Cross-Flow Microfiltration of Bacillus Subtilis Broths under Various Culture Times

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 overall filtration resistance. The mass and average specific filtration resistance of cake can be estimated using a force balance model and empirical equations. The cake structure and thickness are analyzed using SEM. A thicker and more compact cake may be formed under longer broth culture time. Most soluble polysaccharide and protein molecules have the opportunity to penetrate through the cake and membrane into the filtrate because the solute transmissions are measured as high as 0.75–1.0. The influences of operating conditions on the polysaccharide and protein transmissions are negligible. Therefore, to enhance filtration flux by increasing transmembrane pressure or cross-flow velocity is beneficial to improve separation efficiency, especially by increasing cross-flow velocity.     

Determination of cake properties in ultrafiltration of nano‐colloids based on single step‐up pressure filtration test

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     

The preparation and filtration characteristics of Dextran-MnO2 gel particles

Different kinds of Dextran-MnO₂ gel particles are prepared in different conditions, depending on variables such as the molecular weight of dextran and formation temperature. Some physical properties and filtration characteristics of these gel particles are measured and discussed. Although the mean sizes of these gel particles are very close to each other, their filtration characteristics are far different due to their mechanical strength and compressibility. A typical filtration curve of gel particles can be divided into three regions, and a retardation cake compression during the filtration can be observed from the curve. The particle formation temperature has a trivial effect on their mechanical strength. An increase in formation temperature leads to only a slight decrease in particle size. On the other hand, gel particles are formed by using dextrans with three different molecular weights - 70,000 (sample A), 500,000 (sample B) and 2,000,000 Da (sample C) - and are used in filtration experiments. The results show that the molecular weight of dextran plays a major role in determining particle mechanical strength; the sequence of particle hardness is sample B > sample A > sample C. The dynamic analysis method proposed by Hwang and Hseuh [K.J. Hwang, C.L. Hseuh, J. Membr. Sci. 214 (2003) 259] is employed to estimate the local cake properties in a filter cake, e.g., solid compressive pressure, porosity and specific filtration resistance. Because sample C has the highest compressibility, it constructs a cake with the lowest porosity and the highest specific filtration resistance; and consequently, the lowest filtration rate. Although the mechanical strength of sample B is higher than that of sample A, its wider particle size distribution results in lower cake porosity and higher filtration resistance, as well as a lower filtration rate. It could be said that the filtration rate decreases with an increase in the molecular weight of dextran.     

Filtration and Drying Study of Phosphatidylinositol Fraction from De-Oiled Soy Lecithin

Separation and purification of phosphatidylcholine (PC) and phosphatidylinositol (PI) fractions to their utmost purity is still a challenging task industrially due to the molecular and structural complexity of lecithin molecules. The present study deals with the filtration of the PI-rich fraction followed by drying of the cake for its further purification and value addition. Filtration of the PI-rich fraction was carried out under constant pressure conditions followed by the drying of the cake in a vacuum tray dryer. The average specific cake resistance (α_av) as a function of operating pressure was studied and it showed little variation with respect to the applied pressure differential. The initial solvent content in the cake after filtration was found to be in the vicinity of 1 kg solvent/kg dry solid, which was then subsequently reduced in the drying stage to 0.04 kg solvent/kg dry solid. The drying kinetics of the cake was studied at different drying temperatures in a vacuum tray dryer operated under varying degree of vacuum. The drying rate curve showed a prominent falling rate period with the effective solvent diffusivity in the range of 5.4 × 10^?10 m²s^?1 to 1.42 × 10^?9 m²s^?1within the temperature range of 40–60°C. Critical analysis of the dried PI fraction was carried out in terms of phosphatidylinositol content and color. The drying data were analyzed using various models and Das et al.'s model, the modified Henderson and Pabis model, and the Page model were found to provide the best fit sin terms of root mean square error (RMSE), chi square, and correlation coefficient (R ²).     

