Dielectric Analysis of Dynamic Fouling Behavior on Surface of Polyethersulfone Composite Ultrafiltration Membrane

基于界面极化与静电场理论建立了聚醚砜复合超滤膜表面污染层与溶液体系的介电解析模型. 应用参数敏感性分析和介电测量研究了体系中浓差极化层与滤饼层的厚度与电导率对超滤体系介电弛豫的影响机制. 结果表明超滤过程中浓差极化层能快速建立,而滤饼层的形成是一个增长与剥蚀的动态过程;并且浓差极化层与滤饼层的电学性质是影响超滤体系介电谱的关键因素. 比较介电测量与模拟结果证实了建立的超滤体系介电解析模型能够有效可靠地描述与解释超滤体系产生介电驰豫的机制.     

Hollow-fiber membrane adsorber: Mathematical model

Two opposite design strategies for ultrafiltration/microfiltration filters: (1) reduction of concentration polarization and particle deposition to increase permeate velocity and (2) utilization of particle deposition on membrane surface to produce an additional (to permeate) volume of clarified water, are analyzed. It is shown that the first strategy is always associated with additional expenditures in power or other material resources, making it not enough cost-effective to be competitive with non-membrane filtration processes in some water treatment applications. At the same time, the second strategy does not require additional power expenditures and provides high water recovery and cost-effectiveness. The mathematical model describing the performance of hollow-fiber membrane adsorber, which represents a second-strategy filter, is studied. A general form of the particle-deposition equation is introduced, and its terms are analyzed. As a result, its linearized form, looking like a linear equation of reversible adsorption, is chosen. A numerical solution to the system of governing equations is obtained and used to assess the accuracy of approximate solutions. A new approximate solution allowing one to evaluate the adsorber particle retentions with an acceptable accuracy is suggested.     

Charged ultrafiltration membranes of polyacrylonitrile covalently binding amphiphilic quaternary ammonium group: effect of aliphatic hydrophobic groups on the filtration properties

Amphiphilic monomers of chloromethylstyrene derivative were synthesized and then copolymerized with acrylonitrile for the preparation of positively charged ultrafiltration membranes. Four types of the polyacrylonitrile charged membranes having quaternary ammonium group with trimethyl, dimethylbutyl, dimethyloctyl and dimethylstearyl groups were prepared by the phase-inversion method. The ultrafiltration properties of the charged membranes were measured by using dextran molecular probes at various NaCl concentrations. Then, the filtration behavior was compared in these charged membranes. For membranes with trimethyl and dimethylbutyl groups on the quaternary ammonium salt, the values of the molecular weight cutoff and the permeation rate of the solute solution increased when the salt concentration increased. However, for membranes having amphiphilic quaternary ammonium groups with octyl and stearyl aliphatic chains, the filtration properties did not change with the salt concentration. We concluded that a compact conformation of the amphiphilic polyionic segments in the membrane causes stable filtration properties.     

Blocking of capillaries as fouling mechanism for dead-end ultrafiltration

In this paper plugging of capillaries in the potting is investigated. A lot of research has been done on fouling of the membrane surface (pore blocking, cake filtration) but research on other types of membrane fouling like plugging of capillaries is not so common. Experiments were performed with a lab-scale test installation under constant flux conditions with synthetic feed water containing ferric hydroxide flocks as a fouling component. The experiments showed that during operation capillaries became blocked by fouling plugs. The presence of blockages, especially in the potting at the concentrate side of the capillaries, could not be detected by measuring the clean water resistance. However such blockages did result in an increased forward flush pressure. A combination of the clean water resistance and the forward flush pressure is suitable for determining the fouling of a membrane and the effectiveness of a cleaning procedure. The part of the capillaries in the potting is not backwashed and therefore the hydraulic as well as the chemical cleaning is not efficient at this place.     

The use of moment theory to interpret diffusion and sieving measurements in terms of the pore size distribution in heteroporous membranes

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distribution, the hindered diffusivity or the reflection coefficient of large solutes in a heteroporous membrane by measuring the diffusive permeability to a small solute, the hydraulic permeability and one or two additional transport characteristics. These additional measurements involve either the flux of a small solute at Pe1, the hindered diffusivity of a large solute or the reflection coefficient of a large solute at Peå1. Membrane heteroporosity is incroporated in the predicted bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. In this paper, the results from calculations performed with different model membranes containing log-normal pore size distributions are reported. A comparison of the results obtained with three different membranes shows that one can distinguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion coefficient or the reflection coefficient of two different sized solutes. A comparison of the bounds on D and the bounds on σ predicted from different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteristic by measuring a different one.     

Nonlinear two-phase equilibrium model for the binding of arsenic anions to cationic micelles

The binding of arsenic ions to cationic cetylpyridinium chloride (CPC) micelles has been investigated using the semiequilibrium dialysis (SED) technique. In SED experiments, it has been shown that CPC micelles are very effective in binding arsenic ions in the retentate. At the studied pH (pH 8), the unbound and bound arsenic exists primarily as divalent anions (HAsO₄²⁻) while CPC molecules exist as monovalent cations. Therefore, arsenic ions are bound electrostatically to the cationic micelle. The resultant colloid is large enough not to pass through the dialysis membrane, producing a rejection greater than 99.59%. The concentration of the unbound arsenic anions passing through the dialysis membrane is practically the same as the permeate concentration of these species in the analogous micellar-enhanced ultrafiltration (MEUF) experiments. Therefore, a nonlinear equilibrium model which combines thermodynamic relations, charge balance equations, and material balance equations with the Oosawa two-phase polyelectrolyte theory has been developed to correlate the binding of arsenic to CPC micelles in SED and MEUF. It was shown that the equilibrium model successfully accounts for the experimental data in both SED and MEUF in the absence and presence of monovalent (HCO₃¹⁻) and divalent co-ions (HPO₄²⁻). Because of their small sizes (less than 10 nm), micelles should retain their equilibrium shapes in the presence of hydrodynamic shear typically attained in most dynamic processes. Therefore, the equilibrium model can be used to predict separation efficiencies in other ultrafiltration units such as in crossflow ultrafiltration processes.     

