Gregory E. Choryhanna and Alan R. Daza, SEPROTECH SYSTEMS INC. Process Development Division Ottawa, Canada

Transcription

1 SUCCESSFUL IMPLEMENTATION OF MEMBRANE TECHNOLOGY IN DYE ASTEATER RECYCLING Gregory E. Choryhanna and Alan R. Daza, SEPROTECH SYSTEMS INC. Process Development Division Ottawa, Canada ABSTRACT A pilot test was conducted (July to December 1995) at a large textile plant in an effort to evaluate the performance of several types of crossflow membranes. The test employed the use of UF, NF and RO in the processing of wastewater generated from the production of coloured textile fabric. Results generated from this test indicated that an NF system could effectively recover 9% of this feed wastewater for reuse in the plant as process water without any detriment to the quality of the textile fabric. During the test period over 165, gallons of product water was recovered using five types of commercially produced elements. A full scale design incorporating NF followed by UV sterilization along with an RO followed by UV sterilization to provide a higher grade of process water was proposed as a final process train developed during the pilot study. INTRODUCTION One of the largest textile firms in the U.S. was the site of a pilot study conducted by Seprotech Systems Incorporated to evaluate the use of membrane technology, specifically crossflow filtration in the recovery of process wastewater for reuse in the plant. As a large consumer of water, the plant desired to cut down on the use of fresh water and its associated costs by examining technology that could recover as much water as possible from the wastewater generated over the course of a production day. The textile plant's commitment to a ultimate zero discharge mandate dictated that the company would use technology to maximize the reuse of resources and minimize the waste generated in the manufacturing of product from their various plants. The purpose of the pilot study conducted by Seprotech Systems Incorporated was to use membrane technology to maximize the reuse of the wastewater. The pilot program was set up to use commercially produced membranes in the

2 processing of the feed stream in an effort to accurately mimic the functioning of the larger full scale system. Two 8" X 4" membranes were used in a series configuration in the pilot unit rather than the smaller 4" X 4" membranes. This set up would allow the operators of the pilot equipment to generate data that would not be subjected to scale up error and would minimize the assumptions required from scaling up from a 4" test to a 8" full scale design. The test unit would eventually run in a continuous 24 hr period after the optimization of the pilot process. The wastewater processed was drawn from an outside concrete tank that is used as a buffer tank prior to discharge after neutralization. This tank contained the bulk of the rinse and dye water along with any suspended solids that the plant would generate during the day. The volume that would enter this tank ranged from 5k gal to 7k gal per day (45k gal/day, bulk average). The objective of this test was to design a full scale water recovery system capable of processing 4k gal per day. The pilot study would evaluate the pre-treatment of the feed stream by dead end filtration using standard bags at various micron cut-off sizes and a continuous self cleaning screen filter in an effort to remove most of the suspended matter. The bulk of the pilot study would be dedicated to the evaluation of UF, (Ultrafiltration), NF (Nanofiltration) and RO (Reverse Osmosis) membranes in an effort to recover as much of the feed stream as possible with a quality of product that would meet the needs for reuse in the plant as bulk rinse water and high grade rinse water. The pilot study would evaluate the post-treatment of the product stream by sterilization using UV (Ultraviolet) and/or Oxidation by hydrogen peroxide (H22) in an effort to remove the potential growth of bacteria. It was necessary to find a working process train to maximize the recovery of the wastewater and also minimize the capital cost of the full scale equipment and its associated operating costs. The dynamic nature and fluid characteristics of the feed stream presented a great challenge during the operation of the test equipment and would produce some very interesting results. TEST PROTOCOL The pilot system was operated in two modes, i) "8atch" and ii) "Continuous Feed/Bleed (F/B)" configurations in order to collect data for analysis and evaluation. The pilot system consisted of a single Pressure Vessel (PV) with two 8" x 4" diam. membranes (either UF, NF or RO) in series. The inlet stream would be drawn from a orking tank with the Permeate being sent to a collection tank. The Concentrate would be directed back to the working tank in a "Batch" configuration or sent to the concrete pit tank in a continuous Feed/Bleed configuration.

