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The XIGA-concept: A new module system for ultrafiltration
The XIGA-Concept: A New Module System for Ultrafiltration racfitional and conventional technologies for drinking water treatment and purification face severe challenges today and in the future. Increasing amounts of a variety of chemical and microbial contaminants, such as salts, pesticides, herbicides, bacteria and viruses, constantly challenge water distributors in maintaining safe and perfect water qualily for us all. This opens up possibilities for the development and emergence of new technologies to aid in achieving this aim. Conventional treatments use various different lines of operation combining coagulation, settling, flocculation and/or sand filtration, mostly requiring the addition of chemicals. Problems encountered by using chemicals to achieve a constant high water quality, running a highly automized plant operation or handling large variations in the water supply are a just a few of the key issues that regularly arise while reading the literature on water treatment I-4, In this article, H.D.W. Roesink, E. Klezcewski and D.M. Koenhen from X-Flow B.V. discuss the use of a novel module type membrane filtration system which uses permanent, hydrophilic capillary membranes (The Xiga System) In a configuration that can operate efficiently under very low pressures. This offers a viable economical option for water engineers in their choice of an optimal filtration and purification system. Several membrane processes have been evaluated and are still under extensive testing for their suitability as treatment steps in the often very complicated treatment arrangements necessary for drinking water. On a large technical scale, only electrodialysis and reverse osmo-
sis have been used up until now in the treatment of drinking water. Processes such as micro-, ultra-, and nanofiltration are on the emerging front but are not very cost-efficient. It is absolutely necessary to reduce these costs, before the above mentioned membrane processes can make a significant impact in the treatment of surface or ground water streams. Besides this factor, process reliability, reduced energy requirements, and extended membrane life are other factors that have to be established before membrane techniques will find increased usage in the large scale applications typically found in water treatment. Microand ultrafiltration technology (MF, UF) will find their use in the pre-treatment of reverse osmosis or electrodialysis steps to reduce the fouling tendency and hence increase life time of the latter. Also, MF and UF act as an absolute barrier for bacteria and viruses. Nanofiltration has found and will find wide-spread use in the reduction of hardness and control of colour and/or dissolved organics. Membrane processes offer the advantage that they can remove a wide variety of constituents in the water and are, therefore, much more capable than conventional technologies of handling the broad diversity of contaminant categories. In contrast to conventional filtration, UF processes represent a distinct barrier to the passage of particles and constituents larger than the membrane pore size, thus they provide a relatively constant quality permeate or filtrate, despite wide variations in raw water quality. (In fact, their inability to discriminate for a specific constituent turns out here to be of advantage. An ultrafiltration membrane with pores of about 20 nm, will approximately retain
everything included in ?he water, regardless of chemistry or microbiology, as long as the constituents are molecules or particufates larger than 20 nm.) Membrane replacement costs and energy costs constitute an important part of the operational costs for the crossflow MF or UF processes s. Crossflow requirements contribute most to the energy costs. However, it is possible to run an MF or UF process more or less in a dead-end mode. For this process, X-Flow developed a novel module type using permanent, hydrophilic capillary membranes. The module design is based upon an 8” element construction that can be placed in standard available spiralwound pressure vessels, typically used in the RO industry. The module is used as a cleanable, (semi) deadended microfiltration element. The concentrate is flushed only periodically. In addition a regular chemical cleaning and or sanitation schedule is required. Based on the above mentioned approach, the operational costs for UltraFiltration are in the range of 0.25 NLG ($0.4) per m3 of treated water. This very competitive cost-price is at least a factor of ten better than the costs incurred using the traditional CrossFlow technique, i.e., a crossflow velocity of 2 m/s and using modules with a membrane area of 10 m’. Main applications for this dead-end ultrafiltration process are in the (pre)-treatment of surface water, tertiary effluent treatment, and in process water recycling of sandfilter backwash water. THE
MEMBRANE
The adsorption of components from the feed is commonly seen as the first step in the (irreversible) fouling mechanism of membranes. It is generally accepted that hydrophilic membranes
show a mucn lower tendency to adsorb (macromolecular) components from the feed 6. Therefore, it is essential to use nonor low-fouling (i.e. hydrophilic) membranes for the above mentioned deadend processes in order to prevent irreversible fouling. Since chemical cleaning is always required, the chemical resistance is also an important condition. In summary, the following desired properties for membranes are: o Hydrophilic good wettability and low fouling properties. * Narrow pore size distribution and high surface porosity. * Chemical resistance to aflow for a wide range of cleaning agents. o Mechanical strength to withstand feedand backwash pressure. Membranes can be produced either as tubes, or as flat sheets. The tubular configuration (including capillaries and hollow fibres) shows typical features such as: = Good flow conditions. Q High area per volume. * Easy cleaning and sterilization. e Backflush capacity. X-Flow owns patents based on which hydrophilic membranes can be produced “‘.!Jsing different polymeric blends, membranes can be produced with varying levels of hydrophilicity. The hydrophilicity is obtained by using water-soluble polymers such as polyvinylpyrrolidone jPVP)Z while polyethersuifone (PES) is used as a hydrophobic component that gives the membranes its mechanical strength, chemical resistance and thermal stability. Summary of specifications for X-Flow capillary tlF and MF membranes: e Membrane - Capillary; externally or internaily skinned.
