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Continuous increasing discharge of industrial oily wastewater and frequent occurrence of oil spill accidents have taken heavy tolls on global environment and human health. Organic-inorganic modifications can fabricate superhydrophilic/submerged superoleophobic membranes for efficient oil-water separation/treatment though they still suffer from complex operation, non-environmental friendliness, expensive cost or uneven distribution. Herein, a new strategy regarding tannic acid (TA)-Ti(IV) coating and CaCO

While ethylenediamine tetramethylenephosphonic acid (EDTMPA) has been emerged as a stronger chelating agent than ethylene diamine tetraacetic acid (EDTA) for fouling mitigation, and transparent exopolymer particles (TEP) is a major foulant in membrane-based water treatment process, effects of EDTMPA on TEP fouling and the underlying mechanism have been not yet studied. In this study, Flory-Huggins lattice theory was combined with density functional theory (DFT) technology to explore this subject at molecular level. Filtration experiments showed a unimodal pattern of specific filtration resistance (SFR) of TEP sample with Ca

While transparent exopolymer particles (TEP) is a major foulant, and ethylene diamine tetraacetic acid (EDTA) is a strong chelating agent frequently used for fouling mitigation in membrane-based water treatment processes, little has been known about TEP-associated membrane fouling affected by EDTA. This work was performed to investigate roles of EDTA addition in TEP (Ca-alginate gel was used as a TEP model) associated fouling. It was interestingly found that, TEP had rather high specific filtration resistance (SFR) of 2.49 × 10

It is of great importance to propose effective methods to quantify interfacial interaction since it directly determines foulant adhesion and membrane fouling process in membrane bioreactors (MBRs). This study developed a radial basis function (RBF) artificial neural network (ANN) to predict the interfacial interactions with randomly rough membrane surface. The interaction data quantified by the advanced extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) approach were used as the training samples for the RBF networks. It was found that, the computing time consumption for the RBF network prediction was only about 1/50 of that for the advanced XDLVO approach under same conditions, indicating the high efficiency of the RBF ANN method. Meanwhile, the calculation accuracy of the method was acceptable to get reliable results. This study demonstrated the breakthrough of the fundamental methodology related with membrane fouling. The proposed RBF ANN method has broad application prospects in membrane fouling and interface behavior research.

Effects of calcium ions and polyaluminum chloride (PACl) on membrane fouling in coagulation-ultrafiltration (UF) process were investigated in this study. Filtration tests demonstrated three interesting filtration behaviors: 1) high specific filtration resistance (SFR) of alginate solution with low CaCl2 or PACl addition (e. g. 3.51×1015 m·kg -1 under the condition of 1.5 mM CaCl2 addition); 2) unimodal pattern of alginate SFR with PACl or CaCl2 addition alone; 3) synergistic effects between CaCl2 and PACl on alginate SFR. It was found that, the foulant morphological changes driven by the thermodynamic mechanisms based on Flory-Huggins lattice theory take the critical roles in these filtration behaviors. Density functional theory (DFT) calculations showed that initial coordination of Ca2+ and Al3+ ions with alginates tended to form tetrahedron geometry and geometry of coordinating three terminal carboxyl groups, respectively, which facilitated to elongate the alginate chains (without clustering the flocs) and form more stable gel, increasing SFR. Improving Ca2+ and Al3+ dosages triggered transition to other geometries for clustering polymeric network and flocculation, reducing SFR. Due to the higher binding affinity of Ca2+ over Al3+, Ca2+ and Al3+ sequentially take roles of enlarging polymeric network and clustering the coordination compounds, and then facilitate to form large size flocs and reduce SFR, causing the synergistic effects between CaCl2 and PACl additions. The proposed thermodynamic mechanisms satisfactorily explained these interesting fouling behaviors, allowing to further optimize coagulation-UF process.

Membrane fouling represents one of the greatest challenges for widespread applications of polyvinylidene fluoride (PVDF) membranes. This study proposed a new strategy regarding grafting hydroxyethyl acrylate (HEA) via γ ray radiation to improve the antifouling ability of PVDF membranes. The grafted membrane possessed improved antifouling ability for filtration of sodium alginate (SA) solution which was experimentally evidenced by the 92.03% flux recovery ratio (69.68% for the pristine membrane) and 11.54% irreversible fouling ratio (32.76% for the pristine membrane). When subjected to filtration of bovine serum protein (BSA) solution, the grafted membrane showed lower flux decline rate than the pristine membrane, and interestingly, higher BSA solution flux than pure water flux. Thermodynamic analyses indicated that the improved hydrophilicity and the reduced strength of thermodynamic interactions between the grafted membrane and foulants were mainly responsible for the improved antifouling performance. Meanwhile, thermodynamic swelling process affected by ionic strength induced higher BSA solution flux than pure water flux. The revealed thermodynamic mechanisms facilitated to optimize the grafting strategy. This study not only proposed a grafting strategy, but also provided interesting PVDF membranes whose flux can be adjusted by ionic strength, giving important implications for membrane fouling research.

