Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes

Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes

Journal of Membrane Science 186 (2001) 41–52 Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes Rishi Sondhi,...

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Journal of Membrane Science 186 (2001) 41–52

Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes Rishi Sondhi, Ramesh Bhave∗ US Filter, Ceramic Membrane Products, 1750 Filter Drive, DeLand, FL 32724, USA Received 20 July 2000; received in revised form 13 November 2000; accepted 16 November 2000

Abstract Effect of backpulsing on crossflow filtration of different process streams was studied. Laboratory scale experiments were conducted with synthetic electroplating wastewater containing Cr(OH)3 suspension. Porous ceramic membranes of various pore sizes (0.05–5.0 ␮m) were evaluated. Filtration experiments with and without backpulsing show that backpulsing is effective in minimizing membrane fouling. Up to five-fold increase in steady-state permeate flux and 100% flux recovery were observed. Theoretical aspects are reviewed to develop a better understanding of the critical parameters associated with high-pressure backpulsing. Pilot and commercial scale operating results on several industrial applications, such as yeast filtration, process slurry filtration and oily wastewater filtration are presented. Data analysis shows the critical importance of backpulsing in reducing long-term membrane fouling while allowing the realization of high product recovery. Optimization of process parameters with backpulsing typically results in higher flux and reduces the total capital cost required to achieve the desired production rate. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Microfiltration; Ultrafiltration; Ceramic membranes; Backpulsing; Fouling

1. Introduction Transmembrane pressure pulsing or backpulsing (BP) is an effective technique for reducing fouling phenomenon in membranes, improving the overall filtration rate and extending the cleaning interval (the time between two consecutive membrane cleanings). Backpulsing is an in-situ method for cleaning the membrane by periodically reversing the transmembrane pressure. When transmembrane pressure is reversed, permeate liquid is forced back through the membrane to the feed side. This flow reversal ∗ Corresponding author. Tel.: +1-904-822-8000; fax: +1-904-822-8010. E-mail address: [email protected] (R. Bhave).

dislodges deposited foulants, which are then carried out of the membrane module by the tangential flow of retentate or are redeposited on the membrane surface [1]. It should be noted, that backpulsing is most effective in removing deposits on the membrane surface. Should severe pore plugging occur, backpulsing will most likely be ineffective in preventing precipitous flux decline. This type of irreversible fouling may only be corrected by chemical cleaning. There are several parameters associated with backpulsing. Backpulse duration is defined as the amount of time the filtration system operates under negative transmembrane pressure. Pulse amplitude is defined as the absolute value of maximum transmembrane pressure during backpulsing. Backpulse interval is the duration of time in between two consecutive pulses.

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Nomenclature Hm J K L Rm r Si

Greek µ 1P ε

membrane thickness permeate flux Kozeny–Carman constant membrane pore length membrane resistance membrane pore radius internal surface area per unit volume of membrane letters permeate viscosity transmembrane pressure (TMP) membrane porosity

Backpulsing should be distinguished from the more familiar technique of backflushing or backwashing. The fundamental difference between a backpulse and backwash is the speed and force utilized to dislodge accumulated matter on the membrane surface. In backflushing, flow reversal through the membrane occurs for 5–30 s once every 30 min to several hours. In backpulsing, flow reversal occurs every few minutes and reverse high-pressure pulses (up to 10 bar) are applied for very short periods of time (typically <1 s). In addition, backpulsing is a dynamic process and introduces transient effects not found in conventional backflushing. A variety of backpulse devices can be used to produce reverse flow to periodically remove accumulated foulants on the membrane elements. The choice is dependent on the size and number of modules in the filtration loop, along with cost considerations. Typically for each backpulse, about 0.5 l of permeate volume is required per square meter of filtration area. The simplest backpulse device is a pump and bladder assembly. The bladder assembly holds the permeate volume and includes a membrane barrier to prevent direct contact of pressurizing air with permeate. The pump (gear or diaphragm) is connected to the air intake and generates the required air pressure for an effective backpulse. A variation of this approach uses a tank and air compressor. The tank is filled with permeate and pressurized to 80–100 psi. The tank is sized to deliver adequate volume based on the total filtration area. The frequency is set with a timer. The