Influence of Water Compositions and Conditioning on Flux Enhancement in an Immersed Membrane System

Abstract

The objective was to quantify the importance of operational conditions, aeration, and physico‐chemical conditioning on membrane fouling intensity. The suspension filterability was also analysed by using frontal filtration and a cake filtration model. Results pointed out the moderated role of aeration to reduce compound accumulation on the membrane surface. It did not appear as a determining criterion to prevent membrane fouling. In contrast, the physico‐chemical conditioning appeared as a determining criterion to increase critical flux. According to the experimental conditions 200 l/m²/h/bar membrane permeability could be maintained transmembrane pressure (TMP) when filtering stored rainwater. This permeability value was 2–3 times higher than the values obtained without conditioning. Moreover, according to the low turbidity of such stored rainwater and because of the high selectivity of the membrane, the coagulation step, a very low amount of 10 mg/l FeCl₃, was sufficient to intensify the filtration step. This conditioning interest appeared less significant when filtering salted water in immersed membrane systems, but a 20 mg/l FeCl₃ addition appeared sufficient to double the value of critical flux. Nevertheless filtration in frontal mode pointed out the significant impact of physico‐chemical conditioning in reducing the cake deposit hydraulic resistance.     

Ultrafilter Conditions for High‐Level Waste Sludge Processing

Abstract

Optimal filtration conditions were evaluated for the ultrafiltration process planned for pretreating high‐level waste (HLW) sludge in the Hanford Waste Treatment Plant. This sludge must be filtered in the pretreatment process to remove sodium and, consequently, reduce the number of canisters for storage. The evaluation, which was based on Hanford HLW slurry test data, was performed to identify the optimal pressure drop and crossflow velocity for filtration at both high and low solids loading. Results from this analysis indicate that the actual filtration rate achieved is relatively insensitive to these conditions under anticipated operating conditions. The maximum filter flux was obtained by adjusting the system control valve pressure to between 400 kPa and 650 kPa while the filter feed concentration increased from 5 wt% to 20 wt%. However, operating the system with a constant control‐valve pressure drop of 500 kPa resulted in a reduction of less than 1% in the average filter flux. Also, allowing the control valve pressure to swing as much as ±20% resulted in less than a 5% decrease in filter flux. This analysis indicates that a back pressure setting of 500 kPa±100 kPa will give effectively optimal results for the system of interest.     

ON THE SPECIFIC RESISTANCE OF CAKES OF MICROORGANISMS

The mean specific resistance of the cakes of various microorganisms was evaluated by measurement of either a change in the amount of permeate with time or of steady-state flux under constant pressure. The mean specific resistance was different with different shapes and sizes of microorganisms. The large differences arose from different packing structures of the cake. The effect of a filter aid on the filtration rate and cake structure was studied experimentally and theoretically. The effects of a filter aid were best explained by a series model, in which a cake layer composed of microbial cells and a layer of randomly distributed microbial cells and filter aid are packed on the membrane surface in series with respect to the directions of permeation.     

Evaluation of compression-permeability characteristics of microbial cake based on microfiltration data

A method was developed for evaluating the compression-permeability characteristics of microbial cake based on the microfiltration data on microbial suspension. The parameters in newly proposed equations, which were applicable to extremely-high compressible cake, were determined to fit the data of the average specific resistance and average void ratio of the filter cake as functions of the filtration pressure. The calculations based on proposed equations were in good agreement with the compression-permeability cell data such as the local specific resistance and local porosity of the cake represented as functions of the local compressive pressure in all microorganisms used in this study.     

