High Cross-flow Velocity Research Articles

Wastewater treatment using ultrafiltration at a vegetable oil factory

An investigation of wastewater treatment by ultrafiltration (UF) is introduced. In the experiments a OF membrane (UFPHT20-6338) and samples of Behshahr Ind. Co. wastewater as feed were used. The effect of operating conditions such as pressure difference, cross flow velocity, shear stress, temperature, concentration of organic components and pH on permeation flux, flux decline and fouling resistance were studied. The results show that a pressure difference more than 3 bar, high cross flow velocities (depending on economic considerations), a temperature of 30°C and a pH of 9 are the best operating conditions. Analysis of the wastewater treated by OF represents 91%, 87%, 100%, 85% and 40% reduction in COD, TOC, TSS, [P0 4 −3] and [Cl −], respectively. Reduction of the phosphate concentration by UF is very considerable.

In situ manipulation of critical flux in a submerged membrane bioreactor using variable aeration rates, and effects of membrane history

Membrane bioreactors (MBRs) have been increasingly used for municipal wastewater treatment. Systems where the membranes are submerged in the bioreactor, and scoured by the aerating gas stream, are very common and the “Kubota process” is a classic example of this technology [S. Churchouse, Membrane bioreactors: going from laboratory to large scale—problems to clear solutions, in: Presented at Membranes and the Environment, University of Oxford, July 14–17, 2002]. Current treatment plants are designed to cope with three times the average dry weather flow (DWF) of wastewater to accommodate variable flow rates, and hence are over-sized compared to average or “normal” conditions. If throughput in submerged MBRs can be changed readily by manipulation of the aeration rate, due to increased scouring of the membrane and a corresponding increase in critical flux, a smaller plant may be designed compared to the “3×DWF” rule. High throughput operation may be achieved by using a high aeration rate to generate a high crossflow velocity to minimise fouling, and at average or low throughput a low aeration rate may be used to minimise energy consumption. The aim of this work was to explore the feasibility of designing smaller membrane plants by using the aeration rate to manipulate critical flux in situ, and thereby allow variable throughput. It is concluded that (a) permeate flux and aeration rate are important hydrodynamic parameters that must be controlled to avoid excessive membrane fouling; (b) intermittent permeation is an effective membrane cleaning technique for long-term sustainability of flux; (c) manipulation of the critical flux through variation of aeration rate is possible in situ in submerged MBR systems.

Chemical cleaning of reverse osmosis membranes fouled by dye bath effluent of a carpet manufacturing plant

One of the limiting factors in membrane technology is fouling. Fouling results in reducing the due to precipitation of feed materials. This problem may he minimized with manipulation of some operating factors such ashigh cross flow velocity. However fouling occurs even at high cross flow velocity. In this work the chemical cleaning of membranes fouled with textile dyes has been investigated. The optimum operating conditions were obtained for this process. Increasing the applied pressure results in more fouling and declines faster compared to lower pressure. Reverse osmosis membranes fouled with dye bath effluent. The fouled membranes were chemically cleaned. Acids, alkaline, surface active agent and chelants at different concentrations were used as cleaning agents. For determination of cleaning efficiency, flux recovery and resistance removal were calculated for all cleaning agents. Cleaning efficiency depends on the type of the cleaning agents and their concentration. The concentration that provides the highest cleaning efficiency can be considered as the optimum concentration.

Effects of operating conditions on the salt rejection of nanofiltration membranes in reactive dye/salt mixtures

Abstract In this study, DS5 type nanofiltration membrane was tested to evaluate optimal operating conditions for membrane fouling and salt rejection in reactive dye and salt mixtures. Reactive black 5 (RB5) and NaCl were used in the experiments to prepare the synthetic dye and salt mixtures. Effects of pressure, cross flow velocity and pH were investigated. Experiments showed that cross flow velocities had a significant effect on flux values. In addition, effects of cross flow velocity decreased as NaCl concentration increased. At high pH values, low flux values were obtained due to the increase in dye hydrphobicity under alkaline conditions. Variations in pH, pressure and cross flow velocity also affected the salt rejection, which decreased with increasing NaCl concentration. Salt rejection at low cross flow velocities was lower than those conducted at high cross flow velocities and independent of the pressure, especially at low cross flow velocities and high pressures.