3 The following list describes the type of system configuration and membranes in the order that they were tested: Test Configuration Membrane Recovery Type Model 1. Batch 2. Cont. F/B 3. Cont. F/B 4. Batch 5. Cont. F/B 6. Batch 7. Cont. F/B 8. Cont. F/B Nan of ilt r at ion Reverse Osmosis Ultrafiltration Reverse Osmosis Nanofiltration Reverse Osmosis Nanofiltration Nanofiltration A B C B A B D E 95% I 9% 9% 95% ILIV 9% 95% 1II.V 9% 9% Note I: Average Maximized Recovery result Note 11: Maximized Recovery result Note Ill: Maximized Recovery result Note IV: UF Permeate was used as Feed Note V: NF Permeate was used as Feed A consistent time interval was used by the system operator for data collection. The data included the following variables: Temperature; Pressure (Operating, Drop across Membrane and Pre-filter); Flowrates (Feed, Permeate, Concentrate, Recirculation); Conductivities (Feed, Permeate, Concentrate); Samples for Lab Analysis (Feed, Permeate, Concentrate). The respective results of each of the pilot tests were represented graphically as follows: Permeate Flowrate vs. Test Time Conductivity (Feed) vs. Test Time Conductivity (Con cent rat e) vs. Test Time Conductivity (Permeate) vs. Test Time Rejection vs. Test Time Pre-treatment in the form of filter/cartridge bags and a continuous self cleaning screen were evaluated for their efficiency in solids reduction of the raw feed. Post-treatment in the form of UV treatment and Peroxide were evaluated for their efficiency in colour removal and biological activity reduction of the permeate generated throughout the study.

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5 RESULTS & DISCUSSION The results of the pilot study are presented in ten sections representing the pretreatment evaluation followed by the different number of tests that were performed using various selected membranes and the final section will evaluate the post-treatment used on the product water generated during the course of the pilot study. PR E-TR EATMENT The pre-treatment to any membrane system play's a significant role in the operation of the unit both from a productivity point of view and from a foulant/cleaning point of view. The selection of pre-treatment technology varies from fluid to fluid and two means of pre-filtration were selected and evaluated. 1. Cartridge/bag filters were used in series starting at 1 micron followed by 5 micron and ending with a 5 micron pore size. 2. A continuous self cleaning 25 micron screen with a backwash system. Results from the bag tests indicated moderate pressure drops over a long period of time with the eventual caking of solids present in all bag sizes. A particle distribution curve indicated that a good percentage of the solids were less than 2 micron in size. This fraction will pass through the bags but will not clog the feed channel of a spiral membrane. The use of a series of bag filters would be the pre-treatment system for the membranes based on the success demonstrated and the low capital/operating costs associated with this type of pre-filter. Results from testing the continuous filtration screen did not prove that this type of screen was suitable for the feed fluid. Product flowrates achieved did not indicate that this screen would economically act as a pre-treatment for the membrane system. The screen did not exhibit good self cleaning characteristics or any flow recovery after a backwash cycle. Attempts were made to modify flow and head pressure settings but the longitudinal screen pores would get clogged in a short period of time and the internal blade scraping device could not remove this material effectively or consistently. This pretreatment device was discarded as a viable option. TEST # 1 This test comprised of using model 'A' NF membranes on 17 separate batch runs that totalled approximately 25 hrs. testing time and operating with an average fluid temperature of 1 IO O F. The results indicated that the permeate flowrate consistently started in the 6. to 6.5 gpm range and decreased at the end of the batch to a range of

6 4.5 to 5. gpm. The operating pressures during the tests would begin at approximately 35-4 psi and increase towards the end of the runs to approximately 55-7 psi. The feed conductivities of the batch runs ifldicated a starting range of 5-1 pmhos and increased to 3-46 Amhos at the end of the runs. The permeate conductivities would start in the range of pmhos and increased to pmhos at the end of the runs. Rejection results obtained for the runs ranged between %. During the test certain runs indicated inconsistencies with respect to the majority of the results. In Batch Run #I7 the permeate flowrate at the end of the batch decreased to a test low of 4. gpm. After reviewing the data, its respective feed conductivity and operating pressure directly corresponded to test highs of 55 Mmhos and 85 psi, respectively. These two spikes in pressure and dissolved solids loading explain the decrease in permeate flowrate. In Batch Run #8 the permeate conductivity ranged from 11 to 1475 pmhos and its corresponding rejection was between 46 and 6%. This result was due to the possible presence of Sodium (Na') based dyes that have, by design, lower rejection percentage for NF membranes. The test runs indicated that the system recoveries ranged from 84 to 97 % and therefore a 9 % recovery could be achieved consistently. TEST # 2 This test comprised of operating continuously in a Feed and Bleed mode (9% recovery) with model 'B' RO membranes for approximately 7 hrs. and an average fluid temperature of 1 14 deg. F. The results indicated that the permeate flowrate consistently operated in the 5. gpm. The operating pressures during the test began at approximately 22 psi and increase to 56 psi at the end of the run. The increased pressure can be attributed to possible membrane fouling due to changing feed characteristics. The feed conductivities of the test indicated an initial value of 32 pmhos and increasing to 47 pmhos range at the end of the test. This increase in feed conductivity could explain the increase in solids loading to the membrane, increasing operating pressure and subsequently possible membrane fouling. The permeate in this test was subjected to UltraViolet (UV) treatment with conductivity readings taken before and after the UV treatment. The results showed that the permeate conductivities ranged from 15 to 6 /mhos with no UV and a range of 115 to 6 pmhos with UV treatment. Rejection results throughout the test displayed a consistent separation in the 96% range. TEST # 3 This test comprised of operating continuously in a Feed and Bleed mode (9% recovery) with model 'C' UF membranes for approximately 155 hrs. with an average fluid temperature of 119 deg. F. The results indicate that the average permeate flowrate was