disadvantage of this type of device is the relatively slower speed of permeate discharge, inconsistent discharge volume and potential leaks. These factors contribute to the lower efficiency of such devices compared to the backpulse device used in this work. The backpulse valve assembly containing a fixed volume reservoir (such as that used in the laboratory scale unit) is more effective due to the ability to deliver consistent permeate volume at high pressure almost instantaneously. Backpulse devices are available in sizes that provide from 100 ml to 5.7 l of reverse flow. A single assembly can be used to backpulse several modules at one time, whether connected in series or parallel. Backpulsing is of special significance in ceramic membrane filtration because unlike polymeric membranes, ceramic membranes are able to withstand the high pressures associated with backpulsing. Ceramic membranes have been used in several industrial applications for more than 15 years. They are well suited for slurry filtration, oil–water emulsion separations, surface water filtration, aqueous cleaner recovery, as well as food, dairy and beverage (e.g. sugar juice clarification, milk protein concentration, fruit juice clarification, etc.) applications [2]. Some interesting results have been reported on the use of ceramic membranes in crossflow filtration of proteins [3], yeast [4] and other suspensions [5–8]. Several research groups [9–12] have investigated the use of backpulsing with ceramic membranes. However, these studies were limited to bench-scale testing and theoretical analysis of the backpulse technique. This paper endeavors to combine the theoretical aspect of backpulsing with laboratory and industrial data. In addition to bench-scale studies, we report the operating results from pilot testing and commercial scale installations, which illustrate the importance of backpulse in reducing long-term membrane fouling while allowing the realization of high product recovery.

2. Theory During filtration, particles accumulate on the membrane surface, forming a cake or gel layer. At the same time, some particles may adsorb on or block the surface pores. It is believed that the backpulsing process restores the flux by dislodging the particles blocking the membrane pores and those particles forming a

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flow through the membrane can be described by the Kozeny–Carman equation [16] J =

ε3 1P KHm µSi2 (1 − ε)2

(2)

where J is the permeate flux, 1P the applied transmembrane pressure (TMP), µ the permeate viscosity, Hm the membrane thickness, K the Kozeny–Carman constant and Si the internal surface area per unit volume of membrane. For spherical particles and assuming K = 5 [17], the above equation can be written as Fig. 1. Schematic representation of membrane cleaning during backpulsing.

cake on the membrane surface (Fig. 1). It is assumed that the cake layer is instantly lifted and swept into the retentate flow [12]. Thus, only particle fouling on the membrane surface (external fouling) is considered and internal pore plugging effects are assumed to be minimal [12,13]. If the above assumption is valid, then for complete membrane cleaning, the pore-blocking and cake-forming particles should be pushed back into the crossflow and swept away into the retentate flow. The apparent distance traveled by the solvent molecule during pulsing should be greater than the apparent pore length. Then the membrane cleaning time, tc (the time required to dislodge the particles from the membrane surface) can be defined as the ratio of distance traveled by the solvent to the solvent velocity. Considering a limiting case, when the distance traveled by the solvent is assumed to be equal to the pore length L, it can be approximated by using the d’Arcy’s permeability model, assuming a tortuosity factor of 2.5 [14,15] L = 15 (εRm r 2 )

(1)

where µ is the membrane porosity, Rm the membrane resistance and r the pore radius. The particle velocity is governed by hindered transport in the narrow pore. However, when the particle size approximates the pore size, the average particle velocity is equal to the average solvent velocity. This is true for ceramic membranes as the individual ceramic particles are dense. The porosity is created by interstitial space between the solid particles. Assuming the pores to be interstices between close-packed spheres, the

J =

ε3 r 2 1P 45Hm µ(1 − ε)2

(3)

The cleaning time (tc ) can now be written as εRm r 2 /5 ε3 r 2 1P /45Hm µ(1 − ε)2 9µRm Hm (1 − ε)2 = 1P ε2

tc =

(4)

From the above equation, it can be seen that the cleaning time is proportional to permeate viscosity, membrane resistance, membrane thickness and inversely proportional to TMP.

