Metrics & Methods for MF/UF System Optimization After a membrane filtration (i.e., microfiltration [MF] and ultrafiltration [UF]) system is designed, installed, and commissioned, it is essential that the plant is well-maintained in order to proactively identify potential design or equipment problems and ensure its proper operation. However, the strategic use of operational practices to optimize MF/UF system performance and the metrics that are important to track are not always commonly known. Such in-depth understanding may not be conveyed in operator training, and the applicable information is not captured in any one readily accessible reference. Consequently, this article has been prepared by the AWWA Membrane Processes Committee of the Technical and Educational Council, Water Quality and Technology Division to compile the most useful contemporary knowledge regarding the methods and metrics for MF/UF system optimization into a single, concise resource, including essential background on the following topics: • • • • • • • • •
Transmembrane pressure Temperature and temperature correction Permeability Chemical cleaning effectiveness Backwash practices Maintenance cleans Integrity testing Integrity maintenance and repair General & preventative maintenance
This article is not intended to address each of these extensive subjects in a comprehensive manner, but rather it is meant to provide foundational information to serve as both a reference and a platform for building a more extensive and detailed knowledge base with respect to MF/UF system performance, tracking, troubleshooting, and optimization. Transmembrane Pressure Transmembrane pressure (TMP) is a measure of the driving force for membrane filtration. Accordingly, the TMP will increase or decrease in direct proportion to the system flow (or flux). For operation at constant flow (or flux measured in gallons per day per square foot of membrane area, or GFD), the TMP will increase or decrease in response to changing feedwater quality and temperature and membrane fouling. As the temperature decreases, the feedwater becomes more viscous, thereby requiring more pressure (i.e., higher TMP) to maintain the same throughput across the membrane barrier; the converse is also true for increases in feedwater temperature. Similarly, membrane fouling creates resistance, which requires increased TMP to maintain the same flow (or flux); the TMP will likewise decrease for constant flow (or flux) if membrane fouling is reversed by backwashing or chemical cleaning. TMP can be used as an effective tool for membrane filtration system operation monitoring and optimization in several ways. Each MF/UF system has a maximum TMP at which chemical cleaning is required to reverse membrane fouling and reduce the risk of a high pressure differential damaging the membranes. A TMP threshold of about 75–80% of the maximum
(thereby providing a buffer) is recommended. The recommended threshold not only protects the membranes, but also limits operational time at the highest TMPs when power consumption, and therefore typically operational cost, is greatest. In addition, many MF/UF systems can exhibit exponential TMP increases with the build-up of membrane fouling; therefore, as the TMP approaches the recommended cleaning threshold, changes in feedwater quality that augment fouling potential can facilitate very rapid TMP increases. Another important facet of TMP monitoring is the manner in which it increases during a poor feedwater quality event. Typically, the rate of TMP increase will accelerate in this case; however, in many circumstances the rate of TMP increase tends to self-correct after an adverse water quality event has subsided, returning to the previous rate of increase exhibited under more typical conditions. If the rate of TMP increase does not self-correct, then the affected train(s) should be monitored closely, as chemical cleaning may be required sooner than would otherwise have been anticipated as shown in Figure 1. Figure 1: TMP Vs. Time
TMP vs. Time 35
44-Day
25-Day Run
30
Recoverable TMP Increase
(p )
25
Non-Recoverable TMP Increase
20
TMP
15
10
5
0 1
10
20
30
40
45
55
65
70
It is important to note that an increase in TMP over the course of MF/UF system operation is both normal and typical. The goal of an operator or designer should not be to eliminate any type of TMP increase, but rather to allow a gradual and manageable rise with the ultimate TMP goal of ensuring that any such increases are recoverable with reasonable and effective chemical cleans. Every MF/UF system supplier uses a different membrane fiber and module configuration. As such, the typical TMP also varies among different MF/UF systems. The most significant differences are between submerged (vacuum) and encased (pressure) systems. Submerged polymeric MF/UF systems typically operate at relatively low TMP values ranging from about 2 to 10 psi, primarily because of the potential for inducing membrane fiber collapse under
excessive vacuum conditions. Absent this limitation, encased polymeric membrane systems operate over a wider TMP range that extends to higher threshold values from about 5 to 40 psi. Accordingly, encased systems often operate at higher fluxes than submerged systems. A summary of the minimum and maximum TMPs associated with some of the major MF/UF systems on the market is provided in Table 1. Table 1: Summary of Operating TMP Range for Common MF/UF Systems MF/UF System GE-Zenon 500 GE-Zenon 1000 Koch UF Hydranautics UF Pall Corporation MF Siemens – US Filter CMF Siemens – US Filter CMF-S
Encased or Submerged Submerged Submerged Pressure Pressure
Minimum TMP (psi) 2 2 2 4
Maximum TMP (psi) 10 12 30 20
Pressure
7
35
Pressure
5
30
Submerged
2
10
Temperature and Temperature Correction Temperature fluctuations can have a significant effect on MF/UF plant operation because of the associated changes in viscosity, as previously discussed. If the TMP is constant, the increased viscosity at lower temperatures reduces flow (or flux) across the membrane; if the flow (or flux) must remain constant to meet system demand, the TMP must increase to push colder, more viscous water through the membrane pores. While MF/UF systems can be designed to accommodate viscosity variations commensurate with changing water temperature, the effects can be significant during membrane operations and must be taken into account for proper monitoring and optimization. An example of the effects of temperature on membrane flux is shown in Figure 2.
Figure 2: Percent Change in Membrane Flux with Respect to Temperature Variation, Referenced to 20 Degrees C.
