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In this study, we designed three turbulence promoters using three-dimensional (3D) printing technology for cross-flow microfiltration. Designing turbulence promoters with optimal geometry for microfiltration systems remains a key challenge. The effects of the operating conditions, such as cross-flow velocity, transmembrane pressure and geometry of the turbulence promoter on the filtration flux and cake properties were investigated. The distribution of the fluid velocity was examined using computational fluid dynamics (CFD). For normal microfiltration tests without promoters, the filtration flux merely increased by 10% when the cross-flow velocity increased from 0.1 to 0.5 m/s under a transmembrane pressure of 20 kPa. Therefore, the turbulence promoters were individually added into the channel of the microfiltration device to induce vortices in the flow stream and increase cross-flow velocity. Three turbulence promoters with different configurations (circular, diamond and elliptic) were made with a 3D printer. The elliptic type of promoter with a hydraulic angle of 90° displays the flux enhancement by approximately 30–64% under a transmembrane pressure of 20 kPa compared to the normal type of microfiltration, whereas the diamond type of promoter with a hydraulic angle of 60° shows a lower flux enhancement by approximately 7–16% under the same transmembrane pressure. Overall, adding the turbulence promoter to the microfiltration module shows a great potential to significantly mitigate membrane fouling and enhance filtration flux in cross-flow microfiltration.

This study investigated the selection of optimal operating conditions and the design of efficient rotating disk structures for removing particles from seawater. The distributions of fluid velocity and shear stress on the membrane surface under various operating conditions were simulated using computational fluid dynamics. Type 1 rotating disk had two vans, Type 2 rotating disk had four vanes shaped identically to those of the Type 1 rotating disk, and Type 3 rotating disk had two vanes with circular orifices located at the center of each vane. The simulated results indicated that the disk rotation speed is the main factor affecting shear stress on the membrane surface. Cake mass and filtration flux were estimated from shear stress and were calculated using empirical equations. The results showed that flux in Type 1 increased by 28% when the disk rotation speed increased from 1000 to 3000 rpm; the installation of extra vanes caused an ineffective increase in flux at 3000 rpm. Although Type 2 exhibited the highest filtration performance and the lowest cake resistance, Type 3 is the optimal design because it exhibited the highest specific filtration flux among all tested rotating disk types. The rotating disk with two vanes with a circular orifice at a low disk rotation speed demonstrated the optimal design and operating conditions for removing particles from seawater, because it resulted in high filtration fluxes and relatively low power consumption.

For increasing the shear stress on a membrane surface to reduce the fouling caused by solid deposition to improve the membrane filtration operation, various hollow fiber arrangements in a filter channel were investigated using computational fluid dynamics. In the simulation process, the velocity and pressure distributions and the effect of shear stress distributions on the membrane surface were analyzed under various hollow fiber arrangements and operating parameters, including fiber diameter (D), transverse pitch (S T ), and longitudinal pitch (S L ). These parameters lead to a dimensionless Reynolds number Re and the related friction coefficient C fD , which can be used to identify the relationship between different operating conditions. The study results indicated that C fD of the staggered arrangement was approximately 1.5 times greater than that of the aligned arrangement under the same operating conditions. The fouling tendency in the staggered arrangement was less and more favorable than that in the aligned arrangement, and with a higher S L /D ratio and lower S T /D ratio. With the results from this study, a design direction for optimal geometry to prevent membrane fouling and reduce power consumption was proposed, and can be used as the basis for hollow fiber membrane module design in the future.

A dynamic membrane was formed by introducing powder-activated carbon (PAC) prior to the cross-flow microfiltration of seawater. The microbes and organic matter in the seawater were removed efficiently through the interception, adsorption, and biodegradation of the PAC dynamic membrane. The properties of PAC dynamic membranes formed under various operating conditions were analyzed. The relationship between the specific filtration resistance of a dynamic membrane and transmembrane pressure was regressed as a power-type empirical equation for a given cross-flow velocity. An increase in cross-flow velocity led to higher specific filtration resistance because of the smaller particle size distribution. The thickness of a dynamic membrane increased with the transmembrane pressure but decreased with the cross-flow velocity. The thickness was correlated with the ratio of tangential to normal drag forces exerted on the PAC particles on the membrane surface. The filtrate quality for PAC dynamic membrane filtration was analyzed. Humic acid (HA) and turbidity were not detected. The chemical oxygen demand (COD) and dissolved organic carbon (DOC) had maximum removals of approximately 80% and 92%, respectively. The behavior by which HA was adsorbed on a PAC conformed to the Freundlich isotherm, and the modified Wheeler equation produced appropriate estimates of the breakthrough time. The breakthrough time increased with the thickness of the dynamic membrane. This study presents a method by which the filtration performance, such as filtration flux, thickness, and breakthrough time of the dynamic membrane, can be predicted directly from operating conditions.

This study evaluated the separation of particles suspended in power law fluids by applying a 10 mm hydrocyclone. Al2O3 particles were added to an aqueous polyacrylic acid solution to prepare 0.1 vol% suspensions with different flow behaviors. The effects of operating conditions, such as the inlet velocity, pressure drop, split ratio, and fluid flow behavior, on the fluid velocity distribution, particle trajectory, and particle separation efficiency were studied. The study applied a simulation in which the governing equations were coupled using the SIMPLE algorithm; the Reynolds stress model served as a turbulence model. The distributions of the fluid velocity and pressure were simulated using a segregated, steady-state, three-dimensional implicit numerical solver provided by ANSYS 15.0 software. The tangential velocity increased with the flow behavior index, n, at a fixed inlet velocity and split ratio. This effect was attributed to a lower molecular viscosity. The particle trajectories were simulated using a Lagrangian frame. The particle migration in the cylindrical part of the hydrocyclone dominated the particle separation efficiency. A reduction in the fluid n-value engendered a higher effective viscosity and lower tangential velocity, which caused fewer particle rotations and lower centrifugal effects, consequently lowering the opportunity for particles to collect in the underflow. The separation efficiency increased with the inlet velocity and particle size because of a higher centrifugal effect, but the influence of the inlet velocity was extremely small for a low fluid n-value. The total separation efficiency increased by approximately 40% as the n-value increased from 0.51 to 1.00. The simulated particle separation efficiency was highly consistent with experimental data. This verified the reliability of the simulation method and the validity of this method for understanding the effects of fluid flow behavior on particle separation performance.