3. Experimental 3.1. Bench-scale system For laboratory scale studies, Membralox® (US Filter, DeLand, FL) ceramic membranes of various pore sizes (0.05–5.0 ␮m) were used in the filtration experiments. These tubular membranes were 250 mm in length with internal diameter of 7 mm. Filtration experiments with synthetic wastewater were performed using a bench-top filter unit (US Filter 1T1-70) as shown in Fig. 2. The unit was equipped with a 12 l conical bottom feed tank, a 50 psi two-stage centrifugal pump, which feeds a 250 mm membrane housing connected with a backpulse unit. The unit incorporated a tubular heat exchanger, a paddlewheel flow meter with analog display, six valves (V1–V6), and three pressure gauges (PI , PO , and PP ). In filtration experiments, the feed solution was pumped to the filter unit and during forward filtration the pressure

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Fig. 2. Ceramic membrane filtration system.

gradient was maintained by adjusting the flow rate (using valves V1 and V2) of the feed solution. The backpulse unit was attached on the permeate side of the membrane housing. Fig. 3 is the schematic of the particular backpulse device studied in this work. During reverse filtration, a timer initiated pulsing. A solenoid valve, pressurized with nitrogen gas, activated the backpulse unit by directing a nitrogen pulse to the backpulse valve. The backpulse device assembly incorporates a known volume permeate reservoir and a cylindrical piston. When the backpulse

valve is actuated, the piston moves rapidly, displacing permeate and causing a flow reversal through the membrane for a very short duration. The flow reversal removes adsorbed particles and accumulated cake from the membrane deposited during filtration. This very short reverse filtration was followed by a longer forward filtration period during which permeate was collected. Permeate side pressure was measured by a high accuracy sensor (PX 800-100GV, Omega Engineering, Stamford, CT) used in conjunction with data acquisition software (Strawberry Tree, Sunnyvale,

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Fig. 3. Schematic representation of N2 actuated backpulsing apparatus.

CA) and an IBM computer. Backpulse duration could be varied between 0.5 and 30 s and the backpulse interval could be varied between 30 s and 1 h. 3.2. Operating procedures Synthetic wastewater was prepared for laboratory experiments by mixing chromium sulfate (Cr2 (SO4 )3 · 12H2 O), which is the principal component of electroplating wastewater, with de-ionized water and 1N NaOH and stirred overnight to produce a homogenous suspension. The following reaction takes place in water to produce chromium hydroxide particles: Cr 2 (SO4 )3 · 12H2 O + 6NaOH → 2Cr(OH)2 + 3Na2 SO4 + 12H2 O Typical chromium sulfate concentration in the simulated electroplating wastewater was about 50 mg/l and the suspension had a pH of about 7. For the suspension at pH 7, the diameter of the aggregates, measured by a laser zeta-potential and particle size analyzer (Coulter, Delsa 440 SX), ranged from 1.5 to 10 ␮m, with an average of 5 ␮m. Permeate flux through the membrane, reported as l/m2 h (LMH), was measured by collecting the perme-

ate in a graduated cylinder and timing the collection period. Flux was calculated as the amount of permeate collected divided by the product of membrane filtration area and time taken to collect a known volume. The transmembrane pressure (TMP) was calculated as TMP = 21 (PI + PO ) − PP where PI is the inlet pressure, PO the outlet pressure, and PP the permeate side pressure. The total suspended solids (TSS) in the feed (F) and permeate (P) were measured and the rejection coefficient was calculated by R = 1−(P /F ). All laboratory experiments were carried out at a crossflow velocity of 5.7 m/s, unless specified otherwise, and at a temperature of 40◦ C. The Reynolds number was calculated to be well above 4000, implying turbulent flow. Permeate was open to atmosphere (except when backpulse was applied) such that the typical transmembrane pressure (TMP) was 140 kPa (20 psi). Both permeate and retentate were recirculated to the feed tank to maintain the concentration of the feed (total recycle mode). Most experiments were repeated two to four times for each set of conditions and the results discussed in this paper report the average values. Each experiment began with filtering clean tap water through the clean membrane until permeate flux was stabilized.

