Page 92 - ro membanes

4.2 SUBSURFACE INTAKES 75
FIGURE 4.8 Segment of 600-mm intake collector screen.
which saline water is collected; the depth and stability of the coastal zone through which the source water is naturally filtered; underground sources of contamination of the intake area (e.g., landfills, cemeteries, leaking underground oil tanks); and the existence of nearby fresh- water source aquifers, which could be negatively impacted by the intake operations or have measurable effect on the quality of the collected water.
The geological conditions that favor the construction of subsurface intakes are permeable sand and limestone- or dolomite-type geological formations with high transmissivity and depth. Such conditions are not very common in coastal zones worldwide, except for some locations where subsurface intakes have been used successfully such as Malta, portions of the coastal zone of Oman, and some of the Caribbean islands.
Beaches of shallow bays that contain significant amount of mud/alluvial deposits and have limited natural flushing do not favor the use of wells for RO desalination plant intakes. High content of fine solids in the bay seawater in combination with low frequency of bay flushing and low transmissivity of the beach deposits may render shallow bay beaches less desirable or unsuitable for construction of well intakes. If the selected bay is shallow and poorly flushed, the water quality collected by the subsurface intake is likely to be variable and of high content of silt and other substances, which would require significant expendi- tures for source water pretreatment prior to reverse osmosis (RO).
It should be pointed out that both wells and near-shore open intakes use the same seawater as a source. In desalination plants with open intakes, the solids contained in the source seawater are removed in the desalination plant pretreatment filtration system in controlled and closely monitored conditions. In desalination plants with subsurface intakes, the same amount of solids is retained on the ocean floor in the area of source water collection while the filtered water is slowly conveyed through the ocean floor and the beach soils until it reaches the intake collectors.