DIRECT INTEGRITY TESTING
The LT2ESWTRLT2ESWTR: The Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) is an EPA regulation targeting control of Cryptosporidium and other microbial contaminants. It mandates treatment efficiency, i.e., log removal values, and gives credit to various treatment technologies to achieve the needed treatment efficiency. requires at least daily direct integrity testingDirect integrity test: A direct integrity test refers to the process usually performed daily to detect breaches in the membrane system. Integrity tests are pressure-based or marker-based. of MFMicrofiltration (MF) membranes: Microfiltration membranes are typically hollow-fibers with a pore size range of approximately 0.1 – 0.2 μm (nominally 0.1 μm). and UFUltrafiltration (UF) membranes: Ultrafiltration membranes are typically hollow-fibers with a pore size range of approximately 0.01 – 0.05 μm (nominally 0.01 μm). membrane units that are being used to meet the CryptosporidiumCryptosporidium: According to the U.S. Center for Disease Control, Cryptosporidium is a microscopic parasite that causes the diarrheal disease cryptosporidiosis. Both the parasite and the disease are commonly known as "Crypto." There are many species of Cryptosporidium that infect humans and animals. The parasite is protected by an outer shell that allows it to survive outside the body for long periods of time and makes it very tolerant to chlorine disinfection. While this parasite can be spread in several different ways, water (drinking water and recreational water) is the most common method of transmission. Cryptosporidium is one of the most frequent causes of waterborne disease among humans in the United States. requirements of the LT2ESWTR. Direct integrity testing is intended to detect breachesBreach or integrity breach: A breach refers to leakage in the membrane module to due a broken fiber, leaking seal or other leak that can allow microbes and particulates to bypass the membrane and pass directly into the finished water. in the membrane units that would allow unfiltered water to essentially bypass the membrane. Direct integrity testing is applied to the entire membrane module including membranes, seals, potting material, valves and piping, etc. The test in effect verifies the log removal value (LRV)Log removal value (LRV) or log reduction credit: The log removal value or log removal credit is the filtration removal efficiency for a target organism, particulate, or surrogate expressed as log10(feed concentration) – log10(filtrate concentration). that the unit was awarded based on challenge testingChallenge testing: Challenge tests are conducted to determine the efficiency of removal of target particulates such as a microbe or a surrogate in order to calculate the log removal value of the membrane module..
Click this link to see specifics of the direct integrity testing requirements of the LT2ESWTR.
Direct integrity tests can currently be classified as pressure-based tests or marker-based tests. Marker based tests introduce a particulate or molecular marker into the feed stream and then measure its concentration in the filtrateFiltrate: Filtrate is the water that has passed through the membrane. in order to directly assess its removal efficiency; it is similar to challenge testing. Pressure-based tests on the other hand apply a pressure or vacuum to one side of the membrane and then monitor the pressure loss or displacement of air or water in order to determine if a breach has occurred. Regardless of what direct integrity test is used, it must be able to detect a breach of 3 μm or less, i.e., the resolution is 3 μm or less, and it must have a sensitivity capable of verifying a log removal value (LRV) equal to or greater than the LRV awarded through challenge testing. (Note that 3 μm is chosen since it is the lower size range of Cryptosporidium.)
An upper control limitControl limit (CL): A control limit in membrane systems refers to a response from an integrity test, for example exceeding a maximum allowable rate of pressure loss, that triggers a response by the operators such as searching for breaches. (UCL) must be established that serves as a trigger to take the membrane unit off-line for further testing, repair, and maintenance. For a pressure based test, the UCL is an air flow rate or a pressure decay rate which if exceeded triggers action. For a marker-based test, the UCL is the log removal credit awarded by the state. Additionally, the test must be conducted daily although less frequent testing may be approved by the state if sufficient process reliability, etc. can be demonstrated.
Whether a pressure-based or marker-based direct integrity test is used, the log removal established by the test will control the LRV credit awarded to the membrane unit if it is less than the LRV established in challenge testing. If it is greater, then the LRV determined with challenge testing will control.