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Table 1 Summary of typical parameter values for laboratory filtration experiments Membrane characteristics Ceramic membrane tube i.d Tube length Nominal pore size Porosity

7 mm 25 cm 0.05, 0.2, 0.8 ␮m 50%

Feed conditions of sulfate suspension Solid concentration pH Temperature

50 mg/l 7 40◦ C

Typical process parameters Forward filtration pressure Backpulse amplitude Crossflow velocity Backpulse duration Backpulse interval

140 kPa (20 psi) 170 kPa (25 psi) 5.7 m/s 0.5 s 30 s

At this point the feed was changed to the synthetic wastewater. Membranes were reused, and underwent a multi-step cleaning procedure involving 1% (wt./wt.) HNO3 followed with 2% (wt./wt.) NaOH solutions. The system was thoroughly rinsed with clean water prior to the start of a new experiment. The membranes were stored in dilute NaOCl to prevent bacterial growth. Typical values of key operating parameters are given in Table 1. The objective of pilot testing is to obtain data suitable for scale-up. It is, therefore, necessary to operate the unit for a sufficiently long period to obtain information on long-term fouling characteristics. Many industrial applications require continuous filtration at constant filtrate flow. The transmembrane pressure increases as the run progresses to maintain constant flow. For batch processes the filtrate flow is initially high and decreases with concentration factor. The industrial examples discussed later in the paper (see Section 4.3) show filtration performance under both variable flux and constant flux conditions representative of batch and continuous processing, respectively. The backpulse time and duration are set from the start of the run. The filtration cycle is continued until the filtrate flow drops substantially below the starting value, indicating irreversible fouling. At this time, chemical cleaning is performed to remove foulants on the membrane. The cleaning interval between two successive filtration cycles is an important design parameter. The primary purpose for pilot testing is

to establish reproducibility of process flux, cleaning intervals and to validate the cleaning protocol. 4. Results and discussion 4.1. Laboratory filtration results Fig. 4a–c show permeate flux versus time (with and without backpulsing) for synthetic electroplating wastewater filtration using 0.05, 0.2 and 0.8 ␮m membranes, respectively. It was observed that not only was there 100% flux recovery with backpulsing, but the steady-state flux was also two to five times greater than average flux without backpulse. It was observed that filtrate flux could be increased by a factor of 5 for the 0.8 ␮m microfiltration membrane. The 0.2 and 0.5 ␮m ceramic membranes also showed substantial increase in permeate flux (up to 2.5 times) with backpulsing. Thus, the larger the pore diameter, the greater is the effectiveness of backpulse. These results also show that backpulsing is effective in reducing the fouling effect and in maintaining higher steady-state flux. The operating conditions are given in Table 1. In a recent study, Sondhi et al. [18] illustrated the effect of process parameters, crossflow velocity and TMP, on permeate flux. They reported results for wastewater containing 50 mg/l suspended solids using a 0.8 ␮m membrane at crossflow velocity in the range of 3–6.5 m/s and at a fixed feed pressure of 138 kPa (20 psi). All of the operating velocities corresponded to Reynolds number well above 4000, indicating turbulent region. It was seen that, the higher the crossflow velocity, the higher the observed permeate flux. In general, this can be attributed to increased shear (due to high crossflow velocity) reducing the fouling layer on the membrane surface [3,7]. For filtration experiments with the 0.8 ␮m membrane, backpulsing was applied with a fixed pulse interval of 30 s and pulse duration of 0.5 s. Steady-state flux reached a value of 11250 l/m2 h (LMH) at a crossflow velocity of 6.5 m/s. Flux with backpulsing was about four times greater than the steady-state flux without backpulsing. This may be due to the combined effect of high crossflow (scouring action) and backpulsing, that helps to remove the cake layer deposited on the membrane. Transmembrane pressure can have a significant effect on permeate flux. Sondhi et al. [18] also discussed

