- Email: [email protected]
US Army reverse osmosis membrane research programs
Desalination, 99 (1994) 383-399 Elsevier Science B.V. Amsterdam -
383 Printed in The Netherlands
US Army reverse osmosis membrane research programs Edward A. Downing, Amos J. Coleman and Thomas H. Bagwell, Jr. USAmy Tank-Automotive Command, Mobility Technology Center-Belvoir, 10101 Grid&v Road, Suite 104, Fort Belvoir, VA 22060-5818 (USA) Tel (703) 704-3352; Fax (703) 704-3360
SUMMARY
The Water Technology R&D Team of the Mobility Technology Center, US Army Tank Automotive and Armament Command, has the mission to dvelop mobile water purification equipment to support army tactical operations. The current mobile water purifiers use reverse osmosis technology to desalinate raw water. The most significant problem encountered during 15 years of operations is membrane surface fouling. The water technology R&D team has sponsored several efforts to overcome the water production shortfalls caused by membrane fouling.
INTRODUCTION
The Water Technology Research and Development Team of the Tank and Automotive Command (TACOM) has the responsibility to support and improve military field water supply operations. Our team explores new technologies and improves existing equipment to increase the overall combat readiness and operation of military water supply efforts.
OOll-9164/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZOOll-9164(94)00190-l
384
Future systems will operate with fewer operators, less power, and contain smaller and lighter components and subsystems. In 1979 the Army adopted the reverse osmosis (RO) process to desalinate and treat water. Assorted sizes of the RO water purification unit (ROWPU) were developed, tested and found to be effective for producing potable water in the field. The ROWPU system effectively treats most ground and surface water sources including polluted fresh water; seawater; and water contaminated with nuclear, biological and chemical (NBC) agents. Nevertheless, extended operations have uncovered some areas for improvement. A large part of our research activities has been devoted to upgrading components as well making general improvements. The most prominent problem has been a degradation in water production due to an excessive amount of colloidal and/or biofouling of the RO elements. A substantial portion of the Army’s RO research programs is directed toward improving elements by formulating improved polymers for membranes and coagulants; designing improved spacers with antifouling characteristics; evaluating alternate geometric configurations for the present membranes; and devising better techniques for cleaning, preserving and storing elements. Most of the research efforts are contractual and are selected from proposals received from Broad Agency Announcements (BAA) and Small Business Innovation Research (SBIR) solicitations. Small specialty research firms and universities have been used for the most part; however, a few exploratory development projects are carried out inhouse.
THE PROBLEMS
Organic, inorganic and colloidal fouling The most persistent problem with the Army’s ROWPUs has been the reduction in potable water production and the increase in logistical costs caused by RO element fouling from operations on fresh water sources. These fouling problems have been caused by fine silt, silica, iron oxides, calcium carbonate, heavy metal sulfates, high molecular weight organics and biological material collecting on and adhering to the surface of the membrane. Frequent and extensive membrane cleaning procedures must be carried out in order for the ROWPU to remain operational [ 1,2].
38.5
The Army’s ROWPUs, unlike municipal water purification plants, must be capable of making potable water from fresh, brackish and sea waters with unknown compositions. Furthermore, the inherent intermittent mode of operation reduces system performance. The units must also meet stringent size, weight and transportability requirements. Consequently, it has been impossible to design a pretreatment system that will provide high quality feed water for RO modules when operating over the wide range of possible water sources. The estimated effective lifetime of the elements has been reduced from the design goal of 2000 h to a low of 200 h. As a result of this irreversible fouling, a large portion of the Army’s research resources have been devoted to developing technology that will make RO elements resistant to organic, inorganic and colloidal fouling. Biofouling A second problem the Army has is biofouling of the RO elements. Biofouling is the attachment and growth of microorganisms on the surface of the membrane. During normal operations, organic and inorganic matter collects in feed channels and serves as nutrients for microbial growth. Microorganisms that are found naturally in the feed waters attach themselves to the surface of the RO membrane by secreting a polysaccharide fluid. This organic material is extremely difficult to remove. Biological fouling also occurs during storage of both new and used RO elements. Microorganisms contaminate new elements during construction and quality assurance testing. Used elements are contaminated with microbes during operations. The microbes propagate during storage using the organic membrane material as food. Attached microbial growth causes flow restriction, membrane destruction, reduction of active membrane surface area and changes in water chemistry. One method to control microbial growth during operation is to add a biocide to the feed water or frequently clean the element with a disinfectant. The disinfectant presently available in the field is calcium hypochlorite which is used in the post-treatment of the product water. Unfortunately, chlorine attacks and destroys the membrane. The development of chlorineresistant membranes will allow chlorine to be used as a biocide to reduce biofouling of elements during both operation and storage.
