2005. Performance of individual fibers in a submerged hollow fiber bundle. Water Sci. Technol. 51, 165e172. Yoon, S.-H., 2016. Membrane Bioreactor Processes: Principles and Applications. CRC Press, Boca Raton, U.S.A. Yu, Z., Wen, X., Xu, M., Huang, X., 2012. Characteristics of extracellular polymeric substances and bacterial communities in an anaerobic membrane bioreactor coupled with online ultrasound equipment. Bioresour. Technol. 117, 333e340.
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Yue, X., Koh, Y.K.K., Ng, Y.H., 2018. Membrane fouling mitigation by NaClOassisted backwash in anaerobic ceramic membrane bioreactors for the treatment of domestic wastewater. Bioresour. Technol. 268, 622e632. Zhang, J., Xingguo, A.S., Xiao, K., Sun, J., Yan, X., Liang, P., Zhang, X., Huang, X., 2015. Low-voltage electric field applied into MBR for fouling suppression: performance and mechanisms. Chem. Eng. J. 273, 223e230. Zhang, W., Liu, F., Wang, D., Jin, Y., 2018. Impact of reactor configuration on treatment performance and microbial diversity in treating high-strength dyeing wastewater: anaerobic flat-sheet ceramic membrane bioreactor versus upflow anaerobic sludge blanket reactor. Bioresour. Technol. 269, 269e275.
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1. Introduction Fouling is the general term given to the process in which a variety of species (components) present in water or wastewater can gradually increase the resistance of a membrane by their adsorption or deposition onto the membrane surface, or even by complete pore-blocking. Fouling leads to permeate flux decline, which in turn decreases time intervals for membrane cleaning and replacement, both resulting in higher operating costs. Although the Membrane Bio-Reactor (MBR) technology provides significant advantages over the (convenient) Activated Sludge Process (ASP), such as better effluent quality, reduced space requirements, lower production of excess sludge, etc., membrane fouling is regarded as its most serious drawback. In MBRs, fouling increases the operating cost, due to the increased energy consumption and the application of frequent membrane chemical cleaning and, eventually, membrane replacement. Therefore, there is a great interest in investigating the mechanisms, control methods, and challenges of membrane fouling in ceramic MBRs (Gkotsis et al., 2014).
2. Fouling Categories, Mechanisms, and Stages Fouling can be classified according to three different criteria: (1) the nature of the substances which are mainly responsible for membrane fouling (foulant agents), (2) the reversibility (recovery) of flux values after the application of a single cleaning operation, and (3) its specific mechanism. According to the nature of the Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-816822-6.00007-0 Copyright © 2020 Elsevier Inc. All rights reserved.
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foulants, fouling can be classified into: (1) bio-fouling, which designates the formation of bio-films due to the presence of microorganisms and other compounds, which are attached and growing on the membrane surface, (2) organic fouling, which is caused by the presence of organic compounds, such as polysaccharides, proteins, humic substances, grease, oil, surfactants, and other organic biopolymers, and (3) inorganic fouling, which refers to the deposition (“scaling”) of inorganic materials, like salts, metal oxides, etc. According to the reversibility of flux, fouling can be classified into: (1) reversible fouling, when it can be removed by the application of physical cleaning methods, such as aeration with coarse bubbling, backwashing, relaxation, and relevant techniques, (2) irreversible fouling, when it can be removed by the application of chemical cleaning methods, such as the use of NaOCl, NaOH solutions, etc., or (3) irrecoverable fouling, which cannot be removed (recovered) by the application of physical or chemical cleaning methods and results from the long-term use of membranes after a series of consecutive physical and chemical cleaning cycles (Gkotsis et al., 2017, 2014). It is generally accepted that membrane fouling takes place according to one of the following major filtration mechanisms: pore blocking, standard blocking, intermediate blocking, or cake filtration (Fig. 7.1). During the complete pore blocking, it is assumed that each particle reaching the membrane blocks a pore without superimposing over other particles. During the standard blocking, the particles deposit within the membrane pores and the pore volume decreases proportionally to the volume of deposited particles. During the cake filtration, the depositing particles do not block the membrane pores, either because the membrane is dense and there are no pores to block, or because the pores are already covered by other particles and therefore, they are not available to be blocked. Finally, during the intermediate blocking, it is assumed that some particles deposit on other particles, as in the case of cake filtration, while other particles block the membrane pores, as in the case of complete pore blocking (Wang and Tarabara, 2008).
