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Flux recovery of tubular ceramic membranes fouled with whey proteins
Desalination 249 (2009) 293–300
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Flux recovery of tubular ceramic membranes fouled with whey proteins Svetlana Popović ⁎, Spasenija Milanović, Mirela Iličić, Mirjana Djurić, Miodrag Tekić University of Novi Sad, Faculty of Technology, Blv. Cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
i n f o
Article history: Accepted 25 December 2008 Available online 1 October 2009 Keywords: Ceramic membranes Whey proteins Fouling Rinsing Chemical cleaning
a b s t r a c t Membrane process efficiency is governed by the formation of fouling deposits during processing of dairy fluids. Because of fouling with whey proteins, permeate flux can drastically decline during filtration process. This paper describes the flux recovery procedure for ceramic tubular membranes (50 and 200 nm pore sizes) fouled with whey proteins. The results comprehend the effect of rinsing and cleaning agent choice and concentration, on the cleaning efficiency. As chemical cleaning agents, the caustic solution and the commercial detergents P3ultrasil 67 and P3-ultrasil 69 were selected. The observations are that rinsing with deionised water contributes to a flux recovery to a certain degree. For the 50 nm membrane, the choice of the 1.0% (w/w) caustic solution, as cleaning agent, gives the best flux recovery. For the 200 nm, total flux recovery was not observed regardless of the cleaning agent choice and concentration. Cleaning with chosen commercial detergent appeared to be less efficient than cleaning with caustic solution for the chosen ceramic membranes. Also, a mathematical model, proposed in this study, has shown high agreement with experimentally obtained data. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Pressure driven membrane processes are widely used in dairy industry. Membrane processes in the modern dairy industry typically employ one of the two major system configurations: spiral-wound cartridges with polysulphone (PS) and polyethersulphone (PES) membranes or a combination of both, and tubular ceramic membranes based on zirconium or aluminium oxide [1]. As highly efficient separation processes, microfiltration and ultrafiltration find their use in sterilization of milk, separation of casein and whey proteins, concentration of whey proteins and fractionation of whey proteins and clarification of cheese brine. Although microfiltration and ultrafiltration commercially approved their employment in dairy industry, some issues are left to be solved. However, membrane process efficiency is greatly affected by the flux decrease during operation due to the membrane fouling. The flux decline during operation is a consecquence of deposit layer formation (bacteria, casien, whey proteins, ect.) at the membrane surface and in the pores. Consequently, membranes used in dairy industry are usually cleaned, once per day [2], in order to regenerate the permeate flux. Even though researchers are mostly focused on the investigations of the mechanism of fouling with proteins [3–9] some data about membrane cleaning are available [10–21]. Cleaning of membranes
⁎ Corresponding author. Tel.: +381214853675; fax: +38121450413. E-mail addresses: [email protected] (S. Popović), [email protected] (M. Iličić). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.12.060
fouled with milk or whey proteins, made of the different materials (ceramic, stainless steel, PES, PVDF etc.) has been investigated so far [2,10–21]. As cleaning agents, caustic solutions [2,10–17] and formulated detergents [10,15–20] were found to have a positive effect on the flux recovery, while cleaning with acid solution was found to have a negative effect [2,10,17]. Some investigations of cleaning with caustic solutions showed that an amount of sodium hydroxide can be reduced on the range of 0.2– 0.5% (w/w) for the polysulphone membrane [12], stainless steel [10,11] and ceramic membrane [2,10]. Also, in investigation on influence of multiple fouling and cleaning cycles, damage of ceramic membrane with zirconium layer was not observed after cleaning with NaOH [2], while it was observed for PVDF (polyvinylidene difluoride) membrane after cleaning with 0.5% (w/w) NaOH at 55 °C [14]. After investigations of cleaning of polysulphone membrane with commercial detergents it was recommended to apply enzymatic cleaning followed by cleaning with surfactant detergent [20] or to clean with combination of chemicals such as EDTA + SDS + NaOH [17]. Cleaning of inorganic membrane with enzymatic detergent Ultrasil 62 gave the total flux restoration in 20 min with adjustment of some operating parameters, such as pH, [18] while cleaning of ceramic membrane with caustic Ultrasil 11 gave flux recovery of about 80% [10]. A caustic based multicomponent cleaning agent would provide efficient cleaning of inorganic membranes in an optimised cleaning regime [10]. Generally, the choice of cleaning method depends on the module configuration, the chemical and physical resistance of the membrane and ancillary equipment and the nature of the fouling [10]. On the other hand, membrane suppliers, generally, recommend the same
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Fig. 1. Experimental set-up.
cleaning procedure for all tubular ceramic membranes. However, previous fouling mechanism investigation has shown that the fouling depends on the pore size and filtering layer material (alumina, zirconia) [8]. In accordance with these results, we assumed that the cleaning procedures for an alumina and a zirconia membrane should be different. The aim of this work was to study the cleaning of whey protein fouled tubular ceramic membranes, with alumina and zirconia filtering layer, in order to make cleaning procedure more efficient from the point of view of cleaning duration and chemical consumption. 2. Materials and methods 2.1. Experimental set-up All experiments were performed using the microfiltration/ultrafiltration experimental set-up made of stainless steel (Fig. 1). The feed solution was pumped to the membrane module by the rotary vane pump PO511 (Cmf, Italy). Transmembrane pressure across the membrane module and constant flow were adjusted by the bypass valve and the main flow valve. The TMP was monitored by the manometers while the flow was measured by the rotameter. The retentate and permeate were both recycled to the feed tank. The temperature was kept constant and monitored by a digital thermometer in the feed tank. Permeate was collected in a container placed on a digital balance (EW 1500-2M, KERN Germany) and continuously weighted while the data were transferred to a personal computer. 2.2. Membranes Two Membralox monotubular ceramic membranes, 250 mm long, with 7 mm ID and 10 mm OD, were studied (SCT, Bazet, France). The membrane of a 50 nm mean pore size made of ZrO2 filtering layer on an α-alumina support and the membrane of a 200 nm mean pore size made of an α-alumina filtering layer on an α-alumina support, were chosen. The active filtering area of both membranes was 46.2 cm2. 2.3. Fouling A reconstituted whey solution was chosen for the fouling trials. The whey powder composition was as follows: 11.8% (w/w) proteins, 75.0% (w/w) lactose, 3.3% (w/w) fat, 9.5% (w/w) ash and 2.3% (w/w) water (Novosadska mlekara, Srbija). The whey powder was dissolved in deionised water with a concentration of 10 g/L. The natural pH of reconstitute whey solution was 6.0 without adjustment.
