- Email: [email protected]
UF as an alternative pretreatment step for producing drinking water
UF as an alternative pretreatment step for producing drinking water B y W. Doyen (Hemish Institute for Technological Research ( V I T O ) , M o l , B e l g i u m ) , a n d B e t t y B a & and Luc Beeusaert (Centre for Water Research & Co-operation of Flanders ( S V W ) , A n t w e r p , B e l g i u m )
This article describes membrane research which focuses on exploring the use of open uhrafiltration as an alternative to conventional separation technologies used for producing drinking water. Four different water types, originating from three water sources, are investigated. Four open uhrafiltration membranes are used, of which three are of a hollow-fibre configuration while one is tubular. On the one hand it was found that permeate quality was always very good, compared with conventional techniques - - no bacteria, suspended solids or turbidity were present. Moreover, the quality was independent of operational conditions or the membrane type. On the other hand, it was found that operational conditions strongly differ in terms of 'raw' water quality, or the membrane type used. At about the beginning of the 1990s, ultrafiltration (UF) started being investigatedI141 by the drinking-water industry as an alternative to conventional pretreatment techniques. Since UF membranes are physical barriers which are able to efficiently remove suspended particles, turbidity, bacteria, colloids, algae, parasites and viruses for clarification and disinfection purposes, they may offer a number of advantages. Nowadays, conventional pretreatment includes coagulation, flocculation, sedimentation and/or flotation, rapid and slow sand filtration, and 'multimedia' filtration. Here we describe research which is looking at the feasibility of 'semi-dead-end' UF for the same purpose. It is being managed by the Centre for Water Research & Co-operation of Flanders (SVW). SVW brings together the joint research of all the drinking-water companies in Flanders, in particular Antwerpse Waterwerken (AWW),
Provinciale en Intercommunale Drinkwatermaatschappij der Provincie Antwerpen (PIDPA), Tussengemeentelijke Maatschappij der Vlaanderen voor Watervoorziening (TMVW), and Vlaamse Maatschappij voor Watervoorziening (VMW).
Goals Experiments started in mid-1997, and will end during December 2000. 'Semi-dead-end' UF filtration pilot experiments are being conducted at four locations - - consecutively in Oelegem (AWW), Kluizen (VMW), Grobbendonk (PlDPA), and finally in Korbeek-Dijle (VMW). The specific goal for AWW is to explore possible solutions to problems caused by extensive algae growth under conventional separation. For VMW, the specific target is to search for an alternative to conventional treatment, with
chemical consumption at Kluizen being reasonably high. PIDPA is looking at the behaviour of UF with respect to the water source, that is, from a reservoir or taken directly from Canal Albert. Finally, for V M W in Korbeek-Dijle, the aim is to look at the feasibility of using UF as a means of pretreatment.
Materials and methods
Water types
All of the four water types under investigation are classified as surface water. These will now be described. The first water type originates from Canal Albert, which connects the river Maas with the river Schelde. The location is Oelegem (location #1), which is at a distance of some 20 km from Antwerp. Water is pumped to a reservoir. It is characterised by a low content of low molecularweight components, but sometimes it has high algae content. This means that turbidity may range from 0.5-50 NTU. This is AWW's 'raw' water
source.
The second water type originates from Canal Isabella, and from some small local rivers. The location is Kluizen (location #2), which is at a distance of some 10 km from Ghent. The water is also pumped to a reservoir. The water is very clear (with turbidity ranging from 0.2-2 NTU), and it has quite a high content of low molecularweight components, with the biggest fraction
MembraneTechnologyNo. 126
FEATURE
lying between 1 and 3 kD. I51This results in high dissolved organic components (DOC) and high KMnO 4 consumption. This is VMW's 'raw' water source. The third water type is identical to the first water, but it is taken directly from the canal. The means that the turbidity is much higher, and fluctuates to a greater extent (20-80 NTU). The location is Grobbendonk (location #3), which lies roughly 30 km from Antwerp. This is PIDPA's 'raw' water source. The fourth water type originates from the river Dijle. The location is Korbeek-Dijle (location #4), which is around 10 km from Leuven. It is characterised by a low content of low molecular-weight components, but it has a fluctuating turbidity (from 10 to over 100 NTU) because of the presence of clay particles. This is VMW's second 'raw' water source.
Membrane types Five different types of UF membranes were evaluated. Table 1 shows the characteristics of each of these membranes. The first and second membrane types use the so-called XIGA configuration (8-inch diameter, 40- or 60-inch module length). I41 In this configuration replaceable, membrane-module inserts are mounted in an 8-inch diameter pressure vessel, in a similar way to the approach which is adopted in reverse osmosis spiral technology. Permeate collects in a central permeate tube. The first membrane of this type is a polysulfone hollow-fibre product from Koch Membrane Systems (membrane #1). The membrane has an internal fibre diameter of 0.76 mm. The total effective membrane area is 19 m 2, and the given cut-offvalue is 100 kD. This membrane is one of the first 8-inch membrane insert prototypes. The second membrane is a polyethersulfone/polyvinylpyrrolidone blend hollow-fibre product from X-Flow/Norit (membrane #2), with an internal fibre diameter of 0.8 mm. The given cut-off value is between 150 and 200 kD. Two module configurations were tested. The first one had no bypass, while the second has a 12-chamber design. This is a special development for improving back-wash characteristics. The total effective membrane area amounts to 25 m 2 for the first unit and 22 m 2 for the second. The third and fourth membrane types are not mounted in 8-inch pressure vessels. In this case, the membrane and module form an integral unit. The hollow-fibre material for membrane #3 also comprises a polyethersulfone/ polyvinylpyrrolidone blend. It is manufactured by Akzo Nobel, and potted into modules by Stork Friesland. The internal fibre diameter is 0.7 mm. Fibres are potted in eight separate bundles. This is a special development which improves back-wash characteristics. The total effective membrane area is 25.7 m 2. The given pore diameter for this membrane is 20 nm.