Study on ceramic membrane bioreactor with turbulence promoter

Abstract Membrane bioreactor (MBR) is an effective wastewater treatment process in which the membrane separates technology integrated with the activated sludge system. The suspended solids are completely separated from the treated water by the ceramic membrane unit, and all bio-mass are kept in the bioreactor. Because of such advantages as good chemical stability, favorable mechanical strength, good antimicrobe ability and high separation efficiency, the ceramic membrane is suitable for comprising ceramic membrane bioreactor (CMBR) with activated sludge. For the CMBR process, the high energy requirement of recirculation needed to maintain a high cross-flow velocity combined with maintaining the membrane permeability is a major problem hindering the widespread application of membrane separation processes. In order to improve membrane flux, and reduce cyclic volume and energy consumption, studying of the enhancement process is necessary. This work investigated the effect of different kinds of turbulence promoter on flux of ceramic membrane filtration. Three kinds of insert were used in ceramic membrane bioreactor. The flux under the same operation condition increased about 2.5 times with the introduction of the winding insert. Both microfiltration and ultrafiltration gave a steady flux for 30 days without chemical washing. The average reduction rate of COD was greater than 95% for municipal wastewater.

Optimum cleaning-in-place conditions for stainless steel microfiltration membrane fouled by terephthalic acid solids

Terephthalic acid (TPA) is a raw material of polyester fiber and polyethylene terephthalate. When TPA is produced by catalytic air oxidation of p-xylene in the presence of acetic acid solvent, most of produced TPA exists in the form of crystalline suspended solids. A microfiltration process may be used to recover TPA, but the microfilters are subjected to fouling and therefore cleaning-in-place (CIP) regimes need to be developed. In this research, the effects of variations to CIP conditions were investigated on the flux recovery accomplished in a TiO 2-sintered stainless steel microfiltration membrane (0.1 μm pore size) fouled with TPA. The extent of flux recovery was estimated as the ratio of the stabilized flux obtained during CIP to the water flux value achieved under corresponding operational conditions. Based upon batch solubility tests, sodium hydroxide (NaOH) was chosen as the major cleaning agent for the present experiment. The extent of flux recovery increased with increasing NaOH concentration over the range of 3–4% (w/v) NaOH, but decreased at NaOH concentrations above 4%. The flux recovery was favored at high cross-flow velocities, high temperatures and low transmembrane pressures. A high temperature run of cleaning did not produce any adverse effects up to 70 °C. The addition of surfactants (SDS and Tween 80) to the caustic cleaning agent led to a significant reduction in cleaning efficiency.

Controlling fouling in membrane bioreactors operated with a variable throughput

Membrane bioreactors (MBRs) have been used increasingly for municipal wastewater treatment. The current wastewater treatment plants are designed to treat three times the average flow in dry weather (DWF) which covers the expected range of incoming flow rates. If throughput in MBRs can be changed readily by changing the energy input into the system, a smaller plant can be designed. Under varying throughput operation, a high aeration rate is required to generate a high crossflow velocity to minimize fouling. At low flow rates, a low aeration rate is used to minimize energy consumption. The aim of this work is to explore the feasibility of designing smaller membrane plants by varying the throughput. This requires the control of membrane fouling, so that chemical cleaning is not compromised. Fouling is controlled by limiting the membrane flux and also by flushing the surface of the membranes with large air bubbles. Variations of the permeate flux and the aeration rate were varied in this study and their effect on controlling fouling noted. Intermittent permeation while retaining aeration was found to be an effective technique for long-term sustainability of high fluxes.

