High Cross-flow Velocity Research Articles

DYNAMIC MEMBRANE TECHNOLOGY FOR PRINTING WASTEWATER REUSE

As environmental regulations become rigid and the cost of freshwater increases, wastewater is considered as a major resource in China. The paper presented a study on the implementation of the advanced treatment process using dynamic membrane (DM) in reusing of printing wastewater. The DM was well formed by circulating 1.5g/L of PAC in 20 minutes, the trans-membrane pressure of 200 kPa and the cross-flow velocity of 0.75m/s. The printing effluents were treated in effluent treatment plants comprising a physicochemical option followed by biological process. The treated effluent contained chemical oxygen demand (COD), color and turbidity in the range of 45-60 mg/L, 0.030-0.045 (absorbance at 420 nm) and 3-5 NTU. The results showed that the COD, color and turbidity removal efficiencies of the DM permeate were 84%, 85% and 80%, respectively. The wastewater treated by DM was reused as process water and the final concentrated retentate could be discharged directly into sewage treatment works with no additional treatments. Cleaning and regeneration of DM were very convenient if necessary. The proper process was that the polluted DM was cleaned with tap water at high cross-flow velocity. When irreversible pollutants accumulate, it would be rinsed with chemicals tested and the membrane flux would be restored up to 95%. The result showed that DM was considered as a promising method for purification aimed at reuse of printing wastewater, resulting in direct environmental and economic benefits.

Prediction of the circulation velocity in a membrane bioreactor

In this study, the hydrodynamics of a pilot scale membrane bioreactor (MBR) with different configurations for air scouring were investigated experimentally and numerically. The results were analyzed with the aim to derive a correlation between the aeration flow rate and the circulation velocity in the reactor, as it is well known for airlift loop reactors without internals. To achieve a fouling reduction by air scour at a minimal energy input, an airlift loop configuration was chosen because higher cross flow velocities and therefore higher shear rates can be obtained on the membrane surface. The experimental investigations were carried out with water and air in a quasi 2-dimensional model with 2.1 m height, 1.2 m width and 0.1 m depth. The bubble distributions were optically analyzed by video imaging through the transparent walls of the tank. Numerical simulations were used to perform parameter studies by varying geometrical values or operating conditions (e.g. channel width, bubble diameter). A first approach to calculate the circulation velocity depending on the geometry of the flat sheet membrane module and the aeration intensity was derived.

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 t n 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%.

Boundary integral study of nanoparticle flow behaviour in the proximity of a solid wall

Compared with traditional heat transfer fluids, the greater thermal conductivity and better convective heat transfer behaviour with little pressure penalty have made nanofluids one of the most promising emerging technologies in the field of heat transfer applications. There is however only limited knowledge of the mechanisms by which these improvements are obtained and how other factors (particle clustering, migration, interactions with the walls, etc) influence the behaviour of the nanofluids. In this study, a boundary integral formulation is used to simulate the flow of Al2O3/water with 1.05% volume concentration in the vicinity of a plane wall with special attention to particle behaviour and interactions. The simulation includes effects from Brownian motion, van der Waals attraction, electrostatic short range repulsion forces and buoyancy in order to assure physical representation. Viscous drag force and hydrodynamic interaction between the particles are calculated implicitly from the flow field improving the accuracy of the calculations. Results showed that a zone with ∼17% higher concentration (∼0.6% additional increase in thermal conductivity) was created ∼0.3 µm away from the wall; this can help increase the heat conductivity and thus improve heat transfer. High cross flow velocities were also observed which can contribute to mixing in the boundary layer further improving the performance of the nanofluid.

Using a yeast cell layer as a secondary membrane in microfiltration

Microfiltration of yeast suspensions ( Sacch. cerevisiae) was carried out to investigate the impact of a secondary membrane, composed of deposited yeast cells, on the filtration performance. Cross-flow filtration experiments with yeast–water suspensions in a rectangular channel showed the deposition of cells of different sizes, depending on the hydrodynamic parameters of the experiments. In dead-end filtration experiments with yeast–wort suspensions, the presence of the yeast layer provided, for constant filtrate flux, a slower increase in transmembrane pressure and for constant transmembrane pressure a slower decrease of filtrate flux. The positive effect of the yeast presence, which was observed, strongly depends on the yeast concentration. In cross-flow filtration experiments at a given filtrate flux the lower cross-flow velocity enables the formation of the yeast cell layer, which acts as secondary membrane and protects the primary membrane from the fouling agents of the wort, such as proteins and polysaccharides. At higher cross-flow velocity, there is no yeast layer formation, which results in blocking of the membrane through the fouling agents and the transmembrane pressure rises dramatically.

