Vicki Chen

Strategies for controlling biofouling in membrane filtration systems: challenges and opportunities

Biofouling is a critical problem in many membrane filtration processes. This review highlights the emerging strategies for polymer membrane modifications using organic and inorganic additives and surface modification to mitigate foulant deposition and biofilm formation. Constraints and opportunities for future implementation in membrane systems are outlined from the perspectives of water and wastewater treatment applications.

Particle deposition during membrane filtration of colloids: transition between concentration polarization and cake formation

Abstract The transition from concentration polarization to cake formation has been studied for the membrane filtration of colloidal silica by imposing flux and observing the system response. A critical flux ( J crit ) has been measured, below which transmembrane pressure drop, Δ P , is stable for increasing and decreasing flux. The flux–pressure profiles for operations below J crit show little (for MF) or negligible (for UF) hysteresis. Above J crit the pressure has a period of instability for increasing and decreasing flux, and there is significant hysteresis. It appears that once J crit is exceeded, the colloids in the polarized layer form a consolidated cake structure that is slow to depolarize and which reduces the flux. Evidence for cake deposition was obtained from electron micrographs. The depolarization can be increased by crossflow, by washing, and increasing pH. It was observed that the slow incrementation of flux to a given high value can result in significantly lower Δ P than the direct application of that flux. These differences are ascribed to formation of a stagnant, highly concentrated layer near the membrane surface due to consolidation and aggregation of solute resulting from very rapid flux increases.

Challenges and opportunities for mixed-matrix membranes for gas separation

Mixed-Matrix Membranes (MMMs) combining the benefits of both polymeric and inorganic materials have become a focus for the next-generation gas separation membranes. In this review, major challenges surrounding MMMs and the strategies to tackle these challenges are given in detail. The selection criteria of polymeric and inorganic materials are discussed in terms of their physical and chemical compatibility as well as large scale fabrication issues. Major models for separation performance prediction of MMMs are reviewed in terms of their interrelations and limitations. A discussion is provided regarding the future direction of MMMs.

Shell-side mass transfer performance of randomly packed hollow fiber modules

The contribution of flow distribution to shell-side mass transfer performance in randomly packed hollow fiber modules is determined using a random fiber distribution model and the Leveque equation without fitting parameters. Comparisons were made between theoretical results and measurement of oxygen stripping in actual hollow fiber modules in counter-current flow. Modules of packing densities ranging from 8.4 to 70.2% were used. The observed mass transfer coefficients decreased rapidly with increasing packing density until 50% volume fraction was reached and increased again at higher packing densities. The theoretical results showed the same trend but significantly underestimated the mass transfer at low packing densities. The influence of entry and transverse flow increased overall mass transfer at low packing densities over that predicted purely by laminar axial flow. Preliminary work with laser speckle flow visualization of the module shell confirmed these observations. Comparisons with existing literature correlations are discussed, and strategies to assess the level of flow redistribution and flow variation are presented.

The effects of electrolyte concentration and pH on protein aggregation and deposition: critical flux and constant flux membrane filtration

Abstract The performance of membranes used in the separation of solution mixtures containing proteins can be substantially limited by aggregation and deposition inside membrane pores and on their surfaces. In this study flux, stepping and constant flux operation have been used in conjunction with varying electrolyte concentrations and pHs in the microfiltration of aqueous solutions of bovine serum albumin. Experiments were performed to investigate the hydrodynamic and solution conditions during the incipient fouling stage which lead up to eventual cake formation. Results from flux stepping showed that an increase in wall concentration coincides with the onset of an apparent critical flux. This is followed by a time lag before an increase in observed rejection is exhibited. Sub-critical constant flux operation showed that there exists an aggregation and deposition time lag after which the membrane suddenly experiences a rapid increase in hydraulic resistance due to protein aggregates blocking a majority of membrane pores. These incipient fouling conditions were shown to be dependent on pH and ionic strength and were concluded to be the product of a balance between electrostatic, solubility and hydrophobic effects which were manifested in protein–protein and protein–membrane interactions.

The cleaning of ultrafiltration membranes fouled by protein

The relationship between membrane fouling and cleaning was investigated in terms of flow conditions, transmembrane pressures, pH, membrane properties, and cleaning agents using a stirred batch cell and aqueous albumin solution. Fouling was less at the pH extremes than at the isoelectric point for both retentive and partially permeable membranes. Membranes with partial permeability showed a greater tendency for fouling and were less responsive to cleaning than retentive membranes. The results in the stirred cell were shown to be similar to those for a crossflow module under similar operating conditions.

Janus Membranes: Exploring Duality for Advanced Separation

Janus membranes are an emerging class of materials having opposing properties at an interface. This structure results in selective and often novel transport characteristics. In this Minireview, a definition of the Janus membrane, beyond merely asymmetric materials, is introduced and common fabrication strategies are outlined. Also presented are current and potential applications in directional transport, switchable permeation, and performance optimization with detailed mechanisms.

The use of anionic surfactants for reducing fouling of ultrafiltration membranes: their effects and optimization

Pretreatments with anionic surfactants and their applications have been studied for the reduction of fouling during ultrafiltration of proteins. Surfactant chemistry and the composition of mixed surfactants were systematically varied. The initial pure water fluxes were reduced, but the ultrafiltration fluxes were usually enhanced, and flux decline due to fouling reduced. In contrast to the long nonionic surfactants studied previously, the small anionic surfactant (AOT) appears to provide a different means of reducing protein deposition by altering the electrostatic interactions between the protein and membrane surface. When used in conjunction with nonionic surfactants or when polyethylene oxide segments are added to their backbone, the anionic surfactants showed significant flux improvement and fouling resistance compared with that of the single AOT or the nonionic surfactant. The effectiveness of the pretreatment is sensitive to pH and protein charge.

Formation of Ultrathin, Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal-organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF-8) membranes on titania-functionalized porous polymeric supports. The coherent ZIF-8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF-8, with hydrogen permeance up to 201×10(-7)  mol m(-2)  s(-1)  Pa(-1) and an ideal H2 /CO2 selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.

Novel filtration mode for fouling limitation in membrane bioreactors

A novel filtration mode is presented to reduce fouling propensity in membrane bioreactors (MBR). During this mode, an elevated high instantaneous flux (60Lm(-2)h(-1)) is initially applied for a short time (120s), followed by a longer filtration (290s) at lower flux (10.3Lm(-2)h(-1)) and a backwash in each filtration cycle. The mixed mode is expected to limit irreversible fouling as the reversible fouling created during the initial stage appears to protect the membrane. Hydraulic performance and the components of foulants were analyzed and compared with conventional continuous and backwash modes. It was found that the mixed mode featured lower trans-membrane pressure (TMP) after 24h of filtration when compared to other modes. The mixed mode was effective in preventing soluble microbial products (SMP) attaching directly onto the membrane surface, keeping the cake layer weakly compressed, and reducing the mixed liquor suspended solids (MLSS) accumulation on the membrane. This strategy reduced the resistances of both the cake layer and the gel layer. A factorial experimental design was carried out for eight runs with different conditions to identify the major operational parameters affecting the hydraulic performances. The results showed that the value of the flux in the initial high-flux period had the most effect on the performance of the mixed mode: high initial flux (60Lm(-2)h(-1)) led to improved performance.