Quantification of interfacial interactions between particles provides a way to regulate the interface behaviors of particles related with adhesion, aggregation, flotation, flocculation, membrane fouling, etc. Existing methods are based on assumptions of smooth particles although real particle surfaces are rather rough. This study proposed a new method to quantify interfacial interactions between two rough particles. In this study, a rigorous mathematical equation was firstly introduced to construct surface topography. In the framework of surface element integration (SEI) method, the spatial relationship between two rough particles was significantly explored, resulting in establishment of a formula of double integrals for interaction quantification. Thereafter, surface properties of the microbial aggregations obtained from a membrane bioreactor (MBR) were experimentally measured. With these data, the interfacial interactions between two rough microbial aggregations were numerically quantified according to composite Simpson's rule. The new method was compared with Derjaguin approximation (DA) method. It was found that ripple frequency and particle radius had profound effects on the total interfacial interaction. This method has extensive application foreground in interfacial behavior research.

Simulation of randomly rough bioparticle surface is crucial to better understand and control interface behaviors and membrane fouling. Pursuing literature indicated a lack of effective method for simulating random rough bioparticle surface. In this study, a new method which combines Gaussian distribution, Fourier transform, spectrum method and coordinate transformation was proposed to simulate surface topography of foulant bioparticles in a membrane bioreactor (MBR). The natural surface of a foulant bioparticle was found to be irregular and randomly rough. The topography simulated by the new method was quite similar to that of real foulant bioparticles. Moreover, the simulated topography of foulant bioparticles was critically affected by parameters correlation length (l) and root mean square (σ). The new method proposed in this study shows notable superiority over the conventional methods for simulation of randomly rough foulant bioparticles. The ease, facility and fitness of the new method point towards potential applications in interface behaviors and membrane fouling research.

The super anti-adhesion ability of lotus leaf inspires to fabricate anti-adhesion membranes. To achieve this goal, a novel approach to quantify the interfacial forces between a randomly rough particle and membranes with special surface morphology, was developed. In this study, the rough surfaces of foulant particle and membrane were firstly modeled by the modified two-variable Weierstrass-Mandelbrot (WM) function and a rigorous mathematical function, respectively. Thereafter, quantification approach based on the combination of surface element integration (SEI) method and composite Simpson’s rule was deduced. This approach was verified to be feasible to quantify the interfacial forces between a rough particle and a rough flat surface. By using this novel approach, effects of scaled amplitude and water contact angle of membrane surface on interfacial forces were investigated. It was found that, the attractive forces between foulants and membrane surface would rather weaker or even become repulsive with increase in size of asperities and surface hydrophilicity, well explaining the phenomenon regarding the super anti-adhesion ability of lotus leaf. This study gives important implications for fabrication of anti-adhesion membranes.

Fractal roughness is one of the most important properties of a fractal surface. In this study, it was found that, randomly rough membrane surface was a fractal surface, which could be digitally modeled by a modified two-variable Weierstrass-Mandelbrot (WM) function. Fractal roughness of membrane surfaces has a typical power function relation with the statistical roughness of the modeled surface. Assessment of interfacial interactions showed that an increase in fractal roughness of membrane surfaces will strengthen and prolong the interfacial interactions between membranes and foulants, and under conditions in this study, will significantly increase the adhesion propensity of a foulant particle on membrane surface. This interesting result can be attributed to that increase in fractal roughness simultaneously improves separation distance and interaction surface area for adhesion of a foulant particle. This study gives deep insights into interfacial interactions and membrane fouling in MBRs.