Implications of critical flux and cake enhanced osmotic pressure (CEOP) on colloidal fouling in reverse osmosis: Experimental observations

This paper describes the determination of critical flux and the cake enhanced osmotic pressure (CEOP) effect of colloidal silica fouling in reverse osmosis (RO) using a sodium chloride tracer response technique. The tracer technique gave the transient values of concentration polarization, CP, and cake resistance, R f. From these values, the rate of accumulation of deposit, d m f/d t, could be readily determined. The critical flux, J crit, was defined as the flux at which the rate of deposition of colloidal silica on the membrane, became zero, i.e. d m f/d t = 0. It was found that the critical flux concept was applicable to colloidal silica fouling in the RO process. The critical flux was strongly influenced by the crossflow velocity, such that J crit ∼ v 0.4 . In addition, the critical Peclet number, ( J v/ k m) crit, which is a measure of the convective to diffusive forces, to trigger silica fouling was approximately 0.52. Furthermore, it was observed that the amount of particles convected to and finally deposited on the membrane surface, known as the fractional deposition constant, Φ, was strongly influenced by the crossflow velocity. At low crossflow (<0.1 m s −1, shear rate <430 s −1) Φ = 1.0, but at 0.22 m s −1 (shear rate = 940 s −1) Φ dropped to about 0.1. The CEOP effect, as deduced from the transient CP data, was found to be more severe at high flux and low crossflow velocity conditions, due to formation of a thicker cake layer and flux-induced polarization. Even for the small (20 nm) colloids used in this study, the CEOP effect can readily exceed the R f effect due to fouling.

Fouling dynamics modelling in the ultrafiltration of PEGs

The aim of this work was to compare the data on flux decline with time obtained with ultrafiltration experiments of polyethylene glycols (PEGs) performed under different experimental conditions (transmembrane pressures and crossflow velocities) and the theoretical predictions for these experimental conditions obtained with the model proposed by Ho and Zydney (2000). This model considers that two main fouling mechanisms, pore blocking and cake formation, can occur when macromolecules are ultrafiltered. In this work, flux decline predictions were achieved without using empirical parameters dependent on operating conditions. The fouling experiments were performed using monotubular ZrO 2–TiO 2 ceramic membranes with a molecular weight cut off (MWCO) of 15 kDa (Orelis, France). A 5 g/L PEG (35,000 Da molecular weight) aqueous solution was used as feed. The model does not accurately describe the fouling dynamics under every experimental condition tested. However, the best predictions were obtained for high transmembrane pressures (TMPs) and low crossflow velocities when the cake layer is more likely to form.

Effect of ozone dosage and hydrodynamic conditions on the permeate flux in a hybrid ozonation–ceramic ultrafiltration system treating natural waters

The effect of ozone dosage and hydrodynamic conditions on permeate flux in a hybrid ozonation–ceramic ultrafiltration membrane system treating natural water was studied. Ozone injection into the feed water upstream from the membrane module resulted in an increase in the permeate flux over a wide range of operational conditions. At the same ozone dosage, less fouling was observed at higher cross-flow velocities and lower transmembrane pressures (TMP). At lower ozone dosages, the effects of cross-flow velocity and TMP on the extent of fouling were less pronounced. Ozone transfer efficiency from the gas phase to the bulk liquid phase was found to be higher at higher ozone concentrations, cross-flow velocities and TMPs. The accessibility of foulants to ozone at the catalytic membrane surface is believed to be a key factor affecting fouling behavior. The reduction of fouling observed in the hybrid system is largely due to the reaction of ozone or secondary oxidants, such as OH radical, with the foulants rather than to the effect of scouring by the bubbles present in the feed water. The total organic carbon (TOC) concentrations found in the retentate decreased with increasing ozone dosage. TMP and cross-flow velocity had little effect on retentate TOC levels.

