Theoretical analysis of the influence of the axial variation of the transmembrane pressure in cross-flow filtration of rigid spheres

A model of the axial and the radial transmembrane pressure drop in a cylindrical cross-flow filtration module was developed by performing a hydrodynamic analysis of the fluid flow based on the momentum and the continuity equations. Use of this expression for the transmembrane pressure drop together with the resistance model and the concept of shear induced diffusion of the particles at the membrane surface resulted in an expression of the permeate flux. The predictions of the transmembrane pressure drop, the permeate flux and the particles near the membrane surface are discussed for cases with and without the formation of a stagnant layer. The importance of the cylindrical membrane fiber dimensions on the permeate flux is also discussed.     

Preparation,characterization and application of powdered activated carbon‐cellulose acetate phthalate mixed matrix membrane for treatment of steel plant effluent

Preparation, characterization and industrial application of a mixed matrix membrane (MMM) using powdered activated carbon (PAC) in cellulose acetate phthalate (CAP) have been reported in this study. The objective of this work is to fabricate a less energy intensive, highly selective (to phenolic compounds) adsorptive membrane with high throughput in a scalable platform for simultaneous removal of organic as well chemical oxygen demand (COD) from a steel plant effluent. The membrane with 25 wt% PAC has maximum adsorption capacity of phenol 35 mg/g at pH 5.5. Effluent with total phenolic compounds (23 mg/g) and COD of 5200 mg/l is treated in continuous cross‐flow configuration. Breakthrough time is 44 hr for a filtration area of 0.008 m² with total phenol concentration in permeate as per World Health Organization (WHO), 1 mg/l. Throughput of the system is high, 40 l/m² hr at transmembrane pressure drop 276 kPa and cross‐flow rate 20 l/hr. Maximum rejection of phenol is obtained at low pressure and cross‐flow rate. Removal of phenolic compounds is achieved by adsorption by PAC in CAP matrix and satisfactory reduction of COD and complete removal of non‐volatile solids are due of sieving mechanism. A simple chemical regeneration method is proposed to recover the permeate flux beyond 90%. Copyright © 2015 John Wiley & Sons, Ltd.     

Fouling of a ceramic microfiltration membrane by corn starch hydrolysate

The effect of operating parameters on fouling of a ceramic microfiltration membrane by corn starch hydrolysate of 95 dextrose equivalence was studied. Transmembrane pressures above 100 kPa had little or no effect on flux. Cross-flow velocity had a significant beneficial effect. The rate of flux decline was reduced significantly when the feed was adjusted from its natural pH of 4.2 to 10. However, this resulted in a dark brown clarified syrup (permeate). Scanning electron microscopy showed extensive fouling layers on the alumina surface with conventionally processed dextrose solutions and the least fouling layer with corn starch hydrolysate adjusted to pH 10. Maximum steady state flux for unconcentrated hydrolysate at its natural pH was 178 LMH obtained at low transmembrane pressures (103 kPa, 15 psi) and high cross-flow velocities (5 m s⁻¹). Adjustment of the pH to 10 can increase the flux by 40%.     

Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux: application to a membrane bioreactor used for wastewater treatment

Membrane bioreactors for wastewater treatment must operate for long periods without chemical cleaning. This paper investigates the critical flux concept introduced by Field et al. as a means for achieving this goal. Experiments were conducted on a membrane bioreactor containing 600 l of activated sludge, equipped with a 0.25 m² ceramic membrane and located in Compiegne wastewater treatment plant. Hydraulic retention time was set at 24 h and sludge retention time at 60 days, so that suspended solids concentration stabilises at 10 g/l. We conducted two series of tests: at fixed transmembrane pressure (TMP) and at fixed permeate flux, set by a volumetric pump on the permeate. In both cases, velocity was varied from 1 to 5 m/s. In fixed flux tests, the flux was increased by 10 l/h m² increments and the TMP was observed to rise moderately first and then stabilise in about 15 min until a critical value of the flux is reached. Above this critical flux, the TMP rises rapidly and does not stabilise, as in dead-end filtration. The critical flux was found to increase approximately linearly with velocity, reaching about 115 l/h m² at 4 m/s. These data were reproducible at various dates between 30 and 120 days of continuous operation of the bioreactor and permit to know at which flux a membrane bioreactor must be operated. Comparison of constant pressure and constant flux tests under same conditions showed that the critical flux is almost identical to the limiting or pressure independent flux obtained in constant pressure. More generally, constant flux procedure below the critical flux avoids overfouling of the membrane in the initial stage and is more advantageous for membrane bioreactor operation.     