Figure 7.1 Schematic illustration of the four major fouling mechanisms: (A) complete blocking, (B) standard blocking, (C) intermediate blocking, and (D) cake filtration (Wang and Tarabara, 2008).
Chapter 7 Fouling Challenges in Ceramic MBR Systems
In convenient MBR systems, applied for wastewater treatment, fouling usually proceeds by a three-stage process (Fig. 7.2): Stage 1: Conditioning fouling. During the initial stage, strong interactions take place between the membrane surface and the Extracellular Polymeric Substances (EPS) of the mixed liquor. In this stage, rapid irreversible fouling takes place and a passive adsorption of colloids and organic substances has been observed, even during operation under zero flux conditions. In addition, the hydraulic resistance is almost independent of the tangential shear stress and the initial adsorption may account for almost 20%e2000% of the clean membrane resistance, depending on the membrane pore size (Ognier et al., 2002a,b). However, it has been found that the contribution of conditioning fouling to the overall resistance becomes negligible once filtration takes place (Choi et al., 2005). These studies suggest that the initial adsorption of colloids onto cleaned or even new membranes and pore blocking may be expected in MBRs. The intensity of this effect depends mainly on the membrane pore size distribution, the surface chemistry, and the hydrophobicity (Ognier et al., 2002a). Stage 2: Slow/steady fouling. During the second stage, the deposition of EPS on the surface of membrane continues, while microbial bio-flocs also begin to accumulate. As a result, a gel layer is formed, which increases in size, leading to a dense layer of solids on the membrane surface. This layer is gradually compressed resulting in the long-term increase of Trans-Membrane Pressure (TMP), which may be of a linear or slightly exponential form (Judd, 2011).
Figure 7.2 Changes in the spatial and temporal distributions of biopolymer compounds and live/dead cells in the membrane cake layer over the TMP development process (Meng et al., 2017).
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Stage 3: TMP jump. The third stage is characterized by a sharp increase in the TMP values. This increase has been observed in lab-scale MBR systems in which the experimental conditions were completely controlled and, thus, it cannot be attributed to possible abrupt changes in the operating conditions. During this stage, the membrane permeability is already small in the more fouled areas of membrane. As a result, filtration is promoted in the less fouled areas, where the local flux exceeds the critical flux, which is defined as the flux below which no or insignificant fouling is observed. Under these conditions, the fouling rate increases rapidly and exponentially with the increase of flux, leading to a sudden rise in the TMP values, also known as TMP jump. Several models have been proposed to explain the TMP development: a. Inhomogeneous fouling (area-loss) model: This model was proposed to explain the observed TMP profiles in nominally subcritical filtration of upflow anaerobic sludge MBR systems (Cho and Fane, 2002). The TMP jump appears to coincide with the measured loss of local permeability at different positions along the membrane, due to a slow fouling stage, as imposed by the presence of EPS. It is argued that the flux redistribution, which takes place in order to maintain a constant average flux, results in regions of subcritical flux and consequently in the rapid fouling and TMP rise. b. Inhomogeneous fouling (pore-loss) model: Similar TMP development has been observed for the crossflow filtration of a model biopolymer (alginate) in an MBR system (Ye et al., 2005). The obtained data have been explained by a model that involves the redistribution of flux among the open pores. In this case, local flux velocities in these pores may eventually exceed the critical flux of alginate solution, causing the creation of aggregates, which rapidly can block the pores. The model proposed by Ognier et al. (2004) was also based on this idea. While the previous “area-loss” model considers the macroscopic redistribution of flux in the membrane surface, the “pore-loss” model is based on a microscopic (pores’) scale. c. Critical suction pressure model: The two-stage profile of a gradual TMP rise, followed by a more rapid increase of respective values, has been observed in certain studies, based on the “dead-end” filtration concept (operation mode) of fine colloids by the application of an immersed