Influences of fractal dimension (Df) of membrane surface on interfacial interactions related to membrane fouling in a membrane bioreactor were investigated based on thermodynamic methods. It was found that membrane surface had significant fractal features, and its fractal dimension could be characterized by the power spectrum method. The modified Weierstrass-Mandelbrot (WM) function was found to be effective to model the fractal membrane surface, and higher Df corresponded to higher number of fine asperities in the modeled surface. Moreover, the modeled surface roughness exponentially decreased with Df. Interaction calculations according to a novel method showed that the interactions for fractal membrane surface were elongated and weakened as compared with smooth membrane surface. It was interestingly found that the absolute value of total interaction monotonically decreased with Df of membrane surface. As Df is a measure of substance stiffness, this result indicates that softer surface is more susceptible to adhesion by sludge foulant. The results offered new insights into membrane fouling mechanisms and alleviation.

This study provided the first attempt of grafting hydrophobic polyvinylidene fluoride (PVDF) membrane with hydrophilic hydroxyethyl acrylate (HEA) monomer via a radiation grafting method. This grafted membrane showed an enhanced hydrophilicity (10° decrease of water contact angle), water content ratio, settling ability and wettability compared to the control membrane. Interestingly, filtration tests showed an improved dependence of water flux of the grafted membrane on the solution pH in the acidic stage. Atomic force microscopy (AFM) analysis provided in-situ evidence that the reduced surface pore size of the grafted membrane with the solution pH governed such a dependence. It was proposed that, the reduced surface pore size was caused by the swelling of the grafted chain matrix, with the pH increase due to the chemical potential change. It was found that the grafted membrane showed a lower relative flux decreasing rate than the control membrane. Moreover, flux of the bovine serum albumin (BSA) solution was noticeably larger than that of pure water for the grafted membrane. Higher BSA flux than water flux can be explained by the effects of electric double layer compression on the polymeric swelling. This study not only provided a pH-sensitive PVDF membrane potentially useful for various applications, but also proposed novel mechanisms underlying the enhanced performance of the grafted membrane.

In spite of the advantages and popularity of membrane bioreactor (MBR) technology in wastewater treatment, membrane-fouling control is still the major scientific and practical concern of this technology. This chapter reviews the fundamental membrane-fouling mechanisms and the factors affecting membrane fouling in aerobic MBRs. Various membrane-fouling control strategies in MBRs are classified into five groups, and each group is critically summarized. Based on these reviews, future research needs regarding fouling control strategy are proposed.

While water chemistry (e.g., ionic strength, calcium concentration and organic foulants) is the primary property of surface water, its effects on membrane fouling in process of membrane-based water production and seawater pretreatment have not well investigated. In this study, fouling behaviors of alginate solutions in presence of different calcium ion concentration and ionic strength levels were investigated. It was found that alginate solutions complexing with 1.5 mM calcium possessed a remarkably high specific filtration resistance (SFR) (above 3.596 × 1015 m kg−1), and the SFR descended with calcium concentration and increased with ionic strength. A series of characterizations suggested that zeta potential, particle size, viscosity and morphology of alginate solutions were close related with foulant layer microstructure and these fouling behaviors. Based on these characterizations, the thermodynamics described by Flory-Huggins lattice theory was proposed to explain the remarkably high SFR of alginate gel for 1.5 mM calcium level. Meanwhile, preferential intermolecular coordination combined with Flory-Huggins lattice theory was suggested to be responsible for the descend trend of SFR with calcium concentration. Furthermore, electrostatic double layer compression effect together with Flory-Huggins lattice theory could well interpret the increase trend of SFR with ionic strength. This study provided the essential mechanisms underlying effects of ionic strength on alginate fouling in presence of calcium ions, and thus deepened understanding of membrane fouling.

Efficient quantification of interfacial energy related with membrane fouling represents the primary interest in membrane bioreactors (MBRs) as interfacial energy determines foulant layer formation. In this study, radial basis function (RBF) artificial neural networks (ANNs) with five related factors as input variables were applied to quantify interfacial energy with randomly rough membrane surface. It was found that, RBF ANNs could well capture the complex non-linear relationships between the related factors and interfacial energy. RBF ANN quantification showed high regression coefficient and accuracy, suggesting its high capacity to quantify interfacial energy. Compared to at least one-week time consumption of the advanced extensive Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach, quantification by RBF ANNs only took several seconds for a same case, indicating the high efficiency of RBF ANNs. Moreover, the abilities of RBF ANNs can be further improved. The robust RBF ANNs proposed paved a new way to study membrane fouling in MBRs.