RESEARCH
PROGRAMS
The Army’s research program is predominantly centered around improvements to the RO membrane and element through the reduction in and removal of fouling materials from the surface of the RO membrane. The specific projects are: l l l l l l l l l
Development of chlorine-resistant and non-fouling RO membranes Development of improved coagulating agents for water treatment Development of innovative thin-film composite membranes Evaluation of standard RO element feed spacer configuration Development of new RO element feed spacer configuration Development of a new membrane module configuration Identification of commercial biocides for RO element storage Evaluation and selection of biocides for ROWPU storage Development of a membrane separation technique using Donnan exclusion
The list of reports generated from this research has been included in the reference guide. These projects are summarized below. Development of chlorine-resistant and non-fouling RO membranes Southern Research Institute (SRI) recently completed a research project with the objective of developing a thermally stable, chlorine-resistant RO membrane [3]. The SRI approach was to develop a monomer with attached functional groups which have little or no reaction with chlorine and no aliphatic hydrogens, a suspected point of chlorine attack. The monomer was based on a tetra-ester of dis-phenylene-oxazole. The polymer would be created by reacting the monomer with a diamine (Fig. 1) creating a polymer with phenylene, oxazole, imide and amide groups. The investigators could not stabilize the target monomer; however, a variation of the target monomer, a di-amino derivative was developed (Fig. 2). The new monomer had good water stability which could lead to a polymer with high thermal stability, good chlorine resistance, and high water permeability.
387
l+N-R-NH2
Fig. 1. Bis-phenylene-oxazole
polymerization
reaction.
Fig. 2. Di-amino derivative target monomer and desired polymer.
Development of a new~occulentkoagulant agent SRI was cantracted to develop a water soluble copolymer with pendant crown ether groups. The copolymers were poly-diallyldimethyl ammonium chloride (DADMAC) and poly-diallylmethyl-(2-[1-aza-18-crown-61) ethyl ammonium chloride (DAMACEAC) [4]. The heavy metal ion-complexing crown groups were hypothesized to enhance flocculation. A 9:l
388
DADMAUDAMACEAC copolymer was tested on an aqueous solution containing bovine serum albumin as a model contaminant. The copolymer was observed to be more effective in flocculating the contaminant when compared to the control, poly-DADAMC.
Development of innovative thin-film composite membranes Separation Systems Technology, Inc. (SST) was contracted to developed a new “mosaic” thin-film composite membrane deposition process using oxidation-resistant polymers [5]. The mosaic deposition is accomplished by either interfacial polymerization or solution deposition of a monomer inside the pores of the polymeric support material. SST was successful at placing cellulose acetate, sulfonated polyethersulfone, polypiperazine, and polyamide membranes inside the pores of microporous membrane support material. Therefore, membranes can be tailored for a specific separation by placing different polymeric materials inside the microporous structure. Examples of tailored parameters include enhanced selectivity, elevated temperature performance, increased flux and oxidant resistance. SST hypothesized that by placing the membrane inside the pore structure, the thickness of the membrane layer could be reduced resulting in increased water flux and membrane durability (Fig. 3a, 3b). Additional findings included an improved procedure for fabricating the microporous support material. SST developed a method for increasing the pore density, decreasing the pore diameter distribution and retaining the pore structure at the surface of the membrane during temperature curing. SST demonstrated that the mosaic membrane had comparable water permeation and rejection rates to commercially available nanofiltration membranes. Furthermore, the mosaic membranes have the added benefit of oxidant resistance. The polypiperazine membrane had specific rejections of 85 % and 99% on NaCl and MgSO,, respectively. Future research will focus on further characterization of the process variables for both the microporous support structure and the mosaic materials, fabrication of a spiral wound element and demonstration of a low-pressure nanofiltration pilot system.
389
IMosaic Thin Film,
e
Non-porous region
Porous Support Membrane Fig. 3a. Mosaic thin-film membrane.
itional Thin Film
Non-porous region
Porous Support Membrane Fig. 3b. Traditional thin-film membrane.
Evaluation of standard RO element feed spacer contguration SST conducted a rudimentary comparison of the standard Naltex ROF031 (31 mil) spacer (Fig. 4) to the Naltex NWM-045. (45 mil) (Fig. 5) and the Conwed ON 3355 (48 mil) spacer (Fig. 6) [6]. Both Naltex spacers use the “diamond lattice” structure and the Conwed spacer uses the “ladder” configuration. The focus of the study was to compare fouling tendencies, cleanability and water flux of the different spacer configurations.
390
Fig. 4. Standard spacer -
Fig. 5. Alternative
spacer -
Naltex ROF-031 (31 mil).
Naltex NWM-045 (45 mil).
391
Fig. 6. Alternative
spacer - Conwed ON-3355 (48 mil).
SST conducted a fluid dynamic evaluation of each spacer and determined that the linear velocities for each spacer over a range of volume flow rates at fixed exit pressures were very similar; however, the flow resistance was greatest for the Naltex NWM-031 and least for the Conwed ON-3355. Dye tests were conducted using a test cell with a “window”, Flow patterns were recorded on video tape, and no turbulence was observed in any case which correlated with the theoretical calculations. Cleaning efficiency studies were conducted using the window test cell. It was observed that the Conwed spacer permitted better cleaning than either Naltex spacer. Development of a new RO element feed spacer configuration This research effort is a follow-on project to SST’s evaluation of the standard spacer [7]. SST initially conducted a patent search of existing spacer configurations which helped them to identify and rank spacer structural and operational characteristics to optimize spacer design. A subcontractor was hired to develop computer-aided design drawings of the proposed spacer configurations. A second contractor was hired to perform detailed finite element analyses of the mass, momentum and energy balances for each proposed spacer configuration.