Membrane Technology No. 126
Membrane #4 is a polydivinylidene fluoride/ polyvinylpyrrolidone blend tubular membrane, with an internal composite polyester backing material. Its internal diameter is 5.2 ram, and the total effective membrane area is 10 m 2.
(TMP). A special feature of the units is the MEFIAS real-time data-acquisition and steeringsoftware, [61 which was developed by VITO. The units enable the 'semi-dead-end' and cross-flow experiments described here to be carried out automatically.
Membrane filtration units Two specially built membrane units were used for the filtration experiments. These are mounted in a 6 m (20 ft) container. The units include two separate centrifugal pumps (a feed pump and a back-wash pump), each with a capacity of 6 m2/h at 3 bar. Two plastic tanks, with a capacity of 1500 1 and 500 l, respectively, were used to collect the feed and permeate. The feed-water tank is filled automatically with raw water through a float shut-off valve. Retentate can be drained, or fed back to the feed-water tank (cross-flow mode). Permeate overflow is channelled to the drainage system. Before the raw water enters the unit it is pre-filtered over back-washable 300 pm micro-sieves. Data transmitters are used for the measurement of parameters such as pressure, pH, turbidity and temperature, and to measure permeate and retentate flow, and trans-membrane pressure
Methods
and results
Short-term experiments For a short period ( l - 2 months) a number of exploratory tests were initially performed at each location (Oelegem, Kluizen, Grobbendonk and Korbeek-Dijle), to select the membrane types for the long-term tests. The initial work included membrane integrity tests, comparative crossflow experiments, comparative 'semi-dead-end' experiments, exploration of the use ofFeC13, and the evaluation of permeate quality. The tests which were used are now described.
Membrane integrity tests The purpose of this test is to check for membrane or module defects. It is performed using the purewater flux (PX0ffF) and the pressure hold-up test (PHT). PWF gives information about membrane
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condition (clean/fouled), but is not sensitive enough for an integrity check. It is performed with tap water. Figure 1 shows pure-water fluxes versus TMP for the evaluated membrane types. It was found that the permeability (pure-water flux at 1 bar TMP) of membranes ranges between 250 and 1250 l/h.m2.bar. Membrane #1 has the lowest permeability, whereas membrane #3 has the highest. The P H T is based on the natural phenomenon that air will not pass through a wetted hydrophilic membrane, up to a certain pressure. This is the so-called 'bubble-point' pressure (BPP). So, under the BPP the membrane does hold pressure. BPP is inversely proportional to the pore diameter, and for the membrane types used it is far above 3 bar. In Figure 2, results of the PHT at two different pressures (0.5 and 1 bar, respectively) on membrane #3 are shown. From the Figure the pressure decay can be calculated. The allowable pressure decay at 1 bar is a maximum 10 mbar/min. The same test was performed on membrane #4. It was found that this membrane was
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damaged at a differential pressure of 1 bar. This is caused by the membrane being released from its polyester backing material. For this reason membrane #4 was not selected for the long-term tests. However, this problem can be overcome by applying pressure at the retentate side. Also, in normal operation using this membrane, on back-flushing attention should be paid to the air present at the permeate side. It is best to use it only in a vertical position, so that the module is automatically degassed. The PHT was performed several times over the two-year period. All membrane types maintained their integrity over this period. In conclusion, the PHT is quite simple, but it is sensitive and capable of detecting very small defects in membranes.
Comparative'semi-dead-end' experiments In these types of experiments an initial idea of the membrane's behaviour can be gained through 'semi-dead-end' operation on surfacewater. The approach basically consists of alternate filtration cycles with raw water, and
back-wash cycles with permeate, for membrane rinsing. This experiment gives information about the fouling properties of the membrane, with respect to raw water, and the back-wash characteristics of the module. The essential part of the measurement is the evolution of the T M P in each filtration cycle, and with time. As a result, membranes which have the lowest interaction with molecules of the feed, will operate with the most stable TMPs. There are two main aspects of the procedure for this experiment with common 'semi-deadend' operation. First, chemicals are not added to the back-wash liquid (chemical cleaning). [7,8] This is essential, because we want to reveal problems related to adsorption. The second aspect is the flux level which is used. This is much higher than in normal operation. Fluxes of consecutively 100, 150 and 200 1/h.m 2 were each used for 24 h of operation, without intermittent cleaning. This allows quick interpretation. All other filtration parameters (hydraulic load, back-wash flux and backwash volume) were kept constant. Hydraulic load - - the volume of raw water per unit of membrane area and per filtration cycle - - was 25 l/m2.cycle. The back-wash volume was limited to 60 1, and the back-wash flux was fixed at 300 1/h.m 2. As a result, water recovery during the test was always 90%. Filtration-time per cycle was adjusted to the flux level used (7.5 min at 200 l/h.m 2, 10 min at 150 l/h.m 2 and 15 min at 100 l/h.m 2, respectively). This is at constant hydraulic load. The experiments were stopped after reaching a maximum TMP of 1 bar (cutoff value), or after 24 h of operation. Figures 3 and 4 illustrate typical results for such tests on surface-water #3 for membranes #2 and #3. It is clear from these Figures that membrane #2 gives the most stable operation, whereas membrane #3 gives an increasing TMP versus time, at higher flux rates. The effect of cleanwater permeability is also reflected in this test. Membrane #3, with the highest permeability, results in the lowest T M P value (0.12 bar instead of 0.22 bar), at the lowest flux level (100 l/h.m2). The less stable operation of membrane #3 can also be seen at higher flow rates (150 and 200 l/h.m2) at the starting TMP, which approaches those of membrane #2. This test was done on all types of surface water. The results which are published in the literature [7'81 cover similar measurements on all types of membrane, with respect to surfacewaters #1 and #2. It was found that membrane #1 strongly suffered from an interaction with raw feed-water. This was illustrated by the fact that the 1 bar TMP cut-off value was reached after few filtration cycles. This was the reason why membrane #1 was not selected for the longterm test.