Chapter 7 Fouling Challenges in Ceramic MBR Systems
hollow fiber membrane module. At the critical suction pressure, it is suggested that coagulation or collapse occurs at the base of a created cake, as shown by relevant membrane autopsy evaluations (Chang et al., 2005). In this study, the rapid resistance increase, leading to the TMP jump, was attributed to a thin, but dense layer, which was observed close to the membrane surface. d. Percolation theory: According to the percolation theory, the porosity of fouling layer gradually decreases, due to the continuous filtration and material (components) deposition onto and within the already previously deposited layer. At the critical condition, the resistance of the cake layer is reduced, resulting in a rapid TMP increase. This model has been proposed for MBRs (Hermanowicz, 2004), but the suggested rapid change of TMP values (within minutes) cannot always be observed in practice. However, the combination of percolation theory with the inhomogeneous fouling (area-loss) model can account for the observed TMP development. e. Inhomogeneous fiber bundle model: Another manifestation of this TMP profile was observed for a model hollow fibers’ bundle, where the flux from individual fibers was monitored (Yeo and Fane, 2005). The bundle was operated under a constant flux, which was initially evenly distributed among the fibers. However, the average flux became less evenly distributed over the operation time, so that the standard deviation of the fluxes between the individual fibers started to increase rapidly. Consequently, the TMP values rose to maintain the average flux across the fiber bundle, reflecting the increase in the standard deviation of the fluxes. At some point, both TMP and standard deviation rose rapidly. This is believed to be due to flux maldistribution within the bundle, which leads to local pore and flow channel fouling. It was possible to obtain a steadier TMP pattern and standard deviation profiles, when the flow regime around the fibers was more rigorously controlled by applying higher liquid and/or air flow rates. More recently, the TMP jump was also attributed to the poor oxygen transfer within the fouling layer. As a result, bacteria in the biofilm layer can die, releasing extra levels of Soluble Microbial Products (SMP). Experimental data have shown an increase of SMP concentrations at the bottom part of the fouling layer, when the level of dissolved oxygen decreases (Gkotsis et al., 2014; Judd, 2011).
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3. Fouling Control and Mitigation Methods Notwithstanding the significant progress of MBR technology, membrane fouling remains the primary drawback for its universal and full-scale application, since it reduces the system’s productivity, increases the energy requirement for gas scouring and frequency of chemical cleaning, and results in higher membrane replacement costs. The methods which are applied for fouling control and mitigation in MBR systems can be distinguished into conventional methods, which have been implemented since the early years of MBR technology application and include the operation of the system under low fluxes and the application of simple physical or chemical methods, and innovative methods, which are implemented mainly in recent years and include the use of a series of different state-of-the-art materials and methods, such as the use of specific additives, the application of electric field, ultrasound, and/or ozone, the appropriate modifications of membrane surface, etc. It must be noted that the innovative methods are not applied individually, but they are usually used in combination with a conventional method, aiming to result in more effective fouling reduction.
