DESALINATION ELSEVIER
Desalination 117 (1998) 85-94
Methodology for accelerated pre-selection of UF type of membranes for large scale applications W . D o y e n a*, B. B a 6 e b, F. L a m b r e c h t s b, R. L e y s e n a
aVITO (Flemish Institute for Technological Research), Boeretang 200, 2400 Mol, Belgium, Tel.: +32-14-335622, Fax: +32-14-321186, E-mail.
[email protected]; and bSVW (Centre for Water Research), Mechelsesteenweg 64, 2018 Antwerp, Belgium, Tel.: +32-14-335638, Fax: +32-14-321186, E-mail.
[email protected] and
[email protected] Received 7 July 1998; accepted 11 July 1998
Abstract
This paper describes two assessment methods for UF type of membranes for large-scale applications. The combination of those two methods results in quite clear and unambiguous answers to the question what membranes are of interest for long-term testing. With the first method, called dead-end filtration method, information is generated on the suitability of the membrane and on the combination of the membrane material, the module hydraulics and assembly. With this method the evolution of TMP is monitored upon filtration cycles of 20 minutes with raw water at a flux rate of 120 1/h.m2, alternated with backwash cycles with permeate of 40 seconds at 1.2 bar negative TMP. The second method, called cross-flow filtration method, gives exclusively information on the suitability of the membrane material. This is being done by the measurement of the absolute value of the so-called "plateau fluxes" in cross-flow mode at 0.2 m/s linear velocity. For this purpose raw water concentrates are being used. Three "open" UF type of membranes, all three in hollow fibre configuration were assessed with these two methods. It was shown that the PSf based membrane (Koch PM100) reached already after 4 filtration cycles a TMP of 1 bar and showed the lowest plateau flux (25 1/h.m2). This indicated that the membrane suffered from interaction with the raw water. Moreover, it is possible that something was wrong with the hydraulics of this membrane. The two other membranes were PES/PVP based. These membranes showed much less TMP increase over time. The first membrane of this type was X-Flow UFC, the second Stork Friesland Superfil 015-010. It was no problem to operate the first membrane for 18 hours without addition of chemicals for cleaning. The second membrane reached the maximum allowed TMP of 1 bar after 16 hours of operation at the end of the filtration cycle. Moreover, for both membranes a higher plateau flux value (35 l/h.m2) was found. Both observations indicate that this type of membrane material is much more interesting than PSf. It was also shown that the X-Flow membrane gives the lowest absolute TMP values, which is attributed to its higher pure water permeability (740 1/h.m2.bar) as compared to the Stork Friesland membrane (pure water permeability of 350 l/h.m2.bar) and the Koch membrane (pure water permeability of 290 l/h.m2.bar). A last observation was a TMP increase of only 0.1 bar per cycle for the X-Flow membrane, as compared to 0.2 bar for the two others. This observation is in agreement with earlier made FESEM pictures of the inner surfaces. This means that the X-Flow membrane rather acts as a depth filter, whereas the two other membranes act as a surface filter. Keywords:
Ultrafiltration; Drinking water; Cross-flow; Dead-end; Assessment method
*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164/98/$ - See front matter © 1998 Elsevier Science B.V. All fights reserved. P// S0011-9164(98)00072-1
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W. Doyen et al. /Desalination 117 (1998) 85-94
1. Introduction From the beginning of the nineties, ultrafiltration (UF) is being investigated [1-3] in the d r i n k i n g w a t e r i n d u s t r y for clarification and disinfection purposes, since UF membranes are absolute physical barriers able to r e m o v e e f f i c i e n t l y s u s p e n d e d particles, turbidity, bacteria, colloids, algae, parasites, and viruses. As a consequence UF is replacing the conventional pre-treatment techniques like coagulation, sedimentation, flocculation, slow and rapid sand filtration, and m u l t i m e d i a filtration. N o w a d a y s numerous installations have already been constructed. These realisations however, are the outcome of a tedious exploratory testing procedure in order to find the most suitable membrane. The origin of this problem is the fouling sensitivity of this type of membranes. Therefore, membrane selection is mainly based on technical grounds. In recent technical assessment methods the direct testing into the application, is by far the only way to find the most suitable membrane type. In this type of test, the membrane elements are operated at a pre-supposed, economically justified flux level. The assessment methods are then oriented in order to find out the adjusted cleaning methods, just to be able to maintain the flux at this level. At this moment these c l e a n i n g m e t h o d s can only be established by a trial and error method. This is unfortunately a time-consuming activity. This paper describes a methodology to facilitate and a c c e l e r a t e the t e c h n i c a l assessment method and helps to avoid longterm testing of a-priori non-interesting membrane types.