392
The patent search identified one basic technology used by three major spacer manufacturers to fabricate spacers: extrusion. However, it was concluded that injection molding may be a better, although a more costly, method of fabricating uniform spacers. Furthermore, most of the patented designs had not been reduced to practice. SSI designed approximately 20 new feed channel spacers which were compared to a.standard Nelle ROF031 diamond lattice spacer. Twelve of the new designs were based on a ladder configuration, each with a filament thickness of less than 31 mil. The standard spacer configuration and one new configuration (Figs. 7 and 8) were chosen for finite element analysis due to funding constraints. The finite element analysis was developed to determine the spacer’s effects on concentration polarization, fouling rate, pressure drop and cleaning efficiency. Both two- and three-dimensional analyses were performed. Plots of flow velocities in the two dimensional analysis for the standard spacer (Fig. 9) and the new spacer (Fig. 10) were developed as well as pressure and shear plots. One key observation was that the standard spacer velocity pattern identified a recirculation region after each filament which covered approximately 75 % of the distance to the next filament. This same recirculation zone was present in the new spacer configuration; however, it was minimal in length and was isolated from the membrane surface. This effort identified potential improvements to RO spacer configurations and innovative computer analysis techniques.
Fig. 7. Standard
spacer configuration.
393 Transverse Stems NOTE: See Sheet Design 4 - B _ 1 for actual cell dimensions
Direction
of Spiral Winding
Direction $ Flow
Fig. 8a. Innovative spacer design configuration.
Section A - A Transverse
Stem
Parallel Filament
Fig. 8b. Innovative
spacer, magnified view.
Parallel Filaments
Fig. 9. Flow velocity
Fig.
10. Flow velocity
profile of standard
profile
spacer.
of innovative
spacer.
Development of a new RO membrane module configuration A developmental effort by Zenon Environmental, Inc. explored an alternative configuration to spiral wound elements [8]. These modules consist of hollow fibers constructed with existing membrane materials, arranged in a cross-flow pattern (Fig. 11). This innovative configuration is designed to induce turbulence in the feed water, reduce membrane fouling, increase membrane packing density and reduce pretreatment requirements. The technical effort began with the testing of nanofiltration membranes in the new configuration. A comparison of the Zenon configuration (salt rejection of 35%) with two different spiral wound nanofilters (salt rejection of 57% and 90%) was conducted using a 5 micron pre-filter on the spiral wound test loop and a 200 micron pre-filter on the transverse module test loop. After 5000 h of continuous testing, the transverse flow configuration had a higher water flux than the spiral wound nanofilters. Furthermore, the 5 micron cartridge filters required changing on 114 occasions while the 200 micron filter was replaced only twice. The second portion of the effort was to develop an RO membrane in the transverse flow configuration. The initial objective was to develop a fiber that could withstand an operating pressure of 1000 psi. Both reinforced
395
Fig. 11. Transverse
flow hollow fiber membrane module.
and unreinforced support fibers were fabricated from polysulfone, polyether sulfone and polyetheretherketone. In all unreinforced support fiber cases, either the burst pressure was too low or the water permeation rate was too low. For the reinforced fibers, several formulations had sufficient burst pressure and water permeation rates to warrant continued experimentation. The third task was to develop a coating technique capable of placing a thin-film composite membrane on the outside of the fiber, resulting in a membrane module that could produce sufficient amounts of drinking quality water. Two coating methods were investigated: solvent deposition and interfacial polymerization. The solvent deposition method was performed using a Zenon chlorine-resistant membrane coating (LTMX) on both reinforced polyethersulfone and reinforced polysulfone blend fibers, both of which had low sodium chloride rejections. A proven idamime/triacyl chloride interfacial polymerization film was also placed on the reinforced polysulfone fiber; however, salt rejection levels did not exceed 88 % s The final task in this project was to modify the ultrafiltration module to withstand higher pressures for use in desalination processes. Several module modifications were analyzed; however, funding restraints terminated all scheduled work before a solution could be found. Zenon demonstrated that the transverse flow configuration reduces fouling and pretreatment needs. However, there was insufficient evidence to make conclusions concerning the feasibility of using the transverse flow configuration in a RO process.
396
Ident@cation of commercial biocides for RO element storage A phase I SBIR with SST was completed in 1992 with promising results. The initial effort was to identify commercially available biocides through literature searches, chemical company surveys, university surveys and interviews with known microbiological experts 191. After screening the results of the biocide investigation, 16 biocides were chosen for compatibility testing. Included in the 16 biocides (chemicals and processes) were: gamma radiation, quaternary ammonium compounds, ethylene diaminetetraacetic acid (EDTA), benzoic acid, boric acid, sorbic glycerine, sodium acid, salicylic acid, methylchloroisothiazolinone, bisulfite, glutaraldehyde, formaldehyde and several other compounds or combinations of the compounds. The second phase of the effort was to evaluate the compatibility of the biocides with both the Filmtec FT-30 and the Fluid Systems TFC membranes. This test was to determine if the biocide had a detrimental effect on the membrane’s water flux or salt rejection. Initial results found gamma radiation to be compatible with both membrane materials. Compatible chemical b&ides included isothiazolinone, EDTA, sodium benzoate/ EDTA, sodium bisulfite/glycerin and salicylic acid. The third phase of the effort was to evaluate the effectiveness of the biocide to prevent regrowth of bacteria