Comparison of physico-chemical pretreatment methods to seawater reverse osmosis: Detailed analyses of molecular weight distribution of organic matter in initial stage

In desalination, effective pretreatment is the key to reduce membrane fouling that occurs during the seawater reverse osmosis (SWRO) process. However, it is difficult to compare the flux decline after different pretreatments using a small-scale reverse osmosis filtration unit. In this study, we successfully evaluated the effect of pretreatment on SWRO in terms of molecular weight distribution (MWD) of seawater organic matter (SWOM) after 20 h of SWRO operation. Microfiltration (MF), ultrafiltration (UF), ferric chloride (FeCl₃) flocculation and powdered activated carbon (PAC) adsorption, were used as pretreatment. The effluents and the retentates after each pretreatment and 20 h of SWRO operation were characterized in terms of MWD.Although the normalized flux of SWRO showed similar flux decline (J/J₀ = 0.17) with/without pretreatment, SWOM concentration in the retentates after different pretreatments was different in quantity and it increased linearly with time. The slope of the SWOM increase was 0.110, 0.096, 0.077 and 0.059 after MF, FeCl₃ flocculation, UF and PAC adsorption pretreatments, respectively. MW peaks for the seawater used in this study consisted of 1200 Da (biopolymers), 950 Da (fulvic acids), 650 Da (hydrolysates of humic substances), 250 Da (low MW acids) and 90 Da (low MW neutrals and amphiphilics). FeCl₃ flocculation preferentially removed 1200 Da (biopolymers), while PAC adsorption mostly removed 950 Da (fulvic acids). UF and NF removed only a marginal amount of relatively large organics, while RO removed the majority of organics. The intensity of 1200, 950, 650 and 250 Da MW in the RO retentates increased with the RO operation time. The organics of MW around 1200 Da (biopolymers) had a relatively low rate of increase with time compared with those of lower MW. This suggests that the SWOM of 1200 Da MW was preferentially retained on the membrane surface.     

Bacterial attachment on reactive ceramic ultrafiltration membranes

Bacterial attachment is an initial stage in biofilm formation that leads to flux decline in membrane water filtration. This study compares bacterial attachment among three photocatalytic ceramic ultrafiltration membranes for the prevention of biofilm formation. Zirconia ceramic ultrafiltration membranes were dip-coated with anatase and mixed phase titanium dioxide photocatalysts to prevent biofilm growth. The membrane surface was characterized in terms of roughness, hydrophobicity, bacterial cell adhesion, and attached cell viability, all of which are important factors in biofilm formation. The titanium dioxide coatings had minimal impact on the membrane roughness, reduced the hydrophobicity of membranes, prevented Pseudomonas putida attachment, and reduced P. putida viability. Degussa P25 is a particularly promising reactive coating because of its ease of preparation, diminished cell attachment and viability in solutions with low and high organic carbon concentrations, and reduced flux decline. These reactive membranes offer a promising strategy for fouling resistance in water filtration systems.     

Low-pressure membrane filtration of secondary effluent in water reuse: Pre-treatment for fouling reduction

Fouling in the low-pressure membrane filtration of secondary effluent for water reuse can be severe due to the complex nature of the components in the water. Pre-filtration, coagulation and anion exchange resin were investigated as pre-treatments for reducing fouling of microfiltration (MF) and ultrafiltration (UF) membranes in the treatment of activated sludge-lagoon effluent. The key fouling components were determined using several analytical techniques to detect differences in the organic components between the feed and permeate.Pre-filtration (1.5 μm) enhanced the permeate flux for MF by removing particulates, but had little effect for UF. Marked flux improvement was obtained by coagulation pre-treatment at 5 mg L⁻¹ Al³⁺ with internal membrane fouling being substantially alleviated. Anion exchange resin removed >50% of effluent organic matter but did not improve the flux or reduce irreversible membrane fouling. These results, together with detailed organic compositional analyses, showed that the very high-molecular weight organic materials (40–70 kDa) comprised of hydrophilic components such as soluble microbial products, and protein-like extracellular matter were the major cause of membrane fouling.     

Effects of hypochlorite treatment on the surface morphology and mechanical properties of polyethersulfone ultrafiltration membranes

Polyethersulfone membranes are widely used for ultrafiltration and microfiltration especially in the dairy industry, but they are believed to degrade when exposed to the sodium hypochlorite solution that is used to sanitize the processing equipment. Such membranes were exposed to sodium hypochlorite for up to 25,000 ppm-day at 55 °C, and pH 9 and 12. Mechanical properties as measured by dynamic mechanical analysis and tensile strength did not change greatly, but surface properties measured by FTIR-ATR, field emission scanning electron microscopy and associated energy dispersive X-ray spectroscopy detected significant changes in the surface. Surface pitting was observed and was found to be most severe at pH 9. It was thought that pitting was the likely cause of degradation in performance of the membranes and that pits could act as stress raisers leading to cracking.