Comparison of ceramic capillary membrane and ceramic tubular membrane with inserted static mixer

Oily wastewaters are produced in large amounts in many fields of food, mechanical, and other types of industry. In order to protect the environment, wastewaters must not be discharged directly into sewers. First, they must be cleaned at least down to 50 mg L⁻¹ of oil content (according to Hungarian standard). In previous research, the authors found that oil-in-water emulsions can be separated with filtration using ceramic ultrafiltration tubular membranes. The relatively high price of ceramic membranes can be compensated by the fact that this separation process can be significantly intensified by static mixers inside the tubular membranes. New generations of ceramic membranes are the ceramic capillary membranes. These two different types of membranes and their effects on permeate flux, oil retention and specific energy consumption were compared in this work. The results, obtained with a stable oil-in-water emulsion as feed, showed that the use of novel ceramic capillary membranes at optimal operating cross-flow rate and transmembrane pressure is reasonable. The results have also shown the advantage of static mixing in the lumen side of the membrane tube providing a wider range of satisfactory separation level and increased permeate flux.     

Pilot scale membrane separation of electroplating waste water by reverse osmosis

Electroplating waste water containing copper was treated by means of reverse osmosis (RO) membrane separation on a pilot scale. The copper concentration in the untreated waste water was 340 ppm. After the treatment, the concentration in the treated water was below 4 ppm which is the Hong Kong government discharge limit. It is shown that, by increasing transmembrane pressure drop, metal concentrations in the treated water can be further reduced. This study suggests that larger scale operations on treating electroplating waste water by RO membrane separation is possible and effective. Effects of operating variables including transmembrane pressure drop and temperature on membrane separation performance were studied and explained based on the solution-diffusion model. The present study is part of the recent investigation of industrial waste water management sponsored by the Hong Kong Government. The purpose of this project is to provide guidelines to the local industries for waste minimization which is closely monitored by the Hong Kong legislature.     

Removal of suspended clay from water using transmembrane pressure pulsed microfiltration

Transmembrane pressure pulsing (TPP) uses the frequent and periodic reversal of the transmembrane pressure to reduce flux resistances due to membrane fouling. This study examined the effect of TPP on the microfiltration of simulated drinking water (hydrated aluminum silicate solution). Solutions of kaolin clay (0.1–4.0 μm particles, at an approximate concentration of 500 mg l⁻¹ and a turbidity of 402±17 NTU, 0.5 mM CaCl, 2.0 mM NaHCO₃, pH 7.5–7.8) were microfiltered with polyethersulfone (PES) 0.16 μm microfiltration membranes at an operating pressure of 30 kPa. Crossflow shear rates were varied between 165 and 1490 s⁻¹. Pulse frequency was varied between 0.3×10⁻² and 2 Hz, and pulse amplitude was varied between −3 and −16.5 kPa. It was found that the crossflow shear rates did not significantly effect the non-pulsed permeate flux. An optimum pulse amplitude of about 10 kPa was necessary to maximize the permeate flux for pulse frequencies between 0.3×10⁻² and 2.0 Hz. To insure a reduced solute flux, pulse frequencies less than 0.1 Hz were required. These results indicate that TPP can significantly reduce membrane fouling by inorganic particulate materials that are potentially important constituents of natural waters without negatively impacting the rejection of sub-micron particles due to interactions with material accumulated on the membrane.     

On the prediction of permeate flux for nanofiltration of concentrated aqueous solutions with thin-film composite polyamide membranes