on biofouled RO membranes. Fourteen of the initial 15 chemicals were investigated at multiple concentrations and mixture ratios to determine minimum inhibitory concentrations. The biocides which exhibited the greatest degree of growth inhibition included EDTA, sodium benzoate/EDTA, benzalkonium chloride/EDTA, glutaraldehyde, benzalkonium chloride, isothiazolinone and cetyltrimethylammonium toluene sulfonate. The least effective biocides included salicylic acid, sorbic acid and bromochlorodimethyl hydantoin. SST identified a list of candidate biocides for future research and developed a procedure for testing biological growth inhibition. Furthermore, SST identified gamma radiation as a viable method for sterilizing packaged RO elements intended for long-term storage. Evaluation and selection of biocides for ROWPU element storage The promising results from the previous phase I SBIR effort encouraged the Navy, Bureau of Reclamation and SST to combine resources with the Army to expand the biocide effort with the objectives of screening new
397
b&ides, performing long-term storage tests on RO elements and investigating the use of gamma radiation as a post-packaging sterilization technique [lo]. SST modified the effort to include improvements to the test procedures, total of four membrane for evaluation (FT-30, CPA2, TFCL, and CA-B) and a total of 73 potential biocides for evaluation. The initial results indicate that gamma radiation is an effective biocide and is compatible with membranes in combination with inorganic free radical scavengers in solutions. Organic free radical scavengers in combination with the gamma radiation reduced the water flux and salt rejection of the membranes. To date, only 42 of the candidate b&ides have undergone bacteria biocide testing using membrane swatches. The tests included candidate biocide dilutions of 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024:l. Additional dilution tests were performed if complete kills were attained at 1024:l dilution. The most promising biocides identified to date are benzalkonium chloride, EDTA/benzalkonium chloride, MTC-1OWB (trade formula), WSCP (trade formula) and Antocil 1B (trade formula). Spiral wound element storage testing was initiated for candidate biotides. The periods of storage were 2 weeks and 3 months. The candidates with the best bactericide and fungicide properties to date were hydrogen peroxide, sodium bisulfite, EDTA/hydrogen peroxide, EDTA/benzalkonium chloride and sodium bisultite/glycerin. However, the hydrogen peroxide and EDTA/benzalkonium chloride solutions had adverse effects on membrane flux and rejection.
Development of a membrane separation technique wing Donnun exclusion Soane Technologies, Inc. has explored the use of Donnan ion exclusion with thin polyelectrolyte films [ll]. This principle is based on maintaining solution electroneutrality. The process uses a highly charged membrane to repel like charged ionic species. Mobile counter-ions in solution are allowed to permeate the membrane; however, mobile co-ions are not (Fig. 12). At some point an electrochemical equilibrium between the membrane and solution is achieved. Any additional co-ions that are repelled by the membrane cause the counter-ion to be repelled to maintain the electroneutrality [12]. Soane Technologies has been contracted to formulate a highly charged, high water flux membrane that can be used to desalinate seawater.
398
STI’ s Technology Micro-size? contaminants
093 ct
NC
..;.: .._.._.... : .::_;.:.: _.I
I_,
;.:.:,:.:.:.;.
‘.~.~.~_‘_‘.~,~_‘.‘.~_ . . . _ ._. .
Doman Exdusion Principle
r
STh charged polymer
supporting membrane
Direction of solution pow
Fig. 12. Soane Technology charged membrane.
399
To date, Soane Technologies has fabricated a thin-film polyelectrolyte polystyrene sulfonate using either deposition of uncrosslinked polymer from solution or casting techniques. The thin-film composite membrane can be described as a very “loose” hydrated polymer gel with a large water permeation rate coated on a microporous support. The microporous support materials investigated include cellulose, polysulfone and nylon. The results indicate NaCl rejections can be achieved using the fabricated membranes; however, vast improvements to the membranes must be made before this process could be used for seawater desalination.
REFERENCES 1 C.E. Milstead and R.L. Riley, Development of an improved cleaning solution for ROWPU units phase I final report. US Army Report, Contract DAAK70-87-C-0065, 1988. 2 C.E. Milstead and R.L. Riley, Development of an improved cleaning solution for ROWPU units phase II final report. US Army Report, Contract DAAK70-89-C-0055, 1993. 3 A.C. Ibay, Developmentof chlorine-resistantand non-fouling reverse osmosis membranes for water treatment final technical report, US Army Report, Contract DAAK70-92-C0050, 1993. 4 L.P. Tenney, Subject, development of chlorine-resistant and non-fouling reverse osmosis membranes for water treatment. Personal communication, *March 3, 1994. 5 R.L. Riley and C.E. Milstead, Development of oxidation-resistant mosaic membranes with enhanced transport properties. Proposal under Department of Defense Small Business Innovation Research Program, 199 1. 6 R.L. Riley and C.E. Milstead, Evaluation of RO feedwater spacers final report. US Army Report, Contract DAAK70-9 l-P-0581, 1992. 7 C.E. Milstead and R.L. Riley, Development of a new feed channel spacer for reverse osmosis elements, Phase I final report. US Navy Report, prepared under contract N47408-92-C-7011, 1993. 8 K.P. Goodboy, K. Volchek, M. Mahendran, S.T. Pedersen and A. Deutschmann, A foulant-resistant reverse osmosis element based on flow transverse to hollow fibers. US Army Report, Contract DAAK70-92-C-0034, 1993. 9 C.E. Milstead, H.F. Ridgway and R.L. Riley, Identification and evaluation of biocides for ROWPU systems phase I final report. US Army Report, Contract DAAK70-91-C0065, 1992. 10 C.E. Milsted and R.L. Riley, Identification and evaluation of biocides for ROWPU systems, Phase II interum report. US Army Report prepared under contract DAAK70-93c-0007, 1993. 11 G. Fichter, Subject, research synopsis/novel reverse osmosis membranes. Personal communication, March 3, 1994. 12 H. Strathmann, Theory, in: Membrane Handbook, W.S. Winston Ho and K.K. Sirkar (eds.), Van Nostrand Reinhold, New York, 1992.