3.1 Conventional Methods for Fouling Control The conventional methods, which are employed in order to achieve fouling control and mitigation, mainly include the optimal operation of MBR process, i.e., the operation under lower than critical permeate fluxes, and the application of various physical or chemical methods. MBR systems can operate under either a constant flux operation or a constant TMP mode, and fouling is indicated by the increase of TMP values or by the flux reduction, respectively. In the first mode, which is the most common for the MBR systems, it is generally accepted that keeping the flux at high levels favors membrane fouling and thus, must be avoided. This is logical, since the application of lower fluxes can delay the adsorption of foulants and prolong the membrane operating lifetime. On the other hand, the application of very low fluxes is not an economical procedure, because larger membrane surfaces would be required. For this reason, the respective critical flux is usually determined before the start-up of the reactor. Typical flux values in most MBR systems range between 10 and 50 L/m2$h. The most common physical methods include the operation of MBR systems by increasing the aeration rate, the application of backwashing, and the membrane relaxation. In general, the
Chapter 7 Fouling Challenges in Ceramic MBR Systems
increase of aeration rate favors the mitigation of fouling, due to the presence of high shear stresses, which are applied on the membrane surface and can lead to easier removal of deposited microbial bio-flocs. Typical aeration rates range between 0.18 and 7 m3/m2$h, depending on the membrane type and the system’s specific capacity. Backwashing (also known as backflushing) refers to the periodic reversal of the filtration flow in the opposite direction. It effectively removes a significant part of the cake layer from the membrane surface; however, its application is usually limited to hollow fiber or tubular membranes. Typical backwashing flow rates may range between 25 and 40 L/m2$h, although much higher values can be applied in the case of ceramic membranes’ use, which are particularly hydraulic resistant and able to operate even at 150 L/m2$h. Membrane relaxation is the periodic interruption of filtration for a short period of time, usually for a few seconds or minutes. The regular interruptions of filtration rate have shown that the deposited foulants can be diffused from the membrane surface back to the mixed liquor, resulting in the “relaxation” of the membrane and the increase of its operating lifetime. The application of chemical cleaning can remove effectively small particles and soluble components, which have been deposited on the membrane surface, or even inside the membrane pores. The chemical reagents, which are commonly used for chemical cleaning, usually belong to one of the following four categories: (1) acids, such as C2H2O4, C6H8O7, HNO3, HCl, H3PO4, H2SO4, etc., (2) bases, such as NaOH, KOH, Na2CO3, etc., (3) oxidants or disinfectants, such as NaClO, H2O2, O3, etc., and (4) other chemical reagents (e.g., surfactants, complexes, etc.), such as EDTA, DTPA, SDS, etc. The latest trend in chemical cleaning includes the use of chemical reagents from different categories in order to remove all different types of fouling (biofouling, organic, and inorganic fouling) potentially found in an MBR operating system (Yoon, 2016; Wang et al., 2014a,b).
3.2 Innovative Methods for Fouling Control Over the last few years, several novel strategies have been employed to prevent or control effectively the problem of membrane fouling. Most of them include the (bio)chemical mixed liquor modification of properties, such as the addition of specific chemicals (e.g., coagulants, adsorbents, and/or biofilm carriers), the application of ultrasound, electric field, and/or ozone, as well as various membrane surface modifications.
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3.2.1 Use of Additives In recent years, the use of additives is the alternative method mostly applied for fouling control in MBR systems. Fouling mitigation is achieved via the modification of the mixed liquor’s characteristics, which can subsequently improve the filtration process and increase the membrane operation lifetime. The used additives can be classified into three general categories: (1) coagulants/flocculants, (2) adsorbents, and (3) biofilm carriers. The addition of appropriate (inorganic) coagulants can provide positive charges in a (usually) negatively charged biomass and enhance further the flocculation of microbial bio-flocs, which results in the increase of mixed liquor’s biomass filterability. As a result, the TMP rate is maintained at relatively lower levels and the membrane’s operating time is extended. The coagulants, which are usually used in most MBR systems in order to prevent or mitigate fouling, are simple metal Fe or Al salts, such as FeCl3, FeSO4, Al2(SO4)3, etc., as well as inorganic polymeric flocculants (IPF), such as polyferric chloride (PFCl), polyferric sulfate (PFS), polyaluminum chloride (PACl), etc., or organic polymers (flocculants), such as polyacrylamides, specific chitosans, tannins, etc. (Park et al., 2018; Iorhemen et al., 2016). The addition of adsorbents has also been shown to reduce fouling in MBRs. This is not only due to the formation of a porous, low-resistant layer on the membrane surface, but also due to the preliminary adsorption (and removal from the solution) of the substances, which are considered responsible for fouling. Powdered Activated Carbon (PAC), Granular Activated Carbon (GAC), and zeolites are the most commonly applied materials of this category (Nguyen et al., 2014). Bio-Film Carriers or Biocarriers (BFC) are specially designed, small-sized plastic materials, which are added in the mixed liquor of MBR systems at various filling ratios, aiming to mitigate fouling. Fouling mitigation takes place either by the application of high shear stresses near to the membrane surface, caused by their rigorous movement, due to aeration, or/and the decrease of SMP concentration via the enhanced adsorption (and removal) of proteins and polysaccharides content by the deposited biomass onto the biocarriers. Depending on the application, the filling ratio in the bioreactor ranges between 20% and 80% (Yoon, 2016).
