ENHANCED MEMBRANE CLEANING WITH REDUCING AGENTS TO REMOVE IRON FOULING. Introduction

Transcription

1 ENHANCED MEMBRANE CLEANING WITH REDUCING AGENTS TO REMOVE IRON FOULING Korkud Egrican, P.E.*, James C. Lozier, P.E., Srinivas Jalla, P.E.; CH2M J.C. Lan, Robert L. Harris, Jr., Brandon Brown; Gwinnett County, Department of Water Resources * CH2M, 1100 Wayne Avenue, Suite 1150, Silver Spring, MD korkud.egrican@ch2m.com, (240) Keywords: Iron fouling, reducing agents, chemical cleaning, ultrafiltration Introduction Gwinnett County, Georgia, owns and operates the F. Wayne Hill Water Resource Center (FWHWRC) which is currently permitted to produce 60 mgd high quality reclaimed water. The plant includes a 48 mgd ultrafiltration (UF) submerged membrane system that has been operational since 2005, the purpose of which is tertiary phosphorus removal to meet the plant s effluent total phosphorus permit limit of 0.08 mg/l FWHWRC has been experiencing low membrane permeability and production issues. The main issue with permeability and performance is believed to be iron-fouling. Typically ferric addition increases potential for membrane fouling due to colloidal iron oxide formation, and with the combination of organic material may cause membranes to foul irreversible, requiring more rigorous cleaning to remove the material. Increasing frequency of recovery and maintenance chemical cleans using the membrane manufacturer s standard cleaning protocols were not effective in removing the iron fouling which resulted in a narrow transmembrane pressure (TMP) operating window. The County has increased the terminal TMP set point in order to allow for longer production periods between cleans without any real success. The disadvantage of the higher TMP is that the membranes become more fouled resulting in less effective foulant removal with each clean. It has been evident that more rigorous chemical cleans were required to increase permeability of the membranes and production. As part of a 2015 study, various chemical formulations (with reducing agents) were proposed for enhanced recovery cleaning (RC) based on previous experience and literature review. Several of the chemicals would also require ph adjustment to maximize the effectiveness of the reducing agents. At the time this paper was written, the County was in the process of a patent application. Therefore the paper does not mention the names of the chemicals and code names are used when referring to them. In conjunction with the enhanced RC study, membrane autopsies and bench-scale trials were conducted by a third party company on fouled fibers to identify evaluate the effectiveness the candidate cleaning agents and to select the most suitable proprietary cleaner. 1

2 Plant Background The advanced water treatment portion of the FWHWRC, which treats secondary effluent, was constructed in two phases: 20 mgd Phase 1 which utilized chemical clarification (solids contact reactor) followed by granular media filtration (GMF), pre-ozonation (pre-o3), biologically activated carbon (BAC) and post-ozonation (post-o3). 40 mgd (48 mgd peak) Phase 2, comprising a parallel train with chemical clarification, ZW 500 C ultrafiltration (UF) submerged membranes. UF filtrate was then combined with the GMF effluent prior to the expanded pre-o3/bac/post-o3 system. Ferric chloride is used as the coagulant in both chemical clarification systems for the purpose of reducing the orthophosphate level in the Center s treated water in order to comply with surface water discharge standards. A partial process flow diagram is included in Figure 1 to show the GMF and UF systems as well as upstream and downstream processes The UF membrane system consists of 16 trains. Each train is supplied with thirteen ZW 500 C cassettes. Each train s cassettes are submerged in a 45-foot long, 12-foot wide and 11-foot high concrete basin. Space has been allocated in each basin for installation of a fourteenth cassette. The UF system has been operational since December 2005 and the current membrane modules are nearing the end of their useful lives. Figure 1 Partial Plant Process Diagram Note: solids contact clarifiers and recarb clarifiers were originally installed for lime clarification and later converted for use with ferric chloride. The UF system was configured to operate in feed and bleed mode, typical of all ZW 500 systems designed in the early to mid-2000s. The system continues to operate in this mode, 2

