Frequently Asked Questions

1. Q: Why would I choose low-pressure membranes over media filtration for a drinking water application?
  A: Low-pressure membranes are now an accepted alternative to media filtration and are fully compliant with the Long–Term 2 Enhanced Surface Water Treatment Rule. The installed cost of membranes is very comparable to that of granular media filters. However, membranes provide the advantage of an absolute barrier proven to be integral. Therefore, the question today isn’t why choose membranes; the question is why not?

This FAQ supplied by: USFilter

2. Q: Can membranes be applied to challenging water sources such as those requiring enhanced coagulation for total organic carbon (TOC) control or seasonal powdered activated carbon (PAC) addition for taste and odor mitigation?
  A: First generation membranes were designed for turbidity and pathogen removal, but were not well suited to high solids or chemically coagulated feed waters. In the last few years, new membranes and system configurations have been developed to address more challenging water sources, including those containing high solids, chemical addition, or even PAC addition.

This FAQ supplied by: USFilter

3. Q: My community is considering reclaiming wastewater. How do membranes apply?
  A: Membranes now play a prominent role in wastewater reclamation, primarily in two ways. First, membrane filtration can be added to a secondary wastewater treatment plant to provide tertiary effluent for reclamation. Membranes provide a higher, more reliable standard of treatment than media or cloth filters. This is especially important when the reclaimed water is planned for unrestricted use where potential for human contact exists (e.g., irrigation of golf courses, parks or schools). Under those circumstances, it is reassuring to know that there is a positive barrier between pathogens and the public.

Second, membrane bioreactors (MBR) are taking on an increasing role in wastewater reclamation for good reason. The MBR process uses membranes to replace secondary clarifiers. Following the activated sludge process, hollow fiber membranes are immersed in mixed liquor. Vacuum pumps create suction on the membranes which separates the treated effluent from the mixed liquor. The MBR process operates under the same principles as the continuous microfiltration process, except that the system is optimized to operate in mixed liquor.

Because of the higher water quality produced with membranes, the options for reuse are expanded and include:
• Golf course water features and irrigation;
• Irrigation of public and private spaces such as parks, greenbelts and schools;
• Indirect potable reuse;
• Aquifer storage and recovery;
• Salt intrusion barriers in coastal communities; and
• Industrial applications, such as cooling towers and boiler feed water.

This FAQ supplied by: USFilter

4. Q: Why should I consider MBR over secondary effluent filtration?
  A: There are several instances where MBR may meet your needs better than secondary effluent filtration. First, if space for expansion is limited at a facility, then MBR technology offers some significant advantages. Existing clarifiers can be replaced or retrofitted with membranes. Because membrane filtration is unaffected by sludge settleability, the activated sludge process can be optimized for biological activity. Typical MBR mixed liquor suspended solids (MLSS) concentrations are 10,000 to 15,000 mg/l, allowing the activated sludge process to treat a higher flow and load in the same tankage.

Second, when the potential users of reclaimed water are not located near the main wastewater treatment plant(s), or installing distribution systems to convey the reclaimed water to the reuse sites is difficult or expensive, MBR technology offers a unique solution. By locating remote MBR facilities near the reclaimed water users, these problems are avoided. The “satellite” or “scalping” plants can extract or “mine” sewage from large trunkline or interceptor sewers, then deliver the treated effluent directly to the users and return the waste activated sludge to the sewer.

This FAQ supplied by: USFilter

5. Q: Can I expect better quality effluent with MBR technology?
  A: Yes. In general, MBR systems can provide higher quality effluent than settled activated sludge followed by membrane filtration. By removing the operational constraint of sludge settleability, the activated sludge process can be operated at very high MLSS concentrations. This allows for higher solids retention time and leads to more complete biological oxidation. MBR effluent looks, smells, and simply is better than conventional secondary effluent. New data suggests that the very high mean cell residence time of the MBR process results in higher removals of difficult-to-treat compounds such as endocrine disrupters and biopersistent pharmaceuticals.

This FAQ supplied by: USFilter

6. Q: Is an MBR system difficult to operate and maintain?
  A: No. An MBR system is a fully automated process. An operator only needs to enter a few key parameters. The computer monitors the process and provides data trends that tell the operator when routine maintenance is needed.