Percent Change in Membrane Flux
40 30 20 10 20 C Reference
0 -10 -20 -30 -40 0
5
10
20 15 Temperature (°C)
25
30
35
*Based on Mallevialle et al., 1996
For MF/UF membrane systems that operate at varying fluxes throughout the year, 20°C is a generally accepted reference temperature for normalization. Comparing the percent change in flux as the temperature varies from 20°C (at constant TMP) shows that the flux can increase or decrease more than 20% between approximately 10 and 30°C, a normal seasonal range of fluctuation in many climates. Thus, an MF/UF system can produce more filtered water with the same membrane area during warmer times of the year. However, other plant treatment processes and/or overall hydraulics may not necessarily be designed to accommodate this increase in flow. Temperature effects can be normalized to monitor system performance, allowing for the identification of changes attributable to fouling (or in some cases, membrane integrity breaches) independent of temperature variation. There are several well-established temperature correction factors (TCFs) based on an empirical fit of observed water temperature and viscosity data described in the literature. One such TCF described by Mallevialle et al (1996) for temperatures between 5 and 50°C is shown in Eq 1: TCF20 = exp[ –0.0239 (T–20)] (1) in which TCF20 is the temperature correction factor for normalization to 20°C, and T is the actual water temperature in °C. Another widely used methodology for temperature normalization is developed in the US Environmental Protection Agency’s (USEPA’s) Membrane Filtration Guidance Manual (2005), which starts with an expression for viscosity as a function of temperature, as shown in Equation 2:
2
–5
3
µT = 1.784 – (0.0575 × T) + (0.0011 × T ) – (10 × T ) (2) in which T is the ambient temperature in degrees C, and µ is the viscosity in centipoise at that temperature. The TCF is then determined by Eq 3: TCF = µT /µ20 The TCF is applied to normalize operations from the current temperature to the reference temperature (e.g., 20oC, in Eqs 1 and 3). In order to correct flux or TMP to the reference temperature, multiply or divide the value at the actual temperature by the TCF, respectively. This normalization allows system performance to be compared throughout the year irrespective of water temperature. However, it is essential to understand that the normalized flux or TMP does not represent an actual operating condition, but rather what the flux or TMP would be at the reference temperature, all other factors being equal. By comparing normalized data, changes in flux (at constant TMP) or TMP (for constant flow [or flux]) attributable to fouling can be observed. Temperature correction is important for evaluating membrane performance over both long-and short-term operations. In the short term, temperature correction is necessary in order to evaluate membrane fouling between chemical cleanings. For example, diurnal decreases in temperature could be incorrectly interpreted as membrane fouling and result in excessive and unnecessary routine maintenance cleaning events. Variations in temperature on a monthly scale would have a similar effect on the perceived frequency at which more extensive periodic recovery cleaning would be required; in the absence of temperature-corrected operations data, cleaning might be conducted either more often (resulting in higher maintenance costs and potentially reducing membrane life) or less frequently (allowing more extensive fouling to occur between cleanings) than would otherwise be optimal. Over long-term operations, temperature correction is required to evaluate irreversible fouling, because decreases in temperature over several months or multiple seasons – spanning several recovery cleaning events – could be incorrectly interpreted as irreversible fouling. Permeability Permeability, or specific productivity, can be simply explained as the TMP normalized by flux, and is represented by Eq 4: P = Qp/(Am × TMP) (4) In which P = permeability (gfd/psi), Qp = permeate flow rate (gpd), Am = membrane surface area (ft2), and TMP = transmembrane pressure (psi). A more beneficial description of permeability is the measurement of the driving force required to push (pressurized systems) or pull (vacuum systems) water through the membrane. Regarding performance metrics, although it may be similar to measuring the TMP alone, it provides the
added advantage that variations in flow experienced by the system during the operating cycle are essentially removed, providing a true indication of the level of membrane fouling. This is especially relevant for larger facilities in which several trains, or racks, are continually brought online or taken offline or have their flow reduced in response to the varying demands throughout the day. Unlike TMP, the more a membrane becomes fouled, the lower the permeability (because the TMP portion of the equation is on the bottom). Furthermore, unlike either flux or TMP, permeability can be used as a metric for either constant flow or constant pressure systems. Membrane permeability before, during, and after backwash can be an indication of both the severity of fouling and the mechanisms of membrane fouling. If backwash permeability is low, this can be an indication of pore fouling, because the backwash water cannot easily penetrate pores that are plugged with colloidal material. This may be a result of improper coagulation, or a change in the dissolved natural organic matter level in the feedwater. When permeability after the backwash is significantly lower than before the backwash, and permeability rapidly decreases to the prebackwash level, it is an indication of a high degree of cake fouling, because the backwash is breaking the cake on the surface, but it not able to entirely remove it. This is likely caused by large turbidity spikes or upsets in the pretreatment to the membrane system. Examining the permeability at these stages can give an indication of how to correct the fouling. Permeability is also often used as the primary metric for determining when membrane cleaning is necessary, regarding both longer term operation (cleaning cycles) and sudden fouling events. Many membrane manufacturers will use the rate of change of permeability as an indication that rapid fouling has occurred, and can set specific supervisory control and data acquisition logging alarms that warn operations staff that a fouling event has occurred (which cannot be done with TMP alone because rapid changes in flow rate will affect the TMP). Because permeability will remain stable for a stable system (if the temperature remains consistent), when changes of a specific unit of permeability over a specific time (i.e., 0.5 gfd/psi in 60 min) occur, the supervisory control and data acquisition system has been programmed to characterize this as a potential problem, and can warn operations staff to take corrective measures. The level of success for a cleaning event is also measured by permeability, often measured as a percent of permeability restoration as a function of the initial membrane permeability (or a fraction thereof depending on how long the membrane system has been in operation). Membrane manufacturers have provided specific cleaning chemical recipes to achieve a target permeability restoration, and examining the permeability restoration of each specific chemical within the recipe also provides information on the type of membrane fouling. For example, the bar chart in Figure 3 shows that a low level of permeability restoration was achieved with a high pH organic cleaning agent (sodium hypochlorite), and a high level of restoration was achieved with an inorganic acid. This indicates that the fouling is more inorganic in nature, such as pretreatment chemical overdosing or scaling, and operations staff can adjust the cleaning recipe to suit the specific condition.