2.4. Cleaning agents Deionised water was used as rinsing fluid prior to and after the cleaning with an alkaline solution or with an enzymatic detergent developed for the cleaning purposes in dairies. The sodium hydroxide (Lach-ner, Czech Republic) solution of different concentrations was used as alkaline cleaning agent. Along with the caustic cleaning, the cleaning with commercially available detergents P3-ultrasil 67 and P3-ultrasil 69 (Henkel, Germany) was studied. P3-ultrasil 67 consists of two main cleaning components: alkyl amine oxide (15–30%) and proteolytic enzyme (<5%) while P3-ultrasil 69 contains phosphonates (5–15%) and salts of organic acids (5–15%) [18]. The enzymatic cleaning solutions were made using both detergents in the following concentrations: 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67 and 1.2% P3ultrasil 69 + 0.75% P3-ultrasil 67. 2.5. Operating conditions Experiments comprehended the following steps: pure water flux measurement, fouling, rinsing, chemical cleaning, rinsing, and pure water flux measurement. The operating conditions, applied with both membranes, are given in Table 1. Membranes were fouled for 60 min with reconstituted whey solution while both retentate and permeate were recycled in the feed tank. Since it is recommended that the rinsing and cleaning TMP should not be higher than the fouling TMP, it was kept the same for all investigation steps [18]. Furthermore, selected TMP and cross flow velocity should not be as high to induce high sheer stress. Even thou it is recommended to apply higher sheer stress because of surface fouling reduction during the whey solution filtration, during cleaning with enzymatic detergent it may produce a decrease in enzyme activation. Higher cross flow velocity partially prevents surface fouling appearance therefore it is recommended to
Table 1 Operating conditions for the fouling and cleaning procedure. Step
v (m/s)
TMP (kPa)
t (min)
T (°C)
Feed stream
Pure water flux measurement Fouling Rinsing Cleaning Rinsing Pure water flux measurement
1.73
30
30
25
Water
0.43 1.73 1.73 1.73 1.73
30 30 30 30 30
60 30 30 30 30
25 25 50 25 25
Whey (10 g/L) Water NaOH/P3-ultrasil Water Water
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The rinsing efficiency can be estimated using the following equation: Rinsing efficiency =
Jrw 100% Jw ⋅
ð1Þ
The membrane cleaning efficiency can be determined from the following equation: Cleaning efficiency =
Jcw 100% Jw ⋅
ð2Þ
The difference between the pure water fluxes before fouling and after cleaning can occur due to possible appearing of the irreversible fouling. 3. Results and discussion 3.1. Fouling behaviour of membranes The flux decline curves registered during filtration of reconstituted whey solution are shown in Fig. 2a for M50 nm and in Fig. 2b for
Fig. 2. Filtration of a 10 g/L reconstituted whey solution at 25 °C, TMP 30 kPa, and CFV 0.43 ms− 1 for 1 h. Symbols linked with cleaning procedure: (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution, ( ) 0.8% P3ultrasil 69 + 0.5% P3-ultrasil 67, (▽)1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
▹
use the same or the higher cross flow velocity for rinsing and cleaning than that used during fouling [18]. The optimum temperature of enzymatic activity is about 50 °C and the same temperature was chosen for the comparison to caustic cleaning. Rinsing and chemical cleaning was carried out for 30 min, with full recycle. Also, before the rinsing of membrane, the rest apparatus was rinsed while the membrane module was isolated closing valves before and after module. For each experimental step 3 L of the feed solution was used. If the pure water flux was not restored after the examined cleaning procedure, membranes were cleaned according to the standard procedure recommended by the membrane supplier. 2.6. Rinsing and cleaning efficiency Before performing any experiment the initial water flux of clean membrane was measured using pure water. These measurements were used to calculate rinsing and cleaning efficiency. The rinsing efficiency was evaluated in the term of permeate flux recovery. The final flux recovery was based on measurements of permeate flux of pure water through the cleaned membrane.
Fig. 3. Rinsing with pure water: 25 °C, TMP 30kPa, and CFV 1.73 ms− 1 for 30 min. Symbols linked with cleaning procedure: (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution, ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▽)1.2% P3-ultrasil 69+ 0.75% P3-ultrasil 67.