3.2.2 Application of Electric Field, Ultrasound, and/or Ozone The applications which employ an electric field in MBRs can be divided into three categories: (1) Electro-coagulation (EC),
Chapter 7 Fouling Challenges in Ceramic MBR Systems
(2) Electrophoresis (EP), and (3) Microbial Fuel Cell (MFC). In the first two applications, which are most commonly found in MBRs, an external electric field is applied, while in the MFC, the electrical energy is in situ generated within the cell. During EC, the produced cations form aggregates with the negatively charged components of the biomass, resulting in the improvement of biomass filterability. During EP, the negatively charged components of biomass are led toward the positively charged anode, and thus, drawn away from the membrane surface (Hua et al., 2015; Zhang et al., 2015). Ultrasound has been also employed to mitigate fouling in MBRs. It is reported that the tiny bubbles, which are formed by the acoustic waves, can produce high shear stresses on the membrane surface, resulting in the delay or even the removal of the developed cake layer. The most important advantages of this method are that it can be easily combined with other fouling mitigation methods to improve effectiveness and therefore, the use of chemical reagents can be avoided. Ozone is a powerful oxidant that breaks down large polymeric compounds, and thus, it can be used to mitigate fouling in MBRs by reducing (breaking down, disintegrating) at least partly the colloidal or soluble organic substances’ content. In addition, it was reported that ozonation can increase the size of microbial flocs, due to changes in the zeta-potential values, resulting in the increase of sludge filterability (Yu et al., 2012; Liang et al., 2014).
3.2.3 Membrane Surface Modification Several of the membrane surface characteristics, such as hydrophilicity, charge, and roughness, are known to be strongly related to fouling, because they determine the interaction between the membrane and the foulant agents. The majority of commercially available membranes for pressure-driven processes are made from hydrophobic polymers with relatively high thermal, chemical, and mechanical stabilities. However, because of the hydrophobicity of these materials, they are prone to adsorption/interaction with the fouling substances, which are partly hydrophobic. It has been well documented that membranes with hydrophilic surfaces are less susceptible to fouling. Therefore, an increase in the hydrophilicity of membrane surface is often a key goal to reducing membrane fouling by the presence of organic pollutants and/or microorganisms. The membrane surface charge is also another important consideration for reducing membrane fouling. Usually, it is appropriate to employ membranes with a negative surface charge, because the
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biomass is negatively charged as well. As a result, the electrostatic repulsion created between the foulants and the membrane can prevent their deposition on the membrane, thereby to mitigate fouling. Over the last few years, several membrane surface modification methods have been applied in order to increase hydrophilicity and the negative surface charge, and thus, to mitigate fouling in MBRs. These methods include the physical coating/adsorption or grafting on the membrane surface, the use of specifically patterned membranes, the plasma treatment, or other chemical reactions on the membrane surface, or the surface modification by nanoparticles: • The physical coating of a thin layer of water-soluble polymers or surfactants is a flexible technique to optimize the hydrophilicity, smoothness, and surface charge of the membrane surface. Physical coating can take place via filtration, adsorption, or casting techniques. • Membranes with a patterned surface have also become recently an attractive option for fouling mitigation in membrane processes applied for water or wastewater treatment. The origin of this enhanced antifouling behavior is attributed to enhanced shear-induced diffusion, whereby the presence of submicron