Page 217 - ro membanes

200 9. MEMBRANE FILTRATION
submerged. Pressurized membrane systems use membrane elements installed in pressure vessels or housings and the membrane separation process in these systems is driven by of 0.2e2.5 bar of pressure. Submerged systems use membrane modules/cassettes, which are immersed in tanks and operate under a slight negative pressure (vacuum) of typically 0.2e0.8bar. The following issues are recommended to be considered when choosing between submerged and pressurized type of membrane pretreatment system.
9.4.3.1 Handling Source Water Quality Variations
Submerged membrane systems are usually more advantageous when treating saline source water of variable quality in terms of turbidity, such as intake surface waters that expe- rience frequent turbidity fluctuations of 20 NTU or more. Because these membrane systems are located in tanks (vessels) with relatively large retention volume, they can equalize the source water solids load in the tanks and thereby can reduce the impact of water quality fluc- tuations on pretreatment system performance. Because shallow open intakes often yield source water with wide turbidity fluctuations, submerged pretreatment systems are usually more suitable for such applications.
Pressurized membrane systems have limited capacity to retain solids because the individ- ual membrane elements are located in a tight membrane vessel of a very small solid retention volume. Therefore, if a pressurized system is exposed to a large amount of solids, the mem- brane elements and vessels would fill up with solids very quickly, which in turn would trigger frequent membrane backwash and result in destabilization of the membrane system performance. If a pressurized membrane system is overloaded in terms of solids, this system will have to be derated to its capacity to hold solids. Otherwise, such systems will experience very frequent backwashes and ultimately interruption of normal operations.
To address this deficiency of the pressurized membrane systems, some membrane manu- facturers offer membrane modules with adjustable fiber density, which allows customizing membrane system design to the more challenging water quality. Typically, these customized membrane elements have fewer fibers and more empty space within the membrane element and thereby provide more volume to retain higher influent solids loads. However, this cus- tomization is usually at the expense of the installation of more membrane elements and over- all enlarges the size and costs of the membrane system.
Typically, the tanks in which submerged membrane elements are installed provide a min- imum hydraulic retention time of 10e15 min and have an order of magnitude higher volume and capacity available to handle saline source water of elevated turbidity and to temporary store solids. This renders submerged membrane pretreatment systems more suitable for high- turbidity water applications. The aeration scouring, which submerged systems typically apply for backwashing, also improves their tolerance to high solid loads. In addition, because submerged systems usually operate at lower TMP, their rate of membrane fouling is lower and they have more stable operation during transient solid load conditions.
Pressurized membrane pretreatment systems, however, are often more suitable for cold sa- line source waters (i.e., for saline waters of minimum monthly average temperature of 15C or less). Productivity of submerged systems is more sensitive to source water temperature/ viscosity. The maximum trans-membrane operational pressure available for submerged membrane systems is limited to one atmosphere of vacuum, although in practical terms such systems operate at lower maximum TMP (0.7e0.8 bar).