◂
▹
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M200 nm. The sharp flux decline, in the first few minutes, for both membranes can be observed. For the M50 nm, the pseudo-steady state flux was less than 20% of the initial flux value, while, for the M200 nm the pseudo-steady state flux was less than 5% of the initial flux value. Primary high flux declined in the first few minutes due to formation of the gel polarisation layer. Further flux decline can be linked with the appearance of gelling and/or pore blockage. The more intensive decrease of flux observed for the M200 nm can be explained by the more intensive adsorption-related pore blockage. Investigations about mechanism of fouling with whey proteins have been previously published [7,8,12]. 3.2. Efficiency of rinsing Fouling experiment was followed by rinsing with pure water in order to remove fouling deposit bound to the membrane surface. The percentage of flux recovery was around 25% for the M50 nm and around 15% for the M200 nm, obviously low for both membranes (see Fig. 3a and b). The removals of the deposits can take place in two different ways: (i) shearing effect created by the flowing water and (ii) dissolution of the deposits and convection in water [12]. During the rising step, applied shear velocity of 1.73 m/s which has produced turbulent flow (Re = 12,110) was not sufficient to remove fouling deposits. Nevertheless, the rinsing efficiency was higher in the case of M50 nm because fouling occurs at membrane surface as the concentration polarisation layer which can be easily flashed. On the other hand, in the case of the M200 nm prevailing fouling mechanism is the pore blockage so the rinsing of surface deposits has lower effect. However, for both membranes, rinsing with pure water is not sufficient and chemical cleaning is necessary to reach adequate flux restoration. 3.3. Cleaning with caustic solutions The permeate flux curves during caustic cleaning of the 50 nm membrane are shown in Fig. 4. Two concentrations of caustic solution were chosen: 0.2% (w/w) and 1.0% (w/w). As it was expected, the permeate flux, during the cleaning with caustic solutions, increased in the first few minutes and then settled at a constant value. During cleaning with sodium hydroxide solution of the higher concentration, the higher cleaning flux was observed. Determination of the flux recovery was based on the pure water flux measurement after chemical cleaning followed by rinsing. The
Fig. 4. Permeate flux curves during cleaning with sodium hydroxide at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (♦) 0.2% (w/w) NaOH solution, (●) 1.0% (w/w) NaOH solution.
Fig. 5. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, (♦) after cleaning with 0.2% (w/w) NaOH solution, and (●) after cleaning with 1.0% (w/w) NaOH solution.
pure water fluxes of cleaned 50 nm membrane are shown in Fig. 5. The higher the concentration of sodium hydroxide the higher the flux recovery can be observed. The flux recovery was 97% after cleaning with 1.0% (w/w) sodium hydroxide solution and about 88% after cleaning with 0.2% (w/w) sodium hydroxide solution. The permeate flux curves during caustic cleaning of the 200 nm membrane are shown in Fig. 6. Influence of sodium hydroxide concentration was investigated using solutions of the following concentrations: 0.2, 0.4, 0.6, 0.8 and 1.0% (w/w). The permeate flux, during the cleaning with caustic solutions, sharply increased to a maximum in the first few minutes and then very slightly decreased during the rest of the cleaning time. The highest flux maximum was noticed for the 0.6% (w/w) concentration. Appearance of a maximum can be explained by two effects: the primary removal of the concentration polarisation layer, which is relatively weak, and afterwards cleaning process controlled by the swelling of the fouling deposits in pores, as the caustic solution diffused into the deposit matrix [11,12]. The typical shape of the permeate flux curve with an emphasised maximum was observed during caustic cleaning of the
Fig. 6. Permeate flux curves during cleaning with sodium hydroxide at 50 °C, TMP 30 kPa and CFV 1.73 ms−1 for 30 min. (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution.
▹
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Fig. 7. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling and after cleaning with (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution.
▹
297
Fig. 9. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, ( ) after cleaning with 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) after cleaning with 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
metal microfiltration membrane [10,11] and the little less expressed maximum was observed during caustic cleaning of the polysulphone ultrafiltration membrane [12]. The flux recovery decline is enhanced at higher temperatures and NaOH concentrations (pH) but it is absent at zero TMP [10,21] regardless of the membrane material. In the case of ceramic tubular membrane of lower pore size, flux maximum was not observed since there was no considerable fouling deposit in the pores. In the case of ceramic tubular membrane of larger pore size, there was no considerable flux recovery decrease probably because of very low pressure application even though adsorption of proteins in pores was considerable. Determination of the flux recovery was based on the pure water flux measurements after chemical cleaning followed by rinsing. The pure water fluxes of cleaned 200 nm membrane are shown in Fig. 7. The highest flux recovery can be observed for the 0.6% (w/w) concentration of sodium hydroxide. The highest flux recovery was 78% after cleaning with 0.6% (w/w) sodium hydroxide solution. The lowest flux recovery was observed after cleaning with 0.2% (w/w) sodium hydroxide solution.
3.4. Cleaning with commercial detergents
Fig. 8. Permeate flux curves during the cleaning with detergents at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
Fig. 10. Permeate flux curves during the cleaning with detergents at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
The permeate flux patterns during cleaning with P3-ultrasil 67 and P3-ultrasil 69 are shown in Fig. 8 for the 50 nm membrane. The permeate flux during the cleaning with detergents was at the same level regardless of the detergent concentration. The pure water fluxes of cleaned 50 nm membrane are shown in Fig. 9. The highest flux recovery can be observed for the highest concentration of detergents. The flux recovery was 67% after cleaning with solution of higher concentration. The permeate flux patterns during cleaning with P3-ultrasil 67 and P3-ultrasil 69 are shown in Fig. 10 for the 200 nm membrane. In the case of 200 nm membrane, a maximal permeate flux can be observed. The value of maximum was higher for the higher detergent concentration. P3-ultrasil 67 contains alkylamineoxide which is a surface detergent that makes globule around protein enabling action of proteolytic enzyme. Except that, mixture of these two detergents gives the solution of alkaline nature that may also induce gelation or swelling of fouling deposits in the pores.