surface patterns increases the effective shear rate near the membrane surface. • Over the last 2 decades, plasma treatment has been intensively studied, attempting to increase the hydrophilicity and to induce the low-fouling properties for membrane surfaces. Usually, plasma treatment of the membranes can be carried out in three different modes: (1) with non-polymerizable gas molecules, (2) with polymerizable vapors, and (3) with plasma-induced grafting of the polymer chains to the membrane surface. • The introduction of charged groups on the membrane surface can be also a useful approach to mitigate membrane fouling, caused by charged organic compounds. In this type of modification, various chemical reactions may be used for creating different functional groups, such as eSO3 or eCOOH, on the membrane surface. The number of the introduced functional groups and the thickness of the modified surface layer depend mainly on the treatment time, temperature, and concentration of modification agent. • The use of nanoparticles has received particular attention, especially during the last few years, attempting to enhance flux rates and to reduce fouling. Two different methods have been used for preparing modified nanoparticle-based membranes: (1) the deposition of nanoparticles on the membrane
Chapter 7 Fouling Challenges in Ceramic MBR Systems
surface through dipping of a porous support in an aqueous suspension of nanoparticles and (2) the entrapment of nanoparticles in a polymer matrix via a phase inversion method by the addition of the nanoparticles to the casting solution (Gkotsis et al., 2014; Hilal et al., 2012; Kochkodan et al., 2014).
4. Fouling Control in Ceramic MBRs Several research studies have suggested the use of ceramic membranes as an alternative substitute of conventional polymeric membranes, due to their higher stability against chemical cleaning reagents and their structural ability to withstand higher temperatures and mechanical forces, than the respective polymeric membranes, already in wider use for the MBR treatment systems. Nevertheless, and despite their great potential, membrane fouling still remains the major bottleneck for the widespread application of ceramic MBRs, as it increases substantially the operating and capital costs. Therefore, a meticulous study and understanding of membrane fouling in such systems is essential in order to find the most appropriate control methods, which will deal effectively with future challenges. In view of possible damages that the addition of several chemical cleaning agents, such as NaClO, NaOH, ozone, etc., could bring to the polymeric membranes, the ceramic membranes are preferred for their greater stability and integrity under chemical attacks. Zhang et al. (2018) treated high-strength dyeing wastewater by using an anaerobic flat sheet ceramic MBR. Membrane fouling was controlled by the implementation of backwashing as a physical method, while the ceramic membranes were chemically cleaned for 1e2 h with 1000 mg/L of NaOCl, when the TMP was reached 30 kPa. The results showed that the addition of NaOCl was an effective method for fouling control, since the TMP was restored to the initial value after this specific chemical cleaning. During the operation of a MBR system, the TMP values were less than 10 kPa, indicating higher permeability of the flat sheet ceramic membrane. This enabled the system to operate also by using lower energy consumption. Wang et al. (2014a,b) developed a new method, which allowed the backwashing of ceramic membranes at much lower NaClO concentrations (Cbackwashing ¼ 0.05e1.5 mg/L), although it has to be applied more frequently, in order to eliminate the need for a more intensive thorough chemical cleaning (Fig. 7.3). The results showed that there is an optimal backwashing NaClO concentration (0.2 mg/L), which effectively reduced fouling, combining
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Pure water backwashing
NaClO solution backwashing
Permeate
Permeate
Air
Air
Figure 7.3 Schematic illustration of the aerobic ceramic MBR setup (Wang et al., 2014a,b).