383 Printed in The Netherlands
US Army reverse osmosis membrane research programs Edward A. Downing, Amos J. Coleman and Thomas H. Bagwell, Jr. USAmy Tank-Automotive Command, Mobility Technology Center-Belvoir, 10101 Grid&v Road, Suite 104, Fort Belvoir, VA 22060-5818 (USA) Tel (703) 704-3352; Fax (703) 704-3360
SUMMARY
The Water Technology R&D Team of the Mobility Technology Center, US Army Tank Automotive and Armament Command, has the mission to dvelop mobile water purification equipment to support army tactical operations. The current mobile water purifiers use reverse osmosis technology to desalinate raw water. The most significant problem encountered during 15 years of operations is membrane surface fouling. The water technology R&D team has sponsored several efforts to overcome the water production shortfalls caused by membrane fouling.
INTRODUCTION
The Water Technology Research and Development Team of the Tank and Automotive Command (TACOM) has the responsibility to support and improve military field water supply operations. Our team explores new technologies and improves existing equipment to increase the overall combat readiness and operation of military water supply efforts.
OOll-9164/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZOOll-9164(94)00190-l
384
Future systems will operate with fewer operators, less power, and contain smaller and lighter components and subsystems. In 1979 the Army adopted the reverse osmosis (RO) process to desalinate and treat water. Assorted sizes of the RO water purification unit (ROWPU) were developed, tested and found to be effective for producing potable water in the field. The ROWPU system effectively treats most ground and surface water sources including polluted fresh water; seawater; and water contaminated with nuclear, biological and chemical (NBC) agents. Nevertheless, extended operations have uncovered some areas for improvement. A large part of our research activities has been devoted to upgrading components as well making general improvements. The most prominent problem has been a degradation in water production due to an excessive amount of colloidal and/or biofouling of the RO elements. A substantial portion of the Army’s RO research programs is directed toward improving elements by formulating improved polymers for membranes and coagulants; designing improved spacers with antifouling characteristics; evaluating alternate geometric configurations for the present membranes; and devising better techniques for cleaning, preserving and storing elements. Most of the research efforts are contractual and are selected from proposals received from Broad Agency Announcements (BAA) and Small Business Innovation Research (SBIR) solicitations. Small specialty research firms and universities have been used for the most part; however, a few exploratory development projects are carried out inhouse.
THE PROBLEMS
Organic, inorganic and colloidal fouling The most persistent problem with the Army’s ROWPUs has been the reduction in potable water production and the increase in logistical costs caused by RO element fouling from operations on fresh water sources. These fouling problems have been caused by fine silt, silica, iron oxides, calcium carbonate, heavy metal sulfates, high molecular weight organics and biological material collecting on and adhering to the surface of the membrane. Frequent and extensive membrane cleaning procedures must be carried out in order for the ROWPU to remain operational [ 1,2].
38.5
The Army’s ROWPUs, unlike municipal water purification plants, must be capable of making potable water from fresh, brackish and sea waters with unknown compositions. Furthermore, the inherent intermittent mode of operation reduces system performance. The units must also meet stringent size, weight and transportability requirements. Consequently, it has been impossible to design a pretreatment system that will provide high quality feed water for RO modules when operating over the wide range of possible water sources. The estimated effective lifetime of the elements has been reduced from the design goal of 2000 h to a low of 200 h. As a result of this irreversible fouling, a large portion of the Army’s research resources have been devoted to developing technology that will make RO elements resistant to organic, inorganic and colloidal fouling. Biofouling A second problem the Army has is biofouling of the RO elements. Biofouling is the attachment and growth of microorganisms on the surface of the membrane. During normal operations, organic and inorganic matter collects in feed channels and serves as nutrients for microbial growth. Microorganisms that are found naturally in the feed waters attach themselves to the surface of the RO membrane by secreting a polysaccharide fluid. This organic material is extremely difficult to remove. Biological fouling also occurs during storage of both new and used RO elements. Microorganisms contaminate new elements during construction and quality assurance testing. Used elements are contaminated with microbes during operations. The microbes propagate during storage using the organic membrane material as food. Attached microbial growth causes flow restriction, membrane destruction, reduction of active membrane surface area and changes in water chemistry. One method to control microbial growth during operation is to add a biocide to the feed water or frequently clean the element with a disinfectant. The disinfectant presently available in the field is calcium hypochlorite which is used in the post-treatment of the product water. Unfortunately, chlorine attacks and destroys the membrane. The development of chlorineresistant membranes will allow chlorine to be used as a biocide to reduce biofouling of elements during both operation and storage.
