Analysis of RO flux decline due to membrane surface blockage

Recent work has shown that, flux decline due to precipitation of a scaling salt such as CaSO₄ on RO membranes, is governed by a surface blockage rather than a cake filtration mechanism. Crystal growth sites, scattered on the membrane, spread out laterally, increasingly blocking the membrane free flow area. Flux decline, resulting from gradual blockage of the membrane flow area, is analyzed for both laminar and turbulent flow conditions. Predicted results are shown to conform to experimental observations.     

Effect of solution chemistry on membrane resistance and flux decline

Variations in the membrane resistance due to the adsorption of cakes of Glutamicum onto polysulphone membrane surface were evaluated by measuring the change in the permeation of pure water at constant pressure. Relations giving the membrane resistance as a function of contact time at varying bulk concentration, pH and ionic strength were obtained, by analogy with adsorption laws. Static (zero pressure) and dynamic (crossflow) experiments were then compared in order to determine the effect of convective flow and electrostatic interactions on cell adsorption, irreversible resistance and flux decline. Although convective forces tended to increase the amount of material accumulated near the membrane surface, it was electrostatic interactions that strongly affected cell adsorption and irreversible resistance, as evident in the irreversible adsorption and resistance results from the static and dynamic cases. Cell-cell interactions affect the porosity of the cake layer on the membrane, while cell-membrane interactions affect irreversible adsorption onto the membrane. Solution chemistry affects both types of interactions, as evident in the increased irreversible resistance of the cake layer and irreversibly adsorbed cells at the isoelectric point of cells (IEP). Additionally, the irreversible resistance of the fouled layer is dependent on its compactness, which is directly affected by solution chemistry. The flux decline rapidly decreased after the first 10 minutes of filtration. Flux decline is more pronounced at the IEP of the cell, also indicating that fouling and adsorption are strongly dependent on cell-cell and cell-membrane interactions.     

A viscoelastic model for initial flux decline through reverse osmosis membrane

The flux decline through a reverse osmosis membrane is described by assuming that the membrane behaves as a viscoelastic body composed of three elements. This model can be used to predict the flux decline with time for the RO membrane.     

Evaluation of membrane fouling and flux decline related with mass transport in nanofiltration of tartrazine solution

BACKGROUND: This work was carried out to investigate and analyze the interrelated dynamics of mass transport, membrane fouling and flux decline during nanofiltration of tartrazine. A combined application including pore diffusion transport model and a material balance approach was used to model an experimental flux data obtained from different values of pH (3, 5, 7 and 10), feed‐dye concentration (25, 100 and 400 mg L^?1), and transmembrane pressure (1200, 1800 and 2400 kPa). RESULTS: Almost 100% dye solution removal and a permeate flux of 135 L m^?2 h^?1 were obtained for 25 mg L^?1 and 1200 kPa at pH 10. At pH 10, lower membrane fouling was obtained due to the increase of electrostatic repulsion between anionic dye molecules and the more negatively charged membrane surface. Flux decline and membrane fouling increased together with transmembrane pressure and dye concentration. Fouling was found to be directly related to proportional‐permeation coefficient (k_O′) of dye which was identified as the solute passing into the permeate with respect to the amount transported into the membrane from the feed. CONCLUSIONS: For a decrease of pH (10 to 3) and transmembrane pressure (2400 to 1200 kPa) or an increase of feed‐dye concentration (25 to 400 mg L^?1), fewer dye molecules passed into the permeate with respect to the amount transported into the membrane from the feed. This situation depended mainly on the combined influences of the gel layer and fouling in the membrane. Copyright © 2010 Society of Chemical Industry     

Predicting flux decline of reverse osmosis membranes

A mathematical model predicting flux decline of reverse osmosis membranes due to colloidal fouling has been verified. This mathema- tical model is based on the theory of cake or gel filtration and the Modified Fouling Index (MFI). Research was conducted using artificial colloidal solutions and a pilot plant equipped with ultrafiltration membranes. Polystyrene latex spheres, having a size of about 0.05-0.08 μm were used as a foulant.The result of this research was, that the measured and calculated values of flux decline of the ultrafiltration membranes as a function of time agree reasonably well. The difference between the measured and calculated values may be explained by the assumption, that initially blocking filtration occurs instead of cake or gel filtration.     