the direct membrane cleaning with the indirect impact on microbial activities. Yue et al. (2018) also employed the NaClO-assisted backwashing (Cbackwashing ¼ 0.05, 0.25, 1, and 10 mg/L) for the mitigation of fouling in an anaerobic ceramic MBR, which treated domestic wastewater. The results showed that the addition of NaClO at the concentration of 1 mg/L was capable to mitigate the membrane fouling. Mei et al. (2017) used a ceramic membrane to elucidate the role of NaOH backwashing in membrane fouling mitigation and its impact on the bioreactor performance (Fig. 7.4). Ex-situ cleaning tests showed that the addition of NaOH at concentrations between 0.05 and 1.30 mmol/L had a positive impact on the anaerobic biomass, while higher concentrations (>1.30 mmol/L) provoked toxic effects. However, the in-situ cleaning tests revealed that the membrane fouling rate was significantly reduced after backwashing with 10e20 mmol/L of NaOH and the methanogenic activity was also slightly improved. In addition, economic analysis showed that 12 mmol/L was the optimal concentration for fouling control. Sun et al. (2018a,b) investigated the membrane fouling potential of the Dissolved Organic Matter (DOM) and of the halogenated by-products, which were generated during the on-line chemical cleaning of a ceramic MBR by the application of ozone (Fig. 7.5). The results revealed that the released DOM could considerably cause irreversible membrane fouling, with the humic acids fraction having about 500 Da molecular weight, being mainly responsible for the observed fouling. This study offered a better understanding of membrane fouling associated with DOM, which was generated by the addition of ozone during the on-line chemical cleaning, enabling further the design of a ceramic MBR system toward a long-term operation stability.
Chapter 7 Fouling Challenges in Ceramic MBR Systems
Figure 7.4 Schematic illustration of an anaerobic ceramic MBR setup (Mei et al., 2017).
Apart from their remarkable performance in terms of compatibility with aggressive chemical (cleaning) reagents, several studies have shown that the filtration with ceramic membranes was improved in terms of TMP reduction, or of normalized flux increase (i.e., at constant flux conditions or at constant pressure operation mode, respectively) (Jeong et al., 2017). For example, Lee et al. (2013) have found that ceramic membranes present lower fouling propensity than the polymeric membranes, due to the weaker bonding between foulants and membranes. Therefore, not only the required physical and chemical cleaning times can be reduced, but also certain laborious maintenance tasks, such as repair and replacement of polymeric hollow fibers (membranes), can be also eliminated. € ppenbecker et al. (2017) introduced a novel AnMBR system Du with an external cross-flow ceramic membrane, which employed fluidized glass beads as turbulence promoters to mitigate fouling (Fig. 7.6). The reactor treated raw municipal wastewater and three ceramic membranes (two ultrafiltration membranes made from ZrO2 or Al2O3 and one microfiltration membrane made from TiO2) were evaluated in terms of fouling behavior, mechanical resistance, and pollutants’ removal performance during 46
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(A)
Ozone generator
Mass flow controller Ozone destroyer
Oxygen concentraor
Ozone water tank Ozone sensor Pump
Reactor
PC
(B) Permeate pump
Crossflow cell
P
P Pressure Influent pump
transducer
Flow meter
Magnetic stirrer
PC
Digital balance
Figure 7.5 Schematic illustration of the (A) ozone generation and application system for membrane fouling mitigation, and (B) a cross-flow membrane filtration system (Sun et al., 2018b).
Figure 7.6 Schematic illustration of an anaerobic ceramic MBR system (D€ uppenbecker et al., 2017).
Chapter 7 Fouling Challenges in Ceramic MBR Systems
operation days. The results showed that the use of ceramic membranes with the fluidized glass beads content can significantly reduce the membrane fouling. In addition, the mean soluble COD removal rates for the ZrO2, Al2O3, and TiO2 membranes were 50%, 55%, and 41%, respectively. The aforementioned studies underline the superior performance of ceramic membranes in terms of fouling mitigation and removal performance, as compared to most common polymeric membranes in use. Nevertheless, despite their great potential in municipal and industrial wastewater treatment, there are still important challenges that need to be dealt for their full-scale widespread application and promotion.