7 approximately 3.75 gpm during its first 7 hrs. of operation and 2.5 gpm during its remaining 85 hrs. of operation. The operating pressures during the first part of the test seemed to have been relatively steady between 15 and 175 psi. with the second part averaging between 25 and 275 psi. The feed conductivities of the test indicated a range of 6-23 pmhos with an average of 11 pmhos. The results showed that the permeate conductivities ranged from 25 to 9 pmhos with an average of 65 pmhos. Rejection results displayed an average separation occurring in the 4-45% range. TEST # 4 This batch down test comprised of model 'El' RO membranes using UF Permeate, collected from TEST #3, as feed. This test took approximately 2 hrs. to complete with a steady flowrate of 2. gpm and an operating pressure of 3 psi. The feed conductivities of the test indicated an initial value of 79 pmhos to a final value of approximately 145 pmhos. The results indicated an initial value of 23 Dmhos and a final value of 47 pmhos. Rejection results displayed a separation range of approximately 97%. TEST # 5 The same membranes used in TEST # 1 were used in this test, specifically the model 'A' high temperature Nanofiltration membrane (standard spacer) which produced the following results in a feed and bleed mode. The feed and bleed operation was carried out on a continuous basis at a recovery of 9%. Over a time period of 12 hrs the permeate production averaged approximately 5 gpm at a operating temperature of 11 deg. F. Operating pressures averaged approximately 14 psi. The graphical results indicate daily fouling of the membranes as the permeate flow ranged from 7 gpm down to 2 gpm. This cycle was repeated consistently over the operating period and generated cleaning and fast flush sequences that would be used in later test runs. Conductivity data indicated a feed value of approximately 8 pmhos and a concentrate value ranging between 2 and 25 pmhos. Permeate conductivity values ranges between 2 and 4 pmhos for the test duration. Rejection data varied between 8% and 9% during the test run. A small amount of colour was present in the permeate. The presence of colour in the permeate was still acceptable when the water was used as a pre rinse. UV light and peroxide were applied to the permeate for a sterilization step. The synergy of both a high dose of peroxide in combination with the UV light did very little in the removal of colour.

8 The peroxide dose was abandoned due to the high amount of chemical injection and the minimal aesthetic improvement to the permeate. It was noted that after a few days the sample containing the peroxide did lose its colour but due to a long residence time for the change to occur this result further demonstrated the peroxide added no value to the process. TEST # 6 A batch down test (95% recovery) was performed on the NF permeate generated in TEST # 5 using an RO membrane, specifically a model '6' brackish water element. Over a time period of 3 hrs the permeate production averaged approximately 2.25 gpm at a operating temperature of 9 deg. F. Operating pressures averaged approximately 285 psi. The graphical results indicate little or no fouling of the membranes as the permeate flow was stable around the 2 gpm level. This is a normal result as their is virtually no suspended matter in this feed. Conductivity data indicated a feed value of approximately 3 pmhos and a concentrate value increasing to 25 Am hos. Permeate conductivity values ranged between 2 and 6 pmhos for the test duration. Rejection data varied between 92% and 97% during the test run. Permeate was passed through a UV light and a sample was sent for evaluation by the plant staff for use as a high grade water for their rinse operation. Results from this test was positive and adhered to the plant's requirements. TEST # 7 The membranes used in TEST # 7 were, specifically the model 'D' high temperature Nanofiltration membrane which produced the following results in a feed and bleed mode. The feed and bleed operation was carried out on a continuous basis at a recovery of 9%. Over a time period of 15 hrs the permeate production averaged approximately 4 gpm at a operating temperature of 11 deg. F. Operating pressures averaged approximately 16 psi. The graphical results indicate daily fouling of the membranes as the permeate flow ranged from 7 gpm down to 2.5 gpm. This cycle was repeated consistently over the operating period and cleaning and fast flush sequences were used in an effort to maintain these values, note the cleaning and fast flush cycles on the graph. The problem with these membranes was that recovery of the initial starting flux was never achieved and a steady decline of productivity was observed.