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fouled by whey protein was developed by Bird and Bartlett [11. It starts with Darcy's Eq. (2) linked to the total resistance of the fouled membrane (Rtf): ð6Þ
Rtf = Rm + Rf = Rm + Rcp + Rc + Rin
as the sum of the individual resistances: i) the resistance of membrane itself (Rm), ii) the resistance due to concentration polarisation (Rcp), iii) the hydraulic resistance of deposits at the membrane surface (Rc) and iv) the resistance due to in-pore fouling (Rin). The first resistance was calculated from Eq. (1) applied to water flux measurements and following results were obtained: 4.94 × 1011 (m− 1) for M50 nm and 2.12 × 1011 (m− 1) for M200 nm. The second resistance was neglected [11], while the third term of the Eq. (6) might be calculated assuming a second-order change of time derivative of the cake deposit [11] or the first-order change [24], which proved more suitable while leading to the exponential decrease of the cake resistance itself: Fig. 11. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, ( ) after cleaning with 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) after cleaning with 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
dRc = −p1 Rc ⇒ Rc = expð−p1 t + p2 Þ dt
ð7Þ
Finally, the swelling kinetics for in-pore bound material was modelled by application of the modified Carman–Kozeny equation: The pure water fluxes of cleaned 200 nm membrane are shown in Fig. 11. Regardless of the detergent concentration, no considerable difference in the flux recovery can be observed. The flux recovery was 50% after cleaning with 0.5% (w/w) P3-ultrasil 67 and 0.8% (w/w) P3ultrasil 69 solution and 44.5% after cleaning with 0.75% (w/w) P3ultrasil 67 and 1.2% (w/w) P3-ultrasil 69 solution. 3.5. Kinetic model of cleaning To understand deeply the mechanism of fouling, a few mathematical models are available [3,6]. All the same, kinetics of cleaning process might be explained better while processing the cleaning experiment data through an adequate mathematical model. Lee [22] suggested a simple cleaning model, assuming that the flux increaes during cleaning according to the first-order equation, while Zondervan et al. [23] proposed a two-component-balance-model, which follows changes in the fouling state and in the cleaning agent state. However, a particularly interesting model to predict the change in flux during cleaning of a flat plate cross flow microfiltration membrane
Rin =
36hk ð1−εÞ2 l −2 = p3 de ε3 d2e
ð8Þ
where de denotes the effective pore diameter, less than the nominal pore diameter do, according to the equation [11]: de =
δ 2 ðηk2 t + δÞ do −2 ⇒ de = do −ðp4 t + p5 t Þ k2 t + δ
ð9Þ
After substitution of all resistances from Eqs. (7 and 8) into the Eq. (6), a kinetic model appeared. Its parameters p1–p5 should be determined so as to provide the best fit to experimental data: 2
−2
Rtf = Rm + expð−p1 t + p2 Þ + p3 ½do −ðp4 t + p5 t Þ
ð10Þ
By applaying the Levenberg Marquardt method (ORIGIN 6.1) to the flux-data from Figs. 5–7 interesting results were obtained. Here, two examples were presented in Fig. 12.
Fig. 12. Cleaning models (full lines) based on experimental data for the membranes M50 nm (▲) and M200 nm (●), cleaned with 0.2% (w/w) NaOH solution, at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1, for 30 min.
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solution. Also increasing of the commercial detergent concentration had a negative influence on the flux recovery. Cleaning with simple caustic solution appeared to be more efficient for both membranes even though total flux restoration was not achieved for the 200 nm membrane. 4. Conclusion Based on the results of investigations on rinsing and cleaning of 50 nm and 200 nm tubular ceramic membranes, fouled during reconstituted whey solution filtration, the following conclusions can be drawn:
Fig. 13. Flux recovery efficiency for the 50 nm membrane, based on the pure water flux measurements before fouling and after cleaning at the same conditions 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min.
An analysis of the statistical parameters (presented within Fig. 12) has shown a high level of adequacy of the model (10). Also, the presentation of particular resistances has shown that Rc decreases, in both cases, exponentially within a few minutes of the process. Also, Rin decreases slowly (for M50 nm), or remains unchanged, or even increases slowly, (for M200 nm). The mean size of pores of 200 nm is so large that proteins can enter the pores and block their inner surface. Slower and less efficient cleaning of the fouled membranes with 200 nm pores could be connected with this phenomenon. Similar results were obtained on cleaning with detergents.
Rinsing both membranes fouled with whey proteins contributed to a flux recovery to a certain degree. However, experimentally determined fluxes of the rinsed membranes were still low. Chemical cleaning of the 50 nm membrane, by 1.0% (w/w) caustic solution guarantees almost total flux restoration. Chemical cleaning of the 200 nm membrane, by caustic solution, did not allow total flux restoration, regardless of the applied concentration of caustic solution.Application of commercially available detergents in cleaning of both membranes did not allow total flux restoration. The maximum flux recovery for 50 nm membrane was about 75% at higher concentration of detergents. The mathematical model, proposed in this study, has shown high agreement with experimentally obtained data. Acknowledgment This research was supported by the Ministry of Science of Republic of Serbia (project no. 142045). References
3.6. Comparison of caustic and detergent cleaning Cleaning of the M50 nm with 1.0% (w/w) caustic solution gives the 97% flux recovery (Fig. 13). Cleaning of the M50 nm with commercial detergents was less efficient, but the flux recovery increased from 60 to 75% with increasing of the detergent concentration. For the M200 nm, total flux restoration was not achieved, regardless of the chemical cleaning agent and its concentration (Fig. 14). The best flux recovery (about 78%) was achieved after the cleaning with 0.6% (w/w) caustic
Fig. 14. Cleaning efficiency for the 200 nm membrane based, on the pure water flux measurements before fouling and after cleaning at the same conditions 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min.