Membrane cleaning is the primary maintenance needed to keep the system operating at peak form. Cleaning the membranes is recommended every four to six months for a short duration (about four to six hours). This task is performed within the membrane tanks (no need to move the membranes to a designated tank) and is fully automated by the push of a button. The MBR process can easily be observed, diagnosed, and controlled from a distance using proven, reliable remote telemetry systems.

This FAQ supplied by: USFilter

7. Q: How will I know my system is functioning properly?
  A: To be effective, all parts of the membrane system must maintain a physical barrier between feed water and filtrate.

A fully automated pressure decay test (PDT) provides online verification of membrane integrity. And independent testing has proven the PDT as more accurate than turbidimeters or particle counters in finding defects. The PDT works by applying backpressure to the permeate (clean water) side of the membrane system and holding it for a prescribed time period. If the pressure loss is low, you know the system is integral and the barrier is valid. If not, a diagnostic test can be conducted to locate and isolate the integrity breach.

This FAQ supplied by: USFilter

8. Q: What are the requirements for membrane integrity testing in the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR)?
  A: The LT2ESWTR released by the U.S. EPA for public comments in August 2003 requires that membrane systems must undergo direct integrity testing and continuous integrity monitoring for compliance. The removal credit for any membrane system can be only granted as it can be verified by integrity testing.

This FAQ supplied by: Pall Corp.

9. Q: What is the difference between direct and indirect integrity testing?
  A: In general, direct integrity testing is defined as a “physical test” that is able to detect and isolate integrity breach. An example of direct integrity testing is the pressure-hold test. Indirect integrity testing is based on monitoring filtrate water quality. Examples of indirect integrity testing include turbidity monitoring and particle counts. The key difference between direct and indirect integrity testing is the sensitivity of the method: direct integrity testing is typically much more sensitive that indirect integrity testing.

This FAQ supplied by: Pall Corp.

10. Q: How is direct integrity testing regulated in the LT2ESWTR?
  A: EPA does not specify a particular direct integrity testing method. Instead, it requires that all testing methods have to meet three performance criteria: resolution, or the smallest integrity breach that is detectable; sensitivity, or the maximum log reduction value that a testing method can verify; and testing frequency.

This FAQ supplied by: Pall Corp.

11. Q: What are the performance criteria specified in the LT2ESWTR?
  A: The rule specifies the following performance criteria for direct integrity testing:
• A direct integrity testing shall be able to detect a breach equal or less than three micrometers;
• Sensitivity shall be determined based on a site-specific approach; and
• Testing frequency shall be no less than once per day.

This FAQ supplied by: Pall Corp.

12. Q: How can the 3 mm resolution criterion in direct integrity testing be met?
  A: For pressure-hold test, this is simply a matter of applying a pressure high enough to overcome the bubble point of a 3 mm hole plus the backpressure and hydraulic head. The bubble point for a 3 mm hole is about 14 lb per square inch (psi) or 0.95 bars. For example, if a membrane system has no backpressure but has a 7-ft of hydraulic head in the membrane tank or module, the testing pressure to meet 3 mm resolution criterion is 14 psi + 7 ft x 0.45 psi/ft H2O = 17.2 psi. (~1.2 bars).

This FAQ supplied by: Pall Corp.

13. Q: What are factors affecting the test pressure to meet the resolution criteria?
  A: Hydrophobicity (i.e. water-dislike nature) of membrane media and the shape of the hole can affect the testing pressure to meet 3 mm resolution criteria. However, the rule specifies that the most conservative values shall be used as the default when considering resolution. The reasons for that are:
• Hydrophobicity of a membrane media is not routinely measured during the production of membranes due to lack of proper method. Thus, it is not controlled;
• Hydrophobicity of a used membrane media is different from new membrane. There is no easy way to measure it in a plant setting; and
• There is no practical way to predict what shape a membrane breach would be.

As a result, it is scientifically indefensible to use any means except the default values when determining the testing pressure, as required by the rule.

This FAQ supplied by: Pall Corp.