Long-term flux improvement by air sparging and backflushing for a membrane bioreactor, and modeling permeability decline

Fouling remains a major issue for all membrane applications. This study investigated anti-fouling applications for a side-stream membrane bioreactor (MBR) fed with glucose-based synthetic wastewater. Air sparging, backflushing and high cross flow velocity (CFV) were investigated as anti-fouling strategies. For better comparison of longer test runs, an equation to model effects of MLSS and temperature on viscosity was developed. This study showed that for backflushing with a CFV of 5.2 m/s, the permeability is about 3 times higher than for air sparging with CFV of 2 m/s. Long-term investigations of combined air sparging and backflushing compared to conventional membrane filtration at equal CFVs of 2 m/s showed 4 times higher permeability. The long-term permeability decline can be estimated by a power function of the type P= a tⁿ whereby the factor a is related to the initial permeability and the parameter n indicates how rapidly the flux declines. In most cases it was possible to predict the long term flux decline (over a one week or 8 week period) from data for the first two days of filtration with errors of less than 10%.     

Effects of retained natural organic matter (NOM) on NOM rejection and membrane flux decline with nanofiltration and ultrafiltration

Two natural source waters containing natural organic matter (NOM) with different physical and chemical characteristics were dead-end filtered using five types of membranes having different material and geometric properties. In this study retained dissolved organic carbon (DOC) per unit membrane area is introduced as a better parameter compared to permeate volume, time, and delivered DOC to provide a reasonable comparison of NOM rejection and flux-decline trends. Retained DOC/NOM was calculated, which influences NOM concentration polarization at the membrane interface, and transport measurements of NOM rejection and flux decline were made. Molecular weight (MW) distribution measurements (by size exclusion chromatography) were used to calculate the average MW of the NOM. This persuasively demonstrated that the nominal molecular weight cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM composites. The charge density of NOM from the source waters was measured to estimate its effects on NOM rejection and flux decline during filtration. The contact angle of the membranes was used to determine hydrophobic interactions between NOM and membrane. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. ESNA having a nominal MWCO of 200 Daltons showed NOM rejection greater than 95% and flux decline lower than 10% under a condition of a retained DOC of 0.5 mg C/cm² for the feed source waters. The other membranes having larger membrane pores (nominal MWCOs ranging from 8,000 to 20,000 Daltons) than the ESNA showed NOM rejection ranging from 68% to 86% and flux decline ranging from 5% to 17% at the same retained DOC for the waters.     

膜分离过程中强化传质的研究进展

膜分离作为是一种新型的化工分离技术,具有多方面的优点.但是,由于浓差极化和膜污染等问题的存在,造成的渗透通量衰减,阻碍了膜技术在工业领域内的大规模应用.因此,必须采取各种有效的强化措施,减轻或避免浓差极化和膜污染,从而提高渗透通量.本文详细阐述了渗透通量衰减的原因和强化传质的措施及机理,综合考虑提高渗透通量和能耗增加两个相互竞争因素,全面评价和衡量强化效果及其经济性.     

Prediction of permeation flux decline during MF of oily wastewater using genetic programming

Genetic programming is an orderly method for getting computers to regularly solve a problem. The genetic programming creates a computer program from an obtained data and solves the problem. In this work, treatment of oily wastewaters with synthesized mullite ceramic microfiltration membranes was studied and a new approach for modeling of the membrane flux is presented. The model used input parameters for operating conditions (flux and filtration time) and feed oily wastewater quality (oil concentration, temperature, trans-membrane pressure and cross-flow velocity). The genetic programming utilized here delivers a mathematical function for the membrane flux as a function of the independent variables stated above. Parameters for controlling and termination criterion for a run are provided by the user. Result is provided as a tree of functions and terminals. The results thus obtained from the genetic programming model demonstrated good representation of the experimental data with an average error of less than 5%.     