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Glossary de: effective pore diameter (m) do: nominal pore diameter (m) hk: Kozeny factor (1)
J: volume flux (m3/m2s) Jcw: pure water flux of the cleaned membrane (m3/m2s) Jw: pure water flux before fouling experiment (m3/m2s) k2: rate constant (m min− 1) l: thickness of membrane (m) p1–p5: parameters of mathematical model (10) Rc: hydraulic resistance of deposits at the membrane surface (m− 1) Rcp: resistance due to the concentration polarisation (m− 1) Rf: resistance due to the membrane fouling (m− 1) Rin: the resistance due to in-pore fouling (m− 1) Rm: hydraulic resistance of the clean membrane (m− 1) Rtf: total resistance of the fouled membrane (m− 1) t: time (min) TMP: transmembrane pressure (Pa) δ: unswollen in-pore deposite thickness (m) ε: membrane porosity (1) µ: dynamic viscosity (Pa s) η: swelling ratio (1)
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Flux recovery of tubular ceramic membranes fouled with whey proteins Svetlana Popović ⁎, Spasenija Milanović, Mirela Iličić, Mirjana Djurić, Miodrag Tekić University of Novi Sad, Faculty of Technology, Blv. Cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
i n f o
Article history: Accepted 25 December 2008 Available online 1 October 2009 Keywords: Ceramic membranes Whey proteins Fouling Rinsing Chemical cleaning
a b s t r a c t Membrane process efficiency is governed by the formation of fouling deposits during processing of dairy fluids. Because of fouling with whey proteins, permeate flux can drastically decline during filtration process. This paper describes the flux recovery procedure for ceramic tubular membranes (50 and 200 nm pore sizes) fouled with whey proteins. The results comprehend the effect of rinsing and cleaning agent choice and concentration, on the cleaning efficiency. As chemical cleaning agents, the caustic solution and the commercial detergents P3ultrasil 67 and P3-ultrasil 69 were selected. The observations are that rinsing with deionised water contributes to a flux recovery to a certain degree. For the 50 nm membrane, the choice of the 1.0% (w/w) caustic solution, as cleaning agent, gives the best flux recovery. For the 200 nm, total flux recovery was not observed regardless of the cleaning agent choice and concentration. Cleaning with chosen commercial detergent appeared to be less efficient than cleaning with caustic solution for the chosen ceramic membranes. Also, a mathematical model, proposed in this study, has shown high agreement with experimentally obtained data. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Pressure driven membrane processes are widely used in dairy industry. Membrane processes in the modern dairy industry typically employ one of the two major system configurations: spiral-wound cartridges with polysulphone (PS) and polyethersulphone (PES) membranes or a combination of both, and tubular ceramic membranes based on zirconium or aluminium oxide [1]. As highly efficient separation processes, microfiltration and ultrafiltration find their use in sterilization of milk, separation of casein and whey proteins, concentration of whey proteins and fractionation of whey proteins and clarification of cheese brine. Although microfiltration and ultrafiltration commercially approved their employment in dairy industry, some issues are left to be solved. However, membrane process efficiency is greatly affected by the flux decrease during operation due to the membrane fouling. The flux decline during operation is a consecquence of deposit layer formation (bacteria, casien, whey proteins, ect.) at the membrane surface and in the pores. Consequently, membranes used in dairy industry are usually cleaned, once per day [2], in order to regenerate the permeate flux. Even though researchers are mostly focused on the investigations of the mechanism of fouling with proteins [3–9] some data about membrane cleaning are available [10–21]. Cleaning of membranes
⁎ Corresponding author. Tel.: +381214853675; fax: +38121450413. E-mail addresses: [email protected] (S. Popović), [email protected] (M. Iličić). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.12.060
fouled with milk or whey proteins, made of the different materials (ceramic, stainless steel, PES, PVDF etc.) has been investigated so far [2,10–21]. As cleaning agents, caustic solutions [2,10–17] and formulated detergents [10,15–20] were found to have a positive effect on the flux recovery, while cleaning with acid solution was found to have a negative effect [2,10,17]. Some investigations of cleaning with caustic solutions showed that an amount of sodium hydroxide can be reduced on the range of 0.2– 0.5% (w/w) for the polysulphone membrane [12], stainless steel [10,11] and ceramic membrane [2,10]. Also, in investigation on influence of multiple fouling and cleaning cycles, damage of ceramic membrane with zirconium layer was not observed after cleaning with NaOH [2], while it was observed for PVDF (polyvinylidene difluoride) membrane after cleaning with 0.5% (w/w) NaOH at 55 °C [14]. After investigations of cleaning of polysulphone membrane with commercial detergents it was recommended to apply enzymatic cleaning followed by cleaning with surfactant detergent [20] or to clean with combination of chemicals such as EDTA + SDS + NaOH [17]. Cleaning of inorganic membrane with enzymatic detergent Ultrasil 62 gave the total flux restoration in 20 min with adjustment of some operating parameters, such as pH, [18] while cleaning of ceramic membrane with caustic Ultrasil 11 gave flux recovery of about 80% [10]. A caustic based multicomponent cleaning agent would provide efficient cleaning of inorganic membranes in an optimised cleaning regime [10]. Generally, the choice of cleaning method depends on the module configuration, the chemical and physical resistance of the membrane and ancillary equipment and the nature of the fouling [10]. On the other hand, membrane suppliers, generally, recommend the same
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Fig. 1. Experimental set-up.
cleaning procedure for all tubular ceramic membranes. However, previous fouling mechanism investigation has shown that the fouling depends on the pore size and filtering layer material (alumina, zirconia) [8]. In accordance with these results, we assumed that the cleaning procedures for an alumina and a zirconia membrane should be different. The aim of this work was to study the cleaning of whey protein fouled tubular ceramic membranes, with alumina and zirconia filtering layer, in order to make cleaning procedure more efficient from the point of view of cleaning duration and chemical consumption. 2. Materials and methods 2.1. Experimental set-up All experiments were performed using the microfiltration/ultrafiltration experimental set-up made of stainless steel (Fig. 1). The feed solution was pumped to the membrane module by the rotary vane pump PO511 (Cmf, Italy). Transmembrane pressure across the membrane module and constant flow were adjusted by the bypass valve and the main flow valve. The TMP was monitored by the manometers while the flow was measured by the rotameter. The retentate and permeate were both recycled to the feed tank. The temperature was kept constant and monitored by a digital thermometer in the feed tank. Permeate was collected in a container placed on a digital balance (EW 1500-2M, KERN Germany) and continuously weighted while the data were transferred to a personal computer. 2.2. Membranes Two Membralox monotubular ceramic membranes, 250 mm long, with 7 mm ID and 10 mm OD, were studied (SCT, Bazet, France). The membrane of a 50 nm mean pore size made of ZrO2 filtering layer on an α-alumina support and the membrane of a 200 nm mean pore size made of an α-alumina filtering layer on an α-alumina support, were chosen. The active filtering area of both membranes was 46.2 cm2. 2.3. Fouling A reconstituted whey solution was chosen for the fouling trials. The whey powder composition was as follows: 11.8% (w/w) proteins, 75.0% (w/w) lactose, 3.3% (w/w) fat, 9.5% (w/w) ash and 2.3% (w/w) water (Novosadska mlekara, Srbija). The whey powder was dissolved in deionised water with a concentration of 10 g/L. The natural pH of reconstitute whey solution was 6.0 without adjustment.