) Material - Polvethersulfone blend. ) Hydrophilicity - Permanent b Cut-Off - 0.8 micron down to a MWCO of 50 000 Dalton l Inner Diameter - 0.5 - 4 mm l Burst Pressure - > 10 bars THE
MODULE
The use of capillary membranes gives two important advantages: a high membrane area to volume ratio and a good backflush capacity. Based on capillary membranes an 8” element is developed that can be placed in standard 8” pressure vessels, a technology typically found and known in the RO industry for some time now. A bundle of capillaries is embedded in a suitable resin (epoxy, polyurethane). A novelty for this design is that the permeate IS collected in the central tube (again as found in the spiral wound RO and UF soncept), to reduce the pressure drop at the permeate side and to increase the 3ackflush efficiency. Each zlement is lm long (see Figure I), which allows up io six elements to be placed Nn series, creating an Ultra3r Microfiltration module with a membrane area of up [o 150m2, based upon 0.8 mm bore diameter capillaries. Advantages of the new module design: l Compactness: Up to 1000 m* of membrane area per m3 l Low housings costs: Pressure vessel is as standard available l Optimal backflush efficiency: Central permeate tube . Low production costs: Simple, yet effective design THE PROCESS The introduction of CrossFlow Microfiltration (CFMF) as a large-scale filtration process was enabled by the development of highly resistant membranes (polyethersulfone based memcranes, polypropylene, Iolyaramide) and very nuch stimulated by the apllication of the cross-flow system, which had already )een used in ultrafiltration Ipplications. Though the ross-flow effect definitively ‘nhances the performance f ultra- and microfiltration
processes, one major disadvantage hinders the breakthrough for these filtration processes in largescale application: high energy consumption. It is already accepted that UF and MF processes can be performed in a quasi deadend mode. To maintain a stable flux over a long period (typically one month), the concentrate side is flushed periodically with permeate. A typical frequency is every 15 minutes for 30 seconds with a permeate flux of 250-350 l/m*h bar. In addition from time to time (every 2-12 hours) one adds disinfection chemicals (50-200 ppm hydrogenperoxide) to the permeate backwash water. The elements are soaked for a certain period (typically 5-10 minutes) and flushed with permeate. This is called a Chemical Enhanced Backwash (CEB). A more intensive cleaning (e.g., with 500 ppm sodium hypochlorite) is performed based upon a monthly scheme, or at a TMP of 1.5 bar. Advantages of the deadend process:
e . .
Low energy costs Easy to automize. Compact design. RESULTS
The XIGA-concept has been first evaluated using pretreated surface water (Lake Ijsselmeer). It has been clearly demonstrated that on this type of water constant fluxes of 80-90 l/m*h can be achieved with a recovery of 80-90% The XIGA-concept has also been evaluated on sandfilter backwash water. The sandfilter backwash water originates from a groundwater well. Typical features of this type of water are the high loads of iron hydroxide (up to 1000 ppm) and manganese (up to 10 ppm). In this application fluxes are obtained of 90-120 l/m’h, together with a recovery of > 90%, In both applications, the energy consumption is typically 0.2 kWh/m permeate. REFERENCES 1. Filtration and book, Dickenson. Elsevier (1992).
Filters HandCh., 3rd Ed.,
2 Schippers, J C: Application of membrane processes for the preparation of potable water, Seminar Application of rnembrane processes in environmental problems, HdT, Maastricht, NL (19923 3. Aptel, P.: Membrane pressure driven processes in water treatment, i‘n: Membrane Processes in Seoaration and Purification. J.G. Crespo and K. Bijddeke; teds.), Kluwer, (1994). 4. Marquardt K.: Membranprozesse in der Frischwasseraufbereiiung, Getrlnketechnik, No.3, (1992) 90-98. 5. Lazier, J.C.: Membrane processes in municipal water treatment, Proceedings, 12th Membrane TeChnOlOQyfSeparations Planning Conference, Newton (MA) USA, (1994). 6. Mulder, M.H.V.: Energy requirements in membrane separation processes, in: Membrane Processes in Separation and Purifjcation, J.G. Crespo and K. Bijddeker (eds.), Kluwer, (1994). 7. Roesink, H.D.W, et. al.: Characterization of new membrane materials by means of fouling experiments; adsorption of BSA on polyetherimide/polyvinylpyrrolidone membranes, Colloids and Surfaces 55 (1991) 231-243. 8. USP 42798,847; Re 34,296 9. USP 5,076,925
X-Flow B.V., Bedrijvenpark Twente 289, 7602 KK Almelo, The Netherlands. Tel: + 31546 575 202. Fax: +31 546 575 102.
