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68 4. SALINE WATER INTAKES AND PRETREATMENT
FIGURE 4.1 Vertical intake well.
freshwater production. Vertical wells that collect seawater from near-shore coastal aquifers are also referred to as beach wells.
Vertical wells consist of the following key components: casting, well screen, filter pack, well seal, and a surface seal. These wells are equipped with submersible or vertical turbine pump installed inside the well casting. Well casting is a steel or nonmetallic (typically, fiberglass) pipe, which lines the well borehole to protect wells from caving.
The diameter of the casting has to be of size adequate to house the well intake pump and to provide ample room for pump service. While the diameter of the well casting is determined mainly by the well screen size and yield, the well borehole diameter has to be at least 0.1 m (4 in.) larger than the well casting to accommodate the installation of the well seal. Usually, the well casting diameter is between 200 and 1200 mm (8 and 48 in.), and well depth is typically less than 75 m (250 ft).
The well screen is the intake portion of the well and is a sieve-like structure with slotted or perforated openings. The screen is located at a depth corresponding to the water-carrying zone of the aquifer. Screen depth, size of openings, diameter, and length are key well- performance design criteria. These well parameters are selected to maximize well’s safe yield; control well entrance velocity; and to avoid excessive entrance of sand and other particulates, which have a negative impact on the well’s useful life and water quality and on the down- stream pretreatment system. Detailed guideline for selection of water-well castings and screens is provided elsewhere (Roscoe Moss Company, 2012; Voutchkov, 2013).