WATER TREATMENT PLANT SOLVES MEMBRANE INTEGRITY PROBLEMS AND SATISFIES NEW STATE REQUIREMENTS. Abstract

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1 WATER TREATMENT PLANT SOLVES MEMBRANE INTEGRITY PROBLEMS AND SATISFIES NEW STATE REQUIREMENTS Dave Holland, Aqua-Aerobic Systems, Inc., 6306 N. Alpine Rd, Rockford, IL 61111, Trent Diehl, Public Works Director, City of Butler, MO Scott Caothien, Technical Sales Support Manager - Membrane Solutions, BASF/inge, Irvine, CA Abstract As a result of the USEPA s Long Term 2 (LT2) Enhanced Surface Water Treatment Rule, many drinking water facilities drawing from State waterways are now required to meet 99.99% (4-log) removal of Cryptosporidium cysts. To verify this, plants using low-pressure membranes must achieve effluent turbidities below 0.1 NTU and pass daily air integrity tests (AITs). A challenge faced by many of these systems is that they find it difficult to pass AITs due to ever-increasing numbers of fibers that have broken or pulled away from their potting. One such facility was the City of Butler, MO, where pinning broken fibers was a daily activity, sometimes within months of placing new membranes into service. In 2013, the City learned of a fiber with a special honeycomb-like construction that was guaranteed not to break within the first five years of operation, and they decided to try it out. The Missouri Department of Natural Resources (MDNR) agreed to let them replace the membranes in one of their four trains, running the train as a 9-month pilot to see how the new membrane performed. Since startup in April 2014, the system has passed the daily AITs, effluent turbidities have been averaging 0.03 NTU or less, and permeabilities have consistently been above 17 gfd/psi, over 30% higher than the other three trains. This paper details the existing system and its integrity issues and defines how the selected membrane has been able to resolve those issues, meet the LT2 rule, and outperform the existing modules. Project Background Existing Facilities The City of Butler, MO is on the far west side of the state, as shown in Figure 1, a one-hour drive south of Kansas City. The town presently has about 4,100 residents, over 1,600 small businesses (less than ten employees), nearly 400 larger enterprises, and four schools. The City owns and operates the water treatment plant, where water from a surface water impoundment is treated and distributed to the community as well as to four other Public Water Supply Districts. The impoundment is fed from three sources: Butler Lake, Maris de Cygenes River, and Miami Creek. 1

2 Figure 1. Location of Butler, MO Water Treatment Plant The original plant was commissioned in As shown in Figure 2, the treatment process consisted of oxidation, coagulation, ph adjustment, carbon addition, clarification/settling, dualmedia filtration, and disinfection. Waste solids from the clarifiers and filters were sent to a settling pond, which overflowed back to Miami Creek. Figure 2. Original Plant Layout Over the course of the next 35 years, the system was maintained and parts were replaced as needed, but the treatment process remained basically the same. By 2002, the filters were in desperate need of replacement, and the decision was made to replace them with low-pressure hollow-fiber ultrafiltration (UF) membranes, which promised even better water quality than the filters and with less maintenance. The new membrane system was installed, consisting of: (4) disk-type 100-micron strainers to remove the larger particles that could damage the membranes, (4) membrane trains, each with (27) modules, (2) backwash pumps (one operating and one standby), and (1) clean-in-place (CIP) chemical cleaning system. 2

3 The membrane system was commissioned in 2003, rated with a design capacity of 3 MGD. To detect any fiber breaks or other breaches in membrane integrity, the system was set up to perform periodic AITs. During each test, air at psig was applied to the upper feed header on each membrane train, and the filtrate drain valve was opened to allow the air to push the water through the fibers and into the filtrate line, as shown in Figure 3. Figure 3. Air Integrity Test (AIT) The air pressure applied is too low to overcome the surface tension of the water inside the tiny pores of the membrane; therefore, only the very fine air bubbles entrapped in the water should go through the membrane. Once all of the water was displaced from the membrane fibers, the air was turned off and the pressure loss was measured and recorded. A fiber break or any other opening larger than 3 micron - the size of the smallest cryptosporidium cyst would result in a rapid pressure decay; this prevented the train from being returned to service until the breach was corrected and the train passed a subsequent AIT. In order to determine exactly where in the train the breach had occurred, each module was provided with a clear section of piping in its filtrate line (refer to Figure 4); a breach was evidenced by a steady stream of air bubbles. Figure 4. Membrane Modules with Clear Filtrate Piping 3