2. Background of the proposed assessment methods The phenomena behind the flux-losses of a membrane are related to both the fouling properties of the m e m b r a n e and the efficiency of the backwash. On the one hand, fouling is related to the affinity of the
membranes for certain components of the raw water. The efficiency of the backwash, on the other hand, is related to the accessibility of the individual fibres in the module for the backwash liquid. If for instance, the fibres are packed too densely into the module, the fibres in the middle region of the bundle will not be easily backwash-able. This property is related to the hydraulics on the permeate side of the module and to the ease of its degassing. A first possibility to quantify the affinity for components from the raw water is to establish adsorption-isotherms, but this is not directly practical for a membrane user, since e.g. single fibres are usually not available. Therefore, two assessment methods, based on filtration experiments with the raw feed water are being proposed.
3. Description of the proposed assessment methods
3.1. Dead-end filtration method The first assessment method, further called "dead-end filtration method", is based on an altemate filtration cycle with raw water and a backwash cycle with permeate. This method gives information on both the fouling properties of the m e m b r a n e and the backwash-ability of the module. The essential part of this method is the evolution of the starting trans membrane pressure (TMP) in each filtration cycle. This gives information on the easiness of the removal of the formed cake (gel-layer). If the cake is not adsorbing onto the membrane, and the fibres in the module are easily backwashable, the starting TMP in each filtration cycle will be almost identical. If one of these two conditions is not fulfilled, there will be no return to the low starting TMP, and a gradual TMP-increase over time will occur. As a result the membranes with the lowest interaction in the best module assembly will operate with the most stable TMPs. The differences with the common dead-
W. Doyen et al. /Desalination 117 (1998) 85-94
end operation is firstly the exclusion of the addition of chemicals in the backwash liquid (= chemically enhanced backwash) [4]. This is essential since we want to reveal problems related to adsorption. The second difference is the flux level, which is much higher than in normal operation. The membranes were fed only from one side and operated in the flux-controlled mode. The flux level was fixed to 120 1/h.m2. The filtration-time and backwash-time per cycle were respectively 20 minutes and 40 seconds. During backwash the membranes were operated with a negative TMP of 1.2 bar. Typical in this assessment method as explained before was the measurement of the TMP evolution. The e x p e r i m e n t s were stopped after 18 hours of operation, or when a maximum TMP of 1 bar was reached.
3.2. Cross-flow filtration method The second method, further called "crossflow filtration method", is based on filtration experiments with raw water concentrates in the cross-flow filtration mode. In this method only the e f f e c t of the adsorption of c o m p o n e n t s of the raw water is being revealed by the measurement of the so-called "limiting fluxes" or "plateau fluxes". These limiting fluxes are being described in the well-known Film-model [5], which says that upon filtering, starting from a certain TMP, a gel-layer is formed at the membrane surface. The limiting flux is given by Eq. (1). The limiting flux (Jlim) is proportional to the diffusion coefficient (D), the thickness of the formed gel-layer (d), the gel concentration (Cg) and the bulk concentration (Cb).