Membrane Technology No. 126
Comparative cross-flow experiments 40 At locations # l and #2 (Oelegem and Kluizen) a comparative cross-flow experiment was performed. First, raw water concentrates were prepared using the UF membranes. Then, the membranes to be examined were mounted on the filtration system. Subsequently, the flux/TMP profile for a given concentrate was measured under cross-flow conditions at constant linear velocity (for example, 0.2 m/s). This measurement is described in detail further in the literature.[7] Typically a flux/TMP profile consists of two parts - - a horizontal part and a sloping part. The horizontal part is the pressure-independent portion, from which the plateau/limiting flux can be derived. The sloping portion is the pressure-dependent part, and it depends on the membrane's characteristics (cut-off value, surface porosity and permeability). Unfortunately we cannot perform measurements at TMPs below 0.2 bar, so no information was gathered concerning pressure-dependent characteristics. A typical result for membrane #1 covering the first surface-water is shown in Figure 5. Already starting at a T M P of 0.2 bar, the flux is limited to 25 l/h.m 2. A further increase in TMP has no effect on flux. This flux value is the socalled plateau, or limiting flux, of this membrane, for a given feed under given conditions (10°C, 0.2 m/s). In Figure 5 results are also given for other types of membranes which were tested. It is clear that membrane types based on polyethersulfone/polyvinylpyrrolidone (membranes #2 and #3) clearly show higher plateau flux values (35 l/h.m2), compared with the polysulfone-based membrane (25 l/h.m2). This indicates a lower interaction with raw water for both of the membranes which are based on polyethersulfone/polyvinylpyrrolidone. These results confirm most of the observations made by comparative 'semi-deadend' filtration experiments. The findings for the polysulfone-based membrane, in both the 'semidead-end' and cross-flow experiments, are similar, that is, this membrane suffers from stronger adsorption. For surface-water #2, the findings were quite similar, which indicates that the polysulfonebased membrane is a less attractive membrane material for natural-water filtration purposes. For this reason experiments on membrane #1 were stopped, definitively starting from location #3.
Using FeCl 3 The use of coagulation just before UF was also considered. This was started on surface-water #2, containing a high amount of low molecular-weight components. These molecules are prone to foul the internal structure of membranes. The normal operational mode (without FeCI 3 dosing) was
Membrane Technology No. 126
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75 l/h.m 2 at 20 min filtration time. Backwashing took 40 s, resulting in water recovery of 86%. The results of TMP evolution versus time are given in Figure 6. As can be seen here, the TMP fluctuates and rises quite steeply. As a result this operational procedure requires an intensive chemical cleaning procedure. The membranes needed to be cleaned each day with
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@
FEATURE
sampled and analysed on different parameters. It was found that there was hardly any difference in retention for all of the membrane types. The retention for different components is summarised as follows:
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This meant that another filtration procedure had to be found. Initially, experiments were undertaken to find a solution for this problem. It was found that by dosing with FeCl 3 (3 mg Fe3*/l) just in front of the feed pump, the filtration was more stable, and did not show a steep T M P rise with time. This is shown in Figure 7. In this experiment similar filtration and back-wash parameters were used as in the previous one without FeC13 dosing. As a result,
cleaning could be postponed. However, it was found that NaOCI was no longer effective as a cleaning agent. Different acids were tested, and it was found that sulfuric acid was the most effective. The cleaning pH was 1.
Evaluating permeate quality During the 'semi-dead-end' filtration experiments, the respective permeates and retentates of the different membranes were
Membrane #2 was selected for long-term tests at three locations (#1, #2 and #4), because of its superior TMP stability, even without FeCI 3 dosing. Membrane #3 became available from the beginning of 1999, and was selected for longterm tests at location #3, because of its superior pure-water flux. During the long-term tests, all experiments focused on the assessment of seasonal influences on operational parameters, on permeate quality, and on the optimisation of flux and recovery. Also, postponement of cleaning was an important item. Starting with location #2, experiments were done with FeCI3 dosing• Few filtration parameters were fixed - just filtration and back-wash flux, and backwash volume. Values differ between different locations. Table 2 gives an overview of these parameters. Original (starting) values are in parentheses, while optimised values are in bold. If we look through the parameter settings developed for locations #3 and #4, some observations can be made. These are summarised as follows:
MembraneTechnologyNo. 126
Through gaining a better understanding of the filtration process, recovery could be optimised (75% up to 98%). From the optimised flux values we might conclude that either very clear raw water (water type #1 and #2 or the reservoir water), or water contaminated with low molecular-weight components is much more difficult to filter. In the short-term experiments, because FeCI 3 was used, cleaning could be postponed (Figure 7). This was confirmed in the long-term experiments at locations #3 and #4. In Figure 8, the TMP/time profile for location #4 (using membrane #2) is given. It was observed that cleaning could be postponed for two weeks. A two-step cleaning process is being used successfully. It makes use of sodium hypochlorite (200 ppm CI 2) and sulfuric acid (pH = 1). Figure 9 shows the results for membrane #3 at location #3. Here there is a remarkable observation. On using FeCI 3, membrane #3 could also be operated for quite a long time without any cleaning. Cleaning could even be postponed for three weeks. Similar two-step cleaning was applied successfully. This means that specific membrane behaviour, observed during the comparative 'semi-dead-end' experiments described earlier (Figures 3 and 4), and previously in the literature, [7'8] has completely disappeared. From this we may conclude that on using FeCI 3 the type of membrane which is used appears to be of little importance. Thus some conclusions made earlier with respect to this item may have to be revised. The testing procedure which has been described has been applied for more than six months at the time of writing. This proves that membranes can be operated in a stable manner over a long-term period. With respect to permeate quality over the sixmonth period, we can say that it was always equal to results described in short-term experiments. H
Conclusions Ultrafiltration has been shown to be a reliable technique which could replace conventional techniques. Permeate quality is high, and results in a total absence of suspended solids. Dissolved components, however, are not retained. Permeate quality is unaffected by membrane type, the filtration conditions and the raw water composition. By making use of comparative 'semi-dead-end' and cross-flow experiments, membrane types which are less prone to fouling can be identified. Membranes made from a polyethersulfone/polyvinylpyrrolidone blend seem to be most interesting. They have low fouling properties and high permeability. Use of FeC13 facilitates filtration, and helps to optimise recovery and flux level. Moreover, cleaning can be postponed for a period of two to three weeks. A two-step cleaning process with NaOCI/H2SO 4 was found to be efficient for the long-term operation, facing different seasonal influences.