The nanofiltration of binary aqueous solutions of glucose, sucrose and sodium sulfate was investigated using thin-film composite polyamide membranes with different molecular weight cut-off's. The NF experiments, in total recycle mode, were performed in a plate-and-frame module Lab 20 (AlfaLaval), at 22 °C and with a flowrate of 8.2 L/min, using the membranes NF90, NF200 and NF270 from FilmTec (Dow Chemical), for transmembrane pressures between 1 and 6 MPa and with aqueous solutions with osmotic pressures of between 0.5 and 3.0 MPa. The permeate flux was predicted by the osmotic pressure model, using the membrane hydraulic resistance and the solution viscosity inside the membrane pores, and computing the concentration polarization with recourse to a mass-transfer correlation specific for the plate-and-frame module used. The flux predictions, using the pure water viscosity, agree reasonably with the experimental data only for low transmembrane pressures and with the most diluted solutions. For higher transmembrane pressures and for higher solute concentration the predicted fluxes can be as far as 2.5, 4.1 and 9.6 times higher than the experimental one, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. These deviations are strongly reduced when the pure water viscosity is replaced by the solution viscosity adjacent to the membrane. In this case, the maximum deviation between predictions and experiments occurs also for higher transmembrane pressures and for higher solute concentration, but the maximum ratio between predicted values and the experiments were reduced now to 1.8, 2.1 and 2.9, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. Even using the solution viscosity adjacent to the membrane, and for the systems investigated, the osmotic pressure model must used with caution for design purposes because it may over predict the permeate flux by a factor of about 2 when the solute concentration is high.     

Resistance in series model for micellar enhanced ultrafiltration of eosin dye

A study has been performed to quantify the extent of flux decline during micellar enhanced ultrafiltration (MEUF) of an acid dye (eosin red) using hexadecyl (cetyl) pyridinium chloride as the cationic surfactant. Effects of the operating conditions, e.g., transmembrane pressure drop and feed-surfactant-to-dye ratio, on the permeate flux profile and observed retention have been investigated in an unstirred batch ultrafiltration (UF) cell. A simple resistance-in-series model has been used to quantify the flux decline. From the flux decline history, it has been found that the membrane permeability decreases rapidly due to reversible pore blocking and further flux decline is caused by the growth of a gel-type layer over the membrane surface. The different resistances and growth kinetics of the gel layer have been investigated as functions of the operating conditions.     

Amperometric determination of chemical oxygen demand using boron-doped diamond (BDD) sensor

A new application of boron-doped diamond (BDD) electrode was developed for detecting chemical oxygen demand (COD) by amperometric method. The effects of some basic experimental parameters including pH and applied potential on the response of the BDD electrode were investigated and the optimal operating conditions were obtained. In the COD tests of standard samples, a wide linear range of 20–9000 mg l⁻¹ COD and a low detection limit of 7.5 mg l⁻¹ COD were well established with the present approach. Additionally, the BDD sensor was successfully employed to determine the COD of real samples from various chemical or pharmaceutical wastewaters and the performance still kept stable after over 400 measurements. The results obtained indicated that, as compared with the conventional COD determination techniques, the proposed sensor was an environmentally friendly method with the advantages of short analysis period, simplicity, and no requirement of complicated sample pretreatment even for a sample containing relatively high concentration of organic pollutants.     

Application of turbulence promoters in ceramic membrane bioreactor used for municipal wastewater reclamation

In ceramic membrane bioreactor (CMBR), the permeate flux through a multi-channel tubular membrane has been improved by using turbulence promoters with different configurations. It was confirmed that the introduction of inserts led to better flux in comparison with empty tube. Winding inserts with 10 mm pitch and 1.6 mm wire diameter showed better performances than the others did. A 30-day laboratory-scale operation for reclamation of municipal wastewater was studied using the ceramic membrane bioreactor. The flux under the same operation parameters increased from 70 to 175 l m⁻² h⁻¹. The average reduction rate of chemical oxygen demand (COD) was more than 95% for municipal wastewater. The investigation showed that the introduction of winding inserts was effective in increasing permeate flux of a CMBR system, and the effluent quality would not reduce in comparison with empty tube.     

Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

High molar mass polysaccharides (locust bean gum and karaya gum) and effluents from a mechanical pulp mill and a paper mill were nanofiltered with commercially available nanofiltration (NF) membranes. The effect of the filtration conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained.

The critical flux was observed to increase with increasing flow velocity and decreasing concentration. An increase in pH increased the electrostatic repulsion between the membrane and the dissociated (charged) components in the paper mill effluents. As a result, a higher critical flux was obtained and also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate flux deviated from the pure water flux even at the lowest pressure, but increased linearly with pressure until the weak form of the critical flux was exceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface.

In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed. The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the membranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the limiting flux. The pure water fluxes of precleaned membranes after filtration were still higher than the pure water fluxes of the untreated membranes before filtration.     