Pressure-based direct integrity tests. A set of equations has been developed to relate the air pressure necessary to force air through a breach in a membrane module to the size of the breach. The minimum air pressure that must be applied to a membrane in order to overcome capillary forces for air to exit a 3 μm pore is predicted from the following equation derived from bubble-point theory:
Equation 1
Where:
Ptest = minimum test pressure, psi
κ = pore shape correction factor, dimensionless
σ = surface tension at the air-liquid interface, dynes/cm
θ = liquid-membrane contact angle, degrees
BPmax = maximum backpressure on the system during the test, psi
0.193 = constant that includes the defect diameter of 3 μm and unit conversion factors
Note that BPmax, i.e., the maximum backpressure on the membrane, is typically caused by the hydrostatic pressure on the membrane. For a system that is not submerged in water such as a pressure systemPressure system: A pressure membrane system applies a pressure to the feed water side of the membrane in order to force the water through the membrane., then BPmax is most likely zero unless the membrane unit is not drained of water before testing. For a vacuum systemVacuum systems: Vacuum membrane systems are microfiltration or ultrafiltration systems in which the membranes are immersed in a tank of water and a vacuum applied to pull the water through the membrane., it would be the maximum depth of water to the bottom of the membrane assuming that the membranes remain submerged during testing. The values of κ and θ are properties of the membrane which the manufacturer should be able to supply. Conservative values would be κ = 1 and θ = 0. Colder temperatures give more conservative values for σ. Using κ = 1 and θ = 0 and a value for σ of 74.9 dynes/cm for a temperature of 5oC produces the following equation:
Equation 2
This equation predicts that under these conservative conditions the minimum test pressure needed to achieve a 3 μm resolution, i.e., for air to exit a 3 μm breach, is 14.5 psi plus the maximum backpressure. For example, if the bottom of a vacuum membrane is under 7 feet of water (3.0 psi) then the minimum pressure would be 14.5 + 3 =17.5 psi. The EPA Membrane Filtration Guidance Manual (2005) strongly recommends using the conservative value of κ = 1 and θ = 0 unless other values can be defended and even then θ, the contact angle, may change with time and need to be adjusted. Additionally, the equations above assume that applied pressure remains constant during the test but often there is a baseline pressure decay that must be accounted for. One way to estimate the decay is to use the decay established for a fully integral system, e.g., a new membrane unit.
Application. The pressure decay test is the most common method of implementing a pressure-based direct integrity test and is the most commonly used direct integrity test in general for MF and UF systems. In this test, one side of the membrane is drained and a pressure in the range of 4 to 30 psi is applied to that side and then the pressure is monitored for 5 to 10 minutes. The rate at which the pressure decays is determined and if it exceeds an upper control limit (UCL), then the membrane is taken off-line for further testing and repair.
(Click this link for a discussion of the determination of the UCL: determining UCL.)
Note that a vacuum decay test could be used instead of a pressure decay test but a vacuum decay test is applied mostly to spiral-woundSpiral-wound: Spiral-wound membranes is a configuration in which sheets of a semi-permeable membrane, a porous support matrix, and a spacer are wrapped around a central filtrate collector tube. They are typically associated with nanofiltration (NF) and reverse osmosis (RO) membrane processes. nanofiltrationNanofiltration (NF): Nanofiltration is a pressure driven membrane process that uses reverse osmosis to remove dissolved materials. Nanofiltration applications are typically for softening or organics removal. and reverse osmosis (RO)Reverse osmosis (RO): Reverse osmosis is the reverse of the natural osmosis process, i.e., the passage of a water through a semi-permeable membrane from a solution of higher concentration of dissolved solids to a solution of lower concentration. Reverse osmosis applies a high pressure on the higher concentration side of the membrane to drive water through the membrane against the concentration gradient, in order to produce a water with a lower concentration of dissolved solids. membranes rather than MF and UF systems. A typical arrangement for a pressure decay test is shown in the figure below in which the filtrate side of the membrane is pressurized although feed side is possible too depending on the membrane system:
Figure PD.7. Schematic for a pressure decay test.