Acknowledgments
Pressure-assisted electrodialysis Applicant: University of Chicago, USA A process of electrodialysis of aqueous fluids, to transfer dissolved salts from a waste, or other solution identified as a 'diluate' solution, into a concentrate solution is discussed. An electrodialysis unit includes a stack of semi-permeable, ionselective membranes, and an applied electric field which causes ions to pass from the diluate solution to the concentrate solution through the ionselective membranes. The electrodialysis unit defines diluate and concentrate solution compartments. These are separated by alternating cation- and anion-selective membranes. Each
Membrane Technology No. 126
For more information, contact: Wire Doyen,VlTO,
We would like to thank the SVW members for Boeretang 200, B-2400 Mol, Belgium. Tel: +32 1433 their operational and financial support of the 5622, Fax:+32 1432 1186, Email: [email protected] research programme. In particular, personnel of i AWW, PlDPA and V M W are acknowledged. Or contact: Betty Bake or Luc Beeusaer~,SVW, We would also like to thank our colleagues Mechelsesteenweg 64, B-2018 Antwerp, Belgium. from VITO, especially Erwin Van Hoof for Tel: +32 3244 0750, Fax:+32 3248 2742, writing the MEFIAS software, and Bart Emaih [email protected] Molenberghs for helping with the start-up of This article is based on a paper entitled 'Exploration the experiments. of the possibilities of UF as an alternative pretreatment stepfor drinking-water production', which waspresented at the International 1. Blume I. eta/. (1995) Large-scale applications Symposium on Water &Membrane Techniques, for micro- and ultrafiltration in water treatment held in Likge, Belgium on 4-5 April 2000. The - - A new module system. Aachener Membrane edited version which appears here ispublished with Colloquium, Aachen, Germany, March 1995. the permission of the conferenceorganiserand Tribune de l'Eau. For more information, contact: 2. Vos, G. etal. (1996) Membrane filtration as a new technique for recovery of backwash water. Mr D. Van Den Ackerveken, Cebedeau, rue Proceedings of Colloid Science in Membrane Armand St&art 2, B-4000 Likge, Belgium. Tel: +32 4 252 1233. Fax.. +32 4 254 0363. Engineering, Toulouse, France, 1996.
References
I
IIIIII
Patents
3. Baudin I. et al. (1997) L'Apid and Vigneux case studies: first months of operation, Desalination 113(2-3) 273-275. 4. Doyen W. (1997) Latest developments in ultrafiltration for large-scale drinking water applications, Desalination 113(2-3) 165-177. 5. Bade, B. (1998) Karakterisering van organische stoffen in oppervlak-tewater d.m.v. ultrafiltratie. Kiwa/SVW-workshop, Breda, The Netherlands, June 1998. 6. Van Hoof E. et al. (2000) Software controlled pilot testing of pressure driven cross-flow and semi-dead-end membrane filtration. Proceedings of 8th World Filtration Congress, Brighton, UK, April 2000. 7. Doyen W. et al. (1998) Methodology for accelerated pre-selection of UF-type of membranes for large-scale applications, Desalination 117 85-94. 8. Doyen W. (1999) Assessment methods for accelerated selection of UF type of membranes. Proceedings of AWWA Membrane Technology Conference, Long Beach, California, USA, February/March 1999.