14. Q: How is sensitivity of direct integrity testing determined?
  A: The rule specifies that the sensitivity of a membrane system is site-specific. This is because many factors such as the module and the system configurations, sizes, air-tightness of the piping, operating conditions, the parameter settings for integrity testing, and temperature can have an effect on log reduction values. To determine the sensitivity of a membrane system, the log reduction value of the system has to be related to parameter(s) of the direct integrity testing (e.g., pressure decay rate in pressure-hold test) through hydraulic modeling and/or experimental validation.

This FAQ supplied by: Pall Corp.

15. Q: How can the frequency for direct integrity testing be determined?
  A: Given the fact that current practices of direct integrity testing are not continuous and requires down time from production, the determination of testing frequency needs to balance both safeguarding public health and enhancing production. One good approach to determine the testing frequency is a risk-based approach. This approach relates the risk of membrane failure (both the probability of occurrence and consequence of the failure) to testing frequency. Historical record and membrane fatigue test can be used to generate the probability of occurrence of membrane failure. The consequence of the membrane failure can be evaluated through plant validation tests and hydraulic modeling.

This FAQ supplied by: Pall Corp.

16. Q: How is indirect integrity testing regulated and what are important issues?
  A: Indirect integrity testing, such as turbidity monitoring and particle counts, is required to act as continuous monitoring of gross integrity failure due to its low sensitivity. The rule specifies that an upper control limit (UCL) of monitored parameters has to be established. Once the UCL is exceeded, it triggers direct integrity testing. The most important issues of indirect integrity testing are the low sensitivity and poor reliability of the methods, which make establishing UCL very difficult under certain circumstances.

This FAQ supplied by: Pall Corp.

17. Q: How can you include integrity test goals/procedures in specifications to conform to requirements of the LT2ESWTR?
  A: When specifying a membrane system, be sure that direct integrity test methods are specified with defined critical variables that fit the site variables. Assure that the pressure for direct integrity testing is specified to meet the resolution criteria required by the LT2ESWTR, and accounts for system variables such as backpressure and static head. Assure that sensitivity is sufficient to meet or surpass the granted removal credit.
In the case that integrity breaches are found, assure that the system is designed to allow the operator to quickly locate the problematic module or fiber and perform repair or change out operations without significant extra equipment or labor time.

This FAQ supplied by: Pall Corp.

18. Q: What is the difference between a Laser Turbidimeter and a Conventional Turbidimeter?
  A: The difference between a laser turbidimeter and a conventional turbidimeter is in both the light source used and the detection system. Conventional turbidimeters are often referred to as a white-light turbidimeter and they typically conform to the United States Environmental Protection Agency (USEPA) Method 180.1, which is the original method for monitoring turbidity in water. Laser turbidimeters, have a laser-based light source, which is basically a monochromatic light source with a controlled intensity. Second, laser turbidimeters have a higher sensitivity detection system, which is able to detect lower levels of scattered light than conventional turbidimeters. One laser turbidimeter technology, which is represented by the Hach FT660 sc family of instrumentation is also EPA approved. The approval method is known as the Hach Method 10133 or the FilterTrak Method.
This FAQ supplied by: Hach Company.

19. Q: What is the difference between a direct integrity test and a continuous monitoring test?
  A: A direct integrity test is typically a pressure-based test that determines if the membrane unit (rack) is intact, or free of any leaks. Direct integrity tests require that the membrane unit be taken off line to conduct the test. A continuous monitoring test monitors the integrity of a membrane unit through the monitoring of water quality. The direct integrity test is typically more sensitive, but it is an intermittent test, and the continuous monitoring test, though less sensitive, monitors the quality of the membrane system continuously. Typically, direct tests are complemented or supported by continuous monitoring tests.
This FAQ supplied by: Hach Company.

20. Q: Is membrane performance currently regulated for drinking water applications?
  A: All membrane systems that are used to produce drinking water are regulated in some manor. Though there are no current federal regulatory requirements that are specific for membranes, states currently do have monitoring protocol for membrane performance and integrity. The EPA has published the Membrane Guidance Manual as well as a draft version of comment of the Long Term 2 Enhanced Surface Water Treatment Rule. In the absence of federal regulatory requirements, some states are much more proactive in membrane monitoring, while other states currently default to more standardized filter effluent monitoring practices
This FAQ supplied by: Hach Company.