Toward the prediction of porous membrane permeability from morphological data

One of the challenges in membrane technology is predicting permeability in porous membranes for liquid applications in an easy and inexpensive way. This is the aim of this work. To achieve this objective, several techniques can be considered. In this study, a morphological approach from two‐dimensional scanning electron micrographs is proposed. First, numerical membrane morphological parameters have been determined from micrographs by using the QUANTS tool, which applies a texture recognition process. Second, the obtained data have been fit to the Darcy's and Hagen–Poiseuille models to calculate permeations. The QUANTS results have also been compared with the ones obtained through a mercury porosimeter, which is a classic and well‐known methodology. Each parameter of the Hagen–Poiseuille model has been analyzed. A comparison between experimentally measured permeations and calculated ones has been performed. An even easier approach is proposed to predict flow rate with the only knowledge of membrane surface mean pore size. This method is based on cross‐section pore size interpolation by using function fits from surface mean pore sizes. The obtained results show a reasonable agreement between measured and computed results, making this technique a valid approach for predicting membrane permeability. POLYM. ENG. SCI., 56:118–124, 2016. © 2015 Society of Plastics Engineers     

微滤膜通量模型研究进展

对现有的微滤通量模型进行了阐述.首先是基于溶质微粒反向扩散或溶质微粒的横向迁移现象分别阐述了浓差极化类模型和力平衡类模型的发展演变过程;其次是基于溶质（或微粒）在孔内的收缩堵塞及在膜表面上的吸附或沉积特性,对堵塞过滤定律和Darcy 定律及其发展过程分别进行了阐述.再次,基于膜表面上多种效应的协同作用叙述了集成模型的进展情况.最后展望了未来微滤膜通量模型可能的研究发展方向.     

膜生物反应器处理生活污水临界通量的研究

利用“通量阶式递增法”对临界通量进行了测定,得出MBR的3个水动力学操作区:超临界区、临界区和次临界区;在MLSS的质量浓度为6 000 mg/L、曝气量为0.5 m3/h的条件下,膜组件的临界通量区域为10.68～13.86 L/(m2.h),据此确定组件的次临界通量为12 L/(m2.h)。在此基础上研究了次临界通量下的运行特性,试验表明,次临界通量下的膜污染过程具有明显的两阶段特征:第一阶段的TMP呈平缓直线上升,第二阶段的TMP呈剧烈直线上升。     

Determination of true flow curves from capillary rheometer data

The evaluation of polymer melt viscosity versus shear rate has been customarily done by a time-consuming graphical method, which corrects for non-Newtonian behavior and pressure losses at flow transitions. It is shown by mathematical analysis that the flow data in terms of applied pressure and apparent shear rate can be treated by a semigraphical method, in which calculation of the true flow curve can be done with a computer. Moreover, it is possible in some cases to program the data treatment entirely for computer calculation. Results obtained by the proposed method are in excellent agreement with those obtained by the older graphical method.     

Determination of the surface area and porosity of carbon nanotube bundles from a Langmuirian analysis of sub- and supercritical adsorption data

In this paper we revisit the problem of how to obtain reliable information on the porosity and surface area of single-walled carbon nanotube (SWCNT) bundles from adsorption data. We report on adsorption measurements obtained on bundles of high-purity electric-arc nanotubes, with various gases (methane, ethane, ethylene, propane, and propylene) spanning reduced temperatures between 0.8 and 1.6, and pressures from 200 Pa up to 8 MPa, exploring low uptakes as well as multilayering conditions. Using these data we demonstrate the applicability of an analytical procedure for interpreting adsorption isotherms of light alkanes and alkenes on SWCNT bundles in quantitative terms. In particular, its use for the reliable assessment of the surface area and accessible pore volume of the bundles is demonstrated. Our results show that the same set of endohedral sites, as well as the same set of external sites, is available on our sample to all adsorbates investigated. We also show a crossover behavior in endohedral adsorption between the alkene and its saturated counterpart when moving from low pressure to near saturation conditions.     

Initial flux decline of the cellulose acetate butylate membranes with time under RO performance

Initial flux decline, i.e. compaction, has been investigated for the cellulose acetate butylate membrane prepared by varying casting condition. The compaction behavior of membranes prepared in a short evaporation period was different from that prepared in a long evaporation period. Compression tests showed that the membrane thinned by 30 ～ 40% instantaneously when pressurized; afterwards it thinned gradually. This corresponded to the behavior of decline. It appears that a change in decline occurs reflecting membrane structure depending on casting condition. The results were discussed for a pore model based on Kozeny-Carman equation and on the viscoelastic deformation model.     