concentrate solution compartment is pressurised to create a pressure differential between the concentrate solution compartment and the diluate solution compartnaent. Pressurising the concentrate solution compartment reduces the transport of water across the alternating cationand anion-selective membranes. An applied differential pressure of 0.25-1 atm is created between the concentrate and diluate solution compartments. Patent number: W O 00/25903 Inventors:J.N. Hryn, K. Sreenivasarao, E Patsiogiannis, EE.J. Daniels, D. Graziano Publication date: 11 May 2000
Gas separation device Applicant: MG Generon, USA
I
Details of a bore-side feed-gas separation membrane device are given. It contains hollowfibre membranes with discrete flow channels incorporated into the bundle to allow for pressure removal of the permeate gas which collects on the shell-side of the device. The flow channels minimise pressure drop and/or the build-up of back-pressure on the shell-side of the device while maintaining proper counter-current flow patterns in the device. The incorporation of flow channels results in improved efficiency., especially for feedstreams or membrane types which lead to a high permeate flow-rate that results in an excessive shell-side pressure drop. Patent number: WO 00125897 Inventors: E Coan, T. Jeanes, J.A. Jensvold Publication date: 11 May 2000
@
This article describes membrane research which focuses on exploring the use of open uhrafiltration as an alternative to conventional separation technologies used for producing drinking water. Four different water types, originating from three water sources, are investigated. Four open uhrafiltration membranes are used, of which three are of a hollow-fibre configuration while one is tubular. On the one hand it was found that permeate quality was always very good, compared with conventional techniques - - no bacteria, suspended solids or turbidity were present. Moreover, the quality was independent of operational conditions or the membrane type. On the other hand, it was found that operational conditions strongly differ in terms of 'raw' water quality, or the membrane type used. At about the beginning of the 1990s, ultrafiltration (UF) started being investigatedI141 by the drinking-water industry as an alternative to conventional pretreatment techniques. Since UF membranes are physical barriers which are able to efficiently remove suspended particles, turbidity, bacteria, colloids, algae, parasites and viruses for clarification and disinfection purposes, they may offer a number of advantages. Nowadays, conventional pretreatment includes coagulation, flocculation, sedimentation and/or flotation, rapid and slow sand filtration, and 'multimedia' filtration. Here we describe research which is looking at the feasibility of 'semi-dead-end' UF for the same purpose. It is being managed by the Centre for Water Research & Co-operation of Flanders (SVW). SVW brings together the joint research of all the drinking-water companies in Flanders, in particular Antwerpse Waterwerken (AWW),
Provinciale en Intercommunale Drinkwatermaatschappij der Provincie Antwerpen (PIDPA), Tussengemeentelijke Maatschappij der Vlaanderen voor Watervoorziening (TMVW), and Vlaamse Maatschappij voor Watervoorziening (VMW).
Goals Experiments started in mid-1997, and will end during December 2000. 'Semi-dead-end' UF filtration pilot experiments are being conducted at four locations - - consecutively in Oelegem (AWW), Kluizen (VMW), Grobbendonk (PlDPA), and finally in Korbeek-Dijle (VMW). The specific goal for AWW is to explore possible solutions to problems caused by extensive algae growth under conventional separation. For VMW, the specific target is to search for an alternative to conventional treatment, with
chemical consumption at Kluizen being reasonably high. PIDPA is looking at the behaviour of UF with respect to the water source, that is, from a reservoir or taken directly from Canal Albert. Finally, for V M W in Korbeek-Dijle, the aim is to look at the feasibility of using UF as a means of pretreatment.
Materials and methods
Water types
All of the four water types under investigation are classified as surface water. These will now be described. The first water type originates from Canal Albert, which connects the river Maas with the river Schelde. The location is Oelegem (location #1), which is at a distance of some 20 km from Antwerp. Water is pumped to a reservoir. It is characterised by a low content of low molecularweight components, but sometimes it has high algae content. This means that turbidity may range from 0.5-50 NTU. This is AWW's 'raw' water
source.
The second water type originates from Canal Isabella, and from some small local rivers. The location is Kluizen (location #2), which is at a distance of some 10 km from Ghent. The water is also pumped to a reservoir. The water is very clear (with turbidity ranging from 0.2-2 NTU), and it has quite a high content of low molecularweight components, with the biggest fraction
MembraneTechnologyNo. 126
FEATURE
lying between 1 and 3 kD. I51This results in high dissolved organic components (DOC) and high KMnO 4 consumption. This is VMW's 'raw' water source. The third water type is identical to the first water, but it is taken directly from the canal. The means that the turbidity is much higher, and fluctuates to a greater extent (20-80 NTU). The location is Grobbendonk (location #3), which lies roughly 30 km from Antwerp. This is PIDPA's 'raw' water source. The fourth water type originates from the river Dijle. The location is Korbeek-Dijle (location #4), which is around 10 km from Leuven. It is characterised by a low content of low molecular-weight components, but it has a fluctuating turbidity (from 10 to over 100 NTU) because of the presence of clay particles. This is VMW's second 'raw' water source.
Membrane types Five different types of UF membranes were evaluated. Table 1 shows the characteristics of each of these membranes. The first and second membrane types use the so-called XIGA configuration (8-inch diameter, 40- or 60-inch module length). I41 In this configuration replaceable, membrane-module inserts are mounted in an 8-inch diameter pressure vessel, in a similar way to the approach which is adopted in reverse osmosis spiral technology. Permeate collects in a central permeate tube. The first membrane of this type is a polysulfone hollow-fibre product from Koch Membrane Systems (membrane #1). The membrane has an internal fibre diameter of 0.76 mm. The total effective membrane area is 19 m 2, and the given cut-offvalue is 100 kD. This membrane is one of the first 8-inch membrane insert prototypes. The second membrane is a polyethersulfone/polyvinylpyrrolidone blend hollow-fibre product from X-Flow/Norit (membrane #2), with an internal fibre diameter of 0.8 mm. The given cut-off value is between 150 and 200 kD. Two module configurations were tested. The first one had no bypass, while the second has a 12-chamber design. This is a special development for improving back-wash characteristics. The total effective membrane area amounts to 25 m 2 for the first unit and 22 m 2 for the second. The third and fourth membrane types are not mounted in 8-inch pressure vessels. In this case, the membrane and module form an integral unit. The hollow-fibre material for membrane #3 also comprises a polyethersulfone/ polyvinylpyrrolidone blend. It is manufactured by Akzo Nobel, and potted into modules by Stork Friesland. The internal fibre diameter is 0.7 mm. Fibres are potted in eight separate bundles. This is a special development which improves back-wash characteristics. The total effective membrane area is 25.7 m 2. The given pore diameter for this membrane is 20 nm.
Membrane Technology No. 126
Membrane #4 is a polydivinylidene fluoride/ polyvinylpyrrolidone blend tubular membrane, with an internal composite polyester backing material. Its internal diameter is 5.2 ram, and the total effective membrane area is 10 m 2.
