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194 9. MEMBRANE FILTRATION 9.2.4 Membrane Integrity Testing
All membrane pretreatment systems are equipped with integrity testing features that allow detecting occasional loss of membrane filtration capability caused by breaks or punc- tures of the membrane fibers or leafs, cracks of the membrane modules, piping, and connec- tors, and other problems that could occur during membrane production, installation, or operation. The most widely used membrane system integrity test is a pressure-hold/visual test, which is performed while the membrane pretreatment module/rack is offline.
During the pressure hold test, water is purged from the system using filtrate and then air is applied under pressure of 0.3e1.0 bar (4.2e14.2 psi) and decay of air pressure is monitored over time. Typically, the membrane module integrity is adequate when the pressure loss over 5-min period is less than 10% of the initial pressure applied to the membranes. The pres- sure decay test procedures vary for the various commercially available membrane products and configurations and, therefore, the membrane integrity testing system and conditions have to be coordinated with the membrane system supplier/equipment manufacturer.
Besides pressure-hold test, other offline membrane integrity tests used are vacuum hold test, bubble point test, diffusive air flow tests, etc. (AWWA, 2005). In addition, membrane integrity is monitored online by particle passage counting, filter effluent turbidity measure- ments of the individual membrane modules, or acoustic sensing.
The most popular method for online membrane integrity monitoring is the continuous effluent turbidity measurement of the individual membrane modules (trains) comprising the pretreatment system. Usually, the breach of integrity of a given train/module in a system is identified by comparing the filtered effluent turbidity of the train/module to that of the average turbidity from all modules and the turbidity of the other membrane pretreatment racks.
9.3 KEY FILTRATION SYSTEM COMPONENTS 9.3.1 Filter Vessels/Modules
MF and UF filtration membranes are configured in individual functional filtration units referred to as modules. Most commonly used membrane module configurations are hollow fiber, tubular, flat sheet, and spiral. The membrane modules are contained in housings, shells, or cassettes, which are assembled into larger membrane filtration system components, vessels and racks.
9.3.2 Membrane Filtration Media
9.3.2.1 Membrane Materials
Membranes used for saline water pretreatment are typically made of polyethersulfone (PES), polyvinylidene difluoride (PVDF), or polysulfone (PS). All of the membrane products made of these materials are hydrophilic. The PES is the most hydrophilic of all of these ma- terials. Hydrophilic materials have two key advantages: (1) they do wet easily which makes them more permeable for a given pore size; and (2) they have higher resistance to attachment