Microfiltration of skim milk using polymeric membranes for casein concentrate manufacture

Ceramic membranes are classically used for microfiltration of dairy fluids with large wall shear stress values achieved through the use of high crossflow velocities. In this paper, both pilot and laboratory scale studies have been used to investigate the use of polymeric membranes for the production of a casein concentrate from skim milk. Results suggest that very high casein rejections are possible with this approach, but that a low transmembrane pressure is required to facilitate whey protein transmission. As expected, membrane flux increases with both transmembrane pressure and temperature. Permeate composition is essentially independent of temperature, with the notable exception of the κ-casein content, which is detected at 50 °C but not at 10 °C. This leakage of κ-casein at the higher temperature may result from damage to the surface of the casein micelles induced by shear forces in the membrane circuit pump. The work has also shown the usefulness of a laboratory crossflow apparatus for such experiments, with results on this unit consistent with those obtained using a full scale spiral wound element.

Utilization of a shear induced diffusion model to predict permeate flux in the crossflow ultrafiltration of macromolecules

In this paper a shear-induced diffusion model is used to predict permeate flux decline with time in the ultrafiltration of macromolecules. The results estimated by the model were compared with the experimental results obtained in a pilot plant. Tubular ZrO 2–TiO 2 ceramic membranes with a molecular weight cut-off of 15 kDa (Orelis, France) were used. The fouling experiments were performed with 5 g/L PEG (35000 Da molecular weight) aqueous solutions at different transmembrane pressures and crossflow velocities. Flux decline was predicted without the utilization of empirical parameters dependent on the operating conditions. Good predictions were obtained for all time scales in the case of low crossflow velocities. For high crossflow velocities, the results predicted by the model were good only for long time scales.

Modelling of flux decline in crossflow ultrafiltration of macromolecules: comparison between predicted and experimental results

The purpose of this work was to compare the experimental results on flux decline with time in the ultrafiltration of macromolecules with the results predicted by a model based in the resistance in series model. The model considers the osmotic pressure, gel-layer formation and adsorption as responsible for flux decline. In this paper, the change in permeate flux with time was predicted theoretically, without the use of any empirical data. A ceramic ZrO 2-TiO 2 monotubular Carbosep M2 membrane (Orelis, S.A., France) with a molecular weight cut-off of 15 kDa was selected to perform the experiments. Aqueous solutions of PEG 35000 were used as feed. The experimental results obtained at different flow rates and transmembrane pressures (TMPs) were compared with the theoretical predictions. The best predictions were obtained for low crossflow velocities (1 m/s), especially for long time scales. However, for long time scales permeate flux was under predicted for high crossflow velocities (2 and 3 m/s). The reason for these results may be found in the increased possibilities of gel layer formation at low crossflow velocities.

The removal of reactive dye printing compounds using nanofiltration

A synthetically prepared reactive dye print wastewater, mimicking real wastewater obtained from a local textile mill, was treated by nanofiltration using an NFT-50 membrane in a plate and frame module configuration at different cross-flow velocities (0.4, 0.6 and 0.8 m/s) and pressures (2–15 bar). The nanofiltration membrane was evaluated for membrane fouling, permeate flux and its suitability for removing colour, conductivity, Na + ions and COD as a function of operating pressure and feed concentration. The permeate separation efficiency was monitored by measuring the removal efficiency of colour, conductivity, Na + ions and COD retention. The membrane achieved high dye retention for each of the four dyes (from 99.4 to 99.9%) and electrolytes used (63–73%). The retention of organic substances varied between 20 and 50%, depending on the pressure used; higher retentions were achieved at higher pressure and by using higher cross-flow velocities.

Ultrafiltration on Sizing Agent Solution in Tubular-Membrane Module

ABSTRACT Ultrafiltration of an aqueous solution of carboxymethyl cellulose (CMC) was carried out in a tubular-membrane model made of ZrO2/carbon. Water was forced through the macroporous membrane as the permeate, while CMC was concentrated and recovered as the retentate. Correlation equations for calculating the permeate flux of membrane ultrafiltration were derived based on the resistance-in-series model. Correlation results were confirmed by the experimental data. Experimental results showed that the permeate flux increases as transmembrane pressure or fluid velocity increases, but decreases when feed concentration increases. Because membrane ultrafiltration is a pressure-driven process, high cross-flow velocity enhances the mass transfer coefficient of the solute and high solution concentration increases the thickness of the concentration polarization layer.