(TMP). A special feature of the units is the MEFIAS real-time data-acquisition and steeringsoftware, [61 which was developed by VITO. The units enable the 'semi-dead-end' and cross-flow experiments described here to be carried out automatically.
Membrane filtration units Two specially built membrane units were used for the filtration experiments. These are mounted in a 6 m (20 ft) container. The units include two separate centrifugal pumps (a feed pump and a back-wash pump), each with a capacity of 6 m2/h at 3 bar. Two plastic tanks, with a capacity of 1500 1 and 500 l, respectively, were used to collect the feed and permeate. The feed-water tank is filled automatically with raw water through a float shut-off valve. Retentate can be drained, or fed back to the feed-water tank (cross-flow mode). Permeate overflow is channelled to the drainage system. Before the raw water enters the unit it is pre-filtered over back-washable 300 pm micro-sieves. Data transmitters are used for the measurement of parameters such as pressure, pH, turbidity and temperature, and to measure permeate and retentate flow, and trans-membrane pressure
Methods
and results
Short-term experiments For a short period ( l - 2 months) a number of exploratory tests were initially performed at each location (Oelegem, Kluizen, Grobbendonk and Korbeek-Dijle), to select the membrane types for the long-term tests. The initial work included membrane integrity tests, comparative crossflow experiments, comparative 'semi-dead-end' experiments, exploration of the use ofFeC13, and the evaluation of permeate quality. The tests which were used are now described.
Membrane integrity tests The purpose of this test is to check for membrane or module defects. It is performed using the purewater flux (PX0ffF) and the pressure hold-up test (PHT). PWF gives information about membrane
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condition (clean/fouled), but is not sensitive enough for an integrity check. It is performed with tap water. Figure 1 shows pure-water fluxes versus TMP for the evaluated membrane types. It was found that the permeability (pure-water flux at 1 bar TMP) of membranes ranges between 250 and 1250 l/h.m2.bar. Membrane #1 has the lowest permeability, whereas membrane #3 has the highest. The P H T is based on the natural phenomenon that air will not pass through a wetted hydrophilic membrane, up to a certain pressure. This is the so-called 'bubble-point' pressure (BPP). So, under the BPP the membrane does hold pressure. BPP is inversely proportional to the pore diameter, and for the membrane types used it is far above 3 bar. In Figure 2, results of the PHT at two different pressures (0.5 and 1 bar, respectively) on membrane #3 are shown. From the Figure the pressure decay can be calculated. The allowable pressure decay at 1 bar is a maximum 10 mbar/min. The same test was performed on membrane #4. It was found that this membrane was
(~
12 Time (h)
damaged at a differential pressure of 1 bar. This is caused by the membrane being released from its polyester backing material. For this reason membrane #4 was not selected for the long-term tests. However, this problem can be overcome by applying pressure at the retentate side. Also, in normal operation using this membrane, on back-flushing attention should be paid to the air present at the permeate side. It is best to use it only in a vertical position, so that the module is automatically degassed. The PHT was performed several times over the two-year period. All membrane types maintained their integrity over this period. In conclusion, the PHT is quite simple, but it is sensitive and capable of detecting very small defects in membranes.
Comparative'semi-dead-end' experiments In these types of experiments an initial idea of the membrane's behaviour can be gained through 'semi-dead-end' operation on surfacewater. The approach basically consists of alternate filtration cycles with raw water, and
back-wash cycles with permeate, for membrane rinsing. This experiment gives information about the fouling properties of the membrane, with respect to raw water, and the back-wash characteristics of the module. The essential part of the measurement is the evolution of the T M P in each filtration cycle, and with time. As a result, membranes which have the lowest interaction with molecules of the feed, will operate with the most stable TMPs. There are two main aspects of the procedure for this experiment with common 'semi-deadend' operation. First, chemicals are not added to the back-wash liquid (chemical cleaning). [7,8] This is essential, because we want to reveal problems related to adsorption. The second aspect is the flux level which is used. This is much higher than in normal operation. Fluxes of consecutively 100, 150 and 200 1/h.m 2 were each used for 24 h of operation, without intermittent cleaning. This allows quick interpretation. All other filtration parameters (hydraulic load, back-wash flux and backwash volume) were kept constant. Hydraulic load - - the volume of raw water per unit of membrane area and per filtration cycle - - was 25 l/m2.cycle. The back-wash volume was limited to 60 1, and the back-wash flux was fixed at 300 1/h.m 2. As a result, water recovery during the test was always 90%. Filtration-time per cycle was adjusted to the flux level used (7.5 min at 200 l/h.m 2, 10 min at 150 l/h.m 2 and 15 min at 100 l/h.m 2, respectively). This is at constant hydraulic load. The experiments were stopped after reaching a maximum TMP of 1 bar (cutoff value), or after 24 h of operation. Figures 3 and 4 illustrate typical results for such tests on surface-water #3 for membranes #2 and #3. It is clear from these Figures that membrane #2 gives the most stable operation, whereas membrane #3 gives an increasing TMP versus time, at higher flux rates. The effect of cleanwater permeability is also reflected in this test. Membrane #3, with the highest permeability, results in the lowest T M P value (0.12 bar instead of 0.22 bar), at the lowest flux level (100 l/h.m2). The less stable operation of membrane #3 can also be seen at higher flow rates (150 and 200 l/h.m2) at the starting TMP, which approaches those of membrane #2. This test was done on all types of surface water. The results which are published in the literature [7'81 cover similar measurements on all types of membrane, with respect to surfacewaters #1 and #2. It was found that membrane #1 strongly suffered from an interaction with raw feed-water. This was illustrated by the fact that the 1 bar TMP cut-off value was reached after few filtration cycles. This was the reason why membrane #1 was not selected for the longterm test.
