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102 5. PRETREATMENT BY SCREENING
FIGURE 5.6 Wedge-wire screens of Beckton desalination plant, London, UK.
Wedg-wire screens combine very low flow-through velocities (10e15 cm/s/0.3e0.5 ft/s), small slot size, and naturally occurring high screen-surface sweeping velocities to minimize impingement and entrainment. These screens are designed to be placed in a water body where significant prevailing ambient cross-flow current velocities [!0.3 m/s (1.0 fps)] occur over 90% of the time. This high cross-flow velocity allows organisms that would otherwise be impinged on the wedge-wire intake, to be carried away with the flow. Therefore, wedge-wire screens are considered by the USEPA best technology available for impingement and entrainment reduction (WRA, 2011).
An integral part of a typical wedge-wire screen system is an air burst back-flush system, which directs a charge of compressed air to each screen unit to blow off debris back into the water body, where they are carried away from the screen unit by the ambient cross-flow currents.
The screens will need to be installed at a minimum of 1 m (3.3 ft) from the bottom to avoid entrance of sand and silt into the screens. If the intake is located in relatively shallow, tidally influenced area the depth from the bottom is recommended to be increased to a minimum of 2.0 m (6.6 ft) to prevent the entrance of bottom sediments in the intake water.
Typically, the material used for such screens is copper-nickel alloy (Cu/Ni 90/10), supper duplex stainless steel, or titanium. Copper-nicker alloys usually offer optimum combination between reasonable costs and corrosion and erosion resistance.
5.2.3 Construction Costs of Drum- and Band Screens
The graphs presented in Fig. 5.7 provide budgetary-level construction cost estimates for band- and drum screens as a function of the desalination plant feed-water flow. As shown