Membrane filtration has strong potential for bioethanol purification because of its high product selectivity, low energy consumption, and high system flexibility. A method combining microfiltration with cocrystallization was investigated for purifying ethanol from fermentation broth. After yeast removal, unreacted glucose and ethanol must be separated to diminish the product inhibition effect and to enable continuous operation. The simulated fermentation broth was used in experiments to focus salt–sugar cocrystals removal in this study. Sodium chloride (NaCl) was used as a precursor to form crystal nucleus, leading to cocrystalization with glucose. The effect of NaCl concentration and crystallization time on crystal size and growth rate was investigated. The Feret diameters of crystals increased with time, but the crystal growth rate reduced exponentially during crystallization. The optimal mole ratio of NaCl to glucose was determined as 25. Constant pressure microfiltration was subsequently conducted to separate the crystals from ethanol solution. The filtration flux attenuated with time because of membrane fouling, which was majorly attributed to cake formation. The number of crystals increased with the NaCl concentration, resulting in heavier cake mass and therefore higher glucose rejection. Specific filtration resistance was inversely proportional to the square of the crystal size. Moreover, filtration flux increased with the filtration pressure because of the higher driving force. The cake formed by salt–sugar cocrystals was slightly compressible (cake compressibility = 0.23). An increase in salt concentration led to higher crystal formation, higher cake growth, and therefore higher glucose rejection. The optimal NaCl concentration, at which the highest crystal formation and the glucose rejection were obtained, was 165 kg/m3. The proposed method can be used to predict filtration performance from operating conditions and crystal characteristics and applied to other biomass sources in the processes of bioethanol purification.

In this study, three types of vortex finder structures, namely Types A, B, and C, of 10-mm hydrocyclones were designed for improving particle separation efficiency. The Type A vortex finders had uniform but different thicknesses, and the Types B and C had extra conical shapes with different lengths on the outer surfaces. The velocity and pressure distributions in the hydrocyclones were simulated using computational fluid dynamics. The governing equations were coupled using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm, and the second-order pressure-strain coupled with the Reynolds stress model was used in the simulations. When the liquid velocity distribution was simulated, particle trajectories were traced on the basis of a Lagrangian frame considering the hydrodynamic interactions between liquid and particles. Calcium carbonate particles with a density of 2800 kg/m3 and a mean size of 17.8 μm were used as a particulate sample. The simulation methods were verified by comparing the simulation results with the experimental data measured using several selected hydrocyclones. The particle separation efficiencies observed using the different hydrocyclones were compared at an inlet velocity of 10 m/s and a split ratio of 0.6. A thicker vortex finder resulted in higher particle separation efficiency because a higher velocity is maintained in the cylindrical part, but it also resulted in reduced particle residence time and an increased pressure drop through the hydrocyclone. Installing conical structures on the outer surface of the vortex finders was beneficial for improving the particle separation efficiency and reducing the particle cut-size. The sequence of particle separation efficiency observed using the various hydrocyclones was Type C > Type B > Type A. Considering the separation efficiency, pressure drop, and particle cut-size simultaneously, Types B-II and C-II were the optimal designs.

The flocculant-free mechanical deliquoring method has been proposed for obtaining the compressed cake with an extremely low moisture content in expression of municipal excess activated sludge. The method is comprised of the mild ultrasonic pretreatment followed by ultrahigh-pressure expression. The floc breakage induced by ultrasonication greatly facilitated the removal of bound water during ultrahigh-pressure expression. Moreover, the cell disruption caused by ultrasonication induced self-flocculation of the sludge with the aid of the intracellular materials released from within the cells, preventing the souring of dewaterability. The moisture content of the compressed cake produced by expression of ultrasonicated sludge was dramatically reduced to as low as 38.0 wt% at the ultrahigh-pressure of 50 MPa, which exhibited the value much lower than that obtained for untreated sludge, due to the removal of bound water. Analysis of kinetics of ultrahigh-pressure expression for ultrasonicated and untreated sludge showed that the primary consolidation was followed by a three-stage creep phenomenon with different values of the creep constant. The primary consolidation based on the Terzaghi spring analogy accounted for the major portion of moisture removal in this method, and the effect increased with increasing pressure.

Yeast cells were immobilized on calcium alginate beads prepared using different calcium concentrations. The compression properties of the immobilized beads (e.g., softness index and retardation time for compression) were strongly affected by the calcium concentration. The effects of the bead properties on filtration characteristics, such as cake porosity, specific cake filtration resistance, cake compression creeping effect and cake compressibility, were analysed using a dead-end filtration system. The filtration curve of yeast-immobilized beads had an “S” shape, similar to that of soft gel particles. The cake compression behaviour and variation in cake properties were directly reflected on the curve trend. The Voigt in the series model was employed to describe variation in cake porosity with time during a compression. The yeast immobilization increased the bead softness; therefore, the porosity of a cake formed by yeast-immobilized beads was lower than that formed by pure calcium alginate beads. ...

In the pretreatment preceding desalination, fine particles in seawater are removed using cross-flow and rotating-disk dynamic filtration. The effects of operating conditions, such as cross-flow velocity, transmembrane pressure, disk rotation speed, and the clearance between the disk and the membrane, on the filtration flux, cake properties, and power consumption are discussed. An increase in the cross-flow velocity or transmembrane pressure leads to higher filtration flux. The filtration flux and cake thickness can be accurately estimated using a force balance model for the particle deposition associated with the basic filtration equation. A dynamic filtration module is used to increase the filtration flux. The particles are easily swept away from the membrane surface because of the shear stress generated by the rotating disk. The filtration flux therefore increases by either increasing the disk rotation speed or reducing the clearance between the disk and the membrane. The specific filtration flux, defined for indicating the power effectiveness, decreased with an increase in the disk radius, an increase in the disk rotation speed, or a decrease in the clearance between the disk and the membrane because of a drastic increase in power consumption. A smaller disk is more effective in saving energy.

