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70 4. SALINE WATER INTAKES AND PRETREATMENT
FIGURE 4.2 Horizontal Ranney-type intake well.
Ranney intake wells are typically coupled with a pump station installed above the well caisson. The pump station can be designed with submersible pumps to minimize noise levels. However, intakes that consist of mid- and large-size wells most frequently employ horizontal or vertical turbine pumps because these pumps usually have higher energy efficiency and require less power than submersible pumps.
Ranney wells are not as commonly used as vertical wells for seawater intakes. The largest horizontal-well installation is located in Salina Cruz, Mexico (Fig. 4.3) and consists of three wells, which are designed to deliver 168 Lps (3.8 MGD) of seawater, each. The wells are located on the beach and the water quality they collect is not directly suitable for seawater desalination. The source seawater from these wells contains high levels of iron and manga- nese and has to be pretreated in greensand filters prior to SWRO separation.
One potential challenge with Ranney wells is that if the source water contains hydrogen sulfide, this compound will likely be oxidized to elemental sulfur, which could subsequently cause RO membrane fouling (Missimer et al., 2010).
4.2.3 Horizontal Directionally Drilled Wells
The HDD intake wells consist of relatively shallow blank well casting with one or more horizontal perforated screens bored under an angle (typically inclined at 15e20 degrees)