Jlim
=
D d
Cg ln-Cb
(1)
The gel-concentration is the concentration where the flux becomes zero. This is a constant for a given feed. Also the diffusion coefficient is a constant. But since the d i f f u s i o n c o e f f i c i e n t is i n f l u e n c e d by
87
temperature, the temperature is important. So, the only direct factors influencing the absolute value of the limiting or plateau flux are the bulk concentration and the thickness of the gel-layer. The m o r e the bulk concentration d i f f e r s f r o m the gel concentration the higher the limiting flux. The thickness of the gel-layer on the other hand is very difficult to determine. It is determined by the linear velocity at the membrane surface and by the interaction of the membrane with the feed components [6]. So, for c o m p a r i n g the i n t e r a c t i o n behaviour of the different membranes, it is essential to keep all parameters constant, which are influencing the limiting flux. These are respectively the temperature, the bulk concentration and the linear velocity. This "cross-flow filtration" assessment method is performed by measurement of the flux/TMP curves. In general, starting from a certain TMP, upon filtering solutions or suspensions, the flux becomes pressure independent. The membranes showing the highest plateau flux values have the lowest interaction, and viceversa. In practice, this measurement is performed with a raw water concentrate. The reason why a concentrate is being used, is because at low concentrations the plateau fluxes are quite high, and therefore it will be likely not possible to measure them. The absolute concentration level of the concentrate is not so important, but it should be the same for all t e s t e d m e m b r a n e s . If this t y p e of measurements is repeated several times over a o n e - y e a r period, and the v o l u m e t r i c concentration factors are kept constant, this assessment method can be used to get more insight into the contaminants evolution in the raw water over time. This concentrate is prepared by making use of a dummy membrane having a similar pore size to the m e m b r a n e s in the e x p e r i m e n t . The f o r m e d p e r m e a t e is continuously removed and put into the drain. The amount of formed permeate is metered, and the turbidity of the concentrate is monitored. Starting from a raw water
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W. Doyen et al. / Desalination 117 (1998) 85-94
Table I Characteristics of the membranes used in the experiments
Supplier
Membrane-type
Cut-off value
Internal fibre
(given by diameter the suppliers) (mm) (kD) Koch Membrane Systems X-Flow Stork Friesland
PM 100 UFC Superfil E 015-010
100 150-200 20 nm
area (m 2)
Calculated hydraulic cross-section (cm 2)
0.76
19
38
0.8
25
53
1.5
20
84
turbidity of 2.8 NTU, a concentrate is prepared having a turbidity of 130 to 160 NTU. Once this turbidity is reached, the d u m m y m e m b r a n e is r e m o v e d from the filtration system. Subsequently one of the three membrane modules is mounted in the system, containing the raw water concentrate. This m e m b r a n e is a clean m e m b r a n e . S u b s e q u e n t l y the f l u x / T M P c u r v e is measured. At the end of the measurement the membrane is cleaned by a forward flush in order to bring the material sticking to the membrane surface back into suspension in the concentrate. Subsequently the second and the third m e m b r a n e are m o u n t e d and
4. E x p e r i m e n t a l
measured.
-
During the flux/TMP measurement, the temperature is kept at 10°C, and the linear velocity is 0.2 m/s. Filtration is started with the lowest possible TMP (in our case 0.2 bar). This TMP is maintained for 15 minutes for allowing the membrane to be in a steady state. Subsequently, the TMP is increased with steps of 0.1 bar again and again. The end TMP is 1 bar. Each individual TMP level is maintained for 5 minutes. At 1 bar TMP the measurement is prolonged to 15 minutes in order to get first feelings on the fouling properties of the membrane. By c o m b i n i n g the results o f both a s s e s s m e n t m e t h o d s it is possible to distinguish between the fouling properties of the membrane and the backwash-ability of the membrane modules of different suppliers. This feature will be discussed in this paper.
Membrane
4.1. Used membrane types Three UF types of m e m b r a n e s were evaluated. In Table 1 the characteristics of the membranes are given. Two membrane types (Koch Membrane Systems and X-Flow) have the XIGA® configuration (8" diameter, 40" module length) [4]. In this module configuration replaceable m e m b r a n e - m o d u l e inserts are being mounted in an 8" diameter pressure vessel. The first membrane of this type was a polysulfone hollow fibre membrane (PSf m e m b r a n e ) of the Koch M e m b r a n e Systems Company. The internal fibre diameter of this membrane equals 0.76 mm, the total effective membrane area equals 19 m 2 and the given cut-off value is 100 kD. This membrane type is sold under the brand name "Koch PM100 membrane". - The s e c o n d m e m b r a n e o f s i m i l a r configuration was a polyethersulfone / polyvinylpyrrolidone blend hollow fibre membrane (PES/PVP membrane) of the X-Flow Company with an internal fibre diameter of 0.8 mm, a total effective membrane area of 25 m 2 and a given cutoff value between 150 and 200 kD. This membrane type is sold under the brand name "X-Flow UFC membrane".