The determination of chemical oxygen demand in waste-waters with dichromate by flow injection analysis

A simple method is described for the continuous determination of chemical oxygen demand (COD) in wastewater samples by flow injection analysis. Samples are injected into a water stream which merges with an acidic dichromate carrier solution. After reaction in a PTFE coil at 120°C, absorbances are measured at 445 nm. D-Glucose is satisfactory as a standard substance for COD in a variety of wastewaters. A sampling rate of 15 samples per hour can be achieved, and the detection limit and precision are 5 mg l^-1 as COD and 0.4%, respectively. Chloride concentrations of ? 100 mg l^-1 interfere slightly unless silver and/or mercury salts are added. COD values for various wastewater samples correlate well with those obtained by standard methods using dichromate or permanganate.     

Effect of colloids on salt transport in crossflow nanofiltration

The role of colloid deposition on the performance of a salt-rejecting NF membrane was evaluated by modeling salt transport using a two-layer transport model, which quantified the relative contributions of advection and diffusion in the cake and the membrane layers, and the effects of flux on the membrane sieving coefficient. The model was able to accurately describe how the measured permeate concentration, rejection, osmotic pressure, and flux decline varied with time. The two-layer model confirmed that the Peclet number in the cake layer was about an order of magnitude higher than that in the membrane layer, leading to significant concentration polarization at the membrane surface, as shown by others. However, the cake layer also increased overall resistance, which resulted in flux decline during constant pressure operation. Flux decline caused an increase in the actual sieving coefficient, leading to higher solute flux, lower observed rejection, and thus lower the bulk concentration. These coupled phenomena tended to mitigate the increase in concentration polarization caused by the cake. Therefore, as predicted by the model and verified by experiment, the osmotic pressure does not increase monotonically as the cake grows, and in fact can decrease when the cake layer is thick and the flux decline is significant. In our experimental system, the pressure drop across the cake layer, which was proportional to the cake thickness, was significant under the conditions studied. The effects of cake-enhanced osmotic pressure analyzed here are lower than those observed in previous studies, possibly because the transport model employed explicitly accounts for the effect of flux decline due to cake growth on the membrane sieving coefficient, and possibly because we used a somewhat different methodology to estimate cake porosity.     

Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration process is to apply controlled transmembrane pressures to the retentate side of the membrane, with the permeate side open ended. Often the minimum transmembrane pressure available is sufficient to cause membrane fouling in a given system. A membrane rig has been built which monitors transmembrane pressure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detection of trace fouling. Under a variety of low flux conditions fouling was not observed and it was found to be useful to produce an experimentally related definition of two types of critical flux. In the first definition a `strong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the second definition a `weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line differs from that for clean water. This paper describes how the use of this operating mode led to the successful experimental measurements of critical fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experimental evidence in support of a `strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membrane pore size on critical flux measurements for the three types of feed fluids.     

Transmission of bovine albumin under controlled flux ultrafiltration

This work reports the transmission of bovine albumin using 50k MWCO, 100k MWCO and 0.2 μm membranes under controlled fluxes and low transmembrane pressures. With the 50k MWCO membrane, the transmission remained low and when the flux was increased step by step, there was a sharp increase in transmission as the flux reached 50 lm⁻² h⁻¹. The concentration of bovine albumin was estimated at the membrane surface by the classical film theory and did not increase sharply. It is suggested that increasing extensional shear at the higher flux might change the conformation of BSA molecules. This sudden change in rejection was not observed when a 0.2 μm membrane was used. It was also observed that the transmission at iso-electric point of bovine albumin (pH 4.9) was much higher than that at either pH 3.5 or pH 8 under the same operating conditions.     

Effect of ethanol on ultrafiltration of bovine albumin solutions with organic membranes

This paper investigates the ultrafiltration of albumin-ethanol solutions on polysulfone hollow fiber membranes with 30 kDa cut-off. The aim is to identify the mechanisms responsible for the observed permeate flux reduction in presence of ethanol. The variations of permeate flux with transmembrane pressure and wall shear rate fit the usual pattern of flux limitation by concentration polarization. Thus, although ethanol significantly increases the permeate viscosity, the data show that the flux decrease is not a direct consequence of the viscosity increase but rather due to reduced albumin diffusivity which decreases the back transport to the bulk solution. The specific resistance of the albumin layer on the membrane was found to be unaffected by the presence of ethanol. However the fouling potential of our solutions was found to be significantly increased by the addition of ethanol. Thus the observed flux reduction due to ethanol seems to be explained by a combination of a thicker polarization layer caused by reduced back transport and increased membrane fouling. A 10% increase in filtrated volume can be obtained by imposing periodic retrofiltrations which decrease fouling.     