Formation of Mg(OH)2 dynamic membranes for oily water separation: effects of operating conditions

Abstract Formation of a new type of Mg(OH)2 dynamic membrane on α-Al2O3 porous ceramic tubes was investigated. Attention was paid to the effects of operating conditions on the cake layer during formation of a dynamic membrane. The influence of operating parameters such as suspension concentration, cross-flow velocity and transmembrane pressure on the formation was different. It is difficult to form a satisfactory dynamic membrane under the conditions of shorter formation time, higher cross-flow velocity and lower suspension concentration. An increase in transmembrane pressure and suspension concentration resulted in an increase in the dynamic layer and an increase in TOC rejection. However, increasing the cross-flow velocity resulted in a decrease in the dynamic layer and TOC rejection. Under certain conditions, the prepared membrane had a uniform and compact structure. It showed good performance in separating oil from water using a model oil emulsion. A TOC rejection of more than 98% and a permeate flux of more than 1 L·m−2·h−1·kPa−1 were obtained in a batch concentration test.

The purification of protein in cross-flow microfiltration of microbe/protein mixtures

The use of cross-flow microfiltration to purify BSA from yeast/BSA binary suspension is studied. The yeast cake properties, the BSA rejection coefficient and the filtration flux under various operating conditions, such as cross-flow velocity, filtration pressure and suspension pH value, are measured and discussed. The experimental results show that the filter cake plays the major role in determining the filtration resistance, the filtration flux as well as the BSA rejection. The filtration rate increases with the increase of cross-flow velocity, while decreases with the increase of filtration pressure or suspension pH. The BSA rejection coefficient is mainly determined by the sweeping effect on the cake surface and the cake mass. An increase in cross-flow velocity, filtration pressure or suspension pH, causes the BSA rejection coefficient to be higher. Theoretical models are derived to predict the filtration flux and the BSA rejection coefficient at the pseudo-steady state under various conditions. The calculated results agree fairly well with experimental data. On the viewpoint of protein recovery and protein purification flux, pH 3.0, high cross-flow velocity and low filtration pressure is the optimum operating condition for purification of BSA from the binary suspension using cross-flow microfiltration

Optimization of the membrane and pore design for micro-machined membranes

For micro-machined membranes, it is possible to choose pore size, pore geometry and membrane porosity, within certain limits. Different pore geometries (circular, square, slit shaped and triangular pores), particle size to pore size ratios, pore edges and membrane porosities were evaluated with lattice-Boltzmann computer simulations and torque balance considerations for various modes of operation. We focused on hydrodynamic interactions and assumed uncharged neutral surfaces of the particle and the pore. However, the model can easily be extended with additional relations for such interactions in practical systems with defined properties. It was concluded that pore geometry can have a large effect on the flux (up to 60%). Further, the effect of shielding could be quantified. Above a surface coverage of 0.05, the particles effectively shield each other from the flow field, therewith necessitating either a higher cross flow velocity or a lower transmembrane pressure for particle removal. Based on the simulations, an extended criterion for the critical flux was developed, which includes the effects of pore geometry, particle to pore size ratio and membrane porosity. Different optimal membrane choices follow for processes aimed at retention of all particles, and for processes aimed at fractionation of particles into different fractions.

Studies on nanofiltration membrane fouling in the treatment of water solutions containing humic acids

A major factor limiting the use of nanofiltration (NF) in the water treatment process is membrane fouling by natural organic matter. Two NF membranes, Desal 5 DK and DL, which have different hydraulic permeabilities, were compared. Solution chemistries selected for this investigation were concentrated on the common salts found in a variety of wastewaters, including monovalent ions (Na +, Cl −), divalent hardness cations (Ca 2+, Mg 2+) and divalent anions (SO 2− 4). Results of fouling experiments with humic acids demonstrated that the condition of hydrodynamics and the chemical compositions of the feed solutions could significantly influence the fouling tendency. Higher crossflow velocity and lower initial flux mitigated accumulation of humic acid (HA) on the membrane to some extent. The adsorption of humic substances onto the membrane surface was markedly enhanced in the presence of divalent cations due to the formation of HA-complexed divalent cations. The experiment also indicated that the membrane surfaces preferred to adsorb divalent cations rather than divalent anions because the membrane had a negative charge in the higher pH range investigated. In addition, choosing a proper divalent cation chelating agent as a cleaning reagent, especially at basic conditions, can improve the membrane process by dissolving and removing the HA fouling substance.