In this study, we investigated the performance of powdered activated carbon dynamic membranes (PAC DMs) in fouling mitigation strategies during cross-flow microfiltration for high-efficiency seawater pretreatment. By altering the fouling mechanism and water molecule pathway, PAC DMs successfully enhanced the pseudo-steady-state filtration flux and rejection of cross-flow microfiltration by 53% and 29%, respectively, compared to filtration without a PAC DM. Moreover, Fenton oxidation processes (FOPs) application in the DMs’ cross-flow cleaning was studied and showed promising results, recovering up to 28% of the initial filtration flux when used alone and 52% when combined with alkaline cleaning. This study provides insight into the utilization FOPs cleaning technology for PAC DMs cross-flow cleaning, allowing the automation and optimization of DMs operation.

A sophisticated method was developed for evaluating simultaneously and accurately both the average specific resistance and average porosity of the filter cake formed in unstirred dead-end ultrafiltration of nanocolloids such as protein solution and nanosilica sol. In the method, a step-up pressure filtration test was conducted by using a filter with a single-stage reduction in the effective filtration area. The influence of the pressure drop across the cake on not only the average specific cake resistance but also on the average cake porosity of highly compressible filter cake was evaluated using only flux decline data in one dead-end filtration test, taking advantage of the decrease in the cake thickness caused by the pressure increase. As a result, the cake properties were easily determined for a variety of nanocolloids. Constant pressure dead-end ultrafiltration data obtained under various pressures and concentrations were well evaluated based on the method proposed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4426–4436, 2015

In a rotating-disk dynamic filter, the effects of the rotating-disk structure, including the shape and number of vanes, on the distribution of fluid velocity and shear stress acting on the membrane surface were examined using computational fluid dynamics (CFD). Six types of rotating disk were designed and installed above the filter membrane in a filter chamber to study their performance. Three-dimensional fluid flow fields in the filter chamber were simulated for various rotating disks, disk rotation speeds, and feed flow rates by using FLUENT software. The equations of continuity and momentum balance were solved numerically using a finite volume scheme along with the renormalization-group k–ɛ model. The simulated results indicated that the disk structure and rotation speed were the most crucial factors affecting the filtration performance. An optimal disk design for higher permeate fluxes and less membrane cake fouling can be achieved by increasing the shear stress on the membrane surface. A rotating disk equipped with more vanes can generate higher shear stress on the membrane surface for a given rotation speed. However, more power should be supplied to drive the rotating disk. Although the B2 rotating disk showed the highest filtration flux, Type-A2 and Type-C disks had the optimal designs, which were achieved by considering the filtration flux and power consumption simultaneously.

A potential method has been developed for evaluating simultaneously both the average specific resistance and average porosity of the filter cake formed in unstirred dead-end ultrafiltration of nanocolloids such as bovine serum albumin solution and silica sol. The method consists of variable pressure filtration followed by constant pressure filtration. The relation between the average specific cake resistance and the pressure drop across the cake was determined from the evolution of the filtration rate with time in the course of the variable pressure filtration period, based on the compressible cake filtration model. The average porosity was evaluated from the significant flux decline caused by a sudden reduction in the cake surface area in the middle of the constant pressure filtration period. The pressure dependences of both the average specific cake resistance and average cake porosity were obtained from only two runs which differed from each other in the pressure profiles. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3869–3877, 2014

A membrane process is used for bio-ethanol purification from fermentation broths. Yeast cells are separated from a suspension containing yeast, glucose and ethanol using cross-flow microfiltration. Yeast cells are retained by the filter membrane with a mean pore size of 0.45 μm and recycled back into the broth, while most of the glucose and ethanol penetrate through the membrane into the filtrate. The operating condition effects, such as cross-flow velocity and transmembrane pressure, on the filtration flux, cake properties and solute rejections are measured and analyzed. The filtration flux increases over 2-fold as the cross-flow velocity increases from 0.1 to 0.5 m/s and increases 50% as the transmembrane pressure is increased from 20 to 100 kPa at a fixed cross-flow velocity of 0.5 m/s. The experimental data show that the filter cake plays the major role in determining the filtration resistance. The yeast cake exhibits a specific filtration resistance as high as 1014 m/kg and a compressibility of 0.55. The glucose and ethanol rejections are both lower than 8% and increase slightly with increasing cross-flow velocity or transmembrane pressure. The cake formation and solute rejections are analyzed theoretically using a force balance model for particle deposition and the standard capture equation for depth filtration. Semi-empirical equations are derived and used for predicting the filtration flux, cake thickness and solute rejection directly from the operating conditions. The calculated results using the proposed models agree fairly well with the experimental data.

The efficient deliquoring process has been developed for achieving high-rate and high-degree deliquoring of municipal excess activated sludge, which exhibits relatively low dewaterability. In the method developed, low-pressure filtration combined with flocculation led to high-rate deliquoring of sludge due to the formation of large flocs. Subsequently, ultrahigh-pressure expression combined with water permeation through the filter cake which promoted the re-dispersion of flocs in the cake resulted in high-degree deliquoring of the cake. It should be noted that the moisture content in the compressed cake was finally reduced to 31 wt% by expression operation under action of an ultrahigh pressure of 15 MPa. This extremely low value of the cake moisture content implies that the liquid contained within the microorganism cells was partially removed by the mechanical pressure when the ultrahigh pressure is applied to the cake in the course of expression. The relation between the equilibrium porosity in the compressed cake and solid compressive pressure was empirically represented by a power function. The kinetics of ultrahigh-pressure expression such as the time variation of the moisture content in the compressed cake was accurately described by combining the multi-stage creep model with the modified Terzaghi model describing the primary consolidation.

Microalgae are promising feedstock for biodiesel production due to their lipid content. Rotating-disk dynamic microfiltration was used for microalgae concentration in this study. The effects of operating conditions, namely disk rotation speed, suspension feed rate, and transmembrane pressure (TMP), on the filtration flux and cake properties are discussed. Because all microalgae cells were rejected by the filter membrane, the main cause of filtration resistance was highly compressible cake that exhibited a compressibility factor of 0.66. An increase in the disk rotation speed led to 40–300% increase in pseudosteady filtration flux. Applying a high TMP yielded high filtration flux at a low disk rotation speed. However, the opposite result was obtained at a high rotation speed. Nearly the same filtration fluxes were produced at a disk rotation speed of 300 rpm even when the TMPs were different. The flow fields in the rotating-disk dynamic microfilter were simulated using software of computational fluid dynamics, Fluent. The shear stress at the membrane surface is a major factor affecting cake formation. A two order-of-magnitude increase in the shear stress by increasing the disk rotation speed will decrease the cake mass by 80% and enhance the filtration flux to about 10 times in the conditions of this study. A linear relationship between mean cake mass and the ratio of filtration flux to mean shear stress derived from the force balance model was verified using experimental data measured in various operating conditions. The methods proposed by this study provide a way to estimate the cake mass and filtration flux directly from operating conditions.

