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74 4. SALINE WATER INTAKES AND PRETREATMENT
The largest seawater desalination plant with a seabed infiltration system in operation at present is the 50,000 m3/day (13.2 MGD) Fukuoka District RO facility in Japan. This plant has been in operation since 2006. The infiltration bed is 313.6 m long and 64.2 m wide (2 ha/5 acres), and it is designed to collect source seawater flow of 130,000 m3/day (34 MGD). The design infiltration velocity of this intake system is 0.25 m3/m2 h (0.10 gpm/ft2).
The filtration media in the well is configured in three distinctive layers (see Fig. 4.7): a bottomd2.3-m (7.5 ft) layer of graded gravel pack with stone sizes between 20 and 40 mm, which surrounds the horizontal well collectors; in the middled0.3-m (1.0 ft) deep interim layer of finer graded gravel of sizes between 2.5 and 13 mm; and on the topd1.5-m (5.0 ft) layer of natural sand excavated from the ocean bottom. The filtration media is submerged at 11.5 m (37.7 ft) below the ocean surface.
The filtered water collectors are 60-m (200 ft) long, 600-mm (24-in.) diameter polyethylene pipe screens (Fig. 4.8), which are installed at a distance of 5 m (16.4 ft) of each other. The col- lector pipes are designed for inflow velocity of 3 cm/sec (0.1 fps). The screens collect the source water flow into a central pipe with diameter of 1580 mm (62-in.) and length of 1178 m (3860 ft), which conveys it into a two-tank water collection well for pumping to the desalination plant. The collected water is pretreated with UF membrane filtration prior to desalination in SWRO membrane system.
4.2.5 Feasibility Considerations for Subsurface Intakes
4.2.5.1 Intake Site Location and Configuration
Site suitability for the construction of subsurface intake is determined by the following
main factors: the transmissivity/productivity of the geological formation and aquifer from
FIGURE 4.7 Seabed filtration media configuration of the Fukuoka SWRO plant.