21. Q: Why should I use a Laser turbidimeter to monitor membrane water quality?
  A: Laser turbidimeters are more sensitive and more accurate when compared to conventional turbidimeters, measuring in the range of 0.005 to 5.000 NTU. When monitoring membrane integrity, it is important that the instrumentation used is 1) stable, 2) accurate at the low end, and 3) sensitive to fine changes in turbidity. Laser turbidimeters possess all three characteristics when compared to conventional turbidimeters.
This FAQ supplied by: Hach Company.

22. Q: What is the difference between measurement accuracy and sensitivity?
  A: Measurement accuracy is how close an instrument when compared to a known value. Sensitivity is how small a change in a process that can be reliably measured and detected. A test called the limit of detection (LOD) can be used to determine measurement sensitivity.
This FAQ supplied by: Hach Company.

23. Q: What are the major concerns when monitoring membrane effluent quality?
  A: Insure instrumentation is installed at appropriate sample points that insure a homogeneous and representative sample. Sampling at point where the filtrate or permeate water changes direction (such as following a 90-degree elbow) are suggested.
This FAQ supplied by: Hach Company.

24. Q: Instrument bodies (sensor bodies) must be conditioned prior to the establishment of any baseline that is used as a basis for an upper control limit (UCL) or alarm condition. A stable continuous monitoring baseline is represented by a level measurement values that are void of random and un-predicted spikes.
  A: Continuous monitoring methods, like turbidity and particle counting, can be applied to either positive or negative filtration, but both types require different sampling to minimize bubble interference. The following information can be used as guidance for specific types of membrane types.

For UF, NF, and RO systems with no air backwash require the least amount of sampling conditioning because bubble generation during the cleaning processes is minimal. Typically, reverse-flow cleanings do not cause spikes in the measured values. Standard sampling protocols that are provided by the instrument manufacturers are adequate. MF and UF systems that are outside in-filtration processes often use air scour at the within the filtration cycle. For these systems, bubble generation and subsequent bubble interferences can occur. For these systems bubble interferences are minimized through the use of: a) sample chambers that can be pressurized to prevent further outgassing b) Additional bubble traps such as those that contain a larger volume may be added to dampen bubble interference; c) The integration of a signal from the rack that indicates the stoppage of forward filtration flow can be fed to either the plant SCADA system or data logging program. Under this condition data is either ignored or not logged; and d) A time lag should be incorporated into the data logging program to ignore data for the first 2-6 minutes (user changeable) after the completion of an air-scour cleaning procedure. This is to allow for air to vacate sample lines prior to resuming the logging of measurement data.
This FAQ supplied by: Hach Company.

25. Q: What is measurement reliability?
  A: Measurement reliability is providing instrument measurement information that is truly reflective of the process that is being monitored. It is achieved through the elimination of false positive values (a increase in the measured value that did not occur) and false negative values (a change in the measurement process that was not detected). It is possible to have false positives and false negatives with any type of membrane integrity test.
This FAQ supplied by: Hach Company.

26. Q: How can measurement reliability be improved?
  A: Reliability is improved through the selection of the proper sampling location within a membrane system and through the elimination of interferences. Please refer to the section on how to address the major concerns when monitoring membrane effluent quality address. Reliability is improved through the implementation of techniques that reduce or eliminate false positives and false negatives.
This FAQ supplied by: Hach Company.

27. Q: What determines a positive response?
  A: For a continuously monitoring method, a positive response would be characterized by a sudden and increase in the measurement values and / or a sudden increase in the variability of the measurement values. For example, if your membrane effluent was characterized by constant low turbidity of 0.022 NTU and it varies by less than 0.002 NTU suddenly increases to 0.033 NTU and varies by 0.010 NTU, then this could be an indicator of an integrity breach.
This FAQ supplied by: Hach Company.

28. Q: If my membrane encounters a positive response, what should I do?
  A: The operator should further investigate the response. This could be as simple as running a direct integrity test.
This FAQ supplied by: Hach Company.