RESEARCH
PROGRAMS
The Army’s research program is predominantly centered around improvements to the RO membrane and element through the reduction in and removal of fouling materials from the surface of the RO membrane. The specific projects are: l l l l l l l l l
Development of chlorine-resistant and non-fouling RO membranes Development of improved coagulating agents for water treatment Development of innovative thin-film composite membranes Evaluation of standard RO element feed spacer configuration Development of new RO element feed spacer configuration Development of a new membrane module configuration Identification of commercial biocides for RO element storage Evaluation and selection of biocides for ROWPU storage Development of a membrane separation technique using Donnan exclusion
The list of reports generated from this research has been included in the reference guide. These projects are summarized below. Development of chlorine-resistant and non-fouling RO membranes Southern Research Institute (SRI) recently completed a research project with the objective of developing a thermally stable, chlorine-resistant RO membrane [3]. The SRI approach was to develop a monomer with attached functional groups which have little or no reaction with chlorine and no aliphatic hydrogens, a suspected point of chlorine attack. The monomer was based on a tetra-ester of dis-phenylene-oxazole. The polymer would be created by reacting the monomer with a diamine (Fig. 1) creating a polymer with phenylene, oxazole, imide and amide groups. The investigators could not stabilize the target monomer; however, a variation of the target monomer, a di-amino derivative was developed (Fig. 2). The new monomer had good water stability which could lead to a polymer with high thermal stability, good chlorine resistance, and high water permeability.
387
l+N-R-NH2
Fig. 1. Bis-phenylene-oxazole
polymerization
reaction.
Fig. 2. Di-amino derivative target monomer and desired polymer.
Development of a new~occulentkoagulant agent SRI was cantracted to develop a water soluble copolymer with pendant crown ether groups. The copolymers were poly-diallyldimethyl ammonium chloride (DADMAC) and poly-diallylmethyl-(2-[1-aza-18-crown-61) ethyl ammonium chloride (DAMACEAC) [4]. The heavy metal ion-complexing crown groups were hypothesized to enhance flocculation. A 9:l
388
DADMAUDAMACEAC copolymer was tested on an aqueous solution containing bovine serum albumin as a model contaminant. The copolymer was observed to be more effective in flocculating the contaminant when compared to the control, poly-DADAMC.
Development of innovative thin-film composite membranes Separation Systems Technology, Inc. (SST) was contracted to developed a new “mosaic” thin-film composite membrane deposition process using oxidation-resistant polymers [5]. The mosaic deposition is accomplished by either interfacial polymerization or solution deposition of a monomer inside the pores of the polymeric support material. SST was successful at placing cellulose acetate, sulfonated polyethersulfone, polypiperazine, and polyamide membranes inside the pores of microporous membrane support material. Therefore, membranes can be tailored for a specific separation by placing different polymeric materials inside the microporous structure. Examples of tailored parameters include enhanced selectivity, elevated temperature performance, increased flux and oxidant resistance. SST hypothesized that by placing the membrane inside the pore structure, the thickness of the membrane layer could be reduced resulting in increased water flux and membrane durability (Fig. 3a, 3b). Additional findings included an improved procedure for fabricating the microporous support material. SST developed a method for increasing the pore density, decreasing the pore diameter distribution and retaining the pore structure at the surface of the membrane during temperature curing. SST demonstrated that the mosaic membrane had comparable water permeation and rejection rates to commercially available nanofiltration membranes. Furthermore, the mosaic membranes have the added benefit of oxidant resistance. The polypiperazine membrane had specific rejections of 85 % and 99% on NaCl and MgSO,, respectively. Future research will focus on further characterization of the process variables for both the microporous support structure and the mosaic materials, fabrication of a spiral wound element and demonstration of a low-pressure nanofiltration pilot system.
389
IMosaic Thin Film,
e
Non-porous region
Porous Support Membrane Fig. 3a. Mosaic thin-film membrane.
itional Thin Film
Non-porous region
Porous Support Membrane Fig. 3b. Traditional thin-film membrane.
Evaluation of standard RO element feed spacer contguration SST conducted a rudimentary comparison of the standard Naltex ROF031 (31 mil) spacer (Fig. 4) to the Naltex NWM-045. (45 mil) (Fig. 5) and the Conwed ON 3355 (48 mil) spacer (Fig. 6) [6]. Both Naltex spacers use the “diamond lattice” structure and the Conwed spacer uses the “ladder” configuration. The focus of the study was to compare fouling tendencies, cleanability and water flux of the different spacer configurations.
390
Fig. 4. Standard spacer -
Fig. 5. Alternative
spacer -
Naltex ROF-031 (31 mil).
Naltex NWM-045 (45 mil).
391
Fig. 6. Alternative
spacer - Conwed ON-3355 (48 mil).
SST conducted a fluid dynamic evaluation of each spacer and determined that the linear velocities for each spacer over a range of volume flow rates at fixed exit pressures were very similar; however, the flow resistance was greatest for the Naltex NWM-031 and least for the Conwed ON-3355. Dye tests were conducted using a test cell with a “window”, Flow patterns were recorded on video tape, and no turbulence was observed in any case which correlated with the theoretical calculations. Cleaning efficiency studies were conducted using the window test cell. It was observed that the Conwed spacer permitted better cleaning than either Naltex spacer. Development of a new RO element feed spacer configuration This research effort is a follow-on project to SST’s evaluation of the standard spacer [7]. SST initially conducted a patent search of existing spacer configurations which helped them to identify and rank spacer structural and operational characteristics to optimize spacer design. A subcontractor was hired to develop computer-aided design drawings of the proposed spacer configurations. A second contractor was hired to perform detailed finite element analyses of the mass, momentum and energy balances for each proposed spacer configuration.