Bacillus subtilis broths under different culture times are filtered in a cross-flow microfilter. The operating condition effects, such as cross-flow velocity, transmembrane pressure, and broth culture time, on the filtration flux, cake properties, and extracellular polymeric substances (EPS) transmissions are discussed thoroughly. The culture broths contain B. subtilis cells and EPS which is characterized as polysaccharides (hydrocarbons) and proteins. An increase in broth culture time leads to higher concentrations of cells, soluble and extractable EPS. The total protein to polysaccharide concentration ratio in the broths is ca 0.2. However, the soluble polysaccharide concentration is 10-fold higher than that of soluble proteins. The filtration flux increases with increasing cross-flow velocity or transmembrane pressure. However, the impact of cross-flow velocity is more significant. The filter cake resistance formed by B. subtilis cells and EPS flocs plays the most important role in determining the over...

The membrane fouling and filtration performance of Bovine serum albumin (BSA)/dextran cross-flow microfiltration using membranes with different morphologies are analyzed and compared. Three 0.1 μm hydrophilic membranes made of mixed cellulose ester (MCE), polyvinylidene fluoride (PVDF) and polycarbonate (PC) were used in these experiments. The MCE and PVDF membranes had sponge-like structures, while the PC membrane had uniform straight-through circular pores. The membrane morphology, cross-flow velocity and transmembrane pressure effects on the filtration flux; membrane fouling and solute transmissions are discussed. The filtration flux increases with increasing cross-flow velocity or transmembrane pressure. The PVDF membrane produced the highest filtration flux, while the MCE membrane exhibited the lowest. A 15–30% difference in filtration flux may be obtained due to different membrane selection. Comparing various filtration resistances, the cake resistances of PVDF and PC membranes are almost the same but much higher than that for the MCE membrane, especially under high pressure. The membrane blocking resistance sequence is MCE > PVDF > PC membrane. The membrane blocking resistance of MCE and PVDF were quite similar due to their sponge network structures. Membrane fouling was analyzed using SEM and CSLM. BSA aggregates deposited onto the membrane surfaces, while dextran molecules adsorbed on the membrane surface and the membrane pore walls. More BSA/dextran mixtures were observed on the PVDF membrane. The PC membrane fouling was due mainly to pore coverage. Dextran molecules were more likely to deposit onto PVDF and MCE membranes. Both BSA and dextran transmission decreased with increasing cross-flow velocity. Increases in transmembrane pressure lead to lower BSA but higher dextran transmission. The PVDF membrane exhibited the highest solute transmission, while the PC membrane resulted in the lowest. All membranes exhibited higher selectivity for the BSA/dextran mixture under higher pressures, especially the PC membrane.

Glucose and enzyme in a high concentration enzymatic hydrolysis suspension were separated using hollow fiber cross-flow diafiltration. The operating condition effects on the filtration performance are discussed. Over 86% of the enzymes were retained using a 10 kDa polysulfone membrane. Most glucose molecules penetrated through the membrane into the filtrate. The major filtration resistances are due to the cake formation on the membrane surface and enzyme blocking in the membrane pores in addition to the appreciable virgin membrane resistance. The cake mass is correlated with the drag force ratio tangential to the filtration directions and the average specific cake filtration resistance is expressed as a power-function of the transmembrane pressure with a cake compressibility of 0.3. The empirical equation related the resistances due to internal membrane fouling to the operating conditions established by conducting experimental data regression. The filtration flux can be estimated accurately by substituting the results calculated using the empirical equations into the basic filtration equation. An increase in cross-flow velocity or transmembrane pressure leads to higher filtration flux. The filtration flux increases ca 2-fold as the cross-flow velocity increases from 0.3 to 1.5 m/s under a transmembrane pressure of 60 kPa. The filtration flux increases over 3-fold as the transmembrane pressure increases from 20 to 100 kPa. Two kinds of modified operating methods are used to improve the filtration flux. A pulse feeding method may increase the filtration flux by 25%, while the step-increase pressure method improves the filtration flux by over 34%. Forming a thin cake under low pressure in the early filtration period has great potential to significantly mitigate membrane pore blocking and enhance filtration flux.

The single step-up pressure filtration test was developed to determine the pressure dependence of average specific resistance of the cake formed in ultrafiltration of a variety of nano-colloids over a wide range of pressure drops across the cake. The values of the average specific resistance at extremely low pressures were obtained from only the flux decline data through the use of the distinct time variation of the pressure drop across the cake generated by using the ultrafiltration membrane with a high hydraulic resistance under the low filtration pressure in the first step of filtration. The values at higher pressures were obtained from the time variation of the filtration rate induced by a stepwise increase in the pressure. The correlations between the average specific cake resistance and the pressure drop across the cake were evaluated using only the flux decline data for a variety of different proteins and nanoparticles. © 2013 American Institute of Chemical Engineers AIChE J, 60: 289–299, 2014

Several novel hydrocyclones are designed to improve fine particle separation using computational fluid dynamics. The effects of inlet size, number of inlets and top-plate types on the particle separation efficiency and cut-size sharpness are discussed based on the same feed flow rates. The fluid and particle flows are simulated using a segregated, steady-state, 3-dimensional implicit numerical solver supplied by FLUENT software. The governing equations are coupled using the SIMPLE algorithm, while the Reynolds stress model is employed for the hydrocyclone turbulent model due to its' anisotropic nature. Particle trajectories are simulated based on a Lagrangian frame considering the continuous phase interactions. The simulated particle separation efficiencies approximately agree with the available experimental data. The results show that increasing the inlet number and narrowing the inlet width are effective ways to improve the particle separation efficiency due to the increase in fluid velocity in the cylindrical parts of hydrocyclone. A cone-shaped top-plate reduces the fine particle circulation area near the outer surface of overflow conduit, significantly improving the separation efficiency of fine particles. However, increasing the cone angle has a contrary effect because of the decrease in particle residence time. Although installing an extra guide-channel from the inlet may also improve the fine particle separation efficiency, it is not effective for particle classification because of reduced particle cut-size sharpness.