9 Conductivity data indicated a feed value of approximately 7 pmhos and a concentrate value ranging between 1 and 3 ymhos. Permeate conductivity values ranged between 1 and 3 pmhos for the test duration. Rejection data varied but remained in the 9% range for most of the test run. A small amount of colour was present in the permeate and results from the plant lab indicated that the permeate was acceptable for use as a pre rinse in their plant. UV light was applied to the permeate for a sterilization step but no peroxide was used. Generally these membranes functioned well but a steady decrease in flux with no real improvement after a wash indicated that the feed fluid was not compatible with the membrane in this case. The rejection of material and the overall permeate quality were better then the model 'A' membrane. This membrane was not selected for future work and thus would not be considered for full scale design. TEST # 8 The model 'E' membrane used was the same membrane as in TEST # 1 and TEST # 5 with the exception that the feed spacer was a wide variant of the model 'A' high temperature Nanofiltration membrane which produced the following results in a feed and bleed mode. The feed and bleed operation was carried out on a continuous basis at a recovery of 9%. Over a time period of 14 hrs the permeate production averaged approximately 4.5 gpm at a operating temperature of 11 deg. F. Operating pressures averaged approximately 13 psi. The graphical results indicate daily fouling of the membranes as the permeate flow ranged from 5 gpm down to 2 gpm. This cycle was repeated consistently over the operating period and with cleaning and fast flush sequences used to recover and maintain a steady flux. Conductivity data indicated a feed value of approximately 7 ymhos and a concentrate value ranging between 1 and 25 ymhos. Permeate conductivity values ranges between 1 and 4 pmhos for the test duration. Rejection data varied between 7% and 85% during the test run. A small amount of colour was present in the permeate and results from the plant indicated that the permeate was acceptable for use as a pre rinse in their plant. UV light was applied to the permeate for a sterilization step but no peroxide was used. Generally these membranes functioned well but a steady decrease in flux was noticed over time. Improvement after a wash indicated that the feed fluid was compatible with the membrane in this case and the initial flux values could be recovered. The rejection of material and the overall permeate quality was not as good as the model 'D' product. The model 'E' membrane was selected for a full scale size design.

10 CONCLUSIONS & RECOMMENDATIONS 1. The pre-treatment that will be used for the full scale system will be a series of bag/cattridge filters. The pore size will range downward from 1 micron as a initial fraction followed by a 5 and ending with a 5 micron pore size. The continuous screen was discarded due to low productivity and cleaning complications. 2. The use of NF over UF as a initial water recovery system was decided by the lower operating pressure exhibited by the NF and the higher productivity of permeate by the NF when compared to the UF. The NF demonstrated a consistent achievable recovery of 9%. The model 'E' membrane element provided the best overall results. 3. An RO would be used to process a fraction of the NF permeate produced to generate high grade rinse water. The RO demonstrated a recovery rate of 95%. 4. A combination of fast flush sequences and a soap wash would keep the membrane system from fouling. This procedure demonstrated consistent success in keeping the membrane clean. 5. UV post-treatment was selected as a sterilization step to ensure minimal biological activity in both the NF and RO permeate. Hydrogen peroxide was abandoned due to the long retention times and large additions that produced no acceptable product quality enhancement.

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15 UF (Model C) Pilot Test Continuous Feed/Bleed System I- Q CT w a, I I I 1 I I I I I I I I I I I : PILOT TESTING TIME (HRS) MK8.4U

16 Batching NF Permeate with RO - Batch System I 1 I I I O PILOT TESTING TIME (HRS).O 45. MKNFR4

17 Batching NF Permeate with RO I Batch System VI 4. I I 1 I 41.O PILOT TESTING TIME (HRS) 5. MKNFRO1

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19 NF (Model D) Pilot Test Continuous FIB System I I I I I I I 1 I I PILOT TESTING TIME (HRS) MK9.4NTRS

20 NF (Model E) Pilot Test Continuous Feed/Bleed System f 3 2 z cr a, e I I I I I I 1 I I I PILOT TESTING TIME (HRS) MK5.4NFS

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