5. Conclusions and Future Trends Ceramic membranes have outstanding features over most polymeric membranes, because of their remarkable robustness, easier physical and chemical cleaning, and higher life span. In addition, their remarkable separation performance has proved their capability for a wide range of municipal and industrial wastewater treatment and desalination applications. However, there are still important challenges and questions that need to be carefully addressed in the future research in the field of ceramic MBRs, such as the improvement of antifouling properties, the optimization of investment cost by introducing new fabrication and coating technologies, and the improvement of selectivity, water permeability, and packing densities, due to the relevant fragility of ceramic materials (God and Ismail, 2018; Samaei et al., 2018). Given that the ceramic membranes are resistant to harsh environmental (or experimental) operation conditions, several more aggressive chemical cleaning protocols can be effectively used for fouling control in comparison with the polymeric membranes. Furthermore, in the submerged AnMBR configuration, where anaerobic conditions must be maintained, it is difficult to remove the membranes from the bioreactor for cleaning purposes, without influencing the anaerobic environment. Therefore, anaerobic ceramic MBR systems with relatively lower fouling potentials are a challenging issue, when treating municipal or industrial wastewaters. Nevertheless, the practical implementation of ceramic membranes in MBR systems has been limited, because the raw construction materials (i.e., Al2O3, TiO2, ZrO2, or SiC) are still quite expensive. For this reason, research is turning toward the development of cost-effective ceramic membranes, which are made of natural mineral-based
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materials, such as pyrophyllite, dolomite, kaolin, zeolite, or Moroccan clay. Considering that these ceramic materials are cheaper than for e.g., the purified alumina, the natural mineralbased ceramic membranes can be an effective way to reduce their manufacturing costs (Jeong et al., 2017). The use of ozone in order to minimize toxicity issues of several wastewaters, the development of ceramic membranes with high resistance to irradiation and high electrical conductivity with a view to their specific applications in photocatalytic or electrochemical systems, respectively, are also some interesting suggestions for future research. Past studies have also showed that the number of ceramic membrane applications in the mining and pharmaceutical sector is limited in comparison with other industrial sectors, probably due to the complex nature of acid mine drainage, or pharmaceutical wastewaters. Consequently, future research efforts should also focus on the further investigation for different pore sizes of ceramic membranes in combination with the appropriate physico-chemical techniques during the pre- or post-treatment of problematic mining or pharmaceutical effluents. The manufacturing of bio-ceramic materials with selective adsorption properties could also be a promising technique for future applications. These materials may be useful as biological filters, e.g., for air filtration or for wastewater treatment. The incorporation of bio-ceramic modules can be useful in treatment plants fed by industrial wastewaters, while they can contribute to the production of lower amounts of wasted bio-sludge/biosolids as well.
List of Acronyms AnMBR ASP BFC COD DOM DTPA EC EDTA EP EPS GAC IPF MBR MFC PAC PACl
Anaerobic Membrane Bio-Reactor Activated Sludge Process Bio-Film Carriers Chemical Oxygen Demand Dissolved Organic Matter Diethylenetriaminepentaacetic acid Electrocoagulation Ethylenediaminetetraacetic acid Electrophoresis Extracellular Polymeric Substances Granular Activated Carbon Inorganic Polymeric Flocculants Membrane Bio-Reactor Microbial Fuel Cell Powdered Activated Carbon Polyaluminum chloride
Chapter 7 Fouling Challenges in Ceramic MBR Systems
PFCl PFS SDS SMP TMP
Polyferric chloride Polyferric sulfate Sodium Dodecyl Sulfate Soluble Microbial Products Trans-Membrane Pressure
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