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66 4. SALINE WATER INTAKES AND PRETREATMENT 4.1 INTRODUCTION
The saline source water intake is one of the key components of every desalination plant. The primary purpose of the intake is to ensure adequate and consistent flow and quality of source water over the entire useful life of the desalination plant. The type and configura- tion of intake selected for a given desalination project has a significant impact on the nature and quantity of foulants contained in the source water and, therefore on the complexity of the pretreatment system needed to control RO membrane fouling (Henthorne and Boysen, 2015).
Currently, there are two widely used types of desalination plant source water collection facilitiesdsurface (open) and subsurface (groundwater, well) intakes. Open intakes collect source water directly from a surface water body (e.g., ocean, sea, brackish river, or lake) via onshore or offshore intake structure and pipeline connecting this structure to the desali- nation plant. The source water collected by this type of intakes usually contains debris, coarse and fine solids and silt, and aquatic organisms (e.g., fish, algae, bacteria, etc.), which make the pretreatment of this water more complex.
Subsurface intakes tap into brackish groundwater aquifer, or saline coastal aquifer to collect source water for the desalination plant. Source water obtained using subsurface intakes is conveyed through the soils of the aquifer, which naturally filter out debris, marine organisms and some organics, and thereby the RO-membrane fouling potential of the source water is reduced. As a result, the use of subsurface intakes for source water collection usually requires simpler pretreatment than source water collected by open intakes.
4.2 SUBSURFACE INTAKES
Subsurface intakes are the predominant type of source water collection facilities for brackish desalination plants of all sizes. Most BWRO plant intake wells collect water from deep confined aquifers of low to medium salinity (i.e., TDS of 600e3000 mg/L). Typically, deep brackish wells yield source water of low turbidity (<0.4 NTU) and silt content (SDI15 < 2), which can be processed through the RO system with minimal or no pretreatment (usually cartridge or bag filtration only).
Subsurface intakes for seawater desalination plants collect source water from either a saline near-shore (coastal) aquifer and/or from offshore aquifer under the ocean floor. The salinity of the coastal aquifers varies as a result of ocean water tidal movement and of changes in water level and salinity of the fresh and/or brackish groundwater aquifer/s hydraulically connected to the coastal aquifer. As a result, coastal aquifers are typically a source of saline water of TDS concentration lower than that of open-ocean water. For compar- ison, offshore aquifers yield water quality that is usually of the same TDS concentration as that of the ambient seawater. In addition, typically offshore aquifers collect seawater of lower RO-membrane fouling potential than coastal aquifers.
As compared to offshore aquifers, coastal aquifers have the disadvantage that their water quality is more variable and sensitive to changes in the water quality and volume of the fresh groundwater that drains into them. Therefore, for example, rain events could cause significant variations of the source water TDS, turbidity, iron and manganese and oxidationreduction potential, which typically have negative impact on the operation of the pretreatment and RO systems of the desalination plant.