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7.3 SEDIMENTATION TANKS 139
FIGURE 7.1 Sand strainers.
7.2.3 Cyclone Separators
Cyclone separators have found application for removal of sand from groundwater, espe- cially for small desalination plants. In such systems the inlet pressure from the intake well pumps drives the source water into the top of the separator chamber at a tangent, causing rotation and formation of vortex in the center of the separator. The vortex action forces the separation of heavy particles from the water. These particles accumulate at the bottom in a collection chamber from where they are periodically removed. In most recent desalination plant designs, cyclone separators and strainers are replaced by microscreens described in Chapter 5.
7.3 SEDIMENTATION TANKS
7.3.1 Introduction
Sedimentation is typically used upstream of granular media and membrane filters when the membrane-plant source water has daily average turbidity higher than 30 NTU or experi- ences turbidity spikes of 50 NTU or more which continue for a period of over several hours. If sedimentation basins are not provided, large turbidity spikes may cause the pretreatment fil- ters to exceed their solids’ holding capacity (especially if granular media filters are used), which in turn may impact filter pretreatment capacity. If the high solids load continues, the pretreatment filters would enter a condition of continuous backwash, which in turn would render them out of service and effectively will shut down the desalination plant operations.
Sedimentation basins for saline source water pretreatment should be designed to produce settled water with turbidity of less than 2.0 NTU and SDI15 below 6. To achieve this level of