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the effect of TMP on flux during wastewater filtration with a 0.8 ␮m membrane. TMP was in the range of 100–175 kPa and crossflow velocity was at 5.0 m/s. Steady-state flux increased with transmembrane pressure. However, the rate of increase decreased at higher transmembrane pressure of 172 kPa and the flux appeared to level out. This phenomenon could be attributed to the fact that at higher operating pressure, the effect of fouling is more important as the cake layer thickness on the membrane increases. Sondhi et al. [18] reported that when backpulsing was applied, the steady-state flux reached a maximum of 12,300 LMH at a TMP of 170 kPa, which was about 2.6 times greater than the nonpulsed steady-state flux at the same conditions. It should be noted that the high flux values reported in this study are not indicative of the flux values achieved in typical wastewater treatment applications. These values are high due to the dilute nature of synthetic samples. However, dilute feed systems are ideally suited for fundamental analysis and modeling of backpulsing. 4.2. Laboratory backpulse results Fig. 5 shows typical permeate pressure variation during backpulsing. This particular run was done with

Fig. 4. ((a)–(c)) Filtration results with synthetic electroplating water with and without backpulsing for (a) 0.05, (b) 0.2 and (c) 0.8 ␮m membrane.

Fig. 5. Typical permeate side pressure profiles during a backpulse period for a 0.2 ␮m membrane showing two distinct phases: flow reversal (permeate is forced to the feed side) and refilling (permeate refilling of the membrane module).

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a 0.2 ␮m membrane, pulse duration of 0.5 s, and pulse interval of 30 s. It can be seen from the pressure versus time profile that the backpulse process consists of two distinct phases. In the first phase, pressure is applied on the piston which forces permeate back through the membrane. This represents the steep rise in the permeate side pressure as shown in the curve. It is believed that reverse flow and hence, membrane cleaning takes place during this time. In the second phase, the piston remains at the extended position for some time to allow the cylinder to refill with permeate as the piston returns to the original position before the next pulse (refilling time). The backpulse cycle is followed by a longer forward filtration cycle. Levesley and Hoare [10] observed similar permeate side pressure trends during ceramic membrane filtration of yeast suspension with backpulsing. Figs. 6 and 7 show the comparison between the theoretical (solid lines) and experimental (symbols) membrane cleaning time as a function of backpulse amplitude (maximum transmembrane pressure during reverse flow phase of backpulse cycle) and membrane pore size (diameter), respectively. The experimental cleaning time was calculated from the permeate side pressure variation data during the backpulse period. It is assumed that the membrane cleaning takes place during the initial phases of backpulse (flow reversal). Experimental data shown in Fig. 6 were obtained for synthetic wastewater filtration with

Fig. 6. Predicted and experimental cleaning time (tc ) vs. pulse amplitude for a 0.2 ␮m membrane.

Fig. 7. Predicted and experimental cleaning time for different pore size membranes.

a 0.2 ␮m membrane (BP duration = 0.5 s and BP interval = 30 s) at different backpulse amplitudes. Experimental data shown in Fig. 7 were obtained for synthetic wastewater filtration with different pore diameter membranes (BP duration = 0.5 s and BP interval = 30 s). The theoretical cleaning time (Eq. (4)) was calculated by estimating membrane resistance (Rm ) from clean water filtration data and assuming that the resistance to flow is controlled by the top and the intermediate layer of the membrane having an approximate thickness (Hm ) of 50 ␮m [2,19]. Although membrane pore size does not directly influence cleaning time, it affects membrane resistance (Rm ). The larger pore diameter membrane has a lower resistance to flow. The applied transmembrane pressure (1P) was estimated by the pressure sensor on permeate side. The membrane porosity (ε) was assumed to be 50% [2,19]. Typically, the porosities of membrane layer and support range from about 35 to 50% depending on the pore size and ceramic material properties. Membrane cleaning time decreases with an increase in backpulse amplitude and membrane pore diameter. Increasing the pulse amplitude and membrane pore diameter probably results in better and quicker cleaning and hence, less time is required. Note that the zirconia ultrafiltration membrane (0.05 ␮m) has a different cleaning time due to its different membrane structure compared to the alumina microfiltration membranes (0.2, 0.8 and 5.0 ␮m). It can be seen from