sis have been used up until now in the treatment of drinking water. Processes such as micro-, ultra-, and nanofiltration are on the emerging front but are not very cost-efficient. It is absolutely necessary to reduce these costs, before the above mentioned membrane processes can make a significant impact in the treatment of surface or ground water streams. Besides this factor, process reliability, reduced energy requirements, and extended membrane life are other factors that have to be established before membrane techniques will find increased usage in the large scale applications typically found in water treatment. Microand ultrafiltration technology (MF, UF) will find their use in the pre-treatment of reverse osmosis or electrodialysis steps to reduce the fouling tendency and hence increase life time of the latter. Also, MF and UF act as an absolute barrier for bacteria and viruses. Nanofiltration has found and will find wide-spread use in the reduction of hardness and control of colour and/or dissolved organics. Membrane processes offer the advantage that they can remove a wide variety of constituents in the water and are, therefore, much more capable than conventional technologies of handling the broad diversity of contaminant categories. In contrast to conventional filtration, UF processes represent a distinct barrier to the passage of particles and constituents larger than the membrane pore size, thus they provide a relatively constant quality permeate or filtrate, despite wide variations in raw water quality. (In fact, their inability to discriminate for a specific constituent turns out here to be of advantage. An ultrafiltration membrane with pores of about 20 nm, will approximately retain
everything included in ?he water, regardless of chemistry or microbiology, as long as the constituents are molecules or particufates larger than 20 nm.) Membrane replacement costs and energy costs constitute an important part of the operational costs for the crossflow MF or UF processes s. Crossflow requirements contribute most to the energy costs. However, it is possible to run an MF or UF process more or less in a dead-end mode. For this process, X-Flow developed a novel module type using permanent, hydrophilic capillary membranes. The module design is based upon an 8” element construction that can be placed in standard available spiralwound pressure vessels, typically used in the RO industry. The module is used as a cleanable, (semi) deadended microfiltration element. The concentrate is flushed only periodically. In addition a regular chemical cleaning and or sanitation schedule is required. Based on the above mentioned approach, the operational costs for UltraFiltration are in the range of 0.25 NLG ($0.4) per m3 of treated water. This very competitive cost-price is at least a factor of ten better than the costs incurred using the traditional CrossFlow technique, i.e., a crossflow velocity of 2 m/s and using modules with a membrane area of 10 m’. Main applications for this dead-end ultrafiltration process are in the (pre)-treatment of surface water, tertiary effluent treatment, and in process water recycling of sandfilter backwash water. THE
MEMBRANE
The adsorption of components from the feed is commonly seen as the first step in the (irreversible) fouling mechanism of membranes. It is generally accepted that hydrophilic membranes
show a mucn lower tendency to adsorb (macromolecular) components from the feed 6. Therefore, it is essential to use nonor low-fouling (i.e. hydrophilic) membranes for the above mentioned deadend processes in order to prevent irreversible fouling. Since chemical cleaning is always required, the chemical resistance is also an important condition. In summary, the following desired properties for membranes are: o Hydrophilic good wettability and low fouling properties. * Narrow pore size distribution and high surface porosity. * Chemical resistance to aflow for a wide range of cleaning agents. o Mechanical strength to withstand feedand backwash pressure. Membranes can be produced either as tubes, or as flat sheets. The tubular configuration (including capillaries and hollow fibres) shows typical features such as: = Good flow conditions. Q High area per volume. * Easy cleaning and sterilization. e Backflush capacity. X-Flow owns patents based on which hydrophilic membranes can be produced “‘.!Jsing different polymeric blends, membranes can be produced with varying levels of hydrophilicity. The hydrophilicity is obtained by using water-soluble polymers such as polyvinylpyrrolidone jPVP)Z while polyethersuifone (PES) is used as a hydrophobic component that gives the membranes its mechanical strength, chemical resistance and thermal stability. Summary of specifications for X-Flow capillary tlF and MF membranes: e Membrane - Capillary; externally or internaily skinned.
