Page 103 - ro membanes

86 4. SALINE WATER INTAKES AND PRETREATMENT
Typically, the offshore open-ocean intake structure is located several hundred to several thou- sand meters from the shore. The best location of the offshore intake structure in terms of source water quality is at ocean floor depths of 20 m (66 ft) or higher (deep water intake). Debris load in the source water and algal content during red tides at such depths are typically several times lower than that in the surface water or the shallow waters of the tidally influenced near-shore area.
Depending on the plant location and ocean floor formation, installing the intake structure at 20 m (66 ft) depth may require intake pipeline, which is between 200 and 5000 m (1320 and 16,500 ft) long. Because the construction cost for intake pipeline located on the ocean floor is usually very high (between 4 and 10 times higher than the cost of the same size pipe installed inland in the ground), the intake water quality benefits of locating the offshore intake structure in deep waters have to be compared against the costs for construction of the intake structure and pipeline and the cost of the source water pretreatment system.
The best location for an open offshore intake from a lifecycle cost point of view is typically such site where the ocean floor depth of 20 m (66 ft) or more can be reached within 500 m (1650 ft) from the shoreline. If such ocean floor location is not available within a reasonably close vicinity of the RO desalination plant, usually it is more cost-effective to collect source water of inferior water quality and build a more elaborate pretreatment system, than to install a costly offshore intake structure and a long intake pipeline.
Because of the high costs of deep intake structures and long pipelines, many existing SWRO desalination plants with open-ocean offshore intakes are located in shallow near- shore areas where the ocean floor depth is typically between 4 and 8 m (13e26 ft)dsuch intakes are also referred to as shallow water intakes. As a result, plants with shallow water intakes usually have source water with high content of debris, solids, and aquatic organisms, which requires elaborate pretreatment prior to RO-membrane separation.
Offshore intakes that usually extend several hundred meters away from the shoreline and 10e20 m (33e66 ft) below the water surface are typically not influenced by freshwater from surface runoff. Therefore, such intakes usually yield saline water of the same TDS content as that of ambient water.
One exception is offshore intakes located near the entrance of large rivers or other fresh- water bodies into the saline water body (ocean, sea, brackish lake). Since old river beds and associated alluvial aquifers could often extend far beyond the tidal zone, the water qual- ity of such offshore intakes may be influenced by the groundwater quality in the alluvial aquifers, which often is inferior than the open-ocean seawater in terms of content of iron, manganese, and other undesirable contaminants.
4.3.3 Colocated Intakes
The colocated desalination plant intakes are directly connected to the discharge outfall of an adjacently located coastal power plant using seawater for once-through cooling. This configuration allows using the power plant cooling water both as source water for the desa- lination plant and as a blending water to reduce the salinity of the desalination plant concen- trate prior to the discharge to the ocean (see Fig. 4.12).
In seawater-cooled coastal power plants, the seawater enters the power plant intake facil- ities and after screening is pumped through the power plant condensers to cool them and thereby to remove the waste heat generated during the electricity generation process. The