Flux enhancement and cake formation in air-sparged cross-flow microfiltration

The flux enhancement in cross-flow microfiltration of submicron particles by sparged air-bubble is studied. The effects of operating conditions, such as air-bubble velocity, suspension velocity and filtration pressure, on the cake properties and filtration flux are discussed thoroughly. The results show that the pseudo-steady filtration flux increases as the air-bubble velocity and filtration pressure increase. The sparged air-bubble can significantly improve filtration flux, but the flux enhancement is more remarkable in the lower air-bubble velocity region. A gas–liquid two-phase flow model is adopted for estimating the shear stress acting on the membrane surface under various operating conditions. The cake mass can be significantly reduced by increasing the shear stress acting on the membrane surface. However, the SEM analysis illustrates that the particle packing structure becomes more compact as the air-bubble velocity increases. This results in a slightly higher average specific cake filtration resistance under higher air-bubble velocity. Consequently, a minimum flux occurs at the critical shear stress, e.g., τ_w = 1.1 N/m² in this study, when these effects are both taken into consideration. As the shear stress increases by increasing the suspension or gas-bubble velocity, the filtration flux decreases in the low shear stress region but, on the contrary, quickly increases in the high shear stress region. Furthermore, a force balance model is derived for understanding the particle deposition on the membrane surface. The relationship among filtration flux, shear stress and overall filtration resistance is obtained and verified by experimental data.     

Sugar Purification from Enzymatic Rice Straw Hydrolysis Products using Cross-Flow Diafiltration

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 with increasing cross-flow velocity because of the effect of cake reduction. This effect is more significant when the MCE membrane is used. Comparing the sugar mass flux transported into the filtrate, it is more effective for sugar purification by using 10 kD RC membrane and under higher transmembrane pressures.     

Cake properties of nanocolloid evaluated by variable pressure filtration associated with reduction in cake surface area

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     

Improvement of dead-end filtration of biopolymers with pressure electrofiltration

Solid/liquid separation of biopolymers such as polysaccharides and proteins is still a problem not solved sufficiently on a technical scale. The main objective of the presented work was to set up a process strategy in order to improve the recovery of biopolymers by dead-end filtration. One aspect was the investigation of the influence of the pH value, ionic strength and pressure on the filtration kinetics of the dead-end filtration of the polysaccharide xanthan. pH value and ionic strength have an impact on the hydrodynamic radius and the zeta potential of biopolymers, and thus they have an impact on the filter cake structure and the filtration kinetics. The main focus was set on the enhancement of the filtrate flux by an electric field. This process, called pressure electrofiltration, leads to a drastic improvement of the filtration kinetics. The filtration time was thereby reduced from the range of hours down to minutes. Additionally another strategy was followed up, which aimed at an improvement of the specific filter cake resistance by changing process parameters like the pH and the ionic strength. These parameters influence the polymer-polymer and the polymer-water interactions and thus have an influence on the filter cake properties. Due to the great acceleration of the filtration kinetics the pressure electrofiltration serves as an interesting alternative to the cross-flow filtration and the precipitation with alcohol for the separation of biopolymers.     

Pressure centrifugal filtration providing maximum volume of cake

An optimization solution is proposed for the pressure centrifugal filtration, which provides the maximum volume cake in a centrifugal basket with a height-decreasing peripheral zone and cylindrical filtrating strainer of low resistance in order to avoid the formation of a stream. This solution is based on a computer search for the maximal volume cake at determined intervals of the construction and process parameters and avoid filtrate stream formation. The calculations in the example serve as evidence of the efficiency of the method.     

Nanocolloid cake properties determined from step‐up pressure filtration with single‐stage reduction in filtration area

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     

Treatment of Oily Waste Water Using Low-Cost Ceramic Membrane: Flux Decline Mechanism and Economic Feasibility