2.4. Cleaning agents Deionised water was used as rinsing fluid prior to and after the cleaning with an alkaline solution or with an enzymatic detergent developed for the cleaning purposes in dairies. The sodium hydroxide (Lach-ner, Czech Republic) solution of different concentrations was used as alkaline cleaning agent. Along with the caustic cleaning, the cleaning with commercially available detergents P3-ultrasil 67 and P3-ultrasil 69 (Henkel, Germany) was studied. P3-ultrasil 67 consists of two main cleaning components: alkyl amine oxide (15–30%) and proteolytic enzyme (<5%) while P3-ultrasil 69 contains phosphonates (5–15%) and salts of organic acids (5–15%) [18]. The enzymatic cleaning solutions were made using both detergents in the following concentrations: 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67 and 1.2% P3ultrasil 69 + 0.75% P3-ultrasil 67. 2.5. Operating conditions Experiments comprehended the following steps: pure water flux measurement, fouling, rinsing, chemical cleaning, rinsing, and pure water flux measurement. The operating conditions, applied with both membranes, are given in Table 1. Membranes were fouled for 60 min with reconstituted whey solution while both retentate and permeate were recycled in the feed tank. Since it is recommended that the rinsing and cleaning TMP should not be higher than the fouling TMP, it was kept the same for all investigation steps [18]. Furthermore, selected TMP and cross flow velocity should not be as high to induce high sheer stress. Even thou it is recommended to apply higher sheer stress because of surface fouling reduction during the whey solution filtration, during cleaning with enzymatic detergent it may produce a decrease in enzyme activation. Higher cross flow velocity partially prevents surface fouling appearance therefore it is recommended to
Table 1 Operating conditions for the fouling and cleaning procedure. Step
v (m/s)
TMP (kPa)
t (min)
T (°C)
Feed stream
Pure water flux measurement Fouling Rinsing Cleaning Rinsing Pure water flux measurement
1.73
30
30
25
Water
0.43 1.73 1.73 1.73 1.73
30 30 30 30 30
60 30 30 30 30
25 25 50 25 25
Whey (10 g/L) Water NaOH/P3-ultrasil Water Water
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The rinsing efficiency can be estimated using the following equation: Rinsing efficiency =
Jrw 100% Jw ⋅
ð1Þ
The membrane cleaning efficiency can be determined from the following equation: Cleaning efficiency =
Jcw 100% Jw ⋅
ð2Þ
The difference between the pure water fluxes before fouling and after cleaning can occur due to possible appearing of the irreversible fouling. 3. Results and discussion 3.1. Fouling behaviour of membranes The flux decline curves registered during filtration of reconstituted whey solution are shown in Fig. 2a for M50 nm and in Fig. 2b for
Fig. 2. Filtration of a 10 g/L reconstituted whey solution at 25 °C, TMP 30 kPa, and CFV 0.43 ms− 1 for 1 h. Symbols linked with cleaning procedure: (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution, ( ) 0.8% P3ultrasil 69 + 0.5% P3-ultrasil 67, (▽)1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
▹
use the same or the higher cross flow velocity for rinsing and cleaning than that used during fouling [18]. The optimum temperature of enzymatic activity is about 50 °C and the same temperature was chosen for the comparison to caustic cleaning. Rinsing and chemical cleaning was carried out for 30 min, with full recycle. Also, before the rinsing of membrane, the rest apparatus was rinsed while the membrane module was isolated closing valves before and after module. For each experimental step 3 L of the feed solution was used. If the pure water flux was not restored after the examined cleaning procedure, membranes were cleaned according to the standard procedure recommended by the membrane supplier. 2.6. Rinsing and cleaning efficiency Before performing any experiment the initial water flux of clean membrane was measured using pure water. These measurements were used to calculate rinsing and cleaning efficiency. The rinsing efficiency was evaluated in the term of permeate flux recovery. The final flux recovery was based on measurements of permeate flux of pure water through the cleaned membrane.
Fig. 3. Rinsing with pure water: 25 °C, TMP 30kPa, and CFV 1.73 ms− 1 for 30 min. Symbols linked with cleaning procedure: (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution, ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▽)1.2% P3-ultrasil 69+ 0.75% P3-ultrasil 67.
◂
▹
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M200 nm. The sharp flux decline, in the first few minutes, for both membranes can be observed. For the M50 nm, the pseudo-steady state flux was less than 20% of the initial flux value, while, for the M200 nm the pseudo-steady state flux was less than 5% of the initial flux value. Primary high flux declined in the first few minutes due to formation of the gel polarisation layer. Further flux decline can be linked with the appearance of gelling and/or pore blockage. The more intensive decrease of flux observed for the M200 nm can be explained by the more intensive adsorption-related pore blockage. Investigations about mechanism of fouling with whey proteins have been previously published [7,8,12]. 3.2. Efficiency of rinsing Fouling experiment was followed by rinsing with pure water in order to remove fouling deposit bound to the membrane surface. The percentage of flux recovery was around 25% for the M50 nm and around 15% for the M200 nm, obviously low for both membranes (see Fig. 3a and b). The removals of the deposits can take place in two different ways: (i) shearing effect created by the flowing water and (ii) dissolution of the deposits and convection in water [12]. During the rising step, applied shear velocity of 1.73 m/s which has produced turbulent flow (Re = 12,110) was not sufficient to remove fouling deposits. Nevertheless, the rinsing efficiency was higher in the case of M50 nm because fouling occurs at membrane surface as the concentration polarisation layer which can be easily flashed. On the other hand, in the case of the M200 nm prevailing fouling mechanism is the pore blockage so the rinsing of surface deposits has lower effect. However, for both membranes, rinsing with pure water is not sufficient and chemical cleaning is necessary to reach adequate flux restoration. 3.3. Cleaning with caustic solutions The permeate flux curves during caustic cleaning of the 50 nm membrane are shown in Fig. 4. Two concentrations of caustic solution were chosen: 0.2% (w/w) and 1.0% (w/w). As it was expected, the permeate flux, during the cleaning with caustic solutions, increased in the first few minutes and then settled at a constant value. During cleaning with sodium hydroxide solution of the higher concentration, the higher cleaning flux was observed. Determination of the flux recovery was based on the pure water flux measurement after chemical cleaning followed by rinsing. The
Fig. 4. Permeate flux curves during cleaning with sodium hydroxide at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (♦) 0.2% (w/w) NaOH solution, (●) 1.0% (w/w) NaOH solution.