Figure 3: Temperature Corrected Permeability Recovery from Cleaning
* Used with permission from Minneapolis Water Works, Fridley Membrane Pilot Study, 2007
As shown, membrane permeability, when interpreted properly, can be a useful metric in determining the stability of the membrane system. If operations staff members continuously monitor permeability in the situations outlined previously, preventive actions can be taken to ensure less downtime and smoother operation. Chemical Cleaning Effectiveness As previously discussed, permeability recovery is one of the primary metrics for determining the effectiveness of chemical cleaning (clean-in-place [CIP] or recovery clean processes). In order to properly use this metric, it is imperative to first establish a baseline permeability, ideally when the MF/UF membranes are new and mostly unfouled. This baseline can be determined by recording the permeability immediately after several successive chemical cleaning events. However, it is important to account for an initial break-in period when establishing a baseline for new membranes, which typically exhibit a pronounced permeability reduction upon being placed into service. After this initial break-in period, which may last from as little as a few weeks to as long as six months, the postclean permeability tends to stabilize; at this point, an applicable baseline may be fixed as a point of reference over the life of the membranes. If this initial breakin period is not taken into account, the baseline permeability will be artificially high, therefore erroneously making subsequent chemical cleanings appear to be ineffective.
One common and straightforward means of assessing chemical cleaning effectiveness is the use of the clean water flux test, as described in the USEPA Membrane Filtration Guidance Manual (2005). This test involves measuring the TMP as a function of flow (or flux) after each step (e.g., acid, base, and/or chlorine) in the chemical cleaning process. The results are plotted for quick reference, with the slope representing the inverse of permeability, such that lower slopes indicate higher values. Using this graphical analysis, the relative effectiveness of each step in the process can readily be observed, which can indicate the type of fouling experienced. For example, as presented in Figure 4, the largest decrease in slope is attributable to the application of acid in the first step (as shown by the difference between the red and green lines), suggesting that inorganic or particulate fouling may be prevalent. Figure 4: Example of Clean Water Flux Test Results for a Single Cleaning Operation
If the change in slope had been greater after the use of chlorine in the second step (as shown by the difference between the green and black lines), organic or biological agents would have been the more significant foulants. The slope of the line after the second and final step (chlorine— shown by the black line) in the cleaning process can be compared with the baseline (shown by the blue line) to assess the overall effectiveness of this particular cleaning operation. This approach can also be used to determine the order of cleaning chemicals. For example, for some applications permeability recovery may be higher and cleaning more effective if chlorine is used before citric acid whereas for other applications citric acid may be more effective if used ahead of chlorine.
The effectiveness of multiple chemical cleanings can also be compared on a single plot, as shown in Figure 5. Figure 5: Example of Comparative Clean Water Flux Test Results Over Multiple Chemical Cleanings
In this example, the effectiveness of the various chemical cleanings relative to the baseline (again shown by the blue line) seems to vary. However, when these permeability results are corrected for temperature, as shown in Figure 6, it is apparent that each of the cleanings successfully restores the permeability to the baseline level. Thus, although the water temperature does not vary appreciably during the course of implementing the various steps in a single chemical cleaning event, it is important to account for the differences in temperature when comparing the results of multiple cleanings.
Figure 6: Example of Comparative Temperature-Corrected Clean Water Flux Test Results Over Multiple Chemical Cleanings
If the permeability as measured after each chemical cleaning declines over time, the membranes may be irreversibly fouled. For a well-designed and properly maintained membrane system, irreversible fouling should only occur gradually over the course of a number of years. However, irreversible fouling is relative to the particular cleaning regime used. A more aggressive chemical clean (e.g., with higher solution strengths and/or extended soak times) or a cleaning operation using a different combination of chemical agents may remove foulants that would otherwise be considered irreversible by the routine cleaning regime used at a membrane filtration plant. However, frequent aggressive cleans may shorten membrane life. Such apparently irreversible fouling may be caused by fluctuating feedwater quality characteristics (including higher concentrations of and/or changes in contaminants) or prolonged exposure to even the typical water quality for which the system was designed. In some cases, sustained irreversible fouling may be attributed to seasonal variation in raw water quality. When the water quality changes back to that having less propensity for fouling, the membrane system permeability recovers. Periodically using a more aggressive cleaning recipe may be an effective strategy for maintaining MF/UF system permeability, although the frequency of such an operation must be evaluated against any potential adverse impact on membrane life.
In general, an effective chemical clean should restore permeability to within 95% of the established baseline for new membranes after the initial break-in period, with slowly declining effectiveness anticipated over their useful life. MF/UF membranes are typically replaced at the point at which chemical cleaning can no longer recover the permeability to 80% of the baseline. Backwash Practices As with conventional media filters, MF/UF systems are routinely backwashed to remove accumulated foulants, and because backwashing is conducted with filtered water, this process is by far the most significant factor affecting system recovery. However, limiting backwashing in an effort to increase recovery may likewise increase the required frequency of chemical cleaning, lower system productivity, and perhaps reduce membrane life. Thus, although backwashing is often overlooked as a focus area for optimizing MF/UF system performance, it is important to use backwash practices that appropriately balance maximizing plant recovery and minimizing chemical cleaning. MF/UF backwashing is commonly initiated on the basis of time, although TMP increase (or flow [flux] decrease, if the system is operated at constant pressure) and volume of water filtered may also serve as triggers. Filtration cycles between backwash events range from about 20 to 120 min, with backwash durations varying from 0.5 to 5 min. A typical backwash is conducted approximately every 30 min over a 2- to 3-minute span, although these parameters will vary based on water quality. For example, feedwater with higher turbidity or that is prone to algal blooms may increase both the frequency and duration of backwashing. One of the most effective means of reducing the effect of water quality on MF/UF system performance and/or potentially minimizing backwash is the optimization of membrane pretreatment. For example, if improved feedwater quality allows the filtration cycle to be modestly extended from 30 to 36 min while maintaining the same backwash duration and volume of water utilized, a plant operating at 95% recovery could increase benchmark performance to 96% by virtue of wasting 20% less water. However, the benefits to the membrane system and overall plant performance must be balanced by the cost and associated operational effect of pretreatment optimization. In addition to the frequency and duration, the backwash process itself may be optimized by varying the volume of water used and/or the pressure associated with the reverse flow, which is typically much higher than that associated with the forward flow under normal operating conditions. A study by Kennedy et al (1998) indicated that increasing the backwash pressure up to about two and a half times that of the operating pressure resulted in progressively improved restoration of membrane permeability (for backwash pressures tested, ranging from 3 to 23 psi). Further increase in the ratio of backwash to operating pressures was determined to not be costeffective. Similarly, Nakatsuka and Ase (1995) found that the optimum backwash pressure is typically more than two times the operating pressure. If air is used in the backwash process, then the pressure and duration of airflow can also be varied to optimize effectiveness and efficiency.