3 although many ZW 500 plants have been converted to batch or semi-batch mode to reduce aeration energy costs and minimize membrane abrasion. Current Operation and Issues FWHWRC has experienced low membrane permeability and filtrate production from the UF system for several years. On average, the system has only been able to produce approximately 25 mgd of its 48 mgd peak capacity. Iron fouling is considered the primary cause of reduced permeability and production caused by ferric solids carryover from the clarification process and precipitation of soluble iron within the membrane fibers. Coagulants such as ferric chloride and alum are widely used at wastewater treatment plants to achieve high phosphorus removal in the effluent. Typically ferric addition increases potential for membrane fouling due to the colloidal iron oxide formation and, in combination of organic matter, may cause the foulant to become recalcitrant to removal by standard submerged membrane cleaning protocols. In such cases, more effective chemicals used in combination with optimized cleaning protocol are required to remove the foulant and restore permeability. From visual inspection of the membranes, the reddish brown discoloration of the membranes as seen in Figure 2 is a strong indication that oxidized iron is a dominant membrane foulant. Figure 2 A Photo of Representative Cassette Taken on April 8, 2015 Until early 2015, several attempts were taken to mitigate the iron fouling of the membranes. Studies included modifications to the GE-provided maintenance clean (MC) and recovery clean (RC) protocols. The recommendations included conducting four hypochlorite and three citric MCs per week on each train. Recommended RC frequency was 30 days using both hypochlorite and citric acid. Previously chemical cleanings were not regular (more based on terminal TMP) and RC intervals were as long as 60 days. The County implemented these recommendations in the summer of 2014, and with repeated application, train permeability increased but still remained at levels which did not allow much increase in production. 3

4 RCs only reduced transmembrane pressure (TMP) to approximately -5 psi, which results in a narrow TMP operating window given the GE-established terminal TMP setpoint of -8 psi. To garner longer runs between RCs and achieve incrementally more production, the County increased the terminal TMP set point to -10 psi. Currently, the MC regime is four citric acid and three hypochlorite cleanings per week. For each hypochlorite MC, approximately 30 gallons of 12% sodium hypochlorite is required (to achieve a dose of ~50 mg/l) RCs are currently conducted every 30 days. These involve first soaking the membrane modules for 12 hours in a 600 mg/l hypochlorite solution. Following tank drain and flush, the modules are then soaked in citric acid (at ph 2.5) for 6 hours. RCs are conducted at ambient temperature as there is no means to heat the cleaning solutions. Cleaning solutions are aerated for 10 minutes on/10 minutes off to renew the solution in contact with the fibers. There is no recirculation of the cleaning solution within the tank (feed side recirculation), nor through the fiber and back to the tank (permeate recirculation). It has been determined that the frequent and costly MCs, especially with citric acid, were not being effective, and a more rigorous enhanced chemical cleaning was needed to resolve the fouling issue. Proposed Enhanced Cleaning Chemicals Four different chemical formulations were proposed for enhanced recovery cleaning based on CH2M s previous experience and a literature review. These cleaning formulations are presented in Table 1, with each using one or more reducing agents designed to remove recalcitrant ironbased foulants. Table 1 Reducing Agent-Based Formulations for Enhanced Recovery Cleans Enhanced Recovery Clean No. Chemical Formulation Chemical(s) Adjusted Solution ph 1 2 wt % GEN wt % GEN001 & 1 wt. % GEN wt % GEN Proprietary cleaner, PRO001 ~3.0 The performance of the four chemicals was evaluated through single-fiber testing with the two best performing chemicals further evaluated in pilot study on two separate UF trains. Immediate (post-clean) and longer-term permeability recoveries for each train were evaluated to determine which cleaning strategy is most effective to utilize in restoring permeability and filtrate production of the overall UF system. 4

5 Fiber Autopsies Membrane fibers were harvested from five different modules within a cassette for analysis from various locations. In addition, virgin fibers from an unused module were provided to serve as a control. Visual analysis with a 20x Stereoscope confirmed that both the feed and filtrate sides of the fibers were fouled with an orange substance. The Energy Dispersive X-ray analysis also indicated that Iron was the primary inorganic component present on the feed side and filtrate side of the fibers. Aluminum and silicon were also identified but in significantly lesser amounts. In addition, manganese was discovered on the exterior of the fibers. Figure 3 presents an image from the Stereoscope analysis. by Avista Technologies Figure 3 Stereoscope Image of one of the Fiber Samples Chromatic Elemental Imaging (CEI) was performed on one of the fouled fibers and one of the virgin fibers. Analysis of the foulant material on the exterior of the dark orange colored fiber identified primarily a combination of iron oxide, manganese oxide and aluminum hydroxide. Iron and aluminum were also coating the woven interior of the fibers, with iron being the dominant metal oxide. Figure 4 shows the two CEI images from the exterior and interior of one of the fouled fibers. by Avista Technologies by Avista Technologies Figure 4 Fouled Fiber - CEI image of the feed side (exterior) (left) and of the filtrate side (interior) (right) 5