Online monitoring techniques can play a major role in understanding and controlling the mechanisms involved in the fouling of membrane filtration systems. The present study describes the application of an in situ optical method to monitor the growth of a fouling layer using a photointerrupt sensor array. To verify the validity of this method, the effects of the voltage applied, slurry concentration and membrane materials on the signal from the sensor array were examined. In addition, the application of this method in monitoring the growth of the fouling-layer thickness during the filtration of a high-turbidity influent stream under constant flux was studied. The results from three-dimensional (3D) fouling contour studies showed that the average fouling-layer thickness increased proportionally with time during the initial period in response to the rapid deposition of particles on the membrane surface, after which the growth rate of the fouling layer decreased. The 3D fouling contour analyses suggest that this online monitoring technique provides an effective in situ measurement of fouling-layer thickness and distribution. The photo-sensor array proved to be a promising online monitoring technique for scanning membrane surfaces on demand and can be applied to a pilot-scale water treatment unit for use in the real-time profiling of changes in fouling-layer thickness.

The microfiltration characteristics of Bacillus subtilis broths under different culture conditions were studied. In the no medium addition culture condition the major component in the broth was sole B. subtilis cells. The filtration curve of dt / dv vs. v (reciprocal of filtration flux vs. filtrate volume) can be divided into three regions. After a short relaxation time the tangent slope increases drastically. This implies that significant cell deformation and cake compression occurs in the second region. The average specific cake filtration resistance will reach the maximum value at the filtration curve inflection point. The cake compressibility gradually decreases in the third region because most solid compressive pressures are depleted by the formed compact cake. In the medium addition culture condition the B. subtilis cells and extracellular polymer substances (EPS) flocs concentrations in the broth increase markedly with culture time. The filtration curve tangent slope increases continuously during the entire filtration period because of the highly compressible cake. All proteins in the extra-cellular polymeric substances are retained by the filter cake and membrane during a filtration, while most polysaccharides have the opportunity to penetrate into the filtrate when the culture time exceeds 1 day. Polysaccharide rejection increases with increasing filtration pressure for the 1-day culture broth. However, the filtration pressure has a trivial effect on the polysaccharide rejection for longer culture times. The filter cake resistance formed by B. subtilis cells and EPS flocs plays an important role in determining the overall filtration resistance. A longer culture time and lower filtration pressure are beneficial for protein/polysaccharide separation using microfiltration.

Particle deposition in a spiral-wound membrane module was simulated using computational fluid dynamics (CFD). A scheme similar to the Eulerian–Lagrangian numerical method was adopted for the two-phase flow simulation. The effect of curvature on particle transport in four spacer-filled channel configurations with permeable membrane surfaces was analyzed by considering the fluid drag, body force and lift force exerted on the particles. The numerical results showed that there are inherent changes in the particle deposition profile in the spacer-filled channel due to variations in curvature. Comparing the particle deposition profiles and deposition ratios for submerged, zigzag, i-cavity and o-cavity spacer-filled channels showed that, for a given feed velocity and permeation rate, the zigzag-type spacer is best at decreasing the influence of curvature and preventing particle fouling on the membrane. A microscopic understanding derived from the CFD analysis could improve module design and enhance membrane module performance.

The sugars produced by enzymatic hydrolysis of rice straw are separated using cross-flow diafiltration in this study. The effects of membrane type, membrane pore size, cross-flow velocity and transmembrane pressure on the filtration flux, sugar rejection, and sugar mass flux transported to the filtrate are discussed. The filtration flux increases with increasing cross-flow velocity or transmembrane pressure. When the membrane made of mixed cellulose ester (MCE) is used, over 70% filtration resistances are caused by the membrane fouling; while the resistance due to virgin membrane is dominant when regenerated cellulose (RC) membranes are used. A force balance model is applied to relate the filtration flux and filtration resistance to operating conditions. The calculated data of filtration flux based on this model agree fairly well with experimental data. In addition, a theoretical model is used to explain the sugar transmission through the cake and membrane pores. The sugar rejection coefficient decreases ...

The effects of membrane morphology and operating conditions on the particle fouling in dead-end microfiltration are studied to verify the criteria for membrane selection. Three kinds of membranes, Isopore, Durapore and MF-Millipore membranes, with similar mean pore sizes around 0.45 μm are used for filtering 0.15 μm PMMA particles. The blocking index and resistance coefficient used for membrane blocking models are analyzed during microfiltration. The membrane pore blocking is dominant and the blocking index decreases gradually in the early filtration period. The blocking index drops suddenly to zero when a critical condition reaches; and the filtration follows up the cake filtration model thereafter. A membrane blocking chart is established for relating the blocking index and operating conditions for the used membranes. Comparing the results under a fixed filtration pressures, the blocking occurred in Isopore membrane is the most serious. The use of MF-Millipore membrane causes a filter cake to more easily form on the membrane surface. While the use of Durapore membrane results in the highest filtration flux due to its antifouling behavior. To analyze the critical filtration flux and particle accumulation at the changing point of blocking models, the sequence of difficulty in transforming to cake filtration is Isopore > Durapore > MF-Millipore membrane. The force analysis on depositing particles shows that a power relationship exists between the overall filtration resistance and filtration pressure at the critical condition. The critical overall filtration resistance has a sequence of Isopore > Durapore > MF-Millipore membrane under a fixed filtration pressure.

The membrane pore size and operating condition effects on the filtration flux, membrane fouling and solute rejections in cross-flow microfiltration of BSA/dextran binary suspension are studied. Two flat sheet microfiltration membranes with mean pore sizes of 0.1 and 0.025 μm are used as filter media in these experiments. The filtration flux increases with increasing cross-flow velocity and filtration pressure because of less membrane fouling or higher filtration driving force. The filtration resistance caused by membrane fouling is much higher than that caused by concentration polarization or virgin membrane. This impact is more significant under higher transmembrane pressures. The membrane fouling extent due to dextran adsorption is expressed as pore size reduction and fouled layer depth, which can be estimated theoretically based on hydrodynamic models. The dextran rejection decreases with increasing Reynolds number in the membrane pores and approaches constant when the Reynolds number exceeds a critical value. This is attributed to the increase in wall shear stress and molecular deformation in the membrane pores. A higher cross-flow velocity results in higher dextran rejection because of the sweeping effect occurring on the membrane surface. The BSA rejection under various operating conditions can be reasonably explained by the membrane sieving effect. The BSA rejection decreases linearly with increasing pore size in the fouled membrane with no obvious cross-flow velocity effect on BSA rejection found. The effects of the dextran concentration and Reynolds number in the membrane pores on the dextran adsorption layer thickness are also discussed.

