Meijia Zhang

Fouling mechanisms of gel layer in a submerged membrane bioreactor

The fouling mechanisms underlying gel layer formation and its filtration resistance in a submerged membrane bioreactor (MBR) were investigated. It was found that gel layer rather than cake layer was more easily formed when soluble microbial products content in sludge suspension was relatively high. Thermodynamic analyses showed that gel layer formation process should overcome a higher energy barrier as compared with cake layer formation process. However, when separation distance <2.3 nm, attractive interaction energy of gelling foulant-membrane combination was remarkably higher than that of sludge floc-membrane combination. The combined effects were responsible for gel layer formation. Filtration tests showed that specific filtration resistance (SFR) of gel layer was almost 100 times higher than that of cake layer. The unusually high SFR of gel layer could be ascribed to the gelling propensity and osmotic pressure mechanism. These findings shed significant light on fouling mechanisms of gel layer in MBRs.

Membrane fouling in a membrane bioreactor: High filtration resistance of gel layer and its underlying mechanism

A membrane bioreactor (MBR) was continuously operated to investigate mechanisms of fouling caused by the gel layer in this study. Agar was used as a model foulant for gel layer formation, and filtration resistance of gel layers was systematically assessed. The results showed that gel layer possessed unusually high specific filtration resistance (SFR) and high measured porosity as compared with cake layer. Current knowledge cannot explain the contradiction between high filtration resistance and high porosity of gel layer. A new fouling mechanism based on Flory-Huggins theory was then proposed. Filtration resistance of agar gel layer was found to be independent of pH and ionic strength, but linearly increase with gel thickness. The results are accordant with the mechanism deductions. Simulation of the mechanism model showed that the filtration resistance induced by mixing chemical potential variation was comparable to the experimental data of filtration resistance of agar gel layer, indicating that the proposed mechanism is the predominant mechanism responsible for the high filtration resistance of gel layer. The proposed mechanism was further verified from the bound water viewpoint.

A new insight into membrane fouling mechanism in submerged membrane bioreactor: Osmotic pressure during cake layer filtration

Big gap between experimental filtration resistance of cake layer formed on membrane surface and the hydraulic resistance calculated through the Carman-Kozeny equation, suggested the existence of a new membrane fouling mechanism: osmotic pressure during cake layer filtration in SMBR system. An osmotic pressure model based on chemical potential difference was then proposed. Simulation of the model showed that osmotic pressure accounted for the major fraction of total operation pressure, and pH, applied pressure and ionic strength were the key determining factors for osmosis effect. It was found that, variations of osmotic pressure with pH, applied pressure and added ionic strength were well coincident with perditions of models simulation, providing the first direct evidences of the real occurrence of osmosis mechanism and the feasibility of the proposed model. These findings illustrate the essential role of osmotic pressure in filtration resistance, and improve fundamental understanding on membrane fouling in SMBR systems.

Effects of hydrophilicity/hydrophobicity of membrane on membrane fouling in a submerged membrane bioreactor.

The interfacial interactions between sludge foulants and four different types of membranes were assessed based on a new combined calculation method. Effects of membrane surface hydrophilicity/hydrophobicity on the interfacial interactions were investigated. It was found that, membrane surface hydrophilicity/hydrophobicity was not directly relevant to the interfacial interactions with sludge particles. Increasing membrane surface zeta potential could significantly increase the strength of the electrostatic double layer (EL) interaction and the energy barrier. For membrane with a surface roughness of 300nm, the total interaction was continuously repulsive in the separation distance coverage of 0-4nm in this study. The results suggest that, under conditions in this study, designing membranes with a high zeta potential and certain roughness can significantly mitigate membrane fouling, whereas, the strategy of improving membrane surface hydrophilicity cannot alleviate sludge adhesion in the membrane bioreactor.

Thermodynamic analysis of membrane fouling in a submerged membrane bioreactor and its implications

The thermodynamic interactions between membrane and sludge flocs in a submerged membrane bioreactor (MBR) were investigated. It was found that Lewis acid-base (AB) interaction predominated in the total interactions. The interaction energy composition of membrane-sludge flocs combination was quite similar to that of membrane-bovine serum albumin (BSA) combination, indicating the critical role of proteins in adhesion process. Detailed analysis revealed the existence of a repulsive energy barrier in membrane-foulants interaction. Calculation results demonstrated that small flocs possessed higher attractive interaction energy per unit mass, and therefore adhered to membrane surface more easily as compared to large flocs. Meanwhile, initial sludge adhesion would facilitate the following adhesion due to the reduced repulsive energy barrier. Membrane with high electron donor surface tension component was a favor option for membrane fouling abatement. These findings offered new insights into membrane fouling, and also provided significant implications for fouling control in MBRs.