Micellar enhanced ultrafiltration of phenolic derivatives from their mixtures

Micellar enhanced ultrafiltration (MEUF) of different phenolic derivatives including meta-nitrophenol (MNP), catechol (CC), para-nitrophenol (PNP), and beta-napthol (BN) in their binary mixture has been studied. A 1:1 ratio of the mixture of (i) MNP with CC and (ii) PNP with BN is taken for the MEUF experiments using a cationic surfactant, namely, cetyl(hexadecyl)pyridinium chloride (CPC). An organic polyamide membrane with molecular weight cutoff of 1000 is used. Experiments are conducted using an unstirred batch cell and a continuous cross-flow cell. The effects of various operating conditions, e.g., concentrations of surfactant and solute in the feed, transmembrane pressure drop, and cross-flow rate (for cross-flow experiments) on the permeate flux and the observed retention of each solute have been studied in detail. The retention of solutes without using the surfactant varies from 3 to 15% only at a typical feed solute concentration of 0.09 kg/m3, whereas, under the same operating pressure (345 kPa), retention at the end of the experiment increases to about 66 to 99.8% depending on the nature of solute in the batch cell using surfactant micelles (10 kg/m3). Retention of solutes is less in the case of the two-component feed solution compared to the single-component feed solution. An increase in flux to the range of 9 to 16% is realized in cross-flow experiments compared to batch cell flux after one hour of operation.     

Ultrafiltration of stable oil-in-water emulsion by polysulfone membrane

This paper focuses on treatment of oily wastewater coming out from the post-treatment unit of petroleum industries where finely divided oil droplets are uniformly dispersed in large volumes of water. Polysulfone (PSf) membranes which had been modified for higher porosity and hydrophilicity through the use of additives such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) were used for removal of oil from the oily wastewater. The performances of different PSf membranes were evaluated by treating with pure water as well as with laboratory made oil-in-water (o/w) emulsion. Experiments were carried out with 12 such membranes in a semi-batch filtration cell made of Teflon and the influence of operating conditions such as transmembrane pressure and feed properties such as initial oil concentration and pH of feed solution on membrane performance were investigated. Results show that all the parameters play a key role in permeate flux as well as percent oil separation. Also change in morphological properties of membranes due to addition of different molecular weight PVP and PEG are found to have a significant influence on the permeate flow rate and hence subsequent oil removal. The experimental results showed that oil retentions of almost all the membranes were over 90% and oil concentration in the permeate was below 10 mg/L, which met the requirement for discharge. It was concluded that the ultrafiltration (UF) membranes developed in the study were reasonably resistant to fouling and hence the developed PSf membranes may be considered feasible in treating oily wastewater.     

Calcium Ion Coordinated Polyamide Nanofiltration Membrane for Ultrahigh Perm-selectivity Desalination

Improving the permeate flux but retaining the rejection of thin-film composite(TFC) polyamide nanofiltration(NF) membrane is a high requirement for desalination. In this work, a calcium ion(Ca²⁺) coordinated polyamide(PA) NF membrane was prepared by directly adding CaCl₂ to the piperazine(PIP) aqueous solution during the interfacial polymerization process. Due to the coordination interaction between Ca²⁺ and the amide bond in PA active layer, the number of hydrogen bonds in the PA active layer was reduced, causing in turn the decrease of physical cross-linking degree. As a consequence, the pore of the PA active layer was enlarged, prominently enhancing the water permeance of NF membrane. With the increase of CaCl₂ concentration, the pure water flux of TFC NF increased significantly while the rejection of Na₂SO₄ decreased sightly. Compared with TFC NF membrane prepared without CaCl₂, the permeate flux of the Ca²⁺ coordinated polyamide NF membrane prepared under optimal conditions was increased by 3-4 folds with Na₂SO₄ rejection of 95.26%. Meanwhile, such a Ca²⁺ coordinated PA NF membrane showed a better SO₄^2-/Cl^- selectivity.