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w. Doyen et al. / Desalination 117 (1998) 85-94
The third membrane was not mounted in a pressure vessel. In this c o n c e p t the m e m b r a n e and the m o d u l e form an integral part. The single p e r m e a t e connector of this membrane module is positioned externally, in the middle of the module. The m e m b r a n e was also a PES/PVP blend hollow fibre membrane. It is m a n u f a c t u r e d by the Akzo N o b e l Company and potted into the module by the Stork Friesland Company. The internal fibre diameter equals 1.5 mm, the total effective membrane area is 20 m 2 and the given pore diameter is 20 nm. This membrane type is sold under the brand name "Stork Friesland Superfil E 015-010 membrane".
4.2. Membrane filtration unit
A membrane filtration unit, supplied by Norit Membrane Technology, was used for the filtration experiments. The unit contains 2 separate centrifugal pumps (a feed and a b a c k w a s h p u m p ) with a c a p a c i t y of respectively 10 m3/h and 20 m3/h at 3 bar. Two tanks with a capacity of 1,200 1 each are used for collecting the feed and permeate water. The feed water tank is automatically filled with raw water by a float shut-off valve. The retentate can be drained or fed back to the feed water tank ( c r o s s - f l o w mode). Permeate overflow goes to the drain. Before the raw water enters the unit it is pre-filtered over 100 ~tm micro sieves. Different flow-, pressure- and temperature transmitters are foreseen to enable the m e a s u r e m e n t of the permeate flow, the retentate flow, the T M P and the temperature. Two special features of the unit are the unique M E F I A S ® real-time data-acquisition software and the s t e e r i n g - s o f t w a r e for automated operation of the unit. This software was developed by VITO. The unit enables, carrying out of the described deadend and cross-flow assessment methods in an automated manner.
4.3. Location of the experiments
The experiments were performed at the end of January 1998 in O e l e g e m near Antwerp. This is a production site of the Antwerpse Waterwerken (AWW), a major Flemish drinking water company. The raw water was taken from a reservoir, which is fed with water from the Albertkanaal. This canal connects the river Maas (Liege) with the river Schelde (Antwerp). 4.4. Raw water characteristics
The main characteristics of the raw water are summarised in Table 2. Table 2 Overview of the raw water characteristics of the Albertkanaal water pH Suspended solids (SS), mg/1 Turbidity, NTU Chlorophyl a, tag/1 Feofytine a, lag/l Colony number, #/ml E. coli, #/100 ml Dissolved organic carbon (DOC), mg/1 UV absorbance 254 nm, #/m Fe, lag/1 Mn, tag/1 A1, lag/l
7.7-8.1 2 2.4-3.6 4 1 350 5 3.3 9.6 250 20 1 00
5. Results and discussion 5.1. Dead-end filtration method
A typical result of this assessment method is given in Fig. 1 for the X-Flow UFC membrane - PES/PVP membrane. As a result quite stable T M P profiles are observed, indicating that both the backwash-ability of this type of membranes is quite good, and its adsorption for feed water contaminants is low. In Fig. 2 the results of the three evaluated membrane types are summarised.
W. Doyen et al. / Desalination 117 (1998) 85-94
90 1,5 1,2 0,9 0,6 ~- 0,3 ~" 0,0 ~- -0,3 -0,6 -0,9 -1,2 -1,5
Time (h) Fig. 1. Typical T M P profile under dead-end operation at 120 1/h.m2 (measured on the X - F l o w UFC PES/PVP membrane).
1,2 1,0
~- 0,8 t~ e-~
n 0,6 I- 0,4
0,2 0,0 0
6
12
18
Time (h)
Fig. 2. TMP profiles of the PSf and the two PES/PVP membranes at 120 l/h.m 2.