Reactive dye removal in dye/salt mixtures by nanofiltration membranes containing vinylsulphone dyes: effects of feed concentration and cross flow velocity

In this study, DS5 DK type nanofiltration membranes were tested to recycle the reactive dye bath effluents. Reactive black 5 (RB5), reactive orange 16 (RO16), reactive blue 19 (RB19) and NaCl were used in the experiments to prepare the synthetic dye and salt mixtures. Effects of feed concentration, pressure and cross flow velocity on the permeate flux and color removal were investigated. Permeate flux increased with increasing pressure for all NaCl solutions. Dye concentration had a significant effect on flux values. Under the fixed NaCl concentrations the flux decreased with increasing dye concentrations. Dye rejections greater than 99% were achieved. Permeate was almost colorless. A gel layer formed by the rejected dye on membrane surface operates as a resistance to the permeation of dyes due to complete rejection of high molecule weight dyes, especially for the low salt concentrations. The presence of salt concentration has an interesting effect on color removal. Color removal decreased with increasing salt concentration. Cross flow velocities had also a significant effect on flux values. The dye formed agglomerates at high NaCl concentrations. High cross flow velocities decreased this effect.

Effect of Gas-Liquid Ratio on the Membrane Flux in Activated Sludge Process.

The use of membrane bioreactors (MBRs) is rapidly increasing in the field of organic wastewater treatment. In typical MBR applications for activated sludge processes, the aeration for oxygen supply is also used to maintain high cross-flow liquid velocity over the submerged membrane system (SMS). The prime objective of this study was to demonstrate the effect of presence of gas phase fraction in suspension on the performance of membrane filtration. Experimental studies were performed using two different membrane modules of (1) flat sheet membrane cell (FMC) operated outside the feed tank, where the cross flow velocity of liquid and the volume fraction of gas were changed independently, and (2) flat sheet SMS operated in tank. The relationship between the cross flow velocity, volume fraction of gas phase and the trans-membrane pressure (TMP) were determined. It was observed that the rate of increase in TMP did not depend on the presence of gas phase, but depended on the real cross-flow liquid velocity, uL', for both the FMC with and without gas phase and the flat-sheet SMS.

Influence of shear on flux in membrane filtration of integrated pulp and paper mill circulation water

Different ultrafilters and membranes were tested with ground wood mill circulation water from an integrated pulp and paper mill. Both the CR 200/1 (cross rotational, CR-filter™) and the VSEP ® L (vibratory shear enhanced processing) laboratory modules and the CR 1000/10 pilot plant module were investigated. In the pilot plant filtration the effects of the rotor speed, volume reduction factor and pressure were tested. According to the measurements, the lower the pressure and the higher the rotational frequency (CR 200/1) or the vibration amplitude (VSEP L), the less fouling occurred. For the CR-modules the best short time fluxes were obtained with PA 50H and Desal JX membranes, and for the VSEP-module with the PA 50H and C 30F membranes. The C 30F, PA 50H and PES 50H membranes were chosen for the pilot plant filtrations. In the pilot tests at the mill it was observed that the hydrophilic C 30F membranes fouled very little and had fluxes close to 400 l/(m 2h). The more hydrophobic PA 50H and PES 50H membranes were fouled badly, and cleaning only restored flux for a short time. A concentration factor between 10 and 30 did not influence the flux very much, but inside the equipment the membranes handling the highest concentration had a lower flux. Also in the mill site experiments it was shown that the flux should be kept as close to the critical flux as possible and that this could be achieved with a high cross-flow velocity and the lowest pressure possible for the equipment.

Cross-flow microfiltration of submicron microbial suspension

The mechanism of cross-flow microfiltration of submicron microbes is studied. Experiments are carried out using pseudomonas suspensions. The effects of operating conditions on the filtration performance are discussed. The results show that the filtration rates increase with increasing the cross-flow velocity, while decrease with increasing the filtration pressure. The Brownian dynamic simulation method is adopted for simulating the trajectories of microbes in cross-flow microfiltration. The deposition flux of microbes on the membrane surface can then be estimated from the simulated trajectories. Based on the continuity equation of cake compression and Kozeny equation, a dynamic analysis method is developed for simulating the distributions of local cake properties, such as porosity and specific filtration resistance in the filter cake. The deformation of microbes, the area contact between microbes and the creeping effect of cake compression are taken into consideration. The simulated values of average cake properties agree with the experimental data. It can be found that a skin layer controlled the filtration rate will form next to the membrane surface. Although the thickness of the skin layer is only about 10% of the whole cake, it plays the major role on the filtration resistance. Increasing the filtration pressure results in more compact skin layer; therefore, a lower filtration rate is obtained. Moreover, a higher cross-flow velocity will results in a thinner but more compact cake, a higher filtration rate can be given.