Gel layer formation in some cases directly determines membrane fouling extent in membrane bioreactors (MBRs). While hydrogen bonding interactions extensively exist in gelling foulants and sludge suspension, their exact roles in fouling remain unveiled. Filtration results in this study showed that, specific filtration resistance (SFR) of a gel layer formed in the MBR was as high as 2.06 × 1019 m−1·kg−1 at 20 °C, and moreover, SFR of both the real gel and model gel (Poly(N-isopropylacrylamide) (PNIPAM)) decreased with temperature. Fourier-transform infrared spectroscopy (FTIR) analysis indicated that gel samples were abundant of good hydrogen bonding donors/acceptors to form hydrogen bonding, and hydrogen bonding strength decreased with temperature. From viewpoint of free energy, mathematical models depicting roles of hydrogen bonding were proposed. For the first time, contribution level of hydrogen bonding effects to total gel SFR was quantified to be around 20%. These results offered in-depth insights into membrane fouling in MBRs.

Impacts of morphologies of both membrane and foulant on interaction energies related with adhesive fouling in a membrane bioreactor (MBR) were explored by thermodynamic analyses. Interaction energies in three possible interaction scenarios regarding different membrane and foulant morphologies under conditions in this study were quantified according to the thermodynamic methods. It was interestingly found that, strength of total interaction between soluble microbial products (SMPs) and rough membrane was over 20,000 times of that between sludge flocs and rough membrane under same conditions, indicating the extremely higher adhesion ability of SMPs than the large particulate foulants. This result plausibly explained the high fouling propensity of SMPs over sludge flocs. As compared with smooth surfaces, rough surfaces of both membrane and sludge flocs significantly reduced total interaction strength, alleviating adhesive fouling caused by the sludge flocs. Reduce in fractal dimension (Df) of membrane increased adhesive fouling caused by the SMPs, but alleviated adhesive fouling caused by the sludge flocs. These findings gave important implications to better understand and control membrane fouling in MBRs.

While membrane bioreactor (MBR) technology is generally considered as one of the most promising technologies for wastewater treatment and recovery, membrane fouling remains the major obstacle limiting its applications. Interfacial interactions, which critically determine adhesion process and membrane fouling, were investigated in this study. It was found that, natural membrane surface was of a Gaussian surface obeying Gaussian distribution. A Gaussian approach integrating Fourier transform technique, Gaussian distribution and spectrum method was deduced to simulate rough surface topography of membrane. Thereafter, surface element integral (SEI) method, together with composite Simpson rule and triangulation of Gaussian surface was proposed to calculate interfacial interactions. By using the unified method, quantification of interfacial interactions with a Gaussian membrane surface was realized for the first time to date. It was further found that, membrane surface topography had profound impacts on interfacial interactions and adhesive fouling in the MBR. The deduced method can be used to address impacts of various factors on interfacial interactions and adhesive fouling, posing in-depth thermodynamic insights into membrane fouling and pointing towards its widespread potential in fouling research in MBRs.

In this study, mechanisms and roles of gel elasticity in extremely high specific filtration resistance (SFR) were investigated. It was found that, as compared with cake layer in a membrane bioreactor (MBR), real gel layer in the MBR and agar gel possessed extremely high SFR. Foulant characterization showed that foulants were easy to bind water, and agar gel possessed a network structure. Mechanisms based on Flory-Huggins and Flory-Rehner models were deduced to describe the high SFR of agar gel. Model simulation showed that sum of SFR induced by the mixing chemical potential and the elastic chemical potential change is close to that of the agar gel, suggesting feasibility of the deduced models. Gel elasticity accounted for about 13% of total SFR of agar gel under conditions in this study. This study satisfactorily explained the extremely high SFR of gel, and addressed roles of gel elasticity in gel SFR.

In this study, polyvinylidene fluoride (PVDF) microfiltration membrane was coated by dipping the membrane alternatingly in solutions of the polyelectrolytes (poly-diallyldimethylammonium chloride (PDADMAC) and polystyrenesulfonate (PSS)) via layer-by-layer (LBL) self-assembly technique to improve the membrane antifouling ability. Filtration experiments showed that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the remained flux ratio (RFR) of the coated membrane was obviously improved as compared with the control membrane. Characterization of the membranes showed that a polyelectrolyte layer was successfully coated on the membrane surfaces, and the hydrophilicity, surface charge and surface morphology of the coated membrane were changed. Based on the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) approaches, quantification of interfacial interactions between foulants and membranes in three different scenarios was achieved. It was revealed that there existed a repulsive energy barrier when a particle foulant adhered to membrane surface, and the enhanced electrostatic double layer (EL) interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. This study provided a combined solution to membrane modification and interaction energy evaluation related with membrane fouling simultaneously.