Membrane Technology No. 126
Comparative cross-flow experiments 40 At locations # l and #2 (Oelegem and Kluizen) a comparative cross-flow experiment was performed. First, raw water concentrates were prepared using the UF membranes. Then, the membranes to be examined were mounted on the filtration system. Subsequently, the flux/TMP profile for a given concentrate was measured under cross-flow conditions at constant linear velocity (for example, 0.2 m/s). This measurement is described in detail further in the literature.[7] Typically a flux/TMP profile consists of two parts - - a horizontal part and a sloping part. The horizontal part is the pressure-independent portion, from which the plateau/limiting flux can be derived. The sloping portion is the pressure-dependent part, and it depends on the membrane's characteristics (cut-off value, surface porosity and permeability). Unfortunately we cannot perform measurements at TMPs below 0.2 bar, so no information was gathered concerning pressure-dependent characteristics. A typical result for membrane #1 covering the first surface-water is shown in Figure 5. Already starting at a T M P of 0.2 bar, the flux is limited to 25 l/h.m 2. A further increase in TMP has no effect on flux. This flux value is the socalled plateau, or limiting flux, of this membrane, for a given feed under given conditions (10°C, 0.2 m/s). In Figure 5 results are also given for other types of membranes which were tested. It is clear that membrane types based on polyethersulfone/polyvinylpyrrolidone (membranes #2 and #3) clearly show higher plateau flux values (35 l/h.m2), compared with the polysulfone-based membrane (25 l/h.m2). This indicates a lower interaction with raw water for both of the membranes which are based on polyethersulfone/polyvinylpyrrolidone. These results confirm most of the observations made by comparative 'semi-deadend' filtration experiments. The findings for the polysulfone-based membrane, in both the 'semidead-end' and cross-flow experiments, are similar, that is, this membrane suffers from stronger adsorption. For surface-water #2, the findings were quite similar, which indicates that the polysulfonebased membrane is a less attractive membrane material for natural-water filtration purposes. For this reason experiments on membrane #1 were stopped, definitively starting from location #3.
Using FeCl 3 The use of coagulation just before UF was also considered. This was started on surface-water #2, containing a high amount of low molecular-weight components. These molecules are prone to foul the internal structure of membranes. The normal operational mode (without FeCI 3 dosing) was
Membrane Technology No. 126
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75 l/h.m 2 at 20 min filtration time. Backwashing took 40 s, resulting in water recovery of 86%. The results of TMP evolution versus time are given in Figure 6. As can be seen here, the TMP fluctuates and rises quite steeply. As a result this operational procedure requires an intensive chemical cleaning procedure. The membranes needed to be cleaned each day with
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NaOC1 (200 ppm of C12), with a soaking period of 1 h in order to restore the original pure-water permeability. This was very stressing for the membranes, for which was claimed a chlorine tolerance of 200 000 ppm.h. Thus, theoretically the membranes would reach the end of their service life after being cleaned 1000 times, or in 1000 days.
@
FEATURE
sampled and analysed on different parameters. It was found that there was hardly any difference in retention for all of the membrane types. The retention for different components is summarised as follows:
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From this it is clear that UF is a very suitable technique for suspended solids removal; however, some dissolved substances, which can be partially removed by coagulation (for example, PO4), are not removed.
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This meant that another filtration procedure had to be found. Initially, experiments were undertaken to find a solution for this problem. It was found that by dosing with FeCl 3 (3 mg Fe3*/l) just in front of the feed pump, the filtration was more stable, and did not show a steep T M P rise with time. This is shown in Figure 7. In this experiment similar filtration and back-wash parameters were used as in the previous one without FeC13 dosing. As a result,
cleaning could be postponed. However, it was found that NaOCI was no longer effective as a cleaning agent. Different acids were tested, and it was found that sulfuric acid was the most effective. The cleaning pH was 1.
Evaluating permeate quality During the 'semi-dead-end' filtration experiments, the respective permeates and retentates of the different membranes were
Membrane #2 was selected for long-term tests at three locations (#1, #2 and #4), because of its superior TMP stability, even without FeCI 3 dosing. Membrane #3 became available from the beginning of 1999, and was selected for longterm tests at location #3, because of its superior pure-water flux. During the long-term tests, all experiments focused on the assessment of seasonal influences on operational parameters, on permeate quality, and on the optimisation of flux and recovery. Also, postponement of cleaning was an important item. Starting with location #2, experiments were done with FeCI3 dosing• Few filtration parameters were fixed - just filtration and back-wash flux, and backwash volume. Values differ between different locations. Table 2 gives an overview of these parameters. Original (starting) values are in parentheses, while optimised values are in bold. If we look through the parameter settings developed for locations #3 and #4, some observations can be made. These are summarised as follows:
MembraneTechnologyNo. 126
Through gaining a better understanding of the filtration process, recovery could be optimised (75% up to 98%). From the optimised flux values we might conclude that either very clear raw water (water type #1 and #2 or the reservoir water), or water contaminated with low molecular-weight components is much more difficult to filter. In the short-term experiments, because FeCI 3 was used, cleaning could be postponed (Figure 7). This was confirmed in the long-term experiments at locations #3 and #4. In Figure 8, the TMP/time profile for location #4 (using membrane #2) is given. It was observed that cleaning could be postponed for two weeks. A two-step cleaning process is being used successfully. It makes use of sodium hypochlorite (200 ppm CI 2) and sulfuric acid (pH = 1). Figure 9 shows the results for membrane #3 at location #3. Here there is a remarkable observation. On using FeCI 3, membrane #3 could also be operated for quite a long time without any cleaning. Cleaning could even be postponed for three weeks. Similar two-step cleaning was applied successfully. This means that specific membrane behaviour, observed during the comparative 'semi-dead-end' experiments described earlier (Figures 3 and 4), and previously in the literature, [7'8] has completely disappeared. From this we may conclude that on using FeCI 3 the type of membrane which is used appears to be of little importance. Thus some conclusions made earlier with respect to this item may have to be revised. The testing procedure which has been described has been applied for more than six months at the time of writing. This proves that membranes can be operated in a stable manner over a long-term period. With respect to permeate quality over the sixmonth period, we can say that it was always equal to results described in short-term experiments. H
Conclusions Ultrafiltration has been shown to be a reliable technique which could replace conventional techniques. Permeate quality is high, and results in a total absence of suspended solids. Dissolved components, however, are not retained. Permeate quality is unaffected by membrane type, the filtration conditions and the raw water composition. By making use of comparative 'semi-dead-end' and cross-flow experiments, membrane types which are less prone to fouling can be identified. Membranes made from a polyethersulfone/polyvinylpyrrolidone blend seem to be most interesting. They have low fouling properties and high permeability. Use of FeC13 facilitates filtration, and helps to optimise recovery and flux level. Moreover, cleaning can be postponed for a period of two to three weeks. A two-step cleaning process with NaOCI/H2SO 4 was found to be efficient for the long-term operation, facing different seasonal influences.