A first observation on this figure is that the other PES/PVP membrane (Stork Friesland Superfil E 015-010) has a starting-TMP that is at least twice as high as the TMP of the XFlow membrane. The difference in absolute T M P for t h e s e two P E S / P V P b a s e d m e m b r a n e s has to be attributed to the difference in their pure water permeability (PWP). Indeed, separate PWP measurements on these membranes confirm this statement. The P W P of the X-Flow membrane was 740 1/h.m2.bar, and that of the Stork Friesland membrane was 350 1/h.m2.bar.
A second observation on Fig. 2 is that the Stork Friesland Superfil E 015-010 also has a rather limited TMP increase versus time, but after 16 hours of operation at the end of a filtration cycle the 1 bar T M P maximum is reached. This p r o v e s that either the adsorption characteristics or the backwashability of this P E S / P V P m e m b r a n e is somewhat worse. A possible explanation will be given in the chapter dealing with the crossflow assessment method. A third o b s e r v a t i o n is that the P S f membrane shows already a strongly increased
W. Doyen et al. /Desalination 117 (1998) 85-94
starting TMP from the second filtration cycle in contrast with the two PES/PVP based membranes. This trend is continued during the next filtration cycles. Indeed, already after 4 filtration cycles the maximum TMP of 1 bar is reached. The origin of this behaviour has to be sought either in the adsorption characteristics of the membrane material, or in the bad backwash-ability of the module. This will be clarified further in the chapter dealing with the cross-flow assessment method. The reason for the quite high starting TMP is the low PWP of this membrane (290 1/h.m2.bar). A fourth observation that can be made on Fig. 2 is connected with the TMP increase in each filtration cycle. The X-Flow membrane has only a TMP increase of 0.1 bar per filtration cycle, compared to 0.2 bar for the two others. This difference in behaviour is attributed to the difference in surface pore structure. In ref. [4] it was demonstrated by FESEM analysis of the surface structure, that the surface pores of the X-Flow membrane were slit shaped, in contrast to the Koch membrane that has circular pores. The biggest dimension of the slit was about 100 nm, which is 5 times as much as the diameter of the surface pores of the Koch membrane. These two membranes are claimed to have a similar cut-off value (similar pore size), hence this suggests that the Koch membrane should act as a surface filter, whereas the X-Flow membrane as a depth filter. Indeed, this is confirmed by the TMP profiles, demonstrating the much larger "dirt holding capacity" of the X-Flow membrane. 5.2. Cross-flow assessment method
A typical result of the Koch PM100 membrane for this type of measurement is given in Fig. 3. The TMP and the corresponding flux are plotted versus time. At 0.2 bar the membrane starts at a flux level of 75 l/h.m 2 and drops back to 25 l/h.m2 over the 15-minutes interval. Upon each TMP increase the flux slightly increases to about
91
30 1/h.m2, and drops back to 25 1/h.m2 in each 5-minutes measuring interval. This value is the so-called plateau or limiting flux of this membrane for the given feed under the given conditions (10°C, 0.2 m/s). From this measurement, the plateau flux profiles are derived, by selecting at each different TMP the stabilised flux value at the end of the measuring interval. Fig. 4 gives an overview of the derived plateau fluxes of all three tested membrane types. Typical in such graphs are the two parts: the horizontal part and the slope part. The horizontal part is the pressure-independent part from which the plateau flux can be derived; the slope part is the pressure-dependent part that is depending on the membrane characteristics (cut-off value, surface porosity and permeability). Unfortunately by the fact that we cannot measure at TMPs below 0.2 bar, no information is gathered concerning the pressure-dependent characteristics. It is clear that both PES/PVP membrane types clearly show higher plateau flux values (35 l/h.m 2) as compared to the PSf based membrane (25 1/h.m2). This indicates the lower interaction of both PES/PVP based membranes with the raw water. These results confirm most of the observations made by the dead-end filtration method. The findings for the PSf membrane type in the two assessment methods are in the same way: this membrane suffers from stronger adsorption. Hence, the absolute value of the plateau fluxes (measured in cross-flow mode) is lower and the TMP increase in dead-end filtration is much higher because of worse backwash-ability. On the other hand, the reason why the Stork Friesland membrane showed a TMP higher than 1 bar in the deadend filtration method, is no longer the adsorption characteristics of the membrane material but the hydraulics of the module. 5.3. Membrane retention
During the dead-end filtration experiments the respective permeates and retentates of the different membranes were sampled and
W. Doyen et al. / Desalination 117 (1998) 85-94
92
100
1,2
90 1,0
80
70 E
60
LL
40
0,8
.c;
50
O,6 o.. I-
0,4
30 20
0,2
10 0
D,O 0
10
20
30
40
50
60
70
80
Time (rain.)