392
The patent search identified one basic technology used by three major spacer manufacturers to fabricate spacers: extrusion. However, it was concluded that injection molding may be a better, although a more costly, method of fabricating uniform spacers. Furthermore, most of the patented designs had not been reduced to practice. SSI designed approximately 20 new feed channel spacers which were compared to a.standard Nelle ROF031 diamond lattice spacer. Twelve of the new designs were based on a ladder configuration, each with a filament thickness of less than 31 mil. The standard spacer configuration and one new configuration (Figs. 7 and 8) were chosen for finite element analysis due to funding constraints. The finite element analysis was developed to determine the spacer’s effects on concentration polarization, fouling rate, pressure drop and cleaning efficiency. Both two- and three-dimensional analyses were performed. Plots of flow velocities in the two dimensional analysis for the standard spacer (Fig. 9) and the new spacer (Fig. 10) were developed as well as pressure and shear plots. One key observation was that the standard spacer velocity pattern identified a recirculation region after each filament which covered approximately 75 % of the distance to the next filament. This same recirculation zone was present in the new spacer configuration; however, it was minimal in length and was isolated from the membrane surface. This effort identified potential improvements to RO spacer configurations and innovative computer analysis techniques.
Fig. 7. Standard
spacer configuration.
393 Transverse Stems NOTE: See Sheet Design 4 - B _ 1 for actual cell dimensions
Direction
of Spiral Winding
Direction $ Flow
Fig. 8a. Innovative spacer design configuration.
Section A - A Transverse
Stem
Parallel Filament
Fig. 8b. Innovative
spacer, magnified view.
Parallel Filaments
Fig. 9. Flow velocity
Fig.
10. Flow velocity
profile of standard
profile
spacer.
of innovative
spacer.
Development of a new RO membrane module configuration A developmental effort by Zenon Environmental, Inc. explored an alternative configuration to spiral wound elements [8]. These modules consist of hollow fibers constructed with existing membrane materials, arranged in a cross-flow pattern (Fig. 11). This innovative configuration is designed to induce turbulence in the feed water, reduce membrane fouling, increase membrane packing density and reduce pretreatment requirements. The technical effort began with the testing of nanofiltration membranes in the new configuration. A comparison of the Zenon configuration (salt rejection of 35%) with two different spiral wound nanofilters (salt rejection of 57% and 90%) was conducted using a 5 micron pre-filter on the spiral wound test loop and a 200 micron pre-filter on the transverse module test loop. After 5000 h of continuous testing, the transverse flow configuration had a higher water flux than the spiral wound nanofilters. Furthermore, the 5 micron cartridge filters required changing on 114 occasions while the 200 micron filter was replaced only twice. The second portion of the effort was to develop an RO membrane in the transverse flow configuration. The initial objective was to develop a fiber that could withstand an operating pressure of 1000 psi. Both reinforced
395
Fig. 11. Transverse
flow hollow fiber membrane module.
and unreinforced support fibers were fabricated from polysulfone, polyether sulfone and polyetheretherketone. In all unreinforced support fiber cases, either the burst pressure was too low or the water permeation rate was too low. For the reinforced fibers, several formulations had sufficient burst pressure and water permeation rates to warrant continued experimentation. The third task was to develop a coating technique capable of placing a thin-film composite membrane on the outside of the fiber, resulting in a membrane module that could produce sufficient amounts of drinking quality water. Two coating methods were investigated: solvent deposition and interfacial polymerization. The solvent deposition method was performed using a Zenon chlorine-resistant membrane coating (LTMX) on both reinforced polyethersulfone and reinforced polysulfone blend fibers, both of which had low sodium chloride rejections. A proven idamime/triacyl chloride interfacial polymerization film was also placed on the reinforced polysulfone fiber; however, salt rejection levels did not exceed 88 % s The final task in this project was to modify the ultrafiltration module to withstand higher pressures for use in desalination processes. Several module modifications were analyzed; however, funding restraints terminated all scheduled work before a solution could be found. Zenon demonstrated that the transverse flow configuration reduces fouling and pretreatment needs. However, there was insufficient evidence to make conclusions concerning the feasibility of using the transverse flow configuration in a RO process.