Maintenance Cleanings Maintenance cleanings are routine, short-duration processes designed to maximize the time between more extensive recovery cleanings by slowing the progressive increase of TMP (for operation at constant flux) or decline in flux (for operation at constant TMP). In some cases, effective maintenance cleanings have been shown to double the required period between recovery cleanings (Chalmers & Wachinski, 2007). Other common terms for maintenance cleanings include chemically enhanced backwash (CEB) and enhanced flux maintenance (EFM); particular use varies by MF/UF system manufacturer. These terms are functionally synonymous, and each represents a combination of a backwash with one or more cleaning chemicals applied in smaller concentrations over shorter contact times than in the case of a more extensive recovery clean. Typically, maintenance clean are conducted at a frequency ranging from once per day to once per week with a duration on the order of several hours. The selection of cleaning chemicals for both maintenance and recovery cleans is similar and based on the type of foulants in the feedwater, as summarized in Table 2. Table 2: Foulant-Based Selection of Cleaning Chemicals Foulants
Chemical
Inorganics (e.g., Fe and Mn)
Acid (e.g., citric or sulfuric acid)
Organic Materials
Base (e.g., sodium hydroxide)
Biological and Algal
Chlorine / Disinfectant (e.g., sodium hypochlorite)
Particulate Matter
Surfactants
For waters of varying quality, the importance of enabling proper selection and design of the fullscale plant’s chemical systems cannot be over-emphasized. The chemical compatibility of the membranes is also an important consideration. If multiple classes of foulants are problematic, the types of chemicals used may be cycled over a repeating series of maintenance cleans. For example, if the foulants include both significant organic material and low levels of iron and manganese, the maintenance clean sequence may be several caustic cleans followed by one acid clean. In this case, the consecutive series of caustic cleans are designed to remove the predominant organic foulant, whereas the less frequent acid clean removes the precipitated iron and manganese. The mix of cleaning chemicals may also vary during the course of the year as the foulants change, as in the case of a reservoir source subject to periodic turnover. Operators should monitor feedwater quality and membrane performance over time to determine the best maintenance cleaning strategy, as well as whether/when any changes are necessary.
The most direct means to evaluate the effectiveness of maintenance cleans on an ongoing basis is to monitor the TMP before and after each clean, as well as the rate of TMP increase between cleans. (For operation at constant pressure, the flux and rate of flux decrease, respectively, should be monitored.) The application of maintenance cleaning on a daily basis is shown in Figure 7 (Caothien & Wachinski, 2004). Figure 7: Example of Membrane Performance with Daily Maintenance Cleaning
As depicted in the figure, the TMP rises each day as the membrane becomes fouled and is subsequently restored to baseline levels after each daily clean. As flux is increased in two separate step increments, the daily increase in TMP becomes more significant, and the baseline TMP rises. However, in each case, maintenance cleaning effectively stabilizes membrane performance. Maintenance cleanings may be performed with recycled cleaning solution, which helps minimize in-plant water use, maximize recovery, and reduce residuals management costs. Used solution can be refreshed with additional active chemicals and reused a number of times. Moreover, unlike that for recovery cleans, maintenance cleaning solutions would be reused a number of times over the course of days rather than months, simplifying planning and storage. Integrity Testing In providing a robust microporous barrier for particulate matter and pathogenic microorganisms, membranes typically reduce turbidity to levels well below 0.10 ntu and achieve 5- to 7-log removal of Giardia and Cryptosporidium (USEPA, 2001); by virtue of a nominal pore size of about 0.01 µm—an order of magnitude smaller than that for MF membranes—UF can also achieve comparable log removal for viruses. Not only is this level of pathogen and particulate removal substantially in excess of that provided by media filters, but it is also independent of the
feedwater quality, as well, a performance characteristic that media filters lack. However, the reliance on MF/UF to produce such high-quality filtrate is necessarily accompanied by increased risk of compromising this performance with even a small breakthrough of unfiltered feedwater. Thus, monitoring the integrity of membrane filtration systems is a critical component of MF/UF system operation. Direct integrity tests (DITs) are used to challenge the membrane barrier and assess the presence of any breaches. Although there are a number of different DITs described in the literature, almost all commercially available systems used in municipal treatment applications are factoryequipped with the ability to conduct a pressure decay test. This DIT involves pressurizing one side of the membrane barrier and monitoring the rate of decay over a short period of time (e.g., typically 2–5 min). The results of this test (often expressed in units of psi/min) can be translated to an equivalent pathogen log removal value (LRV) using a methodology such as that detailed in the USEPA Membrane Filtration Guidance Manual (2005). A similar approach is described in ASTM standard D 6908-03: Standard Practice for Integrity Testing of Water Filtration Membrane Systems (2003). Although the pressure decay test (like all current DITs) is sensitive for detecting integrity breaches, an MF/UF system cannot produce filtered water during the application of the test. Thus it is common practice to conduct direct integrity testing once per day of operation, a frequency intended to strike an appropriate balance between the desire for operators to be alerted to potential breaches as often as possible with the need to minimize system downtime and maximum productivity. Between DIT applications, less sensitive indirect testing instrumentation that provides only inferential information about the membranes barrier– such as turbidimeters and particle counters–are used to monitor the filtrate on a continuous basis. Accordingly, direct and indirect methods are used in a complimentary manner to provide a continuous assessment of membrane integrity. The effect of a DIT on plant productivity can be reduced by minimizing the duration of the test. For example, decreasing the test time from five to two minutes cuts the time that the system is offline for integrity testing by more than half. Shorter hold times for the pressure decay also generate more conservative results. Because the driving force for pressure decay is higher earlier in the hold time when less air has escaped the system and the pressure is higher, a shorter test will yield results that reflect greater pressure loss per unit of time. By contrast, for the same integrity breach, a longer test will average pressure loss over a period during which the driving force for decay has dissipated to a more significant extent, resulting in smaller measured decay rates over the span of the test. MF/UF system owners should consult with their respective manufacturers to determine an acceptable DIT duration. Although this optimization step may have a small overall impact of system productivity, it may nevertheless be helpful for meeting peak demand and maximizing system efficiency. DIT results are most often used to assess system performance for pathogen removal (i.e., expressed as an LRV) as measured against a specific regulatory requirement. As discussed in the USEPA Membrane Filtration Guidance Manual, both an upper control limit (UCL) and lower control limit (LCL) may be used for this purpose, ensuring that the required removal is achieved on an ongoing basis. If DIT results surpass the LCL, a potential integrity problem is suggested, and operator attention and oversight are typically increased; exceeding the UCL generally triggers automatic shutdown of the affected unit(s) for further diagnostic testing and repair. An