Fig. 3. Typical flux/TMPmeasurement(measuredon the Koch PSf membrane).
45 40 35 ~" 3O E
25
× 20 u. 15 10 5 0 0,0
0,2
0,4
0,6
0,8
1,0
1,2
TMP (bar)
Fig. 4. Flux/TMP curves of both the PSf and the two PES/PVP membranes.
analysed on different parameters. It was found that there was hardly any difference in retention for the three membrane types. The retention for the different components can be summarised as follows: - Very good retention (100% or nearly 100%) for suspended solids, turbidity, bacteria, chlorophyl a and fytoplankton. - Quite good retention (+80%) for feofytine a, Fe, AI and CFU (colony forming units). No retention (0-10%) for dissolved organic components (DOC and UV-
absorbance), different ions (Mn, Sr, Zn, Cu, Ba, Pb, Cd, Cr, Ag, Ni, NH4, NO2, Na, K, SiO2, Ca and Mg) and conductivity.
6.
Conclusions
Since the retention of the three evaluated membrane types for different contaminants in the raw feed water are similar, the assessment of their operation conditions becomes most important.
W. Doyen et al. /Desalination 117 (1998) 85-94
In this paper it has been shown that the combination of the proposed assessment methods results in quite clear and unambiguous answers to the question what membranes are of interest for long-term testing. With the proposed cross-flow assessment method one can evaluate the suitability of the membrane material (in terms of absolute value of the plateau fluxes). The proposed dead-end filtration method, on the other hand, gives information on the suitability of the membrane structure (in terms of starting TMP and TMP evolution in the filtration cycle), and on the combination of the membrane material, the module hydraulics and assembly (in terms of TMP evolution versus time). This information can be used to select membrane types that enable filtration either at higher flux levels, or at longer filtration cycles, or using smaller amounts of cleaning chemicals. Hence, important parameters like the total water recovery and the investment costs of a filtration plant can be optimised by making use of the proposed methods. However, it has to be stressed that longterm testing of the most promising membranes is still necessary in order to identify special aspects of long-term operation. Based on these two assessment methods one can generally conclude that the PES/PVP based membrane types are the most interesting for this surface water.
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Acknowledgements Hereby we firstly would like to thank the SVW for the financial support of the research programme. We also would like to thank our colleagues from VITO, especially Erwin Van Hoof for writing the MEFIAS® software and Bart Molenberghs for helping with the startup of the experiments.
References [1] I. Blume et al., Large scale applications for microand ultrafiltration in water treatment - A new module system, Aachener Membrane Colloquium, March 14-16, 1995. [2] G. Vos, Y. Brekvoort, H. Oosterom, A. Hulsman, and N. Wortel, Membrane filtration as a new technique for recovery of backwash water, Proc. Colloid Science in Membrane Engineering, Toulouse, 1996. [3] I. Baudin, M.R. Chevalier, C. Anselme, S. Cornu, and J.M. Lain6, L'Api6 and Vigneux case studies: First months of operation, Desalination, 113 (1997) 273-275. [4] W. Doyen, Latest developments in ultrafiltration for large-scale drinking water applications, Desalination, 113 (1997) 165-177. [5] M.C. Porter, Concentration polarisation with membrane ultrafiltration, Ind. Eng. Chem. Prod. Res. Develop., 11 (1972) 234-248. [6] M. Mulder, Basic Principles of Membrane Technology, 2nd edition, Kluwer Academic, Dordrecht, 1997.
