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196 9. MEMBRANE FILTRATION
If a hydraulic surge is triggered by the intake pumps and the surge protection of the pipe- line connecting the intake pump station to the feed of the membrane pretreatment system is inadequate, this surge could create pressures along the water flow path well above two times the operating pressure (i.e., over 5 bars), which the membrane material and potting interface may not be able to withstand, if their strength is inadequate. As a result, current practice shows that use of PES pretreatment membranes with low-pressure rating (e.g., 3 bars) in seawater reverse osmosis (SWRO) plants with direct coupling configuration experiences frequent fiber breakages, loss of membrane integrity, and short useful life.
In general, PVDF membrane systems are better suited to handle pressure surges and have enhanced durability and chlorine resistance as compared to PES membranes. However, this type of membrane product is also more expensive and has lower permeability and caustic resistance than PES. In addition, PES membranes have narrower pore size distribution, which could be beneficial in terms of the filtered water quality.
Practical experience to date shows that PVDF membranes are more suitable than PES membranes for the treatment of saline source waters of high biofouling potential such as these of the Persian Gulf and Red Sea. However, PVDF membranes are more costly and despite the obvious disadvantages of PES membranes for such highly fouling waters, to date PES membranes have found wider application because of their lower costs, especially in countries with procurement regulations where equipment and membrane selection are based on the lowest capital costs only and the quality of the products is not factored in the evaluation process (e.g., design-build project procurement).
9.3.2.2 Membrane Geometries
The most widely used membranes in saline water pretreatment have hollow fiber, tubular, and spiral wound geometry. Hollow fiber membranes typically consist of several hundred to several thousand membrane fibers bounded at each end by epoxy or urethane resin and encased in individual modules. Typically, the internal diameter of the membrane fibers is 0.4e1.5 mm.
Depending on the membrane manufacturer, the hollow-fiber (capillary) membrane ele- ments may be operated in an inside-out or outside-in flow pattern. Inside-out mode of oper- ation provides a better control of flow and more uniform flow distribution than outside-in flow pattern. However, outside-in flow pattern usually results in lower headlosses through the module and operation under this pattern is less sensitive to the amount of solids in the saline source water.
Tubular membranes have inner tube diameters, which are an order of magnitude larger than that of hollow fiber membranes (i.e., 1.0e2.5 cm). The individual membrane tubes are placed inside fiberglass-reinforced plastic or stainless steel tube and the two ends of the tube are sealed with gasket or other clamp-type device. The typical flow pattern for these mem- branes is inside-outei.e., the saline source water is introduced into the tube lumens under pressure and it flows through the walls of the membrane tubes into the outside shell of the module.
The key advantages of hollow fiber membrane elements are as follows: (1) high surface area to volume (packing density), which allows the reduction of the overall footprint of the filtration system; (2) easier backwash; (3) filtration at lower feed pressuredTMP is typi- cally 0.2e1.0 bar; (4) lower pressure drop across the membrane modules (0.1e1.0 bar).