6 Virgin fiber CEI analysis was also done as a comparison to observe the extent of the fouling. Figure 5 presents the images from this analysis. Carbon and fluorine are the main components of the PVDF membrane while the woven polyester support is composed of carbon. by Avista Technologies by Avista Technologies Figure 5 Virgin Fiber - CEI images of the feed side (exterior) (left) and of the filtrate side (interior) (right) Bench-scale Single Fiber Cleans A bench-scale chemical study was conducted using three (GEN002, GEN003 and PRO001) of the chemicals listed in Table 1. GEN001, either alone or in combination with GEN002, was eliminated from further consideration due to the potential safety issues associated its use at full scale given the large quantities required to clean the UF cassettes.. As part of the testing, some of the harvested fibers were soaked in solutions of each chemical for a set duration ranging from 6 to 24 hours. A vacuum was applied at the start of the soak period for the 2% PRO001 clean to simulate permeation through the fibers that occurs during full-scale MC and RC. The effect of heating each chemical solution was also evaluated. The average pre-clean permeability was determined to be 17.2 gfd/psi. Post-clean permeabilities ranged from 17.7 to 55.1 gfd/psi with GEN003 and PRO001 providing the highest permeabilities. As expected, heating the solution provided better permeability recovery. Initial permeation did not improve recovery (at least with PRO001). Recoveries increased with increasing soak time to a point; optimal soak time was determined to be hours. Table 2 presents the post clean permeability results. 6

7 No. Chemical and Solution Strength Table 2 Post Clean Permeability Results Temperature Permeability (gfd/psi) Permeability (gfd/psi) 6 Hours 24 Hours 1 1% GEN002 Ambient % GEN002 Ambient % GEN003 Ambient % GEN003 Ambient % GEN C % PRO001 Ambient % PRO001 Ambient % PRO001 Ambient % PRO001 (w/ permeation) Ambient % PRO001 (w/ permeation) 35 C % PRO001 (w/ permeation) 35 C 51.6 Cleaning Demonstration Study Based on the bench scale cleaning study results, an enhanced RC protocol was developed to test the ability of cleans conducted with GEN003 and PRO001 to improve permeability on cassettes from the full-scale UF system. The UF system is fitted with two dip tanks, one on the end of each of the two parallel rows of UF trains (Trains 1-8 and 9-16). The dip tanks, which are equipped with a permeate pump, instrumentation to measure permeate flow and TMP, are designed to allow operation or cleaning of a single cassette (one of thirteen in each UF train). The permeate pump is plumbed to receive water from the tank or from the fiber lumens, allowing for cleaning in either feed or permeate recirculation mode. For the demonstration study, chemical solutions were prepared at predetermined concentrations in the dip tanks, the solution was heated to a target temperature and a cassette was inserted from a given UF train. Each chemical solution was used to clean two cassettes, with six separate solutions required to clean the thirteen cassettes in a train. The enhanced RC utilized a soak time of 11 hours. Permeate flow for the trains was limited to 1,625 gpm flow in order to match the membrane flux that could be achieved when individual cassettes were operated in the dip tanks based on the maximum rated flow capacity of the dip tank permeate pumps. In preparation for the enhanced RCs, UF Trains 4 and 12 (selected as the trains for demonstration study) were first subjected to a standard hypochlorite RC (600 mg/l solution of sodium hypochlorite) to remove organic foulants. Following the completion of the hypochlorite RC, each train was placed in fixed permeate flow mode (at 1,625 gpm) corresponding to 20 gfd 7

8 flux. The post RC permeabilities (corrected to a temperature of 20 o C) were 5.77 and 7.19 gfd/psi for the trains 4 and 12, respectively. Train 4 cassettes were cleaned with 0.2 wt% PRO001 while Train 12 cassettes were cleaned with 2 wt % GEN003. For the last 3 modules, concentration of the PRO001 product was increased to 1 wt% based on incomplete iron color removal from the fibers at 0.2 wt%. Likewise, GEN003 solution concentration was decreased to 1 wt% based on complete color removal at 2 wt %. Both chemicals completely removed the iron color foulant at 1 wt %. Figure 6 and 7 present photos of before and after with the PRO001 and GEN003 cleans. Before Clean After Clean Figure 6 Before and After Photos 0.2 wt% PRO001 Enhanced Clean Before Clean After Clean Figure 7 Before and After Photos 2 wt% GEN003 Enhanced Clean 8