396
Ident@cation of commercial biocides for RO element storage A phase I SBIR with SST was completed in 1992 with promising results. The initial effort was to identify commercially available biocides through literature searches, chemical company surveys, university surveys and interviews with known microbiological experts 191. After screening the results of the biocide investigation, 16 biocides were chosen for compatibility testing. Included in the 16 biocides (chemicals and processes) were: gamma radiation, quaternary ammonium compounds, ethylene diaminetetraacetic acid (EDTA), benzoic acid, boric acid, sorbic glycerine, sodium acid, salicylic acid, methylchloroisothiazolinone, bisulfite, glutaraldehyde, formaldehyde and several other compounds or combinations of the compounds. The second phase of the effort was to evaluate the compatibility of the biocides with both the Filmtec FT-30 and the Fluid Systems TFC membranes. This test was to determine if the biocide had a detrimental effect on the membrane’s water flux or salt rejection. Initial results found gamma radiation to be compatible with both membrane materials. Compatible chemical b&ides included isothiazolinone, EDTA, sodium benzoate/ EDTA, sodium bisulfite/glycerin and salicylic acid. The third phase of the effort was to evaluate the effectiveness of the biocide to prevent regrowth of bacteria on biofouled RO membranes. Fourteen of the initial 15 chemicals were investigated at multiple concentrations and mixture ratios to determine minimum inhibitory concentrations. The biocides which exhibited the greatest degree of growth inhibition included EDTA, sodium benzoate/EDTA, benzalkonium chloride/EDTA, glutaraldehyde, benzalkonium chloride, isothiazolinone and cetyltrimethylammonium toluene sulfonate. The least effective biocides included salicylic acid, sorbic acid and bromochlorodimethyl hydantoin. SST identified a list of candidate biocides for future research and developed a procedure for testing biological growth inhibition. Furthermore, SST identified gamma radiation as a viable method for sterilizing packaged RO elements intended for long-term storage. Evaluation and selection of biocides for ROWPU element storage The promising results from the previous phase I SBIR effort encouraged the Navy, Bureau of Reclamation and SST to combine resources with the Army to expand the biocide effort with the objectives of screening new
397
b&ides, performing long-term storage tests on RO elements and investigating the use of gamma radiation as a post-packaging sterilization technique [lo]. SST modified the effort to include improvements to the test procedures, total of four membrane for evaluation (FT-30, CPA2, TFCL, and CA-B) and a total of 73 potential biocides for evaluation. The initial results indicate that gamma radiation is an effective biocide and is compatible with membranes in combination with inorganic free radical scavengers in solutions. Organic free radical scavengers in combination with the gamma radiation reduced the water flux and salt rejection of the membranes. To date, only 42 of the candidate b&ides have undergone bacteria biocide testing using membrane swatches. The tests included candidate biocide dilutions of 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024:l. Additional dilution tests were performed if complete kills were attained at 1024:l dilution. The most promising biocides identified to date are benzalkonium chloride, EDTA/benzalkonium chloride, MTC-1OWB (trade formula), WSCP (trade formula) and Antocil 1B (trade formula). Spiral wound element storage testing was initiated for candidate biotides. The periods of storage were 2 weeks and 3 months. The candidates with the best bactericide and fungicide properties to date were hydrogen peroxide, sodium bisulfite, EDTA/hydrogen peroxide, EDTA/benzalkonium chloride and sodium bisultite/glycerin. However, the hydrogen peroxide and EDTA/benzalkonium chloride solutions had adverse effects on membrane flux and rejection.
Development of a membrane separation technique wing Donnun exclusion Soane Technologies, Inc. has explored the use of Donnan ion exclusion with thin polyelectrolyte films [ll]. This principle is based on maintaining solution electroneutrality. The process uses a highly charged membrane to repel like charged ionic species. Mobile counter-ions in solution are allowed to permeate the membrane; however, mobile co-ions are not (Fig. 12). At some point an electrochemical equilibrium between the membrane and solution is achieved. Any additional co-ions that are repelled by the membrane cause the counter-ion to be repelled to maintain the electroneutrality [12]. Soane Technologies has been contracted to formulate a highly charged, high water flux membrane that can be used to desalinate seawater.
398
STI’ s Technology Micro-size? contaminants
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Fig. 12. Soane Technology charged membrane.
399
To date, Soane Technologies has fabricated a thin-film polyelectrolyte polystyrene sulfonate using either deposition of uncrosslinked polymer from solution or casting techniques. The thin-film composite membrane can be described as a very “loose” hydrated polymer gel with a large water permeation rate coated on a microporous support. The microporous support materials investigated include cellulose, polysulfone and nylon. The results indicate NaCl rejections can be achieved using the fabricated membranes; however, vast improvements to the membranes must be made before this process could be used for seawater desalination.
REFERENCES 1 C.E. Milstead and R.L. Riley, Development of an improved cleaning solution for ROWPU units phase I final report. US Army Report, Contract DAAK70-87-C-0065, 1988. 2 C.E. Milstead and R.L. Riley, Development of an improved cleaning solution for ROWPU units phase II final report. US Army Report, Contract DAAK70-89-C-0055, 1993. 3 A.C. Ibay, Developmentof chlorine-resistantand non-fouling reverse osmosis membranes for water treatment final technical report, US Army Report, Contract DAAK70-92-C0050, 1993. 4 L.P. Tenney, Subject, development of chlorine-resistant and non-fouling reverse osmosis membranes for water treatment. Personal communication, *March 3, 1994. 5 R.L. Riley and C.E. Milstead, Development of oxidation-resistant mosaic membranes with enhanced transport properties. Proposal under Department of Defense Small Business Innovation Research Program, 199 1. 6 R.L. Riley and C.E. Milstead, Evaluation of RO feedwater spacers final report. US Army Report, Contract DAAK70-9 l-P-0581, 1992. 7 C.E. Milstead and R.L. Riley, Development of a new feed channel spacer for reverse osmosis elements, Phase I final report. US Navy Report, prepared under contract N47408-92-C-7011, 1993. 8 K.P. Goodboy, K. Volchek, M. Mahendran, S.T. Pedersen and A. Deutschmann, A foulant-resistant reverse osmosis element based on flow transverse to hollow fibers. US Army Report, Contract DAAK70-92-C-0034, 1993. 9 C.E. Milstead, H.F. Ridgway and R.L. Riley, Identification and evaluation of biocides for ROWPU systems phase I final report. US Army Report, Contract DAAK70-91-C0065, 1992. 10 C.E. Milsted and R.L. Riley, Identification and evaluation of biocides for ROWPU systems, Phase II interum report. US Army Report prepared under contract DAAK70-93c-0007, 1993. 11 G. Fichter, Subject, research synopsis/novel reverse osmosis membranes. Personal communication, March 3, 1994. 12 H. Strathmann, Theory, in: Membrane Handbook, W.S. Winston Ho and K.K. Sirkar (eds.), Van Nostrand Reinhold, New York, 1992.