1. What are the requirements for membrane integrity testing in the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR)?
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.
2. What is the difference between direct and indirect integrity testing?
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.
3. How is direct integrity testing regulated in the LT2ESWTR?
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.
4. What are the performance criteria specified in the LT2ESWTR?
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.
5. How can the 3 mm resolution criterion in direct integrity testing be met?
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).
6. What are factors affecting the test pressure to meet the resolution criteria?
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 mem- branes due to lack of proper method. Thus, it is not controlled;
- Hydrophobicity of a used mem- brane 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.
7. How is sensitivity of direct integrity testing determined?
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.
8. How can the frequency for direct integrity testing be determined?
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.
9. How is indirect integrity testing regulated and what are important issues?
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.
10. How can you include integrity test goals/procedures in specifications to conform to requirements of the LT2ESWTR?
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.
RSS: WWD Articles
.gif)