Membrane fouling in BSA/dextran binary suspension cross-flow microfiltration under various operating conditions is studied. The mechanisms for membrane fouling based on the SEM and CSLM observations under various suspension concentrations are proposed. BSA aggregates deposit onto the membrane surface while dextran molecules adsorb into the membrane pores, resulting in filtration resistances. The BSA deposition is mainly determined by the drag forces exerted on BSA aggregates on the membrane surface. Dextran adsorption is affected by the dextran concentration and applied pressure. The concentration effects on the filtration flux and filtration resistance are also discussed. The filtration fluxes for BSA/dextran mixtures are located between those for two pure substances and decrease with the increase in either the BSA or dextran concentration. However, the dextran concentration impact is more significant because the filtration resistance caused by membrane blocking is much higher than that caused by cake formation in most conditions. The filtration resistances due to concentration polarization, cake formation and membrane internal fouling can be estimated directly from the operating conditions using the semi-theoretical methods proposed in this study.

The gel particle softness effect on cross-flow microfiltration performance is studied. Three kinds of soft gel particles with a similar size distribution but different softness and compression behaviors were prepared and used in these experiments. The particle softness is the most important factor affecting the pseudo-steady filtration flux compared to the other operating variables in the conditions of this study. A filter cake formed by softer particles is compressed into a more compact structure, consequently, resulting in higher filtration resistance and lower filtration flux. The particle deposition probability is analyzed using a force balance model. An increase in particle softness leads to lower particle deposition probability as well as lighter cake mass due to the drastic flux decay. The cake compression during filtration is described using the Voigt in series model. The cake porosity suddenly decreases and the specific cake filtration resistance drastically increases in the early filtration period. The skin cake layer formation in that period plays the most important role in determining the cross-flow microfiltration performance. The cake mass and average specific filtration resistance calculation results using the proposed methods agree fairly with the available experimental data.

In this study, bovine serum albumin (BSA) molecules are separated from suspended particles using submerged membrane filtration. Several methods, including constant-pressure filtration with backwash, air bubble sparging and a stepwise increase in pressure, are used to reduce filtration resistance and to enhance filtration flux. The effects of filtration pressure, backwash flow rate, backwash duration, air flow rate and stepwise pressure increase on filtration flux, filtration resistances, BSA rejection and BSA production are discussed. The results show that internal fouling of the filter membrane is the most important contributor to the overall filtration resistance, while cake formation is the main determinant of BSA rejection. An increase in filtration pressure leads to lower filtration flux and lower BSA rejection due to more severe internal fouling of the membrane. Although additional periodic backwash or air bubble sparging increases the filtration flux relative to solely constant-pressure filtration, the filtration flux and BSA production when using a stepwise pressure increase is much higher than with the other operations, especially when the latter is combined with a periodic backwash. Moreover, the calculated filtration flux and BSA rejection results for constant-pressure filtration under various filtration pressures agree fairly well with experimental data.

The effect of air-sparging on the performance of cross-flow microfiltration of microbe/protein bio-suspension is studied. A yeast/BSA binary suspension is separated using a two-parallel-plate cross-flow microfilter. The pseudo-steady filtration flux, cake properties and BSA rejection under various operating conditions are measured and discussed. The filtration flux increases with increasing filtration pressure, suspension and air velocities. The cake mass is significantly reduced by sparging air bubbles due to the increase in wall shear stress; however, this impact becomes trivial when air velocity exceeds 0.04 m/s. The average specific cake filtration resistance increases with increasing air velocity in a bubble flow, while suddenly decreases when the flow pattern changes to a slug flow. A drastic increase in BSA rejection is found as air bubbles are sparged into the filter channel. An increase in air flow velocity leads to higher BSA rejection especially in a slug flow regime. Taking filtration flux and BSA rejection into account, the BSA mass flux transported into filtrate decreases with increasing air velocity. Although sparging air bubbles can effectively enhance filtration flux, it is helpless for the separation of BSA from yeast cells. The results also indicate that the fluid flow pattern plays an important role in determining the filtration performance. The cake mass and BSA rejection in different flow regimes can be modeled using a force balance model for particle deposition and the standard capture equation for depth filtration, respectively.

Particle fouling in constant pressure submerged membrane filtration is analyzed. A particulate sample, with a wide size distribution ranging from submicron to micron, is filtered using a flat-sheet membrane module. The effects of filtration pressure and aeration conditions on the particle deposition probability, cake properties and filtration flux are discussed. Particle deposition is analyzed theoretically using a force balance model. The particle size distribution in a filter cake is smaller than that in the original suspension and gradually decreases during a filtration. An increase in filtration pressure leads to a higher filtration flux in the early period of filtration; however, after 3000 s the flux gradually approaches a pseudo-steady value. The filtration flux is significantly enhanced by sparging air bubbles. An increase in air volumetric flow rate or a decrease in bubble size leads to a smaller particle size distribution in the cake. The particle deposition probability, cake mass and average specific filtration resistance calculated by using the proposed models agree fairly well with available experimental data.

Microfiltrations of Saccharomyces cerevisiae yeast particles of various deformabilities were conducted to examine the blocking mechanism due to particle deposition during the initial stage of the microfiltration process. The S. cerevisiae particles were treated with glutaraldehyde to form particles of various deformabilities. To compare differences in the blocking mechanisms among the different particles during microfiltration, a blocking model was adopted to estimate the filtrate flux, which accounts for initial blocking due to pore obstruction and subsequent blocking due to the growth of a cake over these initially blocked regions. The surface morphology of the cake layer on the membrane surface and the blockage by particles in membrane pores was examined by scanning electron microscopy (SEM). Analysis of the blocking mechanism of particles of varying deformability during microfiltration based on the blocking model indicated that softer particles resulted in more severe pore blocking problems and greater flux decline. A comparison of the theoretical results with experimental observations by SEM showed good qualitative agreement.