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Figs. 6 and 7 that, there is a close agreement between the theoretical and experimental cleaning times for different backpulse amplitudes and membrane pore sizes. It can be inferred that the model is applicable to both ultrafiltration and microfiltration processes. The operating conditions are given in Table 1. A minimum pulse amplitude of about 170 kPa (25 psi) was necessary for effective membrane cleaning and to obtain an increase in flux. It was observed that for pulse amplitude <170 kPa, the force acting on the piston was too small to initiate backpulsing. Rodgers and Sparks [20] also reported that for short pulses with low amplitude, significant flow reversal was not observed during ultrafiltration of totally retained solutes using transmembrane pressure pulsing. Figs. 8 and 9 show the membrane cleaning time as a function of pulse duration and interval, respectively. The experimental data for Fig. 8 were obtained with a 0.2 ␮m membrane at feed concentration of 50 and 80 mg/l, and pulse duration ranging from 0.5 to 3 s. The experimental data for Fig. 9 were obtained with a 0.2 ␮m membrane, feed concentration of 50 and 80 mg/l, and pulse intervals ranging from 30 to 160 s. No definite trends were observed in this study and it can be said that the feed concentration did not show any significant effect on the membrane cleaning time as predicted by Eq. (4) (solid lines). Small increases

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Fig. 9. Membrane cleaning time versus pulse interval for a 0.2 ␮m membrane, feed concentration of 50 and 80 mg/l. See Table 1 for operating conditions.

in cleaning time were observed when the pulse interval was increased. This can be expected as longer pulse intervals allow greater time for the accumulation of foulants on the surface thus, increasing the potential to foul the membrane. Further experimentation is necessary to verify these preliminary findings. In particular, it will be useful to study the effect of higher concentration on membrane cleaning time (tc ). 4.3. Industrial scale

Fig. 8. Membrane cleaning time versus pulse duration for a 0.2 ␮m membrane, feed concentration of 50 and 80 mg/l. See Table 1 for operating conditions.

4.3.1. Effect of backpulse on the filtration of dilute yeast suspensions The separation of cells from fermentation broth is a very important unit operation in the recovery of fermentation products. Membrane processes are generally preferred over centrifugation or rotary drum filtration since they minimize product losses. Typically, filtration rates are strongly dependent on broth concentration and TMP. Bhave [5] has discussed the effect of concentration factor and TMP on flux for fermentation broth filtration. Matsumoto et al. [9] studied the effect of backpulse on the filtration of dilute yeast suspensions (prepared from commercial packed wet baker’s yeast to simulate fermentation broth) using the 0.2 ␮m

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Membralox® ceramic membrane. The filtration surface area was 0.2 m2 . The concentration of yeast suspensions ranged from 8 to 30 g/l (dry basis). In the absence of backpulsing, the membranes fouled rapidly and flux dropped from 1100 LMH to about 40 LMH within the first hour of filtration. Backpulse frequency was varied to evaluate its effect on maintaining flux. With a backpulse interval of 1 min an average flux of about 900 LMH could be sustained compared to about 600 LMH at backpulse interval of 5 min. The significant impact of backpulse on flux is clearly evident. Typically, it is seen that a higher pulse frequency significantly improves flux. However, this would also depend on the net flux. At low flux values, high cleaning frequency cannot be used because the amount of permeate generated may not be adequate to maintain the effectiveness of backpulsing. 4.3.2. Effect of backpulsing on the filtration of process slurries The filtration of process slurries containing high concentrations of solids, which may be abrasive in nature, is a prominent industrial application for ceramic membranes. These process slurries often contain a variety of metal (e.g. iron, copper, tungsten, aluminum) compounds and organic constituents (e.g. oils, pigments, wetting agents). The effect of backpulsing on the filtration rate of metallic slurry was demonstrated on the 50 nm Membralox® ceramic ultrafiltration membrane (P37-30, 3 mm channel diameter). Two filtration skids were operated in parallel each containing four modules with a total filtration area of 106 m2 . An important advantage of using tubular multichannel membranes was that recoveries greater than 97% could be achieved. This corresponds to a volumetric concentration factor (VCF) of about 40. Tests were performed to investigate the effect of concentration factor on flux, with and without backpulse. Approximately 200,000 gal of slurry was processed, batchwise, per day. The initial suspended solids ranged between 0.5 and 1% (by volume). At VCF of 40, the average flux without backpulse was about 305 LMH. With backpulsing, the average flux increased to about 365 LMH representing a 20% increase (Fig. 10). This is particularly significant given the concentrated nature of slurry at VCF of 40.