Acknowledgments
Pressure-assisted electrodialysis Applicant: University of Chicago, USA A process of electrodialysis of aqueous fluids, to transfer dissolved salts from a waste, or other solution identified as a 'diluate' solution, into a concentrate solution is discussed. An electrodialysis unit includes a stack of semi-permeable, ionselective membranes, and an applied electric field which causes ions to pass from the diluate solution to the concentrate solution through the ionselective membranes. The electrodialysis unit defines diluate and concentrate solution compartments. These are separated by alternating cation- and anion-selective membranes. Each
Membrane Technology No. 126
For more information, contact: Wire Doyen,VlTO,
We would like to thank the SVW members for Boeretang 200, B-2400 Mol, Belgium. Tel: +32 1433 their operational and financial support of the 5622, Fax:+32 1432 1186, Email: [email protected] research programme. In particular, personnel of i AWW, PlDPA and V M W are acknowledged. Or contact: Betty Bake or Luc Beeusaer~,SVW, We would also like to thank our colleagues Mechelsesteenweg 64, B-2018 Antwerp, Belgium. from VITO, especially Erwin Van Hoof for Tel: +32 3244 0750, Fax:+32 3248 2742, writing the MEFIAS software, and Bart Emaih [email protected] Molenberghs for helping with the start-up of This article is based on a paper entitled 'Exploration the experiments. of the possibilities of UF as an alternative pretreatment stepfor drinking-water production', which waspresented at the International 1. Blume I. eta/. (1995) Large-scale applications Symposium on Water &Membrane Techniques, for micro- and ultrafiltration in water treatment held in Likge, Belgium on 4-5 April 2000. The - - A new module system. Aachener Membrane edited version which appears here ispublished with Colloquium, Aachen, Germany, March 1995. the permission of the conferenceorganiserand Tribune de l'Eau. For more information, contact: 2. Vos, G. etal. (1996) Membrane filtration as a new technique for recovery of backwash water. Mr D. Van Den Ackerveken, Cebedeau, rue Proceedings of Colloid Science in Membrane Armand St&art 2, B-4000 Likge, Belgium. Tel: +32 4 252 1233. Fax.. +32 4 254 0363. Engineering, Toulouse, France, 1996.
References
I
IIIIII
Patents
3. Baudin I. et al. (1997) L'Apid and Vigneux case studies: first months of operation, Desalination 113(2-3) 273-275. 4. Doyen W. (1997) Latest developments in ultrafiltration for large-scale drinking water applications, Desalination 113(2-3) 165-177. 5. Bade, B. (1998) Karakterisering van organische stoffen in oppervlak-tewater d.m.v. ultrafiltratie. Kiwa/SVW-workshop, Breda, The Netherlands, June 1998. 6. Van Hoof E. et al. (2000) Software controlled pilot testing of pressure driven cross-flow and semi-dead-end membrane filtration. Proceedings of 8th World Filtration Congress, Brighton, UK, April 2000. 7. Doyen W. et al. (1998) Methodology for accelerated pre-selection of UF-type of membranes for large-scale applications, Desalination 117 85-94. 8. Doyen W. (1999) Assessment methods for accelerated selection of UF type of membranes. Proceedings of AWWA Membrane Technology Conference, Long Beach, California, USA, February/March 1999.
concentrate solution compartment is pressurised to create a pressure differential between the concentrate solution compartment and the diluate solution compartnaent. Pressurising the concentrate solution compartment reduces the transport of water across the alternating cationand anion-selective membranes. An applied differential pressure of 0.25-1 atm is created between the concentrate and diluate solution compartments. Patent number: W O 00/25903 Inventors:J.N. Hryn, K. Sreenivasarao, E Patsiogiannis, EE.J. Daniels, D. Graziano Publication date: 11 May 2000
Gas separation device Applicant: MG Generon, USA
I
Details of a bore-side feed-gas separation membrane device are given. It contains hollowfibre membranes with discrete flow channels incorporated into the bundle to allow for pressure removal of the permeate gas which collects on the shell-side of the device. The flow channels minimise pressure drop and/or the build-up of back-pressure on the shell-side of the device while maintaining proper counter-current flow patterns in the device. The incorporation of flow channels results in improved efficiency., especially for feedstreams or membrane types which lead to a high permeate flow-rate that results in an excessive shell-side pressure drop. Patent number: WO 00125897 Inventors: E Coan, T. Jeanes, J.A. Jensvold Publication date: 11 May 2000
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