Impact of chemical oxidation on sludge properties and membrane flux in membrane separation bioreactors

The effects of sodium hypochlorite (NaOCl) on sludge properties and membrane permeate flux were studied using a pressurized stirred ultrafiltration cell. Oxidation with NaOCl resulted in sludge solubilization and decreased sludge floc sizes. The sludge dissolution constant was estimated at 0.3 ± 0.1 mg sludge mg^?1 free chlorine under testing conditions. The increases in soluble chemical oxygen demand and total carbohydrate concentration in centrifuged supernatant were less than proportional to chemical dosage. Membrane permeate fluxes were much lower as a result of oxidation. Soluble biopolymers accounted for more than 76% of the total hydraulic resistance during ultrafiltration of oxidized sludge. By contrast, both the settleable sludge flocs and the soluble biopolymers were important contributors to the hydraulic resistance of sludge before oxidation. Given the benefits of NaOCl solution for membrane cleaning and its negative impact on sludge properties, the quantity of NaOCl solution used for maintenance cleaning of membranes should be optimized. Copyright © 2004 Society of Chemical Industry     

Use of data from long-term fertilizer experiments to model plant nitrogen uptake

Correct values for plant N-uptake are particularly important when correctly modelling the dynamics of nitrate leaching from agricultural soils. The general abundance of grain yield statistics represent a potential source of information for modelling plant N-uptake, provided that grain yield dry weight can be used to predict the total plant N-uptake (Vold & Søreng, 1997). In this paper, a nonlinear relationship for plant N-content as a function of grain yield dry weight and fertilizer N-level is derived. Data from long-term field experiments of grain dry weight (g N m^-2) and N-content in grains plus straw (g N m^-2) at harvest was used with nonlinear regression to estimate the parameters of the function. The parameters of a linear function were estimated similarly. Both models obtained could equally well describe the harvested N as a function of fertilizer N-level and grain dry weight, but the year-to-year variation of each fertilizer level was best described by the nonlinear model. An independent dataset, consisting of series of dry weight and N-content of grain yield from farm level experiments conducted in the same region (southeast Norway), was used to validate the nonlinear model.     

Determination of catalyst surface area from desorption characteristics of physisorbed gases

A new experimental method for the measurement of catalyst surface area of supported catalysts has been developed using selective physisorption. The desorption characteristics of a gas are studied separately on the catalyst, the support, and the supported catalyst by carrying out thermal desorption experiments in a continuous flow sorptometer. Differences in the coverage vs. temperature curves, obtained from the thermal desorption experiments, are a measure of the selectivity of the physisorbing gas, and allow calculation of the fraction of total surface area occupied by the catalyst. Two systems have been studied utilizing the thermal desorption with carbon dioxide as adsorbate: potassium carbonate/carbon black and silver/alumina. Supported catalyst surface area was determined for each system; the results were confirmed using physical mixtures of the two components (where the actual area of each component is known) and oxygen chemisorption for the silver/alumina system. The experimental technique allows for straightforward calculation of the catalyst area.     

氯碱离子膜的水传递和水通量

水传递对氯碱离子膜的电化学性能有重要的影响,而膜内水传递过程是电解过程的重要环节。在计算氯碱装置的宏观水通量的基础上,讨论了水传递现象对氯碱离子膜的电化学性能的影响,提出膜的离子透过性赋予膜以电导性,而离子的水传递数决定了氯碱膜的水通量大小。简要介绍了国产氯碱离子膜的基本结构和应用进展,展望了氯碱膜的发展前景。     

Factors influencing critical flux in membrane filtration of activated sludge

Membrane filtration of biomass is usually accompanied by significant flux decline due to cake‐layer formation and fouling. Crossflow filtration with flux controlled by pumping the permeate can produce stable fluxes if a ‘critical flux’ is not exceeded. Below critical flux the transmembrane pressure is typically very low and increases linearly with imposed flux. Above the critical flux the transmembrane pressure rises rapidly signifying cake‐layer formation which is usually accompanied by a continued rise in transmembrane pressure and/or a drop in delivered flux. A range of microfiltration and ultrafiltration membranes with pore sizes from 0.22 to 0.65 µm and molecular weight cut‐off of 100 kDa was used. The feed was an activated sludge mixed liquor with concentration in the range of 3–10 g dm⁻³. The results show that the critical flux depends on feed concentration and crossflow velocity, being higher for higher crossflow velocity or lower feed concentration. Critical flux was also dependent on membrane type, being lower for hydrophobic membranes. Although the transmembrane pressure was higher for the larger pore size membrane, no significant difference in critical flux was observed among different pore size membranes. © 1999 Society of Chemical Industry