Advantages of the pressure decay test are that it can detect breaches in a single fiber or elsewhere in the system that could pass Cryptosporidium, most MF and UF systems come standard with the capability to perform a pressure decay test, it can be automated, and it is widely accepted by state regulatory agencies. Disadvantages are that the membrane unit must be taken off-line to perform the test, it may give false positives if the membrane is not fully wetted such as may occur with new membranes or with hydrophobic membranes or if the test is done immediately after a backwashBackpulse or backwash: 1) Backwash is a procedure in which periodically the flow direction is reversed through the membrane for a short period of time in order to remove particulates accumulated at the membrane surface. 2) Backwash also refers to the waste water produced as a result of the backwash procedure. that includes air, and the test may not be suitable for membranes oriented horizontally due to draining and air venting issues.
An alternative to the pressure-decay test is the diffusive airflow test in which the pressure is kept constant during the test and the airflow through the membrane system is measured. If there are no breaches then the airflow will be very small and will be due to diffusion of air through the membrane pores into the water on the other side of the membrane. This test is reportedly not nearly as widely used as the pressure-decay test described above.
Marker-based direct integrity tests. A marker-based direct integrity test is almost identical to a challenge test and follows most of the same criteria. The 'marker' is typically a particulate that is fed into the feed waterFeed water: The feed water is the water stream applied to the membrane unit. and serves as a surrogate for Cryptosporidium. Since the marker is a surrogate for Cryptosporidium, it must have an effective size less than or equal to 3 μm. The marker of course must also be inert and suitable for potable water. Other criteria for an acceptable marker would be the same as in challenge testing. The measurement method for the marker in the feed and filtrate must be capable of establishing a removal efficiency equal to or better than the LRV assigned to the membrane unit.
Application. A typical configuration for applying a particulate marker is shown in the figure below:
Figure PD.8. Schematic for applying a particulate marker.
As with a pressure-based direct integrity test, a UCL must be established which triggers the membrane unit to be take off-line for further testing and repair. (Click this link for a discussion of the determination of the UCL: determining UCL.)
The advantages of a marker-based direct integrity test are that it gives a direct measure of the system log removal value (LRV), does not necessarily require the membrane unit to be taken off-line, and the particle counting instruments could also potentially be used for indirect integrity testing. Disadvantages are the cost and calibration of the particle counters, costs of the marker, and potential special requirements for disposal of water containing the marker.
Follow-up testing and repair of integrity breaches. When the UCL for either a pressure-based or marker-based direct integrity test is exceeded, the membrane unit must be taken off-line and tested further to locate the breach and repair it. The location of the breach may be able to be indentified or narrowed by visually inspecting for bubbles if the membrane housing is transparent. Lacking that, there may be a clear tubing at the top of a pressure membrane module to inspect for bubbles or in a vacuum system it may be possible to observe bubbles coming to the water surface from the module breached. The specific membrane fibers or seals that have been breached may be able to be identified by removing the housing and submerging the unit in water and pressurizing it to a pressure similar to that used in the direct test and inspecting for bubbles. Alternatively, the membrane unit could be tested without removing the housing by draining the module, pressurizing the shell side of the membrane, removing the end cap of the module, applying a dilute surfactantSurfactant: Surfactants are essentially soaps or detergents that lower the surface tension of water to allow particulates to be more easily removed from surfaces. solution to the open ends of the fibers and observing any air bubble formation at the fiber ends in order to locate the fiber that has been breached. Sonic or acoustic testing has also been proposed which essentially listens for vibrations caused by leaking fibers. This method appears to be not fully developed as yet.
Once the source of the breach has been located, repairs can be performed. Breaches in the hollow fibersHollow-fibers: In hollow-fiber membranes, the membrane consists of a long spaghetti-like tube with a hollow core. The fibers are bundled together with the open ends encased in a resin. Flow can pass through the membrane either from the outside into the hollow membrane center or from the inside to the outside. Hollow fibers are typically used in microfiltration (MF) and ultrafiltration (UF) membrane processes. are repaired by plugging the ends of that fiber with epoxy or a pin which takes that fiber out of service.