4 Persistent Integrity Problems Since being placed in operation, AIT failures steadily increased as membrane fibers broke or pulled away from their potting. Eventually, none of the four membrane trains could pass an AIT without multiple fiber repairs. Since trains with fiber breaks couldn t be used to treat water, the plant often struggled to meet the water demand placed on it by the City and other users. To speed up the repair process, plant personnel devised and built an apparatus shown in Figure 5 that they could connect to a module and use to identify and decommission (pin) any broken fibers. The device clamped on to one end of the module, allowing the module to be filled with water in a horizontal position. Air was then applied to the module s filtrate port and bubbled out through the broken fiber(s). The fiber bore was then plugged with a small plastic pin to prevent the feed water from entering (and passing through) the fiber; the small, oval-shaped tops of the pins can be seen in Figure 5 sticking out from the fiber potting. Figure 5. Apparatus for Fiber Identification and Repair The plant personnel found themselves spending a great deal of time pinning fibers to keep the plant running. Unfortunately, they are not alone in this endeavor. A study of 16 low-pressure drinking-water membrane plants done by Black & Veatch showed that, on average, these plants made nine module repairs per MGD per year [Freeman (2012)]. The study concluded that the present worth of repair costs over 20 years (the typical life of a plant) ranged from % of the equipment price. These repair costs didn t include any direct or indirect expenses resulting from loss of production on the train being repaired (e.g., increased equipment maintenance, additional membrane cleaning, supplemental water purchases etc.). Nor did it consider the noneconomic impact on plant personnel that spend a great deal of their time repairing fibers when they could be doing something much more productive. In addition to failed AITs and high fiber repair costs, there are several other reasons that broken fibers should be avoided: Effluent non-compliance Solids in the backwash water, which could cause irreversible fouling 4

5 Higher downstream disinfection costs To combat the ongoing integrity issues, the City of Butler decided in 2012 to replace the modules in three of their four trains. The original membrane manufacturer had recently introduced a new membrane that they felt would have much fewer integrity issues, but would not easily retrofit into the existing module racks; in addition, it would take extra time (and money) to build the racks. To correct the problem as quickly as possible, the City went with the original model instead. Shortly after being place in service, the new modules began to fail their AITs, and the City knew they would have to go a different route. New Requirements To complicate matters, the State informed the City that, starting in October of 2014, treatment would have to meet the Long Term 2 (LT2) Enhanced Surface Water Treatment Rule, recently enacted by the U.S. Environmental Protection Agency (USEPA). This rule requires that the plant demonstrate 99.99% (4-log) removal of Cryptosporidium cysts, as verified through both daily direct integrity testing (such as an AIT) and continuous indirect integrity testing (such as turbidity monitoring). The guidelines for the AIT would remain the same designed to detect a 3 micron breach in the membrane but now the tests, which were being performed only periodically, would have to be set to occur automatically every day. Effluent turbidities were already being measured, but now would have to be 0.1 NTU or less; higher turbidities lasting 15 minutes will automatically trigger an AIT. Evaluating Alternatives Knowing that the original model of membrane could not reliably meet the new requirements, the City began to look at other options. One alternative would be to use the improved model from the same manufacturer. The advantages would be: some of the existing equipment could be reused, and the manufacturer would probably give them a good price in order to keep the installation. The downsides, however, were that it would require significant skid modifications, require additional equipment (i.e., blowers, PLC hardware), and the membrane model was fairly new and unproven at the time. The other alternative would be to use a proven model from another manufacturer. While this option had a higher probability of delivering a well-performing membrane, modules that operated in a similar fashion to the original modules (where the feed flows from inside the membrane bores to the outside of the fibers) have typically had similar integrity problems, and modules that flow from outside the fibers to inside the bores would require considerable equipment modifications/additions. During their evaluation, the City focused on three main requirements: Robust fibers: the membrane must come with a sound warranty, have a good track record, and demonstrate the ability to pass daily AITs at similar plants. Similar or better performance: the membrane must be able to meet the turbidity requirement (< 0.1 NTU) and meet or exceed cold-water permeabilities of gfd/psi and warm-water permeabilities of gfd/psi. Low cost: the membrane must be installed with minimal system modifications and must demonstrate similar chemical usage 5