Solid-state composites of NaBH 4 and Co-based catalyst have been fabricated for hydrogen generation via a novel mechanochemical technique, i.e. the high-energy ball milling, in which the gravimetric storage capacity of hydrogen has reached 6.7 wt%, meeting the 2010 target of at least 0.06 kg H 2 /kg set by the U.S. Department of Energy (DOE). The active catalysts used in the hydrolysis reaction of sodium borohydride for hydrogen generation are mainly cobalt oxides. Controlled addition of water, namely water used as a limiting agent, to the solid composites of NaBH 4 and Co-based catalyst greatly improves the H 2 storage capacity and resolved the issues of low gravimetric H 2 storage in conventional aqueous system of sodium borohydride. Factors influencing the performance of hydrogen production such as amounts of H 2 O added, catalyst loadings and durations of ball-milling processes are investigated. Moreover the hydrolyzed products of NaBH 4 and spent catalysts are analyzed as well.

Two polymeric membranes, PTFE and PVDF, were employed in a direct contact membrane distillation system to investigate the effects of NaCl concentration, feed temperature, and flow rate on distillate flux and NaCl rejection. The distillate flux increases with increasing the feed flow rate or temperature, PTFE membrane has a higher distillate flux than that of PVDF, and PTFE membrane also has a larger solute rejection than that of PVDF membrane. The decrease of solute rejection by PVDF membrane is owing to the membrane pore wetting, especially at high feed temperature and flow rate. The membrane fouling tests under various feed compositions show that increasing NaCl concentration from 4.5 wt% to 10 wt% or the addition of CaSO4 or BSA into NaCl solution does not cause severe fouling on the PTFE membrane. The distilled water flux recovery (FR) is about 0.96–0.97. The addition of MgCl2 or MgSO4 has a little significant fouling tendency, the distilled water FR is about 0.87–0.88.

The selective deposition of fine particles in a constant-flux submerged microfiltration system is studied. The effects of operating conditions, such as filtration flux, injected air flow rate and air bubble size, on the particle deposition probability, cake mass and average specific cake filtration resistance are discussed. Smaller particles have more opportunity to deposit onto the membrane surface. The particle size distribution in the filter cake increases with increasing the filtration flux or bubble size, but with decreasing the injected air flow rate. The external forces exerted on the depositing particles are analyzed based on hydrodynamics and DLVO theory. The drag forces due to suspension flow, permeate flow, and bubble flow play the major role in determining the particle stability on the membrane surface. Interparticle forces are dominant for submicron particles, while gravitational force increases its weight as particle size exceeds 5 μm. A force balance model is used for estimating the particle deposition probability. An increase in filtration flux or a decrease in air flow rate leads to higher particle deposition probability, heavier cake, or lower average specific cake filtration resistance. The deposition probability is higher by injecting bigger bubbles under a specified air flow rate due to the lower bubble generation frequency. Consequently, a decrease in bubble size causes to lighter cake but higher average specific filtration resistance. The validities of the model predictions for particle deposition probability, cake mass and average specific filtration resistance are demonstrated by experimental data.

Filtration characteristics and membrane fouling in BSA/dextran binary suspension cross-flow microfiltration is studied. An increase in cross-flow velocity or transmembrane pressure leads to higher pseudo-steady filtration flux due to less membrane fouling or higher driving force. The fouled membrane pore size and fouled layer thickness under various conditions are estimated using a theoretical model based on the Hagen–Poiseuille law. The fouled membrane pore size decreases with increasing transmembrane pressure due to ever increasing fouling. The fouled layer thickness becomes thicker and invariant with operating conditions under a lower pressure drop through the fouled membrane. Molecular adsorption occurs on the pore walls until a specific wall shear stress is reached in the membrane pores. However, the fouled layer becomes thinner when the pressure drop exceeds a critical value. An increase in cross-flow velocity results in a larger fouled membrane pore size and a thicker fouled layer in such condition. The dextran molecular deformation under higher transmembrane pressure causes a thinner fouled layer and an ultimate equilibrium adsorption in the membrane pores. Furthermore, an increase in cross-flow velocity leads to higher BSA and dextran rejection due to the sweeping effect on the membrane surface. The BSA rejection increases with transmembrane pressure due to the reduction in membrane pore size, while the decrease in dextran rejection under higher transmembrane pressure is attributed to the molecular deformation. The “coil-stretched” deformation of dextran molecules can be indicated using the Deborah number. The fouled membrane pore size and dextran rejection decrease with increasing Deborah number and remain constant as the Deborah number exceeds a critical value. Taking the filtration flux and solute rejection into consideration, operating a cross-flow microfiltration system under lower cross-flow velocity and higher transmembrane pressure is more efficient from the selectivity and mean dextran mass flux view points.

Consolidation experiments were carried out on pre-consolidated cakes of tofu (soybean curd) and okara (soybean curd refuse), which were soft colloids, by using a compression-permeability (C-P) cell. The variations in the average consolidation ratio of the cake over time, which is a measure of the degree of consolidation, were evaluated on the basis of the data of cake thickness versus time. Both the traditional Terzaghi model and the Terzaghi–Voigt model failed to describe consolidation behaviors of tofu gel and okara cake. A new multi-stage creep model consisted of a number of Voigt elements connected in series has been developed in order to elucidate the complicated consolidation behaviors of tofu and okara. It was shown that the consolidation behaviors of tofu and okara were accurately described using the present model. The consolidation of tofu gel, which has a high water-holding ability, was slower than that of okara, and the fraction of creep consolidation, especially ternary consolidation was more pronounced in tofu than in okara. The model calculations accurately described the dependence of consolidation behavior on consolidation pressure and the type and concentration of coagulants. It was found that the adjustable fitting parameters of consolidation appearing in the model formula remained nearly unaffected by the consolidation pressure in the range investigated. The consolidation rate approached a minimum value in the case of the tofu gel with high firmness prepared with a MgCl2 concentration of 0.05 mol/l.