Fig. 10. Effect of backpulsing on oily wastewater.

4.3.3. Effect of backpulsing on the filtration of oily wastewater The treatment of oily wastewater to remove oils and suspended solids with ceramic membranes is well established. The wastewater typically contains emulsified oils that are difficult to separate with conventional treatment technologies such as coalescers and oil skimmers. Furthermore, many polymeric membranes are unsuitable due to their limited stability in aggressive chemical environments such as highly contaminated oily wastewater (e.g. lube oils, petroleum fractions). In the petrochemical industry, the treatment of produced water with ceramic membranes was demonstrated in a number of land-based and offshore locations [21]. In this application, the produced water is pretreated with coagulants to improve filtration rates. Permeate is free of oils, grease and suspended solids and is suitable for reuse, discharge or deep-well injection. Flux stability using backpulsing is dependent on pretreatment, membrane selection and operating flux. If the operating flux is set too high, the cleaning interval is considerably shorter compared to that realized at lower operating flux. Chen et al. [21] studied the produced water filtration characteristics with 0.2 and 0.5 ␮m Membralox® ceramic membranes. In the absence of backpulsing and feed pretreatment, the runs lasted only a few hours. When backpulse was initiated, with pulse interval in the range of 1–2 min, the 0.2 ␮m membrane fouled in <24 h at high operating

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flux value of 1600 LMH. However, with 0.5 ␮m membrane and backpulsing (at 2 min interval), run duration increased to about 48 h with the same operating flux of 1600 LMH. The cleaning interval can be greatly increased if operating flux is decreased to a sustainable range. This may be accomplished by controlling the rate of increase of membrane resistance (membrane fouling) over time. Long-term flux stability was demonstrated on a commercial scale installation for the treatment of produced water containing 10–100 ppm oils and suspended solids using 0.8 ␮m Membralox® membranes [22]. The 16 module filtration system (∼115 m2 filtration area) handles a continuous flow rate of about 850 m3 per day. It was observed that the operation was extremely stable and an average flux of 300 LMH was maintained over several months. It should be noted, however, that a typical cleaning interval for most commercial microfiltration and ultrafiltration applications ranges between less than a week up to 4 weeks. 5. Conclusions The role of backpulsing is clearly demonstrated with experimental data on the bench-scale, pilot scale and large-scale filtration systems using ceramic membranes. The paper also describes the theoretical aspects governing the process of backpulsing and provides analysis of the critical parameters influencing the effectiveness of backpulse. The results on the filtration of synthetic electroplating wastewater on laboratory scale modules show that the filtrate flux can be increased by a factor of 5 for the 0.8 ␮m membrane. The 0.2 and 0.5 ␮m ceramic membranes also show substantial increases in permeate flux up to 2.5 times with backpulsing. Thus, the larger the pore diameter, the greater is the effectiveness of backpulsing. Permeate flux also increased with increasing crossflow velocity and transmembrane pressure, in the presence of backpulsing. The dependence of membrane cleaning time on backpulse parameters (amplitude, frequency, and duration) and membrane pore size is also discussed. Several industrial applications are discussed that further support the beneficial effects of backpulse in reducing long-term membrane fouling and increasing

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the cleaning interval. Filtration experiments for dilute yeast suspensions with the 0.2 ␮m membrane show that high flux of about 900 LMH can be sustained at backpulse interval of 1 min. In the absence of backpulse the flux decreases rapidly in the first 15 min of filtration. For process slurry filtration, it was shown that backpulse increased flux by 20–25% at concentration factor of 40 (∼97.5% recovery). The industrial scale filtration of oily produced water showed that the flux stability and cleaning interval are strongly dependent on not only backpulse but also on the membrane pore diameter and the absolute value of flux. At low flux values of about 300 LMH, it was shown that cleaning interval could be extended to several months using a 0.8 ␮m membrane. On the other hand, if high flux values are desired, the membrane cleaning frequency could drastically increase.

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