Cleaning strategies for flux recovery of an ultrafiltration membrane fouled by natural organic matter

One of the most common problems encountered in water treatment applications of membranes is fouling. Natural organic matter (NOM) represents a particularly problematic foulant. Membranes may be fouled by relatively hydrophilic and/or hydrophobic NOM components, depending on NOM characteristics, membrane properties, and operating conditions. To maximize flux recovery for an NOM-fouled ultrafiltration membrane (NTR 7410), chemical cleaning and hydraulic rinsing with a relatively high cross-flow velocity were investigated as cleaning strategies. The modification of the membrane surface with either an anionic or a cationic surfactant was also evaluated to minimize membrane fouling and to enhance NOM rejection. Foulants from a hydrophobic NOM source (Orange County ground water (OC-GW)) were cleaned more effectively in terms of permeate flux by acid and caustic cleanings than foulants from a relatively hydrophilic NOM source (Horsetooth surface water (HT-SW)). An anionic surfactant (sodium dodecyl sulfate (SDS)) was not effective as a cleaning agent for foulants from either hydrophobic or hydrophilic NOM sources. High ionic strength cleaning with 0.1 M NaCl was comparatively effective in providing flux recovery for NOM-fouled membranes compared to other chemical cleaning agents. Increased cross-flow velocity and longer cleaning time influenced the efficiency of caustic cleaning, but not high ionic strength cleaning. The membrane was successfully modified only with the cationic surfactant; however, enhanced NOM rejection was accompanied by a significant flux reduction.

Optimization of Hollow-Fiber Design and Low-Pressure Membrane System Operation

An approach is presented for the numerical optimization of low-pressure membrane filtration processes. A multidimensional optimization of an ultrafiltration system is formulated for cost minimization and numerically solved for key optimal design and operating variables. Hollow-fiber ultrafiltration operation under steady-state conditions is assumed and optimized with respect to fiber radius, fiber length, crossflow velocity, transmembrane pressure, and system recovery. Optimizations are performed over variable raw water conditions using a sequential quadratic programming (SQP) algorithm. For typical small to moderately sized low-pressure membrane facilities (≈1 mgd), optimal fiber design and membrane system operation is predicted to be largely influenced by the characteristic dominance of capital costs over operating costs. Thus, total treatment costs tend to be optimal at values of decision variables where permeate fluxes are maximized, within the constraints prescribed by the system, and assuming a fixed membrane cost per unit area. For raw waters demonstrating apparent to significant membrane fouling, optimal membrane treatment is predicted to be achieved by using relatively narrow hollow fibers and relatively high crossflow velocities. For relatively clean raw waters demonstrating very high sustainable permeate fluxes, operating at low crossflow velocities—or perhaps even under the dead-end mode of operation—appears to provide the most cost-effective operation.

Effect of filtration conditions and backflushing on ceramic membrane ultrafiltration of board industry wastewaters