Abstract

This work addresses the applicability of different membrane pore blocking models for the prediction of flux decline mechanisms during dead end microfiltration (MF) of stable oil-in-water (o/w) emulsions using relatively low-cost ceramic membranes. Circular disk type membranes (52.5 mm diameter and 4.5 mm thickness) were prepared by the paste method using locally available low-cost inorganic precursors such as kaolin, quartz, calcium carbonate, sodium carbonate, boric acid, and sodium metasilicate. Characterization of the prepared membrane was done by SEM analysis, porosity determination, and pure water permeation through the membrane. Hydraulic pore diameter, hydraulic permeability, and hydraulic resistance of the membrane was evaluated as 0.7 µm, 1.94 × 10^?6 m³/m²·s·kPa and 5.78 × 10¹¹ m²/m³, respectively. The prepared membrane was used for the treatment of synthetic stable o/w emulsions of 40 and 50 mg/L crude oil concentration in batch mode with varying trans-membrane pressure differentials ranging from 41.37 to 165.47 kPa. The membrane exhibited 96.97% oil rejection efficiency and 21.07 × 10^?6 m³/m²·s permeate flux after 30 min of experimental run at 165.47 kPa trans-membrane pressure for 50 mg/L oil concentration. Different pore blocking, models such as complete pore blocking, standard pore blocking, intermediate pore blocking and cake filtration were used to gain insights into the nature of membrane fouling during permeation. The observed trends for flux decline data convey that the decrease in permeate flux was initially due to intermediate pore blocking (during 1 to 10 minutes of experimental run) followed with cake filtration (during 10 to 30 minutes of experimental run). Based on retail prices of the inorganic precursors, the membrane cost was estimated to be 130 $/m². Finally, preliminary process economic studies for a single stage membrane plant were performed for the application of the prepared membrane in industrial scale treatment of o/w emulsions. A process economics study inferred that the annualized cost of the membrane plant would be 0.098 $/m³ feed for treating 100 m³/day feed with oil concentration of 50 mg/L.     

Experimentelle und theoretische Untersuchungen zur Filtration mit überlagerter Sedimentation in Drucknutschen

Sedimentation superimposed on industrial cake filtration leads to longer filtration times and often has a detrimental effect on subsequent process steps such as washing and demoisturing. The influence of sedimentation is seldom recognised in laboratory filtration experiments. Methods are presented for evaluation of pressure filter experiments with superimposed sedimentation which avoid the error made in the usual evaluation methods. For the case of zone sedimentation the article presents a graphical evaluation and a numerical method of modelling permitting scale up to any desired cake height. In superimposed classifying sedimentation simultaneous measurement of filtrate volume and cake height provide information about local variation in filter cake resistance. It is shown for a model system that, owing to sedimentation, the cake resistance shows a minimum at mean cake height and increases rapidly towards greater heights. The local cake resistance corelates with particle size distributions measured for layers of a horizontally cut filter cake. The method of evaluation presented permints determination of the flow resistance of the uppermost layers of a cake and hence estimation of the gas pressure necessary for demoisturing. The profiles of local filter cake resistance show that the relative cake layering is largely independent of the level of suspension filling. A scale-up model is presented for use in those cases where classifying sedimentation cannot be suppressed.     

Cross‐Flow Microfiltration of Fine Particles Suspended in Polymeric Aqueous Solution

Abstract

The filtration characteristics of cross‐flow microfiltration of fine particles suspended in polymeric aqueous solution are studied. Polymethylmethacrylate (PMMA) submicron particles are suspended in polyacrylamide (PAA) aqueous solution to prepare the suspensions used in experiments. Effects of operating conditions, such as cross‐flow velocity, filtration pressure, and PAA concentration, on the filtration flux and the cake properties are discussed. The results show that an increase in cross‐flow velocity or filtration pressure causes the filtration flux to be higher, but the filtration flux decreases with an increase in PAA concentration. Since the flow behavior indices of three prepared suspensions are almost the same, the average specific filtration resistance of cakes under various cross‐flow velocities and PAA concentrations remain almost constant; and then the cake mass plays a major role in determining the filtration resistance and the filtration flux. A force balance model is derived for particle deposition on the membrane surface. Once the empirical coefficients are obtained from experimental data, the filtration flux at pseudo‐steady state can be predicted accurately.     