example of such diagnostic testing is the examination of specific modules within the membrane unit, coupled with bubble testing to locate specific broken membrane fibers. This diagnostic testing is important not only for isolating and repairing broken fibers, but also for identifying false-positive integrity test results. For example, leaks in piping, valves, or connections upstream of the side of the membrane pressurized during the DIT may yield results suggest a breach even though the actual pathogen/particulate barrier between the feed and filtrates is fully integrated. Because these leaks adversely affect the ability of the DIT to detect more critical membrane breaches that could affect public health, it is important that any such issues be identified and corrected promptly. Diagnostic testing is also important for tracking problematic modules and assessing membrane useful life expectancy based on the rate of breach occurrence. For example, equipment specifications, warranty provisions, and/or operational guidance may indicate that a module should be replaced when 10–20% of the membrane fibers have been pinned or sealed as a result of breaches. Tracking the rate of fiber breakage in each module over time helps project when replacement will be necessary. A detailed module-by-module record of fiber breakage and repair also provides a valuable system profile for helping to diagnose the source and location of recurring problems, thereby allowing expedient repair and the consequent minimization of system downtime. Integrity Maintenance and Repair Because all membrane modules exhibit integrity breaches over time to a greater or lesser degree, effectively managing and repairing these breaches can be an important component of maintaining operational efficiency. Many membrane integrity breaches occur during module manufacture (i.e., defects) or are incurred in the course of shipping and/or installation (i.e., damage), with problems manifesting during initial system start-up or shortly thereafter. Thus, it is especially important to conduct routine and frequent integrity testing during system commissioning, as well as to carefully track the occurrence of breaches, identifying specific modules. These practices are useful not only for the purpose of determining whether a warranty claim may be applicable for any particularly problematic module(s), but also for regulatory compliance and ensuring that the MF/UF system maximizes the protection of public health when filtered water is first introduced into the distribution system. Integrity breaches are not limited to the membrane fibers, but can also occur in seals or the fiber potting material, as well as in system piping or valves. Moreover, the causes of these breaches are not unique to manufacturing defects, routine wear, or small debris particles present in the feedwater (although these are common mechanisms). For example, a seal may be rolled during system maintenance, or the potting may crack because of unusual stresses imparted to the membrane module during operation. Consequently, operators should be cognizant of all potential breach locations and vectors for occurrence when conducting an investigation. In addition, the potential for breaches to occur during maintenance (e.g., a rolled seal or dropped module) underscores the need to handle modules carefully at all times as an important preventive measure. For breaches associated with a module (e.g., at the potting or a fiber), many membrane filtration systems have features such as clear end caps that allow operators to visually locate one or more leaking modules amid an entire train that has been subjected to direct integrity testing. If a
system does not readily allow for visual inspection, sonic testing can be used as a diagnostic tool. In this case, an instrument called an accelerometer can be manually applied to each module in a train to listen for vibrations generated by leaking air. Although accelerometers have been demonstrated to be effective and sensitive tools for detecting integrity breaches, the application of these devices is more labor- and time-intensive than is visual inspection and requires a skilled and experienced operator (Adham et al, 1995). Ambient noise from operation of equipment in proximity to the trains can also pose a challenge to the use of sonic testing. After a leaking module has been identified, it can either be removed from the train to isolate and correct the breach or repaired in situ, depending on the system and the extent of the repair required. In all cases, the breach must be repaired in accordance with the manufacturer’s recommended procedure in order to maintain the system warranty and ensure proper operation. As is the case with in situ breach identification within a module, a simple soap solution may likewise be applied to locate leaks external to module itself, such as in piping and seals. The differences noted previously in terms of the level of effort required for diagnostic testing and repair, which ranges widely among the various commercially available MF/UF systems and can be labor-intensive, highlight the potentially significant cost associated with membrane integrity maintenance, particularly for large facilities with many millions of fibers. Thus, the economics of fiber repair are an important consideration for the operation of any membrane filtration plant. In some cases, because of the high cost of fiber repair or the importance of competing operator duties, a utility may strategically continue to operate with known integrity breaches, provided the MF/UF system still demonstrates on an ongoing basis (via a combination of direct integrity testing and continuous indirect integrity monitoring) that it is able to achieve the minimum pathogen removal credit required by the regulatory agency of jurisdiction. In other words, rather than immediately repair every detectable breach, operators may keep the affected train(s) on-line until a pre-determined threshold of integrity decline is reached, such as the LCL or UCL; once the UCL is reached, the utility would then be required to take the train(s) off-line for diagnostic testing and repair. This operational philosophy would increase system productivity by maximizing on-line time for all trains and should result in cost savings via the economy of scale realized by repairing many fibers (or other breaches) in a single event. If repairs are required on a more frequent basis, as in the case of larger MF/UF systems with more trains and fibers, a utility may opt to contract with the system manufacturer for integrity maintenance on an ongoing basis, thereby relieving operators of this responsibility and allowing them to focus on other critical duties. As previously discussed, it is essential to track integrity breaches over time, including the date/time of detection, the specific location, the cause (if known), and the means and date of repair. This integrity maintenance record can be an important tool to help identify trends, highlight problematic modules(s) (that may be subject to warranty claims), isolate causal factors for recurring breaches, and plan strategically for the timing and extent of conducting repairs, all of which can yield operational cost savings. General and Preventative Maintenance As is the case for all process equipment in a water treatment plant, a proactive and comprehensive maintenance program is essential to ensure the effective and efficient operation of any membrane filtration system. Accordingly, a successful maintenance program not only
protects a utility’s investment in its MF/UF equipment, limiting costly repairs and extending useful life, but also reduces down time that would otherwise adversely affect plant productivity. Notably, such a loss in productivity is quantifiable in two key ways: in addition to the obvious reduction in treated water generated, system down time increases operator labor by diverting attention from important routine practices to unplanned activities. Thus, an investment in general and preventive maintenance is typically more than offset by more efficient operation, longer equipment life, and the avoidance of more costly corrective maintenance. In keeping with the purpose of this article, this section is not intended to provide a comprehensive overview of MF/UF maintenance practices, but rather to briefly underscore some of the most important concepts and highlights significant and/or new information that may not be captured in other references. Any successful maintenance program is rooted in documented practices, including the means of tracking system performance. For a membrane filtration system, important parameters to track include: • • • • •
TMP, permeability, integrity test results (i.e., pressure decay and/or LRV achieved), chemical cleaning effectiveness, and filtrate turbidity.
