As stated elsewhere in this website, indirect integrity monitoringIndirect integrity monitoring:
 Indirect integrity monitoring consists of measuring some water quality parameter of the filtrate, e.g., turbidity or particle concentration, as an indicator of treatment effectiveness. methods rely on measuring water quality parameters such as turbidityTurbidity:
  Turbidity is a measure of  the cloudiness of a water as well as a gross measure of the amount of suspended solids in a water. which may be able to alert the facility to integrity failures in the membrane modules. Any unusual rise in turbidity is potentially a sign of a compromised membrane in which case a direct integrity testDirect 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. would be conducted. Such indirect methods are usually conducted "continuously." Indeed in order to receive treatment credit for 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., 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)./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 systems must meet the indirect integrity monitoring requirements of 40 CFR 141.719. These are:

• Must conduct continuous indirect integrity monitoring on each membrane unit (Note that continuous monitoring is defined as a measurement at least every 15 minutes. Also note that a system that conducts continuous direct integrity monitoring need not conduct indirect monitoring but that would be very rare with current technology),

• Must submit a monthly report to the State summarizing results that triggered direct integrity testing and the corrective action that was taken in each case,

• Must include continuous filtrateFiltrate:
  Filtrate is the water that has passed through the membrane. turbidity monitoring as the indirect integrity monitoring method unless the state approves a different parameter,

• Must be conducted separately on each membrane unit, and

• "If indirect integrity monitoring includes turbidity and if the filtrate turbidity readings are above 0.15 NTUNTU:
  NTU stands for nephelometric turbidity unit and is a measure of turbidity, i.e., the cloudiness of a water, and is a gross measure of the amount of suspended solids in a water. for a period greater than 15 minutes (i.e., two consecutive 15-minute readings above 0.15 NTU), direct integrity testing must immediately be performed on the associated membrane unit," (Note that this same requirement would apply to monitoring any alternative parameter when its corresponding 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.).

Importantly, turbidity may not always be very sensitive to integrity breachesIntegrity breach:
  A breach refers to leakage in the membrane module due to 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.. One study on a Wisconsin utility could not detect noticeable turbidity increases even when a substantial number of fibers were cut. Also, air may become entrained in the system during 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., chemical cleaningClean-in-place (CIP) or in-situ chemical cleaning:
 Clean-in-place is a procedure performed periodically to clean a membrane more thoroughly than backwashing can achieve in order to restore the permeability of the membrane towards baseline levels. The process uses chemicals such as citric acid and chlorine or others to remove accumulated foulants on the membrane., integrity testing or even during production. Thus, turbidimeters with bubble traps are typically used to reduce misleading high turbidity values.

Particle counting/monitoring. Besides turbidity measurements, particle counting/monitoring is also frequently used for indirect integrity monitoring under 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.. Particle counting/monitoring is not specified in the LT2ESWTR but states can approve it as an alternative to turbidity. Although turbidity is easier to measure, there is a complex relationship between turbidity and particle counts since turbidity depends on the number, shape, and refractive index of the particles as well as the light wavelength. Particle counting eliminates some of these limitations and is believed to be more sensitive than turbidity but has been criticized for often giving inconsistent readings. Additionally, the equipment is much more expensive. If particle counting is used then it is recommended that the counters be calibrated to detect particles greater than 3 µmµm:
  A µm stands for one micrometer (or micron) which is a unit of length equal to one millionth of a meter. since Cryptosporidium oocysts are typically 3 to 7 µm. Thus, any increase in particle counts greater than 3 µm would possibly indicate that a breach has occurred that could allow oocysts to pass. Setting a control or action limit similar to the 0.15NTU limit for turbidity is more complex for particle counting/monitoring especially since particle counting/monitoring data can vary significantly between instruments and between membrane units. Therefore site specific limits are needed. Texas sets a site specific control limit based on the 95% confidence interval for all the data collected from a specific unit instrument on a specific membrane unit. The reader is referred to the EPA Membrane Filtration Guidance Manual (USEPA 2005) for additional information.