) Material - Polvethersulfone blend. ) Hydrophilicity - Permanent b Cut-Off - 0.8 micron down to a MWCO of 50 000 Dalton l Inner Diameter - 0.5 - 4 mm l Burst Pressure - > 10 bars THE
MODULE
The use of capillary membranes gives two important advantages: a high membrane area to volume ratio and a good backflush capacity. Based on capillary membranes an 8” element is developed that can be placed in standard 8” pressure vessels, a technology typically found and known in the RO industry for some time now. A bundle of capillaries is embedded in a suitable resin (epoxy, polyurethane). A novelty for this design is that the permeate IS collected in the central tube (again as found in the spiral wound RO and UF soncept), to reduce the pressure drop at the permeate side and to increase the 3ackflush efficiency. Each zlement is lm long (see Figure I), which allows up io six elements to be placed Nn series, creating an Ultra3r Microfiltration module with a membrane area of up [o 150m2, based upon 0.8 mm bore diameter capillaries. Advantages of the new module design: l Compactness: Up to 1000 m* of membrane area per m3 l Low housings costs: Pressure vessel is as standard available l Optimal backflush efficiency: Central permeate tube . Low production costs: Simple, yet effective design THE PROCESS The introduction of CrossFlow Microfiltration (CFMF) as a large-scale filtration process was enabled by the development of highly resistant membranes (polyethersulfone based memcranes, polypropylene, Iolyaramide) and very nuch stimulated by the apllication of the cross-flow system, which had already )een used in ultrafiltration Ipplications. Though the ross-flow effect definitively ‘nhances the performance f ultra- and microfiltration
processes, one major disadvantage hinders the breakthrough for these filtration processes in largescale application: high energy consumption. It is already accepted that UF and MF processes can be performed in a quasi deadend mode. To maintain a stable flux over a long period (typically one month), the concentrate side is flushed periodically with permeate. A typical frequency is every 15 minutes for 30 seconds with a permeate flux of 250-350 l/m*h bar. In addition from time to time (every 2-12 hours) one adds disinfection chemicals (50-200 ppm hydrogenperoxide) to the permeate backwash water. The elements are soaked for a certain period (typically 5-10 minutes) and flushed with permeate. This is called a Chemical Enhanced Backwash (CEB). A more intensive cleaning (e.g., with 500 ppm sodium hypochlorite) is performed based upon a monthly scheme, or at a TMP of 1.5 bar. Advantages of the deadend process:
e . .
Low energy costs Easy to automize. Compact design. RESULTS
The XIGA-concept has been first evaluated using pretreated surface water (Lake Ijsselmeer). It has been clearly demonstrated that on this type of water constant fluxes of 80-90 l/m*h can be achieved with a recovery of 80-90% The XIGA-concept has also been evaluated on sandfilter backwash water. The sandfilter backwash water originates from a groundwater well. Typical features of this type of water are the high loads of iron hydroxide (up to 1000 ppm) and manganese (up to 10 ppm). In this application fluxes are obtained of 90-120 l/m’h, together with a recovery of > 90%, In both applications, the energy consumption is typically 0.2 kWh/m permeate. REFERENCES 1. Filtration and book, Dickenson. Elsevier (1992).
Filters HandCh., 3rd Ed.,
2 Schippers, J C: Application of membrane processes for the preparation of potable water, Seminar Application of rnembrane processes in environmental problems, HdT, Maastricht, NL (19923 3. Aptel, P.: Membrane pressure driven processes in water treatment, i‘n: Membrane Processes in Seoaration and Purification. J.G. Crespo and K. Bijddeke; teds.), Kluwer, (1994). 4. Marquardt K.: Membranprozesse in der Frischwasseraufbereiiung, Getrlnketechnik, No.3, (1992) 90-98. 5. Lazier, J.C.: Membrane processes in municipal water treatment, Proceedings, 12th Membrane TeChnOlOQyfSeparations Planning Conference, Newton (MA) USA, (1994). 6. Mulder, M.H.V.: Energy requirements in membrane separation processes, in: Membrane Processes in Separation and Purifjcation, J.G. Crespo and K. Bijddeker (eds.), Kluwer, (1994). 7. Roesink, H.D.W, et. al.: Characterization of new membrane materials by means of fouling experiments; adsorption of BSA on polyetherimide/polyvinylpyrrolidone membranes, Colloids and Surfaces 55 (1991) 231-243. 8. USP 42798,847; Re 34,296 9. USP 5,076,925
X-Flow B.V., Bedrijvenpark Twente 289, 7602 KK Almelo, The Netherlands. Tel: + 31546 575 202. Fax: +31 546 575 102.