A new method for modeling rough membrane surface and calculation of interfacial interactions.

Membrane fouling control necessitates the establishment of an effective method to assess interfacial interactions between foulants and rough surface membrane. This study proposed a new method which includes a rigorous mathematical equation for modeling membrane surface morphology, and combination of surface element integration (SEI) method and the composite Simpsons approach for assessment of interfacial interactions. The new method provides a complete solution to quantitatively calculate interfacial interactions between foulants and rough surface membrane. Application of this method in a membrane bioreactor (MBR) showed that, high calculation accuracy could be achieved by setting high segment number, and moreover, the strength of three energy components and energy barrier was remarkably impaired by the existence of roughness on the membrane surface, indicating that membrane surface morphology exerted profound effects on membrane fouling in the MBR. Good agreement between calculation prediction and fouling phenomena was found, suggesting the feasibility of this method.

Quantification of interfacial interactions between a rough sludge floc and membrane surface in a membrane bioreactor

Interfacial interactions between foulants and membrane directly determine foulant adhesion and membrane fouling. In this study, surface of sludge foulant particles (flocs) was found to be rough, and could be modeled by a sinusoidal sphere function. A novel method, which combined surface element integration (SEI) method, differential geometry and composite Simpsons rule, was developed to quantify the interfacial interactions between the modeled rough floc surface and membrane surface. Application of the novel method in a membrane bioreactor (MBR) provides broad profiles of quantitative interactions with rough floc surface with separation distance. It is also found that increase in the scaled amplitude of floc surface significantly reduced the interaction strength. Derjaguins approximation (DA) can be regarded as a special case of the novel method, indicating the extensive application prospect of the novel method. The novel method for interaction calculation was verified to be correct and feasible. Finally, roles of the novel method in membrane fouling research were discussed.

Osmotic pressure effect on membrane fouling in a submerged anaerobic membrane bioreactor and its experimental verification

A laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) treating sewage was used to investigate the membrane fouling mechanism. Characterization of cake layer formed on membrane surface showed that cake layer was hydrated, rich of extracellular polymeric substances (EPS) and negative charged with the charge density of 0.21-0.46 meq/kg MLSS. Detailed analysis revealed a new membrane fouling mechanism, osmotic pressure during cake layer filtration process due to the interception of ions. An osmotic pressure model was then developed to elaborate the existence of osmotic pressure and to estimate the contribution of osmotic pressure to membrane fouling. The calculated results showed that osmotic pressure accounted for the largest fraction of total operation pressure, indicating that osmotic pressure generated by the retained ions was one of the major mechanisms responsible for membrane fouling problem in MBRs. These findings provided a new insight into membrane fouling in MBRs.

Realization of quantifying interfacial interactions between a randomly rough membrane surface and a foulant particle

Quantification of interfacial interaction with randomly rough surface is the prerequisite to quantitatively understand and control the interface behaviors such as adhesion, flocculation and membrane fouling. In this study, it was found that membrane surface was randomly rough with obvious fractal characteristics. The randomly rough surface of membrane could be well reconstructed by the fractal geometry represented by a modified Weierstrass-Mandelbrot function. A novel method, which combined composite Simpsons approach, surface element integration method and approximation by computer programming, was developed. By using this method, this study provided the first realization of quantifying interfacial energy between randomly rough surface of membrane and a foulant particle. The calculated interactions with randomly rough surface of membrane were significantly different from those with smooth surface of membrane, indicating the significant effect of surface topography on interactions. This proposed method could be also potentially used to investigate various natural interface environmental phenomena.

Membrane fouling in a submerged membrane bioreactor: effect of pH and its implications.

The effect of pH on membrane fouling in a submerged membrane bioreactor (MBR) was investigated in this study. It was found that, pH increase slightly increased the resistance of virgin membrane and fouled membrane. Pore clogging resistance was quite low, which was not apparently affected by the pH variation. Lower pH resulted in higher adherence of sludge flocs on membrane surface. Thermodynamic analysis showed that a repulsive energy barrier existed in the process of the foulants approaching to membrane surface. This energy barrier would decrease with pH decreased, suggesting the existence of a critical pH below which the repulsive energy barrier would disappear, which would facilitate attachment of the foulants. The resistance of the formed cake layer would significantly increase with the feed pH. This result could be explained by the osmotic pressure mechanism. The obtained findings also provided important implications for membrane fouling mitigation in MBRs.