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4.2 SUBSURFACE INTAKES 67
In contract to brackish desalination plants where subsurface intakes are most common; at present less than 10% of seawater RO desalination plants worldwide use subsurface intakes. Such limited use of subsurface intakes for seawater desalination is mainly due to the fact that most coastal aquifers worldwide are limited in terms of sustainable volume of water that could be collected from them. Such limitation usually stems from the geology (e.g., limited natural water transmissivity) of coastal aquifers and the associated higher construction complexity and costs of subsurface intakes, especially for medium- and large-size desalination plants.
Several types of subsurface intakes have found application for collecting source water for SWRO desalination plants: (1) vertical wells; (2) horizontal directionally drilled (HDD) wells; (3) horizontal Ranney-type wells; and (4) infiltration galleries. The first three types of subsur- face intakes are typically located on the seashore, in a close vicinity to the ocean. Vertical wells are used for both brackish and seawater desalination plants. The other three types of wells have found application mainly for seawater desalination plants.
The source water collected by subsurface intakes is pretreated via slow filtration through the bottom soil formations in the area of source water extraction. Therefore, this source water is expected to be of better quality in terms of solids, slit, oil & grease, and organic and microbial fouling potential, as compared to that collected by open intakes. However, practical experience shows that the notion that subsurface intakes collect better water quality than open intakes only holds true for site-specific conditions: usually when subsurface intakes are located on the shore or under the bottom of well naturally flushed surface water body (e.g., sea, ocean, brackish river); when they are not influenced of surface freshwater (e.g., not located in a confluence of river and ocean); and when they are collecting saline water from an aquifer of uniformly porous structure, such as limestone.
There are numerous small seawater desalination plants located in the Caribbean and several medium-size plants in Malta and Oman, which have such intakes and which require only minimal pretreatment (typically bag filters and/or sand strainers) ahead of RO pretreat- ment. However, the majority of the existing seawater desalination plants worldwide using subsurface intakes require an additional filtration pretreatment step prior to membrane salt separation. In addition, deep, well-configured and positioned open intakes often deliver saline source water of comparable or better quality than shallow coastal wells.
In summary, the assumption that subsurface intakes always deliver better water quality than open intakes does not hold true for all conditions and the selection of the most viable type of intake for a given desalination project has to be based on a detailed life-cycle cost analysis, which accounts for the site-specific project conditions and the impact of the quality of the source water collected by the intake on the configuration and costs of the downstream pretreatment system. The various types of subsurface intakes and their impact on desalina- tion plant pretreatment needs are discussed below.
4.2.1 Vertical Wells
Vertical wells (Fig. 4.1) are the most commonly used type of subsurface intakes at present. Majority of existing BWRO desalination plants worldwide use vertical wells to collect groundwater from brackish aquifers. Similarly, over 30% of small SWRO desalination plants in operation at present apply vertical wells to obtain seawater from coastal aquifers for