6 Plant Upgrade After a thorough evaluation, the City decided in June of 2013 to replace the modules with Aqua- Aerobic MultiBore modules containing a special fiber made by BASF/inge. This membrane was selected because the City found it to best satisfy their requirements. Robust Fibers The primary reason the City made the switch to these membranes was because of their unique honeycomb-like construction, as illustrated in Figure 6. While almost all other hollow-fiber membranes use fibers with single-bores, this fiber is made with seven bores such that it s essentially seven fibers in one. This gives the fiber strength that a single-bore fiber doesn t have. In addition, the multi-bore fiber is made from a single material with different porosities throughout the membrane structure. Many membranes made of two materials with the membrane material layered on the outside of a support structure are very strong, but the outer membrane layer has a tendency to pull away from the support layer over time (delaminate) under the stresses caused by repeated air scours. Figure 6. Multi-bore Fibers Because of the fiber s strength, the membrane manufacturer offers a very unique warranty: if a single fiber breaks within the first five years of operation, the manufacturer will replace the entire module at no charge to the owner. In contrast, other manufacturers won t replace the modules until the effluent turbidity is out of compliance or a certain percentage of fibers break, typically 0.1-1%. While this seems like a small percentage, it amounts to fibers in a typical 14,000-fiber module. In addition to the module s warranty, the membrane has a very high success rate. The same membrane had been installed at over 700 plants in nearly 50,000 total modules treating in excess of 1.3 billion gallons per day (BGD), and only a handful of modules have had broken fibers. As an example, an 11-year-old drinking water installation in Jachenhausen, Germany contains over 54,000 original fibers, and has never had one break. Similar or Better Performance 6

7 With such a large installation base, there is a great deal of data on systems treating surface waters similar to those treated at the Butler plant. For example, a 5.1 MGD system at Guangzhou, China treating river water with an average turbidity of 5 NTU (slightly higher than the water treated at the Butler plant) achieves less than 0.1 NTU and operates consistently at a flux of 48 gfd (82 lmh) and an average trans-membrane pressure (TMP) of 2.2 psi (0.15 bar), resulting in a permeability of 22 gfd/psi (547 lmh/bar) [Berg (2015)]. This is quite better than the average permeability of the original membranes at the Butler facility, 14 gfd/psi, and significantly more than the 9-10 gfd/psi permeability averaged by most outside-in modules. The reason for the higher performance is two-fold. First, the membrane material for both the original and new modules at Butler, polyethersulphone (PES), allows for the creation of moreuniform pores than the material used by most other manufacturers, polyvinylidene difluoride (PVDF). As a result, PES membranes typically have more pores in the same area, which lowers the TMP and increases the permeability. Second, the multi-bore membrane is produced in such a way that the separation layer of very fine 0.02 µm pores which performs the actual filtration is very thin, with the rest of the membrane structure consisting of much larger pores (refer to Figure 7). Figure 7. Fiber Bore (left) with Surrounding Membrane Material With this membrane structure, nearly all of the TMP is occurring at the thin separation layer; once the flow gets past this layer, it moves freely past the other bores and to the outside of the fiber, with very little further pressure loss. The consequence is a lower TMP than other membranes, often reduced 50-70%. Low Cost Another advantage of the inside-out multi-bore modules over the outside-in alternatives was that the operation of the new modules was almost identical to that of the original modules. In addition, the module dimensions and connection locations were the same, with the exception of the filtrate piping. This means that the original membrane racks, backwash pumps, CIP system, and PLC program could be reused. In fact, the only modification required was minor rerouting of the filtrate piping, as shown in Figure 8. Because each new module contained 646 ft 2 (60 m 2 ) of membrane area in contrast to the 500 ft 2 (46 m 2 ) in the original modules, only (21) multi-bore 7