Fig. 5. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, (♦) after cleaning with 0.2% (w/w) NaOH solution, and (●) after cleaning with 1.0% (w/w) NaOH solution.
pure water fluxes of cleaned 50 nm membrane are shown in Fig. 5. The higher the concentration of sodium hydroxide the higher the flux recovery can be observed. The flux recovery was 97% after cleaning with 1.0% (w/w) sodium hydroxide solution and about 88% after cleaning with 0.2% (w/w) sodium hydroxide solution. The permeate flux curves during caustic cleaning of the 200 nm membrane are shown in Fig. 6. Influence of sodium hydroxide concentration was investigated using solutions of the following concentrations: 0.2, 0.4, 0.6, 0.8 and 1.0% (w/w). The permeate flux, during the cleaning with caustic solutions, sharply increased to a maximum in the first few minutes and then very slightly decreased during the rest of the cleaning time. The highest flux maximum was noticed for the 0.6% (w/w) concentration. Appearance of a maximum can be explained by two effects: the primary removal of the concentration polarisation layer, which is relatively weak, and afterwards cleaning process controlled by the swelling of the fouling deposits in pores, as the caustic solution diffused into the deposit matrix [11,12]. The typical shape of the permeate flux curve with an emphasised maximum was observed during caustic cleaning of the
Fig. 6. Permeate flux curves during cleaning with sodium hydroxide at 50 °C, TMP 30 kPa and CFV 1.73 ms−1 for 30 min. (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution.
▹
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Fig. 7. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling and after cleaning with (⋄) 0.2% (w/w), ( ) 0.4% (w/w), (▲) 0.6% (w/w), (□) 0.8% (w/w), (●) 1.0% (w/w) NaOH solution.
▹
297
Fig. 9. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, ( ) after cleaning with 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) after cleaning with 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
metal microfiltration membrane [10,11] and the little less expressed maximum was observed during caustic cleaning of the polysulphone ultrafiltration membrane [12]. The flux recovery decline is enhanced at higher temperatures and NaOH concentrations (pH) but it is absent at zero TMP [10,21] regardless of the membrane material. In the case of ceramic tubular membrane of lower pore size, flux maximum was not observed since there was no considerable fouling deposit in the pores. In the case of ceramic tubular membrane of larger pore size, there was no considerable flux recovery decrease probably because of very low pressure application even though adsorption of proteins in pores was considerable. Determination of the flux recovery was based on the pure water flux measurements after chemical cleaning followed by rinsing. The pure water fluxes of cleaned 200 nm membrane are shown in Fig. 7. The highest flux recovery can be observed for the 0.6% (w/w) concentration of sodium hydroxide. The highest flux recovery was 78% after cleaning with 0.6% (w/w) sodium hydroxide solution. The lowest flux recovery was observed after cleaning with 0.2% (w/w) sodium hydroxide solution.
3.4. Cleaning with commercial detergents
Fig. 8. Permeate flux curves during the cleaning with detergents at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
Fig. 10. Permeate flux curves during the cleaning with detergents at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. ( ) 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
The permeate flux patterns during cleaning with P3-ultrasil 67 and P3-ultrasil 69 are shown in Fig. 8 for the 50 nm membrane. The permeate flux during the cleaning with detergents was at the same level regardless of the detergent concentration. The pure water fluxes of cleaned 50 nm membrane are shown in Fig. 9. The highest flux recovery can be observed for the highest concentration of detergents. The flux recovery was 67% after cleaning with solution of higher concentration. The permeate flux patterns during cleaning with P3-ultrasil 67 and P3-ultrasil 69 are shown in Fig. 10 for the 200 nm membrane. In the case of 200 nm membrane, a maximal permeate flux can be observed. The value of maximum was higher for the higher detergent concentration. P3-ultrasil 67 contains alkylamineoxide which is a surface detergent that makes globule around protein enabling action of proteolytic enzyme. Except that, mixture of these two detergents gives the solution of alkaline nature that may also induce gelation or swelling of fouling deposits in the pores.
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fouled by whey protein was developed by Bird and Bartlett [11. It starts with Darcy's Eq. (2) linked to the total resistance of the fouled membrane (Rtf): ð6Þ
Rtf = Rm + Rf = Rm + Rcp + Rc + Rin
as the sum of the individual resistances: i) the resistance of membrane itself (Rm), ii) the resistance due to concentration polarisation (Rcp), iii) the hydraulic resistance of deposits at the membrane surface (Rc) and iv) the resistance due to in-pore fouling (Rin). The first resistance was calculated from Eq. (1) applied to water flux measurements and following results were obtained: 4.94 × 1011 (m− 1) for M50 nm and 2.12 × 1011 (m− 1) for M200 nm. The second resistance was neglected [11], while the third term of the Eq. (6) might be calculated assuming a second-order change of time derivative of the cake deposit [11] or the first-order change [24], which proved more suitable while leading to the exponential decrease of the cake resistance itself: Fig. 11. Pure water flux curves at 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min. (○) Before fouling, ( ) after cleaning with 0.8% P3-ultrasil 69 + 0.5% P3-ultrasil 67, (▼) after cleaning with 1.2% P3-ultrasil 69 + 0.75% P3-ultrasil 67.