Abstract A biologically treated combined wastewater from a board mill was ultrafiltered with ceramic membranes. The effect of membrane modification, cross-flow velocity, filtration pressure, and backflushing on ultrafiltration was studied. The ceramic membranes were made of α-alumina and their pore size was 100 nm. Some of the membranes were modified with titania and some with magnesia. The modifications were made in order to change the surface properties of the membranes. The cross-flow velocities were from 0.8 to 8 m/s and the filtration pressures from 0.7 bar to 2.3 bar. The backflushing was made at 4 bar with a mixture of air and permeate for 1 s every 1 min. At low cross-flow velocities permeate flux was controlled by the fouling layer formed on the top of the membrane and the permeate fluxes of all the membrane modifications were similar. When the cross-flow velocity was increased the fouling layer decreased and the differences in membrane modifications became clear. The unmodified alumina membrane had the highest permeability, 129 l/(m 2 h bar) and the magnesia-modified membrane had the lowest permeability, 107 l/(m 2 h bar), after 2 h of filtration. The permeability of the titania-modified membrane was close to that of the unmodified membrane. The results showed that when the cross-flow velocity was increased the permeate flux increased. However, it seemed that there existed some optimum cross-flow velocity above which the increase in flux was not significant or the flux began to decrease. Moreover, the increase in filtration pressure increased the permeate flux. There exists an optimum pressure above which the increase of pressure does not increase the flux significantly. With a higher cross-flow velocity the optimum pressure was higher. The backflushing decreased the formation of fouling during filtration. From the biologically treated wastewater, suspended solids and turbidity could be removed almost totally. Moreover, iron, total phosphorous, and biological oxygen demand were reduced with over 60% and in some cases also manganese had retentions over 90%. The increase in permeate flux decreased slightly the retentions of color, iron, chemical and biological oxygen demand, total phosphorous, nitrogen, and the substances absorbing at a wavelength of 280 nm.

Experimental evaluation of a uniform transmembrane pressure crossflow microfiltration unit for the concentration of micellar casein from skim milk

A uniform transmembrane pressure (UTMP) crossflow microfiltration (CFMF) system maintains a low but uniform transmembrane pressure (Δ P TM) with high crossflow velocity (CFV), which reduces fouling and cake build-up, and improves the utilization of available filtration area. A CFMF system, with a 0.2 μm nominal pore size ceramic filter, filtration area 0.184 m 2, was operated in both UTMP and non-UTMP modes. The two modes were compared for their effectiveness in maintaining a steady flux during the separation of casein micelles from skim milk up to a concentration factor (CF) 10 at 50°C. Experiments were performed at an average CFV of 7.2 m s −1 and Δ P TM from 89 to 380 kPa. Up to CF 4 the non-UTMP mode maintained a slightly better flux and process time than the UTMP mode, but reached the minimally acceptable flux (below 0.005 kg m −2 s −1) at CF 6. Depending upon the Δ P TM maintained, the UTMP mode approached the minimal flux at CF 7 or 10 depending upon the combination of Δ P TM and CFV used. Cake resistance ( R cm) was modified to include the effect of an increase in retentate viscosity with concentration. R cm increased for the non-UTMP mode and decreased for the UTMP mode with a decrease in the ratio of permeation flux/wall shear stress ( J p/ τ w) (which occurred as the retentate gets concentrated). This indicated that the cake formed during the non-UTMP mode of operation was more compact and durable (harder to erode) than in the UTMP mode. A central composite rotatable design estimated the optimal operating region at a CFV of 7.1 m s −1 and Δ P TM of 241±10 kPa to achieve maximum flux and a high concentration.

Study on hydraulic characteristics in a submerged membrane bioreactor process

Hydraulic characteristics in a membrane bioreactor are of significance for retarding membrane fouling and maintaining stable operation. Cross flow velocity along the membrane surface and its effect on transmembrane pressure change were experimentally investigated using two pilot-scale submerged membrane bioreactors. A simulation model for calculating cross flow velocity was then developed. Cross flow velocity was a function of aeration intensity. Under the experimental conditions of the present study, there was a critical cross flow velocity of ≈0.3 m s −1. When the cross flow velocity was lower, transmembrane pressure sharply increased. Predicted cross flow velocities using the simulation model derived from energy balance analysis agreed with the measured values very closely. Cross flow velocity is also dependent on the dimensional parameters of bioreactor. For a reactor that has a greater height, a more compact riser and wider channels in the down-comer and bottom, a higher cross flow velocity could be obtained with the same aeration intensity.

SIZING AGENT RECOVERY BY MEMBRANE ULTRAFILTRATION IN HOLLOW-FIBER MODULES

Ultrafiltration of an aqueous solution of polyvinyl alcohol was carried out in an Amicon model H1P30-20 hollow-fiber cartridge made of polysuifone. Fresh water was forced through the macroporous membrane, as the permeate, while polyvinyl alcohol was concentrated and re covered as the retentate. Correlation equations for calculating the permeate flux of membrane ultrafiltration of the polyvinyl alcohol were derived based on the resistance-in-series and exponential models. Correlation results were confirmed by the experimental data, especially for the results obtained from the exponential model. It was found that the permeate flux increases as transmembrane pressure or fluid velocity increases, but decreases when feed concentration increases. Because membrane ultrafiltration is a pressure-driven process, high cross-flow velocity enhances the mass transfer coefficient of the solute and high solution concentration in creases the thickness of the concentration polarization layer.