The effect of air-sparging on the performance of cross-flow microfiltration of size-distributed fine particles is studied. The filtration flux and cake properties under various air velocities are measured and discussed. A higher air velocity leads to a lighter cake due to the higher shear stress acting on the membrane surface, especially under a bubble flow regime. However, the decrease in deposited particle size causes higher average specific cake filtration resistance as well as lower pseudo-steady filtration flux. A force balance model is employed to grasp the particle deposition on the membrane surface. The drag forces due to fluid flows play a major role in determining the particle deposition probability. The interparticle forces are dominant for submicron particles, while the inertial lift and gravitational force increase the weights for those particles larger than 10 µm. The occurrences of a minimum deposited probability for 0.2–0.3 µm particles and a maximum value for ca 2 µm particles ca...

A fast and simple fabrication process, viz. a wet-chemical reduction process, was utilized for preparation of a cobalt-based catalyst to be used for hydrogen generation from alkaline NaBH4 solution. Characteristics of this cobalt-based catalyst were carried out by using various instruments, such as SEM/EDS, XPS, ICP, VSM, BET and TGA. The surface chemistry of the obtained catalyst is mainly cobalt oxides, not cobalt borides. Ferromagnetic property of the cobalt-based catalyst makes it convenient for being recycled from spent NaBH4 solution. The rate of hydrogen generation from catalyzed hydrolysis of alkaline NaBH4 solution was determined as a function of temperature, NaBH4 concentration, and NaOH concentration in the presence of prepared catalysts. In general, stable generation rate of highly pure hydrogen near 200 mL min−1 g−1 was achieved in 100 mL of 5 wt% NaBH4 solution containing 5 wt% NaOH solution and 200 mg of Co/IR-120 catalyst, in which no temperature runoff was observed during the course of hydrogen production. The activation energy of NaBH4 hydrolysis reaction was found at 66.67 kJ mol−1, which was comparable with others reported in the open literatures.

Three kinds of porous gel particles with different compression properties were prepared and used in “dead-end” filtration experiments. Incorporating the conventional cake compressibility index, two additional parameters for particle compression, retardation time and softness index, are introduced to discuss the porous gel particle viscoelastic compression behaviors and filtration characteristics, including the variations of filtration flux, cake mass, cake porosity, water inside the particles and specific cake filtration resistance during filtration. A cake compression mechanism including four stages is proposed to explain the filtration curve trend for d t / d v vs. v for different gel particles. The concave trend in the compression stage is more obvious for gel particles that exhibit longer retardation time or a smaller particle softness index, or under lower filtration pressure. The variation in cake porosity during compression can be described using a Voigt in series model. Cake mass (including dry solid and the internal water) decreases quickly in the early filtration stage because of the rapid percolation of particle internal water, and then increases gradually after the cake compression retardation time due to the formation of new cake layers. A power relationship between the average specific cake filtration resistance and filtration pressure can be regressed. However, the specific cake filtration resistance value varies with time due to both cake and particle compression. A long compression retardation time or a high particle softness index implies that the particles possess high elastic and deformable natures. Consequently, the particles will form an extremely compact and high resistance cake. These effects become trivial when the compression parameters exceed the critical values. In conclusion, the compression behaviors of cakes formed by soft particles can be well interpreted using the relationships between cake compressibility and particle compression properties, such as the retardation time and softness index.

Effect of gas–liquid flow pattern on the performance of air-sparged cross-flow microfiltration of yeast suspension is studied. The pseudo-steady filtration flux and the cake properties under various operating conditions are measured and discussed. The shear stress acting on the membrane surface and the critical condition for particle deposition are analyzed theoretically based on hydrodynamic models. The cake mass is markedly reduced by increasing the wall shear stress. However, the average specific cake filtration resistance increases with increasing the wall shear stress due to more compact cake structure. The increase in the average specific filtration resistance of cake due to air-sparging is more significant in bubbly flows. Consequently, the filtration flux will be increased by air-sparging due to the cake reduction in slug flow microfiltration. However, a contrary result is obtained for bubbly flows due to the drastic increase in the average specific filtration resistance of cake. Therefore, a microfiltration operating under slug flow is more effective to enhance the filtration flux by air-sparging. In addition, the proposed model provides a method to quantitatively relate the filtration flux to operating parameters. The relationships are also strongly dependent on the gas–liquid flow pattern.

The operating condition effects on the cross-flow microfiltration performance of dilute macromolecular suspension were studied. Track-etched membranes of different pore sizes were used to filter blue dextran with a molecular weight of 2000 kDa. Increasing the membrane pore diameter results in higher filtration flux but decreases the dextran rejection coefficient. For filtration using 0.2-μm membrane, an increase in cross-flow velocity or filtration pressure leads to higher filtration flux, but the impact of cross-flow velocity is more significant. A membrane fouling model based on force analysis and membrane pore size reduction caused by dextran adsorption at pseudo-steady state is proposed. The effective membrane pore diameter and pseudo-steady filtration flux under various conditions can then be estimated. On the other hand, the pseudo-steady dextran rejection coefficient increases with increasing cross-flow velocity or decreasing filtration pressure when cross-flow velocity is lower than 0.3 m/s. This can be reasonably explained by the sweeping effect of tangential flow or using the modified gel-polarization model. However, a contrary trend occurs under the conditions with higher cross-flow velocity. This result is attributed to the dextran molecular deformation. The “coil-stretched” deformation of blue dextran molecules can be indicated using the Reynolds number in the permeating flow direction or Deborah number for macromolecular flow through porous media.

Two identical 10-mm hydrocyclone connected in series for improving particle separation efficiency is studied using two particulate samples. The operating variable effects, such as feed rate, split ratio and pressure drop through hydrocyclones on the separation efficiency, energy loss and outlet particle concentration are thoroughly discussed. Some empirical operating equations correlated to dimensionless groups, e.g. , Reynolds number, Euler number, Stokes number and spilt ratio, are proposed that can be employed for evaluating the partial separation efficiency and the hydrocyclone installation effectiveness. The partial separation efficiency of various particulate samples under different operating conditions can be expressed as a unique power-type general Stokes number and split ratio equations. When two hydrocyclones are installed in series, the d 50 value decreases significantly, however, no evident difference can be found for the d 100 cut-size. To connect a second hydrocyclone in series and to operate at a high split ratio in the first hydrocyclone are the optimal conditions from the particle separation efficiency, energy saving and outlet clarity viewpoint. However, the particle classification sharpness cannot be improved by connecting two hydrocyclones in series due to the fish-hook effect.