An ongoing record of these data provides information about historical trends, which are invaluable not only for identifying and diagnosing problems (sometimes before they occur), but also for system optimization. Moreover, the comprehensive picture of system performance yielded by using the data cited above in concert is far more instructive than examining any one metric in isolation, as each provides an important piece of the puzzle for system analysis. Thus, thorough and extensive data collection is strongly recommended. General MF/UF system inspections are valuable for identifying possible operating issues. Consequently, such inspections should be conducted on a monthly basis, at a minimum. One of the most important aspects of a general inspection is determining whether any excessive solids have accumulated in the system, which can ultimately damage the membranes. The accumulation of solids can be caused by underperforming pretreatment processes, prescreen bypass or failure, and/or improper valve cycling. This potential issue is particularly important for submerged membranes, because solids may build up in the tanks if not completely removed in the periodic drains associated with backwashing or cleaning. Membrane tank fill and drain rates can be observed visually; correcting these rates as necessary to conform with the manufacturer’s recommended settings will optimize membrane performance by maintaining production capacity and reducing solids accumulation. In addition, drained tanks should be thoroughly inspected for signs of cracking and deterioration, which may generate particulates that can adversely affect the membrane performance. For both submerged and encased membrane systems, prescreens should periodically be removed and inspected for corrosion and pitting. Hoses, piping, and fittings can easily be visually inspected for integrity, and this practice should be conducted on a routine basis as a preventive measure
even if no breaches are detected via a DIT. Accordingly, this inspection should be applied to the entire membrane system and ancillary equipment rather than simply to those components that might allow for the bypass of unfiltered water around the membrane barrier. Hoses should be examined for cracks and signs of collapse; if necessary, soapy water can be applied to detect potential breaches by checking for bubble formation caused by escaping air. Any damaged hoses should be replaced immediately. Nuts and bolts throughout the membrane system should likewise be checked periodically to ensure proper torque. Fasteners such as cam locks and hose clamps should also be inspected periodically to ensure their proper engagement. In submerged systems with a slack fiber configuration, ensuring that no undue stress is applied to the fibers is necessary for achieving optimum system performance. However, gradual membrane shrinkage over the course of time can cause a reduction in fiber slack. In addition to the effect on performance, reduced slack increases fiber tension, which can augment the occurrence of integrity breaches via premature membrane failure. Thus, for applicable submerged MF/UF systems, each module should be inspected periodically for proper slack in accordance with the manufacturer’s recommendations. As with all inspection results both the qualitative observations and quantitative measurements, as applicable to membrane slack should be recorded for future reference. Modules with reduced slack can be monitored with greater scrutiny for possible breaches, and the resulting data may also help identify the cause and corrective measures associated with any module(s) exhibiting disproportionate shrinkage in the system. In addition, be aware of emerging maintenance challenges that could adversely affect membrane system operations or maintenance. An example of one such contemporary challenge is invasive zebra and quagga mussels, which can cause extensive membrane damage either by growing within the membranes and associated tanks for submerged systems or via shell fragments that might enter the membrane system after bypassing pretreatment processes. If these invasive mussels are detected in a source water feeding the membrane filtration system, certain mitigation options such as incorporating membrane prescreening at a mesh size < 50 µm, periodic heating of the water to temperatures greater than 37ºC and/or the addition of a chemical oxidant to which the mussels are exposed to could prove effective. For any maintenance practice or program, proper documentation is critical. In addition to completed operator log sheets and results of inspections including observations, problems detected, and any corrective measures implemented, a thorough and detailed maintenance record should also incorporate a digital photo or video log. Not only does documentation provide an important history of the membrane system, which may be useful for diagnosing future problems, but it also helps avoid duplication or overlap of maintenance activities. The information captured in this maintenance documentation may also prove useful for sharing instructive and/or innovative insights for the benefit of the water treatment industry in future reference materials much like this article. Summary This report was prepared by the Membrane Processes Committee (MPC) to provide AWWA members with a succinct summary of the most important and practical contemporary knowledge regarding the methods and metrics for MF/ UF system optimization that is not readily available
through other sources. In addition to the cited references, the information provided in this article is based on the collective expertise of the MPC, which consists of more than 50 water treatment professionals with extensive membrane-related experience representing academic researchers, utilities, equipment manufacturers, and consulting engineers. For additional information about this article or the MPC, contact AWWA or the MPC Chair, Brent Alspach, at
[email protected]. Authors Brent Alspach – Arcadis Paul Delphos – Black and Veach Jonathan Pressman – US EPA Jeff Beaty – Ch2M Hill Trevor Cooke – Hatch Mott McDonald Nikolay Voutchkov Jim Schaefer – AECOM Roger Noack – HDR Frank Marascia Daniel Konstanski – URS Corporation 4051 Ogletown Road Newark, DE 19805
[email protected] 302-781-5900 References Adham, S.; Jacangelo, J.; and Laine, J.-M., 1995. Low-Pressure Membranes: Assessing Integrity. Journal AWWA, 87:3:62. ASTM, 2003. D 6908-03: Standard Practice for Integrity Testing of Water Filtration Membrane Systems. http://www.astm.org/Standards/D6908.htm (accessed Dec. 27, 2012). Caothien, S.; Chalmers, S.; & Wachinski, A., 2007. : Membrane Design Flux: Optimization, Not Standardization. Ozwater 2007 Conference and Exhibition. Proc. Australian Water Assn., Sydney, Australia. Kennedy, M.; Kim, S.-M.; Mutenyo, I.; Broens, L.; & Shippers, J., 1998. Intermittent Crossflushing of Hollow Fiber Ultrafiltration Membranes. Desalination, 118:1–3:175. http://dx.doi.org/10.1016/S00119164(98)00121-0. Mallevialle, J.; Odendaal, P.E.; & Wiesner, M.R., 1996. Water Treatment Membrane Processes. McGraw-Hill, New York, N.Y. Nakatsuka, S. & Ase, T., 1995. Ultrafiltration of River Water for Drinking Water Production. Proc. AWWA Membrane Technology Conference, Reno, Nev
USEPA (US Environmental Protection Agency), 2005. Membrane Filtration Guidance Manual. EPA 815-R-06-009. Office of Groundwater and Drinking Water, Cincinnati, Ohio. USEPA, 2001. Low-Pressure Membrane Filtration for Pathogen Removal: Application, Implementation, and Regulatory Issues. EPA 815-C-01-001. http://water.epa.gov/lawsregs/ rulesregs/sdwa/lt2/compliance_membrane-filt.cfm (accessed Dec. 27, 2012).