9 Both chemical cleans were effective in removing the iron foulant from the surface of the membrane fibers, though GEN003 produced more uniform color removal throughout the cassette. The TMP of each cassette was monitored before and after cleaning, but the combination of high solution temperature and low flux resulted in very low TMP values which could not be accurately measured by the dip tank vacuum gauge. To quantify the impact of enhanced RC on fiber permeability, fibers from selected cassettes from Trains 4 and 12 were potted and tested at bench scale. PRO001 post-clean permeabilities ranged from gfd/psi compared with gfd/psi for GEN003 cleans. Other notable conclusions from the demonstration study were: Longer cleaning time did not result in significant improvement in permeability; Reducing GEN003 concentration to 1% had no significant impact on cleaning effectiveness; and Increasing PRO001 concentration to 1% resulted in more uniform color removal across the cassette. Overall, the enhanced RC with GEN003 at 1 and 2 wt% provided the most consistent foulant removal. Near-Term Full-Scale Operation Evaluation After the completion of the enhanced RCs, Trains 4 and 12 were returned to service and the performance of both trains closely monitored. Train 8, which had only received standard RCs, was selected as a control train. All trains were operated at a fixed permeate flow of 2,000 gpm. Train 12 cleaned with GEN003, has performed exceptionally well. Permeability increased from approximately 4 to 7.5 gfd/psi directly following the clean. During the first 30-day run, TMP remained well below 8 psi while permeability remained above 5.6 gfd/psi. This is in contrast to Train 8, which operated at a permeability range of 3.5 to 5.7 gfd/psi. Contrary to the visual observations, Train 4 that was cleaned with PRO001 did not perform as well initially. It also performed worse compared to the control Train 8 but Train 4 performance has improved over time. Initially, an issue with instrumentation was suspected. Figures 8 and 9 present the temperature corrected permeability and temperature-corrected transmembrane pressure for the first thirty day run starting in July 22 nd It should be noted that the portions of the data missing (straight lines) in the figures correspond to the time that the enhanced RCs were conducted. 9

10 Figure 8 Temperature Corrected Permeability Trains 4, 8, and 12 (30-days) Figure 9 Temperature Corrected Transmembrane Pressure Trains 4, 8, and 12 (30-days) 10

11 As seen in the figures above Train 12 performance was exceptional with a starting TMP of 3.5 psi ending at 4.5 psi for the run while the permeabilities ranged from 5.8 to 7.8 gfd/psi. After 14 weeks, Train 12 performance has remained good. The permeability remained at 5.5 gfd/psi and TMP is at 5.5 psi while producing 2,500 gpm (3.6 mgd). Conclusions Trains 4 and 12 have been in operation for almost four months since the enhanced RCs were completed. Train 12, whose cassettes were cleaned using 1 or 2 wt% GEN003, has performed significantly better than the control train (Train 8). It has significantly removed the iron color, increased permeability while maintaining the design flow for the membranes. The plant has reduces the frequency of the maintenance cleans on Train 12 which would result in significant cost savings due to the reduction of citric acid use at the plant. The GEN003 enhanced clean was considered a success. Based on the performance of Train 12, Gwinnett County is considering Enhanced RC for two (2) additional trains to confirm the Train 12 performance. Performance testing and continued monitoring would further validate the effectiveness of the clean. Lastly, the plant is also considering optimizing the membrane system with a potential change of the ferric chloride as a coagulant to an aluminum-based one. This change would potentially reduce the source of fouling and the need for doing frequent enhanced cleans at the plant. Overall process optimization, such as adjusting aeration rate, backpulse frequency, reject flow, and RC chemical concentrations may help the plant further mitigate irreversible iron fouling. Acknowledgements We would like to acknowledge Justin Garmon, Wastewater Manager at FWHWRC and his staff and Lanier Contracting who were instrumental in the all aspects of this study. References 1. Zhenghua Zhang, et al., Cleaning strategies for iron-fouled membranes from submerged membrane bioreactor treatment of wastewaters, Journal of Membrane Science 475 (2015) Gallagher et al., United States Patent Application Publication, Pub. No.: US 2005/ A1 11