Advanced Treatment Combination for Groundwater Recharge of Municipal Wastewater by Nanofiltration and Ozonation

This paper presents the results of research undertaken on an advanced treatment combination for polishing municipal wastewater with the purpose of a safe groundwater recharge. The results of a former study of DWQC initiated this research. It is envisaged that tertiary effluent is nanofiltrated to reject dissolved organic carbon (DOC) and adsorbable organic halogens (AOX) to concentrations less than 2–3 mg DOC/L respectively <2 μg AOX/L. The brine will be given back in a recycling process to the sewage treatment plant after passing an oxidation step. To avoid rising scaling potentials and other negative impacts due to increasing salinity, the rejection characteristics of several NF-membranes were investigated. They show a strong dependence between DOC and sulfate removal. Biofouling on the membrane surface (Desal DK5) can be controlled by higher cross-flow velocities (CFV) of about 1 m/s, however, a suitable pre-treatment like slow sand filtration is required. High water conversion factors result in moderately higher biofouling. This shows that water quality is the main factor responsible for fouling and not the concentration of constituents. Ozonation experiments with the concentrate confirmed an enhanced biodegradability of refractory DOC. At a specific ozone consumption 1,7 mg O3/mg DOC0 the DOC reduction by micro-organisms (aerobic biotest) reaches its maximum after 14 days of biodegradation with a total reduction of 60%.

Experiments on membrane filtration of effluent at wastewater treatment plants in the Netherlands

In the Netherlands interest in advanced treatment is increasing now that almost all wastewater treatment plants apply full biological treatment and nutrient removal. Membrane filtration of WWTP-effluent appears to be a very promising method to remove suspended solids and, in addition, bacteria and viruses. Experiments with a pilot unit consisting of a dual media prefilter and a membrane filter, containing capillary membranes with a pore size of 0.01 μm (ultrafiltration) and working in a semi dead-end mode, gave rather promising results. A flux of 90 l/m2.h was achieved after optimizing the cleaning procedures (TMP 0.6 bar, recovery > 90%). Experiments with microfiltration (transverse flow module) based on the cross-flow principle were successful only at high cross flow velocities (flux 100 l/m2.h, TMP 0.5 bar, recovery 80%). The ultrafiltrate showed an excellent quality with the absence of suspended solids and bacteria. Finally, low costs for energy (NLG 0.02/m3 (1 NLG = 0.5 US$)) and chemicals (NLG 0.01/m3) were calculated. Further optimization will facilitate the applicability of membrane filtration in the near future.

Experiments on membrane filtration of effluent at wastewater treatment plants in the netherlands

In the Netherlands interest in advanced treatment is increasing now that almost all wastewater treatment plants apply full biological treatment and nutrient removal. Membrane filtration of WWTP-effluent appears to be a very promising method to remove suspended solids and, in addition, bacteria and viruses. Experiments with a pilot unit consisting of a dual media prefilter and a membrane filter, containing capillary membranes with a pore size of 0.01 μm (ultrafiltration) and working in a semi dead-end mode, gave rather promising results. A flux of 90 l/m 2.h was achieved after optimizing the cleaning procedures (TMP 0.6 bar, recovery > 90%). Experiments with microfiltration (transverse flow module) based on (he cross-flow principle were successful only at high cross flow velocities (flux 100 l/m 2.h, TMP 0.5 bar, recovery 80%). The ultrafiltrate showed an excellent quality with the absence of suspended solids and bacteria. Finally, low costs for energy (NLG 0.02/m 3 (1 NLG = 0.5 US$)) and chemicals (NLG 0.01/m 3) were calculated. Further optimization will facilitate the applicability of membrane filtration in the near future.