8 modules were needed per membrane train; the train connections vacated by the remaining (6) modules would have to be plugged. Figure 8. Rerouting the Original Filtrate Piping (left) to Accommodate the New Modules (right) Besides the cost savings realized through reuse of existing equipment and piping, chemical costs for the new modules would be identical to that of the original modules. Because the membrane material remained the same (PES), as did the membrane area per train, the new modules would be able to use the same chemical types, quantities, and cleaning protocol. New Membrane Installation After discussions with the MDNR, the City s consultant engineer, Allgeier, Martin and Associates, proposed to the State a pilot study whereby the modules on one of the plant s four UF trains would be replaced and operated for nine months to verify their ability to meet the LT2 rule. The MDNR approved the proposal, and the modules were purchased, installed, and started up in April of Results and Discussion AIT Results Prior to submitting the pilot proposal to the State, the AIT Upper Control Limit (UCL) was calculated using the procedure in the USEPA s Membrane Filtration Guidance Manual [Allgeier (2005)] to be psi/min (6.6 mbar/min). Since startup, the AIT pressure decay for the train with the new modules (Train 2) has consistently been beneath this limit. Table 1 shows the data from one of the AITs; the results are typical of all but two of the daily tests results. As you can see, the pressure decayed 0.3 psi over the 10 minute test, or 0.03 psi/min. As noted in the previous paragraph, there were two days shortly after commissioning of the new modules during which the AIT failed. The reason for the failure was operator error, when the CIP pump was manually turned on with all of the CIP valves open and the line empty; this caused a severe water hammer, bursting the 4-inch PVC CIP piping and breaking some fibers in 8

9 one of the train 2 modules. The module was replaced and all of the AITs have passed since that time. Table 1 AIT Results for 12/2/14 Time Elapsed Pressure (psi) 0 sec sec sec sec sec sec sec sec sec sec sec Effluent Turbidities Since commissioning, the effluent turbidity produced by the train with the new modules (Train 2) has consistently been at or below 0.03 NTU, as shown in Figure 9. During the final four months of the 9-month pilot study, the new modules achieved 10-33% lower effluent turbidities than the existing trains. Train Permeabilities The permeabilties for each train are given in Figure 10 for the final four months of the pilot study. The average value for the new modules (Train 2) during these winter months is approximately 17 gfd/psi, while the permeabilities of the existing trains average about gfd/psi during the same time period. This represents an improvement of 31-42%. There is, however, a noticeable difference between the permeability swings of the new and existing modules. While the permeabilities of the new modules dropped much more between CIPs than those of the existing trains, the new modules fully recovered following each cleaning. While no action is necessary, it may be a good idea to perform more frequent CIPs on the new modules, only with less chemical for each cleaning; this will probably decrease the permeability swings while maintaining the same chemical usage. 9

10 Figure 9. Effluent Turbidities for the Final Four Months of the Pilot Run Figure 10. Train Permeabilities in gfd/psi for the Final Four Months of the Pilot Run 10

11 Conclusions Based on the performance of both the original membranes and the new multi-bore modules over the past one and a half years of operation, there are several conclusions that can be derived from the upgrade of the Butler Water Treatment Plant: 1. The structure of the existing single-bore inside-out PES membrane was insufficient to handle the normal stresses placed on the fibers and their potting. The manufacturer of the original membrane, recognizing this, has since developed a single-bore outside-in PVDF membrane because this material is known to be more flexible than PES. 2. The structure of the new multi-bore inside-out PES membrane has, so far, held up to the normal stresses placed on the modules, with the exception of a single integrity issue caused by excessive water hammer. The reasons for the membrane s superior integrity appear to be its unique honeycomb-like structure and single-material construction. 3. The new membrane has, so far, outperformed the existing membranes in both effluent turbidity and membrane permeability. Turbidities have been 10-33% lower, and permeabilities have been 31-42% higher. The reasons for this are the ability of the PES material to be manufactured with a thin separation layer of small highly-uniform pores and the rest of the membrane with much larger pores. 4. The cost of the membrane upgrade was minimized by reusing much of the existing equipment and making only minor modifications to the filtrate piping. 5. The cost of operating the new membranes has been lower than operating the original membranes, due mostly to the decrease in feed pressure, proportional to the increase in permeability. 6. The new and original membranes use identical operating parameters; therefore, cleaning chemical costs and backwash pump energy costs are the same. 7. The permeability of the new modules drops much more between CIPs than those of the original modules, but the new modules are fully recovering following each cleaning. Performing more frequent CIPs on the new modules using less chemical for each cleaning may decrease the permeability swings while maintaining the same chemical usage. References Allgeier, Steven, et. al. (2005), Establishing Control Limits, Membrane Filtration Guidance Manual, pp through Berg, Peter, and Daniela Calleri (2015), RO pre-treatment of river water for a Chinese power plant, BASF/inge Case Study, page 2. Freeman, Scott, Paul Delphos, Ron Henderson, and Vasu Veerapaneni (2012), Real World Fiber Breakage Rates and Costs, Proceedings of the AWWA/AMTA Membrane Technology Conference & Exposition 2012, pp