Wastewater treatment of a vegetable oil factory by a hybrid ultrafiltration-activated carbon process

In this paper, experimental investigation of wastewater treatment by ultrafiltration (UF) is introduced. In the experiments, a UF membrane (UFPHT20-6338) and samples of Behshahr Ind. Co. wastewater as feed were used. Effect of operating conditions such as pressure difference, cross flow velocity, shear stress, temperature, concentration of organic compounds and pH on permeation flux, flux decline and fouling resistance were studied. Results show that a pressure difference more than 3 × 10 5 Pa (3 bar), a high cross flow velocity (depending on economic considerations), a temperature of 30 °C and a pH of 9 are the best operating conditions. Fouling of the membrane completely follows Hermia's model (cake filtration mechanism). Analysis of the wastewater treated by UF represents 91, 87, 100, 85 and 40% reduction in COD, TOC, TSS, [PO 4 3−] and [Cl −], respectively. Also, treatment of the wastewater by UF-powdered activated carbon (PAC) was studied. According to results, using PAC in feed circulation loop of UF system with a concentration of about 0.1% (W) not only improves water quality but also increases permeation flux. A shear stress induced by PAC on the membrane surface decreases cake layer thickness and as a result increases permeation flux. Analysis of the wastewater treated by UF-PAC represents 94, 93, 100, 99 and 43% reduction in COD, TOC, TSS, [PO 4 −3] and [Cl −], respectively. Reduction of the phosphate concentration by UF-PAC is very considerable. A comparison between the results shows that UF is better than conventional biological method and UF-PAC is better than UF.

Coupling effects of the flow and electric fields in electrostatic precipitators

In this study the variations of the velocity and electric fields in electrohydrodynamic (EHD) flows in a positive single-wire electrostatic precipitator are studied. The fully coupled systems of electrical and hydrodynamical governing equations are solved for both laminar and turbulent flow conditions. Particular attention is given to the flow-electric field coupling effects and the variation of the electrical potential and charge distribution induced by the gas flow field. The presented results show that the effects of airflow on electrical potential and charge density become noticeable at high cross-flow velocities. The electrical conditions in an electrostatic precipitator that was used in an earlier experimental study are also simulated. It is shown that the computational model predictions are in good agreement with the experimental data. A series of parametric studies for a range of parameters for the EHD flows in the single-wire precipitator are performed and the corresponding flow and electric fields are presented. An example of extension of the model to multiwire precipitator is also presented.

Influence of cross-flow velocity on membrane performance during filtration of biological suspension

Through a new experimental procedure, the influence of cross-flow velocity on the formation of fouling layer during microfiltration (MF) and ultrafiltration (UF) of biological suspension with 5 g/l of mixed liquor suspended solids was studied. Based on the resistance-in-series model, filtration resistances caused by adsorption, concentration polarization, and reversible and irreversible fouling were determined. Each filtration resistance was compared with respect to the mass, thickness and density of the corresponding fouling layer (or foulants). Moreover, the effect of fouling layer on permeate quality was investigated. The obtained results showed that permeate flux increased linearly with increasing cross-flow velocity and a high cross-flow velocity was more effective to reduce fouling of MF membrane than that of UF membrane. The formation of a reversible fouling layer was actually prevented by a cross-flow velocity of 3.0 m/s for MF membrane and 2.0 m/s for UF membrane. In all cases, the magnitude of reversible and irreversible filtration resistances matched well the mass and thickness of their corresponding fouling layer. Along with its mass and thickness, the density of fouling layer was another important factor affecting filtration resistance and dissolved organic carbon (DOC) rejection. Distinct fouling layers as well as pore blocking and the narrowed pore itself were the major contributions for enhanced DOC rejection.