◂
dRc = −p1 Rc ⇒ Rc = expð−p1 t + p2 Þ dt
ð7Þ
Finally, the swelling kinetics for in-pore bound material was modelled by application of the modified Carman–Kozeny equation: The pure water fluxes of cleaned 200 nm membrane are shown in Fig. 11. Regardless of the detergent concentration, no considerable difference in the flux recovery can be observed. The flux recovery was 50% after cleaning with 0.5% (w/w) P3-ultrasil 67 and 0.8% (w/w) P3ultrasil 69 solution and 44.5% after cleaning with 0.75% (w/w) P3ultrasil 67 and 1.2% (w/w) P3-ultrasil 69 solution. 3.5. Kinetic model of cleaning To understand deeply the mechanism of fouling, a few mathematical models are available [3,6]. All the same, kinetics of cleaning process might be explained better while processing the cleaning experiment data through an adequate mathematical model. Lee [22] suggested a simple cleaning model, assuming that the flux increaes during cleaning according to the first-order equation, while Zondervan et al. [23] proposed a two-component-balance-model, which follows changes in the fouling state and in the cleaning agent state. However, a particularly interesting model to predict the change in flux during cleaning of a flat plate cross flow microfiltration membrane
Rin =
36hk ð1−εÞ2 l −2 = p3 de ε3 d2e
ð8Þ
where de denotes the effective pore diameter, less than the nominal pore diameter do, according to the equation [11]: de =
δ 2 ðηk2 t + δÞ do −2 ⇒ de = do −ðp4 t + p5 t Þ k2 t + δ
ð9Þ
After substitution of all resistances from Eqs. (7 and 8) into the Eq. (6), a kinetic model appeared. Its parameters p1–p5 should be determined so as to provide the best fit to experimental data: 2
−2
Rtf = Rm + expð−p1 t + p2 Þ + p3 ½do −ðp4 t + p5 t Þ
ð10Þ
By applaying the Levenberg Marquardt method (ORIGIN 6.1) to the flux-data from Figs. 5–7 interesting results were obtained. Here, two examples were presented in Fig. 12.
Fig. 12. Cleaning models (full lines) based on experimental data for the membranes M50 nm (▲) and M200 nm (●), cleaned with 0.2% (w/w) NaOH solution, at 50 °C, TMP 30 kPa and CFV 1.73 ms− 1, for 30 min.
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solution. Also increasing of the commercial detergent concentration had a negative influence on the flux recovery. Cleaning with simple caustic solution appeared to be more efficient for both membranes even though total flux restoration was not achieved for the 200 nm membrane. 4. Conclusion Based on the results of investigations on rinsing and cleaning of 50 nm and 200 nm tubular ceramic membranes, fouled during reconstituted whey solution filtration, the following conclusions can be drawn:
Fig. 13. Flux recovery efficiency for the 50 nm membrane, based on the pure water flux measurements before fouling and after cleaning at the same conditions 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min.
An analysis of the statistical parameters (presented within Fig. 12) has shown a high level of adequacy of the model (10). Also, the presentation of particular resistances has shown that Rc decreases, in both cases, exponentially within a few minutes of the process. Also, Rin decreases slowly (for M50 nm), or remains unchanged, or even increases slowly, (for M200 nm). The mean size of pores of 200 nm is so large that proteins can enter the pores and block their inner surface. Slower and less efficient cleaning of the fouled membranes with 200 nm pores could be connected with this phenomenon. Similar results were obtained on cleaning with detergents.
Rinsing both membranes fouled with whey proteins contributed to a flux recovery to a certain degree. However, experimentally determined fluxes of the rinsed membranes were still low. Chemical cleaning of the 50 nm membrane, by 1.0% (w/w) caustic solution guarantees almost total flux restoration. Chemical cleaning of the 200 nm membrane, by caustic solution, did not allow total flux restoration, regardless of the applied concentration of caustic solution.Application of commercially available detergents in cleaning of both membranes did not allow total flux restoration. The maximum flux recovery for 50 nm membrane was about 75% at higher concentration of detergents. The mathematical model, proposed in this study, has shown high agreement with experimentally obtained data. Acknowledgment This research was supported by the Ministry of Science of Republic of Serbia (project no. 142045). References
3.6. Comparison of caustic and detergent cleaning Cleaning of the M50 nm with 1.0% (w/w) caustic solution gives the 97% flux recovery (Fig. 13). Cleaning of the M50 nm with commercial detergents was less efficient, but the flux recovery increased from 60 to 75% with increasing of the detergent concentration. For the M200 nm, total flux restoration was not achieved, regardless of the chemical cleaning agent and its concentration (Fig. 14). The best flux recovery (about 78%) was achieved after the cleaning with 0.6% (w/w) caustic
Fig. 14. Cleaning efficiency for the 200 nm membrane based, on the pure water flux measurements before fouling and after cleaning at the same conditions 25 °C, TMP 30 kPa and CFV 1.73 ms− 1 for 30 min.
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Glossary de: effective pore diameter (m) do: nominal pore diameter (m) hk: Kozeny factor (1)
J: volume flux (m3/m2s) Jcw: pure water flux of the cleaned membrane (m3/m2s) Jw: pure water flux before fouling experiment (m3/m2s) k2: rate constant (m min− 1) l: thickness of membrane (m) p1–p5: parameters of mathematical model (10) Rc: hydraulic resistance of deposits at the membrane surface (m− 1) Rcp: resistance due to the concentration polarisation (m− 1) Rf: resistance due to the membrane fouling (m− 1) Rin: the resistance due to in-pore fouling (m− 1) Rm: hydraulic resistance of the clean membrane (m− 1) Rtf: total resistance of the fouled membrane (m− 1) t: time (min) TMP: transmembrane pressure (Pa) δ: unswollen in-pore deposite thickness (m) ε: membrane porosity (1) µ: dynamic viscosity (Pa s) η: swelling ratio (1)