Metrics & Methods for MF/UF System Optimization Roger Noack, P.E. S.P.A. HDR
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Authors • This presentation is a summary of a Committee Report written by members of the Membrane Process Committee of AWWA. • The Report was published in the February 2013 issue of JAWWA (Issue 105) and is available through AWWA. • Contributing Authors Included: Roger Noack – HDR Paul Delphos – Black & Veach Jeff Beaty – CH2M Hill Nikolay Voutchkov – Globe Consulting Frank Marascia – City of New Brunswick
Brent Alspach – Arcadis Jonathan Pressman – US EPA Trevor Cooke – Hatch Mott Jim Schaefer – AECOM Daniel Konstanski - URS
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Introduction • • • • • • • • • • •
Overview of MF/UF System Optimization Transmembrane Pressure Temperature & Temperature Correction Permeability Chemical Cleaning Effectiveness Backwash Practices Maintenance Cleans Integrity Testing Integrity Maintenance & Repair General & Preventative Maintenance Questions AWWA/AMTA©
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Overview of MF/UF Optimization • Many MF/UF plants fail to maintain their design outputs. • This is caused by a variety of factors: – – – –
Design problems Equipment problems Operational problems Maintenance problems
• System optimization seeks to proactively identify these issues and address them before they impact plant performance.
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Overview of MF/UF Optimization • System Optimization is realized through 2 primary actions: – Strategic use of key operational Methods. – Tracking of important Metrics. • Unfortunately these operational practices are not always commonly known. • This presentation will provide a brief overview of the key Methods and Metrics for MF/UF Optimization
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Key Metrics • • • •
Transmembrane pressure Temperature and temperature correction Permeability Chemical cleaning effectiveness
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Transmembrane Pressure (TMP) • Measure of the driving force for membrane filtration. • Primarily a function of the porosity of the membrane and the viscosity of the water • Impacted by 3 primary factors: – Feedwater quality – Temperature – Membrane fouling
• Decreases in temperature or blockage of membrane pores increase the TMP AWWA/AMTA©
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Transmembrane Pressure (TMP)
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Transmembrane Pressure (TMP) • Measuring TMP is an effective tool for system operation monitoring & optimization. – Establishing a baseline TMP and then comparing daily measurements can indicate system issues such as: • Damaged membranes • Fouling • Feed water temperature change
• Major loss of TMP can cause pressure differential which can severely damage the membrane. AWWA/AMTA©
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Utilizing TMP • Employing a TMP buffer can reduce the risk of damage to the membranes. – A TMP threshold of 75-80% of the maximum is recommended. – This buffer protects the membrane.
• It is important to note that an increase in TMP over the course of MF/UF system operation is both normal and typical. The goal of an operator or designer should not be to eliminate any type of TMP increase, but rather to allow a gradual and manageable rise with the ultimate TMP goal of ensuring that any such increases are recoverable with reasonable and effective chemical cleans. AWWA/AMTA©
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TMP Vs. Time TMP vs. Time 35
44-Day
25-Day Run
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Recoverable TMP Increase
(p )
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Non-Recoverable TMP Increase
20
TMP
15
10
5
0 1
10
20
30
40
45
55
65
70
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Suggested TMP Operating Ranges MF/UF System GE-Zenon 500 GE-Zenon 1000 Koch UF Hydranautics UF Pall Corporation MF Siemens – US Filter CMF Siemens – US Filter CMF-S
Encased or Submerged Submerged Submerged Pressure Pressure
Minimum TMP (psi) 2 2 2 4
Maximum TMP (psi) 10 12 30 20
Pressure
7
35
Pressure
5
30
Submerged
2
10
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Temperature & Temperature Correction • Temperature impacts MF/UF operations due to associated changes in viscosity. – Higher viscosities will result in lower flux. – Lower viscosities will result in higher flux.
• As the temperature decreases the TMP must be increased in order to maintain the same flux (flow) through the membrane.
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Percent Change in Membrane Flux with Respect to Temperature Variation Percent Change in Membrane Flux
40 30 20 10 20 C Reference
0 -10 -20 -30 -40 0
5
10
15 20 Temperature (°C)
25
30
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Temperature & Temperature Correction • All systems are designed to produce a certain flux within a range of source water temperatures. • Operators and managers need to know the following: – The temperature design range for their system – Yearly fluctuations in source water temperature
• With this data temperatures throughout the year can be normalized and a Temperature Correction Factor (TCF) can be developed.