Effect of Hydrodynamics During Crystallization on Mechanical Dewatering of Salicylic Acid

Mechanical cake dewatering is always desired to reduce the load on thermal dewatering (drying). Any change in the upstream process such as crystallization can have a significant influence on the filtration as well as cake dewatering characteristics. The present study deals with the effect of hydrodynamics (mixing intensity) during salicylic acid crystallization on the air dewatering characteristics in the subsequent pressure filtration. The mixing conditions during crystallization were varied by using three different types of agitators (anchor impeller [AI], curved blade turbine [CBT], and bar turbine [BT]) and by varying the speed of agitation. The effect of operating pressure and dewatering time on the final moisture content of the cake was also studied.

The crystal properties (crystal size and size distribution) were found to vary with the mixing intensity, which further influenced the cake dewatering kinetics as well as the residual moisture content. An AI, which is a laminar flow impeller, produced crystals with a wide size distribution and higher mean particle size, which resulted in cake with high porosity and hence higher moisture content. The high porosity (as well as high cake permeability) caused early air breakthrough, which resulted in ineffective dewatering of cake. Therefore, in this case the residual moisture in cake was found to be higher (27%) even at higher dewatering pressure (1.5 bar gauge) and longer dewatering time (90 s). A BT creates high turbulence during mixing and produced crystals with a relatively narrow size distribution and lower mean particle size, which provided low-porosity cakes. Such cakes could be efficiently dewatered and the final cake moisture content was found to decrease to about 15%, a significant improvement in the filterability of the cake. The dewatering data were modeled according to the correlation between irreducible cake saturation and capillary number for predicting the cake dewatering characteristics (residual moisture as well as dewatering kinetics) and the results were compared with the experimental data.     

CHARACTERISATION OF CAKE COMPRESSIBILITY IN DEAD-END MICROFILTRATION OF MICROBIAL SUSPENSIONS

Culture broths of Saccharomyces cerevisiae and Kluyveromyces marxianus var marxianus NRRLy2415, suspensions of rehydrated active dry bakers yeast and suspensions or calcium carbonate were filtered in dead-end mode at pressures below 200 kPa. In the case of all the microbial suspensions, the specific cake resistance was found to be a linear function of pressure. Cake compression was found to be reversible or weakly irreversible with respect to changes in pressure, i.e., the specific resistance measured at a given pressure was only weakly dependent on whether the filter cake had previously been exposed to higher pressures. The greatest irreversibility effects were obtained with unwashed active dry yeast suspensions, consistent with the breakdown of cell aggregates in these suspensions. The specific cake resistance of calcium carbonate suspensions was found to be a non-linear function of pressure. The compression of calcium carbonate cakes was irreversible, consistent with the breakdown of the large number of particle aggregates in these suspensions.     

Modeling of Particle Fouling and Membrane Blocking in Submerged Membrane Filtration

Abstract

The models of particle fouling and membrane blocking in a submerged membrane filtration are developed in this study. The effects of operating conditions, such as aeration intensity (air flow rate) and filtration pressure, on the filtration flux, membrane blocking, and cake formation are discussed thoroughly. The experimental results show that the filtration resistances due to cake formation and membrane blocking play significant roles in determining the overall filtration resistance, but the latter one is more dominant. An increase in aeration intensity leads the filtration flux to increase due to the reduction of cake formation on the membrane surface. However, a higher filtration pressure causes more severe membrane internal blocking and then to lower filtration flux. The cake properties and the filtration resistance due to membrane blocking are analyzed and can be regressed to empirical functions of filtration pressure. A force balance model for particle deposition on the membrane surface is also derived. In order to estimate the shear stress acting on the membrane surface, the diameter, shape, and rising velocity of air bubbles are analyzed based on hydrodynamics. Once the model coefficients are obtained, the pseudo‐steady filtration flux under various conditions can be estimated by the proposed model and the basic filtration equation. The calculated results agree fairly well with the available experimental data.