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12.2 SWRO MEMBRANE ELEMENTSdKEY TYPES AND PRETREATMENT CONSIDERATIONS 255 12.2.2 Standard Rejection SWRO Membrane Elements
Standard rejection membrane elements are designed to remove up to 99.6% of the salts from the source seawater. These membrane elements are most widely used at present and have found applications in variety of RO-system configurations. Such SWRO membrane el- ements have spacer of 28-mil and are suitable for seawater with low- and moderate fouling potential and well-deigned pretreatment system (see Chapter 11 for design guidelines).
12.2.3 High-Rejection SWRO Membrane Elements
High-rejection membrane elements are designed with tighter membrane structure allowing to increase the mass of rejected ions, and to better reject smaller size ions, such as boron for example. The higher rejection membrane capabilities of 99.75%e99.85% come at a priced10%e20% higher operating pressure. In general, these membrane elements are also more prone to fouling as compared to standard rejection SWRO membrane elements and their use requires more elaborate seawater pretreatment in terms of particulate, colloidal, and microbial foulants.
12.2.4 High-Productivity (Low-Energy) Membrane Elements
High-productivity membrane elements are designed with features to yield more product water per membrane element. These features are: (1) higher surface area and (2) denser mem- brane packing. Increasing active membrane envelope surface area allows gaining significant productivity for the same size (diameter) membrane element. Higher productivity of the membrane elements is obtained by using higher permeability membranes with wider molec- ular channels. Therefore, the salt rejection of these elements is lower than that of standard and high rejection SWRO membranes. The high productivity elements have a standard yield of 9000e12,000 gallons per day (gpd) and salt rejection of 99.2%e99.4%.
Increasing membrane size/diameter can also increase the total active surface area of a membrane element. Although 20-cm (8-in.) SWRO membrane elements are still the “stan- dard” size and are most widely used in large full-scale applications, larger 16- and 18-in. size SWRO-membrane elements are currently available.
Another alternative for improving membrane productivity is increasing the number of membrane leafs packed into the same size (diameter) membrane. This is accomplished either by the use of thinner feed channel spacers, or by improving the element construction. Using thinner feed spacers typically increases the membrane pressure drop. As a result, higher pro- ductivity membrane elements using this approach also have higher operational pressure re- quirements for the same salt rejection level and flux.
Denser membrane-leaf packing makes membranes also more susceptible to fouling and their use requires high-quality source water and more elaborate pretreatment. To address this issue, the newest high-productivity membrane elements actually use wider spacer to compensate for the increased fouling potential and pressure.
The dynamics of the high-productivity (or low energy) membrane element development is illustrated by an example of the development of seawater membranes. In the second half of 1990s the typical 20-cm (8-in.) SWRO-membrane element had a standard productivity of