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Temperature Correction Factors • A TCF is applied to normalize operations from a current temperature to a reference temperature (normally 20 degrees C) • Flux or TMP can be normalized by multiplying or dividing the value at the actual temperature by the TCF respectively. • With this normalization system performance can be compared regardless of temperature. TCF = µT /µ20 AWWA/AMTA©
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Temperature & Temperature Correction Application • Temperature correction is necessary to properly evaluate membrane fouling. – Changes in TMP due to temperature can be erroneously blamed on fouling. – This leads to unnecessary cleaning costs and shortened membrane lifespan.
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Permeability • Permeability is, on the surface, similar to TMP. • It is a measurement of the driving force required to push (pressurized systems) or pull (vacuum systems) water through the membrane. P = Qp/(Am × TMP) (4) P = Permeability (gfd/psi) Q=Permeate Flow Rate (gpd) Am = Membrane Surface Area (ft2) TMP = Transmembrane Pressure (psi)
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Permeability • Advantage over straight TMP measurement: – Variations in flow experienced by the system during the operating cycle are essentially removed
• Membrane permeability before, during, and after backwash can be an indication of both the severity of fouling and the mechanisms of membrane fouling. AWWA/AMTA©
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Permeability • In the following graphic the level of permeability restoration achieved by various chemical cleaning agents is shown. • The level of success of different chemicals indicates the type of fouling. • In the following chart high pH organic cleaning agents were less effective while an inorganic acid was more effective. • This indicates that the fouling is more organic in nature. AWWA/AMTA©
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Permeability
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Chemical Cleaning Effectiveness • Determining the effectiveness of chemical cleanings is one of the most vital metrics an operator should track. • This is, as was just discussed, primarily accomplished through measuring permeability recovery. • In order for these measurements to be effective a baseline permeability must be established. – This baseline can be determined by recording the permeability immediately after successive chemical cleaning events. AWWA/AMTA©
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Chemical Cleaning Effectiveness • A common method for assessing chemical cleaning effectiveness is the clean water flux test. – Described in detail in the USEPA Membrane Filtration Guidance Manual
• Test consists of measuring the TMP as a function of flow after each step in the chemical cleaning process. – When plotted the slope of these measurements represents the inverse of permeability. • e.g. Lower slopes indicate higher permeability.
• The greater the change in slope between chemicals the more effective the chemical was at treating the specific type of fouling. AWWA/AMTA©
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Chemical Cleaning Effectiveness
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Chemical Cleaning Effectiveness • If the change in slope had been greater after the use of chlorine in the second step (as shown by the difference between the green and black lines), organic or biological agents would have been the more significant foulants. • The slope of the line after the second and final step (chlorine—shown by the black line) in the cleaning process can be compared with the baseline (shown by the blue line) to assess the overall effectiveness of this particular cleaning operation.
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Chemical Cleaning Effectiveness
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Chemical Cleaning Effectiveness
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Backwash Practices • As with conventional media filters, MF/UF systems are routinely backwashed to remove accumulated foulants. • MF/UF backwashing is commonly initiated based on one of the following parameters: – – – –
Time TMP increase Flux decrease Change in Volume of Filtered Water
• The backwash process may be optimized by varying the following parameters: – Volume of water used – Pressure associated with the reverse flow • This value is typically much higher than that associated with the forward flow under normal operating conditions. AWWA/AMTA©
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Maintenance Cleanings • Maintenance cleanings are routine, short-duration processes designed to maximize the time between more extensive recovery cleanings by slowing the progressive increase of TMP (for operation at constant flux) or decline in flux (for operation at constant TMP). • In some cases, effective maintenance cleanings have been shown to double the required period between recovery cleanings. • Other common terms for maintenance cleanings include chemically enhanced backwash (CEB) and enhanced flux maintenance (EFM); particular use varies by MF/UF system manufacturer. AWWA/AMTA©
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Foulant Based Selection of Cleaning Chemicals Foulants
Chemical
Inorganics (e.g., Fe and Mn)
Acid (e.g., citric or sulfuric acid)
Organic Materials
Base (e.g., sodium hydroxide)
Biological and Algal
Chlorine / Disinfectant (e.g., sodium hypochlorite)
Particulate Matter
Surfactants
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Integrity Testing • Membranes typically reduce turbidity to levels well below 0.10 ntu and achieve 5- to 7-log removal of Giardia and Cryptosporidium. • Not only is this level of pathogen and particulate removal substantially in excess of that provided by media filters, but it is also independent of the feedwater quality, as well. • However, the reliance on MF/UF to produce such highquality filtrate is necessarily accompanied by increased risk of compromising this performance with even a small breakthrough of unfiltered feedwater
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Integrity Testing • Direct integrity tests (DITs) are used to challenge the membrane barrier and assess the presence of any breaches. • This DIT involves pressurizing one side of the membrane barrier and monitoring the rate of decay over a short period of time (e.g., typically 2–5 min). • Almost all commercially available systems used in municipal treatment applications are factory-equipped with the ability to conduct a pressure decay test. • The results can be translated to an equivalent pathogen log removal value (LRV) using a methodology detailed in the USEPA Membrane Filtration Guidance Manual (2005 AWWA/AMTA©
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Integrity Maintenance & Repair • Because all membrane modules exhibit integrity breaches over time to a greater or lesser degree, effectively managing and repairing these breaches can be an important component of maintaining operational efficiency. • Many membrane integrity breaches occur during module manufacture (i.e., defects) or are incurred in the course of shipping and/or installation (i.e., damage), with problems manifesting during initial system start-up or shortly thereafter. Thus, it is especially important to conduct routine and frequent integrity testing during system commissioning AWWA/AMTA©
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General & Preventative Maintenance • Any successful maintenance program is rooted in documented practices, including the means of tracking system performance. For a membrane filtration system, important parameters to track include: – – – – –
TMP, Permeability, Integrity test results (i.e., pressure decay and/or LRV achieved), Chemical cleaning effectiveness, and Filtrate turbidity.
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Questions?
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